Red Hat Enterprise Linux 5.0.0 Red Hat Enterprise Linux Deployment Guide

Red Hat Enterprise Linux 5.0.0 Red Hat Enterprise Linux Deployment Guide
Red Hat Enterprise Linux 5.0.0
Red Hat Enterprise Linux Deployment
Guide
Red Hat Enterprise Linux 5.0.0: Red Hat Enterprise Linux Deployment Guide
Copyright © 2007 Red Hat, Inc.
This Deployment Guide documents relevant information regarding the deployment, configuration and administration of Red Hat Enterprise Linux 5.0.0.
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Table of Contents
Introduction ............................................................................................................ xxii
1. Document Conventions ............................................................................... xxii
2. Send in Your Feedback ............................................................................... xxv
I. File Systems ........................................................................................................... 1
1. File System Structure ..................................................................................... 2
1. Why Share a Common Structure? ........................................................... 2
2. Overview of File System Hierarchy Standard (FHS) ................................. 2
2.1. FHS Organization ........................................................................ 2
3. Special File Locations Under Red Hat Enterprise Linux ............................ 6
2. The ext3 File System ...................................................................................... 7
1. Features of ext3 ..................................................................................... 7
2. Creating an ext3 File System .................................................................. 7
3. Converting to an ext3 File System ........................................................... 8
4. Reverting to an ext2 File System ............................................................. 9
3. The proc File System ....................................................................................10
1. A Virtual File System .............................................................................10
1.1. Viewing Virtual Files ...................................................................10
1.2. Changing Virtual Files ................................................................11
2. Top-level Files within the proc File System .............................................11
2.1. /proc/apm ..................................................................................11
2.2. /proc/buddyinfo ..........................................................................12
2.3. /proc/cmdline .............................................................................12
2.4. /proc/cpuinfo ..............................................................................13
2.5. /proc/crypto ................................................................................14
2.6. /proc/devices .............................................................................14
2.7. /proc/dma ..................................................................................15
2.8. /proc/execdomains .....................................................................15
2.9. /proc/fb ......................................................................................15
2.10. /proc/filesystems ......................................................................15
2.11. /proc/interrupts .........................................................................16
2.12. /proc/iomem .............................................................................17
2.13. /proc/ioports .............................................................................17
2.14. /proc/kcore ...............................................................................18
2.15. /proc/kmsg ...............................................................................18
2.16. /proc/loadavg ...........................................................................18
2.17. /proc/locks ...............................................................................18
2.18. /proc/mdstat .............................................................................19
2.19. /proc/meminfo ..........................................................................19
2.20. /proc/misc ................................................................................21
2.21. /proc/modules ..........................................................................21
2.22. /proc/mounts ............................................................................22
2.23. /proc/mtrr .................................................................................23
2.24. /proc/partitions .........................................................................23
2.25. /proc/pci ...................................................................................23
2.26. /proc/slabinfo ...........................................................................24
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2.27. /proc/stat ..................................................................................25
2.28. /proc/swaps .............................................................................26
2.29. /proc/sysrq-trigger ....................................................................26
2.30. /proc/uptime .............................................................................26
2.31. /proc/version ............................................................................27
3. Directories within /proc/ .........................................................................27
3.1. Process Directories ....................................................................27
3.2. /proc/bus/ ..................................................................................29
3.3. /proc/driver/ ...............................................................................30
3.4. /proc/fs ......................................................................................30
3.5. /proc/ide/ ...................................................................................30
3.6. /proc/irq/ ....................................................................................32
3.7. /proc/net/ ...................................................................................32
3.8. /proc/scsi/ ..................................................................................33
3.9. /proc/sys/ ...................................................................................35
3.10. /proc/sysvipc/ ...........................................................................46
3.11. /proc/tty/ ..................................................................................47
4. Using the sysctl Command ....................................................................47
5. Additional Resources ............................................................................48
5.1. Installed Documentation .............................................................48
5.2. Useful Websites .........................................................................48
4. Redundant Array of Independent Disks (RAID) ...............................................49
1. What is RAID? ......................................................................................49
2. Who Should Use RAID? ........................................................................49
3. Hardware RAID versus Software RAID ...................................................49
3.1. Hardware RAID ..........................................................................49
3.2. Software RAID ...........................................................................50
4. RAID Levels and Linear Support ............................................................50
5. Configuring Software RAID ....................................................................51
5.1. Creating the RAID Partitions .......................................................52
5.2. Creating the RAID Devices and Mount Points ..............................55
5. Swap Space .................................................................................................61
1. What is Swap Space? ...........................................................................61
2. Adding Swap Space ..............................................................................61
2.1. Extending Swap on an LVM2 Logical Volume ..............................62
2.2. Creating an LVM2 Logical Volume for Swap ................................62
2.3. Creating a Swap File ..................................................................63
3. Removing Swap Space .........................................................................63
3.1. Reducing Swap on an LVM2 Logical Volume ...............................63
3.2. Removing an LVM2 Logical Volume for Swap ..............................64
3.3. Removing a Swap File ................................................................64
4. Moving Swap Space .............................................................................65
6. Managing Disk Storage .................................................................................66
1. Standard Partitions using parted ............................................................66
1.1. Viewing the Partition Table .........................................................67
1.2. Creating a Partition .....................................................................68
1.3. Removing a Partition ..................................................................70
1.4. Resizing a Partition ....................................................................71
2. LVM Partition Management ...................................................................72
7. Implementing Disk Quotas .............................................................................75
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1. Configuring Disk Quotas ........................................................................75
1.1. Enabling Quotas ........................................................................75
1.2. Remounting the File Systems ......................................................76
1.3. Creating the Quota Database Files ..............................................76
1.4. Assigning Quotas per User .........................................................77
1.5. Assigning Quotas per Group .......................................................78
1.6. Setting the Grace Period for Soft Limits .......................................78
2. Managing Disk Quotas ..........................................................................78
2.1. Enabling and Disabling ...............................................................78
2.2. Reporting on Disk Quotas ...........................................................79
2.3. Keeping Quotas Accurate ...........................................................79
3. Additional Resources ............................................................................80
3.1. Installed Documentation .............................................................80
3.2. Related Books ...........................................................................80
8. Access Control Lists ......................................................................................81
1. Mounting File Systems ..........................................................................81
1.1. NFS ..........................................................................................81
2. Setting Access ACLs .............................................................................81
3. Setting Default ACLs .............................................................................83
4. Retrieving ACLs ....................................................................................83
5. Archiving File Systems With ACLs .........................................................83
6. Compatibility with Older Systems ...........................................................84
7. Additional Resources ............................................................................84
7.1. Installed Documentation .............................................................84
7.2. Useful Websites .........................................................................85
9. LVM (Logical Volume Manager) .....................................................................86
1. What is LVM? .......................................................................................86
1.1. What is LVM2? ..........................................................................87
2. LVM Configuration ................................................................................87
3. Automatic Partitioning ...........................................................................88
4. Manual LVM Partitioning ........................................................................89
4.1. Creating the /boot/ Partition .........................................................89
4.2. Creating the LVM Physical Volumes ............................................92
4.3. Creating the LVM Volume Groups ...............................................94
4.4. Creating the LVM Logical Volumes ..............................................95
5. Using the LVM utility system-config-lvm ..................................................98
5.1. Utilizing uninitialized entities ......................................................101
5.2. Adding Unallocated Volumes to a volume group .........................102
5.3. Migrating extents ......................................................................104
5.4. Adding a new hard disk using LVM ............................................106
5.5. Adding a new volume group ......................................................107
5.6. Extending a volume group ........................................................109
5.7. Editing a Logical Volume ..........................................................110
6. Additional Resources ..........................................................................113
6.1. Installed Documentation ...........................................................113
6.2. Useful Websites .......................................................................113
II. Package Management .........................................................................................114
10. Package Management with RPM ...............................................................115
1. RPM Design Goals ..............................................................................115
2. Using RPM .........................................................................................116
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2.1. Finding RPM Packages ............................................................116
2.2. Installing ..................................................................................117
2.3. Uninstalling ..............................................................................118
2.4. Upgrading ................................................................................119
2.5. Freshening ..............................................................................120
2.6. Querying ..................................................................................120
2.7. Verifying ..................................................................................121
3. Checking a Package's Signature ..........................................................122
3.1. Importing Keys .........................................................................122
3.2. Verifying Signature of Packages ................................................123
4. Practical and Common Examples of RPM Usage ..................................123
5. Additional Resources ..........................................................................124
5.1. Installed Documentation ...........................................................124
5.2. Useful Websites .......................................................................124
5.3. Related Books .........................................................................125
11. Package Management Tool .......................................................................126
1. Listing and Analyzing Packages ...........................................................127
2. Installing and Removing Packages .......................................................128
12. Red Hat Network .......................................................................................133
III. Network-Related Configuration ...........................................................................137
13. Network Interfaces ....................................................................................138
1. Network Configuration Files .................................................................138
2. Interface Configuration Files ................................................................139
2.1. Ethernet Interfaces ...................................................................139
2.2. IPsec Interfaces .......................................................................142
2.3. Channel Bonding Interfaces ......................................................143
2.4. Alias and Clone Files ................................................................144
2.5. Dialup Interfaces ......................................................................145
2.6. Other Interfaces .......................................................................146
3. Interface Control Scripts ......................................................................147
4. Network Function Files ........................................................................149
5. Additional Resources ..........................................................................149
5.1. Installed Documentation ...........................................................149
14. Network Configuration ...............................................................................150
1. Overview ............................................................................................151
2. Establishing an Ethernet Connection ....................................................152
3. Establishing an ISDN Connection .........................................................155
4. Establishing a Modem Connection .......................................................156
5. Establishing an xDSL Connection ........................................................158
6. Establishing a Token Ring Connection .................................................161
7. Establishing a Wireless Connection ......................................................164
8. Managing DNS Settings ......................................................................166
9. Managing Hosts ..................................................................................168
10. Working with Profiles .........................................................................169
11. Device Aliases ..................................................................................173
12. Saving and Restoring the Network Configuration .................................174
15. Controlling Access to Services ...................................................................176
1. Runlevels ...........................................................................................177
2. TCP Wrappers ....................................................................................177
2.1. xinetd ......................................................................................178
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3. Services Configuration Tool .................................................................178
4. ntsysv ................................................................................................180
5. chkconfig ............................................................................................182
6. Additional Resources ..........................................................................182
6.1. Installed Documentation ...........................................................183
6.2. Useful Websites .......................................................................183
16. Berkeley Internet Name Domain (BIND) .....................................................184
1. Introduction to DNS .............................................................................184
1.1. Nameserver Zones ...................................................................184
1.2. Nameserver Types ...................................................................185
1.3. BIND as a Nameserver .............................................................185
2. /etc/named.conf ..................................................................................186
2.1. Common Statement Types .......................................................186
2.2. Other Statement Types .............................................................191
2.3. Comment Tags ........................................................................193
3. Zone Files ..........................................................................................193
3.1. Zone File Directives ..................................................................193
3.2. Zone File Resource Records .....................................................194
3.3. Example Zone File ...................................................................197
3.4. Reverse Name Resolution Zone Files ........................................197
4. Using rndc ..........................................................................................198
4.1. Configuring /etc/named.conf .....................................................198
4.2. Configuring /etc/rndc.conf .........................................................199
4.3. Command Line Options ............................................................199
5. Advanced Features of BIND ................................................................200
5.1. DNS Protocol Enhancements ....................................................201
5.2. Multiple Views ..........................................................................201
5.3. Security ...................................................................................201
5.4. IP version 6 .............................................................................202
6. Common Mistakes to Avoid .................................................................202
7. Additional Resources ..........................................................................202
7.1. Installed Documentation ...........................................................202
7.2. Useful Websites .......................................................................203
7.3. Related Books .........................................................................204
17. OpenSSH .................................................................................................205
1. Features of SSH .................................................................................205
1.1. Why Use SSH? ........................................................................205
2. SSH Protocol Versions ........................................................................206
3. Event Sequence of an SSH Connection ................................................206
3.1. Transport Layer ........................................................................207
3.2. Authentication ..........................................................................208
3.3. Channels .................................................................................208
4. Configuring an OpenSSH Server ..........................................................208
4.1. Requiring SSH for Remote Connections ....................................209
5. OpenSSH Configuration Files ..............................................................209
6. Configuring an OpenSSH Client ...........................................................211
6.1. Using the ssh Command ...........................................................211
6.2. Using the scp Command ...........................................................212
6.3. Using the sftp Command ..........................................................212
7. More Than a Secure Shell ...................................................................213
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7.1. X11 Forwarding ........................................................................213
7.2. Port Forwarding .......................................................................213
7.3. Generating Key Pairs ...............................................................214
8. Additional Resources ..........................................................................218
8.1. Installed Documentation ...........................................................218
8.2. Useful Websites .......................................................................218
18. Network File System (NFS) ........................................................................219
1. How It Works ......................................................................................219
1.1. Required Services ....................................................................220
2. NFS Client Configuration .....................................................................221
2.1. Mounting NFS File Systems using /etc/fstab ..............................221
3. autofs .................................................................................................222
3.1. What's new in autofs version 5? ................................................222
3.2. autofs Configuration .................................................................223
3.3. autofs Common Tasks ..............................................................225
4. Common NFS Mount Options ..............................................................228
5. Starting and Stopping NFS ..................................................................230
6. NFS Server Configuration ....................................................................231
6.1. Exporting or Sharing NFS File Systems .....................................232
6.2. Command Line Configuration ....................................................235
6.3. Hostname Formats ...................................................................236
7. The /etc/exports Configuration File .......................................................236
7.1. The exportfs Command ............................................................238
8. Securing NFS .....................................................................................240
8.1. Host Access .............................................................................240
8.2. File Permissions .......................................................................242
9. NFS and portmap ................................................................................242
9.1. Troubleshooting NFS and portmap ............................................242
10. Using NFS over TCP .........................................................................243
11. Additional Resources .........................................................................244
11.1. Installed Documentation .........................................................244
11.2. Useful Websites .....................................................................244
11.3. Related Books ........................................................................245
19. Samba .....................................................................................................246
1. Introduction to Samba .........................................................................246
1.1. Samba Features ......................................................................246
2. Samba Daemons and Related Services ................................................247
2.1. Samba Daemons .....................................................................247
3. Connecting to a Samba Share .............................................................247
3.1. Command Line ........................................................................249
3.2. Mounting the Share ..................................................................250
4. Configuring a Samba Server ................................................................250
4.1. Graphical Configuration ............................................................250
4.2. Command Line Configuration ....................................................255
4.3. Encrypted Passwords ...............................................................256
5. Starting and Stopping Samba ..............................................................256
6. Samba Server Types and the smb.conf File ..........................................257
6.1. Stand-alone Server ..................................................................257
6.2. Domain Member Server ............................................................259
6.3. Domain Controller ....................................................................261
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7. Samba Security Modes .......................................................................263
7.1. User-Level Security ..................................................................263
7.2. Share-Level Security ................................................................264
8. Samba Account Information Databases ................................................265
9. Samba Network Browsing ....................................................................266
9.1. Domain Browsing .....................................................................266
9.2. WINS (Windows Internetworking Name Server) .........................266
10. Samba with CUPS Printing Support ....................................................267
10.1. Simple smb.conf Settings ........................................................267
11. Samba Distribution Programs ............................................................268
12. Additional Resources .........................................................................271
12.1. Installed Documentation .........................................................271
12.2. Related Books ........................................................................272
12.3. Useful Websites .....................................................................272
20. Dynamic Host Configuration Protocol (DHCP) .............................................273
1. Why Use DHCP? ................................................................................273
2. Configuring a DHCP Server .................................................................273
2.1. Configuration File .....................................................................273
2.2. Lease Database .......................................................................277
2.3. Starting and Stopping the Server ...............................................277
2.4. DHCP Relay Agent ...................................................................278
3. Configuring a DHCP Client ..................................................................279
4. Additional Resources ..........................................................................280
4.1. Installed Documentation ...........................................................280
21. Apache HTTP Server ................................................................................281
1. Apache HTTP Server 2.2 .....................................................................281
1.1. Features of Apache HTTP Server 2.2 ........................................281
2. Migrating Apache HTTP Server Configuration Files ...............................282
2.1. Migrating Apache HTTP Server 2.0 Configuration Files ...............282
2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0 .....282
3. Starting and Stopping httpd .................................................................293
4. Apache HTTP Server Configuration .....................................................294
4.1. Basic Settings ..........................................................................295
4.2. Default Settings ........................................................................296
5. Configuration Directives in httpd.conf ...................................................308
5.1. General Configuration Tips .......................................................308
5.2. Configuration Directives for SSL ................................................319
5.3. MPM Specific Server-Pool Directives .........................................320
6. Adding Modules ..................................................................................321
7. Virtual Hosts .......................................................................................322
7.1. Setting Up Virtual Hosts ............................................................322
8. Apache HTTP Secure Server Configuration ..........................................323
8.1. An Overview of Security-Related Packages ...............................323
8.2. An Overview of Certificates and Security ...................................324
8.3. Using Pre-Existing Keys and Certificates ...................................324
8.4. Types of Certificates .................................................................325
8.5. Generating a Key .....................................................................326
8.6. How to configure the server to use the new key ..........................334
9. Additional Resources ..........................................................................334
9.1. Useful Websites .......................................................................334
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22. FTP ..........................................................................................................336
1. The File Transport Protocol .................................................................336
1.1. Multiple Ports, Multiple Modes ...................................................336
2. FTP Servers .......................................................................................337
2.1. vsftpd ......................................................................................337
3. Files Installed with vsftpd .....................................................................338
4. Starting and Stopping vsftpd ................................................................338
4.1. Starting Multiple Copies of vsftpd ..............................................339
5. vsftpd Configuration Options ................................................................340
5.1. Daemon Options ......................................................................341
5.2. Log In Options and Access Controls ..........................................341
5.3. Anonymous User Options .........................................................342
5.4. Local User Options ...................................................................343
5.5. Directory Options .....................................................................344
5.6. File Transfer Options ................................................................345
5.7. Logging Options .......................................................................346
5.8. Network Options ......................................................................347
6. Additional Resources ..........................................................................349
6.1. Installed Documentation ...........................................................349
6.2. Useful Websites .......................................................................350
23. Email ........................................................................................................351
1. Email Protocols ...................................................................................351
1.1. Mail Transport Protocols ...........................................................351
1.2. Mail Access Protocols ..............................................................352
2. Email Program Classifications ..............................................................354
2.1. Mail Transport Agent ................................................................354
2.2. Mail Delivery Agent ..................................................................354
2.3. Mail User Agent .......................................................................355
3. Mail Transport Agents .........................................................................355
3.1. Sendmail .................................................................................355
3.2. Postfix .....................................................................................359
3.3. Fetchmail .................................................................................361
4. Mail Transport Agent (MTA) Configuration ............................................365
5. Mail Delivery Agents ...........................................................................366
5.1. Procmail Configuration .............................................................367
5.2. Procmail Recipes .....................................................................368
6. Mail User Agents ................................................................................373
6.1. Securing Communication ..........................................................373
7. Additional Resources ..........................................................................375
7.1. Installed Documentation ...........................................................375
7.2. Useful Websites .......................................................................376
7.3. Related Books .........................................................................376
24. Lightweight Directory Access Protocol (LDAP) ............................................377
1. Why Use LDAP? .................................................................................377
1.1. OpenLDAP Features ................................................................377
2. LDAP Terminology ..............................................................................378
3. OpenLDAP Daemons and Utilities ........................................................379
3.1. NSS, PAM, and LDAP ..............................................................381
3.2. PHP4, LDAP, and the Apache HTTP Server ..............................381
3.3. LDAP Client Applications ..........................................................382
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4. OpenLDAP Configuration Files ............................................................382
5. The /etc/openldap/schema/ Directory ...................................................382
6. OpenLDAP Setup Overview .................................................................383
6.1. Editing /etc/openldap/slapd.conf ................................................384
7. Configuring a System to Authenticate Using OpenLDAP ........................385
7.1. PAM and LDAP ........................................................................386
7.2. Migrating Old Authentication Information to LDAP Format ...........386
8. Migrating Directories from Earlier Releases ..........................................387
9. Additional Resources ..........................................................................387
9.1. Installed Documentation ...........................................................387
9.2. Useful Websites .......................................................................389
9.3. Related Books .........................................................................389
25. Authentication Configuration ......................................................................390
1. User Information .................................................................................390
2. Authentication .....................................................................................393
3. Options ..............................................................................................395
4. Command Line Version .......................................................................397
IV. System Configuration .........................................................................................400
26. Console Access ........................................................................................401
1. Disabling Shutdown Via CtrlAltDel ........................................................401
2. Disabling Console Program Access ......................................................402
3. Defining the Console ...........................................................................402
4. Making Files Accessible From the Console ...........................................402
5. Enabling Console Access for Other Applications ...................................403
6. The floppy Group ................................................................................404
27. The sysconfig Directory .............................................................................405
1. Files in the /etc/sysconfig/ Directory .....................................................405
1.1. /etc/sysconfig/amd ...................................................................405
1.2. /etc/sysconfig/apmd ..................................................................405
1.3. /etc/sysconfig/arpwatch ............................................................405
1.4. /etc/sysconfig/authconfig ...........................................................405
1.5. /etc/sysconfig/autofs .................................................................406
1.6. /etc/sysconfig/clock ..................................................................406
1.7. /etc/sysconfig/desktop ..............................................................407
1.8. /etc/sysconfig/dhcpd .................................................................408
1.9. /etc/sysconfig/exim ...................................................................408
1.10. /etc/sysconfig/firstboot ............................................................408
1.11. /etc/sysconfig/gpm ..................................................................408
1.12. /etc/sysconfig/hwconf ..............................................................409
1.13. /etc/sysconfig/i18n ..................................................................409
1.14. /etc/sysconfig/init ....................................................................409
1.15. /etc/sysconfig/ip6tables-config .................................................410
1.16. /etc/sysconfig/iptables-config ...................................................410
1.17. /etc/sysconfig/irda ...................................................................410
1.18. /etc/sysconfig/keyboard ..........................................................411
1.19. /etc/sysconfig/kudzu ...............................................................411
1.20. /etc/sysconfig/named ..............................................................412
1.21. /etc/sysconfig/netdump ...........................................................412
1.22. /etc/sysconfig/network ............................................................412
1.23. /etc/sysconfig/ntpd ..................................................................412
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1.24. /etc/sysconfig/radvd ................................................................413
1.25. /etc/sysconfig/samba ..............................................................413
1.26. /etc/sysconfig/selinux ..............................................................413
1.27. /etc/sysconfig/sendmail ...........................................................413
1.28. /etc/sysconfig/spamassassin ...................................................414
1.29. /etc/sysconfig/squid ................................................................414
1.30. /etc/sysconfig/system-config-selinux ........................................414
1.31. /etc/sysconfig/system-config-users ..........................................414
1.32. /etc/sysconfig/system-logviewer ..............................................414
1.33. /etc/sysconfig/tux ....................................................................414
1.34. /etc/sysconfig/vncservers ........................................................415
1.35. /etc/sysconfig/xinetd ...............................................................415
2. Directories in the /etc/sysconfig/ Directory .............................................415
3. Additional Resources ..........................................................................416
3.1. Installed Documentation ...........................................................416
28. Date and Time Configuration .....................................................................417
1. Time and Date Properties ....................................................................417
2. Network Time Protocol (NTP) Properties ..............................................418
3. Time Zone Configuration .....................................................................420
29. Keyboard Configuration .............................................................................422
30. The X Window System ..............................................................................423
1. The X11R7.1 Release .........................................................................423
2. Desktop Environments and Window Managers .....................................424
2.1. Desktop Environments ..............................................................424
2.2. Window Managers ...................................................................425
3. X Server Configuration Files ................................................................426
3.1. xorg.conf .................................................................................426
4. Fonts ..................................................................................................432
4.1. Fontconfig ................................................................................433
4.2. Core X Font System .................................................................434
5. Runlevels and X ..................................................................................436
5.1. Runlevel 3 ...............................................................................436
5.2. Runlevel 5 ...............................................................................437
6. Additional Resources ..........................................................................438
6.1. Installed Documentation ...........................................................438
6.2. Useful Websites .......................................................................438
31. X Window System Configuration ................................................................439
1. Display Settings ..................................................................................439
2. Display Hardware Settings ...................................................................440
3. Dual Head Display Settings .................................................................441
32. Users and Groups .....................................................................................443
1. User and Group Configuration .............................................................443
1.1. Adding a New User ..................................................................444
1.2. Modifying User Properties .........................................................446
1.3. Adding a New Group ................................................................447
1.4. Modifying Group Properties .......................................................448
2. User and Group Management Tools .....................................................449
2.1. Command Line Configuration ....................................................449
2.2. Adding a User ..........................................................................449
2.3. Adding a Group ........................................................................450
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Red Hat Enterprise Linux 5.0.0
2.4. Password Aging .......................................................................451
2.5. Explaining the Process .............................................................453
3. Standard Users ...................................................................................455
4. Standard Groups ................................................................................456
5. User Private Groups ............................................................................459
5.1. Group Directories .....................................................................459
6. Shadow Passwords .............................................................................460
7. Additional Resources ..........................................................................460
7.1. Installed Documentation ...........................................................460
33. Printer Configuration .................................................................................462
1. Adding a Local Printer .........................................................................463
2. Adding an IPP Printer ..........................................................................464
3. Adding a Samba (SMB) Printer ............................................................465
4. Adding a JetDirect Printer ....................................................................467
5. Selecting the Printer Model and Finishing .............................................468
5.1. Confirming Printer Configuration ...............................................469
6. Printing a Test Page ............................................................................469
7. Modifying Existing Printers ...................................................................469
7.1. The Settings Tab ......................................................................469
7.2. The Policies Tab ......................................................................470
7.3. The Access Control Tab ...........................................................471
7.4. The Printer and Job OptionsTab ................................................472
8. Managing Print Jobs ...........................................................................473
9. Additional Resources ..........................................................................474
9.1. Installed Documentation ...........................................................475
9.2. Useful Websites .......................................................................475
34. Automated Tasks ......................................................................................476
1. Cron ...................................................................................................476
1.1. Configuring Cron Tasks ............................................................476
1.2. Controlling Access to Cron ........................................................478
1.3. Starting and Stopping the Service .............................................478
2. At and Batch .......................................................................................478
2.1. Configuring At Jobs ..................................................................478
2.2. Configuring Batch Jobs .............................................................479
2.3. Viewing Pending Jobs ..............................................................480
2.4. Additional Command Line Options .............................................480
2.5. Controlling Access to At and Batch ............................................480
2.6. Starting and Stopping the Service .............................................480
3. Additional Resources ..........................................................................480
3.1. Installed Documentation ...........................................................480
35. Log Files ..................................................................................................482
1. Locating Log Files ...............................................................................482
2. Viewing Log Files ................................................................................482
3. Adding a Log File ................................................................................484
4. Monitoring Log Files ............................................................................485
V. System Monitoring ..............................................................................................489
36. SystemTap ...............................................................................................490
1. Introduction ........................................................................................490
2. Implementation ...................................................................................490
3. Using SystemTap ................................................................................491
xiv
Red Hat Enterprise Linux 5.0.0
3.1. Tracing ....................................................................................491
37. Gathering System Information ....................................................................493
1. System Processes ..............................................................................493
2. Memory Usage ...................................................................................495
3. File Systems .......................................................................................496
4. Hardware ...........................................................................................497
5. Additional Resources ..........................................................................500
5.1. Installed Documentation ...........................................................500
38. OProfile ....................................................................................................501
1. Overview of Tools ...............................................................................501
2. Configuring OProfile ............................................................................502
2.1. Specifying the Kernel ................................................................502
2.2. Setting Events to Monitor ..........................................................503
2.3. Separating Kernel and User-space Profiles ................................505
3. Starting and Stopping OProfile .............................................................506
4. Saving Data ........................................................................................507
5. Analyzing the Data ..............................................................................507
5.1. Using opreport .........................................................................508
5.2. Using opreport on a Single Executable ......................................508
5.3. Getting more detailed output on the modules .............................509
5.4. Using opannotate .....................................................................510
6. Understanding /dev/oprofile/ ................................................................510
7. Example Usage ..................................................................................511
8. Graphical Interface ..............................................................................511
9. Additional Resources ..........................................................................513
9.1. Installed Docs ..........................................................................513
9.2. Useful Websites .......................................................................514
VI. Kernel and Driver Configuration ..........................................................................515
39. Manually Upgrading the Kernel ..................................................................516
1. Overview of Kernel Packages ..............................................................516
2. Preparing to Upgrade ..........................................................................517
3. Downloading the Upgraded Kernel .......................................................518
4. Performing the Upgrade ......................................................................519
5. Verifying the Initial RAM Disk Image .....................................................519
6. Verifying the Boot Loader ....................................................................520
6.1. x86 Systems ............................................................................520
6.2. Itanium Systems ......................................................................520
6.3. IBM S/390 and IBM System z Systems ......................................521
6.4. IBM eServer iSeries Systems ....................................................521
6.5. IBM eServer pSeries Systems ...................................................522
40. General Parameters and Modules ..............................................................523
1. Kernel Module Utilities .........................................................................523
2. Persistent Module Loading ..................................................................525
3. Specifying Module Parameters ............................................................526
4. Storage parameters ............................................................................526
5. Ethernet Parameters ...........................................................................532
5.1. Using Multiple Ethernet Cards ...................................................539
5.2. The Channel Bonding Module ...................................................539
6. Additional Resources ..........................................................................542
6.1. Installed Documentation ...........................................................542
xv
Red Hat Enterprise Linux 5.0.0
6.2. Useful Websites .......................................................................542
VII. Security And Authentication ...............................................................................544
41. Security Overview .....................................................................................545
1. Introduction to Security ........................................................................545
1.1. What is Computer Security? ......................................................545
1.2. Security Controls ......................................................................547
1.3. Conclusion ...............................................................................548
2. Vulnerability Assessment .....................................................................548
2.1. Thinking Like the Enemy ...........................................................549
2.2. Defining Assessment and Testing .............................................549
2.3. Evaluating the Tools .................................................................551
3. Attackers and Vulnerabilities ................................................................553
3.1. A Quick History of Hackers .......................................................553
3.2. Threats to Network Security ......................................................554
3.3. Threats to Server Security ........................................................555
3.4. Threats to Workstation and Home PC Security ...........................557
4. Common Exploits and Attacks .............................................................558
5. Security Updates ................................................................................561
5.1. Updating Packages ..................................................................561
42. Securing Your Network ..............................................................................567
1. Workstation Security ...........................................................................567
1.1. Evaluating Workstation Security ................................................567
1.2. BIOS and Boot Loader Security .................................................567
1.3. Password Security ...................................................................569
1.4. Administrative Controls .............................................................575
1.5. Available Network Services .......................................................582
1.6. Personal Firewalls ....................................................................586
1.7. Security Enhanced Communication Tools ..................................586
2. Server Security ...................................................................................587
2.1. Securing Services With TCP Wrappers and xinetd .....................587
2.2. Securing Portmap ....................................................................591
2.3. Securing NIS ...........................................................................592
2.4. Securing NFS ..........................................................................594
2.5. Securing the Apache HTTP Server ............................................595
2.6. Securing FTP ...........................................................................596
2.7. Securing Sendmail ...................................................................599
2.8. Verifying Which Ports Are Listening ...........................................600
3. Single Sign-on (SSO) ..........................................................................601
3.1. Introduction ..............................................................................601
3.2. Getting Started with your new Smart Card .................................603
3.3. How Smart Card Enrollment Works ...........................................604
3.4. How Smart Card Login Works ...................................................605
3.5. Configuring Firefox to use Kerberos for SSO ..............................606
4. Pluggable Authentication Modules (PAM) .............................................609
4.1. Advantages of PAM ..................................................................609
4.2. PAM Configuration Files ...........................................................609
4.3. PAM Configuration File Format .................................................609
4.4. Sample PAM Configuration Files ...............................................612
4.5. Creating PAM Modules .............................................................614
4.6. PAM and Administrative Credential Caching ..............................614
xvi
Red Hat Enterprise Linux 5.0.0
4.7. PAM and Device Ownership .....................................................616
4.8. Additional Resources ................................................................617
5. TCP Wrappers and xinetd ....................................................................618
5.1. TCP Wrappers .........................................................................619
5.2. TCP Wrappers Configuration Files ............................................621
5.3. xinetd ......................................................................................628
5.4. xinetd Configuration Files .........................................................628
5.5. Additional Resources ................................................................634
6. Kerberos ............................................................................................635
6.1. What is Kerberos? ....................................................................635
6.2. Kerberos Terminology ..............................................................637
6.3. How Kerberos Works ................................................................638
6.4. Kerberos and PAM ...................................................................640
6.5. Configuring a Kerberos 5 Server ...............................................640
6.6. Configuring a Kerberos 5 Client .................................................642
6.7. Domain-to-Realm Mapping .......................................................644
6.8. Setting Up Secondary KDCs .....................................................644
6.9. Setting Up Cross Realm Authentication .....................................645
6.10. Additional Resources ..............................................................649
7. Virtual Private Networks (VPNs) ...........................................................650
7.1. How Does a VPN Work? ...........................................................651
7.2. VPNs and Red Hat Enterprise Linux ..........................................651
7.3. IPsec .......................................................................................651
7.4. Creating an IPsec Connection ...................................................652
7.5. IPsec Installation ......................................................................652
7.6. IPsec Host-to-Host Configuration ..............................................653
7.7. IPsec Network-to-Network Configuration ....................................659
7.8. Starting and Stopping an IPsec Connection ...............................666
8. Firewalls .............................................................................................666
8.1. Netfilter and IPTables ...............................................................668
8.2. Basic Firewall Configuration ......................................................668
8.3. Using IPTables ........................................................................672
8.4. Common IPTables Filtering .......................................................674
8.5. FORWARD and NAT Rules ......................................................675
8.6. Malicious Software and Spoofed IP Addresses ...........................677
8.7. IPTables and Connection Tracking ............................................678
8.8. IPv6 ........................................................................................679
8.9. Additional Resources ................................................................679
9. IPTables .............................................................................................680
9.1. Packet Filtering ........................................................................680
9.2. Differences Between IPTables and IPChains .............................682
9.3. Command Options for IPTables ................................................683
9.4. Saving IPTables Rules .............................................................692
9.5. IPTables Control Scripts ...........................................................693
9.6. IPTables and IPv6 ....................................................................695
9.7. Additional Resources ................................................................695
43. Security and SELinux ................................................................................697
1. Access Control Mechanisms (ACMs) ....................................................697
1.1. Discretionary Access Control (DAC) ..........................................697
1.2. Access Control Lists (ACLs) ......................................................697
xvii
Red Hat Enterprise Linux 5.0.0
1.3. Mandatory Access Control (MAC) .............................................697
1.4. Role-based Access Control (RBAC) ..........................................697
1.5. Multi-Level Security (MLS) ........................................................698
1.6. Multi-Category Security (MCS) ..................................................698
2. Introduction to SELinux .......................................................................698
2.1. SELinux Overview ....................................................................698
2.2. Files Related to SELinux ...........................................................699
2.3. Additional Resources ................................................................703
3. Brief Background and History of SELinux .............................................704
4. Multi-Category Security (MCS) .............................................................704
4.1. Introduction ..............................................................................704
4.2. Applications for Multi-Category Security .....................................705
4.3. SELinux Security Contexts ........................................................705
5. Getting Started with Multi-Category Security (MCS) ..............................706
5.1. Introduction ..............................................................................706
5.2. Comparing SELinux and Standard Linux User Identities .............706
5.3. Configuring Categories .............................................................707
5.4. Assigning Categories to Users ..................................................708
5.5. Assigning Categories to Files ....................................................709
6. Multi-Level Security (MLS) ...................................................................711
6.1. Why Multi-Level? ......................................................................711
6.2. Security Levels, Objects and Subjects .......................................713
6.3. MLS Policy ..............................................................................714
6.4. LSPP Certification ....................................................................715
7. SELinux Policy Overview .....................................................................715
7.1. What is the SELinux Policy? .....................................................715
7.2. Where is the Policy? .................................................................716
7.3. The Role of Policy in the Boot Process ......................................718
7.4. Object Classes and Permissions ...............................................719
8. Targeted Policy Overview ....................................................................720
8.1. What is the Targeted Policy? ....................................................720
8.2. Files and Directories of the Targeted Policy ...............................720
8.3. Understanding the Users and Roles in the Targeted Policy .........721
44. Working With SELinux ...............................................................................723
1. End User Control of SELinux ...............................................................723
1.1. Moving and Copying Files .........................................................723
1.2. Checking the Security Context of a Process, User, or File Object 724
1.3. Relabeling a File or Directory ....................................................725
1.4. Creating Archives That Retain Security Contexts ........................728
2. Administrator Control of SELinux ..........................................................729
2.1. Viewing the Status of SELinux ..................................................729
2.2. Relabeling a File System ..........................................................730
2.3. Managing NFS Home Directories ..............................................731
2.4. Granting Access to a Directory or a Tree ...................................732
2.5. Backing Up and Restoring the System .......................................732
2.6. Enabling or Disabling Enforcement ............................................732
2.7. Enable or Disable SELinux .......................................................735
2.8. Changing the Policy .................................................................736
2.9. Specifying the Security Context of Entire File Systems ...............738
2.10. Changing the Security Category of a File or User .....................739
xviii
Red Hat Enterprise Linux 5.0.0
2.11. Running a Command in a Specific Security Context .................739
2.12. Useful Commands for Scripts ..................................................739
2.13. Changing to a Different Role ...................................................740
2.14. When to Reboot .....................................................................740
3. Analyst Control of SELinux ..................................................................740
3.1. Enabling Kernel Auditing ...........................................................740
3.2. Dumping and Viewing Logs .......................................................741
45. Customizing SELinux Policy .......................................................................742
1. Introduction ........................................................................................742
1.1. Modular Policy .........................................................................742
2. Building a Local Policy Module .............................................................743
2.1. Using audit2allow to Build a Local Policy Module ........................743
2.2. Analyzing the Type Enforcement (TE) File .................................743
2.3. Loading the Policy Package ......................................................744
46. References ...............................................................................................745
VIII. Red Hat Training And Certification ....................................................................747
47. Red Hat Training and Certification ..............................................................748
1. Three Ways to Train ............................................................................748
2. Microsoft Certified Professional Resource Center ..................................748
48. Certification Tracks ...................................................................................749
1. Free Pre-assessment tests ..................................................................749
49. RH033: Red Hat Linux Essentials ...............................................................750
1. Course Description ..............................................................................750
1.1. Prerequisites ............................................................................750
1.2. Goal ........................................................................................750
1.3. Audience .................................................................................750
1.4. Course Objectives ....................................................................750
1.5. Follow-on Courses ...................................................................751
50. RH035: Red Hat Linux Essentials for Windows Professionals ......................752
1. Course Description ..............................................................................752
1.1. Prerequisites ............................................................................752
1.2. Goal ........................................................................................752
1.3. Audience .................................................................................752
1.4. Course Objectives ....................................................................752
1.5. Follow-on Courses ...................................................................753
51. RH133: Red Hat Linux System Administration and Red Hat Certified Technician
(RHCT) Certification ........................................................................................754
1. Course Description ..............................................................................754
1.1. Prerequisites ............................................................................754
1.2. Goal ........................................................................................754
1.3. Audience .................................................................................754
1.4. Course Objectives ....................................................................754
1.5. Follow-on Courses ...................................................................755
52. RH202 RHCT EXAM - The fastest growing credential in all of Linux. ............756
1. Course Description ..............................................................................756
1.1. Prerequisites ............................................................................756
53. RH253 Red Hat Linux Networking and Security Administration .....................757
1. Course Description ..............................................................................757
1.1. Prerequisites ............................................................................757
1.2. Goal ........................................................................................757
xix
Red Hat Enterprise Linux 5.0.0
1.3. Audience .................................................................................757
1.4. Course Objectives ....................................................................757
1.5. Follow-on Courses ...................................................................758
54. RH300: RHCE Rapid track course (and RHCE exam) .................................759
1. Course Description ..............................................................................759
1.1. Prerequisites ............................................................................759
1.2. Goal ........................................................................................759
1.3. Audience .................................................................................759
1.4. Course Objectives ....................................................................759
1.5. Follow-on Courses ...................................................................759
55. RH302 RHCE EXAM .................................................................................761
1. Course Description ..............................................................................761
1.1. Prerequisites ............................................................................761
1.2. Content ...................................................................................761
56. RHS333: RED HAT enterprise security: network services ............................762
1. Course Description ..............................................................................762
1.1. Prerequisites ............................................................................762
1.2. Goal ........................................................................................762
1.3. Audience .................................................................................762
1.4. Course Objectives ....................................................................762
1.5. Follow-on Courses ...................................................................763
57. RH401: Red Hat Enterprise Deployment and systems management .............764
1. Course Description ..............................................................................764
1.1. Prerequisites ............................................................................764
1.2. Goal ........................................................................................764
1.3. Audience .................................................................................764
1.4. Course Objectives ....................................................................764
1.5. Follow-on Courses ...................................................................765
58. RH423: Red Hat Enterprise Directory services and authentication ................766
1. Course Description ..............................................................................766
1.1. Prerequisites ............................................................................766
1.2. Goal ........................................................................................766
1.3. Audience .................................................................................766
1.4. Course Objectives ....................................................................766
1.5. Follow-on Courses ...................................................................767
59. SE Linux Courses .....................................................................................768
1. RHS427: Introduction to SELinux and Red Hat Targeted Policy .............768
1.1. Audience .................................................................................768
1.2. Course Summary .....................................................................768
2. RHS429: Red Hat Enterprise SE Linux Policy Administration .................768
60. RH436: Red Hat Enterprise storage management .......................................769
1. Course Description ..............................................................................769
1.1. Prerequisites ............................................................................769
1.2. Goal ........................................................................................769
1.3. Audience .................................................................................769
1.4. Course Objectives ....................................................................769
1.5. Follow-on Courses ...................................................................770
61. RH442: Red Hat Enterprise system monitoring and performance tuning .......771
1. Course Description ..............................................................................771
1.1. Prerequisites ............................................................................771
xx
Red Hat Enterprise Linux 5.0.0
1.2. Goal ........................................................................................771
1.3. Audience .................................................................................771
1.4. Course Objectives ....................................................................771
1.5. Follow-on Courses ...................................................................772
62. Red Hat Enterprise Linux Developer Courses .............................................773
1. RHD143: Red Hat Linux Programming Essentials .................................773
2. RHD221 Red Hat Linux Device Drivers ................................................773
3. RHD236 Red Hat Linux Kernel Internals ...............................................773
4. RHD256 Red Hat Linux Application Development and Porting ...............773
63. JBoss Courses ..........................................................................................774
1. RHD161 JBoss and EJB3 for Java .......................................................774
1.1. Prerequisites ............................................................................774
2. RHD163 JBoss for Web Developers .....................................................774
2.1. Prerequisites ............................................................................774
3. RHD167: JBOSS - HIBERNATE ESSENTIALS .....................................775
3.1. Prerequisites ............................................................................775
3.2. Course Summary .....................................................................775
4. RHD267: JBOSS - ADVANCED HIBERNATE .......................................775
4.1. Prerequisites ............................................................................776
5. RHD261:JBOSS for advanced J2EE developers ...................................776
5.1. Prerequisites ............................................................................776
6. RH336: JBOSS for Administrators ........................................................777
6.1. Prerequisites ............................................................................777
6.2. Course Summary .....................................................................777
7. RHD439: JBoss Clustering ..................................................................778
7.1. Prerequisites ............................................................................778
8. RHD449: JBoss jBPM .........................................................................778
8.1. Description ..............................................................................779
8.2. Prerequisites ............................................................................779
9. RHD451 JBoss Rules ..........................................................................779
9.1. Prerequisites ............................................................................779
xxi
Introduction
Welcome to the Red Hat Enterprise Linux Deployment Guide.
The Red Hat Enterprise Linux Deployment Guide contains information on how to customize
your Red Hat Enterprise Linux system to fit your needs. If you are looking for a comprehensive,
task-oriented guide for configuring and customizing your system, this is the manual for you.
This manual discusses many intermediate topics such as the following:
•
Setting up a network interface card (NIC)
•
Configuring a Virtual Private Network (VPN)
•
Configuring Samba shares
•
Managing your software with RPM
•
Determining information about your system
•
Upgrading your kernel
This manual is divided into the following main categories:
•
File systems
•
Package management
•
Network-related configuration
•
System configuration
•
System monitoring
•
Kernel and Driver Configuration
•
Security and Authentication
•
Red Hat Training and Certification
This guide assumes you have a basic understanding of your Red Hat Enterprise Linux system.
If you need help installing Red Hat Enterprise Linux, refer to the Red Hat Enterprise Linux Installation Guide.
1. Document Conventions
In this manual, certain words are represented in different fonts, typefaces, sizes, and weights.
This highlighting is systematic; different words are represented in the same style to indicate their
inclusion in a specific category. The types of words that are represented this way include the following:
command
xxii
1. Document Conventions
Linux commands (and other operating system commands, when used) are represented this
way. This style should indicate to you that you can type the word or phrase on the command line and press Enter to invoke a command. Sometimes a command contains words
that would be displayed in a different style on their own (such as file names). In these
cases, they are considered to be part of the command, so the entire phrase is displayed as
a command. For example:
Use the cat testfile command to view the contents of a file, named testfile, in the current working directory.
file name
File names, directory names, paths, and RPM package names are represented this way.
This style indicates that a particular file or directory exists with that name on your system.
Examples:
The .bashrc file in your home directory contains bash shell definitions and aliases for your
own use.
The /etc/fstab file contains information about different system devices and file systems.
Install the webalizer RPM if you want to use a Web server log file analysis program.
application
This style indicates that the program is an end-user application (as opposed to system software). For example:
Use Mozilla to browse the Web.
key
A key on the keyboard is shown in this style. For example:
To use Tab completion to list particular files in a directory, type ls, then a character, and finally the Tab key. Your terminal displays the list of files in the working directory that begin
with that character.
key-combination
A combination of keystrokes is represented in this way. For example:
The Ctrl-Alt-Backspace key combination exits your graphical session and returns you to
the graphical login screen or the console.
text found on a GUI interface
A title, word, or phrase found on a GUI interface screen or window is shown in this style.
Text shown in this style indicates a particular GUI screen or an element on a GUI screen
(such as text associated with a checkbox or field). Example:
Select the Require Password checkbox if you would like your screensaver to require a
password before stopping.
top level of a menu on a GUI screen or window
A word in this style indicates that the word is the top level of a pulldown menu. If you click
on the word on the GUI screen, the rest of the menu should appear. For example:
Under File on a GNOME terminal, the New Tab option allows you to open multiple shell
xxiii
1. Document Conventions
prompts in the same window.
Instructions to type in a sequence of commands from a GUI menu look like the following example:
Go to Applications (the main menu on the panel) => Programming => Emacs Text Editor to start the Emacs text editor.
button on a GUI screen or window
This style indicates that the text can be found on a clickable button on a GUI screen. For example:
Click on the Back button to return to the webpage you last viewed.
computer output
Text in this style indicates text displayed to a shell prompt such as error messages and responses to commands. For example:
The ls command displays the contents of a directory. For example:
Desktop
about.html
Mail
backupfiles
logs
mail
paulwesterberg.png
reports
The output returned in response to the command (in this case, the contents of the directory)
is shown in this style.
prompt
A prompt, which is a computer's way of signifying that it is ready for you to input something,
is shown in this style. Examples:
$
#
[[email protected] stephen]$
leopard login:
user input
Text that the user types, either on the command line or into a text box on a GUI screen, is
displayed in this style. In the following example, text is displayed in this style:
To boot your system into the text based installation program, you must type in the text command at the boot: prompt.
<replaceable>
Text used in examples that is meant to be replaced with data provided by the user is displayed in this style. In the following example, <version-number> is displayed in this style:
The directory for the kernel source is /usr/src/kernels/<version-number>/, where
<version-number> is the version and type of kernel installed on this system.
Additionally, we use several different strategies to draw your attention to certain pieces of information. In order of urgency, these items are marked as a note, tip, important, caution, or
xxiv
2. Send in Your Feedback
warning. For example:
Note
Remember that Linux is case sensitive. In other words, a rose is not a ROSE is not
a rOsE.
Tip
The directory /usr/share/doc/ contains additional documentation for packages installed on your system.
Important
If you modify the DHCP configuration file, the changes do not take effect until you
restart the DHCP daemon.
Caution
Do not perform routine tasks as root — use a regular user account unless you
need to use the root account for system administration tasks.
Warning
Be careful to remove only the necessary partitions. Removing other partitions
could result in data loss or a corrupted system environment.
2. Send in Your Feedback
If you find an error in the Red Hat Enterprise Linux Deployment Guide, or if you have thought of
a way to make this manual better, we would like to hear from you! Submit a report in Bugzilla
(http://bugzilla.redhat.com/bugzilla/) against the component Deployment_Guide.
If you have a suggestion for improving the documentation, try to be as specific as possible. If
you have found an error, include the section number and some of the surrounding text so we
can find it easily.
xxv
Part I. File Systems
File system refers to the files and directories stored on a computer. A file system can have different formats called file system types. These formats determine how the information is stored
as files and directories. Some file system types store redundant copies of the data, while some
file system types make hard drive access faster. This part discusses the ext3, swap, RAID, and
LVM file system types. It also discusses the parted utility to manage partitions and access control lists (ACLs) to customize file permissions.
Chapter 1. File System Structure
1. Why Share a Common Structure?
The file system structure is the most basic level of organization in an operating system. Almost
all of the ways an operating system interacts with its users, applications, and security model are
dependent upon the way it organizes files on storage devices. Providing a common file system
structure ensures users and programs are able to access and write files.
File systems break files down into two logical categories:
•
Shareable vs. unsharable files
•
Variable vs. static files
Shareable files are those that can be accessed locally and by remote hosts; unsharable files are
only available locally. Variable files, such as documents, can be changed at any time; static
files, such as binaries, do not change without an action from the system administrator.
The reason for looking at files in this manner is to help correlate the function of the file with the
permissions assigned to the directories which hold them. The way in which the operating system and its users interact with a given file determines the directory in which it is placed, whether
that directory is mounted with read-only or read/write permissions, and the level of access each
user has to that file. The top level of this organization is crucial. Access to the underlying directories can be restricted or security problems could manifest themselves if, from the top level
down, it does not adhere to a rigid structure.
2. Overview of File System Hierarchy Standard (FHS)
Red Hat Enterprise Linux uses the Filesystem Hierarchy Standard (FHS) file system structure,
which defines the names, locations, and permissions for many file types and directories.
The FHS document is the authoritative reference to any FHS-compliant file system, but the
standard leaves many areas undefined or extensible. This section is an overview of the standard and a description of the parts of the file system not covered by the standard.
Compliance with the standard means many things, but the two most important are compatibility
with other compliant systems and the ability to mount a /usr/ partition as read-only. This
second point is important because the directory contains common executables and should not
be changed by users. Also, since the /usr/ directory is mounted as read-only, it can be mounted from the CD-ROM or from another machine via a read-only NFS mount.
2.1. FHS Organization
The directories and files noted here are a small subset of those specified by the FHS document.
Refer to the latest FHS document for the most complete information.
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2.1. FHS Organization
The complete standard is available online at http://www.pathname.com/fhs/
[http://www.pathname.com/fhs].
2.1.1. The /boot/ Directory
The /boot/ directory contains static files required to boot the system, such as the Linux kernel.
These files are essential for the system to boot properly.
Warning
Do not remove the /boot/ directory. Doing so renders the system unbootable.
2.1.2. The /dev/ Directory
The /dev/ directory contains device nodes that either represent devices that are attached to the
system or virtual devices that are provided by the kernel. These device nodes are essential for
the system to function properly. The udev demon takes care of creating and removing all these
device nodes in /dev/.
Devices in the /dev directory and subdirectories are either character (providing only a serial
stream of input/output) or block (accessible randomly). Character devices include mouse, keyboard, modem while block devices include hard disk, floppy drive etc. If you have GNOME or
KDE installed in your system, devices such as external drives or cds are automatically detected
when connected (e.g via usb) or inserted (e.g via CD or DVD drive) and a popup window displaying the contents is automatically displayed. Files in the /dev directory are essential for the
system to function properly. Examples of common files in the /dev include:
/dev/hda - the master device on primary IDE channel./dev/hdb - the slave device on primary IDE channel./dev
2.1.3. The /etc/ Directory
The /etc/ directory is reserved for configuration files that are local to the machine. No binaries
are to be placed in /etc/. Any binaries that were once located in /etc/ should be placed into /
sbin/ or /bin/.
Examples of directories in /etc are the X11/ and skel/:
/etc |- X11/ |- skel/
The /etc/X11/ directory is for X Window System configuration files, such as xorg.conf. The /
etc/skel/ directory is for "skeleton" user files, which are used to populate a home directory
when a user is first created. Applications also store their configuration files in this directory and
may reference them when they are executed.
2.1.4. The /lib/ Directory
The /lib/ directory should contain only those libraries needed to execute the binaries in /bin/
and /sbin/. These shared library images are particularly important for booting the system and
executing commands within the root file system.
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2.1. FHS Organization
2.1.5. The /media/ Directory
The /media/ directory contains subdirectories used as mount points for removeable media such
as usb storage media, DVDs, CD-ROMs, and Zip disks.
2.1.6. The /mnt/ Directory
The /mnt/ directory is reserved for temporarily mounted file systems, such as NFS file system
mounts. For all removeable media, please use the /media/ directory. Automatically detected removeable media will be mounted in the /media directory.
Note
The /mnt directory must not be used by installation programs.
2.1.7. The /opt/ Directory
The /opt/ directory provides storage for most application software packages.
A package placing files in the /opt/ directory creates a directory bearing the same name as the
package. This directory, in turn, holds files that otherwise would be scattered throughout the file
system, giving the system administrator an easy way to determine the role of each file within a
particular package.
For example, if sample is the name of a particular software package located within the /opt/ directory, then all of its files are placed in directories inside the /opt/sample/ directory, such as /
opt/sample/bin/ for binaries and /opt/sample/man/ for manual pages.
Packages that encompass many different sub-packages, data files, extra fonts, clipart etc are
also located in the /opt/ directory, giving that large package a way to organize itself. In this
way, our sample package may have different tools that each go in their own sub-directories,
such as /opt/sample/tool1/ and /opt/sample/tool2/, each of which can have their own bin/,
man/, and other similar directories.
2.1.8. The /proc/ Directory
The /proc/ directory contains special files that either extract information from or send information to the kernel. Examples include system memory, cpu information, hardware configuration
etc.
Due to the great variety of data available within /proc/ and the many ways this directory can be
used to communicate with the kernel, an entire chapter has been devoted to the subject. For
more information, refer to Chapter 3, The proc File System.
2.1.9. The /sbin/ Directory
The /sbin/ directory stores executables used by the root user. The executables in /sbin/ are
used at boot time, for system administration and to perform system recovery operations. Of this
directory, the FHS says:
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2.1. FHS Organization
contains binaries essential for booting, restoring, recovering, and/or repairing the system in addition to the binaries in /bin. Programs executed after /
usr/ is known to be mounted (when there are no problems) are generally
placed into /usr/sbin. Locally-installed system administration programs should
be placed into /usr/local/sbin.
/sbin
At a minimum, the following programs should be in /sbin/:
arp, clock, halt, init, fsck.*, grub, ifconfig, mingetty, mkfs.*, mkswap, reboot, route, shutdown, swapoff,
2.1.10. The /srv/ Directory
The /srv/ directory contains site-specific data served by your system running Red Hat Enterprise Linux. This directory gives users the location of data files for a particular service, such as
FTP, WWW, or CVS. Data that only pertains to a specific user should go in the /home/ directory.
2.1.11. The /sys/ Directory
The /sys/ directory utilizes the new sysfs virtual file system specific to the 2.6 kernel. With the
increased support for hot plug hardware devices in the 2.6 kernel, the /sys/ directory contains
information similarly held in /proc/, but displays a hierarchical view of specific device information in regards to hot plug devices.
2.1.12. The /usr/ Directory
The /usr/ directory is for files that can be shared across multiple machines. The /usr/ directory
is often on its own partition and is mounted read-only. At a minimum, the following directories
should be subdirectories of /usr/:
/usr |- bin/ |- etc/ |- games/ |- include/ |- kerberos/ |- lib/ |- libexec/ |- local/ |- sbin/ |- share/ |
Under the /usr/ directory, the bin/ subdirectory contains executables, etc/ contains systemwide configuration files, games is for games, include/ contains C header files, kerberos/ contains
binaries and other Kerberos-related files, and lib/ contains object files and libraries that are not
designed to be directly utilized by users or shell scripts. The libexec/ directory contains small
helper programs called by other programs, sbin/ is for system administration binaries (those
that do not belong in the /sbin/ directory), share/ contains files that are not architecture-specific, src/ is for source code.
2.1.13. The /usr/local/ Directory
The FHS says:
The /usr/local hierarchy is for use by the system administrator when installing
software locally. It needs to be safe from being overwritten when the system
software is updated. It may be used for programs and data that are shareable
among a group of hosts, but not found in /usr.
The /usr/local/ directory is similar in structure to the /usr/ directory. It has the following subdirectories, which are similar in purpose to those in the /usr/ directory:
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3. Special File Locations Under Red Hat Enterprise Linux
/usr/local |- bin/ |- etc/ |- games/ |- include/ |- lib/ |- libexec/ |- sbin/ |- share/ |- src/
In Red Hat Enterprise Linux, the intended use for the /usr/local/ directory is slightly different
from that specified by the FHS. The FHS says that /usr/local/ should be where software that is
to remain safe from system software upgrades is stored. Since software upgrades can be performed safely with RPM Package Manager (RPM), it is not necessary to protect files by putting
them in /usr/local/. Instead, the /usr/local/ directory is used for software that is local to the
machine.
For instance, if the /usr/ directory is mounted as a read-only NFS share from a remote host, it
is still possible to install a package or program under the /usr/local/ directory.
2.1.14. The /var/ Directory
Since the FHS requires Linux to mount /usr/ as read-only, any programs that write log files or
need spool/ or lock/ directories should write them to the /var/ directory. The FHS states /var/
is for:
...variable data files. This includes spool directories and files, administrative and
logging data, and transient and temporary files.
Below are some of the directories found within the /var/ directory:
/var |- account/ |- arpwatch/ |- cache/ |- crash/ |- db/ |- empty/ |- ftp/ |- gdm/ |- kerberos/ |- lib/ |-
System log files, such as messages and lastlog, go in the /var/log/ directory. The /
var/lib/rpm/ directory contains RPM system databases. Lock files go in the /var/lock/ directory, usually in directories for the program using the file. The /var/spool/ directory has subdirectories for programs in which data files are stored.
3. Special File Locations Under Red Hat Enterprise Linux
Red Hat Enterprise Linux extends the FHS structure slightly to accommodate special files.
Most files pertaining to RPM are kept in the /var/lib/rpm/ directory. For more information on
RPM, refer to the chapter Chapter 10, Package Management with RPM.
The /var/cache/yum/ directory contains files used by the Package Updater, including RPM
header information for the system. This location may also be used to temporarily store RPMs
downloaded while updating the system. For more information about Red Hat Network, refer to
the documentation online at https://rhn.redhat.com/.
Another location specific to Red Hat Enterprise Linux is the /etc/sysconfig/ directory. This directory stores a variety of configuration information. Many scripts that run at boot time use the
files in this directory. Refer to Chapter 27, The sysconfig Directory for more information about
what is within this directory and the role these files play in the boot process.
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Chapter 2. The ext3 File System
The default file system is the journaling ext3 file system.
1. Features of ext3
The ext3 file system is essentially an enhanced version of the ext2 file system. These improvements provide the following advantages:
Availability
After an unexpected power failure or system crash (also called an unclean system shutdown), each mounted ext2 file system on the machine must be checked for consistency by
the e2fsck program. This is a time-consuming process that can delay system boot time significantly, especially with large volumes containing a large number of files. During this time,
any data on the volumes is unreachable.
The journaling provided by the ext3 file system means that this sort of file system check is
no longer necessary after an unclean system shutdown. The only time a consistency check
occurs using ext3 is in certain rare hardware failure cases, such as hard drive failures. The
time to recover an ext3 file system after an unclean system shutdown does not depend on
the size of the file system or the number of files; rather, it depends on the size of the journal
used to maintain consistency. The default journal size takes about a second to recover, depending on the speed of the hardware.
Data Integrity
The ext3 file system prevents loss of data integrity in the event that an unclean system shutdown occurs. The ext3 file system allows you to choose the type and level of protection that
your data receives. By default, the ext3 volumes are configured to keep a high level of data
consistency with regard to the state of the file system.
Speed
Despite writing some data more than once, ext3 has a higher throughput in most cases than
ext2 because ext3's journaling optimizes hard drive head motion. You can choose from
three journaling modes to optimize speed, but doing so means trade-offs in regards to data
integrity if the system was to fail.
Easy Transition
It is easy to migrate from ext2 to ext3 and gain the benefits of a robust journaling file system
without reformatting. Refer to Section 3, “Converting to an ext3 File System” for more on
how to perform this task.
The following sections walk you through the steps for creating and tuning ext3 partitions. For
ext2 partitions, skip the partitioning and formating sections below and go directly to Section 3,
“Converting to an ext3 File System”.
2. Creating an ext3 File System
After installation, it is sometimes necessary to create a new ext3 file system. For example, if you
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3. Converting to an ext3 File System
add a new disk drive to the system, you may want to partition the drive and use the ext3 file system.
The steps for creating an ext3 file system are as follows:
1.
Format the partition with the ext3 file system using mkfs.
2.
Label the partition using e2label.
3. Converting to an ext3 File System
The tune2fs allows you to convert an ext2 filesystem to ext3.
Note
Always use the e2fsck utility to check your filesystem before and after using
tune2fs. A default installation of Red Hat Enterprise Linux uses ext3 for all file systems.
To convert an ext2 filesystem to ext3, log in as root and type the following command in a terminal:
/sbin/tune2fs -j <block_device>
where <block_device> contains the ext2 filesystem you wish to convert.
A valid block device could be one of two types of entries:
•
A mapped device — A logical volume in a volume group, for example, /
dev/mapper/VolGroup00-LogVol02.
•
A static device — A traditional storage volume, for example, /dev/hdbX, where hdb is a storage device name and X is the partition number.
Issue the df command to display mounted file systems.
For the remainder of this section, the sample commands use the following value for the block
device:
/dev/mapper/VolGroup00-LogVol02
You must recreate the initrd image so that it will contain the ext3 kernel module. To create this,
run the mkinitrd program. For information on using the mkinitrd command, type man mkinitrd.
Also, make sure your GRUB configuration loads the initrd.
If you fail to make this change, the system still boots, but the file system is mounted as ext2 instead of ext3.
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4. Reverting to an ext2 File System
4. Reverting to an ext2 File System
If you wish to revert a partition from ext3 to ext2 for any reason, you must first unmount the partition by logging in as root and typing,
umount /dev/mapper/VolGroup00-LogVol02
Next, change the file system type to ext2 by typing the following command as root:
/sbin/tune2fs -O ^has_journal /dev/mapper/VolGroup00-LogVol02
Check the partition for errors by typing the following command as root:
/sbin/e2fsck -y /dev/mapper/VolGroup00-LogVol02
Then mount the partition again as ext2 file system by typing:
mount -t ext2 /dev/mapper/VolGroup00-LogVol02/mount/point
In the above command, replace /mount/point with the mount point of the partition.
Next, remove the .journal file at the root level of the partition by changing to the directory
where it is mounted and typing:
rm -f .journal
You now have an ext2 partition.
If you want to permanently change the partition to ext2, remember to update the /etc/fstab file.
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Chapter 3. The
proc
File System
The Linux kernel has two primary functions: to control access to physical devices on the computer and to schedule when and how processes interact with these devices. The /proc/ directory — also called the proc file system — contains a hierarchy of special files which represent
the current state of the kernel — allowing applications and users to peer into the kernel's view of
the system.
Within the /proc/ directory, one can find a wealth of information detailing the system hardware
and any processes currently running. In addition, some of the files within the /proc/ directory
tree can be manipulated by users and applications to communicate configuration changes to the
kernel.
1. A Virtual File System
Under Linux, all data are stored as files. Most users are familiar with the two primary types of
files: text and binary. But the /proc/ directory contains another type of file called a virtual file. It
is for this reason that /proc/ is often referred to as a virtual file system.
These virtual files have unique qualities. Most of them are listed as zero bytes in size and yet
when one is viewed, it can contain a large amount of information. In addition, most of the time
and date settings on virtual files reflect the current time and date, indicative of the fact they are
constantly updated.
Virtual files such as /proc/interrupts, /proc/meminfo, /proc/mounts, and /proc/partitions
provide an up-to-the-moment glimpse of the system's hardware. Others, like the /
proc/filesystems file and the /proc/sys/ directory provide system configuration information and
interfaces.
For organizational purposes, files containing information on a similar topic are grouped into virtual directories and sub-directories. For instance, /proc/ide/ contains information for all physical IDE devices. Likewise, process directories contain information about each running process on
the system.
1.1. Viewing Virtual Files
By using the cat, more, or less commands on files within the /proc/ directory, users can immediately access enormous amounts of information about the system. For example, to display the
type of CPU a computer has, type cat /proc/cpuinfo to receive output similar to the following:
processor : 0 vendor_id : AuthenticAMD cpu family : 5 model : 9 model name : AMD-K6(tm) 3D+ Processor stepp
When viewing different virtual files in the /proc/ file system, some of the information is easily
understandable while some is not human-readable. This is in part why utilities exist to pull data
from virtual files and display it in a useful way. Examples of these utilities include lspci, apm,
free, and top.
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1.2. Changing Virtual Files
Note
Some of the virtual files in the /proc/ directory are readable only by the root user.
1.2. Changing Virtual Files
As a general rule, most virtual files within the /proc/ directory are read-only. However, some
can be used to adjust settings in the kernel. This is especially true for files in the /proc/sys/
subdirectory.
To change the value of a virtual file, use the echo command and a greater than symbol (>) to redirect the new value to the file. For example, to change the hostname on the fly, type:
echo www.example.com > /proc/sys/kernel/hostname
Other files act as binary or Boolean switches. Typing cat /proc/sys/net/ipv4/ip_forward returns either a 0 or a 1. A 0 indicates that the kernel is not forwarding network packets. Using the
echo command to change the value of the ip_forward file to 1 immediately turns packet forwarding on.
Tip
Another command used to alter settings in the /proc/sys/ subdirectory is /
sbin/sysctl. For more information on this command, refer to Section 4, “Using the
sysctl Command”
For a listing of some of the kernel configuration files available in the /proc/sys/ subdirectory,
refer to Section 3.9, “/proc/sys/”.
2. Top-level Files within the
proc
File System
Below is a list of some of the more useful virtual files in the top-level of the /proc/ directory.
Note
In most cases, the content of the files listed in this section are not the same as
those installed on your machine. This is because much of the information is specific to the hardware on which Red Hat Enterprise Linux is running for this documentation effort.
2.1. /proc/apm
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2.2. /proc/buddyinfo
This file provides information about the state of the Advanced Power Management (APM) system and is used by the apm command. If a system with no battery is connected to an AC power
source, this virtual file would look similar to the following:
1.16 1.2 0x07 0x01 0xff 0x80 -1% -1 ?
Running the apm
-v
command on such a system results in output similar to the following:
APM BIOS 1.2 (kernel driver 1.16ac) AC on-line, no system battery
For systems which do not use a battery as a power source, apm is able do little more than put
the machine in standby mode. The apm command is much more useful on laptops. For example,
the following output is from the command cat /proc/apm on a laptop while plugged into a power
outlet:
1.16 1.2 0x03 0x01 0x03 0x09 100% -1 ?
When the same laptop is unplugged from its power source for a few minutes, the content of the
apm file changes to something like the following:
1.16 1.2 0x03 0x00 0x00 0x01 99% 1792 min
The apm
-v
command now yields more useful data, such as the following:
APM BIOS 1.2 (kernel driver 1.16) AC off-line, battery status high: 99% (1 day, 5:52)
2.2. /proc/buddyinfo
This file is used primarily for diagnosing memory fragmentation issues. Using the buddy algorithm, each column represents the number of pages of a certain order (a certain size) that are
available at any given time. For example, for zone DMA (direct memory access), there are 90 of
2^(0*PAGE_SIZE) chunks of memory. Similarly, there are 6 of 2^(1*PAGE_SIZE) chunks, and 2
of 2^(2*PAGE_SIZE) chunks of memory available.
The DMA row references the first 16 MB on a system, the HighMem row references all memory
greater than 4 GB on a system, and the Normal row references all memory in between.
The following is an example of the output typical of /proc/buddyinfo:
Node 0, zone
Node 0, zone
Node 0, zone
DMA
Normal
HighMem
90
1650
2
6
310
0
2
5
0
1
0
1
1
0
1
...
...
...
2.3. /proc/cmdline
This file shows the parameters passed to the kernel at the time it is started. A sample /
proc/cmdline file looks like the following:
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2.4. /proc/cpuinfo
ro root=/dev/VolGroup00/LogVol00 rhgb quiet 3
This tells us that the kernel is mounted read-only (signified by (ro)), located on the first logical
volume (LogVol00) of the first volume group (/dev/VolGroup00). LogVol00 is the equivalent of a
disk partition in a non-LVM system (Logical Volume Management), just as /dev/VolGroup00 is
similar in concept to /dev/hda1, but much more extensible.
For more information on LVM used in Red Hat Enterprise Linux, refer to http://www.tldp.org/HOWTO/LVM-HOWTO/index.html.
Next, rhgb signals that the rhgb package has been installed, and graphical booting is supported,
assuming /etc/inittab shows a default runlevel set to id:5:initdefault:.
Finally, quiet indicates all verbose kernel messages are suppressed at boot time.
2.4. /proc/cpuinfo
This virtual file identifies the type of processor used by your system. The following is an example of the output typical of /proc/cpuinfo:
processor: 0
vendor_id: GenuineIntel
cpu family: 15
model: 2
model name: Intel(R) Xeon(TM) CPU 2.40GHz
stepping: 7 cpu
MHz: 2392.371
cache size: 512 KB
physical id: 0
siblings: 2
runqueue: 0
fdiv_bug: no
hlt_bug: no
f00f_bug: no
coma_bug: no
fpu: yes
fpu_exception: yes
cpuid level: 2
wp: yes
flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca
bogomips: 4771.02
•
processor
•
cpu family
•
model name
•
cpu MHz
cmov pat pse36 clflush dts acpi mmx fxsr s
— Provides each processor with an identifying number. On systems that have one
processor, only a 0 is present.
— Authoritatively identifies the type of processor in the system. For an Intelbased system, place the number in front of "86" to determine the value. This is particularly
helpful for those attempting to identify the architecture of an older system such as a 586,
486, or 386. Because some RPM packages are compiled for each of these particular architectures, this value also helps users determine which packages to install.
— Displays the common name of the processor, including its project name.
— Shows the precise speed in megahertz for the processor to the thousandths
decimal place.
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2.5. /proc/crypto
•
cache size
•
siblings
•
flags
— Displays the amount of level 2 memory cache available to the processor.
— Displays the number of sibling CPUs on the same physical CPU for architectures which use hyper-threading.
— Defines a number of different qualities about the processor, such as the presence
of a floating point unit (FPU) and the ability to process MMX instructions.
2.5. /proc/crypto
This file lists all installed cryptographic ciphers used by the Linux kernel, including additional details for each. A sample /proc/crypto file looks like the following:
name
module
type
blocksize
digestsize
name
module
type
blocksize
digestsize
:
:
:
:
:
:
:
:
:
:
sha1
kernel
digest
64
20
md5
md5
digest
64
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2.6. /proc/devices
This file displays the various character and block devices currently configured (not including
devices whose modules are not loaded). Below is a sample output from this file:
Character devices:
1 mem
4 /dev/vc/0
4 tty
4 ttyS
5 /dev/tty
5 /dev/console
5 /dev/ptmx
7 vcs
10 misc
13 input
29 fb
36 netlink
128 ptm
136 pts
180 usb
Block devices:
1 ramdisk
3 ide0
9 md
22 ide1
253 device-mapper
254 mdp
The output from /proc/devices includes the major number and name of the device, and is
broken into two major sections: Character devices and Block devices.
Character devices are similar to block devices, except for two basic differences:
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2.7. /proc/dma
1.
Character devices do not require buffering. Block devices have a buffer available, allowing
them to order requests before addressing them. This is important for devices designed to
store information — such as hard drives — because the ability to order the information before writing it to the device allows it to be placed in a more efficient order.
2.
Character devices send data with no preconfigured size. Block devices can send and receive information in blocks of a size configured per device.
For more information about devices refer to the following installed documentation:
/usr/share/doc/kernel-doc-<version>/Documentation/devices.txt
2.7. /proc/dma
This file contains a list of the registered ISA DMA channels in use. A sample /proc/dma files
looks like the following:
4: cascade
2.8. /proc/execdomains
This file lists the execution domains currently supported by the Linux kernel, along with the
range of personalities they support.
0-0
Linux
[kernel]
Think of execution domains as the "personality" for an operating system. Because other binary
formats, such as Solaris, UnixWare, and FreeBSD, can be used with Linux, programmers can
change the way the operating system treats system calls from these binaries by changing the
personality of the task. Except for the PER_LINUX execution domain, different personalities can be
implemented as dynamically loadable modules.
2.9. /proc/fb
This file contains a list of frame buffer devices, with the frame buffer device number and the
driver that controls it. Typical output of /proc/fb for systems which contain frame buffer devices
looks similar to the following:
0 VESA VGA
2.10. /proc/filesystems
This file displays a list of the file system types currently supported by the kernel. Sample output
from a generic /proc/filesystems file looks similar to the following:
nodev
nodev
nodev
nodev
sysfs
rootfs
bdev
proc
15
2.11. /proc/interrupts
nodev
nodev
nodev
nodev
nodev
nodev
nodev
nodev
nodev
ext2
nodev
nodev
iso9660
nodev
ext3
nodev
nodev
sockfs
binfmt_misc
usbfs
usbdevfs
futexfs
tmpfs
pipefs
eventpollfs
devpts
ramfs
hugetlbfs
mqueue
rpc_pipefs
autofs
The first column signifies whether the file system is mounted on a block device. Those beginning with nodev are not mounted on a device. The second column lists the names of the file systems supported.
The mount command cycles through the file systems listed here when one is not specified as an
argument.
2.11. /proc/interrupts
This file records the number of interrupts per IRQ on the x86 architecture. A standard /
proc/interrupts looks similar to the following:
CPU0
0:
80448940
1:
174412
2:
0
8:
1
10:
410964
12:
60330
14:
1314121
15:
5195422
NMI:
0
ERR:
0
XT-PIC
XT-PIC
XT-PIC
XT-PIC
XT-PIC
XT-PIC
XT-PIC
XT-PIC
timer
keyboard
cascade
rtc
eth0
PS/2 Mouse
ide0
ide1
For a multi-processor machine, this file may look slightly different:
CPU0
CPU1
0: 1366814704
0
1:
128
340
2:
0
0
8:
0
1
12:
5323
5793
13:
1
0
16:
11184294
15940594
20:
8450043
11120093
30:
10432
10722
31:
23
22
NMI:
0
ERR:
0
XT-PIC
IO-APIC-edge
XT-PIC
IO-APIC-edge
IO-APIC-edge
XT-PIC
IO-APIC-level
IO-APIC-level
IO-APIC-level
IO-APIC-level
timer
keyboard
cascade
rtc
PS/2 Mouse
fpu
Intel EtherExpress Pro 10/100 Ethernet
megaraid
aic7xxx
aic7xxx
The first column refers to the IRQ number. Each CPU in the system has its own column and its
16
2.12. /proc/iomem
own number of interrupts per IRQ. The next column reports the type of interrupt, and the last
column contains the name of the device that is located at that IRQ.
Each of the types of interrupts seen in this file, which are architecture-specific, mean something
different. For x86 machines, the following values are common:
•
XT-PIC
— This is the old AT computer interrupts.
•
IO-APIC-edge
•
IO-APIC-level
— The voltage signal on this interrupt transitions from low to high, creating an
edge, where the interrupt occurs and is only signaled once. This kind of interrupt, as well as
the IO-APIC-level interrupt, are only seen on systems with processors from the 586 family
and higher.
— Generates interrupts when its voltage signal is high until the signal is low
again.
2.12. /proc/iomem
This file shows you the current map of the system's memory for each physical device:
00000000-0009fbff
0009fc00-0009ffff
000a0000-000bffff
000c0000-000c7fff
000f0000-000fffff
00100000-07ffffff
00100000-00291ba8
00291ba9-002e09cb
e0000000-e3ffffff
e4000000-e4003fff
e5000000-e57fffff
e8000000-e8ffffff
e8000000-e8ffffff
ea000000-ea00007f
ea000000-ea00007f
:
:
:
:
:
:
:
:
:
:
:
:
:
:
:
System RAM
reserved
Video RAM area
Video ROM
System ROM
System RAM
Kernel code
Kernel data
VIA Technologies, Inc. VT82C597 [Apollo VP3] e4000000-e7ffffff : PCI Bus #01
Matrox Graphics, Inc. MGA G200 AGP
Matrox Graphics, Inc. MGA G200 AGP
PCI Bus #01
Matrox Graphics, Inc. MGA G200 AGP
Digital Equipment Corporation DECchip 21140 [FasterNet]
tulip ffff0000-ffffffff : reserved
The first column displays the memory registers used by each of the different types of memory.
The second column lists the kind of memory located within those registers and displays which
memory registers are used by the kernel within the system RAM or, if the network interface card
has multiple Ethernet ports, the memory registers assigned for each port.
2.13. /proc/ioports
The output of /proc/ioports provides a list of currently registered port regions used for input or
output communication with a device. This file can be quite long. The following is a partial listing:
0000-001f
0020-003f
0040-005f
0060-006f
0070-007f
0080-008f
00a0-00bf
00c0-00df
00f0-00ff
0170-0177
:
:
:
:
:
:
:
:
:
:
dma1
pic1
timer
keyboard
rtc
dma page reg
pic2
dma2
fpu
ide1
17
2.14. /proc/kcore
01f0-01f7
02f8-02ff
0376-0376
03c0-03df
03f6-03f6
03f8-03ff
0cf8-0cff
d000-dfff
e000-e00f
e000-e007
e008-e00f
e800-e87f
e800-e87f
:
:
:
:
:
:
:
:
:
:
:
:
:
ide0
serial(auto)
ide1
vga+
ide0
serial(auto)
PCI conf1
PCI Bus #01
VIA Technologies, Inc. Bus Master IDE
ide0
ide1
Digital Equipment Corporation DECchip 21140 [FasterNet]
tulip
The first column gives the I/O port address range reserved for the device listed in the second
column.
2.14. /proc/kcore
This file represents the physical memory of the system and is stored in the core file format. Unlike most /proc/ files, kcore displays a size. This value is given in bytes and is equal to the size
of the physical memory (RAM) used plus 4 KB.
The contents of this file are designed to be examined by a debugger, such as gdb, and is not human readable.
Caution
Do not view the /proc/kcore virtual file. The contents of the file scramble text output on the terminal. If this file is accidentally viewed, press Ctrl-C to stop the process and then type reset to bring back the command line prompt.
2.15. /proc/kmsg
This file is used to hold messages generated by the kernel. These messages are then picked up
by other programs, such as /sbin/klogd or /bin/dmesg.
2.16. /proc/loadavg
This file provides a look at the load average in regard to both the CPU and IO over time, as well
as additional data used by uptime and other commands. A sample /proc/loadavg file looks similar to the following:
0.20 0.18 0.12 1/80 11206
The first three columns measure CPU and IO utilization of the last one, five, and 10 minute periods. The fourth column shows the number of currently running processes and the total number
of processes. The last column displays the last process ID used.
2.17. /proc/locks
18
2.18. /proc/mdstat
This file displays the files currently locked by the kernel. The contents of this file contain internal
kernel debugging data and can vary tremendously, depending on the use of the system. A
sample /proc/locks file for a lightly loaded system looks similar to the following:
1:
2:
3:
4:
5:
6:
7:
POSIX
FLOCK
POSIX
POSIX
POSIX
POSIX
POSIX
ADVISORY
ADVISORY
ADVISORY
ADVISORY
ADVISORY
ADVISORY
ADVISORY
WRITE
WRITE
WRITE
WRITE
WRITE
WRITE
WRITE
3568
3517
3452
3443
3326
3175
3056
fd:00:2531452
fd:00:2531448
fd:00:2531442
fd:00:2531440
fd:00:2531430
fd:00:2531425
fd:00:2548663
0
0
0
0
0
0
0
EOF
EOF
EOF
EOF
EOF
EOF
EOF
Each lock has its own line which starts with a unique number. The second column refers to the
class of lock used, with FLOCK signifying the older-style UNIX file locks from a flock system call
and POSIX representing the newer POSIX locks from the lockf system call.
The third column can have two values: ADVISORY or MANDATORY. ADVISORY means that the lock
does not prevent other people from accessing the data; it only prevents other attempts to lock it.
MANDATORY means that no other access to the data is permitted while the lock is held. The fourth
column reveals whether the lock is allowing the holder READ or WRITE access to the file. The fifth
column shows the ID of the process holding the lock. The sixth column shows the ID of the file
being locked, in the format of MAJOR-DEVICE:MINOR-DEVICE:INODE-NUMBER. The seventh and eighth
column shows the start and end of the file's locked region.
2.18. /proc/mdstat
This file contains the current information for multiple-disk, RAID configurations. If the system
does not contain such a configuration, then /proc/mdstat looks similar to the following:
Personalities :
read_ahead not set unused devices: <none>
This file remains in the same state as seen above unless a software RAID or md device is
present. In that case, view /proc/mdstat to find the current status of mdX RAID devices.
The /proc/mdstat file below shows a system with its md0 configured as a RAID 1 device, while it
is currently re-syncing the disks:
Personalities : [linear] [raid1] read_ahead 1024 sectors
md0: active raid1 sda2[1] sdb2[0] 9940 blocks [2/2] [UU] resync=1% finish=12.3min algorithm 2 [3/3] [UUU]
unused devices: <none>
2.19. /proc/meminfo
This is one of the more commonly used files in the /proc/ directory, as it reports a large amount
of valuable information about the systems RAM usage.
The following sample /proc/meminfo virtual file is from a system with 256 MB of RAM and 512
MB of swap space:
MemTotal:
MemFree:
Buffers:
255908 kB
69936 kB
15812 kB
19
2.19. /proc/meminfo
Cached:
115124
SwapCached:
0
Active:
92700
Inactive:
63792
HighTotal:
0
HighFree:
0
LowTotal:
255908
LowFree:
69936
SwapTotal:
524280
SwapFree:
524280
Dirty:
4
Writeback:
0
Mapped:
42236
Slab:
25912
Committed_AS:
118680
PageTables:
1236
VmallocTotal: 3874808
VmallocUsed:
1416
VmallocChunk: 3872908
HugePages_Total:
0
HugePages_Free:
0
Hugepagesize:
4096
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
kB
Much of the information here is used by the free, top, and ps commands. In fact, the output of
the free command is similar in appearance to the contents and structure of /proc/meminfo. But
by looking directly at /proc/meminfo, more details are revealed:
•
MemTotal
— Total amount of physical RAM, in kilobytes.
•
MemFree
— The amount of physical RAM, in kilobytes, left unused by the system.
•
Buffers
— The amount of physical RAM, in kilobytes, used for file buffers.
•
Cached
•
SwapCached
•
Active
•
Inactive
•
HighTotal
•
LowTotal
•
SwapTotal
•
SwapFree
•
Dirty
— The amount of physical RAM, in kilobytes, used as cache memory.
— The amount of swap, in kilobytes, used as cache memory.
— The total amount of buffer or page cache memory, in kilobytes, that is in active
use. This is memory that has been recently used and is usually not reclaimed for other purposes.
— The total amount of buffer or page cache memory, in kilobytes, that are free and
available. This is memory that has not been recently used and can be reclaimed for other
purposes.
and HighFree — The total and free amount of memory, in kilobytes, that is not directly mapped into kernel space. The HighTotal value can vary based on the type of kernel
used.
and LowFree — The total and free amount of memory, in kilobytes, that is directly
mapped into kernel space. The LowTotal value can vary based on the type of kernel used.
— The total amount of swap available, in kilobytes.
— The total amount of swap free, in kilobytes.
— The total amount of memory, in kilobytes, waiting to be written back to the disk.
20
2.20. /proc/misc
•
Writeback
— The total amount of memory, in kilobytes, actively being written back to the
disk.
•
Mapped
— The total amount of memory, in kilobytes, which have been used to map devices,
files, or libraries using the mmap command.
•
Slab
•
Committed_AS
•
PageTables
— The total amount of memory, in kilobytes, used by the kernel to cache data structures for its own use.
— The total amount of memory, in kilobytes, estimated to complete the workload. This value represents the worst case scenario value, and also includes swap memory.
— The total amount of memory, in kilobytes, dedicated to the lowest page table
level.
•
VMallocTotal
— The total amount of memory, in kilobytes, of total allocated virtual address
space.
•
VMallocUsed
— The total amount of memory, in kilobytes, of used virtual address space.
•
VMallocChunk
— The largest contiguous block of memory, in kilobytes, of available virtual ad-
dress space.
•
HugePages_Total
— The total number of hugepages for the system. The number is derived
by dividing Hugepagesize by the megabytes set aside for hugepages specified in /
proc/sys/vm/hugetlb_pool. This statistic only appears on the x86, Itanium, and AMD64 architectures.
•
HugePages_Free
•
Hugepagesize
— The total number of hugepages available for the system. This statistic
only appears on the x86, Itanium, and AMD64 architectures.
— The size for each hugepages unit in kilobytes. By default, the value is 4096
KB on uniprocessor kernels for 32 bit architectures. For SMP, hugemem kernels, and
AMD64, the default is 2048 KB. For Itanium architectures, the default is 262144 KB. This
statistic only appears on the x86, Itanium, and AMD64 architectures.
2.20. /proc/misc
This file lists miscellaneous drivers registered on the miscellaneous major device, which is
device number 10:
63 device-mapper 175 agpgart 135 rtc 134 apm_bios
The first column is the minor number of each device, while the second column shows the driver
in use.
2.21. /proc/modules
This file displays a list of all modules loaded into the kernel. Its contents vary based on the configuration and use of your system, but it should be organized in a similar manner to this sample
/proc/modules file output:
21
2.22. /proc/mounts
Note
This example has been reformatted into a readable format. Most of this information
can also be viewed via the /sbin/lsmod command.
nfs
lockd
nls_utf8
vfat
fat
autofs4
sunrpc
3c59x
uhci_hcd
md5
ipv6
ext3
jbd
dm_mod
170109 0 51593
1 nfs,
1729
0 12097
0 38881
1 vfat,
20293
2 140453 3 nfs,lockd,
33257
0 28377
0 3777
1 211845 16 92585
2 65625
1 ext3,
46677
3 -
Live
Live
Live
Live
Live
Live
Live
Live
Live
Live
Live
Live
Live
Live
0x129b0000
0x128b0000
0x12830000
0x12823000
0x1287b000
0x1284f000
0x12954000
0x12871000
0x12869000
0x1282c000
0x128de000
0x12886000
0x12857000
0x12833000
The first column contains the name of the module.
The second column refers to the memory size of the module, in bytes.
The third column lists how many instances of the module are currently loaded. A value of zero
represents an unloaded module.
The fourth column states if the module depends upon another module to be present in order to
function, and lists those other modules.
The fifth column lists what load state the module is in: Live, Loading, or Unloading are the only
possible values.
The sixth column lists the current kernel memory offset for the loaded module. This information
can be useful for debugging purposes, or for profiling tools such as oprofile.
2.22. /proc/mounts
This file provides a list of all mounts in use by the system:
rootfs / rootfs rw 0 0
/proc /proc proc rw,nodiratime 0 0 none
/dev ramfs rw 0 0
/dev/mapper/VolGroup00-LogVol00 / ext3 rw 0 0
none /dev ramfs rw 0 0
/proc /proc proc rw,nodiratime 0 0
/sys /sys sysfs rw 0 0
none /dev/pts devpts rw 0 0
usbdevfs /proc/bus/usb usbdevfs rw 0 0
/dev/hda1 /boot ext3 rw 0 0
none /dev/shm tmpfs rw 0 0
none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0
sunrpc /var/lib/nfs/rpc_pipefs rpc_pipefs rw 0 0
The output found here is similar to the contents of /etc/mtab, except that /proc/mount is more
22
2.23. /proc/mtrr
up-to-date.
The first column specifies the device that is mounted, the second column reveals the mount
point, and the third column tells the file system type, and the fourth column tells you if it is mounted read-only (ro) or read-write (rw). The fifth and sixth columns are dummy values designed to
match the format used in /etc/mtab.
2.23. /proc/mtrr
This file refers to the current Memory Type Range Registers (MTRRs) in use with the system. If
the system architecture supports MTRRs, then the /proc/mtrr file may look similar to the following:
reg00: base=0x00000000 (
0MB), size= 256MB: write-back, count=1
reg01: base=0xe8000000 (3712MB), size= 32MB: write-combining, count=1
MTRRs are used with the Intel P6 family of processors (Pentium II and higher) and control processor access to memory ranges. When using a video card on a PCI or AGP bus, a properly
configured /proc/mtrr file can increase performance more than 150%.
Most of the time, this value is properly configured by default. More information on manually configuring this file can be found locally at the following location:
/usr/share/doc/kernel-doc-<version>/Documentation/mtrr.txt
2.24. /proc/partitions
This file contains partition block allocation information. A sampling of this file from a basic system looks similar to the following:
major minor #blocks name
3
0
19531250 hda
3
1
104391 hda1
3
2
19422585 hda2
253
0
22708224 dm-0
253
1
524288 dm-1
Most of the information here is of little importance to the user, except for the following columns:
•
major
— The major number of the device with this partition. The major number in the /
corresponds with the block device ide0, in /proc/devices.
proc/partitions, (3),
•
minor
— The minor number of the device with this partition. This serves to separate the partitions into different physical devices and relates to the number at the end of the name of the
partition.
•
#blocks
•
name
— Lists the number of physical disk blocks contained in a particular partition.
— The name of the partition.
2.25. /proc/pci
23
2.26. /proc/slabinfo
This file contains a full listing of every PCI device on the system. Depending on the number of
PCI devices, /proc/pci can be rather long. A sampling of this file from a basic system looks similar to the following:
Bus
Bus
Bus
Bus
Bus
Bus
Bus
Bus
0,
0,
0,
0,
0,
0,
0,
0,
device
device
device
device
device
device
device
device
0, function 0: Host bridge: Intel Corporation 440BX/ZX - 82443BX/ZX Host bridge (rev 3). Mas
1, function 0: PCI bridge: Intel Corporation 440BX/ZX - 82443BX/ZX AGP bridge (rev 3).
Mas
4, function 0: ISA bridge: Intel Corporation 82371AB PIIX4 ISA (rev 2).
4, function 1: IDE interface: Intel Corporation 82371AB PIIX4 IDE (rev 1). Master Capable. L
4, function 2: USB Controller: Intel Corporation 82371AB PIIX4 USB (rev 1). IRQ 5. Master Ca
4, function 3: Bridge: Intel Corporation 82371AB PIIX4 ACPI (rev 2). IRQ 9.
9, function 0: Ethernet controller: Lite-On Communications Inc LNE100TX (rev 33). IRQ 5. Mas
12, function 0: VGA compatible controller: S3 Inc. ViRGE/DX or /GX (rev 1). IRQ 11. Master
This output shows a list of all PCI devices, sorted in the order of bus, device, and function. Beyond providing the name and version of the device, this list also gives detailed IRQ information
so an administrator can quickly look for conflicts.
Tip
To get a more readable version of this information, type:
/sbin/lspci -vb
2.26. /proc/slabinfo
This file gives full information about memory usage on the slab level. Linux kernels greater than
version 2.2 use slab pools to manage memory above the page level. Commonly used objects
have their own slab pools.
Instead of parsing the highly verbose /proc/slabinfo file manually, the /usr/bin/slabtop program displays kernel slab cache information in real time. This program allows for custom configurations, including column sorting and screen refreshing.
A sample screen shot of /usr/bin/slabtop usually looks like the following example:
Active / Total Objects (% used)
Active / Total Slabs (% used)
Active / Total Caches (% used)
Active / Total Size (% used)
Minimum / Average / Maximum Object
OBJS
ACTIVE USE
OBJ
SIZE
44814 43159 96%
0.62K
7469
36900 34614 93%
0.05K
492
35213 33124 94%
0.16K
1531
7364
6463 87%
0.27K
526
2585
1781 68%
0.08K
55
2263
2116 93%
0.12K
73
1904
1125 59%
0.03K
16
1666
768 46%
0.03K
14
1512
1482 98%
0.44K
168
1464
1040 71%
0.06K
24
1320
820 62%
0.19K
66
678
587 86%
0.02K
3
:
:
:
:
:
133629 / 147300 (90.7%)
11492 / 11493 (100.0%)
77 / 121 (63.6%)
41739.83K / 44081.89K (94.7%)
0.01K / 0.30K / 128.00K
SLABS OBJ/SLAB CACHE SIZE NAME
6
29876K ext3_inode_cache
75
1968K buffer_head
23
6124K dentry_cache
14
2104K radix_tree_node
47
220K vm_area_struct
31
292K size-128
119
64K size-32
119
56K anon_vma
9
672K inode_cache
61
96K size-64
20
264K filp
226
12K dm_io
24
2.27. /proc/stat
678
576
528
492
465
452
420
305
290
264
260
260
587 86%
574 99%
514 97%
372 75%
314 67%
331 73%
420 100%
256 83%
4
1%
264 100%
256 98%
256 98%
0.02K
0.47K
0.50K
0.09K
0.25K
0.02K
0.19K
0.06K
0.01K
4.00K
0.19K
0.75K
3
72
66
12
31
2
21
5
1
264
13
52
226
8
8
41
15
226
20
61
290
1
20
5
12K
288K
264K
48K
124K
8K
84K
20K
4K
1056K
52K
208K
dm_tio
proc_inode_cache
size-512
bio
size-256
biovec-1
skbuff_head_cache
biovec-4
revoke_table
size-4096
biovec-16
biovec-64
Some of the more commonly used statistics in /proc/slabinfo that are included into /
usr/bin/slabtop include:
•
OBJS
— The total number of objects (memory blocks), including those in use (allocated), and
some spares not in use.
•
ACTIVE
•
USE
•
OBJ SIZE
•
SLABS
•
OBJ/SLAB
•
CACHE SIZE
•
NAME
— The number of objects (memory blocks) that are in use (allocated).
— Percentage of total objects that are active. ((ACTIVE/OBJS)(100))
— The size of the objects.
— The total number of slabs.
— The number of objects that fit into a slab.
— The cache size of the slab.
— The name of the slab.
For more information on the /usr/bin/slabtop program, refer to the slabtop man page.
2.27. /proc/stat
This file keeps track of a variety of different statistics about the system since it was last restarted. The contents of /proc/stat, which can be quite long, usually begins like the following example:
cpu 259246 7001 60190 34250993 137517 772 0
cpu0 259246 7001 60190 34250993 137517 772 0
intr 354133732 347209999 2272 0 4 4 0 0 3 1 1249247 0 0 80143 0 422626 5169433
ctxt 12547729
btime 1093631447
processes 130523
procs_running 1
procs_blocked 0
preempt 5651840
cpu 209841 1554 21720 118519346 72939 154 27168
cpu0 42536 798 4841 14790880 14778 124 3117
cpu1 24184 569 3875 14794524 30209 29 3130
cpu2 28616 11 2182 14818198 4020 1 3493
cpu3 35350 6 2942 14811519 3045 0 3659
cpu4 18209 135 2263 14820076 12465 0 3373
cpu5 20795 35 1866 14825701 4508 0 3615
cpu6 21607 0 2201 14827053 2325 0 3334
cpu7 18544 0 1550 14831395 1589 0 3447
25
2.28. /proc/swaps
intr 15239682 14857833 6 0 6 6 0 5 0 1 0 0 0 29 0 2 0 0 0 0 0 0 0 94982 0 286812
ctxt 4209609
btime 1078711415
processes 21905
procs_running 1
procs_blocked 0
Some of the more commonly used statistics include:
•
cpu
— Measures the number of jiffies (1/100 of a second for x86 systems) that the system
has been in user mode, user mode with low priority (nice), system mode, idle task, I/O wait,
IRQ (hardirq), and softirq respectively. The IRQ (hardirq) is the direct response to a hardware event. The IRQ takes minimal work for queuing the "heavy" work up for the softirq to
execute. The softirq runs at a lower priority than the IRQ and therefore may be interrupted
more frequently. The total for all CPUs is given at the top, while each individual CPU is listed
below with its own statistics. The following example is a 4-way Intel Pentium Xeon configuration with multi-threading enabled, therefore showing four physical processors and four virtual
processors totaling eight processors.
•
page
— The number of memory pages the system has written in and out to disk.
•
swap
— The number of swap pages the system has brought in and out.
•
intr
— The number of interrupts the system has experienced.
•
btime
— The boot time, measured in the number of seconds since January 1, 1970, otherwise known as the epoch.
2.28. /proc/swaps
This file measures swap space and its utilization. For a system with only one swap partition, the
output of /proc/swaps may look similar to the following:
Filename
/dev/mapper/VolGroup00-LogVol01
Type
partition
Size
524280
Used
0
Priority
-1
While some of this information can be found in other files in the /proc/ directory, /proc/swap
provides a snapshot of every swap file name, the type of swap space, the total size, and the
amount of space in use (in kilobytes). The priority column is useful when multiple swap files are
in use. The lower the priority, the more likely the swap file is to be used.
2.29. /proc/sysrq-trigger
Using the echo command to write to this file, a remote root user can execute most System Request Key commands remotely as if at the local terminal. To echo values to this file, the /
proc/sys/kernel/sysrq must be set to a value other than 0. For more information about the System Request Key, refer to Section 3.9.3, “/proc/sys/kernel/”.
Although it is possible to write to this file, it cannot be read, even by the root user.
2.30. /proc/uptime
26
2.31. /proc/version
This file contains information detailing how long the system has been on since its last restart.
The output of /proc/uptime is quite minimal:
350735.47 234388.90
The first number is the total number of seconds the system has been up. The second number is
how much of that time the machine has spent idle, in seconds.
2.31. /proc/version
This file specifies the version of the Linux kernel and gcc in use, as well as the version of Red
Hat Enterprise Linux installed on the system:
Linux version 2.6.8-1.523 ([email protected]) (gcc version 3.4.1 20040714 \
(Red Hat Enterprise Linux 3.
This information is used for a variety of purposes, including the version data presented when a
user logs in.
3. Directories within
/proc/
Common groups of information concerning the kernel are grouped into directories and subdirectories within the /proc/ directory.
3.1. Process Directories
Every /proc/ directory contains a number of directories with numerical names. A listing of them
may be similar to the following:
dr-xr-xr-x
dr-xr-xr-x
dr-xr-xr-x
dr-xr-xr-x
dr-xr-xr-x
dr-xr-xr-x
dr-xr-xr-x
dr-xr-xr-x
3
3
3
3
3
3
3
3
root
root
xfs
daemon
root
apache
rpc
rpcuser
root
root
xfs
daemon
root
apache
rpc
rpcuser
0
0
0
0
0
0
0
0
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
13
13
13
13
13
13
13
13
01:28
01:28
01:28
01:28
01:28
01:28
01:28
01:28
1
1010
1087
1123
11307
13660
637
666
These directories are called process directories, as they are named after a program's process
ID and contain information specific to that process. The owner and group of each process directory is set to the user running the process. When the process is terminated, its /proc/ process
directory vanishes.
Each process directory contains the following files:
•
cmdline
•
cwd
•
environ
— Contains the command issued when starting the process.
— A symbolic link to the current working directory for the process.
— A list of the environment variables for the process. The environment variable is
given in all upper-case characters, and the value is in lower-case characters.
27
3.1. Process Directories
•
exe
•
fd
— A symbolic link to the executable of this process.
— A directory containing all of the file descriptors for a particular process. These are given in numbered links:
total 0
lrwx-----lrwx-----lrwx-----lrwx-----lrwx-----lrwx-----lrwx-----lrwx------
•
1
1
1
1
1
1
1
1
root
root
root
root
root
root
root
root
root
root
root
root
root
root
root
root
64
64
64
64
64
64
64
64
May
May
May
May
May
May
May
May
8
8
8
8
8
8
8
8
11:31
11:31
11:31
11:31
11:31
11:31
11:31
11:31
0
1
2
3
4
5
6
7
->
->
->
->
->
->
->
->
/dev/null
/dev/null
/dev/null
/dev/ptmx
socket:[7774817]
/dev/ptmx
socket:[7774829]
/dev/ptmx
— A list of memory maps to the various executables and library files associated with
this process. This file can be rather long, depending upon the complexity of the process, but
sample output from the sshd process begins like the following:
maps
08048000-08086000
08086000-08088000
08088000-08095000
40000000-40013000
40013000-40014000
40031000-40038000
40038000-40039000
40039000-4003a000
4003a000-4003c000
4003c000-4003d000
r-xp
rw-p
rwxp
r-xp
rw-p
r-xp
rw-p
rw-p
r-xp
rw-p
00000000 03:03 391479
/usr/sbin/sshd
0003e000 03:03 391479/usr/sbin/sshd
00000000 00:00 0
0000000 03:03 293205/lib/ld-2.2.5.so
00013000 03:03 293205/lib/ld-2.2.5.so
00000000 03:03 293282/lib/libpam.so.0.75
00006000 03:03 293282/lib/libpam.so.0.75
00000000 00:00 0
00000000 03:03 293218/lib/libdl-2.2.5.so
00001000 03:03 293218/lib/libdl-2.2.5.so
•
mem
— The memory held by the process. This file cannot be read by the user.
•
root
— A link to the root directory of the process.
•
stat
— The status of the process.
•
statm
— The status of the memory in use by the process. Below is a sample /proc/statm
file:
263 210 210 5 0 205 0
The seven columns relate to different memory statistics for the process. From left to right,
they report the following aspects of the memory used:
1.
Total program size, in kilobytes.
2.
Size of memory portions, in kilobytes.
3.
Number of pages that are shared.
4.
Number of pages that are code.
5.
Number of pages of data/stack.
6.
Number of library pages.
28
3.2. /proc/bus/
7.
•
Number of dirty pages.
— The status of the process in a more readable form than stat or statm. Sample output for sshd looks similar to the following:
status
Name:sshd
State:S (sleeping)
Tgid:797
Pid:797
PPid:1
TracerPid:0
Uid:0000
Gid:0000
FDSize:32
Groups:
VmSize:
3072 kB
VmLck:
0 kB
VmRSS:
840 kB
VmData:
104 kB
VmStk:
12 kB
VmExe:
300 kB
VmLib:
2528 kB
SigPnd:0000000000000000
SigBlk:0000000000000000
SigIgn:8000000000001000
SigCgt:0000000000014005
CapInh:0000000000000000
CapPrm:00000000fffffeff
CapEff:00000000fffffeff
The information in this output includes the process name and ID, the state (such as S
(sleeping) or R (running)), user/group ID running the process, and detailed data regarding
memory usage.
3.1.1. /proc/self/
The /proc/self/ directory is a link to the currently running process. This allows a process to
look at itself without having to know its process ID.
Within a shell environment, a listing of the /proc/self/ directory produces the same contents as
listing the process directory for that process.
3.2. /proc/bus/
This directory contains information specific to the various buses available on the system. For example, on a standard system containing PCI and USB buses, current data on each of these
buses is available within a subdirectory within /proc/bus/ by the same name, such as /
proc/bus/pci/.
The subdirectories and files available within /proc/bus/ vary depending on the devices connected to the system. However, each bus type has at least one directory. Within these bus directories are normally at least one subdirectory with a numerical name, such as 001, which contain
binary files.
For example, the /proc/bus/usb/ subdirectory contains files that track the various devices on
any USB buses, as well as the drivers required for them. The following is a sample listing of a /
29
3.3. /proc/driver/
proc/bus/usb/
directory:
total 0 dr-xr-xr-x
-r--r--r-1 root
-r--r--r-1 root
1 root
root
root
root
0 May
0 May
0 May 3 16:25 001
3 16:25 devices
3 16:25 drivers
The /proc/bus/usb/001/ directory contains all devices on the first USB bus and the devices file
identifies the USB root hub on the motherboard.
The following is a example of a /proc/bus/usb/devices file:
T:
B:
D:
P:
S:
S:
C:*
I:
E:
Bus=01 Lev=00 Prnt=00 Port=00 Cnt=00 Dev#= 1 Spd=12 MxCh= 2
Alloc= 0/900 us ( 0%), #Int= 0, #Iso= 0
Ver= 1.00 Cls=09(hub ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
Vendor=0000 ProdID=0000 Rev= 0.00
Product=USB UHCI Root Hub
SerialNumber=d400
#Ifs= 1 Cfg#= 1 Atr=40 MxPwr= 0mA
If#= 0 Alt= 0 #EPs= 1 Cls=09(hub ) Sub=00 Prot=00 Driver=hub
Ad=81(I) Atr=03(Int.) MxPS=
8 Ivl=255ms
3.3. /proc/driver/
This directory contains information for specific drivers in use by the kernel.
A common file found here is rtc which provides output from the driver for the system's Real
Time Clock (RTC), the device that keeps the time while the system is switched off. Sample output from /proc/driver/rtc looks like the following:
rtc_time
rtc_date
rtc_epoch
alarm
DST_enable
BCD
24hr
square_wave
alarm_IRQ
update_IRQ
periodic_IRQ
periodic_freq
batt_status
:
:
:
:
:
:
:
:
:
:
:
:
:
16:21:00
2004-08-31
1900
21:16:27
no
yes
yes
no
no
no
no
1024
okay
For more information about the RTC, refer to the following installed documentation:
/usr/share/doc/kernel-doc-<version>/Documentation/rtc.txt.
3.4. /proc/fs
This directory shows which file systems are exported. If running an NFS server, typing cat /
proc/fs/nfsd/exports displays the file systems being shared and the permissions granted for
those file systems. For more on file system sharing with NFS, refer to Chapter 18, Network File
System (NFS).
3.5. /proc/ide/
30
3.5. /proc/ide/
This directory contains information about IDE devices on the system. Each IDE channel is represented as a separate directory, such as /proc/ide/ide0 and /proc/ide/ide1. In addition, a
drivers file is available, providing the version number of the various drivers used on the IDE
channels:
ide-floppy version 0.99.
newide ide-cdrom version 4.61
ide-disk version 1.18
Many chipsets also provide a file in this directory with additional data concerning the drives connected through the channels. For example, a generic Intel PIIX4 Ultra 33 chipset produces the /
proc/ide/piix file which reveals whether DMA or UDMA is enabled for the devices on the IDE
channels:
Intel PIIX4 Ultra 33 Chipset.
------------- Primary Channel ---------------- Secondary Channel ------------enabled
enabled
------------- drive0 --------- drive1 -------- drive0 ---------- drive1 -----DMA enabled:
yes
no
yes
no
UDMA enabled:
yes
no
no
no
UDMA enabled:
2
X
X
X
UDMA DMA PIO
Navigating into the directory for an IDE channel, such as ide0, provides additional information.
The channel file provides the channel number, while the model identifies the bus type for the
channel (such as pci).
3.5.1. Device Directories
Within each IDE channel directory is a device directory. The name of the device directory corresponds to the drive letter in the /dev/ directory. For instance, the first IDE drive on ide0 would
be hda.
Note
There is a symbolic link to each of these device directories in the /proc/ide/ directory.
Each device directory contains a collection of information and statistics. The contents of these
directories vary according to the type of device connected. Some of the more useful files common to many devices include:
•
cache
— The device cache.
•
capacity
•
driver
•
geometry
— The capacity of the device, in 512 byte blocks.
— The driver and version used to control the device.
— The physical and logical geometry of the device.
31
3.6. /proc/irq/
•
media
— The type of device, such as a disk.
•
model
— The model name or number of the device.
•
settings
— A collection of current device parameters. This file usually contains quite a bit of
useful, technical information. A sample settings file for a standard IDE hard disk looks similar to the following:
name
---acoustic
address
bios_cyl
bios_head
bios_sect
bswap
current_speed
failures
init_speed
io_32bit
keepsettings
lun
max_failures
multcount
nice1
nowerr
number
pio_mode
unmaskirq
using_dma
wcache
value
----0
0
38752
16
63
0
68
0
68
0
0
0
1
16
1
0
0
write-only
0
1
1
min
--0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
max
--254
2
65535
255
63
1
70
65535
70
3
1
7
65535
16
1
1
3
255
1
1
1
mode
---rw
rw
rw
rw
rw
r
rw
rw
rw
rw
rw
rw
rw
rw
rw
rw
rw
w
rw
rw
rw
3.6. /proc/irq/
This directory is used to set IRQ to CPU affinity, which allows the system to connect a particular
IRQ to only one CPU. Alternatively, it can exclude a CPU from handling any IRQs.
Each IRQ has its own directory, allowing for the individual configuration of each IRQ. The /
proc/irq/prof_cpu_mask file is a bitmask that contains the default values for the smp_affinity file
in the IRQ directory. The values in smp_affinity specify which CPUs handle that particular IRQ.
For more information about the /proc/irq/ directory, refer to the following installed documentation:
/usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt
3.7. /proc/net/
This directory provides a comprehensive look at various networking parameters and statistics.
Each directory and virtual file within this directory describes aspects of the system's network
configuration. Below is a partial list of the /proc/net/ directory:
•
— Lists the kernel's ARP table. This file is particularly useful for connecting a hardware
address to an IP address on a system.
arp
32
3.8. /proc/scsi/
•
atm/
directory — The files within this directory contain Asynchronous Transfer Mode (ATM)
settings and statistics. This directory is primarily used with ATM networking and ADSL cards.
•
dev
•
dev_mcast
•
igmp
•
ip_conntrack
— Lists the various network devices configured on the system, complete with transmit
and receive statistics. This file displays the number of bytes each interface has sent and received, the number of packets inbound and outbound, the number of errors seen, the number of packets dropped, and more.
— Lists Layer2 multicast groups on which each device is listening.
— Lists the IP multicast addresses which this system joined.
— Lists tracked network connections for machines that are forwarding IP con-
nections.
•
ip_tables_names
— Lists the types of iptables in use. This file is only present if iptables is
active on the system and contains one or more of the following values: filter, mangle, or
nat.
•
ip_mr_cache
•
ip_mr_vif
•
netstat
•
psched
•
raw
•
route
•
rt_cache
•
snmp
•
sockstat
•
tcp
•
tr_rif
•
udp
•
unix
•
wireless
— Lists the multicast routing cache.
— Lists multicast virtual interfaces.
— Contains a broad yet detailed collection of networking statistics, including TCP
timeouts, SYN cookies sent and received, and much more.
— Lists global packet scheduler parameters.
— Lists raw device statistics.
— Lists the kernel's routing table.
— Contains the current routing cache.
— List of Simple Network Management Protocol (SNMP) data for various networking
protocols in use.
— Provides socket statistics.
— Contains detailed TCP socket information.
— Lists the token ring RIF routing table.
— Contains detailed UDP socket information.
— Lists UNIX domain sockets currently in use.
— Lists wireless interface data.
3.8. /proc/scsi/
This directory is analogous to the /proc/ide/ directory, but it is for connected SCSI devices.
The primary file in this directory is /proc/scsi/scsi, which contains a list of every recognized
SCSI device. From this listing, the type of device, as well as the model name, vendor, SCSI
33
3.8. /proc/scsi/
channel and ID data is available.
For example, if a system contains a SCSI CD-ROM, a tape drive, a hard drive, and a RAID controller, this file looks similar to the following:
Attached devices:
Host: scsi1
Channel: 00
Id: 05
Lun: 00
Vendor: NEC
Model: CD-ROM DRIVE:466
Rev: 1.06
Type:
CD-ROM
ANSI SCSI revision: 02
Host: scsi1
Channel: 00
Id: 06
Lun: 00
Vendor: ARCHIVE
Model: Python 04106-XXX
Rev: 7350
Type:
Sequential-Access
ANSI SCSI revision: 02
Host: scsi2
Channel: 00
Id: 06
Lun: 00
Vendor: DELL
Model: 1x6 U2W SCSI BP
Rev: 5.35
Type:
Processor
ANSI SCSI revision: 02
Host: scsi2
Channel: 02
Id: 00
Lun: 00
Vendor: MegaRAID
Model: LD0 RAID5 34556R
Rev: 1.01
Type:
Direct-Access
ANSI SCSI revision: 02
Each SCSI driver used by the system has its own directory within /proc/scsi/, which contains
files specific to each SCSI controller using that driver. From the previous example, aic7xxx/ and
megaraid/ directories are present, since two drivers are in use. The files in each of the directories typically contain an I/O address range, IRQ information, and statistics for the SCSI controller
using that driver. Each controller can report a different type and amount of information. The Adaptec AIC-7880 Ultra SCSI host adapter's file in this example system produces the following
output:
Adaptec AIC7xxx driver version: 5.1.20/3.2.4
Compile Options:
TCQ Enabled By Default : Disabled
AIC7XXX_PROC_STATS
: Enabled
AIC7XXX_RESET_DELAY
: 5
Adapter Configuration:
SCSI Adapter: Adaptec AIC-7880 Ultra SCSI host adapter
Ultra Narrow Controller
PCI MMAPed
I/O Base: 0xfcffe000
Adapter SEEPROM Config: SEEPROM found and used.
Adaptec SCSI BIOS: Enabled
IRQ: 30
34
3.9. /proc/sys/
SCBs: Active 0, Max Active 1, Allocated 15, HW 16, Page 255
Interrupts: 33726
BIOS Control Word: 0x18a6
Adapter Control Word: 0x1c5f
Extended Translation: Enabled
Disconnect Enable Flags: 0x00ff
Ultra Enable Flags: 0x0020
Tag Queue Enable Flags: 0x0000
Ordered Queue Tag Flags: 0x0000
Default Tag Queue Depth: 8
Tagged Queue By Device array for aic7xxx
host instance 1:
{255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255}
Actual queue depth per device for aic7xxx host instance 1:
{1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}
Statistics:
(scsi1:0:5:0) Device using Narrow/Sync transfers at 20.0 MByte/sec, offset 15
Transinfo settings: current(12/15/0/0), goal(12/15/0/0), user(12/15/0/0)
Total transfers 0 (0 reads and 0 writes)
< 2K
2K+
4K+
8K+
16K+
32K+
64K+
128K+
Reads:
0
0
0
0
0
0
0
0
Writes:
0
0
0
0
0
0
0
0
(scsi1:0:6:0) Device using Narrow/Sync transfers at 10.0 MByte/sec, offset 15
Transinfo settings: current(25/15/0/0), goal(12/15/0/0), user(12/15/0/0)
Total transfers 132 (0 reads and 132 writes)
< 2K
2K+
4K+
8K+
16K+
32K+
64K+
128K+
Reads:
0
0
0
0
0
0
0
0
Writes:
0
0
0
1
131
0
0
0
This output reveals the transfer speed to the SCSI devices connected to the controller based on
channel ID, as well as detailed statistics concerning the amount and sizes of files read or written
by that device. For example, this controller is communicating with the CD-ROM at 20 megabytes per second, while the tape drive is only communicating at 10 megabytes per second.
3.9. /proc/sys/
The /proc/sys/ directory is different from others in /proc/ because it not only provides information about the system but also allows the system administrator to immediately enable and disable kernel features.
Caution
Use caution when changing settings on a production system using the various files
in the /proc/sys/ directory. Changing the wrong setting may render the kernel unstable, requiring a system reboot.
For this reason, be sure the options are valid for that file before attempting to
change any value in /proc/sys/.
A good way to determine if a particular file can be configured, or if it is only designed to provide
information, is to list it with the -l option at the shell prompt. If the file is writable, it may be used
to configure the kernel. For example, a partial listing of /proc/sys/fs looks like the following:
-r--r--r--rw-r--r--
1 root
1 root
root
root
0 May 10 16:14 dentry-state
0 May 10 16:14 dir-notify-enable
35
3.9. /proc/sys/
-r--r--r--rw-r--r--r--r--r--
1 root
1 root
1 root
root
root
root
0 May 10 16:14 dquot-nr
0 May 10 16:14 file-max
0 May 10 16:14 file-nr
In this listing, the files dir-notify-enable and file-max can be written to and, therefore, can be
used to configure the kernel. The other files only provide feedback on current settings.
Changing a value within a /proc/sys/ file is done by echoing the new value into the file. For example, to enable the System Request Key on a running kernel, type the command:
echo 1 > /proc/sys/kernel/sysrq
This changes the value for sysrq from 0 (off) to 1 (on).
A few /proc/sys/ configuration files contain more than one value. To correctly send new values
to them, place a space character between each value passed with the echo command, such as
is done in this example:
echo 4 2 45 > /proc/sys/kernel/acct
Note
Any configuration changes made using the echo command disappear when the
system is restarted. To make configuration changes take effect after the system is
rebooted, refer to Section 4, “Using the sysctl Command”.
The /proc/sys/ directory contains several subdirectories controlling different aspects of a running kernel.
3.9.1. /proc/sys/dev/
This directory provides parameters for particular devices on the system. Most systems have at
least two directories, cdrom/ and raid/. Customized kernels can have other directories, such as
parport/, which provides the ability to share one parallel port between multiple device drivers.
The cdrom/ directory contains a file called info, which reveals a number of important CD-ROM
parameters:
CD-ROM information, Id:
drive name:
drive speed:
drive # of slots:
Can close tray:
Can open tray:
Can lock tray:
Can change speed:
Can select disk:
Can read multisession:
Can read MCN:
Reports media changed:
Can play audio:
cdrom.c 3.20 2003/12/17
hdc
48
1
1
1
1
1
0
1
1
1
1
36
3.9. /proc/sys/
Can
Can
Can
Can
Can
Can
Can
Can
write CD-R:
write CD-RW:
read DVD:
write DVD-R:
write DVD-RAM:
read MRW:
write MRW:
write RAM:
0
0
0
0
0
0
0
0
This file can be quickly scanned to discover the qualities of an unknown CD-ROM. If multiple
CD-ROMs are available on a system, each device is given its own column of information.
Various files in /proc/sys/dev/cdrom, such as autoclose and checkmedia, can be used to control
the system's CD-ROM. Use the echo command to enable or disable these features.
If RAID support is compiled into the kernel, a /proc/sys/dev/raid/ directory becomes available
with at least two files in it: speed_limit_min and speed_limit_max. These settings determine the
acceleration of RAID devices for I/O intensive tasks, such as resyncing the disks.
3.9.2. /proc/sys/fs/
This directory contains an array of options and information concerning various aspects of the file
system, including quota, file handle, inode, and dentry information.
The binfmt_misc/ directory is used to provide kernel support for miscellaneous binary formats.
The important files in /proc/sys/fs/ include:
•
dentry-state
— Provides the status of the directory cache. The file looks similar to the fol-
lowing:
574115293945000
The first number reveals the total number of directory cache entries, while the second number displays the number of unused entries. The third number tells the number of seconds
between when a directory has been freed and when it can be reclaimed, and the fourth
measures the pages currently requested by the system. The last two numbers are not used
and display only zeros.
•
dquot-nr
— Lists the maximum number of cached disk quota entries.
•
file-max
•
file-nr
•
overflowgid
•
super-max
•
super-nr
— Lists the maximum number of file handles that the kernel allocates. Raising the
value in this file can resolve errors caused by a lack of available file handles.
— Lists the number of allocated file handles, used file handles, and the maximum
number of file handles.
and overflowuid — Defines the fixed group ID and user ID, respectively, for use
with file systems that only support 16-bit group and user IDs.
— Controls the maximum number of superblocks available.
— Displays the current number of superblocks in use.
37
3.9. /proc/sys/
3.9.3. /proc/sys/kernel/
This directory contains a variety of different configuration files that directly affect the operation of
the kernel. Some of the most important files include:
•
— Controls the suspension of process accounting based on the percentage of free
space available on the file system containing the log. By default, the file looks like the following:
acct
4230
The first value dictates the percentage of free space required for logging to resume, while
the second value sets the threshold percentage of free space when logging is suspended.
The third value sets the interval, in seconds, that the kernel polls the file system to see if logging should be suspended or resumed.
•
— Controls the capability bounding settings, which provides a list of capabilities
for any process on the system. If a capability is not listed here, then no process, no matter
how privileged, can do it. The idea is to make the system more secure by ensuring that certain things cannot happen, at least beyond a certain point in the boot process.
cap-bound
For a valid list of values for this virtual file, refer to the following installed documentation:
/lib/modules/<kernel-version>/build/include/linux/capability.h.
•
— Controls whether Ctrl-Alt-Delete gracefully restarts the computer using
or forces an immediate reboot without syncing the dirty buffers to disk (1).
ctrl-alt-del
init (0)
•
domainname
— Configures the system domain name, such as example.com.
•
exec-shield
— Configures the Exec Shield feature of the kernel. Exec Shield provides protection against certain types of buffer overflow attacks.
There are two possible values for this virtual file:
•
0
— Disables Exec Shield.
•
1
— Enables Exec Shield. This is the default value.
Important
If a system is running security-sensitive applications that were started while Exec Shield was disabled, these applications must be restarted when Exec Shield
is enabled in order for Exec Shield to take effect.
•
— Enables location randomization of various items in memory. This
helps deter potential attackers from locating programs and daemons in memory. Each time a
program or daemon starts, it is put into a different memory location each time, never in a
exec-shield-randomize
38
3.9. /proc/sys/
static or absolute memory address.
There are two possible values for this virtual file:
•
0
— Disables randomization of Exec Shield. This may be useful for application debugging
purposes.
•
1
— Enables randomization of Exec Shield. This is the default value. Note: The execfile must also be set to 1 for exec-shield-randomize to be effective.
shield
•
hostname
•
hotplug
•
modprobe
•
msgmax
•
msgmnb
— Configures the system hostname, such as www.example.com.
— Configures the utility to be used when a configuration change is detected by the
system. This is primarily used with USB and Cardbus PCI. The default value of /
sbin/hotplug should not be changed unless testing a new program to fulfill this role.
— Sets the location of the program used to load kernel modules. The default value
is /sbin/modprobe which means kmod calls it to load the module when a kernel thread calls
kmod.
— Sets the maximum size of any message sent from one process to another and is
set to 8192 bytes by default. Be careful when raising this value, as queued messages
between processes are stored in non-swappable kernel memory. Any increase in msgmax
would increase RAM requirements for the system.
— Sets the maximum number of bytes in a single message queue. The default is
16384.
•
msgmni
— Sets the maximum number of message queue identifiers. The default is 16.
•
osrelease
•
ostype
•
overflowgid
•
panic
•
printk
— Lists the Linux kernel release number. This file can only be altered by changing
the kernel source and recompiling.
— Displays the type of operating system. By default, this file is set to Linux, and this
value can only be changed by changing the kernel source and recompiling.
and overflowuid — Defines the fixed group ID and user ID, respectively, for use
with system calls on architectures that only support 16-bit group and user IDs.
— Defines the number of seconds the kernel postpones rebooting when the system
experiences a kernel panic. By default, the value is set to 0, which disables automatic rebooting after a panic.
— This file controls a variety of settings related to printing or logging error messages.
Each error message reported by the kernel has a loglevel associated with it that defines the
importance of the message. The loglevel values break down in this order:
•
0
— Kernel emergency. The system is unusable.
•
1
— Kernel alert. Action must be taken immediately.
•
2
— Condition of the kernel is considered critical.
39
3.9. /proc/sys/
•
3
— General kernel error condition.
•
4
— General kernel warning condition.
•
5
— Kernel notice of a normal but significant condition.
•
6
— Kernel informational message.
•
7
— Kernel debug-level messages.
Four values are found in the printk file:
6
4
1
7
Each of these values defines a different rule for dealing with error messages. The first value,
called the console loglevel, defines the lowest priority of messages printed to the console.
(Note that, the lower the priority, the higher the loglevel number.) The second value sets the
default loglevel for messages without an explicit loglevel attached to them. The third value
sets the lowest possible loglevel configuration for the console loglevel. The last value sets
the default value for the console loglevel.
•
random/
directory — Lists a number of values related to generating random numbers for the
kernel.
•
rtsig-max
— Configures the maximum number of POSIX real-time signals that the system
may have queued at any one time. The default value is 1024.
•
rtsig-nr
•
sem
•
shmall
•
shmmax
•
shmmni
•
sysrq
— Lists the current number of POSIX real-time signals queued by the kernel.
— Configures semaphore settings within the kernel. A semaphore is a System V IPC object that is used to control utilization of a particular process.
— Sets the total amount of shared memory that can be used at one time on the system, in bytes. By default, this value is 2097152.
— Sets the largest shared memory segment size allowed by the kernel, in bytes. By
default, this value is 33554432. However, the kernel supports much larger values than this.
— Sets the maximum number of shared memory segments for the whole system, in
bytes. By default, this value is 4096
— Activates the System Request Key, if this value is set to anything other than zero
(0), the default.
The System Request Key allows immediate input to the kernel through simple key combinations. For example, the System Request Key can be used to immediately shut down or restart a system, sync all mounted file systems, or dump important information to the console.
To initiate a System Request Key, type Alt-SysRq-<system request code> . Replace <system
request code> with one of the following system request codes:
•
— Disables raw mode for the keyboard and sets it to XLATE (a limited keyboard mode
which does not recognize modifiers such as Alt, Ctrl, or Shift for all keys).
r
40
3.9. /proc/sys/
•
k
— Kills all processes active in a virtual console. Also called Secure Access Key (SAK),
it is often used to verify that the login prompt is spawned from init and not a trojan copy
designed to capture usernames and passwords.
•
b
•
c
•
o
— Shuts off the system.
•
s
— Attempts to sync disks attached to the system.
•
u
— Attempts to unmount and remount all file systems as read-only.
•
p
— Outputs all flags and registers to the console.
•
t
— Outputs a list of processes to the console.
•
m
— Outputs memory statistics to the console.
•
0
through 9 — Sets the log level for the console.
•
e
— Kills all processes except init using SIGTERM.
•
i
— Kills all processes except init using SIGKILL.
•
l
•
h
— Reboots the kernel without first unmounting file systems or syncing disks attached to
the system.
— Crashes the system without first unmounting file systems or syncing disks attached
to the system.
— Kills all processes using SIGKILL (including init). The system is unusable after issuing this System Request Key code.
— Displays help text.
This feature is most beneficial when using a development kernel or when experiencing system freezes.
Caution
The System Request Key feature is considered a security risk because an unattended console provides an attacker with access to the system. For this reason, it is turned off by default.
Refer to /usr/share/doc/kernel-doc-<version>/Documentation/sysrq.txt for more information about the System Request Key.
•
sysrq-key
— Defines the key code for the System Request Key (84 is the default).
•
sysrq-sticky
— Defines whether the System Request Key is a chorded key combination.
The accepted values are as follows:
41
3.9. /proc/sys/
•
0
— Alt-SysRq and the system request code must be pressed simultaneously. This is
the default value.
•
1
— Alt-SysRq must be pressed simultaneously, but the system request code can be
pressed anytime before the number of seconds specified in /
proc/sys/kernel/sysrq-timer elapses.
•
sysrq-timer
— Specifies the number of seconds allowed to pass before the system request
code must be pressed. The default value is 10.
•
tainted
•
0
•
1
•
2
— Indicates whether a non-GPL module is loaded.
— No non-GPL modules are loaded.
— At least one module without a GPL license (including modules with no license) is
loaded.
— At least one module was force-loaded with the command insmod
•
threads-max
•
version
-f.
— Sets the maximum number of threads to be used by the kernel, with a default
value of 2048.
— Displays the date and time the kernel was last compiled. The first field in this file,
such as #3, relates to the number of times a kernel was built from the source base.
3.9.4. /proc/sys/net/
This directory contains subdirectories concerning various networking topics. Various configurations at the time of kernel compilation make different directories available here, such as ethernet/, ipv4/, ipx/, and ipv6/. By altering the files within these directories, system administrators
are able to adjust the network configuration on a running system.
Given the wide variety of possible networking options available with Linux, only the most common /proc/sys/net/ directories are discussed.
The /proc/sys/net/core/ directory contains a variety of settings that control the interaction
between the kernel and networking layers. The most important of these files are:
•
message_burst
— Sets the amount of time in tenths of a second required to write a new
warning message. This setting is used to mitigate Denial of Service (DoS) attacks. The default setting is 50.
•
message_cost
— Sets a cost on every warning message. The higher the value of this file
(default of 5), the more likely the warning message is ignored. This setting is used to mitigate
DoS attacks.
The idea of a DoS attack is to bombard the targeted system with requests that generate errors and fill up disk partitions with log files or require all of the system's resources to handle
the error logging. The settings in message_burst and message_cost are designed to be modified based on the system's acceptable risk versus the need for comprehensive logging.
42
3.9. /proc/sys/
•
netdev_max_backlog
— Sets the maximum number of packets allowed to queue when a particular interface receives packets faster than the kernel can process them. The default value
for this file is 300.
•
optmem_max
•
rmem_default
•
rmem_max
•
wmem_default
•
wmem_max
— Configures the maximum ancillary buffer size allowed per socket.
— Sets the receive socket buffer default size in bytes.
— Sets the receive socket buffer maximum size in bytes.
— Sets the send socket buffer default size in bytes.
— Sets the send socket buffer maximum size in bytes.
The /proc/sys/net/ipv4/ directory contains additional networking settings. Many of these settings, used in conjunction with one another, are useful in preventing attacks on the system or
when using the system to act as a router.
Caution
An erroneous change to these files may affect remote connectivity to the system.
The following is a list of some of the more important files within the /proc/sys/net/ipv4/ directory:
•
icmp_destunreach_rate, icmp_echoreply_rate, icmp_paramprob_rate,
and
icmp_timeexeed_rate — Set the maximum ICMP send packet rate, in 1/100 of a second, to
hosts under certain conditions. A setting of 0 removes any delay and is not a good idea.
•
icmp_echo_ignore_all
•
ip_default_ttl
•
ip_forward
•
ip_local_port_range
•
tcp_syn_retries
•
tcp_retries1
and icmp_echo_ignore_broadcasts — Allows the kernel to ignore ICMP
ECHO packets from every host or only those originating from broadcast and multicast addresses, respectively. A value of 0 allows the kernel to respond, while a value of 1 ignores
the packets.
— Sets the default Time To Live (TTL), which limits the number of hops a
packet may make before reaching its destination. Increasing this value can diminish system
performance.
— Permits interfaces on the system to forward packets to one other. By default,
this file is set to 0. Setting this file to 1 enables network packet forwarding.
— Specifies the range of ports to be used by TCP or UDP when a local
port is needed. The first number is the lowest port to be used and the second number specifies the highest port. Any systems that expect to require more ports than the default 1024
to 4999 should use a range from 32768 to 61000.
— Provides a limit on the number of times the system re-transmits a SYN
packet when attempting to make a connection.
— Sets the number of permitted re-transmissions attempting to answer an in-
43
3.9. /proc/sys/
coming connection. Default of 3.
•
tcp_retries2
— Sets the number of permitted re-transmissions of TCP packets. Default of
15.
The file called
/usr/share/doc/kernel-doc-<version>/Documentation/networking/ ip-sysctl.txt
contains a complete list of files and options available in the /proc/sys/net/ipv4/ directory.
A number of other directories exist within the /proc/sys/net/ipv4/ directory and each covers a
different aspect of the network stack. The /proc/sys/net/ipv4/conf/ directory allows each system interface to be configured in different ways, including the use of default settings for unconfigured devices (in the /proc/sys/net/ipv4/conf/default/ subdirectory) and settings that override all special configurations (in the /proc/sys/net/ipv4/conf/all/ subdirectory).
The /proc/sys/net/ipv4/neigh/ directory contains settings for communicating with a host directly connected to the system (called a network neighbor) and also contains different settings
for systems more than one hop away.
Routing over IPV4 also has its own directory, /proc/sys/net/ipv4/route/. Unlike conf/ and
neigh/, the /proc/sys/net/ipv4/route/ directory contains specifications that apply to routing
with any interfaces on the system. Many of these settings, such as max_size, max_delay, and
min_delay, relate to controlling the size of the routing cache. To clear the routing cache, write
any value to the flush file.
Additional information about these directories and the possible values for their configuration files
can be found in:
/usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt
3.9.5. /proc/sys/vm/
This directory facilitates the configuration of the Linux kernel's virtual memory (VM) subsystem.
The kernel makes extensive and intelligent use of virtual memory, which is commonly referred
to as swap space.
The following files are commonly found in the /proc/sys/vm/ directory:
•
— Configures block I/O debugging when enabled. All read/write and block dirtying operations done to files are logged accordingly. This can be useful if diagnosing disk
spin up and spin downs for laptop battery conservation. All output when block_dump is enabled can be retrieved via dmesg. The default value is 0.
block_dump
Tip
If block_dump is enabled at the same time as kernel debugging, it is prudent to
stop the klogd daemon, as it generates erroneous disk activity caused by
44
3.9. /proc/sys/
block_dump.
•
dirty_background_ratio
— Starts background writeback of dirty data at this percentage of
total memory, via a pdflush daemon. The default value is 10.
•
dirty_expire_centisecs
•
dirty_ratio
•
dirty_writeback_centisecs
•
laptop_mode
— Defines when dirty in-memory data is old enough to be eligible
for writeout. Data which has been dirty in-memory for longer than this interval is written out
next time a pdflush daemon wakes up. The default value is 3000, expressed in hundredths of
a second.
— Starts active writeback of dirty data at this percentage of total memory for the
generator of dirty data, via pdflush. The default value is 40.
— Defines the interval between pdflush daemon wakeups, which
periodically writes dirty in-memory data out to disk. The default value is 500, expressed in
hundredths of a second.
— Minimizes the number of times that a hard disk needs to spin up by keeping
the disk spun down for as long as possible, therefore conserving battery power on laptops.
This increases efficiency by combining all future I/O processes together, reducing the frequency of spin ups. The default value is 0, but is automatically enabled in case a battery on
a laptop is used.
This value is controlled automatically by the acpid daemon once a user is notified battery
power is enabled. No user modifications or interactions are necessary if the laptop supports
the ACPI (Advanced Configuration and Power Interface) specification.
For more information, refer to the following installed documentation:
/usr/share/doc/kernel-doc-<version>/Documentation/laptop-mode.txt
•
— Determines how aggressive the kernel is in defending lower
memory allocation zones. This is effective when utilized with machines configured with highmem memory space enabled. The default value is 0, no protection at all. All other integer values are in megabytes, and lowmem memory is therefore protected from being allocated by
users.
lower_zone_protection
For more information, refer to the following installed documentation:
/usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt
•
max_map_count
— Configures the maximum number of memory map areas a process may
have. In most cases, the default value of 65536 is appropriate.
•
min_free_kbytes
•
nr_hugepages
— Forces the Linux VM (virtual memory manager) to keep a minimum number of kilobytes free. The VM uses this number to compute a pages_min value for each lowmem zone in the system. The default value is in respect to the total memory on the machine.
— Indicates the current number of configured hugetlb pages in the kernel.
45
3.10. /proc/sysvipc/
For more information, refer to the following installed documentation:
/usr/share/doc/kernel-doc-<version>/Documentation/vm/hugetlbpage.txt
•
nr_pdflush_threads
— Indicates the number of pdflush daemons that are currently running.
This file is read-only, and should not be changed by the user. Under heavy I/O loads, the default value of two is increased by the kernel.
•
overcommit_memory
— Configures the conditions under which a large memory request is accepted or denied. The following three modes are available:
•
0
— The kernel performs heuristic memory over commit handling by estimating the
amount of memory available and failing requests that are blatantly invalid. Unfortunately,
since memory is allocated using a heuristic rather than a precise algorithm, this setting
can sometimes allow available memory on the system to be overloaded. This is the default setting.
•
1
•
2
— The kernel performs no memory over commit handling. Under this setting, the potential for memory overload is increased, but so is performance for memory intensive tasks
(such as those executed by some scientific software).
— The kernel fails requests for memory that add up to all of swap plus the percent of
physical RAM specified in /proc/sys/vm/overcommit_ratio. This setting is best for those
who desire less risk of memory overcommitment.
Note
This setting is only recommended for systems with swap areas larger than
physical memory.
•
overcommit_ratio
— Specifies the percentage of physical RAM considered when /
proc/sys/vm/overcommit_memory is set to 2. The default value is 50.
•
page-cluster
•
swappiness
— Sets the number of pages read in a single attempt. The default value of 3,
which actually relates to 16 pages, is appropriate for most systems.
— Determines how much a machine should swap. The higher the value, the
more swapping occurs. The default value, as a percentage, is set to 60.
All kernel-based documentation can be found in the following locally installed location:
/usr/share/doc/kernel-doc-<version>/Documentation/,
which contains additional information.
3.10. /proc/sysvipc/
This directory contains information about System V IPC resources. The files in this directory relate to System V IPC calls for messages (msg), semaphores (sem), and shared memory (shm).
46
4. Using the sysctl Command
3.11. /proc/tty/
This directory contains information about the available and currently used tty devices on the
system. Originally called teletype devices, any character-based data terminals are called tty
devices.
In Linux, there are three different kinds of tty devices. Serial devices are used with serial connections, such as over a modem or using a serial cable. Virtual terminals create the common
console connection, such as the virtual consoles available when pressing Alt-<F-key> at the
system console. Pseudo terminals create a two-way communication that is used by some higher
level applications, such as XFree86. The drivers file is a list of the current tty devices in use, as
in the following example:
serial
serial
pty_slave
pty_master
pty_slave
pty_master
/dev/vc/0
/dev/ptmx
/dev/console
/dev/tty
unknown
/dev/cua
/dev/ttyS
/dev/pts
/dev/ptm
/dev/ttyp
/dev/pty
/dev/vc/0
/dev/ptmx
/dev/console
/dev/tty
/dev/vc/%d
5
4
136
128
3
2
4
5
5
5
4
64-127
64-127
0-255
0-255
0-255
0-255
0
2
1
0
1-63
serial:callout
serial
pty:slave
pty:master
pty:slave
pty:master
system:vtmaster
system
system:console
system:/dev/tty
console
The /proc/tty/driver/serial file lists the usage statistics and status of each of the serial tty
lines.
In order for tty devices to be used as network devices, the Linux kernel enforces line discipline
on the device. This allows the driver to place a specific type of header with every block of data
transmitted over the device, making it possible for the remote end of the connection to a block
of data as just one in a stream of data blocks. SLIP and PPP are common line disciplines, and
each are commonly used to connect systems to one other over a serial link.
Registered line disciplines are stored in the ldiscs file, and more detailed information is available within the ldisc/ directory.
4. Using the
sysctl
Command
The /sbin/sysctl command is used to view, set, and automate kernel settings in the /proc/sys/
directory.
For a quick overview of all settings configurable in the /proc/sys/ directory, type the /
sbin/sysctl -a command as root. This creates a large, comprehensive list, a small portion of
which looks something like the following:
net.ipv4.route.min_delay = 2 kernel.sysrq = 0 kernel.sem = 250
32000
32
128
This is the same information seen if each of the files were viewed individually. The only difference is the file location. For example, the /proc/sys/net/ipv4/route/min_delay file is listed as
net.ipv4.route.min_delay, with the directory slashes replaced by dots and the proc.sys portion
assumed.
47
5. Additional Resources
The sysctl command can be used in place of echo to assign values to writable files in the /
proc/sys/ directory. For example, instead of using the command
echo 1 > /proc/sys/kernel/sysrq
use the equivalent sysctl command as follows:
sysctl -w kernel.sysrq="1"
kernel.sysrq = 1
While quickly setting single values like this in /proc/sys/ is helpful during testing, this method
does not work as well on a production system as special settings within /proc/sys/ are lost
when the machine is rebooted. To preserve custom settings, add them to the /etc/sysctl.conf
file.
Each time the system boots, the init program runs the /etc/rc.d/rc.sysinit script. This script
contains a command to execute sysctl using /etc/sysctl.conf to determine the values passed
to the kernel. Any values added to /etc/sysctl.conf therefore take effect each time the system
boots.
5. Additional Resources
Below are additional sources of information about proc file system.
5.1. Installed Documentation
Some of the best documentation about the proc file system is installed on the system by default.
•
/usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt
— Contains as-
sorted, but limited, information about all aspects of the /proc/ directory.
•
/usr/share/doc/kernel-doc-<version>/Documentation/sysrq.txt
— An overview of System
Request Key options.
•
/usr/share/doc/kernel-doc-<version>/Documentation/sysctl/
— A directory containing a
variety of sysctl tips, including modifying values that concern the kernel (kernel.txt), accessing file systems (fs.txt), and virtual memory use (vm.txt).
•
/usr/share/doc/kernel-doc-<version>/Documentation/networking/ip-sysctl.txt
— A de-
tailed overview of IP networking options.
5.2. Useful Websites
•
http://www.linuxhq.com/ — This website maintains a complete database of source, patches,
and documentation for various versions of the Linux kernel.
48
Chapter 4. Redundant Array of
Independent Disks (RAID)
The basic idea behind RAID is to combine multiple small, inexpensive disk drives into an array
to accomplish performance or redundancy goals not attainable with one large and expensive
drive. This array of drives appears to the computer as a single logical storage unit or drive.
1. What is RAID?
RAID allows information to access several disks. RAID uses techniques such as disk striping
(RAID Level 0), disk mirroring (RAID Level 1), and disk striping with parity (RAID Level 5) to
achieve redundancy, lower latency, increased bandwidth, and maximized ability to recover from
hard disk crashes.
RAID consistently distributes data across each drive in the array. RAID then breaks down the
data into consistently-sized chunks (commonly 32K or 64k, although other values are acceptable). Each chunk is then written to a hard drive in the RAID array according to the RAID level
employed. When the data is read, the process is reversed, giving the illusion that the multiple
drives in the array are actually one large drive.
2. Who Should Use RAID?
System Administrators and others who manage large amounts of data would benefit from using
RAID technology. Primary reasons to deploy RAID include:
•
Enhances speed
•
Increases storage capacity using a single virtual disk
•
Minimizes disk failure
3. Hardware RAID versus Software RAID
There are two possible RAID approaches: Hardware RAID and Software RAID.
3.1. Hardware RAID
The hardware-based array manages the RAID subsystem independently from the host. It
presents a single disk per RAID array to the host.
A Hardware RAID device connects to the SCSI controller and presents the RAID arrays as a
single SCSI drive. An external RAID system moves all RAID handling "intelligence" into a controller located in the external disk subsystem. The whole subsystem is connected to the host via
a normal SCSI controller and appears to the host as a single disk.
RAID controller cards function like a SCSI controller to the operating system, and handle all the
49
3.2. Software RAID
actual drive communications. The user plugs the drives into the RAID controller (just like a normal SCSI controller) and then adds them to the RAID controllers configuration, and the operating system won't know the difference.
3.2. Software RAID
Software RAID implements the various RAID levels in the kernel disk (block device) code. It offers the cheapest possible solution, as expensive disk controller cards or hot-swap chassis 1 are
not required. Software RAID also works with cheaper IDE disks as well as SCSI disks. With
today's faster CPUs, Software RAID outperforms Hardware RAID.
The Linux kernel contains an MD driver that allows the RAID solution to be completely hardware
independent. The performance of a software-based array depends on the server CPU performance and load.
To learn more about Software RAID, here are the key features:
•
Threaded rebuild process
•
Kernel-based configuration
•
Portability of arrays between Linux machines without reconstruction
•
Backgrounded array reconstruction using idle system resources
•
Hot-swappable drive support
•
Automatic CPU detection to take advantage of certain CPU optimizations
4. RAID Levels and Linear Support
RAID supports various configurations, including levels 0, 1, 4, 5, and linear. These RAID types
are defined as follows:
•
Level 0 — RAID level 0, often called "striping," is a performance-oriented striped data mapping technique. This means the data being written to the array is broken down into strips and
written across the member disks of the array, allowing high I/O performance at low inherent
cost but provides no redundancy. The storage capacity of a level 0 array is equal to the total
capacity of the member disks in a Hardware RAID or the total capacity of member partitions
in a Software RAID.
•
Level 1 — RAID level 1, or "mirroring," has been used longer than any other form of RAID.
Level 1 provides redundancy by writing identical data to each member disk of the array,
leaving a "mirrored" copy on each disk. Mirroring remains popular due to its simplicity and
high level of data availability. Level 1 operates with two or more disks that may use parallel
access for high data-transfer rates when reading but more commonly operate independently
to provide high I/O transaction rates. Level 1 provides very good data reliability and improves
performance for read-intensive applications but at a relatively high cost. 2 The storage capacity of the level 1 array is equal to the capacity of one of the mirrored hard disks in a Hard-
1
A hot-swap chassis allows you to remove a hard drive without having to power-down your system.
50
5. Configuring Software RAID
ware RAID or one of the mirrored partitions in a Software RAID.
•
Level 4 — Level 4 uses parity 3 concentrated on a single disk drive to protect data. It is better suited to transaction I/O rather than large file transfers. Because the dedicated parity disk
represents an inherent bottleneck, level 4 is seldom used without accompanying technologies such as write-back caching. Although RAID level 4 is an option in some RAID partitioning schemes, it is not an option allowed in Red Hat Enterprise Linux RAID installations. 4
The storage capacity of Hardware RAID level 4 is equal to the capacity of member disks,
minus the capacity of one member disk. The storage capacity of Software RAID level 4 is
equal to the capacity of the member partitions, minus the size of one of the partitions if they
are of equal size.
•
Level 5 — This is the most common type of RAID. By distributing parity across some or all of
an array's member disk drives, RAID level 5 eliminates the write bottleneck inherent in level
4. The only performance bottleneck is the parity calculation process. With modern CPUs and
Software RAID, that usually is not a very big problem. As with level 4, the result is asymmetrical performance, with reads substantially outperforming writes. Level 5 is often used with
write-back caching to reduce the asymmetry. The storage capacity of Hardware RAID level 5
is equal to the capacity of member disks, minus the capacity of one member disk. The storage capacity of Software RAID level 5 is equal to the capacity of the member partitions,
minus the size of one of the partitions if they are of equal size.
•
Linear RAID — Linear RAID is a simple grouping of drives to create a larger virtual drive. In
linear RAID, the chunks are allocated sequentially from one member drive, going to the next
drive only when the first is completely filled. This grouping provides no performance benefit,
as it is unlikely that any I/O operations will be split between member drives. Linear RAID also
offers no redundancy and, in fact, decreases reliability — if any one member drive fails, the
entire array cannot be used. The capacity is the total of all member disks.
5. Configuring Software RAID
Users can configure Software RAID during the graphical installation process (Disk Druid), the
text-based installation process, or during a kickstart installation.This chapter covers Software
RAID configuration during the installation process using the Disk Druid application.
•
Apply software RAID partitions to the physical hard drives.
To add a boot partition (/boot/) to a RAID partition, ensure it is on a RAID1 partiton.
•
Creating RAID devices from the software RAID partitions.
•
Optional: Configuring LVM from the RAID devices.
•
Creating file systems from the RAID devices.
2
RAID level 1 comes at a high cost because you write the same information to all of the disks in the array, which
wastes drive space. For example, if you have RAID level 1 set up so that your root (/) partition exists on two 40G
drives, you have 80G total but are only able to access 40G of that 80G. The other 40G acts like a mirror of the first 40G.
3
Parity information is calculated based on the contents of the rest of the member disks in the array. This information
can then be used to reconstruct data when one disk in the array fails. The reconstructed data can then be used to satisfy I/O requests to the failed disk before it is replaced and to repopulate the failed disk after it has been replaced.
4
RAID level 4 takes up the same amount of space as RAID level 5, but level 5 has more advantages. For this reason,
level 4 is not supported.
51
5.1. Creating the RAID Partitions
Note
Although this procedure covers installating with a GUI application, system administrators can do the same with text-based installation.
Configuration of software RAID must be done manually in Disk Druid during the
installation process.
These examples use two 9.1 GB SCSI drives (/dev/sda and /dev/sdb) to illustrate the creation
of simple RAID1 configurations. They detail how to create a simple RAID 1 configuration by implementing multiple RAID devices.
On the Disk Partitioning Setup screen, select Manually partition with Disk Druid.
5.1. Creating the RAID Partitions
In a typical situation, the disk drives are new or are formatted. Both drives are shown as raw
devices with no partition configuration in Figure 4.1, “Two Blank Drives, Ready For Configuration”.
Figure 4.1. Two Blank Drives, Ready For Configuration
1.
In Disk Druid, choose RAID to enter the software RAID creation screen.
52
5.1. Creating the RAID Partitions
2.
Choose Create a software RAID partition to create a RAID partition as shown in Figure 4.2, “RAID Partition Options”. Note that no other RAID options (such as entering a
mount point) are available until RAID partitions, as well as RAID devices, are created.
Figure 4.2. RAID Partition Options
3.
A software RAID partition must be constrained to one drive. For Allowable Drives, select
the drive to use for RAID. If you have multiple drives, by default all drives are selected and
you must deselect the drives you do not want.
53
5.1. Creating the RAID Partitions
Figure 4.3. Adding a RAID Partition
4.
Enter the size that you want the partition to be.
5.
Select Fixed Size to specify partition size. Select Fill all space up to (MB) and enter a
value (in MB) to specify partition size range. Select Fill to maximum allowable size to allow maximum available space of the hard disk. Note that if you make more than one space
growable, they share the available free space on the disk.
6.
Select Force to be a primary partition if you want the partition to be a primary partition. A
primary partition is one of the first four partitions on the hard drive. If unselected, the partition is created as a logical partition. If other operating systems are already on the system,
unselecting this option should be considered. For more information on primary versus logical/extended partitions, refer to the appendix section of the Red Hat Enterprise Linux Installation Guide.
7.
Repeat these steps to create as many partitions as you need for your partitions.
Repeat these steps to create as many partitions as needed for your RAID setup. Notice that all
the partitions do not have to be RAID partitions. For example, you can configure only the /boot/
partition as a software RAID device, leaving the root partition (/), /home/, and swap as regular
file systems. Figure 4.4, “RAID 1 Partitions Ready, Pre-Device and Mount Point Creation”
shows successfully allocated space for the RAID 1 configuration (for /boot/), which is now
ready for RAID device and mount point creation:
54
5.2. Creating the RAID Devices and Mount Points
Figure 4.4. RAID 1 Partitions Ready, Pre-Device and Mount Point Creation
5.2. Creating the RAID Devices and Mount Points
Once you create all of your partitions as Software RAID partitions, you must create the RAID
device and mount point.
1.
Select the RAID button on the Disk Druid main partitioning screen (refer to Figure 4.5,
“RAID Options”).
2.
Figure 4.5, “RAID Options” appears. Select Create a RAID device.
55
5.2. Creating the RAID Devices and Mount Points
Figure 4.5. RAID Options
3.
Next, Figure 4.6, “Making a RAID Device and Assigning a Mount Point” appears, where you
can make a RAID device and assign a mount point.
56
5.2. Creating the RAID Devices and Mount Points
Figure 4.6. Making a RAID Device and Assigning a Mount Point
4.
Select a mount point.
5.
Choose the file system type for the partition. At this point you can either configure a dynamic LVM file system or a traditional static ext2/ext3 file system. For more information on configuring LVM on a RAID device, select physical volume (LVM). If LVM is not required, continue on with the following instructions.
6.
Select a device name such as md0 for the RAID device.
7.
Choose your RAID level. You can choose from RAID 0, RAID 1, and RAID 5.
Note
If you are making a RAID partition of /boot/, you must choose RAID level 1,
and it must use one of the first two drives (IDE first, SCSI second). If you are
not creating a seperate RAID partition of /boot/, and you are making a RAID
partition for the root file system (/), it must be RAID level 1 and must use one
of the first two drives (IDE first, SCSI second).
57
5.2. Creating the RAID Devices and Mount Points
Figure 4.7. The /boot/ Mount Error
8.
The RAID partitions created appear in the RAID Members list. Select which of these partitions should be used to create the RAID device.
9.
If configuring RAID 1 or RAID 5, specify the number of spare partitions. If a software RAID
partition fails, the spare is automatically used as a replacement. For each spare you want to
specify, you must create an additional software RAID partition (in addition to the partitions
for the RAID device). Select the partitions for the RAID device and the partition(s) for the
spare(s).
10. After clicking OK, the RAID device appears in the Drive Summary list.
11. Repeat this chapter's entire process for configuring additional partitions, devices, and
mount points, such as the root partition (/), /home/, or swap.
After completing the entire configuration, the figure as shown in Figure 4.8, “Final Sample RAID
Configuration” resembles the default configuration, except for the use of RAID.
58
5.2. Creating the RAID Devices and Mount Points
Figure 4.8. Final Sample RAID Configuration
The figure as shown in Figure 4.9, “Final Sample RAID With LVM Configuration” is an example
of a RAID and LVM configuration.
59
5.2. Creating the RAID Devices and Mount Points
Figure 4.9. Final Sample RAID With LVM Configuration
You can continue with your installation process. Refer to the Red Hat Enterprise Linux Installation Guide for further instructions.
60
Chapter 5. Swap Space
1. What is Swap Space?
Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system
needs more memory resources and the RAM is full, inactive pages in memory are moved to the
swap space. While swap space can help machines with a small amount of RAM, it should not
be considered a replacement for more RAM. Swap space is located on hard drives, which have
a slower access time than physical memory.
Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of
swap partitions and swap files.
Swap should equal 2x physical RAM for up to 2 GB of physical RAM, and then an additional 1x
physical RAM for any amount above 2 GB, but never less than 32 MB.
So, if:
M = Amount of RAM in GB, and S = Amount of swap in GB, then
If M < 2
S = M *2
Else
S = M + 2
Using this formula, a system with 2 GB of physical RAM would have 4 GB of swap, while one
with 3 GB of physical RAM would have 5 GB of swap. Creating a large swap space partition can
be especially helpful if you plan to upgrade your RAM at a later time.
For systems with really large amounts of RAM (more than 32 GB) you can likely get away with a
smaller swap partition (around 1x, or less, of physical RAM).
Important
File systems and LVM2 volumes assigned as swap space cannot be in use when
being modified. For example, no system processes can be assigned the swap
space, as well as no amount of swap should be allocated and used by the kernel.
Use the free and cat /proc/swaps commands to verify how much and where swap
is in use.
The best way to achieve swap space modifications is to boot your system in rescue mode, and then follow the instructions (for each scenario) in the remainder of
this chapter. Refer to the Red Hat Enterprise Linux Installation Guide for instructions on booting into rescue mode. When prompted to mount the file system, select
Skip.
2. Adding Swap Space
61
2.1. Extending Swap on an LVM2 Logical Volume
Sometimes it is necessary to add more swap space after installation. For example, you may upgrade the amount of RAM in your system from 128 MB to 256 MB, but there is only 256 MB of
swap space. It might be advantageous to increase the amount of swap space to 512 MB if you
perform memory-intense operations or run applications that require a large amount of memory.
You have three options: create a new swap partition, create a new swap file, or extend swap on
an existing LVM2 logical volume. It is recommended that you extend an existing logical volume.
2.1. Extending Swap on an LVM2 Logical Volume
To extend an LVM2 swap logical volume (assuming /dev/VolGroup00/LogVol01 is the volume
you want to extend):
1.
Disable swapping for the associated logical volume:
# swapoff -v /dev/VolGroup00/LogVol01
2.
Resize the LVM2 logical volume by 256 MB:
# lvm lvresize /dev/VolGroup00/LogVol01 -L +256M
3.
Format the new swap space:
# mkswap /dev/VolGroup00/LogVol01
4.
Enable the extended logical volume:
# swapon -va
5.
Test that the logical volume has been extended properly:
# cat /proc/swaps # free
2.2. Creating an LVM2 Logical Volume for Swap
To add a swap volume group (assuming /dev/VolGroup00/LogVol02 is the swap volume you
want to add):
1.
Create the LVM2 logical volume of size 256 MB:
# lvm lvcreate VolGroup00 -n LogVol02 -L 256M
2.
Format the new swap space:
# mkswap /dev/VolGroup00/LogVol02
3.
Add the following entry to the /etc/fstab file:
/dev/VolGroup00/LogVol02 swap swap defaults 0 0
4.
Enable the extended logical volume:
# swapon -va
62
2.3. Creating a Swap File
5.
Test that the logical volume has been extended properly:
# cat /proc/swaps # free
2.3. Creating a Swap File
To add a swap file:
1.
Determine the size of the new swap file in megabytes and multiply by 1024 to determine
the number of blocks. For example, the block size of a 64 MB swap file is 65536.
2.
At a shell prompt as root, type the following command with count being equal to the desired
block size:
dd if=/dev/zero of=/swapfile bs=1024 count=65536
3.
Setup the swap file with the command:
mkswap /swapfile
4.
To enable the swap file immediately but not automatically at boot time:
swapon /swapfile
5.
To enable it at boot time, edit /etc/fstab to include the following entry:
/swapfile swap swap defaults 0 0
The next time the system boots, it enables the new swap file.
6.
After adding the new swap file and enabling it, verify it is enabled by viewing the output of
the command cat /proc/swaps or free.
3. Removing Swap Space
Sometimes it can be prudent to reduce swap space after installation. For example, say you
downgraded the amount of RAM in your system from 1 GB to 512 MB, but there is 2 GB of
swap space still assigned. It might be advantageous to reduce the amount of swap space to 1
GB, since the larger 2 GB could be wasting disk space.
You have three options: remove an entire LVM2 logical volume used for swap, remove a swap
file, or reduce swap space on an existing LVM2 logical volume.
3.1. Reducing Swap on an LVM2 Logical Volume
To reduce an LVM2 swap logical volume (assuming /dev/VolGroup00/LogVol01 is the volume
you want to extend):
1.
Disable swapping for the associated logical volume:
# swapoff -v /dev/VolGroup00/LogVol01
63
3.2. Removing an LVM2 Logical Volume for Swap
2.
Reduce the LVM2 logical volume by 512 MB:
# lvm lvreduce /dev/VolGroup00/LogVol01 -L -512M
3.
Format the new swap space:
# mkswap /dev/VolGroup00/LogVol01
4.
Enable the extended logical volume:
# swapon -va
5.
Test that the logical volume has been reduced properly:
# cat /proc/swaps # free
3.2. Removing an LVM2 Logical Volume for Swap
The swap logical volume cannot be in use (no system locks or processes on the volume). The
easiest way to achieve this it to boot your system in rescue mode. Refer to for instructions on
booting into rescue mode. When prompted to mount the file system, select Skip.
To remove a swap volume group (assuming /dev/VolGroup00/LogVol02 is the swap volume you
want to remove):
1.
Disable swapping for the associated logical volume:
# swapoff -v /dev/VolGroup00/LogVol02
2.
Remove the LVM2 logical volume of size 512 MB:
# lvm lvremove /dev/VolGroup00/LogVol02
3.
Remove the following entry from the /etc/fstab file:
/dev/VolGroup00/LogVol02 swap swap defaults 0 0
4.
Test that the logical volume has been extended properly:
# cat /proc/swaps # free
3.3. Removing a Swap File
To remove a swap file:
1.
At a shell prompt as root, execute the following command to disable the swap file (where /
swapfile is the swap file):
# swapoff -v /swapfile
2.
Remove its entry from the /etc/fstab file.
64
4. Moving Swap Space
3.
Remove the actual file:
# rm /swapfile
4. Moving Swap Space
To move swap space from one location to another, follow the steps for removing swap space,
and then follow the steps for adding swap space.
65
Chapter 6. Managing Disk Storage
1. Standard Partitions using
parted
The utility parted allows users to:
•
View the existing partition table
•
Change the size of existing partitions
•
Add partitions from free space or additional hard drives
If you want to view the system's disk space usage or monitor the disk space usage, refer to
Section 3, “File Systems”.
By default, the parted package is included when installing Red Hat Enterprise Linux. To start
parted, log in as root and type the command parted /dev/sda at a shell prompt (where /dev/sda
is the device name for the drive you want to configure).
A device containing a partition must not be in use if said partition is to be removed or resized.
Similarly, when creating a new partition on a device, said device must not be in use.
For a device to not be in use, none of the partitions on the device can be mounted, and any
swap space on the device must not be enabled.
As well, the partition table should not be modified while it is in use because the kernel may not
properly recognize the changes. If the partition table does not match the actual state of the
mounted partitions, information could be written to the wrong partition, resulting in lost and overwritten data.
The easiest way to achieve this it to boot your system in rescue mode. When prompted to
mount the file system, select Skip.
Alternately, if the drive does not contain any partitions in use (system processes that use or lock
the file system from being unmounted), you can unmount them with the umount command and
turn off all the swap space on the hard drive with the swapoff command.
Table 6.1, “parted commands” contains a list of commonly used parted commands. The sections that follow explain some of these commands and arguments in more detail.
Command
Description
check minor-num
Perform a simple check of the file system
cp fromto
Copy file system from one partition to another;
from and to are the minor numbers of the partitions
help
Display list of available commands
mktable label
Create a disk label for the partition table
66
1.1. Viewing the Partition Table
Command
Description
mkfs minor-numfile-system-type
Create a file system of type file-system-type
mkpart part-typefs-typestart-mbend-mb
Make a partition without creating a new file
system
mkpartfs part-typefs-typestart-mbend-mb
Make a partition and create the specified file
system
move minor-numstart-mbend-mb
Move the partition
name minor-numname
Name the partition for Mac and PC98 disklabels only
print
Display the partition table
quit
Quit parted
rescuestart-mbend-mb
Rescue a lost partition from start-mb to endmb
resize minor-numstart-mbend-mb
Resize the partition from start-mb to end-mb
rm minor-num
Remove the partition
select device
Select a different device to configure
set minor-numflagstate
Set the flag on a partition; state is either on or
off
toggle [NUMBER [FLAG]
Toggle the state of FLAG on partition NUMBER
unit UNIT
Set the default unit to UNIT
Table 6.1. parted commands
1.1. Viewing the Partition Table
After starting parted, use the command print to view the partition table. A table similar to the
following appears:
Model: ATA ST3160812AS (scsi)
Disk /dev/sda: 160GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number
1
2
3
4
5
6
7
Start
32.3kB
107MB
105GB
107GB
107GB
133GB
133GB
End
107MB
105GB
107GB
160GB
133GB
133GB
160GB
Size
107MB
105GB
2147MB
52.9GB
26.2GB
107MB
26.6GB
Type
primary
primary
primary
extended
logical
logical
logical
File system
ext3
ext3
linux-swap
root
ext3
ext3
Flags
boot
lvm
67
1.2. Creating a Partition
The first line contains the disk type, manufacturer, model number and interface, and the second
line displays the disk label type. The remaining output below the fourth line shows the partition
table.
In the partition table, the Minor number is the partition number. For example, the partition with
minor number 1 corresponds to /dev/sda1. The Start and End values are in megabytes. Valid
Type are metadata, free, primary, extended, or logical. The Filesystem is the file system type,
which can be any of the following:
•
ext2
•
ext3
•
fat16
•
fat32
•
hfs
•
jfs
•
linux-swap
•
ntfs
•
reiserfs
•
hp-ufs
•
sun-ufs
•
xfs
If a Filesystem of a device shows no value, this means that its file system type is unknown.
The Flags column lists the flags set for the partition. Available flags are boot, root, swap, hidden, raid, lvm, or lba.
Tip
To select a different device without having to restart parted, use the select command followed by the device name (for example, /dev/sda). Doing so allows you to
view or configure the partition table of a device.
1.2. Creating a Partition
Warning
68
1.2. Creating a Partition
Do not attempt to create a partition on a device that is in use.
Before creating a partition, boot into rescue mode (or unmount any partitions on the device and
turn off any swap space on the device).
Start parted, where /dev/sda is the device on which to create the partition:
parted /dev/sda
View the current partition table to determine if there is enough free space:
print
If there is not enough free space, you can resize an existing partition. Refer to Section 1.4,
“Resizing a Partition” for details.
1.2.1. Making the Partition
From the partition table, determine the start and end points of the new partition and what partition type it should be. You can only have four primary partitions (with no extended partition) on a
device. If you need more than four partitions, you can have three primary partitions, one extended partition, and multiple logical partitions within the extended. For an overview of disk partitions, refer to the appendix An Introduction to Disk Partitions in the Red Hat Enterprise Linux Installation Guide.
For example, to create a primary partition with an ext3 file system from 1024 megabytes until
2048 megabytes on a hard drive type the following command:
mkpart primary ext3 1024 2048
Tip
If you use the mkpartfs command instead, the file system is created after the partition is created. However, parted does not support creating an ext3 file system.
Thus, if you wish to create an ext3 file system, use mkpart and create the file system with the mkfs command as described later.
The changes start taking place as soon as you press Enter, so review the command before executing to it.
After creating the partition, use the print command to confirm that it is in the partition table with
the correct partition type, file system type, and size. Also remember the minor number of the
new partition so that you can label it. You should also view the output of
cat /proc/partitions
to make sure the kernel recognizes the new partition.
69
1.3. Removing a Partition
1.2.2. Formating the Partition
The partition still does not have a file system. Create the file system:
/sbin/mkfs -t ext3 /dev/sda6
Warning
Formatting the partition permanently destroys any data that currently exists on the
partition.
1.2.3. Labeling the Partition
Next, give the partition a label. For example, if the new partition is /dev/sda6 and you want to label it /work:
e2label /dev/sda6 /work
By default, the installation program uses the mount point of the partition as the label to make
sure the label is unique. You can use any label you want.
1.2.4. Creating the Mount Point
As root, create the mount point:
mkdir /work
1.2.5. Add to /etc/fstab
As root, edit the /etc/fstab file to include the new partition. The new line should look similar to
the following:
LABEL=/work /work ext3 defaults 1 2
The first column should contain LABEL= followed by the label you gave the partition. The second
column should contain the mount point for the new partition, and the next column should be the
file system type (for example, ext3 or swap). If you need more information about the format,
read the man page with the command man fstab.
If the fourth column is the word defaults, the partition is mounted at boot time. To mount the
partition without rebooting, as root, type the command:
mount /work
1.3. Removing a Partition
70
1.4. Resizing a Partition
Warning
Do not attempt to remove a partition on a device that is in use.
Before removing a partition, boot into rescue mode (or unmount any partitions on the device
and turn off any swap space on the device).
Start parted, where /dev/sda is the device on which to remove the partition:
parted /dev/sda
View the current partition table to determine the minor number of the partition to remove:
print
Remove the partition with the command rm. For example, to remove the partition with minor
number 3:
rm 3
The changes start taking place as soon as you press Enter, so review the command before
committing to it.
After removing the partition, use the print command to confirm that it is removed from the partition table. You should also view the output of
cat /proc/partitions
to make sure the kernel knows the partition is removed.
The last step is to remove it from the /etc/fstab file. Find the line that declares the removed
partition, and remove it from the file.
1.4. Resizing a Partition
Warning
Do not attempt to resize a partition on a device that is in use.
Before resizing a partition, boot into rescue mode (or unmount any partitions on the device and
turn off any swap space on the device).
Start parted, where /dev/sda is the device on which to resize the partition:
parted /dev/sda
View the current partition table to determine the minor number of the partition to resize as well
as the start and end points for the partition:
71
2. LVM Partition Management
print
To resize the partition, use the resize command followed by the minor number for the partition,
the starting place in megabytes, and the end place in megabytes. For example:
resize 3 1024 2048
Warning
A partition cannot be made larger than the space available on the device
After resizing the partition, use the print command to confirm that the partition has been resized correctly, is the correct partition type, and is the correct file system type.
After rebooting the system into normal mode, use the command df to make sure the partition
was mounted and is recognized with the new size.
2. LVM Partition Management
The following commands can be found by issuing lvm
help
at a command prompt.
Command
Description
dumpconfig
Dump the active configuration
formats
List the available metadata formats
help
Display the help commands
lvchange
Change the attributes of logical volume(s)
lvcreate
Create a logical volume
lvdisplay
Display information about a logical volume
lvextend
Add space to a logical volume
lvmchange
Due to use of the device mapper, this command has been deprecated
lvmdiskscan
List devices that may be used as physical
volumes
lvmsadc
Collect activity data
lvmsar
Create activity report
lvreduce
Reduce the size of a logical volume
lvremove
Remove logical volume(s) from the system
lvrename
Rename a logical volume
lvresize
Resize a logical volume
72
2. LVM Partition Management
Command
Description
lvs
Display information about logical volumes
lvscan
List all logical volumes in all volume groups
pvchange
Change attributes of physical volume(s)
pvcreate
Initialize physical volume(s) for use by LVM
pvdata
Display the on-disk metadata for physical
volume(s)
pvdisplay
Display various attributes of physical
volume(s)
pvmove
Move extents from one physical volume to another
pvremove
Remove LVM label(s) from physical volume(s)
pvresize
Resize a physical volume in use by a volume
group
pvs
Display information about physical volumes
pvscan
List all physical volumes
segtypes
List available segment types
vgcfgbackup
Backup volume group configuration
vgcfgrestore
Restore volume group configuration
vgchange
Change volume group attributes
vgck
Check the consistency of a volume group
vgconvert
Change volume group metadata format
vgcreate
Create a volume group
vgdisplay
Display volume group information
vgexport
Unregister a volume group from the system
vgextend
Add physical volumes to a volume group
vgimport
Register exported volume group with system
vgmerge
Merge volume groups
vgmknodes
Create the special files for volume group
devices in /dev/
vgreduce
Remove a physical volume from a volume
group
vgremove
Remove a volume group
vgrename
Rename a volume group
73
2. LVM Partition Management
Command
Description
vgs
Display information about volume groups
vgscan
Search for all volume groups
vgsplit
Move physical volumes into a new volume
group
version
Display software and driver version information
Table 6.2. LVM commands
74
Chapter 7. Implementing Disk
Quotas
Disk space can be restricted by implementing disk quotas which alert a system administrator
before a user consumes too much disk space or a partition becomes full.
Disk quotas can be configured for individual users as well as user groups. This makes it possible to manage the space allocated for user-specific files (such as email) separately from the
space allocated to the projects a user works on (assuming the projects are given their own
groups).
In addition, quotas can be set not just to control the number of disk blocks consumed but to control the number of inodes (data structures that contain information about files in UNIX file systems). Because inodes are used to contain file-related information, this allows control over the
number of files that can be created.
The quota RPM must be installed to implement disk quotas.
Note
For more information on installing RPM packages, refer to Part II, “Package Management”.
1. Configuring Disk Quotas
To implement disk quotas, use the following steps:
1.
Enable quotas per file system by modifying the /etc/fstab file.
2.
Remount the file system(s).
3.
Create the quota database files and generate the disk usage table.
4.
Assign quota policies.
Each of these steps is discussed in detail in the following sections.
1.1. Enabling Quotas
As root, using a text editor, edit the /etc/fstab file. Add the usrquota and/or grpquota options to
the file systems that require quotas:
/dev/VolGroup00/LogVol00 / ext3 defaults 1 1 LABEL=/boot /boot ext3 defaults 1 2 none /dev/pts devpts gid=
In this example, the /home file system has both user and group quotas enabled.
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1.2. Remounting the File Systems
Note
The following examples assume that a separate /home partition was created during
the installation of Red Hat Enterprise Linux. The root (/) partition can be used for
setting quota policies in the /etc/fstab file.
1.2. Remounting the File Systems
After adding the usrquota and/or grpquota options, remount each file system whose fstab entry
has been modified. If the file system is not in use by any process, use one of the following methods:
•
Issue the umount command followed by the mount command to remount the file system.(See
the man page for both umount and mount for the specific syntax for mounting and unmounting
various filesystem types.)
•
Issue the mount -o remount <file-system> command (where <file-system> is the name of
the file system) to remount the file system. For example, to remount the /home file system,
the command to issue is mount -o remount /home.
If the file system is currently in use, the easiest method for remounting the file system is to reboot the system.
1.3. Creating the Quota Database Files
After each quota-enabled file system is remounted, the system is capable of working with disk
quotas. However, the file system itself is not yet ready to support quotas. The next step is to run
the quotacheck command.
The quotacheck command examines quota-enabled file systems and builds a table of the current
disk usage per file system. The table is then used to update the operating system's copy of disk
usage. In addition, the file system's disk quota files are updated.
To create the quota files (aquota.user and aquota.group) on the file system, use the -c option of
the quotacheck command. For example, if user and group quotas are enabled for the /home file
system, create the files in the /home directory:
quotacheck -cug /home
The -c option specifies that the quota files should be created for each file system with quotas
enabled, the -u option specifies to check for user quotas, and the -g option specifies to check
for group quotas.
If neither the -u or -g options are specified, only the user quota file is created. If only -g is specified, only the group quota file is created.
After the files are created, run the following command to generate the table of current disk usage per file system with quotas enabled:
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1.4. Assigning Quotas per User
quotacheck -avug
The options used are as follows:
•
a
— Check all quota-enabled, locally-mounted file systems
•
v
— Display verbose status information as the quota check proceeds
•
u
— Check user disk quota information
•
g
— Check group disk quota information
After quotacheck has finished running, the quota files corresponding to the enabled quotas (user
and/or group) are populated with data for each quota-enabled locally-mounted file system such
as /home.
1.4. Assigning Quotas per User
The last step is assigning the disk quotas with the edquota command.
To configure the quota for a user, as root in a shell prompt, execute the command:
edquota username
Perform this step for each user who needs a quota. For example, if a quota is enabled in /
etc/fstab for the /home partition (/dev/VolGroup00/LogVol02 in the example below) and the command edquota testuser is executed, the following is shown in the editor configured as the default for the system:
Disk quotas for user testuser (uid 501): Filesystem blocks soft hard inodes soft hard /dev/VolGroup00/LogV
Note
The text editor defined by the EDITOR environment variable is used by edquota. To
change the editor, set the EDITOR environment variable in your ~/.bash_profile file
to the full path of the editor of your choice.
The first column is the name of the file system that has a quota enabled for it. The second
column shows how many blocks the user is currently using. The next two columns are used to
set soft and hard block limits for the user on the file system. The inodes column shows how
many inodes the user is currently using. The last two columns are used to set the soft and hard
inode limits for the user on the file system.
The hard block limit is the absolute maximum amount of disk space that a user or group can
use. Once this limit is reached, no further disk space can be used.
The soft block limit defines the maximum amount of disk space that can be used. However, unlike the hard limit, the soft limit can be exceeded for a certain amount of time. That time is
known as the grace period. The grace period can be expressed in seconds, minutes, hours,
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1.5. Assigning Quotas per Group
days, weeks, or months.
If any of the values are set to 0, that limit is not set. In the text editor, change the desired limits.
For example:
Disk quotas for user testuser (uid 501): Filesystem blocks soft hard inodes soft hard /dev/VolGroup00/LogVo
To verify that the quota for the user has been set, use the command:
quota testuser
1.5. Assigning Quotas per Group
Quotas can also be assigned on a per-group basis. For example, to set a group quota for the
devel group (the group must exist prior to setting the group quota), use the command:
edquota -g devel
This command displays the existing quota for the group in the text editor:
Disk quotas for group devel (gid 505): Filesystem blocks soft hard inodes soft hard /dev/VolGroup00/LogVol
Modify the limits, then save the file.
To verify that the group quota has been set, use the command:
quota -g devel
1.6. Setting the Grace Period for Soft Limits
If soft limits are set for a given quota (whether inode or block and for either users or groups) the
grace period, or amount of time a soft limit can be exceeded, should be set with the command:
edquota -t
While other edquota commands operate on a particular user's or group's quota, the -t option operates on every filesystem with quotas enabled.
2. Managing Disk Quotas
If quotas are implemented, they need some maintenance — mostly in the form of watching to
see if the quotas are exceeded and making sure the quotas are accurate.
Of course, if users repeatedly exceed their quotas or consistently reach their soft limits, a system administrator has a few choices to make depending on what type of users they are and how
much disk space impacts their work. The administrator can either help the user determine how
to use less disk space or increase the user's disk quota.
2.1. Enabling and Disabling
It is possible to disable quotas without setting them to 0. To turn all user and group quotas off,
use the following command:
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2.2. Reporting on Disk Quotas
quotaoff -vaug
If neither the -u or -g options are specified, only the user quotas are disabled. If only -g is specified, only group quotas are disabled. The -v switch causes verbose status information to display as the command executes.
To enable quotas again, use the quotaon command with the same options.
For example, to enable user and group quotas for all file systems, use the following command:
quotaon -vaug
To enable quotas for a specific file system, such as /home, use the following command:
quotaon -vug /home
If neither the -u or -g options are specified, only the user quotas are enabled. If only -g is specified, only group quotas are enabled.
2.2. Reporting on Disk Quotas
Creating a disk usage report entails running the repquota utility. For example, the command
repquota /home produces this output:
*** Report for user quotas on device /dev/mapper/VolGroup00-LogVol02 Block grace time: 7days; Inode grace t
To view the disk usage report for all (option -a) quota-enabled file systems, use the command:
repquota -a
While the report is easy to read, a few points should be explained. The -- displayed after each
user is a quick way to determine whether the block or inode limits have been exceeded. If either
soft limit is exceeded, a + appears in place of the corresponding -; the first - represents the
block limit, and the second represents the inode limit.
The grace columns are normally blank. If a soft limit has been exceeded, the column contains a
time specification equal to the amount of time remaining on the grace period. If the grace period
has expired, none appears in its place.
2.3. Keeping Quotas Accurate
Whenever a file system is not unmounted cleanly (due to a system crash, for example), it is necessary to run quotacheck. However, quotacheck can be run on a regular basis, even if the system has not crashed. Running the following command periodically keeps the quotas more accurate (the options used have been described in Section 1.1, “Enabling Quotas”):
quotacheck -avug
The easiest way to run it periodically is to use cron. As root, either use the crontab -e command
to schedule a periodic quotacheck or place a script that runs quotacheck in any one of the following directories (using whichever interval best matches your needs):
79
3. Additional Resources
•
/etc/cron.hourly
•
/etc/cron.daily
•
/etc/cron.weekly
•
/etc/cron.monthly
The most accurate quota statistics can be obtained when the file system(s) analyzed are not in
active use. Thus, the cron task should be schedule during a time where the file system(s) are
used the least. If this time is various for different file systems with quotas, run quotacheck for
each file system at different times with multiple cron tasks.
Refer to Chapter 34, Automated Tasks for more information about configuring cron.
3. Additional Resources
For more information on disk quotas, refer to the following resources.
3.1. Installed Documentation
•
The quotacheck, edquota, repquota, quota, quotaon, and quotaoff man pages
3.2. Related Books
•
Red Hat Enterprise Linux Introduction to System Administration; Red Hat, Inc. — Available
at http://www.redhat.com/docs/ and on the Documentation CD, this manual contains background information on storage management (including disk quotas) for new Red Hat Enterprise Linux system administrators.
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Chapter 8. Access Control Lists
Files and directories have permission sets for the owner of the file, the group associated with
the file, and all other users for the system. However, these permission sets have limitations. For
example, different permissions cannot be configured for different users. Thus, Access Control
Lists (ACLs) were implemented.
The Red Hat Enterprise Linux 5 kernel provides ACL support for the ext3 file system and NFSexported file systems. ACLs are also recognized on ext3 file systems accessed via Samba.
Along with support in the kernel, the acl package is required to implement ACLs. It contains the
utilities used to add, modify, remove, and retrieve ACL information.
The cp and mv commands copy or move any ACLs associated with files and directories.
1. Mounting File Systems
Before using ACLs for a file or directory, the partition for the file or directory must be mounted
with ACL support. If it is a local ext3 file system, it can mounted with the following command:
mount -t ext3 -o acl <device-name><partition>
For example:
mount -t ext3 -o acl /dev/VolGroup00/LogVol02 /work
Alternatively, if the partition is listed in the /etc/fstab file, the entry for the partition can include
the acl option:
LABEL=/work /work ext3 acl 1 2
If an ext3 file system is accessed via Samba and ACLs have been enabled for it, the ACLs are
recognized because Samba has been compiled with the --with-acl-support option. No special
flags are required when accessing or mounting a Samba share.
1.1. NFS
By default, if the file system being exported by an NFS server supports ACLs and the NFS client
can read ACLs, ACLs are utilized by the client system.
To disable ACLs on NFS shares when configuring the server, include the no_acl option in the /
etc/exports file. To disable ACLs on an NFS share when mounting it on a client, mount it with
the no_acl option via the command line or the /etc/fstab file.
2. Setting Access ACLs
There are two types of ACLs: access ACLs and default ACLs. An access ACL is the access
control list for a specific file or directory. A default ACL can only be associated with a directory; if
a file within the directory does not have an access ACL, it uses the rules of the default ACL for
the directory. Default ACLs are optional.
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3. Setting Default ACLs
ACLs can be configured:
1.
Per user
2.
Per group
3.
Via the effective rights mask
4.
For users not in the user group for the file
The setfacl utility sets ACLs for files and directories. Use the -m option to add or modify the
ACL of a file or directory:
setfacl -m <rules><files>
Rules (<rules>) must be specified in the following formats. Multiple rules can be specified in the
same command if they are separated by commas.
u:<uid>:<perms>
Sets the access ACL for a user. The user name or UID may be specified. The user may be
any valid user on the system.
g:<gid>:<perms>
Sets the access ACL for a group. The group name or GID may be specified. The group may
be any valid group on the system.
m:<perms>
Sets the effective rights mask. The mask is the union of all permissions of the owning group
and all of the user and group entries.
o:<perms>
Sets the access ACL for users other than the ones in the group for the file.
White space is ignored. Permissions (<perms>) must be a combination of the characters r, w, and
x for read, write, and execute.
If a file or directory already has an ACL, and the setfacl command is used, the additional rules
are added to the existing ACL or the existing rule is modified.
For example, to give read and write permissions to user andrius:
setfacl -m u:andrius:rw /project/somefile
To remove all the permissions for a user, group, or others, use the -x option and do not specify
any permissions:
setfacl -x <rules><files>
For example, to remove all permissions from the user with UID 500:
setfacl -x u:500 /project/somefile
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4. Retrieving ACLs
3. Setting Default ACLs
To set a default ACL, add d: before the rule and specify a directory instead of a file name.
For example, to set the default ACL for the /share/ directory to read and execute for users not
in the user group (an access ACL for an individual file can override it):
setfacl -m d:o:rx /share
4. Retrieving ACLs
To determine the existing ACLs for a file or directory, use the getfacl command. In the example
below, the getfacl is used to determine the existing ACLs for a file.
getfacl home/john/picture.png
The above command returns the following output:
# file: home/john/picture.png # owner: john # group: john user::rw- group::r-- other::r--
If a directory with a default ACL is specified, the default ACL is also displayed as illustrated below.
[[email protected] /]$ getfacl home/sales/# file: home/sales/ # owner: john # group: john user::rw- user:barryg:r--
5. Archiving File Systems With ACLs
Warning
The tar and dump commands do not backup ACLs.
The star utility is similar to the tar utility in that it can be used to generate archives of files;
however, some of its options are different. Refer to Table 8.1, “Command Line Options for star”
for a listing of more commonly used options. For all available options, refer to the star man
page. The star package is required to use this utility.
Option
Description
-c
Creates an archive file.
-n
Do not extract the files; use in conjunction with -x to
show what extracting the files does.
-r
Replaces files in the archive. The files are written to the
end of the archive file, replacing any files with the same
path and file name.
-t
Displays the contents of the archive file.
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6. Compatibility with Older Systems
Option
Description
-u
Updates the archive file. The files are written to the end
of the archive if they do not exist in the archive or if the
files are newer than the files of the same name in the
archive. This option only work if the archive is a file or
an unblocked tape that may backspace.
-x
Extracts the files from the archive. If used with -U and a
file in the archive is older than the corresponding file on
the file system, the file is not extracted.
-help
Displays the most important options.
-xhelp
Displays the least important options.
-/
Do not strip leading slashes from file names when extracting the files from an archive. By default, they are
striped when files are extracted.
-acl
When creating or extracting, archive or restore any
ACLs associated with the files and directories.
Table 8.1. Command Line Options for star
6. Compatibility with Older Systems
If an ACL has been set on any file on a given file system, that file system has the ext_attr attribute. This attribute can be seen using the following command:
tune2fs -l <filesystem-device>
A file system that has acquired the ext_attr attribute can be mounted with older kernels, but
those kernels do not enforce any ACLs which have been set.
Versions of the e2fsck utility included in version 1.22 and higher of the e2fsprogs package
(including the versions in Red Hat Enterprise Linux 2.1 and 4) can check a file system with the
ext_attr attribute. Older versions refuse to check it.
7. Additional Resources
Refer to the follow resources for more information.
7.1. Installed Documentation
•
acl
man page — Description of ACLs
•
getfacl
man page — Discusses how to get file access control lists
•
setfacl
man page — Explains how to set file access control lists
84
7.2. Useful Websites
•
star
man page — Explains more about the star utility and its many options
7.2. Useful Websites
•
http://acl.bestbits.at/ — Website for ACLs
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Chapter 9. LVM (Logical Volume
Manager)
1. What is LVM?
LVM is a tool for logical volume management which includes allocating disks, striping, mirroring
and resizing logical volumes.
With LVM, a hard drive or set of hard drives is allocated to one or more physical volumes. LVM
physical volumes can be placed on other block devices which might span two or more disks.
The physical volumes are combined into logical volumes, with the exception of the /boot/ partition. The /boot/ partition cannot be on a logical volume group because the boot loader cannot
read it. If the root (/) partition is on a logical volume, create a separate /boot/ partition which is
not a part of a volume group.
Since a physical volume cannot span over multiple drives, to span over more than one drive,
create one or more physical volumes per drive.
Figure 9.1. Logical Volumes
The volume groups can be divided into logical volumes, which are assigned mount points, such
as /home and / and file system types, such as ext2 or ext3. When "partitions" reach their full capacity, free space from the volume group can be added to the logical volume to increase the
size of the partition. When a new hard drive is added to the system, it can be added to the
volume group, and partitions that are logical volumes can be increased in size.
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1.1. What is LVM2?
Figure 9.2. Logical Volumes
On the other hand, if a system is partitioned with the ext3 file system, the hard drive is divided
into partitions of defined sizes. If a partition becomes full, it is not easy to expand the size of the
partition. Even if the partition is moved to another hard drive, the original hard drive space has
to be reallocated as a different partition or not used.
To learn how to configure LVM during the installation process, refer to Section 2, “LVM Configuration”.
1.1. What is LVM2?
LVM version 2, or LVM2, is the default for Red Hat Enterprise Linux 5, which uses the device
mapper driver contained in the 2.6 kernel. LVM2 can be upgraded from versions of Red Hat Enterprise Linux running the 2.4 kernel.
2. LVM Configuration
LVM can be configured during the graphical installation process, the text-based installation process, or during a kickstart installation. You can use the system-config-lvm utility to create your
own LVM configuration post-installation. The next two sections focus on using Disk Druid during installation to complete this task. The third section introduces the LVM utility (system-config-lvm) which allows you to manage your LVM volumes in X windows or graphically.
Read Section 1, “What is LVM?” first to learn about LVM. An overview of the steps required to
configure LVM include:
•
Creating physical volumes from the hard drives.
•
Creating volume groups from the physical volumes.
•
Creating logical volumes from the volume groups and assign the logical volumes mount
points.
87
3. Automatic Partitioning
Two 9.1 GB SCSI drives (/dev/sda and /dev/sdb) are used in the following examples. They detail how to create a simple configuration using a single LVM volume group with associated logical volumes during installation.
3. Automatic Partitioning
On the Disk Partitioning Setup screen, select Automatically partition.
For Red Hat Enterprise Linux, LVM is the default method for disk partitioning. If you do not wish
to have LVM implemented, or if you require RAID partitioning, manual disk partitioning through
Disk Druid is required.
The following properties make up the automatically created configuration:
•
The /boot/ partition resides on its own non-LVM partition. In the following example, it is the
first partition on the first drive (/dev/sda1). Bootable partitions cannot reside on LVM logical
volumes.
•
A single LVM volume group (VolGroup00) is created, which spans all selected drives and all
remaining space available. In the following example, the remainder of the first drive (/
dev/sda2), and the entire second drive (/dev/sdb1) are allocated to the volume group.
•
Two LVM logical volumes (LogVol00 and LogVol01) are created from the newly created
spanned volume group. In the following example, the recommended swap space is automatically calculated and assigned to LogVol01, and the remainder is allocated to the root file system, LogVol00.
88
4. Manual LVM Partitioning
Figure 9.3. Automatic LVM Configuration With Two SCSI Drives
Note
If enabling quotas are of interest to you, it may be best to modify the automatic
configuration to include other mount points, such as /home/ or /var/, so that each
file system has its own independent quota configuration limits.
In most cases, the default automatic LVM partitioning is sufficient, but advanced
implementations could warrant modification or manual configuration of the partition
tables.
Note
If you anticipate future memory upgrades, leaving some free space in the volume
group would allow for easy future expansion of the swap space logical volume on
the system; in which case, the automatic LVM configuration should be modified to
leave available space for future growth.
4. Manual LVM Partitioning
The following section explains how to manually configure LVM for Red Hat Enterprise Linux. Because there are numerous ways to manually configure a system with LVM, the following example is similar to the default configuration done in Section 3, “Automatic Partitioning”.
On the Disk Partitioning Setup screen, select Manually partition with Disk Druid.
4.1. Creating the /boot/ Partition
In a typical situation, the disk drives are new, or formatted clean. The following figure, Figure 9.4, “Two Blank Drives, Ready For Configuration”, shows both drives as raw devices with no
partitioning configured.
89
4.1. Creating the /boot/ Partition
Figure 9.4. Two Blank Drives, Ready For Configuration
Warning
The /boot/ partition cannot reside on an LVM volume because the GRUB boot
loader cannot read it.
1.
Select New.
2.
Select /boot from the Mount Point pulldown menu.
3.
Select ext3 from the File System Type pulldown menu.
4.
Select only the sda checkbox from the Allowable Drives area.
5.
Leave 100 (the default) in the Size (MB) menu.
6.
Leave the Fixed size (the default) radio button selected in the Additional Size Options
area.
7.
Select Force to be a primary partition to make the partition be a primary partition. A
primary partition is one of the first four partitions on the hard drive. If unselected, the partition is created as a logical partition. If other operating systems are already on the system,
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4.1. Creating the /boot/ Partition
unselecting this option should be considered. For more information on primary versus logical/extended partitions, refer to the appendix section of the Red Hat Enterprise Linux Installation Guide.
Refer to Figure 9.5, “Creation of the Boot Partition” to verify your inputted values:
Figure 9.5. Creation of the Boot Partition
Click OK to return to the main screen. The following figure displays the boot partition correctly
set:
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4.2. Creating the LVM Physical Volumes
Figure 9.6. The /boot/ Partition Displayed
4.2. Creating the LVM Physical Volumes
Once the boot partition is created, the remainder of all disk space can be allocated to LVM partitions. The first step in creating a successful LVM implementation is the creation of the physical
volume(s).
1.
Select New.
2.
Select physical volume (LVM) from the File System Type pulldown menu as shown in
Figure 9.7, “Creating a Physical Volume”.
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4.2. Creating the LVM Physical Volumes
Figure 9.7. Creating a Physical Volume
3.
You cannot enter a mount point yet (you can once you have created all your physical
volumes and then all volume groups).
4.
A physical volume must be constrained to one drive. For Allowable Drives, select the drive
on which the physical volume are created. If you have multiple drives, all drives are selected, and you must deselect all but one drive.
5.
Enter the size that you want the physical volume to be.
6.
Select Fixed size to make the physical volume the specified size, select Fill all space up
to (MB) and enter a size in MBs to give range for the physical volume size, or select Fill to
maximum allowable size to make it grow to fill all available space on the hard disk. If you
make more than one growable, they share the available free space on the disk.
7.
Select Force to be a primary partition if you want the partition to be a primary partition.
8.
Click OK to return to the main screen.
Repeat these steps to create as many physical volumes as needed for your LVM setup. For example, if you want the volume group to span over more than one drive, create a physical
volume on each of the drives. The following figure shows both drives completed after the repeated process:
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4.3. Creating the LVM Volume Groups
Figure 9.8. Two Physical Volumes Created
4.3. Creating the LVM Volume Groups
Once all the physical volumes are created, the volume groups can be created:
1.
Click the LVM button to collect the physical volumes into volume groups. A volume group is
basically a collection of physical volumes. You can have multiple logical volumes, but a
physical volume can only be in one volume group.
Note
There is overhead disk space reserved in the volume group. The volume
group size is slightly less than the total of physical volume sizes.
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4.4. Creating the LVM Logical Volumes
Figure 9.9. Creating an LVM Volume Group
2.
Change the Volume Group Name if desired.
3.
All logical volumes inside the volume group must be allocated in physical extent (PE) units.
A physical extent is an allocation unit for data.
4.
Select which physical volumes to use for the volume group.
4.4. Creating the LVM Logical Volumes
Create logical volumes with mount points such as /, /home/, and swap space. Remember that /
boot cannot be a logical volume. To add a logical volume, click the Add button in the Logical
Volumes section. A dialog window as shown in Figure 9.10, “Creating a Logical Volume” appears.
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4.4. Creating the LVM Logical Volumes
Figure 9.10. Creating a Logical Volume
Repeat these steps for each volume group you want to create.
Tip
You may want to leave some free space in the volume group so you can expand
the logical volumes later. The default automatic configuration does not do this, but
this manual configuration example does — approximately 1 GB is left as free
space for future expansion.
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4.4. Creating the LVM Logical Volumes
Figure 9.11. Pending Logical Volumes
Click OK to apply the volume group and all associated logical volumes.
The following figure shows the final manual configuration:
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5. Using the LVM utility system-config-lvm
Figure 9.12. Final Manual Configuration
5. Using the LVM utility
system-config-lvm
The LVM utility allows you to manage logical volumes within X windows or graphically. You can
access the application by selecting from your menu panel System => Administration => Logical Volume Management. Alternatively you can start the Logical Volume Management utility
by typing system-config-lvm from a terminal.
In the example used in this section, the following are the details for the volume group that was
created during the installation:
/boot - (Ext3) file system. Displayed under 'Uninitialized Entities'. (DO NOT initialize this partition).
LogVol00 - (LVM) contains the (/) directory (312 extents).
LogVol02 - (LVM) contains the (/home) directory (128 extents).
LogVol03 - (LVM) swap (28 extents).
The logical volumes above were created in disk entity /dev/hda2 while /boot was created in /
dev/hda1. The system also consists of 'Uninitialised Entities' which are illustrated in Figure 9.17,
“Uninitialized Entities”. The figure below illustrates the main window in the LVM utility. The logical and the physical views of the above configuration are illustrated below. The three logical
volumes exist on the same physical volume (hda2).
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5. Using the LVM utility system-config-lvm
Figure 9.13. Main LVM Window
The figure below illustrates the physical view for the volume. In this window, you can select and
remove a volume from the volume group or migrate extents from the volume to another volume
group. Steps to migrate extents are discussed in Figure 9.22, “Migrate Extents”.
Figure 9.14. Physical View Window
The figure below illustrates the logical view for the selected volume group. The logical volume
size is also indicated with the individual logical volume sizes illustrated.
99
5. Using the LVM utility system-config-lvm
Figure 9.15. Logical View Window
On the left side column, you can select the individual logical volumes in the volume group to
view more details about each. In this example the objective is to rename the logical volume
name for 'LogVol03' to 'Swap'. To perform this operation select the respective logical volume
and click on the Edit Properties button. This will display the Edit Logical Volume window from
which you can modify the Logical volume name, size (in extents) and also use the remaining
space available in a logical volume group. The figure below illustrates this.
Please note that this logical volume cannot be changed in size as there is currently no free
space in the volume group. If there was remaining space, this option would be enabled (see
Figure 9.31, “Edit logical volume”). Click on the OK button to save your changes (this will remount the volume). To cancel your changes click on the Cancel button. To revert to the last
snapshot settings click on the Revert button. A snapshot can be created by clicking on the Create Snapshot button on the LVM utility window. If the selected logical volume is in use by the
system (for example) the / (root) directory, this task will not be successful as the volume cannot
be unmounted.
100
5.1. Utilizing uninitialized entities
Figure 9.16. Edit Logical Volume
5.1. Utilizing uninitialized entities
'Uninitialized Entities' consist of unpartitioned space and non LVM file systems. In this example
partitions 3, 4, 5, 6 and 7 were created during installation and some unpartitioned space was left
on the hard disk. Please view each partition and ensure that you read the 'Properties for Disk
Entity' on the right column of the window to ensure that you do not delete critical data. In this example partition 1 cannot be initialized as it is /boot. Uninitialized entities are illustrated below.
Figure 9.17. Uninitialized Entities
In this example, partition 3 will be initialized and added to an existing volume group. To initialize
a partition or unpartioned space, select the partition and click on the Initialize Entity button.
101
5.2. Adding Unallocated Volumes to a volume group
Once initialized, a volume will be listed in the 'Unallocated Volumes' list.
5.2. Adding Unallocated Volumes to a volume group
Once initialized, a volume will be listed in the 'Unallocated Volumes' list. The figure below illustrates an unallocated partition (Partition 3). The respective buttons at the bottom of the window
allow you to:
•
create a new volume group,
•
add the unallocated volume to an existing volume group,
•
remove the volume from LVM.
To add the volume to an existing volume group, click on the Add to Existing Volume Group
button.
Figure 9.18. Unallocated Volumes
Clicking on the Add to Existing Volume Group button will display a pop up window listing the
existing volume groups to which you can add the physical volume you are about to initialize. A
volume group may span across one or more hard disks. In this example only one volume group
exists as illustrated below.
102
5.2. Adding Unallocated Volumes to a volume group
Figure 9.19. Add physical volume to volume group
Once added to an existing volume group the new logical volume is automatically added to the
unused space of the selected volume group. You can use the unused space to:
•
create a new logical volume (click on the Create New Logical Volume(s) button,
•
select one of the existing logical volumes and increase the extents (see Section 5.6,
“Extending a volume group”),
•
select an existing logical volume and remove it from the volume group by clicking on the Remove Selected Logical Volume(s) button. Please note that you cannot select unused
space to perform this operation.
The figure below illustrates the logical view of 'VolGroup00' after adding the new volume group.
103
5.3. Migrating extents
Figure 9.20. Logical view of volume group
In the figure below, the uninitialized entities (partitions 3, 5, 6 and 7) were added to 'VolGroup00'.
Figure 9.21. Logical view of volume group
5.3. Migrating extents
To migrate extents from a physical volume, select the volume and click on the Migrate Selected Extent(s) From Volume button. Please note that you need to have a sufficient number of
free extents to migrate extents within a volume group. An error message will be displayed if you
do not have a sufficient number of free extents. To resolve this problem, please extend your
volume group (see Section 5.6, “Extending a volume group”). If a sufficient number of free extents is detected in the volume group, a pop up window will be displayed from which you can select the destination for the extents or automatically let LVM choose the physical volumes (PVs)
to migrate them to. This is illustrated below.
104
5.3. Migrating extents
Figure 9.22. Migrate Extents
The figure below illustrates a migration of extents in progress. In this example, the extents were
migrated to 'Partition 3'.
Figure 9.23. Migrating extents in progress
Once the extents have been migrated, unused space is left on the physical volume. The figure
105
5.4. Adding a new hard disk using LVM
below illustrates the physical and logical view for the volume group. Please note that the extents
of LogVol00 which were initially in hda2 are now in hda3. Migrating extents allows you to move
logical volumes in case of hard disk upgrades or to manage your disk space better.
Figure 9.24. Logical and physical view of volume group
5.4. Adding a new hard disk using LVM
In this example, a new IDE hard disk was added. The figure below illustrates the details for the
new hard disk. From the figure below, the disk is uninitialized and not mounted. To initialize a
partition, click on the Initialize Entity button. For more details, see Section 5.1, “Utilizing uninitialized entities”. Once initialized, LVM will add the new volume to the list of unallocated volumes
as illustrated in Figure 9.26, “Create new volume group”.
106
5.5. Adding a new volume group
Figure 9.25. Uninitialized hard disk
5.5. Adding a new volume group
Once initialized, LVM will add the new volume to the list of unallocated volumes where you can
add it to an existing volume group or create a new volume group. You can also remove the
volume from LVM. The volume if removed from LVM will be listed in the list of 'Uninitialized Entities' as illustrated in Figure 9.25, “Uninitialized hard disk”. In this example, a new volume group
was created as illustrated below.
Figure 9.26. Create new volume group
107
5.5. Adding a new volume group
Once created a new volume group will be displayed in the list of existing volume groups as illustrated below. The logical view will display the new volume group with unused space as no logical volumes have been created. To create a logical volume, select the volume group and click on
the Create New Logical Volume button as illustrated below. Please select the extents you wish
to use on the volume group. In this example, all the extents in the volume group were used to
create the new logical volume.
Figure 9.27. Create new logical volume
The figure below illustrates the physical view of the new volume group. The new logical volume
named 'Backups' in this volume group is also listed.
108
5.6. Extending a volume group
Figure 9.28. Physical view of new volume group
5.6. Extending a volume group
In this example, the objective was to extend the new volume group to include an uninitialized
entity (partition). This was to increase the size or number of extents for the volume group. To
extend the volume group, click on the Extend Volume Group button. This will display the 'Extend Volume Group' window as illustrated below. On the 'Extend Volume Group' window, you
can select disk entities (partitions) to add to the volume group. Please ensure that you check
the contents of any 'Uninitialized Disk Entities' (partitions) to avoid deleting any critical data (see
Figure 9.25, “Uninitialized hard disk”). In the example, the disk entity (partition) /dev/hda6 was
selected as illustrated below.
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5.7. Editing a Logical Volume
Figure 9.29. Select disk entities
Once added, the new volume will be added as 'Unused Space' in the volume group. The figure
below illustrates the logical and physical view of the volume group after it was extended.
Figure 9.30. Logical and physical view of an extended volume group
5.7. Editing a Logical Volume
The LVM utility allows you to select a logical volume in the volume group and modify its name,
size and specify filesystem options. In this example, the logical volume named 'Backups" was
extended onto the remaining space for the volume group.
Clicking on the Edit Properties button will display the 'Edit Logical Volume' popup window from
which you can edit the properties of the logical volume. On this window, you can also mount the
volume after making the changes and mount it when the system is rebooted. Please note that
you should indicate the mount point. If the mount point you specify does not exist, a popup window will be displayed prompting you to create it. The 'Edit Logical Volume' window is illustrated
below.
110
5.7. Editing a Logical Volume
Figure 9.31. Edit logical volume
If you wish to mount the volume, select the 'Mount' checkbox indicating the preferred mount
point. To mount the volume when the system is rebooted, select the 'Mount when rebooted'
checkbox. In this example, the new volume will be mounted in /mnt/backups. This is illustrated in
the figure below.
111
5.7. Editing a Logical Volume
Figure 9.32. Edit logical volume - specifying mount options
The figure below illustrates the logical and physical view of the volume group after the logical
volume was extended to the unused space. Please note in this example that the logical volume
named 'Backups' spans across two hard disks. A volume can be stripped across two or more
physical devices using LVM.
112
6. Additional Resources
Figure 9.33. Edit logical volume
6. Additional Resources
Use these sources to learn more about LVM.
6.1. Installed Documentation
•
rpm -qd lvm
— This command shows all the documentation available from the lvm package,
including man pages.
•
lvm help
— This command shows all LVM commands available.
6.2. Useful Websites
•
http://sources.redhat.com/lvm2 — LVM2 webpage, which contains an overview, link to the
mailing lists, and more.
•
http://tldp.org/HOWTO/LVM-HOWTO/ — LVM HOWTO from the Linux Documentation
Project.
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Part II. Package Management
All software on a Red Hat Enterprise Linux system is divided into RPM packages which can be
installed, upgraded, or removed. This part describes how to manage the RPM packages on a
Red Hat Enterprise Linux system using graphical and command line tools.
Chapter 10. Package Management
with RPM
The RPM Package Manager (RPM) is an open packaging system, which runs on Red Hat Enterprise Linux as well as other Linux and UNIX systems. Red Hat, Inc. encourages other
vendors to use RPM for their own products. RPM is distributed under the terms of the GPL.
The utility works only with packages built for processing by the rpm package. For the end user,
RPM makes system updates easy. Installing, uninstalling, and upgrading RPM packages can be
accomplished with short commands. RPM maintains a database of installed packages and their
files, so you can invoke powerful queries and verifications on your system. If you prefer a graphical interface, you can use the Package Management Tool to perform many RPM commands.
Refer to Chapter 11, Package Management Tool for details.
Important
When installing a package, please ensure it is compatible with your operating system and architecture. This can usually be determined by checking the package
name.
During upgrades, RPM handles configuration files carefully, so that you never lose your customizations — something that you cannot accomplish with regular .tar.gz files.
For the developer, RPM allows you to take software source code and package it into source
and binary packages for end users. This process is quite simple and is driven from a single file
and optional patches that you create. This clear delineation between pristine sources and your
patches along with build instructions eases the maintenance of the package as new versions of
the software are released.
Note
Because RPM makes changes to your system, you must be logged in as root to install, remove, or upgrade an RPM package.
1. RPM Design Goals
To understand how to use RPM, it can be helpful to understand the design goals of RPM:
Upgradability
With RPM, you can upgrade individual components of your system without completely reinstalling. When you get a new release of an operating system based on RPM (such as Red
Hat Enterprise Linux), you do not need to reinstall on your machine (as you do with operat115
2. Using RPM
ing systems based on other packaging systems). RPM allows intelligent, fully-automated,
in-place upgrades of your system. Configuration files in packages are preserved across upgrades, so you do not lose your customizations. There are no special upgrade files needed
to upgrade a package because the same RPM file is used to install and upgrade the package on your system.
Powerful Querying
RPM is designed to provide powerful querying options. You can do searches through your
entire database for packages or just for certain files. You can also easily find out what package a file belongs to and from where the package came. The files an RPM package contains are in a compressed archive, with a custom binary header containing useful information about the package and its contents, allowing you to query individual packages quickly
and easily.
System Verification
Another powerful RPM feature is the ability to verify packages. If you are worried that you
deleted an important file for some package, you can verify the package. You are then notified of any anomalies, if any — at which point, you can reinstall the package if necessary.
Any configuration files that you modified are preserved during reinstallation.
Pristine Sources
A crucial design goal was to allow the use of pristine software sources, as distributed by the
original authors of the software. With RPM, you have the pristine sources along with any
patches that were used, plus complete build instructions. This is an important advantage for
several reasons. For instance, if a new version of a program is released, you do not necessarily have to start from scratch to get it to compile. You can look at the patch to see what
you might need to do. All the compiled-in defaults, and all of the changes that were made to
get the software to build properly, are easily visible using this technique.
The goal of keeping sources pristine may seem important only for developers, but it results
in higher quality software for end users, too.
2. Using RPM
RPM has five basic modes of operation (not counting package building): installing, uninstalling,
upgrading, querying, and verifying. This section contains an overview of each mode. For complete details and options, try rpm --help or man rpm. You can also refer to Section 5, “Additional
Resources” for more information on RPM.
2.1. Finding RPM Packages
Before using any RPM packages, you must know where to find them. An Internet search returns
many RPM repositories, but if you are looking for RPM packages built by Red Hat, they can be
found at the following locations:
•
The Red Hat Enterprise Linux CD-ROMs
•
The Red Hat Errata Page available at http://www.redhat.com/apps/support/errata/
•
Red Hat Network — Refer to Chapter 12, Red Hat Network for more details on Red Hat Network.
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2.2. Installing
2.2. Installing
RPM packages typically have file names like foo-1.0-1.i386.rpm. The file name includes the
package name (foo), version (1.0), release (1), and architecture (i386). To install a package, log
in as root and type the following command at a shell prompt:
rpm -Uvh foo-1.0-1.i386.rpm
If the installation is successful, the following output is displayed:
Preparing... ########################################### [100%] 1:foo ####################################
As you can see, RPM prints out the name of the package and then prints a succession of hash
marks as a progress meter while the package is installed.
The signature of a package is checked automatically when installing or upgrading a package.
The signature confirms that the package was signed by an authorized party. For example, if the
verification of the signature fails, an error message such as the following is displayed:
error: V3 DSA signature: BAD, key ID 0352860f
If it is a new, header-only, signature, an error message such as the following is displayed:
error: Header V3 DSA signature: BAD, key ID 0352860f
If you do not have the appropriate key installed to verify the signature, the message contains
the word NOKEY such as:
warning: V3 DSA signature: NOKEY, key ID 0352860f
Refer to Section 3, “Checking a Package's Signature” for more information on checking a package's signature.
Warning
If you are installing a kernel package, you should use rpm
Chapter 39, Manually Upgrading the Kernel for details.
-ivh
instead. Refer to
2.2.1. Package Already Installed
If a package of the same name and version is already installed, the following output is displayed:
Preparing... ########################################### [100%] package foo-1.0-1 is already installed
However, if you want to install the package anyway, you can use the --replacepkgs option,
which tells RPM to ignore the error:
rpm -ivh --replacepkgs foo-1.0-1.i386.rpm
This option is helpful if files installed from the RPM were deleted or if you want the original con117
2.3. Uninstalling
figuration files from the RPM to be installed.
2.2.2. Conflicting Files
If you attempt to install a package that contains a file which has already been installed by another package, the following is displayed:
Preparing... ########################################### [100%] file /usr/bin/foo from install of foo-1.0-
To make RPM ignore this error, use the --replacefiles option:
rpm -ivh --replacefiles foo-1.0-1.i386.rpm
2.2.3. Unresolved Dependency
RPM packages may sometimes depend on other packages, which means that they require other packages to be installed to run properly. If you try to install a package which has an unresolved dependency, output similar to the following is displayed:
error: Failed dependencies: bar.so.2 is needed by foo-1.0-1 Suggested resolutions: bar-2.0.20-3.i386.rpm
If you are installing a package from the Red Hat Enterprise Linux CD-ROM set, it usually suggest the package(s) needed to resolve the dependency. Find the suggested package(s) on the
Red Hat Enterprise Linux CD-ROMs or from Red Hat Network , and add it to the command:
rpm -ivh foo-1.0-1.i386.rpm bar-2.0.20-3.i386.rpm
If installation of both packages is successful, output similar to the following is displayed:
Preparing... ########################################### [100%] 1:foo ####################################
If it does not suggest a package to resolve the dependency, you can try the --redhatprovides
option to determine which package contains the required file. You need the rpmdb-redhat package installed to use this option.
rpm -q --redhatprovides bar.so.2
If the package that contains bar.so.2 is in the installed database from the rpmdb-redhat package, the name of the package is displayed:
bar-2.0.20-3.i386.rpm
To force the installation anyway (which is not recommended since the package may not run correctly), use the --nodeps option.
2.3. Uninstalling
Uninstalling a package is just as simple as installing one. Type the following command at a shell
prompt:
rpm -e foo
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2.4. Upgrading
Note
Notice that we used the package namefoo, not the name of the original package
filefoo-1.0-1.i386.rpm. To uninstall a package, replace foo with the actual package name of the original package.
You can encounter a dependency error when uninstalling a package if another installed package depends on the one you are trying to remove. For example:
error: Failed dependencies: foo is needed by (installed) bar-2.0.20-3.i386.rpm
To make RPM ignore this error and uninstall the package anyway (which may break the package dependent on it) use the --nodeps option.
2.4. Upgrading
Upgrading a package is similar to installing one. Type the following command at a shell prompt:
rpm -Uvh foo-2.0-1.i386.rpm
As part of upgrading a package, RPM automatically uninstalls any old versions of the foo package. Note that -U will also install a package even when there are no previous versions of the
package installed.
Tip
It is not advisable to use the -U option for installing kernel packages, because RPM
replaces the previous kernel package. This does not affect a running system, but if
the new kernel is unable to boot during your next restart, there would be no other
kernel to boot instead.
Using the -i option adds the kernel to your GRUB boot menu (/etc/grub.conf).
Similarly, removing an old, unneeded kernel removes the kernel from GRUB.
Because RPM performs intelligent upgrading of packages with configuration files, you may see
a message like the following:
saving /etc/foo.conf as /etc/foo.conf.rpmsave
This message means that changes you made to the configuration file may not be forward compatible with the new configuration file in the package, so RPM saved your original file and installed a new one. You should investigate the differences between the two configuration files
and resolve them as soon as possible, to ensure that your system continues to function properly.
If you attempt to upgrade to a package with an older version number (that is, if a more updated
version of the package is already installed), the output is similar to the following:
119
2.5. Freshening
package foo-2.0-1 (which is newer than foo-1.0-1) is already installed
To force RPM to upgrade anyway, use the --oldpackage option:
rpm -Uvh --oldpackage foo-1.0-1.i386.rpm
2.5. Freshening
Freshening is similar to upgrading, except that only existent packages are upgraded. Type the
following command at a shell prompt:
rpm -Fvh foo-1.2-1.i386.rpm
RPM's freshen option checks the versions of the packages specified on the command line
against the versions of packages that have already been installed on your system. When a newer version of an already-installed package is processed by RPM's freshen option, it is upgraded
to the newer version. However, RPM's freshen option does not install a package if no previously-installed package of the same name exists. This differs from RPM's upgrade option, as an
upgrade does install packages whether or not an older version of the package was already installed.
Freshening works for single packages or package groups. If you have just downloaded a large
number of different packages, and you only want to upgrade those packages that are already
installed on your system, freshening does the job. Thus, you do not have to delete any unwanted packages from the group that you downloaded before using RPM.
In this case, issue the following command:
rpm -Fvh *.rpm
RPM automatically upgrades only those packages that are already installed.
2.6. Querying
The RPM database stores information about all RPM packages installed in your system. It is
stored in the directory /var/lib/rpm/, and is used to query what packages are installed, what
versions each package is, and any changes to any files in the package since installation, among
others.
To query this database, use the -q option. The rpm -q package name command displays the
package name, version, and release number of the installed package package name. For example, using rpm -q foo to query installed package foo might generate the following output:
foo-2.0-1
You can also use the following Package Selection Options with -q to further refine or qualify
your query:
•
-a
— queries all currently installed packages.
•
-f <filename>
— queries the RPM database for which package owns <filename>. When
specifying a file, specify the absolute path of the file (for example, rpm -f /bin/ls).
120
2.7. Verifying
•
-p <packagefile>
— queries the uninstalled package <packagefile>.
There are a number of ways to specify what information to display about queried packages. The
following options are used to select the type of information for which you are searching. These
are called Information Query Options.
•
-i
displays package information including name, description, release, size, build date, install
date, vendor, and other miscellaneous information.
•
-l
displays the list of files that the package contains.
•
-s
displays the state of all the files in the package.
•
-d
displays a list of files marked as documentation (man pages, info pages, READMEs, etc.).
•
-c
displays a list of files marked as configuration files. These are the files you edit after installation to adapt and customize the package to your system (for example, sendmail.cf,
passwd, inittab, etc.).
For options that display lists of files, add -v to the command to display the lists in a familiar ls
l format.
-
2.7. Verifying
Verifying a package compares information about files installed from a package with the same information from the original package. Among other things, verifying compares the size, MD5
sum, permissions, type, owner, and group of each file.
The command rpm -V verifies a package. You can use any of the Package Verify Options listed
for querying to specify the packages you wish to verify. A simple use of verifying is rpm -V foo,
which verifies that all the files in the foo package are as they were when they were originally installed. For example:
•
To verify a package containing a particular file:
rpm -Vf /usr/bin/foo
In this example, /usr/bin/foo is the absolute path to the file used to query a package.
•
To verify ALL installed packages throughout the system:
rpm -Va
•
To verify an installed package against an RPM package file:
rpm -Vp foo-1.0-1.i386.rpm
This command can be useful if you suspect that your RPM databases are corrupt.
If everything verified properly, there is no output. If there are any discrepancies, they are displayed. The format of the output is a string of eight characters (a c denotes a configuration file)
and then the file name. Each of the eight characters denotes the result of a comparison of one
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3. Checking a Package's Signature
attribute of the file to the value of that attribute recorded in the RPM database. A single period
(.) means the test passed. The following characters denote specific discrepancies:
•
5
— MD5 checksum
•
S
— file size
•
L
— symbolic link
•
T
— file modification time
•
D
— device
•
U
— user
•
G
— group
•
M
— mode (includes permissions and file type)
•
?
— unreadable file
If you see any output, use your best judgment to determine if you should remove the package,
reinstall it, or fix the problem in another way.
3. Checking a Package's Signature
If you wish to verify that a package has not been corrupted or tampered with, examine only the
md5sum by typing the following command at a shell prompt (where <rpm-file> is the file name
of the RPM package):
rpm -K --nosignature <rpm-file>
The message <rpm-file>: md5 OK is displayed. This brief message means that the file was not
corrupted by the download. To see a more verbose message, replace -K with -Kvv in the command.
On the other hand, how trustworthy is the developer who created the package? If the package
is signed with the developer's GnuPG key, you know that the developer really is who they say
they are.
An RPM package can be signed using Gnu Privacy Guard (or GnuPG), to help you make certain your downloaded package is trustworthy.
GnuPG is a tool for secure communication; it is a complete and free replacement for the encryption technology of PGP, an electronic privacy program. With GnuPG, you can authenticate the
validity of documents and encrypt/decrypt data to and from other recipients. GnuPG is capable
of decrypting and verifying PGP 5.x files as well.
During installation, GnuPG is installed by default. That way you can immediately start using
GnuPG to verify any packages that you receive from Red Hat. Before doing so, you must first
import Red Hat's public key.
3.1. Importing Keys
122
3.2. Verifying Signature of Packages
To verify Red Hat packages, you must import the Red Hat GPG key. To do so, execute the following command at a shell prompt:
rpm --import /usr/share/rhn/RPM-GPG-KEY
To display a list of all keys installed for RPM verification, execute the command:
rpm -qa gpg-pubkey*
For the Red Hat key, the output includes:
gpg-pubkey-db42a60e-37ea5438
To display details about a specific key, use rpm
command:
-qi
followed by the output from the previous
rpm -qi gpg-pubkey-db42a60e-37ea5438
3.2. Verifying Signature of Packages
To check the GnuPG signature of an RPM file after importing the builder's GnuPG key, use the
following command (replace <rpm-file> with the filename of the RPM package):
rpm -K <rpm-file>
If all goes well, the following message is displayed: md5 gpg
the package has been verified, and that it is not corrupt.
OK.
This means that the signature of
4. Practical and Common Examples of RPM
Usage
RPM is a useful tool for both managing your system and diagnosing and fixing problems. The
best way to make sense of all of its options is to look at some examples.
•
Perhaps you have deleted some files by accident, but you are not sure what you deleted. To
verify your entire system and see what might be missing, you could try the following command:
rpm -Va
If some files are missing or appear to have been corrupted, you should probably either reinstall the package or uninstall and then re-install the package.
•
At some point, you might see a file that you do not recognize. To find out which package
owns it, enter:
rpm -qf /usr/bin/ggv
The output would look like the following:
ggv-2.6.0-2
123
5. Additional Resources
•
We can combine the above two examples in the following scenario. Say you are having
problems with /usr/bin/paste. You would like to verify the package that owns that program,
but you do not know which package owns paste. Enter the following command,
rpm -Vf /usr/bin/paste
and the appropriate package is verified.
•
Do you want to find out more information about a particular program? You can try the following command to locate the documentation which came with the package that owns that program:
rpm -qdf /usr/bin/free
The output would be similar to the following:
/usr/share/doc/procps-3.2.3/BUGS /usr/share/doc/procps-3.2.3/FAQ /usr/share/doc/procps-3.2.3/NEWS /usr/s
•
You may find a new RPM, but you do not know what it does. To find information about it, use
the following command:
rpm -qip crontabs-1.10-7.noarch.rpm
The output would be similar to the following:
Name : crontabs Relocations: (not relocatable) Version : 1.10 Vendor: Red Hat, Inc. Release : 7 Build D
•
Perhaps you now want to see what files the crontabs RPM installs. You would enter the following:
rpm -qlp crontabs-1.10-5.noarch.rpm
The output is similar to the following:
/etc/cron.daily /etc/cron.hourly /etc/cron.monthly /etc/cron.weekly /etc/crontab /usr/bin/run-parts
These are just a few examples. As you use RPM, you may find more uses for it.
5. Additional Resources
RPM is an extremely complex utility with many options and methods for querying, installing, upgrading, and removing packages. Refer to the following resources to learn more about RPM.
5.1. Installed Documentation
•
rpm --help
•
man rpm
— This command displays a quick reference of RPM parameters.
— The RPM man page gives more detail about RPM parameters than the rpm
-help command.
5.2. Useful Websites
124
-
5.3. Related Books
•
http://www.rpm.org/ — The RPM website.
•
http://www.redhat.com/mailman/listinfo/rpm-list/ — The RPM mailing list is archived here. To
subscribe, send mail to <[email protected]> with the word subscribe in the subject line.
5.3. Related Books
•
Red Hat RPM Guide by Eric Foster-Johnson; Wiley, John & Sons, Incorporated — This book
is a comprehensive guide to RPM, from installing packages to building RPMs.
125
Chapter 11. Package Management
Tool
If you prefer to use a graphical interface to view and manage packages in your system, you can
use the Package Management Tool, better known as pirut. This tool allows you to perform basic package management of your system through an easy-to-use interface to remove installed
packages or download (and install) packages compatible to your system. It also allows you to
view what packages are installed in your system and which ones are available for download
from Red Hat Network. In addition, the Package Management Tool also automatically resolves
any critical dependencies when you install or remove packages in the same way that the rpm
command does.
Note
While the Package Management Tool can automatically resolve dependencies
during package installation and removal, it cannot perform a forced install / remove
the same way that rpm -e --nodeps or rpm -U --nodeps can.
The X Window System is required to run the Package Management Tool. To start the application, go to Applications (the main menu on the panel) => Add/Remove Software. Alternatively, you can type the commands system-config-packages or pirut at shell prompt.
Figure 11.1. Package Management Tool
126
1. Listing and Analyzing Packages
1. Listing and Analyzing Packages
You can use the Package Management Tool to search and list all packages installed in your
system, as well as any packages available for you to download. The Browse, Search, and List
tabs present different options in viewing, analyzing, installing or removing packages.
The Browse tab allows you to view packages by group. In Figure 11.1, “Package Management
Tool”, the left window shows the different package group types you can choose from (for example, Desktop Environments, Applications, Development and more). When a package group
type is selected, the right window displays the different package groups of that type.
To view what packages are included in a package group, click Optional packages. Installed
packages are checked.
Figure 11.2. Optional Packages
The List tab displays a list of packages installed or available for download. Packages already
installed in your system are marked with a green check (
).
By default, the All packages option above the main window is selected; this specifies that all
packages be displayed. Use the Installed packages option to display only packages that are
already installed in your system, and the Available packages option to view what packages
you can download and install.
The Search tab allows you to use keywords to search for particular packages. This tab also allows you to view a short description of a package. To do so, simply select a package and click
127
2. Installing and Removing Packages
the Package Details button below the main window.
2. Installing and Removing Packages
To install a package available for download, click the checkbox beside the package name.
When you do so, an installation icon (
) appears beside its checkbox. This indicates that the
package is queued for download and installation. You can select multiple packages to download
and install; once you have made your selection, click the Apply button.
Figure 11.3. Package installation
If there are any package dependencies for your selected downloads, the Package Management Tool will notify you accordingly. Click Details to view what additional packages are
needed. To proceed with downloading and installing the package (along with all other dependent packages) click Continue.
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2. Installing and Removing Packages
Figure 11.4. Package dependencies: installation
Removing a package can be done in a similar manner. To remove a package installed in your
system, click the checkbox beside the package name. The green check appearing beside the
package name will be replaced by a package removal icon (
). This indicates that the package is queued for removal; you can also select multiple packages to be removed at the same
time. Once you have selected the packages you want to remove, click the Apply button.
129
2. Installing and Removing Packages
Figure 11.5. Package removal
Note that if any other installed packages are dependent on the package you are removing, they
will be removed as well. The Package Management Tool will notify you if there are any such
dependencies. Click Details to view what packages are dependent on the one you are removing. To proceed with removing your selected package/s (along with all other dependent packages) click Continue.
130
2. Installing and Removing Packages
Figure 11.6. Package dependencies: removal
You can install and remove multiple packages by selecting packages to be installed / removed
and then clicking Apply. The Package selections window displays the number of packages to
be installed and removed.
131
2. Installing and Removing Packages
Figure 11.7. Installing and removing packages simultaneously
132
Chapter 12. Red Hat Network
Red Hat Network is an Internet solution for managing one or more Red Hat Enterprise Linux
systems. All Security Alerts, Bug Fix Alerts, and Enhancement Alerts (collectively known as Errata Alerts) can be downloaded directly from Red Hat using the Package Updater standalone
application or through the RHN website available at https://rhn.redhat.com/.
Figure 12.1. Your RHN
Red Hat Network saves you time because you receive email when updated packages are released. You do not have to search the Web for updated packages or security alerts. By default,
Red Hat Network installs the packages as well. You do not have to learn how to use RPM or
worry about resolving software package dependencies; RHN does it all.
Red Hat Network features include:
•
Errata Alerts — learn when Security Alerts, Bug Fix Alerts, and Enhancement Alerts are issued for all the systems in your network
133
Figure 12.2. Relevant Errata
•
Automatic email notifications — Receive an email notification when an Errata Alert is issued
for your system(s)
•
Scheduled Errata Updates — Schedule delivery of Errata Updates
•
Package installation — Schedule package installation on one or more systems with the click
of a button
•
Package Updater — Use the Package Updater to download the latest software packages
for your system (with optional package installation)
•
Red Hat Network website — Manage multiple systems, downloaded individual packages,
and schedule actions such as Errata Updates through a secure Web browser connection
from any computer
Caution
You must activate your Red Hat Enterprise Linux product before registering your
system with Red Hat Network to make sure your system is entitled to the correct
services. To activate your product, go to:
http://www.redhat.com/apps/activate/
134
After activating your product, register it with Red Hat Network to receive Errata Updates. The registration process gathers information about the system that is required to notify you of updates.
For example, a list of packages installed on the system is compiled so you are only notified
about updates that are relevant to your system.
The first time the system is booted, the Software Update Setup Assistant prompts you to register. If you did not register then, select Applications (the main menu on the panel) => System Tools => Package Updater on your desktop to start the registration process. Alternately,
execute the command yum update from a shell prompt.
Figure 12.3. Registering with RHN
After registering, use one of the following methods to start receiving updates:
•
Select Applications (the main menu on the panel) => System Tools => Package Updater
on your desktop
•
Execute the command yum from a shell prompt
135
•
Use the RHN website at https://rhn.redhat.com/
•
Click on the package icon when it appears in the panel to launch the Package Updater.
For more detailed instructions, refer to the documentation available at:
http://www.redhat.com/docs/manuals/RHNetwork/
Tip
Red Hat Enterprise Linux includes a convenient panel icon that displays visible
alerts when there is an update for your Red Hat Enterprise Linux system. This panel icon is not present if no updates are available.
136
Part III. Network-Related
Configuration
After explaining how to configure the network, this part discusses topics related to networking
such as how to allow remote logins, share files and directories over the network, and set up a
Web server.
Chapter 13. Network Interfaces
Under Red Hat Enterprise Linux, all network communications occur between configured software interfaces and physical networking devices connected to the system.
The configuration files for network interfaces are located in the /
etc/sysconfig/network-scripts/ directory. The scripts used to activate and deactivate these
network interfaces are also located here. Although the number and type of interface files can
differ from system to system, there are three categories of files that exist in this directory:
1.
Interface configuration files
2.
Interface control scripts
3.
Network function files
The files in each of these categories work together to enable various network devices.
This chapter explores the relationship between these files and how they are used.
1. Network Configuration Files
Before delving into the interface configuration files, let us first itemize the primary configuration
files used in network configuration. Understanding the role these files play in setting up the network stack can be helpful when customizing a Red Hat Enterprise Linux system.
The primary network configuration files are as follows:
/etc/hosts
The main purpose of this file is to resolve hostnames that cannot be resolved any other
way. It can also be used to resolve hostnames on small networks with no DNS server. Regardless of the type of network the computer is on, this file should contain a line specifying
the IP address of the loopback device (127.0.0.1) as localhost.localdomain. For more information, refer to the hosts man page.
/etc/resolv.conf
This file specifies the IP addresses of DNS servers and the search domain. Unless configured to do otherwise, the network initialization scripts populate this file. For more information about this file, refer to the resolv.conf man page.
/etc/sysconfig/network
This file specifies routing and host information for all network interfaces. For more information about this file and the directives it accepts, refer to Section 1.22,
“/etc/sysconfig/network”.
/etc/sysconfig/network-scripts/ifcfg-<interface-name>
For each network interface, there is a corresponding interface configuration script. Each of
these files provide information specific to a particular network interface. Refer to Section 2,
“Interface Configuration Files” for more information on this type of file and the directives it
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2. Interface Configuration Files
accepts.
Warning
The /etc/sysconfig/networking/ directory is used by the Network Administration
Tool (system-config-network) and its contents should not be edited manually. Using only one method for network configuration is strongly encouraged, due to the
risk of configuration deletion.
For more information about configuring network interfaces using the Network Administration Tool, refer to Chapter 14, Network Configuration
2. Interface Configuration Files
Interface configuration files control the software interfaces for individual network devices. As the
system boots, it uses these files to determine what interfaces to bring up and how to configure
them. These files are usually named ifcfg-<name>, where <name> refers to the name of the
device that the configuration file controls.
2.1. Ethernet Interfaces
One of the most common interface files is ifcfg-eth0, which controls the first Ethernet network
interface card or NIC in the system. In a system with multiple NICs, there are multiple
ifcfg-eth<X> files (where <X> is a unique number corresponding to a specific interface). Because each device has its own configuration file, an administrator can control how each interface functions individually.
The following is a sample ifcfg-eth0 file for a system using a fixed IP address:
DEVICE=eth0 BOOTPROTO=none ONBOOT=yes NETWORK=10.0.1.0 NETMASK=255.255.255.0 IPADDR=10.0.1.27 USERCTL=no
The values required in an interface configuration file can change based on other values. For example, the ifcfg-eth0 file for an interface using DHCP looks different because IP information is
provided by the DHCP server:
DEVICE=eth0 BOOTPROTO=dhcp ONBOOT=yes
The Network Administration Tool (system-config-network) is an easy way to make changes
to the various network interface configuration files (refer to Chapter 14, Network Configuration
for detailed instructions on using this tool).
However, it is also possible to manually edit the configuration files for a given network interface.
Below is a listing of the configurable parameters in an Ethernet interface configuration file:
BOOTPROTO=<protocol>
where <protocol> is one of the following:
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2.1. Ethernet Interfaces
•
none
— No boot-time protocol should be used.
•
bootp
•
dhcp
— The BOOTP protocol should be used.
— The DHCP protocol should be used.
BROADCAST=<address>
where <address> is the broadcast address. This directive is deprecated, as the value is calculated automatically with ifcalc.
DEVICE=<name>
where <name> is the name of the physical device (except for dynamically-allocated PPP
devices where it is the logical name).
DHCP_HOSTNAME
Use this option only if the DHCP server requires the client to specify a hostname before receiving an IP address.
DNS{1,2}=<address>
where <address> is a name server address to be placed in /etc/resolv.conf if the PEERDNS
directive is set to yes.
ETHTOOL_OPTS=<options>
where <options> are any device-specific options supported by ethtool. For example, if you
wanted to force 100Mb, full duplex:
ETHTOOL_OPTS="autoneg off speed 100 duplex full"
Note
Changing speed or duplex settings almost always requires disabling autonegotiation with the autoneg off option. This needs to be stated first, as the option
entries are order-dependent.
GATEWAY=<address>
where <address> is the IP address of the network router or gateway device (if any).
HWADDR=<MAC-address>
where <MAC-address> is the hardware address of the Ethernet device in the form
AA:BB:CC:DD:EE:FF. This directive is useful for machines with multiple NICs to ensure that
the interfaces are assigned the correct device names regardless of the configured load order for each NIC's module. This directive should not be used in conjunction with MACADDR.
IPADDR=<address>
where <address> is the IP address.
MACADDR=<MAC-address>
where <MAC-address> is the hardware address of the Ethernet device in the form
AA:BB:CC:DD:EE:FF. This directive is used to assign a MAC address to an interface, overriding the one assigned to the physical NIC. This directive should not be used in conjunction
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2.1. Ethernet Interfaces
with HWADDR.
MASTER=<bond-interface>
where <bond-interface> is the channel bonding interface to which the Ethernet interface is
linked.
This directive is used in conjunction with the SLAVE directive.
Refer to Section 2.3, “Channel Bonding Interfaces” for more information about channel
bonding interfaces.
NETMASK=<mask>
where <mask> is the netmask value.
NETWORK=<address>
where <address> is the network address. This directive is deprecated, as the value is calculated automatically with ifcalc.
ONBOOT=<answer>
where <answer> is one of the following:
•
yes
•
no
— This device should be activated at boot-time.
— This device should not be activated at boot-time.
PEERDNS=<answer>
where <answer> is one of the following:
•
yes
— Modify /etc/resolv.conf if the DNS directive is set. If using DHCP, then yes is the
default.
•
no
— Do not modify /etc/resolv.conf.
SLAVE=<bond-interface>
where <bond-interface> is one of the following:
•
yes
— This device is controlled by the channel bonding interface specified in the MASTER
directive.
•
no
— This device is not controlled by the channel bonding interface specified in the MASTER directive.
This directive is used in conjunction with the MASTER directive.
Refer to Section 2.3, “Channel Bonding Interfaces” for more about channel bonding interfaces.
SRCADDR=<address>
where <address> is the specified source IP address for outgoing packets.
USERCTL=<answer>
where <answer> is one of the following:
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2.2. IPsec Interfaces
•
yes
•
no
— Non-root users are allowed to control this device.
— Non-root users are not allowed to control this device.
2.2. IPsec Interfaces
The following example shows the ifcfg file for a network-to-network IPsec connection for LAN
A. The unique name to identify the connection in this example is ipsec1, so the resulting file is
named /etc/sysconfig/network-scripts/ifcfg-ipsec1.
TYPE=IPsec ONBOOT=yes IKE_METHOD=PSK SRCNET=192.168.1.0/24 DSTNET=192.168.2.0/24 DST=X.X.X.X
In the example above, X.X.X.X is the publicly routable IP address of the destination IPsec
router.
Below is a listing of the configurable parameters for an IPsec interface:
DST=<address>
where <address> is the IP address of the IPsec destination host or router. This is used for
both host-to-host and network-to-network IPsec configurations.
DSTNET=<network>
where <network> is the network address of the IPsec destination network. This is only used
for network-to-network IPsec configurations.
SRC=<address>
where <address> is the IP address of the IPsec source host or router. This setting is optional
and is only used for host-to-host IPsec configurations.
SRCNET=<network>
where <network> is the network address of the IPsec source network. This is only used for
network-to-network IPsec configurations.
TYPE=<interface-type>
where <interface-type> is IPSEC. Both applications are part of the ipsec-tools package.
If manual key encryption with IPsec is being used, refer to
(replace <version-number> with
the version of the initscripts package installed) for configuration parameters.
/usr/share/doc/initscripts-<version-number>/sysconfig.txt
The racoon IKEv1 key management daemon negotiates and configures a set of parameters for
IPSec. It can use preshared keys, RSA signatures, or GSS-API. If racoon is used to automatically manage key encryption, the following options are required:
IKE_METHOD=<encryption-method>
where <encryption-method> is either PSK, X509, or GSSAPI. If PSK is specified, the IKE_PSK
parameter must also be set. If X509 is specified, the IKE_CERTFILE parameter must also be
set.
IKE_PSK=<shared-key>
where <shared-key> is the shared, secret value for the PSK (preshared keys) method.
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2.3. Channel Bonding Interfaces
IKE_CERTFILE=<cert-file>
where <cert-file> is a valid X.509 certificate file for the host.
IKE_PEER_CERTFILE=<cert-file>
where <cert-file> is a valid X.509 certificate file for the remote host.
IKE_DNSSEC=<answer>
where <answer> is yes. The racoon daemon retrieves the remote host's X.509 certificate via
DNS. If a IKE_PEER_CERTFILE is specified, do not include this parameter.
For more information about the encryption algorithms available for IPsec, refer to the setkey
man page. For more information about racoon, refer to the racoon and racoon.conf man pages.
2.3. Channel Bonding Interfaces
Red Hat Enterprise Linux allows administrators to bind multiple network interfaces together into
a single channel using the bonding kernel module and a special network interface called a channel bonding interface. Channel bonding enables two or more network interfaces to act as one,
simultaneously increasing the bandwidth and providing redundancy.
To create a channel bonding interface, create a file in the /etc/sysconfig/network-scripts/ directory called ifcfg-bond<N>, replacing <N> with the number for the interface, such as 0.
The contents of the file can be identical to whatever type of interface is getting bonded, such as
an Ethernet interface. The only difference is that the DEVICE= directive must be bond<N>, replacing <N> with the number for the interface.
The following is a sample channel bonding configuration file:
DEVICE=bond0 BOOTPROTO=none ONBOOT=yes NETWORK=10.0.1.0 NETMASK=255.255.255.0 IPADDR=10.0.1.27 USERCTL=no
After the channel bonding interface is created, the network interfaces to be bound together must
be configured by adding the MASTER= and SLAVE= directives to their configuration files. The configuration files for each of the channel-bonded interfaces can be nearly identical.
For example, if two Ethernet interfaces are being channel bonded, both eth0 and eth1 may look
like the following example:
DEVICE=eth<N> BOOTPROTO=none ONBOOT=yes MASTER=bond0 SLAVE=yes USERCTL=no
In this example, replace <N> with the numerical value for the interface.
For a channel bonding interface to be valid, the kernel module must be loaded. To ensure that
the module is loaded when the channel bonding interface is brought up, add the following line to
/etc/modprobe.conf:
alias bond<N> bonding
Replace <N> with the number of the interface, such as 0. For each configured channel bonding
interface, there must be a corresponding entry in /etc/modprobe.conf.
Once /etc/modprobe.conf is configured — and the channel bonding interface and network interfaces are configured — the ifup command can be used to bring up the channel bonding inter-
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2.4. Alias and Clone Files
face.
Important
Important aspects of the channel bonding interface are controlled through the kernel module. For more information about controlling the bonding modules, refer to
Section 5.2, “The Channel Bonding Module”.
2.4. Alias and Clone Files
Two lesser-used types of interface configuration files are alias and clone files.
Alias interface configuration files, which are used to bind multiple addresses to a single interface, use the ifcfg-<if-name>:<alias-value> naming scheme.
For example, an ifcfg-eth0:0 file could be configured to specify DEVICE=eth0:0 and a static IP
address of 10.0.0.2, serving as an alias of an Ethernet interface already configured to receive its
IP information via DHCP in ifcfg-eth0. Under this configuration, eth0 is bound to a dynamic IP
address, but the same physical network card can receive requests via the fixed, 10.0.0.2 IP address.
Caution
Alias interfaces do not support DHCP.
A clone interface configuration file should use the following naming convention:
ifcfg-<if-name>-<clone-name>. While an alias file allows multiple addresses for an existing interface, a clone file is used to specify additional options for an interface. For example, a standard
DHCP Ethernet interface called eth0, may look similar to this:
DEVICE=eth0 ONBOOT=yes BOOTPROTO=dhcp
Since the default value for the USERCTL directive is no if it is not specified, users cannot bring this
interface up and down. To give users the ability to control the interface, create a clone by copying ifcfg-eth0 to ifcfg-eth0-user and add the following line to ifcfg-eth0-user:
USERCTL=yes
This way a user can bring up the eth0 interface using the /sbin/ifup eth0-user command because the configuration options from ifcfg-eth0 and ifcfg-eth0-user are combined. While this
is a very basic example, this method can be used with a variety of options and interfaces.
The easiest way to create alias and clone interface configuration files is to use the graphical
Network Administration Tool. For more information on using this tool, refer to Chapter 14,
Network Configuration.
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2.5. Dialup Interfaces
2.5. Dialup Interfaces
If you are connecting to the Internet via a dialup connection, a configuration file is necessary for
the interface.
PPP interface files are named using the following format:
ifcfg-ppp<X>
where <X> is a unique number corresponding to a specific interface.
The PPP interface configuration file is created automatically when wvdial, the Network Administration Tool or Kppp is used to create a dialup account. It is also possible to create and edit
this file manually.
The following is a typical ifcfg-ppp0 file:
DEVICE=ppp0 NAME=test WVDIALSECT=test MODEMPORT=/dev/modem LINESPEED=115200 PAPNAME=test USERCTL=true ONBOO
Serial Line Internet Protocol (SLIP) is another dialup interface, although it is used less frequently. SLIP files have interface configuration file names such as ifcfg-sl0.
Other options that may be used in these files include:
DEFROUTE=<answer>
where <answer> is one of the following:
•
yes
•
no
— Set this interface as the default route.
— Do not set this interface as the default route.
DEMAND=<answer>
where <answer> is one of the following:
•
yes
— This interface allows pppd to initiate a connection when someone attempts to use
it.
•
no
— A connection must be manually established for this interface.
IDLETIMEOUT=<value>
where <value> is the number of seconds of idle activity before the interface disconnects itself.
INITSTRING=<string>
where <string> is the initialization string passed to the modem device. This option is primarily used in conjunction with SLIP interfaces.
LINESPEED=<value>
where <value> is the baud rate of the device. Possible standard values include 57600, 38400,
19200, and 9600.
MODEMPORT=<device>
where <device> is the name of the serial device that is used to establish the connection for
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2.6. Other Interfaces
the interface.
MTU=<value>
where <value> is the Maximum Transfer Unit (MTU) setting for the interface. The MTU
refers to the largest number of bytes of data a frame can carry, not counting its header information. In some dialup situations, setting this to a value of 576 results in fewer packets
dropped and a slight improvement to the throughput for a connection.
NAME=<name>
where <name> is the reference to the title given to a collection of dialup connection configurations.
PAPNAME=<name>
where <name> is the username given during the Password Authentication Protocol (PAP) exchange that occurs to allow connections to a remote system.
PERSIST=<answer>
where <answer> is one of the following:
•
yes
— This interface should be kept active at all times, even if deactivated after a modem hang up.
•
no
— This interface should not be kept active at all times.
REMIP=<address>
where <address> is the IP address of the remote system. This is usually left unspecified.
WVDIALSECT=<name>
where <name> associates this interface with a dialer configuration in /etc/wvdial.conf. This
file contains the phone number to be dialed and other important information for the interface.
2.6. Other Interfaces
Other common interface configuration files include the following:
ifcfg-lo
A local loopback interface is often used in testing, as well as being used in a variety of applications that require an IP address pointing back to the same system. Any data sent to the
loopback device is immediately returned to the host's network layer.
Warning
The loopback interface script, /etc/sysconfig/network-scripts/ifcfg-lo,
should never be edited manually. Doing so can prevent the system from operating correctly.
ifcfg-irlan0
An infrared interface allows information between devices, such as a laptop and a printer, to
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3. Interface Control Scripts
flow over an infrared link. This works in a similar way to an Ethernet device except that it
commonly occurs over a peer-to-peer connection.
ifcfg-plip0
A Parallel Line Interface Protocol (PLIP) connection works much the same way as an Ethernet device, except that it utilizes a parallel port.
ifcfg-tr0
Token Ring topologies are not as common on Local Area Networks (LANs) as they once
were, having been eclipsed by Ethernet.
3. Interface Control Scripts
The interface control scripts activate and deactivated system interfaces. There are two primary
interface control scripts that call on control scripts located in the /
etc/sysconfig/network-scripts/ directory: /sbin/ifdown and /sbin/ifup.
The ifup and ifdown interface scripts are symbolic links to scripts in the /sbin/ directory. When
either of these scripts are called, they require the value of the interface to be specified, such as:
ifup eth0
Caution
The ifup and ifdown interface scripts are the only scripts that the user should use
to bring up and take down network interfaces.
The following scripts are described for reference purposes only.
Two files used to perform a variety of network initialization tasks during the process of bringing
up a network interface are /etc/rc.d/init.d/functions and /
etc/sysconfig/network-scripts/network-functions. Refer to Section 4, “Network Function Files”
for more information.
After verifying that an interface has been specified and that the user executing the request is allowed to control the interface, the correct script brings the interface up or down. The following
are common interface control scripts found within the /etc/sysconfig/network-scripts/ directory:
ifup-aliases
Configures IP aliases from interface configuration files when more than one IP address is
associated with an interface.
and ifdown-ippp
Brings ISDN interfaces up and down.
ifup-ippp
and ifdown-ipsec
Brings IPsec interfaces up and down.
ifup-ipsec
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4. Network Function Files
and ifdown-ipv6
Brings IPv6 interfaces up and down.
ifup-ipv6
ifup-ipx
Brings up an IPX interface.
ifup-plip
Brings up a PLIP interface.
ifup-plusb
Brings up a USB interface for network connections.
and ifdown-post
Contains commands to be executed after an interface is brought up or down.
ifup-post
and ifdown-ppp
Brings a PPP interface up or down.
ifup-ppp
ifup-routes
Adds static routes for a device as its interface is brought up.
and ifup-sit
Contains function calls related to bringing up and down an IPv6 tunnel within an IPv4 connection.
ifdown-sit
and ifdown-sl
Brings a SLIP interface up or down.
ifup-sl
ifup-wireless
Brings up a wireless interface.
Warning
Removing or modifying any scripts in the /etc/sysconfig/network-scripts/ directory can cause interface connections to act irregularly or fail. Only advanced users
should modify scripts related to a network interface.
The easiest way to manipulate all network scripts simultaneously is to use the /sbin/service
command on the network service (/etc/rc.d/init.d/network), as illustrated the following command:
/sbin/service network <action>
Here, <action> can be either start, stop, or restart.
To view a list of configured devices and currently active network interfaces, use the following
command:
/sbin/service network status
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5. Additional Resources
4. Network Function Files
Red Hat Enterprise Linux makes use of several files that contain important common functions
used to bring interfaces up and down. Rather than forcing each interface control file to contain
these functions, they are grouped together in a few files that are called upon when necessary.
The /etc/sysconfig/network-scripts/network-functions file contains the most commonly used
IPv4 functions, which are useful to many interface control scripts. These functions include contacting running programs that have requested information about changes in the status of an interface, setting hostnames, finding a gateway device, verifying whether or not a particular
device is down, and adding a default route.
As the functions required for IPv6 interfaces are different from IPv4 interfaces, a /
etc/sysconfig/network-scripts/network-functions-ipv6 file exists specifically to hold this information. The functions in this file configure and delete static IPv6 routes, create and remove
tunnels, add and remove IPv6 addresses to an interface, and test for the existence of an IPv6
address on an interface.
5. Additional Resources
The following are resources which explain more about network interfaces.
5.1. Installed Documentation
/usr/share/doc/initscripts-<version>/sysconfig.txt
A guide to available options for network configuration files, including IPv6 options not
covered in this chapter.
/usr/share/doc/iproute-<version>/ip-cref.ps
This file contains a wealth of information about the ip command, which can be used to manipulate routing tables, among other things. Use the ggv or kghostview application to view
this file.
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Chapter 14. Network Configuration
To communicate with each other, computers must have a network connection. This is accomplished by having the operating system recognize an interface card (such as Ethernet, ISDN
modem, or token ring) and configuring the interface to connect to the network.
The Network Administration Tool can be used to configure the following types of network interfaces:
•
Ethernet
•
ISDN
•
modem
•
xDSL
•
token ring
•
CIPE
•
wireless devices
It can also be used to configure IPsec connections, manage DNS settings, and manage the /
etc/hosts file used to store additional hostnames and IP address combinations.
To use the Network Administration Tool, you must have root privileges. To start the application, go to the Applications (the main menu on the panel) => System Settings => Network, or
type the command system-config-network at a shell prompt (for example, in an XTerm or a
GNOME terminal). If you type the command, the graphical version is displayed if X is running;
otherwise, the text-based version is displayed.
To use the command line version, execute the command system-config-network-cmd
root to view all of the options.
150
--help
as
1. Overview
Figure 14.1. Network Administration Tool
Tip
Use the Red Hat Hardware Compatibility List (http://hardware.redhat.com/hcl/) to
determine if Red Hat Enterprise Linux supports your hardware device.
1. Overview
To configure a network connection with the Network Administration Tool, perform the following steps:
1.
Add a network device associated with the physical hardware device.
2.
Add the physical hardware device to the hardware list, if it does not already exist.
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2. Establishing an Ethernet Connection
3.
Configure the hostname and DNS settings.
4.
Configure any hosts that cannot be looked up through DNS.
This chapter discusses each of these steps for each type of network connection.
2. Establishing an Ethernet Connection
To establish an Ethernet connection, you need a network interface card (NIC), a network cable
(usually a CAT5 cable), and a network to connect to. Different networks are configured to use
different network speeds; make sure your NIC is compatible with the network to which you want
to connect.
To add an Ethernet connection, follow these steps:
1.
Click the Devices tab.
2.
Click the New button on the toolbar.
3.
Select Ethernet connection from the Device Type list, and click Forward.
4.
If you have already added the network interface card to the hardware list, select it from the
Ethernet card list. Otherwise, select Other Ethernet Card to add the hardware device.
Note
The installation program detects supported Ethernet devices and prompts you
to configure them. If you configured any Ethernet devices during the installation, they are displayed in the hardware list on the Hardware tab.
5.
If you selected Other Ethernet Card, the Select Ethernet Adapter window appears. Select the manufacturer and model of the Ethernet card. Select the device name. If this is the
system's first Ethernet card, select eth0 as the device name; if this is the second Ethernet
card, select eth1 (and so on). The Network Administration Tool also allows you to configure the resources for the NIC. Click Forward to continue.
6.
In the Configure Network Settings window shown in Figure 14.2, “Ethernet Settings”,
choose between DHCP and a static IP address. If the device receives a different IP address each time the network is started, do not specify a hostname. Click Forward to continue.
7.
Click Apply on the Create Ethernet Device page.
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2. Establishing an Ethernet Connection
Figure 14.2. Ethernet Settings
After configuring the Ethernet device, it appears in the device list as shown in Figure 14.3,
“Ethernet Device”.
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3. Establishing an ISDN Connection
Figure 14.3. Ethernet Device
Be sure to select File => Save to save the changes.
After adding the Ethernet device, you can edit its configuration by selecting the device from the
device list and clicking Edit. For example, when the device is added, it is configured to start at
boot time by default. To change this setting, select to edit the device, modify the Activate
device when computer starts value, and save the changes.
When the device is added, it is not activated immediately, as seen by its Inactive status. To activate the device, select it from the device list, and click the Activate button. If the system is
configured to activate the device when the computer starts (the default), this step does not have
to be performed again.
If you associate more than one device with an Ethernet card, the subsequent devices are
device aliases. A device alias allows you to setup multiple virtual devices for one physical
device, thus giving the one physical device more than one IP address. For example, you can
configure an eth1 device and an eth1:1 device. For details, refer to Section 11, “Device Aliases”.
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3. Establishing an ISDN Connection
3. Establishing an ISDN Connection
An ISDN connection is an Internet connection established with a ISDN modem card through a
special phone line installed by the phone company. ISDN connections are popular in Europe.
To add an ISDN connection, follow these steps:
1.
Click the Devices tab.
2.
Click the New button on the toolbar.
3.
Select ISDN connection from the Device Type list, and click Forward.
4.
Select the ISDN adapter from the pulldown menu. Then configure the resources and D
channel protocol for the adapter. Click Forward to continue.
Figure 14.4. ISDN Settings
5.
If your Internet Service Provider (ISP) is in the pre-configured list, select it. Otherwise, enter
the required information about your ISP account. If you do not know the values, contact
your ISP. Click Forward.
6.
In the IP Settings window, select the Encapsulation Mode and whether to obtain an IP
address automatically or to set a static IP instead. Click Forward when finished.
7.
On the Create Dialup Connection page, click Apply.
After configuring the ISDN device, it appears in the device list as a device with type ISDN as
shown in Figure 14.5, “ISDN Device”.
Be sure to select File => Save to save the changes.
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4. Establishing a Modem Connection
After adding the ISDN device, you can edit its configuration by selecting the device from the
device list and clicking Edit. For example, when the device is added, it is configured not to start
at boot time by default. Edit its configuration to modify this setting. Compression, PPP options,
login name, password, and more can be changed.
When the device is added, it is not activated immediately, as seen by its Inactive status. To activate the device, select it from the device list, and click the Activate button. If the system is
configured to activate the device when the computer starts (the default), this step does not have
to be performed again.
Figure 14.5. ISDN Device
4. Establishing a Modem Connection
A modem can be used to configure an Internet connection over an active phone line. An Internet Service Provider (ISP) account (also called a dial-up account) is required.
To add a modem connection, follow these steps:
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4. Establishing a Modem Connection
1.
Click the Devices tab.
2.
Click the New button on the toolbar.
3.
Select Modem connection from the Device Type list, and click Forward.
4.
If there is a modem already configured in the hardware list (on the Hardware tab), the Network Administration Tool assumes you want to use it to establish a modem connection. If
there are no modems already configured, it tries to detect any modems in the system. This
probe might take a while. If a modem is not found, a message is displayed to warn you that
the settings shown are not values found from the probe.
5.
After probing, the window in Figure 14.6, “Modem Settings” appears.
Figure 14.6. Modem Settings
6.
Configure the modem device, baud rate, flow control, and modem volume. If you do not
know these values, accept the defaults if the modem was probed successfully. If you do not
have touch tone dialing, uncheck the corresponding checkbox. Click Forward.
7.
If your ISP is in the pre-configured list, select it. Otherwise, enter the required information
about your ISP account. If you do not know these values, contact your ISP. Click Forward.
8.
On the IP Settings page, select whether to obtain an IP address automatically or whether
to set one statically. Click Forward when finished.
9.
On the Create Dialup Connection page, click Apply.
After configuring the modem device, it appears in the device list with the type Modem as shown in
Figure 14.7, “Modem Device”.
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5. Establishing an xDSL Connection
Figure 14.7. Modem Device
Be sure to select File => Save to save the changes.
After adding the modem device, you can edit its configuration by selecting the device from the
device list and clicking Edit. For example, when the device is added, it is configured not to start
at boot time by default. Edit its configuration to modify this setting. Compression, PPP options,
login name, password, and more can also be changed.
When the device is added, it is not activated immediately, as seen by its Inactive status. To activate the device, select it from the device list, and click the Activate button. If the system is
configured to activate the device when the computer starts (the default), this step does not have
to be performed again.
5. Establishing an xDSL Connection
DSL stands for Digital Subscriber Lines. There are different types of DSL such as ADSL, IDSL,
and SDSL. The Network Administration Tool uses the term xDSL to mean all types of DSL
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5. Establishing an xDSL Connection
connections.
Some DSL providers require that the system is configured to obtain an IP address through DHCP with an Ethernet card. Some DSL providers require you to configure a PPPoE
(Point-to-Point Protocol over Ethernet) connection with an Ethernet card. Ask your DSL provider
which method to use.
If you are required to use DHCP, refer to Section 2, “Establishing an Ethernet Connection” to
configure your Ethernet card.
If you are required to use PPPoE, follow these steps:
1.
Click the Devices tab.
2.
Click the New button.
3.
Select xDSL connection from the Device Type list, and click Forward.
4.
If your Ethernet card is in the hardware list, select the Ethernet Device from the pulldown
menu from the page shown in Figure 14.8, “xDSL Settings”. Otherwise, the Select Ethernet Adapter window appears.
Note
The installation program detects supported Ethernet devices and prompts you
to configure them. If you configured any Ethernet devices during the installation, they are displayed in the hardware list on the Hardware tab.
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5. Establishing an xDSL Connection
Figure 14.8. xDSL Settings
5.
If the Select Ethernet Adapter window appears, select the manufacturer and model of the
Ethernet card. Select the device name. If this is the system's first Ethernet card, select eth0
as the device name; if this is the second Ethernet card, select eth1 (and so on). The Network Administration Tool also allows you to configure the resources for the NIC. Click
Forward to continue.
6.
Enter the Provider Name, Login Name, and Password. If you have a T-Online account,
instead of entering a Login Name and Password in the default window, click the T-Online
Account Setup button and enter the required information. Click Forward to continue.
7.
On the Create DSL Connection page, click Apply.
After configuring the DSL connection, it appears in the device list as shown in Figure 14.7,
“Modem Device”.
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6. Establishing a Token Ring Connection
Figure 14.9. xDSL Device
Be sure to select File => Save to save the changes.
After adding the xDSL connection, you can edit its configuration by selecting the device from the
device list and clicking Edit. For example, when the device is added, it is configured not to start
at boot time by default. Edit its configuration to modify this setting.
When the device is added, it is not activated immediately, as seen by its Inactive status. To activate the device, select it from the device list, and click the Activate button. If the system is
configured to activate the device when the computer starts (the default), this step does not have
to be performed again.
6. Establishing a Token Ring Connection
A token ring network is a network in which all the computers are connected in a circular pattern.
A token, or a special network packet, travels around the token ring and allows computers to
send information to each other.
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6. Establishing a Token Ring Connection
Tip
For more information on using token rings under Linux, refer to the Linux Token
Ring Project website available at http://www.linuxtr.net/.
To add a token ring connection, follow these steps:
1.
Click the Devices tab.
2.
Click the New button on the toolbar.
3.
Select Token Ring connection from the Device Type list and click Forward.
4.
If you have already added the token ring card to the hardware list, select it from the Tokenring card list. Otherwise, select Other Tokenring Card to add the hardware device.
5.
If you selected Other Tokenring Card, the Select Token Ring Adapter window as shown
in Figure 14.10, “Token Ring Settings” appears. Select the manufacturer and model of the
adapter. Select the device name. If this is the system's first token ring card, select tr0; if this
is the second token ring card, select tr1 (and so on). The Network Administration Tool
also allows the user to configure the resources for the adapter. Click Forward to continue.
Figure 14.10. Token Ring Settings
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7. Establishing a Wireless Connection
6.
On the Configure Network Settings page, choose between DHCP and static IP address.
You may specify a hostname for the device. If the device receives a dynamic IP address
each time the network is started, do not specify a hostname. Click Forward to continue.
7.
Click Apply on the Create Tokenring Device page.
After configuring the token ring device, it appears in the device list as shown in Figure 14.11,
“Token Ring Device”.
Figure 14.11. Token Ring Device
Be sure to select File => Save to save the changes.
After adding the device, you can edit its configuration by selecting the device from the device list
and clicking Edit. For example, you can configure whether the device is started at boot time.
When the device is added, it is not activated immediately, as seen by its Inactive status. To activate the device, select it from the device list, and click the Activate button. If the system is
configured to activate the device when the computer starts (the default), this step does not have
to be performed again.
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7. Establishing a Wireless Connection
7. Establishing a Wireless Connection
Wireless Ethernet devices are becoming increasingly popular. The configuration is similar to the
Ethernet configuration except that it allows you to configure settings such as the SSID and key
for the wireless device.
To add a wireless Ethernet connection, follow these steps:
1.
Click the Devices tab.
2.
Click the New button on the toolbar.
3.
Select Wireless connection from the Device Type list and click Forward.
4.
If you have already added the wireless network interface card to the hardware list, select it
from the Wireless card list. Otherwise, select Other Wireless Card to add the hardware
device.
Note
The installation program usually detects supported wireless Ethernet devices
and prompts you to configure them. If you configured them during the installation, they are displayed in the hardware list on the Hardware tab.
5.
If you selected Other Wireless Card, the Select Ethernet Adapter window appears. Select the manufacturer and model of the Ethernet card and the device. If this is the first Ethernet card for the system, select eth0; if this is the second Ethernet card for the system, select eth1 (and so on). The Network Administration Tool also allows the user to configure
the resources for the wireless network interface card. Click Forward to continue.
6.
On the Configure Wireless Connection page as shown in Figure 14.12, “Wireless Settings”, configure the settings for the wireless device.
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7. Establishing a Wireless Connection
Figure 14.12. Wireless Settings
7.
On the Configure Network Settings page, choose between DHCP and static IP address.
You may specify a hostname for the device. If the device receives a dynamic IP address
each time the network is started, do not specify a hostname. Click Forward to continue.
8.
Click Apply on the Create Wireless Device page.
After configuring the wireless device, it appears in the device list as shown in Figure 14.13,
“Wireless Device”.
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8. Managing DNS Settings
Figure 14.13. Wireless Device
Be sure to select File => Save to save the changes.
After adding the wireless device, you can edit its configuration by selecting the device from the
device list and clicking Edit. For example, you can configure the device to activate at boot time.
When the device is added, it is not activated immediately, as seen by its Inactive status. To activate the device, select it from the device list, and click the Activate button. If the system is
configured to activate the device when the computer starts (the default), this step does not have
to be performed again.
8. Managing DNS Settings
The DNS tab allows you to configure the system's hostname, domain, name servers, and
search domain. Name servers are used to look up other hosts on the network.
If the DNS server names are retrieved from DHCP or PPPoE (or retrieved from the ISP), do not
add primary, secondary, or tertiary DNS servers.
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8. Managing DNS Settings
If the hostname is retrieved dynamically from DHCP or PPPoE (or retrieved from the ISP), do
not change it.
Figure 14.14. DNS Configuration
Note
The name servers section does not configure the system to be a name server. Instead, it configures which name servers to use when resolving IP addresses to
hostnames and vice-versa.
Warning
If the hostname is changed and system-config-network is started on the local host,
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9. Managing Hosts
you may not be able to start another X11 application. As such, you may have to relogin to a new desktop session.
9. Managing Hosts
The Hosts tab allows you to add, edit, or remove hosts from the /etc/hosts file. This file contains IP addresses and their corresponding hostnames.
When your system tries to resolve a hostname to an IP address or tries to determine the hostname for an IP address, it refers to the /etc/hosts file before using the name servers (if you are
using the default Red Hat Enterprise Linux configuration). If the IP address is listed in the /
etc/hosts file, the name servers are not used. If your network contains computers whose IP addresses are not listed in DNS, it is recommended that you add them to the /etc/hosts file.
To add an entry to the /etc/hosts file, go to the Hosts tab, click the New button on the toolbar,
provide the requested information, and click OK. Select File => Save or press Ctrl-S to save
the changes to the /etc/hosts file. The network or network services do not need to be restarted
since the current version of the file is referred to each time an address is resolved.
Warning
Do not remove the localhost entry. Even if the system does not have a network
connection or have a network connection running constantly, some programs need
to connect to the system via the localhost loopback interface.
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10. Working with Profiles
Figure 14.15. Hosts Configuration
Tip
To change lookup order, edit the /etc/host.conf file. The line order hosts, bind
specifies that /etc/hosts takes precedence over the name servers. Changing the
line to order bind, hosts configures the system to resolve hostnames and IP addresses using the name servers first. If the IP address cannot be resolved through
the name servers, the system then looks for the IP address in the /etc/hosts file.
10. Working with Profiles
Multiple logical network devices can be created for each physical hardware device. For example, if you have one Ethernet card in your system (eth0), you can create logical network
devices with different nicknames and different configuration options, all to be specifically associ-
169
10. Working with Profiles
ated with eth0.
Logical network devices are different from device aliases. Logical network devices associated
with the same physical device must exist in different profiles and cannot be activated simultaneously. Device aliases are also associated with the same physical hardware device, but device
aliases associated with the same physical hardware can be activated at the same time. Refer to
Section 11, “Device Aliases” for details about creating device aliases.
Profiles can be used to create multiple configuration sets for different networks. A configuration
set can include logical devices as well as hosts and DNS settings. After configuring the profiles,
you can use the Network Administration Tool to switch back and forth between them.
By default, there is one profile called Common. To create a new profile, select Profile => New
from the pull-down menu, and enter a unique name for the profile.
You are now modifying the new profile as indicated by the status bar at the bottom of the main
window.
Click on an existing device already in the list and click the Copy button to copy the existing
device to a logical network device. If you use the New button, a network alias is created, which
is incorrect. To change the properties of the logical device, select it from the list and click Edit.
For example, the nickname can be changed to a more descriptive name, such as eth0_office,
so that it can be recognized more easily.
In the list of devices, there is a column of checkboxes labeled Profile. For each profile, you can
check or uncheck devices. Only the checked devices are included for the currently selected profile. For example, if you create a logical device named eth0_office in a profile called Office and
want to activate the logical device if the profile is selected, uncheck the eth0 device and check
the eth0_office device.
For example, Figure 14.16, “Office Profile” shows a profile called Office with the logical device
eth0_office. It is configured to activate the first Ethernet card using DHCP.
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10. Working with Profiles
Figure 14.16. Office Profile
Notice that the Home profile as shown in Figure 14.17, “Home Profile” activates the eth0_home
logical device, which is associated with eth0.
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10. Working with Profiles
Figure 14.17. Home Profile
You can also configure eth0 to activate in the Office profile only and to activate a PPP (modem)
device in the Home profile only. Another example is to have the Common profile activate eth0
and an Away profile activate a PPP device for use while traveling.
To activate a profile at boot time, modify the boot loader configuration file to include the netprofile=<profilename> option. For example, if the system uses GRUB as the boot loader and /
boot/grub/grub.conf contains:
title Red Hat Enterprise Linux (2.6.9-5.EL) root (hd0,0) kernel /vmlinuz-2.6.9-5.EL ro root=/dev/VolGroup00
Modify it to the following (where <profilename> is the name of the profile to be activated at boot
time):
title Red Hat Enterprise Linux (2.6.9-5.EL) root (hd0,0) kernel /vmlinuz-2.6.9-5.EL ro root=/dev/VolGroup00
To switch profiles after the system has booted, go to Applications (the main menu on the panel)
=> System Tools => Network Device Control (or type the command system-control-network)
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11. Device Aliases
to select a profile and activate it. The activate profile section only appears in the Network
Device Control interface if more than the default Common interface exists.
Alternatively, execute the following command to enable a profile (replace <profilename> with the
name of the profile):
system-config-network-cmd --profile <profilename> --activate
11. Device Aliases
Device aliases are virtual devices associated with the same physical hardware, but they can be
activated at the same time to have different IP addresses. They are commonly represented as
the device name followed by a colon and a number (for example, eth0:1). They are useful if you
want to have multiple IP addresses for a system that only has one network card.
After configuring the Ethernet device —such as eth0 —to use a static IP address (DHCP does
not work with aliases), go to the Devices tab and click New. Select the Ethernet card to configure with an alias, set the static IP address for the alias, and click Apply to create it. Since a
device already exists for the Ethernet card, the one just created is the alias, such as eth0:1.
Warning
If you are configuring an Ethernet device to have an alias, neither the device nor
the alias can be configured to use DHCP. You must configure the IP addresses
manually.
Figure 14.18, “Network Device Alias Example” shows an example of one alias for the eth0
device. Notice the eth0:1 device — the first alias for eth0. The second alias for eth0 would have
the device name eth0:2, and so on. To modify the settings for the device alias, such as whether
to activate it at boot time and the alias number, select it from the list and click the Edit button.
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12. Saving and Restoring the Network Configuration
Figure 14.18. Network Device Alias Example
Select the alias and click the Activate button to activate the alias. If you have configured multiple profiles, select which profiles in which to include it.
To verify that the alias has been activated, use the command /sbin/ifconfig. The output should
show the device and the device alias with different IP addresses:
eth0 Link encap:Ethernet HWaddr 00:A0:CC:60:B7:G4 inet addr:192.168.100.5 Bcast:192.168.100.255 Mask:255.25
12. Saving and Restoring the Network Configuration
The command line version of Network Administration Tool can be used to save the system's
network configuration to a file. This file can then be used to restore the network settings to a
Red Hat Enterprise Linux system.
This feature can be used as part of an automated backup script, to save the configuration be174
12. Saving and Restoring the Network Configuration
fore upgrading or reinstalling, or to copy the configuration to a different Red Hat Enterprise
Linux system.
To save, or export, the network configuration of a system to the file /tmp/network-config, execute the following command as root:
system-config-network-cmd -e > /tmp/network-config
To restore, or import, the network configuration from the file created from the previous command, execute the following command as root:
system-config-network-cmd -i -c -f /tmp/network-config
The -i option means to import the data, the -c option means to clear the existing configuration
prior to importing, and the -f option specifies that the file to import is as follows.
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Chapter 15. Controlling Access to
Services
Maintaining security on your system is extremely important, and one approach for this task is to
manage access to system services carefully. Your system may need to provide open access to
particular services (for example, httpd if you are running a Web server). However, if you do not
need to provide a service, you should turn it off to minimize your exposure to possible bug exploits.
There are several different methods for managing access to system services. Choose which
method of management to use based on the service, your system's configuration, and your level
of Linux expertise.
The easiest way to deny access to a service is to turn it off. Both the services managed by
xinetd and the services in the /etc/rc.d/init.d hierarchy (also known as SysV services) can
be configured to start or stop using three different applications:
Services Configuration Tool
This is a graphical application that displays a description of each service, displays whether
each service is started at boot time (for runlevels 3, 4, and 5), and allows services to be
started, stopped, and restarted.
ntsysv
This is a text-based application that allows you to configure which services are started at
boot time for each runlevel. Non-xinetd services can not be started, stopped, or restarted
using this program.
chkconfig
This is a command line utility that allows you to turn services on and off for the different runlevels. Non-xinetd services can not be started, stopped, or restarted using this utility.
You may find that these tools are easier to use than the alternatives — editing the numerous
symbolic links located in the directories below /etc/rc.d by hand or editing the xinetd configuration files in /etc/xinetd.d.
Another way to manage access to system services is by using iptables to configure an IP firewall. If you are a new Linux user, note that iptables may not be the best solution for you. Setting up iptables can be complicated, and is best tackled by experienced Linux system administrators.
On the other hand, the benefit of using iptables is flexibility. For example, if you need a customized solution which provides certain hosts access to certain services, iptables can provide it for
you. Refer to Section 8.1, “Netfilter and IPTables” and Section 8.3, “Using IPTables” for more information about iptables.
Alternatively, if you are looking for a utility to set general access rules for your home machine,
and/or if you are new to Linux, try the Security Level Configuration Tool (system-config-selinux), which allows you to select the security level for your system, similar to the Fire-
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1. Runlevels
wall Configuration screen in the installation program.
Refer to Section 8, “Firewalls” for more information.
Important
When you allow access for new services, always remember that both the firewall
and SELinux need to be configured as well. One of the most common mistakes
committed when configuring a new service is neglecting to implement the necessary firewall configuration and SELinux policies to allow access for it. Refer to Section 8.2, “Basic Firewall Configuration” for more information.
1. Runlevels
Before you can configure access to services, you must understand Linux runlevels. A runlevel is
a state, or mode, that is defined by the services listed in the directory /etc/rc.d/rc<x>.d, where
<x> is the number of the runlevel.
The following runlevels exist:
•
0 — Halt
•
1 — Single-user mode
•
2 — Not used (user-definable)
•
3 — Full multi-user mode
•
4 — Not used (user-definable)
•
5 — Full multi-user mode (with an X-based login screen)
•
6 — Reboot
If you use a text login screen, you are operating in runlevel 3. If you use a graphical login
screen, you are operating in runlevel 5.
The default runlevel can be changed by modifying the /etc/inittab file, which contains a line
near the top of the file similar to the following:
id:5:initdefault:
Change the number in this line to the desired runlevel. The change does not take effect until
you reboot the system.
2. TCP Wrappers
Many UNIX system administrators are accustomed to using TCP wrappers to manage access to
certain network services. Any network services managed by xinetd (as well as any program
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2.1. xinetd
with built-in support for libwrap) can use TCP wrappers to manage access. xinetd can use the /
etc/hosts.allow and /etc/hosts.deny files to configure access to system services. As the
names imply, hosts.allow contains a list of rules that allow clients to access the network services controlled by xinetd, and hosts.deny contains rules to deny access. The hosts.allow file
takes precedence over the hosts.deny file. Permissions to grant or deny access can be based
on individual IP address (or hostnames) or on a pattern of clients. Refer to hosts_access in section 5 of the man pages (man 5 hosts_access) for details.
2.1. xinetd
To control access to Internet services, use xinetd, which is a secure replacement for inetd. The
xinetd daemon conserves system resources, provides access control and logging, and can be
used to start special-purpose servers. xinetd can also be used to grant or deny access to particular hosts, provide service access at specific times, limit the rate of incoming connections, limit
the load created by connections, and more.
runs constantly and listens on all ports for the services it manages. When a connection
request arrives for one of its managed services, xinetd starts up the appropriate server for that
service.
xinetd
The configuration file for xinetd is /etc/xinetd.conf, but the file only contains a few defaults
and an instruction to include the /etc/xinetd.d directory. To enable or disable an xinetd service, edit its configuration file in the /etc/xinetd.d directory. If the disable attribute is set to yes,
the service is disabled. If the disable attribute is set to no, the service is enabled. You can edit
any of the xinetd configuration files or change its enabled status using the Services Configuration Tool, ntsysv, or chkconfig. For a list of network services controlled by xinetd, review the
contents of the /etc/xinetd.d directory with the command ls /etc/xinetd.d.
3. Services Configuration Tool
The Services Configuration Tool is a graphical application developed by Red Hat to configure
which SysV services in the /etc/rc.d/init.d directory are started at boot time (for runlevels 3,
4, and 5) and which xinetd services are enabled. It also allows you to start, stop, and restart
SysV services as well as reload xinetd.
To start the Services Configuration Tool from the desktop, go to the Applications (the main
menu on the panel) => System Settings => Server Settings => Services or type the command system-config-services at a shell prompt (for example, in an XTerm or a GNOME terminal).
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3. Services Configuration Tool
Figure 15.1. Services Configuration Tool
The Services Configuration Tool displays the current runlevel as well as the runlevel you are
currently editing. To edit a different runlevel, select Edit Runlevel from the pulldown menu and
select runlevel 3, 4, or 5. Refer to Section 1, “Runlevels” for a description of runlevels.
The Services Configuration Tool lists the services from the /etc/rc.d/init.d directory as well
as the services controlled by xinetd. Click on the name of the service from the list on the lefthand side of the application to display a brief description of that service as well as the status of
the service. If the service is not an xinetd service, the status window shows whether the service
is currently running. If the service is controlled by xinetd, the status window displays the phrase
xinetd service.
To start, stop, or restart a service immediately, select the service from the list and click the appropriate button on the toolbar (or choose the action from the Actions pulldown menu). If the
service is an xinetd service, the action buttons are disabled because they cannot be started or
stopped individually.
If you enable/disable an xinetd service by checking or unchecking the checkbox next to the ser-
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4. ntsysv
vice name, you must select File => Save Changes from the pulldown menu (or the Save button
above the tabs) to reload xinetd and immediately enable/disable the xinetd service that you
changed. xinetd is also configured to remember the setting. You can enable/disable multiple
xinetd services at a time and save the changes when you are finished.
For example, assume you check rsync to enable it in runlevel 3 and then save the changes. The
rsync service is immediately enabled. The next time xinetd is started, rsync is still enabled.
Note
When you save changes to xinetd services, xinetd is reloaded, and the changes
take place immediately. When you save changes to other services, the runlevel is
reconfigured, but the changes do not take effect immediately.
To enable a non-xinetd service to start at boot time for the currently selected runlevel, check
the box beside the name of the service in the list. After configuring the runlevel, apply the
changes by selecting File => Save Changes from the pulldown menu. The runlevel configuration is changed, but the runlevel is not restarted; thus, the changes do not take place immediately.
For example, assume you are configuring runlevel 3. If you change the value for the httpd service from checked to unchecked and then select Save Changes, the runlevel 3 configuration
changes so that httpd is not started at boot time. However, runlevel 3 is not reinitialized, so httpd is still running. Select one of following options at this point:
1.
Stop the httpd service — Stop the service by selecting it from the list and clicking the Stop
button. A message appears stating that the service was stopped successfully.
2.
Reinitialize the runlevel — Reinitialize the runlevel by going to a shell prompt and typing the
command telinit x (where x is the runlevel number; in this example, 3.). This option is recommended if you change the Start at Boot value of multiple services and want to activate
the changes immediately.
3.
Do nothing else — You do not have to stop the httpd service. You can wait until the system
is rebooted for the service to stop. The next time the system is booted, the runlevel is initialized without the httpd service running.
To add a service to a runlevel, select the runlevel from the Edit Runlevel pulldown menu, and
then select Actions => Add Service. To delete a service from a runlevel, select the runlevel
from the Edit Runlevel pulldown menu, select the service to be deleted from the list on the left,
and select Actions => Delete Service.
4. ntsysv
The ntsysv utility provides a simple interface for activating or deactivating services. You can
use ntsysv to turn an xinetd-managed service on or off. You can also use ntsysv to configure
runlevels. By default, only the current runlevel is configured. To configure a different runlevel,
specify one or more runlevels with the --level option. For example, the command ntsysv -
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4. ntsysv
-level 345
configures runlevels 3, 4, and 5.
The ntsysv interface works like the text mode installation program. Use the up and down arrows to navigate up and down the list. The space bar selects/unselects services and is also
used to "press" the Ok and Cancel buttons. To move between the list of services and the Ok
and Cancel buttons, use the Tab key. An asterisk (*) signifies that a service is set to on. Pressing the F1 key displays a short description of the selected service.
Figure 15.2. The ntsysv utility
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5. chkconfig
Warning
Services managed by xinetd are immediately affected by ntsysv. For all other services, changes do not take effect immediately. You must stop or start the individual
service with the command service <daemon> stop (where <daemon> is the name of
the service you want to stop; for example, httpd). Replace stop with start or restart to start or restart the service.
5.
chkconfig
The chkconfig command can also be used to activate and deactivate services. The chkconfig -list command displays a list of system services and whether they are started (on) or stopped
(off) in runlevels 0-6. At the end of the list is a section for the services managed by xinetd.
If the chkconfig --list command is used to query a service managed by xinetd, it displays
whether the xinetd service is enabled (on) or disabled (off). For example, the command chkconfig --list rsync returns the following output:
rsync on
As shown, rsync is enabled as an xinetd service. If xinetd is running, rsync is enabled.
If you use chkconfig --list to query a service in /etc/rc.d, that service's settings for each runlevel are displayed. For example, the command chkconfig --list httpd returns the following
output:
httpd 0:off 1:off 2:on 3:on 4:on 5:on 6:off
can also be used to configure a service to be started (or not) in a specific runlevel.
For example, to turn nscd off in runlevels 3, 4, and 5, use the following command:
chkconfig
chkconfig --level 345 nscd off
Warning
Services managed by xinetd are immediately affected by chkconfig. For example,
if xinetd is running while rsync is disabled, and the command chkconfig rsync on
is executed, then rsync is immediately enabled without having to restart xinetd
manually. Changes for other services do not take effect immediately after using
chkconfig. You must stop or start the individual service with the command service
<daemon> stop (where <daemon> is the name of the service you want to stop; for example, httpd). Replace stop with start or restart to start or restart the service.
6. Additional Resources
182
6.1. Installed Documentation
For more information, refer to the following resources.
6.1. Installed Documentation
•
The man pages for ntsysv, chkconfig, xinetd, and xinetd.conf.
•
man 5 hosts_access
— The man page for the format of host access control files (in section 5
of the man pages).
6.2. Useful Websites
•
http://www.xinetd.org — The xinetd webpage. It contains sample configuration files and a
more detailed list of features.
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Chapter 16. Berkeley Internet Name
Domain (BIND)
On most modern networks, including the Internet, users locate other computers by name. This
frees users from the daunting task of remembering the numerical network address of network
resources. The most effective way to configure a network to allow such name-based connections is to set up a Domain Name Service (DNS) or a nameserver, which resolves hostnames
on the network to numerical addresses and vice versa.
This chapter reviews the nameserver included in Red Hat Enterprise Linux and the Berkeley Internet Name Domain (BIND) DNS server, with an emphasis on the structure of its configuration
files and how it may be administered both locally and remotely.
Note
BIND is also known as the service named in Red Hat Enterprise Linux. You can
manage it via the Services Configuration Tool (system-config-service).
1. Introduction to DNS
DNS associates hostnames with their respective IP addresses, so that when users want to connect to other machines on the network, they can refer to them by name, without having to remember IP addresses.
Use of DNS and FQDNs also has advantages for system administrators, allowing the flexibility
to change the IP address for a host without affecting name-based queries to the machine. Conversely, administrators can shuffle which machines handle a name-based query.
DNS is normally implemented using centralized servers that are authoritative for some domains
and refer to other DNS servers for other domains.
When a client host requests information from a nameserver, it usually connects to port 53. The
nameserver then attempts to resolve the FQDN based on its resolver library, which may contain
authoritative information about the host requested or cached data from an earlier query. If the
nameserver does not already have the answer in its resolver library, it queries other nameservers, called root nameservers, to determine which nameservers are authoritative for the FQDN in
question. Then, with that information, it queries the authoritative nameservers to determine the
IP address of the requested host. If a reverse lookup is performed, the same procedure is used,
except that the query is made with an unknown IP address rather than a name.
1.1. Nameserver Zones
On the Internet, the FQDN of a host can be broken down into different sections. These sections
are organized into a hierarchy (much like a tree), with a main trunk, primary branches, secondary branches, and so forth. Consider the following FQDN:
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1.2. Nameserver Types
bob.sales.example.com
When looking at how an FQDN is resolved to find the IP address that relates to a particular system, read the name from right to left, with each level of the hierarchy divided by periods (.). In
this example, com defines the top level domain for this FQDN. The name example is a subdomain under com, while sales is a sub-domain under example. The name furthest to the left, bob,
identifies a specific machine hostname.
Except for the hostname, each section is called a zone, which defines a specific namespace. A
namespace controls the naming of the sub-domains to its left. While this example only contains
two sub-domains, an FQDN must contain at least one sub-domain but may include many more,
depending upon how the namespace is organized.
Zones are defined on authoritative nameservers through the use of zone files (which describe
the namespace of that zone), the mail servers to be used for a particular domain or sub-domain,
and more. Zone files are stored on primary nameservers (also called master nameservers),
which are truly authoritative and where changes are made to the files, and secondary
nameservers (also called slave nameservers), which receive their zone files from the primary
nameservers. Any nameserver can be a primary and secondary nameserver for different zones
at the same time, and they may also be considered authoritative for multiple zones. It all depends on how the nameserver is configured.
1.2. Nameserver Types
There are four primary nameserver configuration types:
master
Stores original and authoritative zone records for a namespace, and answers queries about
the namespace from other nameservers.
slave
Answers queries from other nameservers concerning namespaces for which it is considered
an authority. However, slave nameservers get their namespace information from master
nameservers.
caching-only
Offers name-to-IP resolution services, but is not authoritative for any zones. Answers for all
resolutions are cached in memory for a fixed period of time, which is specified by the retrieved zone record.
forwarding
Forwards requests to a specific list of nameservers for name resolution. If none of the specified nameservers can perform the resolution, the resolution fails.
A nameserver may be one or more of these types. For example, a nameserver can be a master
for some zones, a slave for others, and only offer forwarding resolutions for others.
1.3. BIND as a Nameserver
BIND performs name resolution services through the /usr/sbin/named daemon. BIND also includes an administration utility called /usr/sbin/rndc. More information about rndc can be found
185
2. /etc/named.conf
in Section 4, “Using rndc”.
BIND stores its configuration files in the following locations:
/etc/named.conf
The configuration file for the named daemon
directory
The named working directory which stores zone, statistic, and cache files
/var/named/
Note
If you have installed the bind-chroot package, the BIND service will run in the /
var/named/chroot environment. All configuration files will be moved there. As such,
named.conf will be located in /var/named/chroot/etc/named.conf, and so on.
Tip
If you have installed the caching-nameserver package, the default configuration file
is /etc/named.caching-nameserver.conf. To override this default configuration, you
can create your own custom configuration file in /etc/named.conf. BIND will use
the /etc/named.conf custom file instead of the default configuration file after you restart.
The next few sections review the BIND configuration files in more detail.
2.
/etc/named.conf
The named.conf file is a collection of statements using nested options surrounded by opening
and closing ellipse characters, { }. Administrators must be careful when editing named.conf to
avoid syntax errors as many seemingly minor errors prevent the named service from starting.
A typical named.conf file is organized similar to the following example:
<statement-1> ["<statement-1-name>"] [<statement-1-class>] { <option-1>; <option-2>; <option-N>; }; <statem
2.1. Common Statement Types
The following types of statements are commonly used in /etc/named.conf:
2.1.1. acl Statement
The acl statement (or access control statement) defines groups of hosts which can then be permitted or denied access to the nameserver.
An acl statement takes the following form:
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2.1. Common Statement Types
acl <acl-name> { <match-element>; [<match-element>; ...] };
In this statement, replace <acl-name> with the name of the access control list and replace
<match-element> with a semi-colon separated list of IP addresses. Most of the time, an individual
IP address or IP network notation (such as 10.0.1.0/24) is used to identify the IP addresses
within the acl statement.
The following access control lists are already defined as keywords to simplify configuration:
•
any
— Matches every IP address
•
localhost
— Matches any IP address in use by the local system
•
localnets
— Matches any IP address on any network to which the local system is connected
•
none
— Matches no IP addresses
When used in conjunction with other statements (such as the options statement), acl statements can be very useful in preventing the misuse of a BIND nameserver.
The following example defines two access control lists and uses an options statement to define
how they are treated by the nameserver:
acl black-hats { 10.0.2.0/24; 192.168.0.0/24; }; acl red-hats { 10.0.1.0/24; }; options { blackhole { black
This example contains two access control lists, black-hats and red-hats. Hosts in the blackhats list are denied access to the nameserver, while hosts in the red-hats list are given normal
access.
2.1.2. include Statement
The include statement allows files to be included in a named.conf file. In this way, sensitive configuration data (such as keys) can be placed in a separate file with restrictive permissions.
An include statement takes the following form:
include "<file-name>"
In this statement, <file-name> is replaced with an absolute path to a file.
2.1.3. options Statement
The options statement defines global server configuration options and sets defaults for other
statements. It can be used to specify the location of the named working directory, the types of
queries allowed, and much more.
The options statement takes the following form:
options { <option>; [<option>; ...] };
In this statement, the <option> directives are replaced with a valid option.
The following are commonly used options:
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2.1. Common Statement Types
allow-query
Specifies which hosts are allowed to query this nameserver. By default, all hosts are allowed to query. An access control list, or collection of IP addresses or networks, may be
used here to allow only particular hosts to query the nameserver.
allow-recursion
Similar to allow-query, this option applies to recursive queries. By default, all hosts are allowed to perform recursive queries on the nameserver.
blackhole
Specifies which hosts are not allowed to query the server.
directory
Specifies the named working directory if different from the default value, /var/named/.
forwarders
Specifies a list of valid IP addresses for nameservers where requests should be forwarded
for resolution.
forward
Specifies the forwarding behavior of a forwarders directive.
The following options are accepted:
•
first
— Specifies that the nameservers listed in the forwarders directive be queried before named attempts to resolve the name itself.
•
only
— Specifies that named does not attempt name resolution itself in the event that
queries to nameservers specified in the forwarders directive fail.
listen-on
Specifies the network interface on which named listens for queries. By default, all interfaces
are used.
Using this directive on a DNS server which also acts a gateway, BIND can be configured to
only answer queries that originate from one of the networks.
The following is an example of a listen-on directive:
options { listen-on { 10.0.1.1; }; };
In this example, only requests that arrive from the network interface serving the private network (10.0.1.1) are accepted.
notify
Controls whether named notifies the slave servers when a zone is updated. It accepts the following options:
•
yes
•
no
— Notifies slave servers.
— Does not notify slave servers.
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2.1. Common Statement Types
•
— Only notifies slave servers specified in an also-notify list within a zone
statement.
explicit
pid-file
Specifies the location of the process ID file created by named.
root-delegation-only
Turns on the enforcement of delegation properties in top-level domains (TLDs) and root
zones with an optional exclude list. Delegation is the process of dividing a single zone into
multiple subzones. In order to create a delegated zone, items known as NS records are
used. NameServer records (delegation records) announce the authoritative nameservers for
a particular zone.
The following root-delegation-only example specifies an exclude list of TLDs from whom
undelegated responses are expected and trusted:
options { root-delegation-only exclude { "ad"; "ar"; "biz"; "cr"; "cu"; "de"; "dm"; "id"; "lu"; "lv"; "
statistics-file
Specifies an alternate location for statistics files. By default, named statistics are saved to the
/var/named/named.stats file.
There are several other options also available, many of which rely upon one another to work
properly. Refer to the BIND 9 Administrator Reference Manual referenced in Section 7.1,
“Installed Documentation” and the bind.conf man page for more details.
2.1.4. zone Statement
A zone statement defines the characteristics of a zone, such as the location of its configuration
file and zone-specific options. This statement can be used to override the global options statements.
A zone statement takes the following form:
zone <zone-name><zone-class> { <zone-options>; [<zone-options>; ...] };
In this statement, <zone-name> is the name of the zone, <zone-class> is the optional class of the
zone, and <zone-options> is a list of options characterizing the zone.
The <zone-name> attribute for the zone statement is particularly important. It is the default value
assigned for the $ORIGIN directive used within the corresponding zone file located in the /
var/named/ directory. The named daemon appends the name of the zone to any non-fully qualified domain name listed in the zone file.
Note
If you have installed the caching-nameserver package, the default configuration file
will be in /etc/named.rfc1912.zones.
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2.1. Common Statement Types
For example, if a zone statement defines the namespace for example.com, use example.com as
the <zone-name> so it is placed at the end of hostnames within the example.com zone file.
For more information about zone files, refer to Section 3, “Zone Files”.
The most common zone statement options include the following:
allow-query
Specifies the clients that are allowed to request information about this zone. The default is
to allow all query requests.
allow-transfer
Specifies the slave servers that are allowed to request a transfer of the zone's information.
The default is to allow all transfer requests.
allow-update
Specifies the hosts that are allowed to dynamically update information in their zone. The default is to deny all dynamic update requests.
Be careful when allowing hosts to update information about their zone. Do not enable this
option unless the host specified is completely trusted. In general, it is better to have an administrator manually update the records for a zone and reload the named service.
file
Specifies the name of the file in the named working directory that contains the zone's configuration data.
masters
Specifies the IP addresses from which to request authoritative zone information and is used
only if the zone is defined as typeslave.
notify
Specifies whether or not named notifies the slave servers when a zone is updated. This directive accepts the following options:
•
yes
— Notifies slave servers.
•
no
•
explicit
— Does not notify slave servers.
— Only notifies slave servers specified in an also-notify list within a zone
statement.
type
Defines the type of zone.
Below is a list of valid options:
•
— Enforces the delegation status of infrastructure zones such as COM,
NET, or ORG. Any answer that is received without an explicit or implicit delegation is
treated as NXDOMAIN. This option is only applicable in TLDs or root zone files used in recursive or caching implementations.
delegation-only
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2.2. Other Statement Types
•
forward
•
hint
•
master
•
slave
— Forwards all requests for information about this zone to other nameservers.
— A special type of zone used to point to the root nameservers which resolve queries when a zone is not otherwise known. No configuration beyond the default is necessary with a hint zone.
— Designates the nameserver as authoritative for this zone. A zone should be
set as the master if the zone's configuration files reside on the system.
— Designates the nameserver as a slave server for this zone. Also specifies the
IP address of the master nameserver for the zone.
zone-statistics
Configures named to keep statistics concerning this zone, writing them to either the default
location (/var/named/named.stats) or the file listed in the statistics-file option in the server statement. Refer to Section 2.2, “Other Statement Types” for more information about the
server statement.
2.1.5. Sample zone Statements
Most changes to the /etc/named.conf file of a master or slave nameserver involves adding,
modifying, or deleting zone statements. While these zone statements can contain many options,
most nameservers require only a small subset to function efficiently. The following zone statements are very basic examples illustrating a master-slave nameserver relationship.
The following is an example of a zone statement for the primary nameserver hosting example.com
(192.168.0.1):
zone "example.com" IN { type master; file "example.com.zone"; allow-update { none; }; };
In the statement, the zone is identified as example.com, the type is set to master, and the named
service is instructed to read the /var/named/example.com.zone file. It also tells named not to allow
any other hosts to update.
A slave server's zone statement for example.com is slightly different from the previous example.
For a slave server, the type is set to slave and in place of the allow-update line is a directive
telling named the IP address of the master server.
The following is an example slave server zone statement for example.com zone:
zone "example.com" { type slave; file "example.com.zone"; masters { 192.168.0.1; }; };
This zone statement configures named on the slave server to query the master server at the
192.168.0.1 IP address for information about the example.com zone. The information that the
slave server receives from the master server is saved to the /var/named/example.com.zone file.
2.2. Other Statement Types
The following is a list of lesser used statement types available within named.conf:
controls
Configures various security requirements necessary to use the rndc command to administer
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2.2. Other Statement Types
the named service.
Refer to Section 4.1, “Configuring /etc/named.conf” to learn more about how the controls
statement is structured and what options are available.
key "<key-name>"
Defines a particular key by name. Keys are used to authenticate various actions, such as
secure updates or the use of the rndc command. Two options are used with key:
•
algorithm <algorithm-name>
•
secret "<key-value>"
— The type of algorithm used, such as dsa or hmac-md5.
— The encrypted key.
Refer to Section 4.2, “Configuring /etc/rndc.conf” for instructions on how to write a key statement.
logging
Allows for the use of multiple types of logs, called channels. By using the channel option
within the logging statement, a customized type of log can be constructed — with its own
file name (file), size limit (size), versioning (version), and level of importance (severity).
Once a customized channel is defined, a category option is used to categorize the channel
and begin logging when named is restarted.
By default, named logs standard messages to the syslog daemon, which places them in /
var/log/messages. This occurs because several standard channels are built into BIND with
various severity levels, such as default_syslog (which handles informational logging messages) and default_debug (which specifically handles debugging messages). A default category, called default, uses the built-in channels to do normal logging without any special
configuration.
Customizing the logging process can be a very detailed process and is beyond the scope of
this chapter. For information on creating custom BIND logs, refer to the BIND 9 Administrator Reference Manual referenced in Section 7.1, “Installed Documentation”.
server
Specifies options that affect how named should respond to remote nameservers, especially
with regard to notifications and zone transfers.
The transfer-format option controls whether one resource record is sent with each message (one-answer) or multiple resource records are sent with each message (many-answers).
While many-answers is more efficient, only newer BIND nameservers understand it.
trusted-keys
Contains assorted public keys used for secure DNS (DNSSEC). Refer to Section 5.3,
“Security” for more information concerning BIND security.
view "<view-name>"
Creates special views depending upon which network the host querying the nameserver is
on. This allows some hosts to receive one answer regarding a zone while other hosts receive totally different information. Alternatively, certain zones may only be made available to
particular trusted hosts while non-trusted hosts can only make queries for other zones.
Multiple views may be used, but their names must be unique. The match-clients option
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2.3. Comment Tags
specifies the IP addresses that apply to a particular view. Any options statement may also
be used within a view, overriding the global options already configured for named. Most view
statements contain multiple zone statements that apply to the match-clients list. The order
in which view statements are listed is important, as the first view statement that matches a
particular client's IP address is used.
Refer to Section 5.2, “Multiple Views” for more information about the view statement.
2.3. Comment Tags
The following is a list of valid comment tags used within named.conf:
•
//
•
#
•
/*
— When placed at the beginning of a line, that line is ignored by named.
— When placed at the beginning of a line, that line is ignored by named.
and */ — When text is enclosed in these tags, the block of text is ignored by named.
3. Zone Files
Zone files contain information about a namespace and are stored in the named working directory
(/var/named/) by default. Each zone file is named according to the file option data in the zone
statement, usually in a way that relates to the domain in question and identifies the file as containing zone data, such as example.com.zone.
Note
If you have installed the bind-chroot package, the BIND service will run in the /
var/named/chroot environment. All configuration files will be moved there. As such,
you can find the zone files in /var/named/chroot/var/named.
Each zone file may contain directives and resource records. Directives tell the nameserver to
perform tasks or apply special settings to the zone. Resource records define the parameters of
the zone and assign identities to individual hosts. Directives are optional, but resource records
are required to provide name service to a zone.
All directives and resource records should be entered on individual lines.
Comments can be placed after semicolon characters (;) in zone files.
3.1. Zone File Directives
Directives begin with the dollar sign character ($) followed by the name of the directive. They
usually appear at the top of the zone file.
The following are commonly used directives:
$INCLUDE
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3.2. Zone File Resource Records
Configures named to include another zone file in this zone file at the place where the directive appears. This allows additional zone settings to be stored apart from the main zone file.
$ORIGIN
Appends the domain name to unqualified records, such as those with the hostname and
nothing more.
For example, a zone file may contain the following line:
$ORIGIN example.com.
Any names used in resource records that do not end in a trailing period (.) are appended
with example.com.
Note
The use of the $ORIGIN directive is unnecessary if the zone is specified in /
etc/named.conf because the zone name is used as the value for the $ORIGIN
directive by default.
$TTL
Sets the default Time to Live (TTL) value for the zone. This is the length of time, in seconds,
that a zone resource record is valid. Each resource record can contain its own TTL value,
which overrides this directive.
Increasing this value allows remote nameservers to cache the zone information for a longer
period of time, reducing the number of queries for the zone and lengthening the amount of
time required to proliferate resource record changes.
3.2. Zone File Resource Records
The primary component of a zone file is its resource records.
There are many types of zone file resource records. The following are used most frequently:
A
This refers to the Address record, which specifies an IP address to assign to a name, as in
this example:
<host> IN A <IP-address>
If the <host> value is omitted, then an A record points to a default IP address for the top of
the namespace. This system is the target for all non-FQDN requests.
Consider the following A record examples for the example.com zone file:
server1 IN A 10.0.1.3 IN A 10.0.1.5
Requests for example.com are pointed to 10.0.1.3 or 10.0.1.5.
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3.2. Zone File Resource Records
CNAME
This refers to the Canonical Name record, which maps one name to another. This type of
record can also be referred to as an alias record.
The next example tells named that any requests sent to the <alias-name> should point to the
host, <real-name>. CNAME records are most commonly used to point to services that use a
common naming scheme, such as www for Web servers.
<alias-name> IN CNAME <real-name>
In the following example, an A record binds a hostname to an IP address, while a CNAME record points the commonly used www hostname to it.
server1 IN A 10.0.1.5 www IN CNAME server1
MX
This refers to the Mail eXchange record, which tells where mail sent to a particular
namespace controlled by this zone should go.
IN MX <preference-value><email-server-name>
Here, the <preference-value> allows numerical ranking of the email servers for a
namespace, giving preference to some email systems over others. The MX resource record
with the lowest <preference-value> is preferred over the others. However, multiple email
servers can possess the same value to distribute email traffic evenly among them.
The <email-server-name> may be a hostname or FQDN.
IN MX 10 mail.example.com. IN MX 20 mail2.example.com.
In this example, the first mail.example.com email server is preferred to the
mail2.example.com email server when receiving email destined for the example.com domain.
NS
This refers to the NameServer record, which announces the authoritative nameservers for a
particular zone.
The following illustrates the layout of an NS record:
IN NS <nameserver-name>
Here, <nameserver-name> should be an FQDN.
Next, two nameservers are listed as authoritative for the domain. It is not important whether
these nameservers are slaves or if one is a master; they are both still considered authoritative.
IN NS dns1.example.com. IN NS dns2.example.com.
PTR
This refers to the PoinTeR record, which is designed to point to another part of the
namespace.
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3.2. Zone File Resource Records
records are primarily used for reverse name resolution, as they point IP addresses back
to a particular name. Refer to Section 3.4, “Reverse Name Resolution Zone Files” for more
examples of PTR records in use.
PTR
SOA
This refers to the Start Of Authority resource record, which proclaims important authoritative
information about a namespace to the nameserver.
Located after the directives, an SOA resource record is the first resource record in a zone file.
The following shows the basic structure of an SOA resource record:
@ IN SOA <primary-name-server><hostmaster-email> ( <serial-number><time-to-refresh><time-to-retry><time
The @ symbol places the $ORIGIN directive (or the zone's name, if the $ORIGIN directive is not
set) as the namespace being defined by this SOA resource record. The hostname of the
primary nameserver that is authoritative for this domain is the <primary-name-server> directive, and the email of the person to contact about this namespace is the <hostmaster-email>
directive.
The <serial-number> directive is a numerical value incremented every time the zone file is
altered to indicate it is time for named to reload the zone. The <time-to-refresh> directive is
the numerical value slave servers use to determine how long to wait before asking the master nameserver if any changes have been made to the zone. The <serial-number> directive
is a numerical value used by the slave servers to determine if it is using outdated zone data
and should therefore refresh it.
The <time-to-retry> directive is a numerical value used by slave servers to determine the
length of time to wait before issuing a refresh request in the event that the master
nameserver is not answering. If the master has not replied to a refresh request before the
amount of time specified in the <time-to-expire> directive elapses, the slave servers stop
responding as an authority for requests concerning that namespace.
The <minimum-TTL> directive is the amount of time other nameservers cache the zone's information.
When configuring BIND, all times are specified in seconds. However, it is possible to use
abbreviations when specifying units of time other than seconds, such as minutes (M), hours
(H), days (D), and weeks (W). The table in Table 16.1, “Seconds compared to other time
units” shows an amount of time in seconds and the equivalent time in another format.
Seconds
Other Time Units
60
1M
1800
30M
3600
1H
10800
3H
21600
6H
43200
12H
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3.3. Example Zone File
Seconds
Other Time Units
86400
1D
259200
3D
604800
1W
31536000
365D
Table 16.1. Seconds compared to other time units
The following example illustrates the form an SOA resource record might take when it is populated with real values.
@ IN SOA dns1.example.com. hostmaster.example.com. ( 2001062501 ; serial 21600 ; refresh after 6 hours
3.3. Example Zone File
Seen individually, directives and resource records can be difficult to grasp. However, when
placed together in a single file, they become easier to understand.
The following example shows a very basic zone file.
$ORIGIN example.com. $TTL 86400 @ IN SOA dns1.example.com. hostmaster.example.com. ( 2001062501 ; serial 21
In this example, standard directives and SOA values are used. The authoritative nameservers are
set as dns1.example.com and dns2.example.com, which have A records that tie them to 10.0.1.1
and 10.0.1.2, respectively.
The email servers configured with the MX records point to server1 and server2 via CNAME records.
Since the server1 and server2 names do not end in a trailing period (.), the $ORIGIN domain is
placed after them, expanding them to server1.example.com and server2.example.com. Through
the related A resource records, their IP addresses can be determined.
FTP and Web services, available at the standard ftp.example.com and www.example.com names,
are pointed at the appropriate servers using CNAME records.
This zone file would be called into service with a zone statement in the named.conf similar to the
following:
zone "example.com" IN { type master; file "example.com.zone"; allow-update { none; }; };
3.4. Reverse Name Resolution Zone Files
A reverse name resolution zone file is used to translate an IP address in a particular namespace
into an FQDN. It looks very similar to a standard zone file, except that PTR resource records are
used to link the IP addresses to a fully qualified domain name.
The following illustrates the layout of a PTR record:
<last-IP-digit> IN PTR <FQDN-of-system>
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4. Using rndc
The <last-IP-digit> is the last number in an IP address which points to a particular system's
FQDN.
In the following example, IP addresses 10.0.1.1 through 10.0.1.6 are pointed to corresponding
FQDNs. It can be located in /var/named/example.com.rr.zone.
$ORIGIN 1.0.10.in-addr.arpa. $TTL 86400 @ IN SOA dns1.example.com. hostmaster.example.com. ( 2001062501 ; s
This zone file would be called into service with a zone statement in the named.conf file similar to
the following:
zone "1.0.10.in-addr.arpa" IN { type master; file "example.com.rr.zone"; allow-update { none; }; };
There is very little difference between this example and a standard zone statement, except for
the zone name. Note that a reverse name resolution zone requires the first three blocks of the
IP address reversed followed by .in-addr.arpa. This allows the single block of IP numbers used
in the reverse name resolution zone file to be associated with the zone.
4. Using
rndc
BIND includes a utility called rndc which allows command line administration of the named daemon from the localhost or a remote host.
In order to prevent unauthorized access to the named daemon, BIND uses a shared secret key
authentication method to grant privileges to hosts. This means an identical key must be present
in both /etc/named.conf and the rndc configuration file, /etc/rndc.conf.
Note
If you have installed the bind-chroot package, the BIND service will run in the /
var/named/chroot environment. All configuration files will be moved there. As such,
the rndc.conf file is located in /var/named/chroot/etc/rndc.conf.
Note that since the rndc utility does not run in a chroot environment, /
etc/rndc.conf is a symlink to /var/named/chroot/etc/rndc.conf.
4.1. Configuring /etc/named.conf
In order for rndc to connect to a named service, there must be a controls statement in the BIND
server's /etc/named.conf file.
The controls statement, shown in the following example, allows rndc to connect from the localhost.
controls { inet 127.0.0.1 allow { localhost; } keys { <key-name>; }; };
This statement tells named to listen on the default TCP port 953 of the loopback address and allow rndc commands coming from the localhost, if the proper key is given. The <key-name> specifies a name in the key statement within the /etc/named.conf file. The next example illustrates a
198
4.2. Configuring /etc/rndc.conf
sample key statement.
key "<key-name>" { algorithm hmac-md5; secret "<key-value>"; };
In this case, the <key-value> uses the HMAC-MD5 algorithm. Use the following command to
generate keys using the HMAC-MD5 algorithm:
dnssec-keygen -a hmac-md5 -b <bit-length> -n HOST <key-file-name>
A key with at least a 256-bit length is a good idea. The actual key that should be placed in the
<key-value> area can be found in the <key-file-name> file generated by this command.
Warning
Because /etc/named.conf is world-readable, it is advisable to place the key statement in a separate file, readable only by root, and then use an include statement
to reference it. For example:
include "/etc/rndc.key";
4.2. Configuring /etc/rndc.conf
The key is the most important statement in /etc/rndc.conf.
key "<key-name>" { algorithm hmac-md5; secret "<key-value>"; };
The <key-name> and <key-value> should be exactly the same as their settings in
/etc/named.conf.
To match the keys specified in the target server's /etc/named.conf, add the following lines to /
etc/rndc.conf.
options { default-server localhost; default-key "<key-name>"; };
This directive sets a global default key. However, the rndc configuration file can also specify different keys for different servers, as in the following example:
server localhost { key "<key-name>"; };
Important
Make sure that only the root user can read or write to the /etc/rndc.conf file.
For more information about the /etc/rndc.conf file, refer to the rndc.conf man page.
4.3. Command Line Options
199
5. Advanced Features of BIND
An rndc command takes the following form:
rndc <options><command><command-options>
When executing rndc on a properly configured localhost, the following commands are available:
•
halt
— Stops the named service immediately.
•
querylog
•
refresh
•
reload
— Logs all queries made to this nameserver.
— Refreshes the nameserver's database.
— Reloads the zone files but keeps all other previously cached responses. This command also allows changes to zone files without losing all stored name resolutions.
If changes made only affect a specific zone, reload only that specific zone by adding the
name of the zone after the reload command.
•
stats
•
stop
— Dumps the current named statistics to the /var/named/named.stats file.
— Stops the server gracefully, saving any dynamic update and Incremental Zone
Transfers (IXFR) data before exiting.
Occasionally, it may be necessary to override the default settings in the /etc/rndc.conf file. The
following options are available:
•
-c <configuration-file>
•
-p <port-number>
— Specifies the alternate location of a configuration file.
— Specifies a port number to use for the rndc connection other than the
default port 953.
•
-s <server>
— Specifies a server other than the default-server listed in /etc/rndc.conf.
•
-y <key-name>
— Specifies a key other than the default-key option in /etc/rndc.conf.
Additional information about these options can be found in the rndc man page.
5. Advanced Features of BIND
Most BIND implementations only use named to provide name resolution services or to act as an
authority for a particular domain or sub-domain. However, BIND version 9 has a number of advanced features that allow for a more secure and efficient DNS service.
Caution
Some of these advanced features, such as DNSSEC, TSIG, and IXFR (which are
defined in the following section), should only be used in network environments with
nameservers that support the features. If the network environment includes nonBIND or older BIND nameservers, verify that each advanced feature is supported
before attempting to use it.
200
5.1. DNS Protocol Enhancements
All of the features mentioned are discussed in greater detail in the BIND 9 Administrator Reference Manual referenced in Section 7.1, “Installed Documentation”.
5.1. DNS Protocol Enhancements
BIND supports Incremental Zone Transfers (IXFR), where a slave nameserver only downloads
the updated portions of a zone modified on a master nameserver. The standard transfer process requires that the entire zone be transferred to each slave nameserver for even the smallest
change. For very popular domains with very lengthy zone files and many slave nameservers,
IXFR makes the notification and update process much less resource-intensive.
Note that IXFR is only available when using dynamic updating to make changes to master zone
records. If manually editing zone files to make changes, Automatic Zone Transfer (AXFR) is
used. More information on dynamic updating is available in the BIND 9 Administrator Reference
Manual referenced in Section 7.1, “Installed Documentation”.
5.2. Multiple Views
Through the use of the view statement in named.conf, BIND can present different information depending on which network a request originates from.
This is primarily used to deny sensitive DNS entries from clients outside of the local network,
while allowing queries from clients inside the local network.
The view statement uses the match-clients option to match IP addresses or entire networks
and give them special options and zone data.
5.3. Security
BIND supports a number of different methods to protect the updating and transfer of zones, on
both master and slave nameservers:
DNSSEC
Short for DNS SECurity, this feature allows for zones to be cryptographically signed with a
zone key.
In this way, the information about a specific zone can be verified as coming from a
nameserver that has signed it with a particular private key, as long as the recipient has that
nameserver's public key.
BIND version 9 also supports the SIG(0) public/private key method of message authentication.
TSIG
Short for Transaction SIGnatures, this feature allows a transfer from master to slave only
after verifying that a shared secret key exists on both nameservers.
This feature strengthens the standard IP address-based method of transfer authorization.
An attacker would not only need to have access to the IP address to transfer the zone, but
they would also need to know the secret key.
201
5.4. IP version 6
BIND version 9 also supports TKEY, which is another shared secret key method of authorizing zone transfers.
5.4. IP version 6
BIND version 9 supports name service in IP version 6 (IPv6) environments through the use of A6
zone records.
If the network environment includes both IPv4 and IPv6 hosts, use the lwresd lightweight resolver daemon on all network clients. This daemon is a very efficient, caching-only nameserver
which understands the new A6 and DNAME records used under IPv6. Refer to the lwresd man
page for more information.
6. Common Mistakes to Avoid
It is very common for beginners to make mistakes when editing BIND configuration files. Be
sure to avoid the following issues:
•
Take care to increment the serial number when editing a zone file.
If the serial number is not incremented, the master nameserver has the correct, new information, but the slave nameservers are never notified of the change and do not attempt to refresh their data of that zone.
•
Be careful to use ellipses and semi-colons correctly in the /etc/named.conf file.
An omitted semi-colon or unclosed ellipse section can cause named to refuse to start.
•
Remember to place periods (.) in zone files after all FQDNs and omit them on hostnames.
A period at the end of a domain name denotes a fully qualified domain name. If the period is
omitted, then named appends the name of the zone or the $ORIGIN value to complete it.
•
If a firewall is blocking connections from the named program to other nameservers, edit its
configuration file.
By default, BIND version 9 uses random ports above 1024 to query other nameservers.
Some firewalls, however, expect all nameservers to communicate using only port 53. To
force named to use port 53, add the following line to the options statement of
/etc/named.conf:
query-source address * port 53;
7. Additional Resources
The following sources of information provide additional resources regarding BIND.
7.1. Installed Documentation
BIND features a full range of installed documentation covering many different topics, each
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7.2. Useful Websites
placed in its own subject directory. For each item below, replace <version-number> with the version of bind installed on the system:
/usr/share/doc/bind-<version-number>/
This directory lists the most recent features.
/usr/share/doc/bind-<version-number>/arm/
This directory contains the BIND 9 Administrator Reference Manual in HTML and SGML
formats, which details BIND resource requirements, how to configure different types of
nameservers, how to perform load balancing, and other advanced topics. For most new
users of BIND, this is the best place to start.
/usr/share/doc/bind-<version-number>/draft/
This directory contains assorted technical documents that review issues related to DNS service and propose some methods to address them.
/usr/share/doc/bind-<version-number>/misc/
This directory contains documents designed to address specific advanced issues. Users of
BIND version 8 should consult the migration document for specific changes they must
make when moving to BIND 9. The options file lists all of the options implemented in BIND
9 that are used in /etc/named.conf.
/usr/share/doc/bind-<version-number>/rfc/
This directory provides every RFC document related to BIND.
There are also a number of man pages for the various applications and configuration files involved with BIND. The following lists some of the more important man pages.
Administrative Applications
•
— Explains the different options available when using the rndc command to
control a BIND nameserver.
man rndc
Server Applications
•
man named
— Explores assorted arguments that can be used to control the BIND
nameserver daemon.
•
man lwresd
— Describes the purpose of and options available for the lightweight resolver
daemon.
Configuration Files
•
man named.conf
— A comprehensive list of options available within the named configura-
tion file.
•
man rndc.conf
— A comprehensive list of options available within the rndc configuration
file.
7.2. Useful Websites
203
7.3. Related Books
•
http://www.isc.org/index.pl?/sw/bind/ — The home page of the BIND project containing information about current releases as well as a PDF version of the BIND 9 Administrator Reference Manual.
•
http://www.redhat.com/mirrors/LDP/HOWTO/DNS-HOWTO.html — Covers the use of BIND
as a resolving, caching nameserver and the configuration of various zone files necessary to
serve as the primary nameserver for a domain.
7.3. Related Books
•
DNS and BIND by Paul Albitz and Cricket Liu; O'Reilly & Associates — A popular reference
that explains both common and esoteric BIND configuration options, as well as providing
strategies for securing a DNS server.
•
The Concise Guide to DNS and BIND by Nicolai Langfeldt; Que — Looks at the connection
between multiple network services and BIND, with an emphasis on task-oriented, technical
topics.
204
Chapter 17. OpenSSH
SSH™ (or Secure SHell) is a protocol which facilitates secure communications between two
systems using a client/server architecture and allows users to log into server host systems remotely. Unlike other remote communication protocols, such as FTP or Telnet, SSH encrypts the
login session, rendering the connection difficult for intruders to collect unencrypted passwords.
SSH is designed to replace older, less secure terminal applications used to log into remote
hosts, such as telnet or rsh. A related program called scp replaces older programs designed to
copy files between hosts, such as rcp. Because these older applications do not encrypt passwords transmitted between the client and the server, avoid them whenever possible. Using secure methods to log into remote systems decreases the risks for both the client system and the
remote host.
1. Features of SSH
The SSH protocol provides the following safeguards:
•
After an initial connection, the client can verify that it is connecting to the same server it had
connected to previously.
•
The client transmits its authentication information to the server using strong, 128-bit encryption.
•
All data sent and received during a session is transferred using 128-bit encryption, making
intercepted transmissions extremely difficult to decrypt and read.
•
The client can forward X115 applications from the server. This technique, called X11 forwarding, provides a secure means to use graphical applications over a network.
Because the SSH protocol encrypts everything it sends and receives, it can be used to secure
otherwise insecure protocols. Using a technique called port forwarding, an SSH server can become a conduit to securing otherwise insecure protocols, like POP, and increasing overall system and data security.
The OpenSSH server and client can also be configured to create a tunnel similar to a virtual
private network for traffic between server and client machines.
Finally, OpenSSH servers and clients can be configured to authenticate using the GSSAPI implementation of the Kerberos network authentication protocol. For more information on configuring Kerberos authentication services, refer to Section 6, “Kerberos”.
Red Hat Enterprise Linux includes the general OpenSSH package (openssh) as well as the
OpenSSH server (openssh-server) and client (openssh-clients) packages. Note, the OpenSSH
packages require the OpenSSL package (openssl) which installs several important cryptographic libraries, enabling OpenSSH to provide encrypted communications.
1.1. Why Use SSH?
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205
2. SSH Protocol Versions
cept, and re-route network traffic in an effort to gain access to a system. In general terms, these
threats can be categorized as follows:
•
Interception of communication between two systems — In this scenario, the attacker can be
somewhere on the network between the communicating parties, copying any information
passed between them. The attacker may intercept and keep the information, or alter the information and send it on to the intended recipient.
This attack can be mounted through the use of a packet sniffer — a common network utility.
•
Impersonation of a particular host — Using this strategy, an attacker's system is configured
to pose as the intended recipient of a transmission. If this strategy works, the user's system
remains unaware that it is communicating with the wrong host.
This attack can be mounted through techniques known as DNS poisoning6 or IP spoofing7.
Both techniques intercept potentially sensitive information and, if the interception is made for
hostile reasons, the results can be disastrous.
If SSH is used for remote shell login and file copying, these security threats can be greatly diminished. This is because the SSH client and server use digital signatures to verify their identity.
Additionally, all communication between the client and server systems is encrypted. Attempts to
spoof the identity of either side of a communication does not work, since each packet is encrypted using a key known only by the local and remote systems.
2. SSH Protocol Versions
The SSH protocol allows any client and server programs built to the protocol's specifications to
communicate securely and to be used interchangeably.
Two varieties of SSH (version 1 and version 2) currently exist. The OpenSSH suite under Red
Hat Enterprise Linux uses SSH version 2 which has an enhanced key exchange algorithm not
vulnerable to the exploit in version 1. However, the OpenSSH suite does support version 1 connections.
Important
It is recommended that only SSH version 2-compatible servers and clients are
used whenever possible.
3. Event Sequence of an SSH Connection
The following series of events help protect the integrity of SSH communication between two
hosts.
6
DNS poisoning occurs when an intruder cracks a DNS server, pointing client systems to a maliciously duplicated host.
IP spoofing occurs when an intruder sends network packets which falsely appear to be from a trusted host on the network.
7
206
3.1. Transport Layer
1.
A cryptographic handshake is made so that the client can verify that it is communicating
with the correct server.
2.
The transport layer of the connection between the client and remote host is encrypted using
a symmetric cipher.
3.
The client authenticates itself to the server.
4.
The remote client interacts with the remote host over the encrypted connection.
3.1. Transport Layer
The primary role of the transport layer is to facilitate safe and secure communication between
the two hosts at the time of authentication and during subsequent communication. The transport
layer accomplishes this by handling the encryption and decryption of data, and by providing integrity protection of data packets as they are sent and received. The transport layer also
provides compression, speeding the transfer of information.
Once an SSH client contacts a server, key information is exchanged so that the two systems
can correctly construct the transport layer. The following steps occur during this exchange:
•
Keys are exchanged
•
The public key encryption algorithm is determined
•
The symmetric encryption algorithm is determined
•
The message authentication algorithm is determined
•
The hash algorithm is determined
During the key exchange, the server identifies itself to the client with a unique host key. If the
client has never communicated with this particular server before, the server's host key is unknown to the client and it does not connect. OpenSSH gets around this problem by accepting
the server's host key. This is done after the user is notified and has both accepted and verified
the new host key. In subsequent connections, the server's host key is checked against the
saved version on the client, providing confidence that the client is indeed communicating with
the intended server. If, in the future, the host key no longer matches, the user must remove the
client's saved version before a connection can occur.
Caution
It is possible for an attacker to masquerade as an SSH server during the initial
contact since the local system does not know the difference between the intended
server and a false one set up by an attacker. To help prevent this, verify the integrity of a new SSH server by contacting the server administrator before connecting
for the first time or in the event of a host key mismatch.
SSH is designed to work with almost any kind of public key algorithm or encoding format. After
207
3.2. Authentication
an initial key exchange creates a hash value used for exchanges and a shared secret value, the
two systems immediately begin calculating new keys and algorithms to protect authentication
and future data sent over the connection.
After a certain amount of data has been transmitted using a given key and algorithm (the exact
amount depends on the SSH implementation), another key exchange occurs, generating another set of hash values and a new shared secret value. Even if an attacker is able to determine
the hash and shared secret value, this information is only useful for a limited period of time.
3.2. Authentication
Once the transport layer has constructed a secure tunnel to pass information between the two
systems, the server tells the client the different authentication methods supported, such as using a private key-encoded signature or typing a password. The client then tries to authenticate
itself to the server using one of these supported methods.
SSH servers and clients can be configured to allow different types of authentication, which gives
each side the optimal amount of control. The server can decide which encryption methods it
supports based on its security model, and the client can choose the order of authentication
methods to attempt from the available options.
3.3. Channels
After a successful authentication over the SSH transport layer, multiple channels are opened via
a technique called multiplexing8. Each of these channels handles communication for different
terminal sessions and for forwarded X11 sessions.
Both clients and servers can create a new channel. Each channel is then assigned a different
number on each end of the connection. When the client attempts to open a new channel, the clients sends the channel number along with the request. This information is stored by the server
and is used to direct communication to that channel. This is done so that different types of sessions do not affect one another and so that when a given session ends, its channel can be
closed without disrupting the primary SSH connection.
Channels also support flow-control, which allows them to send and receive data in an orderly
fashion. In this way, data is not sent over the channel until the client receives a message that
the channel is open.
The client and server negotiate the characteristics of each channel automatically, depending on
the type of service the client requests and the way the user is connected to the network. This allows great flexibility in handling different types of remote connections without having to change
the basic infrastructure of the protocol.
4. Configuring an OpenSSH Server
To run an OpenSSH server, you must first make sure that you have the proper RPM packages
installed. The openssh-server package is required and is dependent on the openssh package.
8
A multiplexed connection consists of several signals being sent over a shared, common medium. With SSH, different
channels are sent over a common secure connection.
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4.1. Requiring SSH for Remote Connections
The OpenSSH daemon uses the configuration file /etc/ssh/sshd_config. The default configuration file should be sufficient for most purposes. If you want to configure the daemon in ways not
provided by the default sshd_config, read the sshd man page for a list of the keywords that can
be defined in the configuration file.
To start the OpenSSH service, use the command /sbin/service sshd start. To stop the
OpenSSH server, use the command /sbin/service sshd stop. If you want the daemon to start
automatically at boot time, refer to Chapter 15, Controlling Access to Services for information on
how to manage services.
If you reinstall, the reinstalled system creates a new set of identification keys. Any clients who
had connected to the system with any of the OpenSSH tools before the reinstall will see the following message:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@
WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!
@
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
If you want to keep the host keys generated for the system, backup the /etc/ssh/ssh_host*key*
files and restore them after the reinstall. This process retains the system's identity, and when clients try to connect to the system after the reinstall, they will not receive the warning message.
4.1. Requiring SSH for Remote Connections
For SSH to be truly effective, using insecure connection protocols, such as Telnet and FTP,
should be prohibited. Otherwise, a user's password may be protected using SSH for one session, only to be captured later while logging in using Telnet.
Some services to disable include:
•
telnet
•
rsh
•
rlogin
•
vsftpd
To disable insecure connection methods to the system, use the command line program chkconfig, the ncurses-based program /usr/sbin/ntsysv, or the Services Configuration Tool (system-config-services) graphical application. All of these tools require root level access.
For more information on runlevels and configuring services with chkconfig, /usr/sbin/ntsysv,
and the Services Configuration Tool, refer to Chapter 15, Controlling Access to Services.
5. OpenSSH Configuration Files
OpenSSH has two different sets of configuration files: one for client programs (ssh, scp, and
sftp) and one for the server daemon (sshd).
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5. OpenSSH Configuration Files
System-wide SSH configuration information is stored in the /etc/ssh/ directory:
•
moduli
— Contains Diffie-Hellman groups used for the Diffie-Hellman key exchange which is
critical for constructing a secure transport layer. When keys are exchanged at the beginning
of an SSH session, a shared, secret value is created which cannot be determined by either
party alone. This value is then used to provide host authentication.
•
ssh_config
•
sshd_config
•
ssh_host_dsa_key
•
ssh_host_dsa_key.pub
•
ssh_host_key
— The system-wide default SSH client configuration file. It is overridden if one is
also present in the user's home directory (~/.ssh/config).
— The configuration file for the sshd daemon.
— The DSA private key used by the sshd daemon.
— The DSA public key used by the sshd daemon.
— The RSA private key used by the sshd daemon for version 1 of the SSH pro-
tocol.
•
ssh_host_key.pub
— The RSA public key used by the sshd daemon for version 1 of the SSH
protocol.
•
ssh_host_rsa_key
— The RSA private key used by the sshd daemon for version 2 of the SSH
protocol.
•
ssh_host_rsa_key.pub
— The RSA public key used by the sshd for version 2 of the SSH pro-
tocol.
User-specific SSH configuration information is stored in the user's home directory within the
~/.ssh/ directory:
•
authorized_keys
— This file holds a list of authorized public keys for servers. When the client
connects to a server, the server authenticates the client by checking its signed public key
stored within this file.
•
id_dsa
•
id_dsa.pub
•
id_rsa
•
id_rsa.pub
•
identity
•
identity.pub
•
known_hosts
— Contains the DSA private key of the user.
— The DSA public key of the user.
— The RSA private key used by ssh for version 2 of the SSH protocol.
— The RSA public key used by ssh for version 2 of the SSH protocol
— The RSA private key used by ssh for version 1 of the SSH protocol.
— The RSA public key used by ssh for version 1 of the SSH protocol.
— This file contains DSA host keys of SSH servers accessed by the user. This
file is very important for ensuring that the SSH client is connecting the correct SSH server.
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6. Configuring an OpenSSH Client
Important
If an SSH server's host key has changed, the client notifies the user that the
connection cannot proceed until the server's host key is deleted from the
known_hosts file using a text editor. Before doing this, however, contact the system administrator of the SSH server to verify the server is not compromised.
Refer to the ssh_config and sshd_config man pages for information concerning the various directives available in the SSH configuration files.
6. Configuring an OpenSSH Client
To connect to an OpenSSH server from a client machine, you must have the openssh-clients
and openssh packages installed on the client machine.
6.1. Using the ssh Command
The ssh command is a secure replacement for the rlogin, rsh, and telnet commands. It allows
you to log in to a remote machine as well as execute commands on a remote machine.
Logging in to a remote machine with ssh is similar to using telnet. To log in to a remote machine named penguin.example.net, type the following command at a shell prompt:
ssh penguin.example.net
The first time you ssh to a remote machine, you will see a message similar to the following:
The authenticity of host 'penguin.example.net' can't be established.
DSA key fingerprint is 94:68:3a:3a:bc:f3:9a:9b:01:5d:b3:07:38:e2:11:0c.
Are you sure you want to continue connecting (yes/no)?
Type yes to continue. This will add the server to your list of known hosts (~/.ssh/known_hosts)
as seen in the following message:
Warning: Permanently added 'penguin.example.net' (RSA) to the list of known hosts.
Next, you will see a prompt asking for your password for the remote machine. After entering
your password, you will be at a shell prompt for the remote machine. If you do not specify a
username the username that you are logged in as on the local client machine is passed to the
remote machine. If you want to specify a different username, use the following command:
ssh [email protected]
You can also use the syntax ssh
-l username penguin.example.net.
The ssh command can be used to execute a command on the remote machine without logging
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6.2. Using the scp Command
in to a shell prompt. The syntax is ssh hostnamecommand. For example, if you want to execute the
command ls /usr/share/doc on the remote machine penguin.example.net, type the following
command at a shell prompt:
ssh penguin.example.net ls /usr/share/doc
After you enter the correct password, the contents of the remote directory /usr/share/doc will be
displayed, and you will return to your local shell prompt.
6.2. Using the scp Command
The scp command can be used to transfer files between machines over a secure, encrypted
connection. It is similar to rcp.
The general syntax to transfer a local file to a remote system is as follows:
scp <localfile>[email protected]:<remotefile>
The <localfile> specifies the source including path to the file, such as /var/log/maillog. The
<remotefile> specifies the destination, which can be a new filename such as /
tmp/hostname-maillog. For the remote system, if you do not have a preceding /, the path will be
relative to the home directory of username, typically /home/username/.
To transfer the local file shadowman to the home directory of your account on penguin.example.net, type the following at a shell prompt (replace username with your username):
scp shadowman [email protected]:shadowman
This will transfer the local file shadowman to /home/username/shadowman on penguin.example.net.
Alternately, you can leave off the final shadowman in the scp command.
The general syntax to transfer a remote file to the local system is as follows:
scp [email protected]:<remotefile><newlocalfile>
The <remotefile> specifies the source including path, and <newlocalfile> specifies the destination including path.
Multiple files can be specified as the source files. For example, to transfer the contents of the
directory downloads/ to an existing directory called uploads/ on the remote machine penguin.example.net, type the following at a shell prompt:
scp downloads/* [email protected]:uploads/
6.3. Using the sftp Command
The sftp utility can be used to open a secure, interactive FTP session. It is similar to ftp except
that it uses a secure, encrypted connection. The general syntax is sftp [email protected]
Once authenticated, you can use a set of commands similar to those used by FTP. Refer to the
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7. More Than a Secure Shell
sftp
sftp
man page for a list of these commands. To read the man page, execute the command man
at a shell prompt. The sftp utility is only available in OpenSSH version 2.5.0p1 and higher.
7. More Than a Secure Shell
A secure command line interface is just the beginning of the many ways SSH can be used. Given the proper amount of bandwidth, X11 sessions can be directed over an SSH channel. Or, by
using TCP/IP forwarding, previously insecure port connections between systems can be
mapped to specific SSH channels.
7.1. X11 Forwarding
Opening an X11 session over an SSH connection is as easy as connecting to the SSH server
using the -Y option and running an X program on a local machine.
ssh -Y <user>@example.com
When an X program is run from the secure shell prompt, the SSH client and server create a new
secure channel, and the X program data is sent over that channel to the client machine transparently.
X11 forwarding can be very useful. For example, X11 forwarding can be used to create a secure, interactive session of the Printer Configuration Tool. To do this, connect to the server
using ssh and type:
system-config-printer &
After supplying the root password for the server, the Printer Configuration Tool appears and
allows the remote user to safely configure printing on the remote system.
7.2. Port Forwarding
SSH can secure otherwise insecure TCP/IP protocols via port forwarding. When using this technique, the SSH server becomes an encrypted conduit to the SSH client.
Port forwarding works by mapping a local port on the client to a remote port on the server. SSH
can map any port from the server to any port on the client; port numbers do not need to match
for this technique to work.
To create a TCP/IP port forwarding channel which listens for connections on the localhost, use
the following command:
ssh -L local-port:remote-hostname:[email protected]
Note
Setting up port forwarding to listen on ports below 1024 requires root level access.
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7.3. Generating Key Pairs
To check email on a server called mail.example.com using POP3 through an encrypted connection, use the following command:
ssh -L 1100:mail.example.com:110 mail.example.com
Once the port forwarding channel is in place between the client machine and the mail server,
direct a POP3 mail client to use port 1100 on the localhost to check for new mail. Any requests
sent to port 1100 on the client system are directed securely to the mail.example.com server.
If mail.example.com is not running an SSH server, but another machine on the same network is,
SSH can still be used to secure part of the connection. However, a slightly different command is
necessary:
ssh -L 1100:mail.example.com:110 other.example.com
In this example, POP3 requests from port 1100 on the client machine are forwarded through the
SSH connection on port 22 to the SSH server, other.example.com. Then, other.example.com
connects to port 110 on mail.example.com to check for new mail. Note, when using this technique only the connection between the client system and other.example.com SSH server is secure.
Port forwarding can also be used to get information securely through network firewalls. If the
firewall is configured to allow SSH traffic via its standard port (22) but blocks access to other
ports, a connection between two hosts using the blocked ports is still possible by redirecting
their communication over an established SSH connection.
Note
Using port forwarding to forward connections in this manner allows any user on the
client system to connect to that service. If the client system becomes compromised, the attacker also has access to forwarded services.
System administrators concerned about port forwarding can disable this functionality on the server by specifying a No parameter for the AllowTcpForwarding line in /
etc/ssh/sshd_config and restarting the sshd service.
7.3. Generating Key Pairs
If you do not want to enter your password every time you use ssh, scp, or sftp to connect to a
remote machine, you can generate an authorization key pair.
Keys must be generated for each user. To generate keys for a user, use the following steps as
the user who wants to connect to remote machines. If you complete the steps as root, only root
will be able to use the keys.
Starting with OpenSSH version 3.0, ~/.ssh/authorized_keys2, ~/.ssh/known_hosts2, and /
etc/ssh_known_hosts2 are obsolete. SSH Protocol 1 and 2 share the ~/.ssh/authorized_keys,
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7.3. Generating Key Pairs
~/.ssh/known_hosts,
and /etc/ssh/ssh_known_hosts files.
Red Hat Enterprise Linux 5.0.0 uses SSH Protocol 2 and RSA keys by default.
Tip
If you reinstall and want to save your generated key pair, backup the .ssh directory
in your home directory. After reinstalling, copy this directory back to your home directory. This process can be done for all users on your system, including root.
7.3.1. Generating an RSA Key Pair for Version 2
Use the following steps to generate an RSA key pair for version 2 of the SSH protocol. This is
the default starting with OpenSSH 2.9.
1.
To generate an RSA key pair to work with version 2 of the protocol, type the following command at a shell prompt:
ssh-keygen -t rsa
Accept the default file location of ~/.ssh/id_rsa. Enter a passphrase different from your account password and confirm it by entering it again.
The public key is written to ~/.ssh/id_rsa.pub. The private key is written to ~/.ssh/id_rsa.
Never distribute your private key to anyone.
2.
Change the permissions of the .ssh directory using the following command:
chmod 755 ~/.ssh
3.
Copy the contents of ~/.ssh/id_rsa.pub into the file ~/.ssh/authorized_keys on the machine to which you want to connect. If the file ~/.ssh/authorized_keys exist, append the
contents of the file ~/.ssh/id_rsa.pub to the file ~/.ssh/authorized_keys on the other machine.
4.
Change the permissions of the authorized_keys file using the following command:
chmod 644 ~/.ssh/authorized_keys
5.
If you are running GNOME or are running in a graphical desktop with GTK2+ libraries installed, skip to Section 7.3.4, “Configuring ssh-agent with a GUI”. If you are not running the
X Window System, skip to Section 7.3.5, “Configuring ssh-agent”.
7.3.2. Generating a DSA Key Pair for Version 2
Use the following steps to generate a DSA key pair for version 2 of the SSH Protocol.
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7.3. Generating Key Pairs
1.
To generate a DSA key pair to work with version 2 of the protocol, type the following command at a shell prompt:
ssh-keygen -t dsa
Accept the default file location of ~/.ssh/id_dsa. Enter a passphrase different from your account password and confirm it by entering it again.
Tip
A passphrase is a string of words and characters used to authenticate a user.
Passphrases differ from passwords in that you can use spaces or tabs in the
passphrase. Passphrases are generally longer than passwords because they
are usually phrases instead of a single word.
The public key is written to ~/.ssh/id_dsa.pub. The private key is written to ~/.ssh/id_dsa.
It is important never to give anyone the private key.
2.
Change the permissions of the .ssh directory with the following command:
chmod 755 ~/.ssh
3.
Copy the contents of ~/.ssh/id_dsa.pub into the file ~/.ssh/authorized_keys on the machine to which you want to connect. If the file ~/.ssh/authorized_keys exist, append the
contents of the file ~/.ssh/id_dsa.pub to the file ~/.ssh/authorized_keys on the other machine.
4.
Change the permissions of the authorized_keys file using the following command:
chmod 644 ~/.ssh/authorized_keys
5.
If you are running GNOME or a graphical desktop environment with the GTK2+ libraries installed, skip to Section 7.3.4, “Configuring ssh-agent with a GUI”. If you are not running the
X Window System, skip to Section 7.3.5, “Configuring ssh-agent”.
7.3.3. Generating an RSA Key Pair for Version 1.3 and 1.5
Use the following steps to generate an RSA key pair, which is used by version 1 of the SSH
Protocol. If you are only connecting between systems that use DSA, you do not need an RSA
version 1.3 or RSA version 1.5 key pair.
1.
To generate an RSA (for version 1.3 and 1.5 protocol) key pair, type the following command at a shell prompt:
ssh-keygen -t rsa1
216
7.3. Generating Key Pairs
Accept the default file location (~/.ssh/identity). Enter a passphrase different from your
account password. Confirm the passphrase by entering it again.
The public key is written to ~/.ssh/identity.pub. The private key is written to
~/.ssh/identity. Do not give anyone the private key.
2.
Change the permissions of your .ssh directory and your key with the commands chmod
~/.ssh and chmod 644 ~/.ssh/identity.pub.
3.
Copy the contents of ~/.ssh/identity.pub into the file ~/.ssh/authorized_keys on the machine to which you wish to connect. If the file ~/.ssh/authorized_keys does not exist, you
can copy the file ~/.ssh/identity.pub to the file ~/.ssh/authorized_keys on the remote machine.
4.
If you are running GNOME, skip to Section 7.3.4, “Configuring ssh-agent with a GUI”. If you
are not running GNOME, skip to Section 7.3.5, “Configuring ssh-agent”.
755
7.3.4. Configuring ssh-agent with a GUI
The ssh-agent utility can be used to save your passphrase so that you do not have to enter it
each time you initiate an ssh or scp connection. If you are using GNOME, the gnome-ssh-askpass
package contains the application used to prompt you for your passphrase when you log in to
GNOME and save it until you log out of GNOME. You will not have to enter your password or
passphrase for any ssh or scp connection made during that GNOME session. If you are not using GNOME, refer to Section 7.3.5, “Configuring ssh-agent”.
To save your passphrase during your GNOME session, follow the following steps:
1.
You will need to have the package gnome-ssh-askpass installed; you can use the command
rpm -q openssh-askpass to determine if it is installed or not. If it is not installed, install it from
your Red Hat Enterprise Linux CD-ROM set, from a Red Hat FTP mirror site, or using Red
Hat Network.
2.
Select Main Menu Button (on the Panel) => Preferences => More Preferences => Sessions, and click on the Startup Programs tab. Click Add and enter /usr/bin/ssh-add in
the Startup Command text area. Set it a priority to a number higher than any existing commands to ensure that it is executed last. A good priority number for ssh-add is 70 or higher.
The higher the priority number, the lower the priority. If you have other programs listed, this
one should have the lowest priority. Click Close to exit the program.
3.
Log out and then log back into GNOME; in other words, restart X. After GNOME is started,
a dialog box will appear prompting you for your passphrase(s). Enter the passphrase requested. If you have both DSA and RSA key pairs configured, you will be prompted for
both. From this point on, you should not be prompted for a password by ssh, scp, or sftp.
7.3.5. Configuring ssh-agent
The ssh-agent can be used to store your passphrase so that you do not have to enter it each
time you make a ssh or scp connection. If you are not running the X Window System, follow
these steps from a shell prompt. If you are running GNOME but you do not want to configure it
217
8. Additional Resources
to prompt you for your passphrase when you log in (refer to Section 7.3.4, “Configuring sshagent with a GUI”), this procedure will work in a terminal window, such as an XTerm. If you are
running X but not GNOME, this procedure will work in a terminal window. However, your passphrase will only be remembered for that terminal window; it is not a global setting.
1.
At a shell prompt, type the following command:
exec /usr/bin/ssh-agent $SHELL
2.
Then type the command:
ssh-add
and enter your passphrase(s). If you have more than one key pair configured, you will be
prompted for each one.
3.
When you log out, your passphrase(s) will be forgotten. You must execute these two commands each time you log in to a virtual console or open a terminal window.
8. Additional Resources
The OpenSSH and OpenSSL projects are in constant development, and the most up-to-date information for them is available from their websites. The man pages for OpenSSH and OpenSSL
tools are also good sources of detailed information.
8.1. Installed Documentation
•
The ssh, scp, sftp, sshd, and ssh-keygen man pages — These man pages include information on how to use these commands as well as all the parameters that can be used with
them.
8.2. Useful Websites
•
http://www.openssh.com/ — The OpenSSH FAQ page, bug reports, mailing lists, project
goals, and a more technical explanation of the security features.
•
http://www.openssl.org/ — The OpenSSL FAQ page, mailing lists, and a description of the
project goal.
•
http://www.freessh.org/ — SSH client software for other platforms.
218
Chapter 18. Network File System
(NFS)
A Network File System (NFS) allows remote hosts to mount file systems over a network and interact with those file systems as though they are mounted locally. This enables system administrators to consolidate resources onto centralized servers on the network.
This chapter focuses on fundamental NFS concepts and supplemental information.
1. How It Works
Currently, there are three versions of NFS. NFS version 2 (NFSv2) is older and is widely supported. NFS version 3 (NFSv3) has more features, including 64bit file handles, Safe Async
writes and more robust error handling. NFS version 4 (NFSv4) works through firewalls and on
the Internet, no longer requires portmapper, supports ACLs, and utilizes stateful operations.
Red Hat Enterprise Linux supports NFSv2, NFSv3, and NFSv4 clients, and when mounting a
file system via NFS, Red Hat Enterprise Linux uses NFSv3 by default, if the server supports it.
All versions of NFS can use Transmission Control Protocol (TCP) running over an IP network,
with NFSv4 requiring it. NFSv2 and NFSv3 can use the User Datagram Protocol (UDP) running
over an IP network to provide a stateless network connection between the client and server.
When using NFSv2 or NFSv3 with UDP, the stateless UDP connection under normal conditions
has less Protocol overhead than TCP which can translate into better performance on very clean,
non-congested networks. The NFS server sends the client a file handle after the client is authorized to access the shared volume. This file handle is an opaque object stored on the server's
side and is passed along with RPC requests from the client. The NFS server can be restarted
without affecting the clients and the cookie remains intact. However, because UDP is stateless,
if the server goes down unexpectedly, UDP clients continue to saturate the network with requests for the server. For this reason, TCP is the preferred protocol when connecting to an NFS
server.
NFSv4 has no interaction with portmapper, rpc.mountd, rpc.lockd, and rpc.statd, since protocol
support has been incorporated into the v4 protocol. NFSv4 listens on the well known TCP port
(2049) which eliminates the need for the portmapper interaction. The mounting and locking protocols have been incorpated into the V4 protocol which eliminates the need for interaction with
rpc.mountd and rpc.lockd.
Note
TCP is the default transport protocol for NFS under Red Hat Enterprise Linux. UDP
can be used for compatibility purposes as needed, but is not recommended for
wide usage.
All the RPC/NFS daemon have a '-p' command line option that can set the port,
making firewall configuration easier.
219
1.1. Required Services
After the client is granted access by TCP wrappers, the NFS server refers to its configuration
file, /etc/exports, to determine whether the client is allowed to access any of the exported file
systems. Once access is granted, all file and directory operations are available to the user.
Important
In order for NFS to work with a default installation of Red Hat Enterprise Linux with
a firewall enabled, IPTables with the default TCP port 2049 must be configured.
Without proper IPTables configuration, NFS does not function properly.
The NFS initialization script and rpc.nfsd process now allow binding to any specified port during system start up. However, this can be error prone if the port is
unavailable or conflicts with another daemon.
1.1. Required Services
Red Hat Enterprise Linux uses a combination of kernel-level support and daemon processes to
provide NFS file sharing. All NFS versions rely on Remote Procedure Calls (RPC) between clients and servers. RPC services under Linux are controlled by the portmap service. To share or
mount NFS file systems, the following services work together, depending on which version of
NFS is implemented:
•
nfs
— (/sbin/service nfs start) starts the NFS server and the appropriate RPC processes
to service requests for shared NFS file systems.
•
nfslock
•
portmap
— (/sbin/service nfslock start) is a mandatory service that starts the appropriate
RPC processes to allow NFS clients to lock files on the server.
— accepts port reservations from local RPC services. These ports are then made
available (or advertised) so the corresponding remote RPC services access them. portmap
responds to requests for RPC services and sets up connections to the requested RPC service. This is not used with NFSv4.
The following RPC processes facilitate NFS services:
•
rpc.mountd
— This process receives mount requests from NFS clients and verifies the requested file system is currently exported. This process is started automatically by the nfs
service and does not require user configuration. This is not used with NFSv4.
•
rpc.nfsd
•
rpc.lockd
— Allows explicit NFS versions and protocols the server advertises to be defined. It
works with the Linux kernel to meet the dynamic demands of NFS clients, such as providing
server threads each time an NFS client connects. This process corresponds to the nfs service.
— allows NFS clients to lock files on the server. If rpc.lockd is not started, file
locking will fail. rpc.lockd implements the Network Lock Manager (NLM) protocol. This process corresponds to the nfslock service. This is not used with NFSv4.
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2. NFS Client Configuration
•
rpc.statd
— This process implements the Network Status Monitor (NSM) RPC protocol
which notifies NFS clients when an NFS server is restarted without being gracefully brought
down. This process is started automatically by the nfslock service and does not require user
configuration. This is not used with NFSv4.
•
rpc.rquotad
•
rpc.idmapd
— This process provides user quota information for remote users. This process
is started automatically by the nfs service and does not require user configuration.
— This process provides NFSv4 client and server upcalls which map between
on-the-wire NFSv4 names (which are strings in the form of [email protected]) and local UIDs
and GIDs. For idmapd to function with NFSv4, the /etc/idmapd.conf must be configured. This
service is required for use with NFSv4.
2. NFS Client Configuration
NFS shares are mounted on the client side using the mount command. The format of the command is as follows:
mount -t <nfs-type> -o <options><host>:</remote/export></local/directory>
Replace <nfs-type> with either nfs for NFSv2 or NFSv3 servers, or nfs4 for NFSv4 servers. Replace <options> with a comma separated list of options for the NFS file system (refer to Section 4, “Common NFS Mount Options” for details). Replace <host> with the remote host,
</remote/export> with the remote directory being mounted, and </local/directory> with the
local directory where the remote file system is to be mounted.
Refer to the mount man page for more details.
If accessing an NFS share by manually issuing the mount command, the file system must be remounted manually after the system is rebooted. Red Hat Enterprise Linux offers two methods
for mounting remote file systems automatically at boot time: the /etc/fstab file or the autofs
service.
2.1. Mounting NFS File Systems using /etc/fstab
An alternate way to mount an NFS share from another machine is to add a line to the /
etc/fstab file. The line must state the hostname of the NFS server, the directory on the server
being exported, and the directory on the local machine where the NFS share is to be mounted.
You must be root to modify the /etc/fstab file.
The general syntax for the line in /etc/fstab is as follows:
server:/usr/local/pub /pub nfs rsize=8192,wsize=8192,timeo=14,intr
The mount point /pub must exist on the client machine before this command can be executed.
After adding this line to /etc/fstab on the client system, type the command mount /pub at a shell
prompt, and the mount point /pub is mounted from the server.
The /etc/fstab file is referenced by the netfs service at boot time, so lines referencing NFS
shares have the same effect as manually typing the mount command during the boot process.
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3. autofs
A sample /etc/fstab line to mount an NFS export looks like the following example:
<server>:</remote/export></local/directory><nfs-type><options> 0 0
Replace <server> with the hostname, IP address, or fully qualified domain name of the server
exporting the file system.
Replace </remote/export> with the path to the exported directory.
Replace </local/directory> with the local file system on which the exported directory is mounted. This mount point must exist before /etc/fstab is read or the mount fails.
Replace <nfs-type> with either nfs for NFSv2 or NFSv3 servers, or nfs4 for NFSv4 servers.
Replace <options> with a comma separated list of options for the NFS file system (refer to Section 4, “Common NFS Mount Options” for details). Refer to the fstab man page for additional information.
3.
autofs
One drawback to using /etc/fstab is that, regardless of how infrequently a user accesses the
NFS mounted file system, the system must dedicate resources to keep the mounted file system
in place. This is not a problem with one or two mounts, but when the system is maintaining
mounts to many systems at one time, overall system performance can be affected. An alternative to /etc/fstab is to use the kernel-based automount utility. An automounter consists of two
components. One is a kernel module that implements a file system, while the other is a userspace daemon that performs all of the other functions. The automount utility can mount and unmount NFS file systems automatically (on demand mounting) therefore saving system resources. The automount utility can be used to mount other file systems including AFS, SMBFS,
CIFS and local file systems.
uses /etc/auto.master (master map) as its default primary configuration file. This can be
changed to use another supported network source and name using the autofs configuration (in /
etc/sysconfig/autofs) in conjunction with the Name Service Switch mechanism. An instance of
the version 4 daemon was run for each mount point configured in the master map and so it
could be run manually from the command line for any given mount point. This is not possible
with version 5 because it uses a single daemon to manage all configured mount points, so all
automounts must be configured in the master map. This is in line with the usual requirements of
other industry standard automounters. Mount point, hostname, exported directory, and options
can all be specified in a set of files (or other supported network sources) rather than configuring
them manually for each host. Please ensure that you have the autofs package installed if you
wish to use this service.
autofs
3.1. What's new in autofs version 5?
Direct map support
Autofs direct maps provide a mechanism to automatically mount file systems at arbitrary
points in the file system hierarchy. A direct map is denoted by a mount point of "/-" in the
master map. Entries in a direct map contain an absolute path name as a key (instead of the
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3.2. autofs Configuration
relative path names used in indirect maps).
Lazy mount and unmount support
Multimount map entries describe a hierarchy of mount points under a single key. A good example of this is the "-hosts" map, commonly used for automounting all exports from a host
under "/net/<host>" as a multi-mount map entry. When using the "-hosts" map, an 'ls' of "/
net/<host>" will mount autofs trigger mounts for each export from <host> and mount and expire them as they are accessed. This can greatly reduce the number of active mounts
needed when accessing a server with a large number of exports.
Enhanced LDAP support
The Lightweight Directory Access Protocol, or LDAP, support in autofs version 5 has been
enhanced in several ways with respect to autofs version 4. The autofs configuration file (/
etc/sysconfig/autofs) provides a mechanism to specify the autofs schema that a site implements, thus precluding the need to determine this via trial and error in the application itself. In addition, authenticated binds to the LDAP server are now supported, using most
mechanisms supported by the common LDAP server implementations. A new configuration
file has been added for this support: /etc/autofs_ldap_auth.conf. The default configuration
file is self-documenting, and uses an XML format.
Proper use of the Name Service Switch (nsswitch) configuration.
The Name Service Switch configuration file exists to provide a means of determining from
where specific configuration data comes. The reason for this configuration is to allow administrators the flexibility of using the back-end database of choice, while maintaining a uniform
software interface to access the data. While the version 4 automounter is becoming increasingly better at handling the name service switch configuration, it is still not complete. Autofs
version 5, on the other hand, is a complete implementation. See the manual page for nsswitch.conf for more information on the supported syntax of this file. Please note that not all
nss databases are valid map sources and the parser will reject ones that are invalid. Valid
sources are files, yp, nis, nisplus, ldap and hesiod.
Multiple master map entries per autofs mount point
One thing that is frequently used but not yet mentioned is the handling of multiple master
map entries for the direct mount point "/-". The map keys for each entry are merged and behave as one map.
An example is seen in the connectathon test maps for the direct mounts below:
/- /tmp/auto_dcthon
/- /tmp/auto_test3_direct
/- /tmp/auto_test4_direct
3.2. autofs Configuration
The primary configuration file for the automounter is /etc/auto.master, also referred to as the
master map which may be changed as described in the introduction section above. The master
map lists autofs-controlled mount points on the system, and their corresponding configuration
files or network sources known as automount maps. The format of the master map is as follows:
<mount-point> <map-name> <options>
where:
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3.2. autofs Configuration
•
mount-point
•
map-name
•
options
is the autofs mount point e.g /home.
is the name of a map source which contains a list of mount points, and the file system location from which those mount points should be mounted. The syntax for a map entry
is described below.
if supplied, will apply to all entries in the given map provided they don't themselves
have options specified. This behavior is different from autofs version 4 where the options
where cumulative. This has been changed to meet our primary goal of mixed environment
compatibility.
The following is a sample /etc/auto.master file:
$ cat /etc/auto.master /home /etc/auto.misc
The general format of maps is similar to the master map, however the "options" appear between
the mount point and the location instead of at the end of the entry as in the master map:
<mount-point> [<options>] <location>
where:
•
<mount-point>
is the autofs mount point. This can be a single directory name for an indirect
mount or the full path of the mount point for direct mounts. Each direct and indirect map
entry key (<mount-point> above) may be followed by a space separated list of offset directories (sub directory names each beginning with a "/") making them what is known as a mutlimount entry.
•
<options> if supplied, are the mount options for the map entries that do not specify their own
options.
•
<location> is the file system location such as a local file system path (preceded with the Sun
map format escape character ":" for map names beginning with "/"), an NFS file system or
other valid file system location.
The following is a sample map file:
$ cat /etc/auto.misc payroll -fstype=nfs personnel:/dev/hda3 sales -fstype=ext3 :/dev/hda4
The first column in a map file indicates the autofs mount point (sales and payroll from the server called personnel). The second column indicates the options for the autofs mount while the
third column indicates the source of the mount. Following the above configuration, the autofs
mount points will be /home/payroll and /home/sales. The -fstype= option is often omitted and is
generally not needed for correct operation.
The automounter will create the directories if they do not exist. If the directories exist before the
automounter was started, the automounter will not remove them when it exits. You can start or
restart the automount daemon by issuing the following command:
$/sbin/service autofs start
or $/sbin/service autofs restart
Using the above configuration, if a process requires access to an autofs unmounted directory
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3.3. autofs Common Tasks
such as /home/payroll/2006/July.sxc, the automount daemon automatically mounts the directory. If a timeout is specified, the directory will automatically be unmounted if the directory is not
accessed for the timeout period.
You can view the status of the automount daemon by issuing the following command in your
terminal:
$/sbin/service/autofs status
3.3. autofs Common Tasks
3.3.1. Overriding or augmenting site configuration files
It can be useful to override site defaults for a specific mount point on a client system. For example, assuming that the automounter maps are stored in NIS and the /etc/nsswitch.conf file
has the following directive:
automount:
files nis
and the NIS auto.master map file contains the following:
/home auto.home
Also assume the NIS auto.home map contains the following:
beth
joe
*
fileserver.example.com:/export/home/beth
fileserver.example.com:/export/home/joe
fileserver.example.com:/export/home/&
and the file map /etc/auto.home does not exist.
For the above example, lets assume that the client system needs to mount home directories
from a different server. In this case, the client will need to use the following /etc/auto.master
map:
/home /etc/auto.home2
+auto.master
And the /etc/auto.home2 map contains the entry:
*
labserver.example.com:/export/home/&
Because only the first occurrence of a mount point is processed, /home will contain the contents
of /etc/auto.home2 instead of the NIS auto.home map.
Alternatively, if you just want to augment the site-wide
auto.home
map with a few entries, create a /etc/auto.home file map, and in it put your new entries and at
the end, include the NIS auto.home map. Then the /etc/auto.home file map might look similar
to:
mydir someserver:/export/mydir
+auto.home
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3.3. autofs Common Tasks
Given the NIS auto.home map listed above, an ls of /home would now give:
$ ls /home
bethjoemydir
This last example works as expected because autofs knows not to include the contents of a file
map of the same name as the one it is reading and so moves on to the next map source in the
nsswitch configuration.
3.3.2. Using LDAP to Store Automounter Maps
LDAP client libraries must be installed on all systems which are to retrieve automounter maps
from LDAP. On RHEL 5, the openldap package should be installed automatically as a dependency of the automounter. To configure LDAP access, modify /etc/openldap/ldap.conf. Ensure
that BASE and URI are set appropriately for your site. Please also ensure that the schema is
set in the configuration.
The most recently established schema for storing automount maps in LDAP is described by
rfc2307bis. To use this schema it is necessary to set it in the autofs configuration (/
etc/sysconfig/autofs) by removing the comment characters from the schema definition. For example:
DEFAULT_MAP_OBJECT_CLASS="automountMap"
DEFAULT_ENTRY_OBJECT_CLASS="automount"
DEFAULT_MAP_ATTRIBUTE="automountMapName"
DEFAULT_ENTRY_ATTRIBUTE="automountKey"
DEFAULT_VALUE_ATTRIBUTE="automountInformation"
Ensure that these are the only schema entries not commented in the configuration. Please also
note that the automountKey replaces the cn attribute in the rfc2307bis schema. An LDIF of a
sample configuration is described below:
#
#
#
#
#
#
#
extended LDIF
LDAPv3
base <> with scope subtree
filter: (&(objectclass=automountMap)(automountMapName=auto.master))
requesting: ALL
# auto.master, example.com
dn: automountMapName=auto.master,dc=example,dc=com
objectClass: top
objectClass: automountMap
automountMapName: auto.master
#
#
#
#
#
#
#
extended LDIF
LDAPv3
base <automountMapName=auto.master,dc=example,dc=com> with scope subtree
filter: (objectclass=automount)
requesting: ALL
# /home, auto.master, example.com
dn: automountMapName=auto.master,dc=example,dc=com
objectClass: automount
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3.3. autofs Common Tasks
cn: /home
automountKey: /home
automountInformation: auto.home
#
#
#
#
#
#
#
extended LDIF
LDAPv3
base <> with scope subtree
filter: (&(objectclass=automountMap)(automountMapName=auto.home))
requesting: ALL
# auto.home, example.com
dn: automountMapName=auto.home,dc=example,dc=com
objectClass: automountMap
automountMapName: auto.home
#
#
#
#
#
#
#
extended LDIF
LDAPv3
base <automountMapName=auto.home,dc=example,dc=com> with scope subtree
filter: (objectclass=automount)
requesting: ALL
# foo, auto.home, example.com
dn: automountKey=foo,automountMapName=auto.home,dc=example,dc=com
objectClass: automount
automountKey: foo
automountInformation: filer.example.com:/export/foo
# /, auto.home, example.com
dn: automountKey=/,automountMapName=auto.home,dc=example,dc=com
objectClass: automount
automountKey: /
automountInformation: filer.example.com:/export/&
3.3.3. Adapting Autofs v4 Maps To Autofs v5
v4 Multi-map entries. Autofs version 4 introduced the notion of a multi-map entry in the master
map. A multi-map entry is of the form:
<mount-point> <maptype1> <mapname1> <options1> -- <maptype2> <mapname2> <options2> -- ...
Any number of maps can be combined into a single map in this manner. This feature is no
longer present in v5. This is because Version 5 supports included maps which can be used to
attain the same results. Consider the following multi-map example: /home file /etc/auto.home
-- nis auto.home
This can be replaced by the following configuration for v5:
/etc/nsswitch.conf
must list:
automount: files nis
/etc/auto.master
/home
should contain:
auto.home
/etc/auto.home
should contain:
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4. Common NFS Mount Options
<entries for the home directory>
+auto.home
In this way, the entries from /etc/auto.home and the nis auto.home map are combined.
Multiple master maps. In autofs version 4, it is possible to merge the contents of master maps
from each source, such as files, nis, hesiod, and LDAP. The version 4 automounter looks for a
master map for each of the sources listed in /etc/nsswitch.conf. The map is read if it exists and
its contents are merged into one large auto.master map.
In version 5, this is no longer the behaviour. Only the first master map found from the list of
sources in nsswitch.conf is consulted. If it is desirable to merge the contents of multiple master
maps, included maps can be used. Consider the following example:
/etc/nsswitch.conf:
automount: files nis
/etc/auto.master:
/home /etc/auto.home
+auto.master
The above configuration will merge the contents of the file-based auto.master and the NISbased auto.master. However, because included map entries are only allowed in file maps, there
is no way to include both an NIS auto.master and an LDAP auto.master.
This limitation can be overcome by creating a master maps that have a different name in the
source. In the example above if we had an LDAP master map named auto.master.ldap we
could also add "+auto.master.ldap" to the file based master map and provided that "ldap" is listed as a source in our nsswitch configuration it would also be included.
4. Common NFS Mount Options
Beyond mounting a file system via NFS on a remote host, other options can be specified at the
time of the mount to make it easier to use. These options can be used with manual mount commands, /etc/fstab settings, and autofs.
The following are options commonly used for NFS mounts:
•
fsid=num
— Forces the file handle and file attributes settings on the wire to be num, instead of
a number derived from the major and minor number of the block device on the mounted file
system. The value 0 has special meaning when used with NFSv4. NFSv4 has a concept of a
root of the overall exported file system. The export point exported with fsid=0 is used as this
root.
•
hard
or soft — Specifies whether the program using a file via an NFS connection should
stop and wait (hard) for the server to come back online, if the host serving the exported file
system is unavailable, or if it should report an error (soft).
If hard is specified, the user cannot terminate the process waiting for the NFS communication to resume unless the intr option is also specified.
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4. Common NFS Mount Options
If soft is specified, the user can set an additional timeo=<value> option, where <value> specifies the number of seconds to pass before the error is reported.
Note
Using soft mounts is not recommended as they can generate I/O errors in very
congested networks or when using a very busy server.
•
intr
— Allows NFS requests to be interrupted if the server goes down or cannot be reached.
•
nfsvers=2
•
noacl
•
nolock
•
noexec
•
nosuid
•
port=num
or nfsvers=3 — Specifies which version of the NFS protocol to use. This is useful
for hosts that run multiple NFS servers. If no version is specified, NFS uses the highest supported version by the kernel and mount command. This option is not supported with NFSv4
and should not be used.
— Turns off all ACL processing. This may be needed when interfacing with older versions of Red Hat Enterprise Linux, Red Hat Linux, or Solaris, since the most recent ACL
technology is not compatible with older systems.
— Disables file locking. This setting is occasionally required when connecting to older
NFS servers.
— Prevents execution of binaries on mounted file systems. This is useful if the system is mounting a non-Linux file system via NFS containing incompatible binaries.
— Disables set-user-identifier or set-group-identifier bits. This prevents remote users
from gaining higher privileges by running a setuid program.
— Specifies the numeric value of the NFS server port. If num is 0 (the default), then
queries the remote host's portmapper for the port number to use. If the remote host's
NFS daemon is not registered with its portmapper, the standard NFS port number of TCP
2049 is used instead.
mount
•
rsize=num
and wsize=num — These settings speed up NFS communication for reads (rsize)
and writes (wsize) by setting a larger data block size, in bytes, to be transferred at one time.
Be careful when changing these values; some older Linux kernels and network cards do not
work well with larger block sizes. For NFSv2 or NFSv3, the default values for both parameters is set to 8192. For NFSv4, the default values for both parameters is set to 32768.
•
sec=mode
— Specifies the type of security to utilize when authenticating an NFS connection.
is the default setting, which uses local UNIX UIDs and GIDs by means of AUTH_SYS to authenticate NFS operations.
sec=sys
sec=krb5
uses Kerberos V5 instead of local UNIX UIDs and GIDs to authenticate users.
uses Kerberos V5 for user authentication and performs integrity checking of NFS
operations using secure checksums to prevent data tampering.
sec=krb5i
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5. Starting and Stopping NFS
uses Kerberos V5 for user authentication, integrity checking, and encrypts NFS
traffic to prevent traffic sniffing. This is the most secure setting, but it also has the most performance overhead involved.
sec=krb5p
•
tcp
— Specifies for the NFS mount to use the TCP protocol.
•
udp
— Specifies for the NFS mount to use the UDP protocol.
Many more options are listed on the mount and nfs man pages.
5. Starting and Stopping NFS
To run an NFS server, the portmap service must be running. To verify that portmap is active, type
the following command as root:
/sbin/service portmap status
If the portmap service is running, then the nfs service can be started. To start an NFS server, as
root type:
/sbin/service nfs start
Note
also has to be started for both the NFS client and server to function properly. To start NFS locking as root type: /sbin/service nfslock start. If NFS is set
to start at boot, please ensure that nfslock also starts by running chkconfig --list
nfslock. If nfslock is not set to on, this implies that you will need to manually run
the /sbin/service nfslock start each time the computer starts. To set nfslock to
automatically start on boot, type the following command in a terminal chkconfig
nfslock on.
nfslock
To stop the server, as root, type:
/sbin/service nfs stop
The restart option is a shorthand way of stopping and then starting NFS. This is the most efficient way to make configuration changes take effect after editing the configuration file for NFS.
To restart the server, as root, type:
/sbin/service nfs restart
The condrestart (conditional restart) option only starts nfs if it is currently running. This option is
useful for scripts, because it does not start the daemon if it is not running.
To conditionally restart the server, as root, type:
/sbin/service nfs condrestart
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6. NFS Server Configuration
To reload the NFS server configuration file without restarting the service, as root, type:
/sbin/service nfs reload
By default, the nfs service does not start automatically at boot time. To configure the NFS to
start up at boot time, use an initscript utility, such as /sbin/chkconfig, /usr/sbin/ntsysv, or the
Services Configuration Tool program. Refer to Chapter 15, Controlling Access to Services for
more information regarding these tools.
6. NFS Server Configuration
There are three ways to configure an NFS server under Red Hat Enterprise Linux: using the
NFS Server Configuration Tool (system-config-nfs), manually editing its configuration file (/
etc/exports), or using the /usr/sbin/exportfs command.
To use the NFS Server Configuration Tool, you must be running X Windows, have root privileges, and have the system-config-nfs RPM package installed. To start the application, click on
System => Administration => Server Settings => NFS. You can also type the command system-config-nfs in a terminal. The NFS Server Configuration tool window is illustrated below.
Figure 18.1. NFS Server Configuration Tool
Based on certain firewall settings, you may need to configure the NFS daemon processes to
use specific networking ports. The NFS server settings allows you to specify the ports for each
process instead of using the random ports assigned by the portmapper. You can set the NFS
Server settings by clicking on the Server Settings button. The figure below illustrates the NFS
Server Settings window.
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6.1. Exporting or Sharing NFS File Systems
Figure 18.2. NFS Server Settings
6.1. Exporting or Sharing NFS File Systems
Sharing or serving files from an NFS server is known as exporting the directories. The NFS
Server Configuration Tool can be used to configure a system as an NFS server.
To add an NFS share, click the Add button. The dialog box shown in Figure 18.3, “Add Share”
appears.
The Basic tab requires the following information:
•
Directory — Specify the directory to share, such as /tmp.
•
Host(s) — Specify the host(s) with which to share the directory. Refer to Section 6.3,
“Hostname Formats” for an explanation of possible formats.
•
Basic permissions — Specify whether the directory should have read-only or read/write
permissions.
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6.1. Exporting or Sharing NFS File Systems
Figure 18.3. Add Share
The General Options tab allows the following options to be configured:
Figure 18.4. NFS General Options
•
Allow connections from port 1024 and higher — Services started on port numbers less
than 1024 must be started as root. Select this option to allow the NFS service to be started
by a user other than root. This option corresponds to insecure.
•
Allow insecure file locking — Do not require a lock request. This option corresponds to insecure_locks.
•
Disable subtree checking — If a subdirectory of a file system is exported, but the entire file
system is not exported, the server checks to see if the requested file is in the subdirectory
exported. This check is called subtree checking. Select this option to disable subtree checking. If the entire file system is exported, selecting to disable subtree checking can increase
the transfer rate. This option corresponds to no_subtree_check.
•
Sync write operations on request — Enabled by default, this option does not allow the
server to reply to requests before the changes made by the request are written to the disk.
This option corresponds to sync. If this is not selected, the async option is used.
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6.1. Exporting or Sharing NFS File Systems
•
Force sync of write operations immediately — Do not delay writing to disk. This option
corresponds to no_wdelay.
•
Hide filesystems beneath turns the nohide option on or off. When the nohide option is off,
nested directories are revealed. The clients can therefore navigate through a filesystem from
the parent without noticing any changes.
•
Export only if mounted sets the mountpoint option which allows a directory to be exported
only if it has been mounted.
•
Optional Mount Point specifies the path to an optional mount point. Click on the Browse to
navigate to the preferred mount point or type the path if known.
•
Set explicit Filesystem ID: sets the fsid=X option. This is mainly used in a clustered setup.
Using a consistent filesystem ID in all clusters avoids having stale NFS filehandles.
Figure 18.5. NFS User Access
The User Access tab allows the following options to be configured:
•
Treat remote root user as local root — By default, the user and group IDs of the root user
are both 0. Root squashing maps the user ID 0 and the group ID 0 to the user and group IDs
of anonymous so that root on the client does not have root privileges on the NFS server. If
this option is selected, root is not mapped to anonymous, and root on a client has root priv-
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6.2. Command Line Configuration
ileges to exported directories. Selecting this option can greatly decrease the security of the
system. Do not select it unless it is absolutely necessary. This option corresponds to
no_root_squash.
•
Treat all client users as anonymous users — If this option is selected, all user and group
IDs are mapped to the anonymous user. This option corresponds to all_squash.
•
Specify local user ID for anonymous users — If Treat all client users as anonymous users is selected, this option lets you specify a user ID for the anonymous user. This
option corresponds to anonuid.
•
Specify local group ID for anonymous users — If Treat all client users as anonymous users is selected, this option lets you specify a group ID for the anonymous user.
This option corresponds to anongid.
To edit an existing NFS share, select the share from the list, and click the Properties button. To
delete an existing NFS share, select the share from the list, and click the Delete button.
After clicking OK to add, edit, or delete an NFS share from the list, the changes take place immediately — the server daemon is restarted and the old configuration file is saved as /
etc/exports.bak. The new configuration is written to /etc/exports.
The NFS Server Configuration Tool reads and writes directly to the /etc/exports configuration
file. Thus, the file can be modified manually after using the tool, and the tool can be used after
modifying the file manually (provided the file was modified with correct syntax).
The next this section discusses manually editing /etc/exports and using the /
usr/sbin/exportfs command to export NFS file systems.
6.2. Command Line Configuration
If you prefer editing configuration files using a text editor or if you do not have the X Window
System installed, you can modify the configuration file directly.
The /etc/exports file controls what directories the NFS server exports. Its format is as follows:
directoryhostname(options)
The only option that needs to be specified is one of sync or async (sync is recommended). If sync
is specified, the server does not reply to requests before the changes made by the request are
written to the disk.
For example,
/misc/export speedy.example.com(sync)
would allow users from speedy.example.com to mount /misc/export with the default read-only
permissions, but,
/misc/export speedy.example.com(rw,sync)
would allow users from speedy.example.com to mount /misc/export with read/write privileges.
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6.3. Hostname Formats
Refer to Section 6.3, “Hostname Formats” for an explanation of possible hostname formats.
Caution
Be careful with spaces in the /etc/exports file. If there are no spaces between the
hostname and the options in parentheses, the options apply only to the hostname.
If there is a space between the hostname and the options, the options apply to the
rest of the world. For example, examine the following lines:
/misc/export speedy.example.com(rw,sync) /misc/export speedy.example.com (rw,sync)
The first line grants users from speedy.example.com read-write access and denies
all other users. The second line grants users from speedy.example.com read-only
access (the default) and allows the rest of the world read-write access.
Each time you change /etc/exports, you must inform the NFS daemon of the change, or reload
the configuration file with the following command:
/sbin/service nfs reload
6.3. Hostname Formats
The host(s) can be in the following forms:
•
Single machine — A fully qualified domain name (that can be resolved by the server), hostname (that can be resolved by the server), or an IP address.
•
Series of machines specified with wildcards — Use the * or ? character to specify a string
match. Wildcards are not to be used with IP addresses; however, they may accidentally work
if reverse DNS lookups fail. When specifying wildcards in fully qualified domain names, dots
(.) are not included in the wildcard. For example, *.example.com includes one.example.com
but does not include one.two.example.com.
•
IP networks — Use a.b.c.d/z, where a.b.c.d is the network and z is the number of bits in
the netmask (for example 192.168.0.0/24). Another acceptable format is a.b.c.d/netmask,
where a.b.c.d is the network and netmask is the netmask (for example,
192.168.100.8/255.255.255.0).
•
Netgroups — In the format @group-name, where group-name is the NIS netgroup name.
7. The
/etc/exports
Configuration File
The /etc/exports file controls which file systems are exported to remote hosts and specifies options. Blank lines are ignored, comments can be made by starting a line with the hash mark (#),
and long lines can be wrapped with a backslash (\). Each exported file system should be on its
own individual line, and any lists of authorized hosts placed after an exported file system must
be separated by space characters. Options for each of the hosts must be placed in parentheses
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7. The /etc/exports Configuration File
directly after the host identifier, without any spaces separating the host and the first parenthesis.
Valid host types are gss/krb5gss/krb5i and gss/krb5p.
A line for an exported file system has the following structure:
<export><host1>(<options>) <hostN>(<options>)...
In this structure, replace <export> with the directory being exported, replace <host1> with the
host or network to which the export is being shared, and replace <options> with the options for
that host or network. Additional hosts can be specified in a space separated list.
The following methods can be used to specify host names:
•
single host — Where one particular host is specified with a fully qualified domain name,
hostname, or IP address.
•
wildcards — Where a * or ? character is used to take into account a grouping of fully qualified domain names that match a particular string of letters. Wildcards should not be used
with IP addresses; however, it is possible for them to work accidentally if reverse DNS lookups fail.
Be careful when using wildcards with fully qualified domain names, as they tend to be more
exact than expected. For example, the use of *.example.com as a wildcard allows
sales.example.com to access an exported file system, but not bob.sales.example.com. To
match both possibilities both *.example.com and *.*.example.com must be specified.
•
IP networks — Allows the matching of hosts based on their IP addresses within a larger network. For example, 192.168.0.0/28 allows the first 16 IP addresses, from 192.168.0.0 to
192.168.0.15, to access the exported file system, but not 192.168.0.16 and higher.
•
netgroups — Permits an NIS netgroup name, written as @<group-name>, to be used. This effectively puts the NIS server in charge of access control for this exported file system, where
users can be added and removed from an NIS group without affecting /etc/exports.
In its simplest form, the /etc/exports file only specifies the exported directory and the hosts permitted to access it, as in the following example:
/exported/directory bob.example.com
In the example, bob.example.com can mount /exported/directory/. Because no options are specified in this example, the following default NFS options take effect:
•
ro
— Mounts of the exported file system are read-only. Remote hosts are not able to make
changes to the data shared on the file system. To allow hosts to make changes to the file
system, the read/write (rw) option must be specified.
•
wdelay
•
root_squash
— Causes the NFS server to delay writing to the disk if it suspects another write request is imminent. This can improve performance by reducing the number of times the disk
must be accessed by separate write commands, reducing write overhead. The no_wdelay option turns off this feature, but is only available when using the sync option.
— Prevents root users connected remotely from having root privileges and assigns them the user ID for the user nfsnobody. This effectively "squashes" the power of the
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7.1. The exportfs Command
remote root user to the lowest local user, preventing unauthorized alteration of files on the
remote server. Alternatively, the no_root_squash option turns off root squashing. To squash
every remote user, including root, use the all_squash option. To specify the user and group
IDs to use with remote users from a particular host, use the anonuid and anongid options, respectively. In this case, a special user account can be created for remote NFS users to
share and specify (anonuid=<uid-value>,anongid=<gid-value>), where <uid-value> is the
user ID number and <gid-value> is the group ID number.
Important
By default, access control lists (ACLs) are supported by NFS under Red Hat Enterprise Linux. To disable this feature, specify the no_acl option when exporting the
file system.
Each default for every exported file system must be explicitly overridden. For example, if the rw
option is not specified, then the exported file system is shared as read-only. The following is a
sample line from /etc/exports which overrides two default options:
/another/exported/directory 192.168.0.3(rw,sync)
In this example 192.168.0.3 can mount /another/exported/directory/ read/write and all transfers to disk are committed to the disk before the write request by the client is completed.
Additionally, other options are available where no default value is specified. These include the
ability to disable sub-tree checking, allow access from insecure ports, and allow insecure file
locks (necessary for certain early NFS client implementations). Refer to the exports man page
for details on these lesser used options.
Warning
The format of the /etc/exports file is very precise, particularly in regards to use of
the space character. Remember to always separate exported file systems from
hosts and hosts from one another with a space character. However, there should
be no other space characters in the file except on comment lines.
For example, the following two lines do not mean the same thing:
/home bob.example.com(rw) /home bob.example.com (rw)
The first line allows only users from bob.example.com read/write access to the /home
directory. The second line allows users from bob.example.com to mount the directory as read-only (the default), while the rest of the world can mount it read/write.
7.1. The exportfs Command
Every file system being exported to remote users via NFS, as well as the access level for those
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7.1. The exportfs Command
file systems, are listed in the /etc/exports file. When the nfs service starts, the /
usr/sbin/exportfs command launches and reads this file, passes control to rpc.mountd (if NFSv2 or NFSv3) for the actual mounting process, then to rpc.nfsd where the file systems are
then available to remote users.
When issued manually, the /usr/sbin/exportfs command allows the root user to selectively export or unexport directories without restarting the NFS service. When given the proper options,
the /usr/sbin/exportfs command writes the exported file systems to /var/lib/nfs/xtab. Since
rpc.mountd refers to the xtab file when deciding access privileges to a file system, changes to
the list of exported file systems take effect immediately.
The following is a list of commonly used options available for /usr/sbin/exportfs:
•
-r
— Causes all directories listed in /etc/exports to be exported by constructing a new export list in /etc/lib/nfs/xtab. This option effectively refreshes the export list with any
changes that have been made to /etc/exports.
•
-a
•
-o file-systems
•
-i
•
-u
•
-v
— Causes all directories to be exported or unexported, depending on what other options
are passed to /usr/sbin/exportfs. If no other options are specified, /usr/sbin/exportfs exports all file systems specified in /etc/exports.
— Specifies directories to be exported that are not listed in /etc/exports.
Replace file-systems with additional file systems to be exported. These file systems must
be formatted in the same way they are specified in /etc/exports. Refer to Section 7, “The /
etc/exports Configuration File” for more information on /etc/exports syntax. This option is
often used to test an exported file system before adding it permanently to the list of file systems to be exported.
— Ignores /etc/exports; only options given from the command line are used to define exported file systems.
— Unexports all shared directories. The command /usr/sbin/exportfs -ua suspends
NFS file sharing while keeping all NFS daemons up. To re-enable NFS sharing, type exportfs -r.
— Verbose operation, where the file systems being exported or unexported are displayed
in greater detail when the exportfs command is executed.
If no options are passed to the /usr/sbin/exportfs command, it displays a list of currently exported file systems.
For more information about the /usr/sbin/exportfs command, refer to the exportfs man page.
7.1.1. Using exportfs with NFSv4
The exportfs command is used in maintaining the NFS table of exported file systems. When
typed in a terminal with no arguments, the exportfs command shows all the exported directories.
Since NFSv4 no longer utilizes the rpc.mountd protocol as was used in NFSv2 and NFSv3, the
mounting of file systems has changed.
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8. Securing NFS
An NFSv4 client now has the ability to see all of the exports served by the NFSv4 server as a
single file system, called the NFSv4 pseudo-file system. On Red Hat Enterprise Linux, the
pseudo-file system is identified as a single, real file system, identified at export with the fsid=0
option.
For example, the following commands could be executed on an NFSv4 server:
mkdir /exports mkdir /exports/opt mkdir /exports/etc mount --bind /usr/local/opt /exports/opt mount --bind
In this example, clients are provided with multiple file systems to mount, by using the --bind option which creates unbreakeable links.
Because of the pseudo-file systems feature, NFS version 2, 3 and 4 export configurations are
not always compatible. For example, given the following directory tree:
/home /home/sam /home/john /home/joe
and the export:
/home *(rw,fsid=0,sync)
Using NFS version 2,3 and 4 the following would work:
mount server:/home /mnt/home ls /mnt/home/joe
Using v4 the following would work:
mount -t nfs4 server:/ /mnt/home ls /mnt/home/joe
The difference being "server:/home" and "server:/". To make the exports configurations compatible for all version, one needs to export (read only) the root filesystem with an fsid=0. The
fsid=0 signals the NFS server that this export is the root.
/ *(ro,fsid=0) /home *(rw,sync,nohide)
Now with these exports, both "mount
/mnt/home" will work as expected.
server:/home /mnt/home"
and "mount
-t nfs server:/home
8. Securing NFS
NFS is well suited for sharing entire file systems with a large number of known hosts in a transparent manner. However, with ease of use comes a variety of potential security problems.
The following points should be considered when exporting NFS file systems on a server or
mounting them on a client. Doing so minimizes NFS security risks and better protects data on
the server.
8.1. Host Access
Depending on which version of NFS you plan to implement, depends on your existing network
environment, and your security concerns. The following sections explain the differences
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8.1. Host Access
between implementing security measures with NFSv2, NFSv3, and NFSv4. If at all possible,
use of NFSv4 is recommended over other versions of NFS.
8.1.1. Using NFSv2 or NFSv3
NFS controls who can mount an exported file system based on the host making the mount request, not the user that actually uses the file system. Hosts must be given explicit rights to
mount the exported file system. Access control is not possible for users, other than through file
and directory permissions. In other words, once a file system is exported via NFS, any user on
any remote host connected to the NFS server can access the shared data. To limit the potential
risks, administrators often allow read-only access or squash user permissions to a common
user and group ID. Unfortunately, these solutions prevent the NFS share from being used in the
way it was originally intended.
Additionally, if an attacker gains control of the DNS server used by the system exporting the
NFS file system, the system associated with a particular hostname or fully qualified domain
name can be pointed to an unauthorized machine. At this point, the unauthorized machine is the
system permitted to mount the NFS share, since no username or password information is exchanged to provide additional security for the NFS mount.
Wildcards should be used sparingly when exporting directories via NFS as it is possible for the
scope of the wildcard to encompass more systems than intended.
It is also possible to restrict access to the portmap service via TCP wrappers. Access to ports
used by portmap, rpc.mountd, and rpc.nfsd can also be limited by creating firewall rules with
iptables.
For more information on securing NFS and portmap, refer to Section 9, “IPTables”.
8.1.2. Using NFSv4
The release of NFSv4 brought a revolution to authentication and security to NFS exports. NFSv4 mandates the implementation of the RPCSEC_GSS kernel module, the Kerberos version 5
GSS-API mechanism, SPKM-3, and LIPKEY. With NFSv4, the mandatory security mechanisms
are oriented towards authenticating individual users, and not client machines as used in NFSv2
and NFSv3.
Note
It is assumed that a Kerberos ticket-granting server (KDC) is installed and configured correctly, prior to configuring an NFSv4 server. Kerberos is a network authentication system which allows clients and servers to authenticate to each other
through use of symmetric encryption and a trusted third party, the KDC.
NFSv4 includes ACL support based on the Microsoft Windows NT model, not the POSIX model,
because of its features and because it is widely deployed. NFSv2 and NFSv3 do not have support for native ACL attributes.
Another important security feature of NFSv4 is its removal of the rpc.mountd daemon. The
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8.2. File Permissions
rpc.mountd
daemon presented possible security holes because of the way it dealt with filehand-
lers.
For more information on the RPCSEC_GSS framework, including how rpc.svcgssd and
rpc.gssd inter operate, refer to http://www.citi.umich.edu/projects/nfsv4/gssd/.
8.2. File Permissions
Once the NFS file system is mounted read/write by a remote host, the only protection each
shared file has is its permissions. If two users that share the same user ID value mount the
same NFS file system, they can modify each others files. Additionally, anyone logged in as root
on the client system can use the su - command to become a user who could access particular
files via the NFS share.
By default, access control lists (ACLs) are supported by NFS under Red Hat Enterprise Linux. It
is not recommended that this feature be disabled.
The default behavior when exporting a file system via NFS is to use root squashing. This sets
the user ID of anyone accessing the NFS share as the root user on their local machine to a
value of the server's nfsnobody account. Never turn off root squashing.
If exporting an NFS share as read-only, consider using the all_squash option, which makes
every user accessing the exported file system take the user ID of the nfsnobody user.
9. NFS and
portmap
Note
The following section only applies to NFSv2 or NFSv3 implementations that require the portmap service for backward compatibility.
The portmapper maps RPC services to the ports they are listening on. RPC processes notify
portmap when they start, registering the ports they are listening on and the RPC program numbers they expect to serve. The client system then contacts portmap on the server with a particular RPC program number. The portmap service redirects the client to the proper port number so
it can communicate with the requested service.
Because RPC-based services rely on portmap to make all connections with incoming client requests, portmap must be available before any of these services start.
The portmap service uses TCP wrappers for access control, and access control rules for portmap
affect all RPC-based services. Alternatively, it is possible to specify access control rules for
each of the NFS RPC daemons. The man pages for rpc.mountd and rpc.statd contain information regarding the precise syntax for these rules.
9.1. Troubleshooting NFS and portmap
Because portmap provides coordination between RPC services and the port numbers used to
communicate with them, it is useful to view the status of current RPC services using portmap
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10. Using NFS over TCP
when troubleshooting. The rpcinfo command shows each RPC-based service with port numbers, an RPC program number, a version number, and an IP protocol type (TCP or UDP).
To make sure the proper NFS RPC-based services are enabled for portmap, issue the following
command as root:
rpcinfo -p
The following is sample output from this command:
program vers proto port 100000 2 tcp 111 portmapper 100000 2 udp 111 portmapper 100021 1 udp 32774 nlockmg
If one of the NFS services does not start up correctly, portmap is unable to map RPC requests
from clients for that service to the correct port. In many cases, if NFS is not present in rpcinfo
output, restarting NFS causes the service to correctly register with portmap and begin working.
For instructions on starting NFS, refer to Section 5, “Starting and Stopping NFS”.
Other useful options are available for the rpcinfo command. Refer to the rpcinfo man page for
more information.
10. Using NFS over TCP
The default transport protocol for NFSv4 is TCP; however, the Red Hat Enterprise Linux 5 kernel includes support for NFS over UDP. To use NFS over UDP, include the -o udp option to
mount when mounting the NFS-exported file system on the client system.
There are three ways to configure an NFS file system export. On demand via the command line
(client side), automatically via the /etc/fstab file (client side), and automatically via autofs configuration files, such as /etc/auto.master and /etc/auto.misc (server side with NIS).
For example, on demand via the command line (client side):
mount -o udp shadowman.example.com:/misc/export /misc/local
When the NFS mount is specified in /etc/fstab (client side):
server:/usr/local/pub /pub nfs rsize=8192,wsize=8192,timeo=14,intr,udp
When the NFS mount is specified in an autofs configuration file for a NIS server, available for
NIS enabled workstations:
myproject -rw,soft,intr,rsize=8192,wsize=8192,udp penguin.example.net:/proj52
Since the default is TCP, if the -o
accessed via TCP.
udp
option is not specified, the NFS-exported file system is
The advantages of using TCP include the following:
•
Improved connection durability, thus less NFS
•
Performance gain on heavily loaded networks because TCP acknowledges every packet,
unlike UDP which only acknowledges completion.
stale file handles
243
messages.
11. Additional Resources
•
TCP has better congestion control than UDP. On a very congested network, UDP packets
are the first packets that are dropped. This means that if NFS is writing data (in 8K chunks)
all of that 8K must be retransmitted over UDP. Because of TCP's reliability, only parts of that
8K data are transmitted at a time.
•
Error detection. When a TCP connection breaks (due to the server being unavailable) the client stops sending data and restarts the connection process once the server becomes available. With UDP, since it's connection-less, the client continues to pound the network with
data until the server reestablishes a connection.
The main disadvantage is that there is a very small performance hit due to the overhead associated with the TCP protocol.
11. Additional Resources
Administering an NFS server can be a challenge. Many options, including quite a few not mentioned in this chapter, are available for exporting or mounting NFS shares. Consult the following
sources for more information.
11.1. Installed Documentation
•
/usr/share/doc/nfs-utils-<version-number>/
— Replace <version-number> with the version
number of the NFS package installed. This directory contains a wealth of information about
the NFS implementation for Linux, including a look at various NFS configurations and their
impact on file transfer performance.
•
man mount
•
man fstab
— Contains a comprehensive look at mount options for both NFS server and client configurations.
— Gives details for the format of the /etc/fstab file used to mount file systems at
boot-time.
•
man nfs
— Provides details on NFS-specific file system export and mount options.
•
man exports
— Shows common options used in the /etc/exports file when exporting NFS
file systems.
11.2. Useful Websites
•
http://nfs.sourceforge.net/ — The home of the Linux NFS project and a great place for
project status updates.
•
http://www.citi.umich.edu/projects/nfsv4/linux/ — An NFSv4 for Linux 2.6 kernel resource.
•
http://www.nfsv4.org [http://www.nfsv4.org/] — The home of NFS version 4 and all related
standards.
•
http://www.vanemery.com/Linux/NFSv4/NFSv4-no-rpcsec.html — Describes the details of
NFSv4 with Fedora Core 2, which includes the 2.6 kernel.
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11.3. Related Books
•
http://www.nluug.nl/events/sane2000/papers/pawlowski.pdf — An excellent whitepaper on
the features and enhancements of the NFS Version 4 protocol.
•
http://wiki.autofs.net — The Autofs wiki, discussions, documentation and enhancements.
11.3. Related Books
•
Managing NFS and NIS by Hal Stern, Mike Eisler, and Ricardo Labiaga; O'Reilly & Associates — Makes an excellent reference guide for the many different NFS export and mount
options available.
•
NFS Illustrated by Brent Callaghan; Addison-Wesley Publishing Company — Provides comparisons of NFS to other network file systems and shows, in detail, how NFS communication
occurs.
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Chapter 19. Samba
Samba is an open source implementation of the Server Message Block (SMB) protocol. It allows the networking of Microsoft Windows®, Linux, UNIX, and other operating systems together, enabling access to Windows-based file and printer shares. Samba's use of SMB allows it to
appear as a Windows server to Windows clients.
1. Introduction to Samba
The third major release of Samba, version 3.0.0, introduced numerous improvements from prior
versions, including:
•
The ability to join an Active Directory domain by means of LDAP and Kerberos
•
Built in Unicode support for internationalization
•
Support for Microsoft Windows XP Professional client connections to Samba servers without
needing local registry hacking
•
Two new documents developed by the Samba.org team, which include a 400+ page reference manual, and a 300+ page implementation and integration manual. For more information about these published titles, refer to Section 12.2, “Related Books”.
1.1. Samba Features
Samba is a powerful and versatile server application. Even seasoned system administrators
must know its abilities and limitations before attempting installation and configuration.
What Samba can do:
•
Serve directory trees and printers to Linux, UNIX, and Windows clients
•
Assist in network browsing (with or without NetBIOS)
•
Authenticate Windows domain logins
•
Provide Windows Internet Name Service (WINS) name server resolution
•
Act as a Windows NT®-style Primary Domain Controller (PDC)
•
Act as a Backup Domain Controller (BDC) for a Samba-based PDC
•
Act as an Active Directory domain member server
•
Join a Windows NT/2000/2003 PDC
What Samba cannot do:
•
Act as a BDC for a Windows PDC (and vice versa)
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2. Samba Daemons and Related Services
•
Act as an Active Directory domain controller
2. Samba Daemons and Related Services
The following is a brief introduction to the individual Samba daemons and services.
2.1. Samba Daemons
Samba is comprised of three daemons (smbd, nmbd, and winbindd). Two services (smb and windbind) control how the daemons are started, stopped, and other service-related features. Each
daemon is listed in detail, as well as which specific service has control over it.
smbd.
The smbd server daemon provides file sharing and printing services to Windows clients. In
addition, it is responsible for user authentication, resource locking, and data sharing through the
SMB protocol. The default ports on which the server listens for SMB traffic are TCP ports 139
and 445.
The smbd daemon is controlled by the smb service.
nmbd.
The nmbd server daemon understands and replies to NetBIOS name service requests
such as those produced by SMB/CIFS in Windows-based systems. These systems include Windows 95/98/ME, Windows NT, Windows 2000, Windows XP, and LanManager clients. It also
participates in the browsing protocols that make up the Windows Network Neighborhood view.
The default port that the server listens to for NMB traffic is UDP port 137.
The nmbd daemon is controlled by the smb service.
winbindd.
The winbind service resolves user and group information on a server running Windows NT 2000 or Windows Server 2003. This makes Windows user / group information understandable by UNIX platforms. This is achieved by using Microsoft RPC calls, Pluggable Authentication Modules (PAM), and the Name Service Switch (NSS). This allows Windows NT domain
users to appear and operate as UNIX users on a UNIX machine. Though bundled with the
Samba distribution, the winbind service is controlled separately from the smb service.
The winbindd daemon is controlled by the winbind service and does not require the smb service
to be started in order to operate. Winbindd is also used when Samba is an Active Directory
member, and may also be used on a Samba domain controller (to implement nested groups
and/or interdomain trust). Because winbind is a client-side service used to connect to Windows
NT-based servers, further discussion of winbind is beyond the scope of this manual.
Note
You may refer to Section 11, “Samba Distribution Programs” for a list of utilities included in the Samba distribution.
3. Connecting to a Samba Share
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3. Connecting to a Samba Share
You can use Nautilus to view available Samba shares on your network. Select Places (on the
Panel) => Network Servers to view a list of Samba workgroups on your network. You can also
type smb: in the File => Open Location bar of Nautilus to view the workgroups.
As shown in Figure 19.1, “SMB Workgroups in Nautilus”, an icon appears for each available
SMB workgroup on the network.
Figure 19.1. SMB Workgroups in Nautilus
Double-click one of the workgroup icons to view a list of computers within the workgroup.
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3.1. Command Line
Figure 19.2. SMB Machines in Nautilus
As you can see from Figure 19.2, “SMB Machines in Nautilus”, there is an icon for each machine within the workgroup. Double-click on an icon to view the Samba shares on the machine.
If a username and password combination is required, you are prompted for them.
Alternately, you can also specify the Samba server and sharename in the Location: bar for
Nautilus using the following syntax (replace <servername> and <sharename> with the appropriate
values):
smb://<servername>/<sharename>
3.1. Command Line
To query the network for Samba servers, use the findsmb command. For each server found, it
displays its IP address, NetBIOS name, workgroup name, operating system, and SMB server
version.
To connect to a Samba share from a shell prompt, type the following command:
smbclient //<hostname>/<sharename> -U <username>
Replace <hostname> with the hostname or IP address of the Samba server you want to connect
to, <sharename> with the name of the shared directory you want to browse, and <username> with
the Samba username for the system. Enter the correct password or press Enter if no password
is required for the user.
If you see the smb:\> prompt, you have successfully logged in. Once you are logged in, type
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3.2. Mounting the Share
for a list of commands. If you wish to browse the contents of your home directory, replace
with your username. If the -U switch is not used, the username of the current user is
passed to the Samba server.
help
sharename
To exit smbclient, type exit at the smb:\> prompt.
3.2. Mounting the Share
Sometimes it is useful to mount a Samba share to a directory so that the files in the directory
can be treated as if they are part of the local file system.
To mount a Samba share to a directory, create create a directory to mount it to (if it does not
already exist), and execute the following command as root:
mount -t cifs -o <username>,<password> //<servername>/<sharename>/mnt/point/
This command mounts <sharename> from <servername> in the local directory /mnt/point/. For
more information about mounting a samba share, refer to man mount.cifs.
4. Configuring a Samba Server
The default configuration file (/etc/samba/smb.conf) allows users to view their home directories
as a Samba share. It also shares all printers configured for the system as Samba shared printers. In other words, you can attach a printer to the system and print to it from the Windows machines on your network.
4.1. Graphical Configuration
To configure Samba using a graphical interface, use the Samba Server Configuration Tool.
For command line configuration, skip to Section 4.2, “Command Line Configuration”.
The Samba Server Configuration Tool is a graphical interface for managing Samba shares,
users, and basic server settings. It modifies the configuration files in the /etc/samba/ directory.
Any changes to these files not made using the application are preserved.
To use this application, you must be running the X Window System, have root privileges, and
have the system-config-samba RPM package installed. To start the Samba Server Configuration Tool from the desktop, go to the System (on the Panel) => Administration => Server
Settings => Samba or type the command system-config-samba at a shell prompt (for example,
in an XTerm or a GNOME terminal).
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4.1. Graphical Configuration
Figure 19.3. Samba Server Configuration Tool
Note
The Samba Server Configuration Tool does not display shared printers or the
default stanza that allows users to view their own home directories on the Samba
server.
4.1.1. Configuring Server Settings
The first step in configuring a Samba server is to configure the basic settings for the server and
a few security options. After starting the application, select Preferences => Server Settings
from the pulldown menu. The Basic tab is displayed as shown in Figure 19.4, “Configuring Basic Server Settings”.
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4.1. Graphical Configuration
Figure 19.4. Configuring Basic Server Settings
On the Basic tab, specify which workgroup the computer should be in as well as a brief description of the computer. They correspond to the workgroup and server string options in smb.conf.
Figure 19.5. Configuring Security Server Settings
The Security tab contains the following options:
•
Authentication Mode — This corresponds to the security option. Select one of the following types of authentication.
•
ADS — The Samba server acts as a domain member in an Active Directory Domain
(ADS) realm. For this option, Kerberos must be installed and configured on the server,
and Samba must become a member of the ADS realm using the net utility, which is part
of the samba-client package. Refer to the net man page for details. This option does not
configure Samba to be an ADS Controller. Specify the realm of the Kerberos server in
the Kerberos Realm field.
Note
The Kerberos Realm field must be supplied in all uppercase letters, such
as EXAMPLE.COM.
Using a Samba server as a domain member in an ADS realm assumes
proper configuration of Kerberos, including the /etc/krb5.conf file.
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4.1. Graphical Configuration
•
Domain — The Samba server relies on a Windows NT Primary or Backup Domain Controller to verify the user. The server passes the username and password to the Controller
and waits for it to return. Specify the NetBIOS name of the Primary or Backup Domain
Controller in the Authentication Server field.
The Encrypted Passwords option must be set to Yes if this is selected.
•
Server — The Samba server tries to verify the username and password combination by
passing them to another Samba server. If it can not, the server tries to verify using the
user authentication mode. Specify the NetBIOS name of the other Samba server in the
Authentication Server field.
•
Share — Samba users do not have to enter a username and password combination on a
per Samba server basis. They are not prompted for a username and password until they
try to connect to a specific shared directory from a Samba server.
•
User — (Default) Samba users must provide a valid username and password on a per
Samba server basis. Select this option if you want the Windows Username option to
work. Refer to Section 4.1.2, “Managing Samba Users” for details.
•
Encrypt Passwords — This option must be enabled if the clients are connecting from a system with Windows 98, Windows NT 4.0 with Service Pack 3, or other more recent versions
of Microsoft Windows. The passwords are transfered between the server and the client in an
encrypted format instead of as a plain-text word that can be intercepted. This corresponds to
the encrypted passwords option. Refer to Section 4.3, “Encrypted Passwords” for more information about encrypted Samba passwords.
•
Guest Account — When users or guest users log into a Samba server, they must be
mapped to a valid user on the server. Select one of the existing usernames on the system to
be the guest Samba account. When guests log in to the Samba server, they have the same
privileges as this user. This corresponds to the guest account option.
After clicking OK, the changes are written to the configuration file and the daemon is restarted;
thus, the changes take effect immediately.
4.1.2. Managing Samba Users
The Samba Server Configuration Tool requires that an existing user account be active on the
system acting as the Samba server before a Samba user can be added. The Samba user is associated with the existing user account.
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4.1. Graphical Configuration
Figure 19.6. Managing Samba Users
To add a Samba user, select Preferences => Samba Users from the pulldown menu, and click
the Add User button. In the Create New Samba User window select a Unix Username from
the list of existing users on the local system.
If the user has a different username on a Windows machine and needs to log into the Samba
server from the Windows machine, specify that Windows username in the Windows Username
field. The Authentication Mode on the Security tab of the Server Settings preferences must
be set to User for this option to work.
Also, configure a Samba Password for the Samba User and confirm it by typing it again. Even
if you opt to use encrypted passwords for Samba, it is recommended that the Samba passwords
for all users are different from their system passwords.
To edit an existing user, select the user from the list, and click Edit User. To delete an existing
Samba user, select the user, and click the Delete User button. Deleting a Samba user does not
delete the associated system user account.
The users are modified immediately after clicking the OK button.
4.1.3. Adding a Share
To create a Samba share, click the Add button from the main Samba configuration window.
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4.2. Command Line Configuration
Figure 19.7. Adding a Share
The Basic tab configures the following options:
•
Directory — The directory to share via Samba. The directory must exist before it can be
entered here.
•
Share name — The actual name of the share that is seen from remote machines. By default, it is the same value as Directory, but can be configured.
•
Descriptions — A brief description of the share.
•
Writable — Enables users to read and write to the shared directory
•
Visible — Grants read-only rights to users for the shared directory.
On the Access tab, select whether to allow only specified users to access the share or whether
to allow all Samba users to access the share. If you select to allow access to specific users, select the users from the list of available Samba users.
The share is added immediately after clicking OK.
4.2. Command Line Configuration
Samba uses /etc/samba/smb.conf as its configuration file. If you change this configuration file,
the changes do not take effect until you restart the Samba daemon with the command service
smb restart.
To specify the Windows workgroup and a brief description of the Samba server, edit the following lines in your smb.conf file:
workgroup = WORKGROUPNAME
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4.3. Encrypted Passwords
server string = BRIEF COMMENT ABOUT SERVER
Replace WORKGROUPNAME with the name of the Windows workgroup to which this machine should
belong. The BRIEF COMMENT ABOUT SERVER is optional and is used as the Windows comment
about the Samba system.
To create a Samba share directory on your Linux system, add the following section to your
smb.conf file (after modifying it to reflect your needs and your system):
[sharename]
comment = Insert a comment here
path = /home/share/
valid users = tfox carole
public = no
writable = yes
printable = no
create mask = 0765
The above example allows the users tfox and carole to read and write to the directory /
home/share, on the Samba server, from a Samba client.
4.3. Encrypted Passwords
Encrypted passwords are enabled by default because it is more secure to do so. To create a
user with an encrypted password, use the command smbpasswd -a <username>.
5. Starting and Stopping Samba
To start a Samba server, type the following command in a shell prompt while logged in as root:
/sbin/service smb start
Important
To set up a domain member server, you must first join the domain or Active Directory using the net join command before starting the smb service.
To stop the server, type the following command in a shell prompt while logged in as root:
/sbin/service smb stop
The restart option is a quick way of stopping and then starting Samba. This is the most reliable
way to make configuration changes take effect after editing the configuration file for Samba.
Note that the restart option starts the daemon even if it was not running originally.
To restart the server, type the following command in a shell prompt while logged in as root:
/sbin/service smb restart
The condrestart (conditional restart) option only starts smb on the condition that it is currently
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6. Samba Server Types and the smb.conf File
running. This option is useful for scripts, because it does not start the daemon if it is not running.
Note
When the smb.conf file is changed, Samba automatically reloads it after a few
minutes. Issuing a manual restart or reload is just as effective.
To conditionally restart the server, type the following command as root:
/sbin/service smb condrestart
A manual reload of the smb.conf file can be useful in case of a failed automatic reload by the smb
service. To ensure that the Samba server configuration file is reloaded without restarting the
service, type the following command as root:
/sbin/service smb reload
By default, the smb service does not start automatically at boot time. To configure Samba to start
at boot time, use an initscript utility, such as /sbin/chkconfig, /usr/sbin/ntsysv, or the Services
Configuration Tool program. Refer to Chapter 15, Controlling Access to Services for more information regarding these tools.
6. Samba Server Types and the
smb.conf
File
Samba configuration is straightforward. All modifications to Samba are done in the /
etc/samba/smb.conf configuration file. Although the default smb.conf file is well documented, it
does not address complex topics such as LDAP, Active Directory, and the numerous domain
controller implementations.
The following sections describe the different ways a Samba server can be configured. Keep in
mind your needs and the changes required to the smb.conf file for a successful configuration.
6.1. Stand-alone Server
A stand-alone server can be a workgroup server or a member of a workgroup environment. A
stand-alone server is not a domain controller and does not participate in a domain in any way.
The following examples include several anonymous share-level security configurations and one
user-level security configuration. For more information on share-level and user-level security
modes, refer to Section 7, “Samba Security Modes”.
6.1.1. Anonymous Read-Only
The following smb.conf file shows a sample configuration needed to implement anonymous
read-only file sharing. The security = share parameter makes a share anonymous. Note, security levels for a single Samba server cannot be mixed. The security directive is a global
Samba parameter located in the [global] configuration section of the smb.conf file.
[global]
workgroup = DOCS
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6.1. Stand-alone Server
netbios name = DOCS_SRV
security = share
[data]
comment = Documentation Samba Server
path = /export
read only = Yes
guest only = Yes
6.1.2. Anonymous Read/Write
The following smb.conf file shows a sample configuration needed to implement anonymous
read/write file sharing. To enable anonymous read/write file sharing, set the read only directive
to no. The force user and force group directives are also added to enforce the ownership of
any newly placed files specified in the share.
Note
Although having an anonymous read/write server is possible, it is not recommended. Any files placed in the share space, regardless of user, are assigned the
user/group combination as specified by a generic user (force user) and group
(force group) in the smb.conf file.
[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = share
[data]
comment = Data
path = /export
force user = docsbot
force group = users
read only = No
guest ok = Yes
6.1.3. Anonymous Print Server
The following smb.conf file shows a sample configuration needed to implement an anonymous
print server. Setting browseable to no as shown does not list the printer in Windows Network
Neighborhood. Although hidden from browsing, configuring the printer explicitly is possible. By
connecting to DOCS_SRV using NetBIOS, the client can have access to the printer if the client is
also part of the DOCS workgroup. It is also assumed that the client has the correct local printer
driver installed, as the use client driver directive is set to Yes. In this case, the Samba server
has no responsibility for sharing printer drivers to the client.
[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = share
printcap name = cups
disable spools= Yes
show add printer wizard = No
printing = cups
[printers]
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6.2. Domain Member Server
comment = All Printers
path = /var/spool/samba
guest ok = Yes
printable = Yes
use client driver = Yes
browseable = Yes
6.1.4. Secure Read/Write File and Print Server
The following smb.conf file shows a sample configuration needed to implement a secure read/
write print server. Setting the security directive to user forces Samba to authenticate client connections. Notice the [homes] share does not have a force user or force group directive as the
[public] share does. The [homes] share uses the authenticated user details for any files created
as opposed to the force user and force group in [public].
[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = user
printcap name = cups
disable spools = Yes
show add printer wizard = No
printing = cups
[homes]
comment = Home Directories
valid users = %S
read only = No
browseable = No
[public]
comment = Data
path = /export
force user = docsbot
force group = users
guest ok = Yes
[printers]
comment = All Printers
path = /var/spool/samba
printer admin = john, ed, @admins
create mask = 0600
guest ok = Yes
printable = Yes
use client driver = Yes
browseable = Yes
6.2. Domain Member Server
A domain member, while similar to a stand-alone server, is logged into a domain controller
(either Windows or Samba) and is subject to the domain's security rules. An example of a domain member server would be a departmental server running Samba that has a machine account on the Primary Domain Controller (PDC). All of the department's clients still authenticate
with the PDC, and desktop profiles and all network policy files are included. The difference is
that the departmental server has the ability to control printer and network shares.
6.2.1. Active Directory Domain Member Server
The following smb.conf file shows a sample configuration needed to implement an Active Directory domain member server. In this example, Samba authenticates users for services being run
locally but is also a client of the Active Directory. Ensure that your kerberos realm parameter is
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6.2. Domain Member Server
shown in all caps (for example realm = EXAMPLE.COM). Since Windows 2000/2003 requires Kerberos for Active Directory authentication, the realm directive is required. If Active Directory and
Kerberos are running on different servers, the password server directive may be required to help
the distinction.
[global]
realm = EXAMPLE.COM
security = ADS
encrypt passwords = yes
# Optional. Use only if Samba cannot determine the Kerberos server automatically.
password server = kerberos.example.com
In order to join a member server to an Active Directory domain, the following steps must be
completed:
•
Configuration of the smb.conf file on the member server
•
Configuration of Kerberos, including the /etc/krb5.conf file, on the member server
•
Creation of the machine account on the Active Directory domain server
•
Association of the member server to the Active Directory domain
To create the machine account and join the Windows 2000/2003 Active Directory, Kerberos
must first be initialized for the member server wishing to join the Active Directory domain. To
create an administrative Kerberos ticket, type the following command as root on the member
server:
kinit [email protected]
The kinit command is a Kerberos initialization script that references the Active Directory administrator account and Kerberos realm. Since Active Directory requires Kerberos tickets, kinit
obtains and caches a Kerberos ticket-granting ticket for client/server authentication. For more
information on Kerberos, the /etc/krb5.conf file, and the kinit command, refer to Section 6,
“Kerberos”.
To join an Active Directory server (windows1.example.com), type the following command as
root on the member server:
net ads join -S windows1.example.com -U administrator%password
Since the machine windows1 was automatically found in the corresponding Kerberos realm (the
kinit command succeeded), the net command connects to the Active Directory server using its
required administrator account and password. This creates the appropriate machine account on
the Active Directory and grants permissions to the Samba domain member server to join the domain.
Note
Since security = ads and not security = user is used, a local password backend
such as smbpasswd is not needed. Older clients that do not support security = ads
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6.3. Domain Controller
are authenticated as if security = domain had been set. This change does not affect functionality and allows local users not previously in the domain.
6.2.2. Windows NT4-based Domain Member Server
The following smb.conf file shows a sample configuration needed to implement a Windows
NT4-based domain member server. Becoming a member server of an NT4-based domain is
similar to connecting to an Active Directory. The main difference is NT4-based domains do not
use Kerberos in their authentication method, making the smb.conf file simpler. In this instance,
the Samba member server functions as a pass through to the NT4-based domain server.
[global]
workgroup = DOCS
netbios name = DOCS_SRV
security = domain
[homes]
comment = Home Directories
valid users = %S
read only = No
browseable = No
[public]
comment = Data
path = /export
force user = docsbot
force group = users
guest ok = Yes
Having Samba as a domain member server can be useful in many situations. There are times
where the Samba server can have other uses besides file and printer sharing. It may be beneficial to make Samba a domain member server in instances where Linux-only applications are required for use in the domain environment. Administrators appreciate keeping track of all machines in the domain, even if not Windows-based. In the event the Windows-based server hardware is deprecated, it is quite easy to modify the smb.conf file to convert the server to a Sambabased PDC. If Windows NT-based servers are upgraded to Windows 2000/2003, the smb.conf
file is easily modifiable to incorporate the infrastructure change to Active Directory if needed.
Important
After configuring the smb.conf file, join the domain before starting Samba by typing
the following command as root:
net rpc join -U administrator%password
Note that the -S option, which specifies the domain server hostname, does not need to be
stated in the net rpc join command. Samba uses the hostname specified by the workgroup directive in the smb.conf file instead of it being stated explicitly.
6.3. Domain Controller
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6.3. Domain Controller
A domain controller in Windows NT is functionally similar to a Network Information Service (NIS)
server in a Linux environment. Domain controllers and NIS servers both host user/group information databases as well as related services. Domain controllers are mainly used for security, including the authentication of users accessing domain resources. The service that maintains the
user/group database integrity is called the Security Account Manager (SAM). The SAM database is stored differently between Windows and Linux Samba-based systems, therefore SAM
replication cannot be achieved and platforms cannot be mixed in a PDC/BDC environment.
In a Samba environment, there can be only one PDC and zero or more BDCs.
Important
Samba cannot exist in a mixed Samba/Windows domain controller environment
(Samba cannot be a BDC of a Windows PDC or vice versa). Alternatively, Samba
PDCs and BDCs can coexist.
6.3.1. Primary Domain Controller (PDC) using tdbsam
The simplest and most common implementation of a Samba PDC uses the tdbsam password
database backend. Planned to replace the aging smbpasswd backend, tdbsam has numerous improvements that are explained in more detail in Section 8, “Samba Account Information Databases”. The passdb backend directive controls which backend is to be used for the PDC.
[global]
workgroup = DOCS
netbios name = DOCS_SRV
passdb backend = tdbsam
security = user
add user script = /usr/sbin/useradd -m %u
delete user script = /usr/sbin/userdel -r %u
add group script = /usr/sbin/groupadd %g
delete group script = /usr/sbin/groupdel %g
add user to group script = /usr/sbin/usermod -G %g %u
add machine script = /usr/sbin/useradd -s /bin/false -d /dev/null
# The following specifies the default logon script
# Per user logon scripts can be specified in the user
# account using pdbedit logon script = logon.bat
# This sets the default profile path.
# Set per user paths with pdbedit
logon drive = H:
domain logons = Yes
os level = 35
preferred master = Yes
domain master = Yes
[homes]
comment = Home Directories
valid users = %S
read only = No
[netlogon]
comment = Network Logon Service
path = /var/lib/samba/netlogon/scripts
browseable = No
read only = No
# For profiles to work, create a user directory under the
# path shown. mkdir -p /var/lib/samba/profiles/john
[Profiles]
262
-g machines %u
7. Samba Security Modes
comment = Roaming Profile Share
path = /var/lib/samba/profiles
read only = No
browseable = No
guest ok = Yes
profile acls = Yes
# Other resource shares ... ...
Note
If you need more than one domain controller or have more than 250 users, do not
use a tdbsam authentication backend. LDAP is recommended in these cases.
6.3.2. Primary Domain Controller (PDC) with Active Directory
Although it is possible for Samba to be a member of an Active Directory, it is not possible for
Samba to operate as an Active Directory domain controller.
7. Samba Security Modes
There are only two types of security modes for Samba, share-level and user-level, which are
collectively known as security levels. Share-level security can only be implemented in one way,
while user-level security can be implemented in one of four different ways. The different ways of
implementing a security level are called security modes.
7.1. User-Level Security
User-level security is the default setting for Samba. Even if the security = user directive is not
listed in the smb.conf file, it is used by Samba. If the server accepts the client's username/password, the client can then mount multiple shares without specifying a password for each instance. Samba can also accept session-based username/password requests. The client maintains multiple authentication contexts by using a unique UID for each logon.
In smb.conf, the security
= user
directive that sets user-level security is:
[GLOBAL]
...
security = user
...
The following sections describe other implementations of user-level security.
7.1.1. Domain Security Mode (User-Level Security)
In domain security mode, the Samba server has a machine account (domain security trust account) and causes all authentication requests to be passed through to the domain controllers.
The Samba server is made into a domain member server by using the following directives in
smb.conf:
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7.2. Share-Level Security
[GLOBAL]
...
security = domain
workgroup = MARKETING
...
7.1.2. Active Directory Security Mode (User-Level Security)
If you have an Active Directory environment, it is possible to join the domain as a native Active
Directory member. Even if a security policy restricts the use of NT-compatible authentication
protocols, the Samba server can join an ADS using Kerberos. Samba in Active Directory member mode can accept Kerberos tickets.
In smb.conf, the following directives make Samba an Active Directory member server:
[GLOBAL]
...
security = ADS
realm = EXAMPLE.COM
password server = kerberos.example.com
...
7.1.3. Server Security Mode (User-Level Security)
Server security mode was previously used when Samba was not capable of acting as a domain
member server.
Note
It is highly recommended to not use this mode since there are numerous security
drawbacks.
In smb.conf, the following directives enable Samba to operate in server security mode:
[GLOBAL]
...
encrypt passwords = Yes
security = server
password server = "NetBIOS_of_Domain_Controller"
...
7.2. Share-Level Security
With share-level security, the server accepts only a password without an explicit username from
the client. The server expects a password for each share, independent of the username. There
have been recent reports that Microsoft Windows clients have compatibility issues with sharelevel security servers. Samba developers strongly discourage use of share-level security.
In smb.conf, the security
= share
directive that sets share-level security is:
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8. Samba Account Information Databases
[GLOBAL]
...
security = share
...
8. Samba Account Information Databases
The latest release of Samba offers many new features including new password database
backends not previously available. Samba version 3.0.0 fully supports all databases used in
previous versions of Samba. However, although supported, many backends may not be suitable
for production use.
The following is a list different backends you can use with Samba. Other backends not listed
here may also be available.
Plain Text
Plain text backends are nothing more than the /etc/passwd type backends. With a plain text
backend, all usernames and passwords are sent unencrypted between the client and the
Samba server. This method is very unsecure and is not recommended for use by any
means. It is possible that different Windows clients connecting to the Samba server with
plain text passwords cannot support such an authentication method.
smbpasswd
A popular backend used in previous Samba packages, the smbpasswd backend utilizes a
plain ASCII text layout that includes the MS Windows LanMan and NT account, and encrypted password information. The smbpasswd backend lacks the storage of the Windows NT/
2000/2003 SAM extended controls. The smbpasswd backend is not recommended because it
does not scale well or hold any Windows information, such as RIDs for NT-based groups.
The tdbsam backend solves these issues for use in a smaller database (250 users), but is
still not an enterprise-class solution.
ldapsam_compat
The ldapsam_compat backend allows continued OpenLDAP support for use with upgraded
versions of Samba. This option normally used when migrating to Samba 3.0.
tdbsam
The tdbsam backend provides an ideal database backend for local servers, servers that do
not need built-in database replication, and servers that do not require the scalability or complexity of LDAP. The tdbsam backend includes all of the smbpasswd database information as
well as the previously-excluded SAM information. The inclusion of the extended SAM data
allows Samba to implement the same account and system access controls as seen with
Windows NT/2000/2003-based systems.
The tdbsam backend is recommended for 250 users at most. Larger organizations should require Active Directory or LDAP integration due to scalability and possible network infrastructure concerns.
ldapsam
The ldapsam backend provides an optimal distributed account installation method for
Samba. LDAP is optimal because of its ability to replicate its database to any number of
servers using the OpenLDAP slurpd daemon. LDAP databases are light-weight and scal265
9. Samba Network Browsing
able, and as such are preferred by large enterprises.
If you are upgrading from a previous version of Samba to 3.0, note that the /
usr/share/doc/samba-<version>/LDAP/samba.schema has changed. This file contains the attribute syntax definitions and objectclass definitions that the ldapsam backend will need in order to function properly.
As such, if you are using the ldapsam backend for your Samba server, you will need to configure slapd to include this schema file. Refer to Section 5, “The /etc/openldap/schema/ Directory” for directions on how to do this.
Note
You will need to have the openldap-server package installed if you want to use
the ldapsam backend.
mysqlsam
The mysqlsam backend uses a MySQL-based database backend. This is useful for sites that
already implement MySQL. At present, mysqlsam is now packed in a module separate from
Samba, and as such is not officially supported by Samba.
9. Samba Network Browsing
Network browsing enables Windows and Samba servers to appear in the Windows Network
Neighborhood. Inside the Network Neighborhood, icons are represented as servers and if
opened, the server's shares and printers that are available are displayed.
Network browsing capabilities require NetBIOS over TCP/IP. NetBIOS-based networking uses
broadcast (UDP) messaging to accomplish browse list management. Without NetBIOS and
WINS as the primary method for TCP/IP hostname resolution, other methods such as static files
(/etc/hosts) or DNS, must be used.
A domain master browser collates the browse lists from local master browsers on all subnets so
that browsing can occur between workgroups and subnets. Also, the domain master browser
should preferably be the local master browser for its own subnet.
9.1. Domain Browsing
By default, a Windows server PDC for a domain is also the domain master browser for that domain. A Samba server must note be set up as a domain master server in this type of situation
For subnets that do not include the Windows server PDC, a Samba server can be implemented
as a local master browser. Configuring the smb.conf for a local master browser (or no browsing
at all) in a domain controller environment is the same as workgroup configuration.
9.2. WINS (Windows Internetworking Name Server)
Either a Samba server or a Windows NT server can function as a WINS server. When a WINS
server is used with NetBIOS enabled, UDP unicasts can be routed which allows name resolu266
10. Samba with CUPS Printing Support
tion across networks. Without a WINS server, the UDP broadcast is limited to the local subnet
and therefore cannot be routed to other subnets, workgroups, or domains. If WINS replication is
necessary, do not use Samba as your primary WINS server, as Samba does not currently support WINS replication.
In a mixed NT/2000/2003 server and Samba environment, it is recommended that you use the
Microsoft WINS capabilities. In a Samba-only environment, it is recommended that you use only
one Samba server for WINS.
The following is an example of the smb.conf file in which the Samba server is serving as a WINS
server:
[global]
wins support = Yes
Tip
All servers (including Samba) should connect to a WINS server to resolve NetBIOS names. Without WINS, browsing only occurs on the local subnet. Furthermore, even if a domain-wide list is somehow obtained, hosts cannot be resolved
for the client without WINS.
10. Samba with CUPS Printing Support
Samba allows client machines to share printers connected to the Samba server. In addition,
Samba also allows client machines to send documents built in Linux to Windows printer shares.
Although there are other printing systems that function with Red Hat Enterprise Linux, CUPS
(Common UNIX Print System) is the recommended printing system due to its close integration
with Samba.
10.1. Simple smb.conf Settings
The following example shows a very basic smb.conf configuration for CUPS support:
[global]
load printers = Yes
printing = cups
printcap name = cups
[printers]
comment = All Printers
path = /var/spool/samba/print
printer = IBMInfoP
browseable = No
public = Yes
guest ok = Yes
writable = No
printable = Yes
printer admin = @ntadmins
[print$]
comment = Printer Drivers Share
path = /var/lib/samba/drivers
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11. Samba Distribution Programs
write list = ed, john
printer admin = ed, john
Other printing configurations are also possible. To add additional security and privacy for printing confidential documents, users can have their own print spooler not located in a public path.
If a job fails, other users would not have access to the file.
The print$ share contains printer drivers for clients to access if not available locally. The print$
share is optional and may not be required depending on the organization.
Setting browseable to Yes enables the printer to be viewed in the Windows Network Neighborhood, provided the Samba server is set up correctly in the domain/workgroup.
11. Samba Distribution Programs
findsmb. findsmb <subnet_broadcast_address>
The findsmb program is a Perl script which reports information about SMB-aware systems on a
specific subnet. If no subnet is specified the local subnet is used. Items displayed include IP address, NetBIOS name, workgroup or domain name, operating system, and version.
The following example shows the output of executing findsmb as any valid user on a system:
findsmb
IP ADDR
NETBIOS NAME WORKGROUP/OS/VERSION
-----------------------------------------------------------------10.1.59.25
VERVE
[MYGROUP] [Unix] [Samba 3.0.0-15]
10.1.59.26
STATION22
[MYGROUP] [Unix] [Samba 3.0.2-7.FC1]
10.1.56.45
TREK
+[WORKGROUP] [Windows 5.0] [Windows 2000 LAN Manager]
10.1.57.94
PIXEL
[MYGROUP] [Unix] [Samba 3.0.0-15]
10.1.57.137
MOBILE001
[WORKGROUP] [Windows 5.0] [Windows 2000 LAN Manager]
10.1.57.141
JAWS
+[KWIKIMART] [Unix] [Samba 2.2.7a-security-rollup-fix]
10.1.56.159
FRED
+[MYGROUP] [Unix] [Samba 3.0.0-14.3E]
10.1.59.192
LEGION
*[MYGROUP] [Unix] [Samba 2.2.7-security-rollup-fix]
10.1.56.205
NANCYN
+[MYGROUP] [Unix] [Samba 2.2.7a-security-rollup-fix]
net. net <protocol> <function> <misc_options> <target_options>
The net utility is similar to the net utility used for Windows and MS-DOS. The first argument is
used to specify the protocol to use when executing a command. The <protocol> option can be
ads, rap, or rpc for specifying the type of server connection. Active Directory uses ads,
Win9x/NT3 uses rap, and Windows NT4/2000/2003 uses rpc. If the protocol is omitted, net
automatically tries to determine it.
The following example displays a list the available shares for a host named wakko:
net -l share -S wakko
Password:
Enumerating shared resources (exports) on remote server:
Share name
---------data
tmp
IPC$
ADMIN$
Type
---Disk
Disk
IPC
IPC
Description
----------Wakko data share
Wakko tmp share
IPC Service (Samba Server)
IPC Service (Samba Server)
268
11. Samba Distribution Programs
The following example displays a list of Samba users for a host named wakko:
net -l user -S wakko
root password:
User name
Comment
----------------------------andriusb
Documentation
joe
Marketing
lisa
Sales
nmblookup. nmblookup <options> <netbios_name>
The nmblookup program resolves NetBIOS names into IP addresses. The program broadcasts
its query on the local subnet until the target machine replies.
Here is an example:
nmblookup trek
querying trek on 10.1.59.255
10.1.56.45 trek<00>
pdbedit. pdbedit <options>
The pdbedit program manages accounts located in the SAM database. All backends are supported including smbpasswd, LDAP, NIS+, and the tdb database library.
The following are examples of adding, deleting, and listing users:
pdbedit -a kristin
new password:
retype new password:
Unix username:
kristin
NT username:
Account Flags:
[U
]
User SID:
S-1-5-21-1210235352-3804200048-1474496110-2012
Primary Group SID:
S-1-5-21-1210235352-3804200048-1474496110-2077
Full Name: Home Directory:
\\wakko\kristin
HomeDir Drive:
Logon Script:
Profile Path:
\\wakko\kristin\profile
Domain:
WAKKO
Account desc:
Workstations: Munged
dial:
Logon time:
0
Logoff time:
Mon, 18 Jan 2038 22:14:07 GMT
Kickoff time:
Mon, 18 Jan 2038 22:14:07 GMT
Password last set:
Thu, 29 Jan 2004 08:29:28
GMT Password can change: Thu, 29 Jan 2004 08:29:28 GMT
Password must change: Mon, 18 Jan 2038 22:14:07 GMT pdbedit -v -L kristin
Unix username:
kristin
NT username:
Account Flags:
[U
]
User SID:
S-1-5-21-1210235352-3804200048-1474496110-2012
Primary Group SID:
S-1-5-21-1210235352-3804200048-1474496110-2077
Full Name:
Home Directory:
\\wakko\kristin
HomeDir Drive:
Logon Script:
Profile Path:
\\wakko\kristin\profile
Domain:
WAKKO
Account desc:
Workstations: Munged
dial:
269
11. Samba Distribution Programs
Logon time:
Logoff time:
Kickoff time:
Password last set:
Password can change:
Password must change:
andriusb:505:
joe:503:
lisa:504:
kristin:506: pdbedit
0
Mon,
Mon,
Thu,
Thu,
Mon,
18
18
29
29
18
-x joe
Jan
Jan
Jan
Jan
Jan
2038
2038
2004
2004
2038
22:14:07
22:14:07
08:29:28
08:29:28
22:14:07
GMT
GMT
GMT
GMT
GMT
pdbedit -L
pdbedit -L
andriusb:505: lisa:504: kristin:506:
rpcclient. rpcclient <server> <options>
The rpcclient program issues administrative commands using Microsoft RPCs, which provide
access to the Windows administration graphical user interfaces (GUIs) for systems management. This is most often used by advanced users that understand the full complexity of Microsoft RPCs.
smbcacls. smbcacls <//server/share> <filename> <options>
The smbcacls program modifies Windows ACLs on files and directories shared by the Samba
server.
smbclient. smbclient <//server/share> <password> <options>
The smbclient program is a versatile UNIX client which provides functionality similar to ftp.
smbcontrol. smbcontrol -i <options>
smbcontrol <options> <destination> <messagetype> <parameters>
The smbcontrol program sends control messages to running smbd or nmbd daemons. Executing
smbcontrol -i runs commands interactively until a blank line or a 'q' is entered.
smbpasswd. smbpasswd <options> <username> <password>
The smbpasswd program manages encrypted passwords. This program can be run by a superuser to change any user's password as well as by an ordinary user to change their own Samba
password.
smbspool. smbspool <job> <user> <title> <copies> <options> <filename>
The smbspool program is a CUPS-compatible printing interface to Samba. Although designed for
use with CUPS printers, smbspool can work with non-CUPS printers as well.
smbstatus. smbstatus <options>
The smbstatus program displays the status of current connections to a Samba server.
smbtar. smbtar <options>
The smbtar program performs backup and restores of Windows-based share files and directories to a local tape archive. Though similar to the tar command, the two are not compatible.
testparm. testparm <options> <filename> <hostname IP_address>
270
12. Additional Resources
The testparm program checks the syntax of the smb.conf file. If your smb.conf file is in the default location (/etc/samba/smb.conf) you do not need to specify the location. Specifying the hostname and IP address to the testparm program verifies that the hosts.allow and host.deny files
are configured correctly. The testparm program also displays a summary of your smb.conf file
and the server's role (stand-alone, domain, etc.) after testing. This is convenient when debugging as it excludes comments and concisely presents information for experienced administrators
to read.
For example:
testparm
Load smb config files from /etc/samba/smb.conf
Processing section "[homes]"
Processing section "[printers]"
Processing section "[tmp]"
Processing section "[html]"
Loaded services file OK.
Server role: ROLE_STANDALONE
Press enter to see a dump of your service definitions <enter>
# Global parameters
[global]
workgroup = MYGROUP
server string = Samba Server
security = SHARE
log file = /var/log/samba/%m.log
max log size = 50
socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
dns proxy = No
[homes]
comment = Home Directories
read only = No
browseable = No
[printers]
comment = All Printers
path = /var/spool/samba
printable = Yes
browseable = No
[tmp]
comment = Wakko tmp
path = /tmp
guest only = Yes
[html]
comment = Wakko www
path = /var/www/html
force user = andriusb
force group = users
read only = No
guest only = Yes
wbinfo. wbinfo <options>
The wbinfo program displays information from the winbindd daemon. The winbindd daemon
must be running for wbinfo to work.
12. Additional Resources
The following sections give you the means to explore Samba in greater detail.
12.1. Installed Documentation
271
12.2. Related Books
•
— All additional files included with the Samba distribution. This includes all helper scripts, sample configuration files, and documentation.
/usr/share/doc/samba-<version-number>/
This directory also contains online versions of The Official Samba-3 HOWTO-Collection and
Samba-3 by Example, both of which are cited below.
12.2. Related Books
•
The Official Samba-3 HOWTO-Collection by John H. Terpstra and Jelmer R. Vernooij; Prentice Hall — The official Samba-3 documentation as issued by the Samba development team.
This is more of a reference guide than a step-by-step guide.
•
Samba-3 by Example by John H. Terpstra; Prentice Hall — This is another official release issued by the Samba development team which discusses detailed examples of OpenLDAP,
DNS, DHCP, and printing configuration files. This has step-by-step related information that
helps in real-world implementations.
•
Using Samba, 2nd Edition by Jay T's, Robert Eckstein, and David Collier-Brown; O'Reilly —
A good resource for novice to advanced users, which includes comprehensive reference material.
12.3. Useful Websites
•
http://www.samba.org/ — Homepage for the Samba distribution and all official documentation created by the Samba development team. Many resources are available in HTML and
PDF formats, while others are only available for purchase. Although many of these links are
not Red Hat Enterprise Linux specific, some concepts may apply.
•
http://samba.org/samba/archives.html [http://us1.samba.org/samba/archives.html] — Active
email lists for the Samba community. Enabling digest mode is recommended due to high
levels of list activity.
•
Samba newsgroups — Samba threaded newsgroups, such as gmane.org, that use the
NNTP protocol are also available. This an alternative to receiving mailing list emails.
•
http://samba.idealx.org/ — Idealx.org distributes installation and configuration scripts for integration of Samba and OpenLDAP. These are highly recommended for assisting in managing LDAP related resources. The scripts can be found at /usr/share/doc/samba-version_number/LDAP/smbldap-tools or can be downloaded from the Idealx website.
272
Chapter 20. Dynamic Host
Configuration Protocol (DHCP)
Dynamic Host Configuration Protocol (DHCP) is a network protocol that automatically assigns
TCP/IP information to client machines. Each DHCP client connects to the centrally located DHCP server, which returns that client's network configuration (including the IP address, gateway,
and DNS servers).
1. Why Use DHCP?
DHCP is useful for automatic configuration of client network interfaces. When configuring the client system, the administrator chooses DHCP instead of specifying an IP address, netmask,
gateway, or DNS servers. The client retrieves this information from the DHCP server. DHCP is
also useful if an administrator wants to change the IP addresses of a large number of systems.
Instead of reconfiguring all the systems, he can just edit one DHCP configuration file on the
server for the new set of IP addresses. If the DNS servers for an organization changes, the
changes are made on the DHCP server, not on the DHCP clients. When the administrator restarts the network or reboots the clients, the changes will go into effect.
If an organization has a functional DHCP server properly connected to a network, laptops and
other mobile computer users can move these devices from office to office.
2. Configuring a DHCP Server
To configure a DHCP server, you must create the dhcpd.conf configuration file in the /etc/ directory. A sample file can be found at /usr/share/doc/dhcp-<version>/dhcpd.conf.sample.
DHCP also uses the file /var/lib/dhcpd/dhcpd.leases to store the client lease database. Refer
to Section 2.2, “Lease Database” for more information.
2.1. Configuration File
The first step in configuring a DHCP server is to create the configuration file that stores the network information for the clients.Use this file to declare options and global options for client systems.
The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are
case-insensitive and lines beginning with a hash mark (#) are considered comments.
Two DNS update schemes are currently implemented — the ad-hoc DNS update mode and the
interim DHCP-DNS interaction draft update mode. If and when these two are accepted as part
of the Internet Engineering Task Force (IETF) standards process, there will be a third mode —
the standard DNS update method. You must configure the DNS server for compatibility with
these schemes. Version 3.0b2pl11 and previous versions used the ad-hoc mode; however, it
has been deprecated. To keep the same behavior, add the following line to the top of the configuration file:
ddns-update-style ad-hoc;
273
2.1. Configuration File
To use the recommended mode, add the following line to the top of the configuration file:
ddns-update-style interim;
Refer to the dhcpd.conf man page for details about the different modes.
There are two types of statements in the configuration file:
•
Parameters — State how to perform a task, whether to perform a task, or what network configuration options to send to the client.
•
Declarations — Describe the topology of the network, describe the clients, provide addresses for the clients, or apply a group of parameters to a group of declarations.
The parameters that start with the keyword option are reffered to as options. These options control DHCP options; whereas, parameters configure values that are not optional or control how
the DHCP server behaves.
Parameters (including options) declared before a section enclosed in curly brackets ({ }) are
considered global parameters. Global parameters apply to all the sections below it.
Important
If the configuration file is changed, the changes do not take effect until the DHCP
daemon is restarted with the command service dhcpd restart.
Tip
Instead of changing a DHCP configuration file and restarting the service each time,
using the omshell command provides an interactive way to connect to, query, and
change the configuration of a DHCP server. By using omshell, all changes can be
made while the server is running. For more information on omshell, refer to the omshell man page.
In Example 20.1, “Subnet Declaration”, the routers, subnet-mask, domain-name, domainname-servers, and time-offset options are used for any host statements declared below it.
Additionally, a subnet can be declared, a subnet declaration must be included for every subnet
in the network. If it is not, the DHCP server fails to start.
In this example, there are global options for every DHCP client in the subnet and a range declared. Clients are assigned an IP address within the range.
subnet 192.168.1.0 netmask 255.255.255.0 {
option routers
192.168.1.254;
274
2.1. Configuration File
option subnet-mask
255.255.255.0;
option domain-name
option domain-name-servers
"example.com";
192.168.1.1;
option time-offset
-18000;
# Eastern Standard Time
range 192.168.1.10 192.168.1.100;
}
Example 20.1. Subnet Declaration
All subnets that share the same physical network should be declared within a shared-network
declaration as shown in Example 20.2, “Shared-network Declaration”. Parameters within the
shared-network, but outside the enclosed subnet declarations, are considered to be global parameters. The name of the shared-network must be a descriptive title for the network, such as using the title 'test-lab' to describe all the subnets in a test lab environment.
shared-network name {
option domain-name
"test.redhat.com";
option domain-name-servers
ns1.redhat.com, ns2.redhat.com;
option routers
192.168.0.254;
more parameters for EXAMPLE shared-network
subnet 192.168.1.0 netmask 255.255.252.0 {
parameters for subnet
range 192.168.1.1 192.168.1.254;
}
subnet 192.168.2.0 netmask 255.255.252.0 {
parameters for subnet
range 192.168.2.1 192.168.2.254;
}
}
Example 20.2. Shared-network Declaration
As demonstrated in Example 20.3, “Group Declaration”, the group declaration is used to apply
global parameters to a group of declarations. For example, shared networks, subnets, and
hosts can be grouped.
group {
option routers
option subnet-mask
192.168.1.254;
255.255.255.0;
option domain-name
option domain-name-servers
"example.com";
192.168.1.1;
option time-offset
-18000;
# Eastern Standard Time
host apex {
option host-name "apex.example.com";
hardware ethernet 00:A0:78:8E:9E:AA;
fixed-address 192.168.1.4;
}
275
2.1. Configuration File
host raleigh {
option host-name "raleigh.example.com";
hardware ethernet 00:A1:DD:74:C3:F2;
fixed-address 192.168.1.6;
}
}
Example 20.3. Group Declaration
To configure a DHCP server that leases a dynamic IP address to a system within a subnet,
modify Example 20.4, “Range Parameter” with your values. It declares a default lease time,
maximum lease time, and network configuration values for the clients. This example assigns IP
addresses in the range 192.168.1.10 and 192.168.1.100 to client systems.
default-lease-time 600;
max-lease-time 7200;
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.1.255;
option routers 192.168.1.254;
option domain-name-servers 192.168.1.1, 192.168.1.2;
option domain-name "example.com";
subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.10 192.168.1.100;
}
Example 20.4. Range Parameter
To assign an IP address to a client based on the MAC address of the network interface card,
use the hardware ethernet parameter within a host declaration. As demonstrated in Example 20.5, “Static IP Address using DHCP”, the host apex declaration specifies that the network interface card with the MAC address 00:A0:78:8E:9E:AA always receives the IP address
192.168.1.4.
Note that the optional parameter host-name can also be used to assign a host name to the client.
host apex {
option host-name "apex.example.com";
hardware ethernet 00:A0:78:8E:9E:AA;
fixed-address 192.168.1.4;
}
Example 20.5. Static IP Address using DHCP
Tip
276
2.2. Lease Database
The sample configuration file provided can be used as a starting point and custom
configuration options can be added to it. To copy it to the proper location, use the
following command:
cp /usr/share/doc/dhcp-<version-number>/dhcpd.conf.sample /etc/dhcpd.conf
(where <version-number> is the DHCP version number).
For a complete list of option statements and what they do, refer to the dhcp-options man page.
2.2. Lease Database
On the DHCP server, the file /var/lib/dhcpd/dhcpd.leases stores the DHCP client lease database. Do not change this file. DHCP lease information for each recently assigned IP address is
automatically stored in the lease database. The information includes the length of the lease, to
whom the IP address has been assigned, the start and end dates for the lease, and the MAC
address of the network interface card that was used to retrieve the lease.
All times in the lease database are in Coordinated Universal Time (UTC), not local time.
The lease database is recreated from time to time so that it is not too large. First, all known
leases are saved in a temporary lease database. The dhcpd.leases file is renamed dhcpd.leases~ and the temporary lease database is written to dhcpd.leases.
The DHCP daemon could be killed or the system could crash after the lease database has been
renamed to the backup file but before the new file has been written. If this happens, the dhcpd.leases file does not exist, but it is required to start the service. Do not create a new lease
file. If you do, all old leases are lost which causes many problems. The correct solution is to rename the dhcpd.leases~ backup file to dhcpd.leases and then start the daemon.
2.3. Starting and Stopping the Server
Important
When the DHCP server is started for the first time, it fails unless the dhcpd.leases
file exists. Use the command touch /var/lib/dhcpd/dhcpd.leases to create the file
if it does not exist.
If the same server is also running BIND as a DNS server, this step is not necessary, as starting the named service automatically checks for a dhcpd.leases file.
To start the DHCP service, use the command /sbin/service
server, use the command /sbin/service dhcpd stop.
dhcpd start.
To stop the DHCP
By default, the DHCP service does not start at boot time. To configure the daemon to start automatically at boot time, refer to Chapter 15, Controlling Access to Services.
277
2.4. DHCP Relay Agent
If more than one network interface is attached to the system, but the DHCP server should only
be started on one of the interfaces, configure the DHCP server to start only on that device. In /
etc/sysconfig/dhcpd, add the name of the interface to the list of DHCPDARGS:
# Command line options here
DHCPDARGS=eth0
This is useful for a firewall machine with two network cards. One network card can be configured as a DHCP client to retrieve an IP address to the Internet. The other network card can
be used as a DHCP server for the internal network behind the firewall. Specifying only the network card connected to the internal network makes the system more secure because users can
not connect to the daemon via the Internet.
Other command line options that can be specified in /etc/sysconfig/dhcpd include:
•
-p <portnum>
— Specifies the UDP port number on which dhcpd should listen. The default is
port 67. The DHCP server transmits responses to the DHCP clients at a port number one
greater than the UDP port specified. For example, if the default port 67 is used, the server
listens on port 67 for requests and responses to the client on port 68. If a port is specified
here and the DHCP relay agent is used, the same port on which the DHCP relay agent
should listen must be specified. Refer to Section 2.4, “DHCP Relay Agent” for details.
•
-f
•
-d
•
-cf <filename>
— Runs the daemon as a foreground process. This is mostly used for debugging.
— Logs the DHCP server daemon to the standard error descriptor. This is mostly used for
debugging. If this is not specified, the log is written to /var/log/messages.
— Specifies the location of the configuration file. The default location is /
etc/dhcpd.conf.
•
-lf <filename>
— Specifies the location of the lease database file. If a lease database file
already exists, it is very important that the same file be used every time the DHCP server is
started. It is strongly recommended that this option only be used for debugging purposes on
non-production machines. The default location is /var/lib/dhcpd/dhcpd.leases.
•
-q
— Do not print the entire copyright message when starting the daemon.
2.4. DHCP Relay Agent
The DHCP Relay Agent (dhcrelay) allows for the relay of DHCP and BOOTP requests from a
subnet with no DHCP server on it to one or more DHCP servers on other subnets.
When a DHCP client requests information, the DHCP Relay Agent forwards the request to the
list of DHCP servers specified when the DHCP Relay Agent is started. When a DHCP server returns a reply, the reply is broadcast or unicast on the network that sent the original request.
The DHCP Relay Agent listens for DHCP requests on all interfaces unless the interfaces are
specified in /etc/sysconfig/dhcrelay with the INTERFACES directive.
To start the DHCP Relay Agent, use the command service
278
dhcrelay start.
3. Configuring a DHCP Client
3. Configuring a DHCP Client
The first step for configuring a DHCP client is to make sure the kernel recognizes the network
interface card. Most cards are recognized during the installation process and the system is configured to use the correct kernel module for the card. If a card is added after installation,
Kudzu9 will recognize it and prompt you for the proper kernel module (Be sure to check the
Hardware Compatibility List at http://hardware.redhat.com/hcl/). If either the installation program
or kudzu does not recognize the network card, you can load the correct kernel module (refer to
Chapter 40, General Parameters and Modules for details).
To configure a DHCP client manually, modify the /etc/sysconfig/network file to enable networking and the configuration file for each network device in the /etc/sysconfig/network-scripts directory. In this directory, each device should have a configuration file named ifcfg-eth0, where
eth0 is the network device name.
The /etc/sysconfig/network file should contain the following line:
NETWORKING=yes
The NETWORKING variable must be set to yes if you want networking to start at boot time.
The /etc/sysconfig/network-scripts/ifcfg-eth0 file should contain the following lines:
DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
A configuration file is needed for each device to be configured to use DHCP.
Other options for the network script includes:
•
DHCP_HOSTNAME
— Only use this option if the DHCP server requires the client to specify a
hostname before receiving an IP address. (The DHCP server daemon in Red Hat Enterprise
Linux does not support this feature.)
•
PEERDNS=<answer>,
•
yes
•
no
where <answer> is one of the following:
— Modify /etc/resolv.conf with information from the server. If using DHCP, then yes
is the default.
— Do not modify /etc/resolv.conf.
•
SRCADDR=<address>,
•
USERCTL=<answer>,
•
yes
•
no
where <address> is the specified source IP address for outgoing packets.
where <answer> is one of the following:
— Non-root users are allowed to control this device.
— Non-root users are not allowed to control this device.
9
Kudzu is a hardware probing tool run at system boot time to determine what hardware has been added or removed
from the system.
279
4. Additional Resources
If you prefer using a graphical interface, refer to Chapter 14, Network Configuration for instructions on using the Network Administration Tool to configure a network interface to use DHCP.
Tip
For advanced configurations of client DHCP options such as protocol timing, lease
requirements and requests, dynamic DNS support, aliases, as well as a wide variety of values to override, prepend, or append to client-side configurations, refer to
the dhclient and dhclient.conf man pages.
4. Additional Resources
For additional configuration options, refer to the following resources.
4.1. Installed Documentation
•
dhcpd
man page — Describes how the DHCP daemon works.
•
dhcpd.conf
•
dhcpd.leases
man page — Explains how to configure the DHCP configuration file; includes
some examples.
man page — Explains how to configure the DHCP leases file; includes some
examples.
•
dhcp-options
man page — Explains the syntax for declaring DHCP options in dhcpd.conf; includes some examples.
•
dhcrelay
•
/usr/share/doc/dhcp-<version>/
man page — Explains the DHCP Relay Agent and its configuration options.
— Contains sample files, README files, and release notes
for current versions of the DHCP service.
280
Chapter 21. Apache HTTP Server
The Apache HTTP Server is a robust, commercial-grade open source Web server developed by
the Apache Software Foundation (http://www.apache.org/). Red Hat Enterprise Linux includes
the Apache HTTP Server 2.2 as well as a number of server modules designed to enhance its
functionality.
The default configuration file installed with the Apache HTTP Server works without alteration for
most situations. This chapter outlines many of the directives found within its configuration file (/
etc/httpd/conf/httpd.conf) to aid those who require a custom configuration or need to convert
a configuration file from the older Apache HTTP Server 1.3 format.
Warning
If using the graphical HTTP Configuration Tool (system-config-httpd ), do not
hand edit the Apache HTTP Server's configuration file as the HTTP Configuration
Tool regenerates this file whenever it is used.
1. Apache HTTP Server 2.2
There are important differences between the Apache HTTP Server 2.2 and version 2.0 (version
2.0 shipped with Red Hat Enterprise Linux 4 and earlier). This section reviews some of the features of Apache HTTP Server 2.2 and outlines important changes. If you are upgrading from
version 1.3, you should also read the instructions on migrating from version 1.3 to version 2.0.
For instructions on migrating a version 1.3 configuration file to the 2.0 format, refer to Section 2.2, “Migrating Apache HTTP Server 1.3 Configuration Files to 2.0”.
1.1. Features of Apache HTTP Server 2.2
Apache HTTP Server 2.2 features the following improvements over version 2.0 :
•
Improved caching modules (mod_cache, mod_disk_cache, mod_mem_cache).
•
A new structure for authentication and authorization support, replacing the authentication
modules provided in previous versions.
•
Support for proxy load balancing (mod_proxy_balancer)
•
support for handling large files (namely, greater than 2GB) on 32-bit platforms
The following changes have been made to the default httpd configuration:
•
The mod_cern_meta and mod_asis modules are no longer loaded by default.
•
The mod_ext_filter module is now loaded by default.
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2. Migrating Apache HTTP Server Configuration Files
If upgrading from a previous release of Red Hat Enterprise Linux, the httpd configuration will
need to be updated for httpd 2.2. For more information, refer to http://httpd.apache.org/docs/2.2/upgrading.html
2. Migrating Apache HTTP Server Configuration Files
2.1. Migrating Apache HTTP Server 2.0 Configuration Files
This section outlines migration from version 2.0 to 2.2. If you are migrating from version 1.3,
please refer to Section 2.2, “Migrating Apache HTTP Server 1.3 Configuration Files to 2.0”.
•
Configuration files and startup scripts from version 2.0 need minor adjustments particularly in
module names which may have changed. Third party modules which worked in version 2.0
can also work in version 2.2 but need to be recompiled before you load them. Key modules
that need to be noted are authentication and authorization modules. For each of the modules which has been renamed the LoadModule
[http://httpd.apache.org/docs/2.2/mod/mod_so.html#loadmodule] line will need to be updated.
•
The mod_userdir module will only act on requests if you provide a UserDir directive indicating
a directory name. If you wish to maintain the procedures used in version 2.0, add the directive UserDir public_html in your configuration file.
•
To enable SSL, edit the httpd.conf file adding the necessary mod_ssl directives. Use
apachectl start as apachectl startssl is unavailable in version 2.2. You can view an example of SSL configuration for httpd in conf/extra/httpd-ssl.conf.
•
To test your configuration it is advisable to use service
configuration errors.
httpd configtest
which will detect
More information on upgrading from version 2.0 to 2.2 can be found on http://httpd.apache.org/docs/2.2/upgrading.html.
2.2. Migrating Apache HTTP Server 1.3 Configuration Files
to 2.0
This section details migrating an Apache HTTP Server 1.3 configuration file to be utilized by
Apache HTTP Server 2.0.
If upgrading to Red Hat Enterprise Linux 5 from Red Hat Enterprise Linux 2.1, note that the new
stock configuration file for the Apache HTTP Server 2.0 package is installed as /
etc/httpd/conf/httpd.conf.rpmnew and the original version 1.3 httpd.conf is left untouched. It is
entirely up to you whether to use the new configuration file and migrate the old settings to it, or
use the existing file as a base and modify it to suit; however, some parts of the file have
changed more than others and a mixed approach is generally the best. The stock configuration
files for both version 1.3 and 2.0 are divided into three sections.
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
If the /etc/httpd/conf/httpd.conf file is a modified version of the newly installed default and a
saved a copy of the original configuration file is available, it may be easiest to invoke the diff
command, as in the following example (logged in as root):
diff -u httpd.conf.orig httpd.conf | less
This command highlights any modifications made. If a copy of the original file is not available,
extract it from an RPM package using the rpm2cpio and cpio commands, as in the following example:
rpm2cpio apache-<version-number>.i386.rpm | cpio -i --make
In the above command, replace <version-number> with the version number for the apache package.
Finally, it is useful to know that the Apache HTTP Server has a testing mode to check for configuration errors. To use access it, type the following command:
apachectl configtest
2.2.1. Global Environment Configuration
The global environment section of the configuration file contains directives which affect the overall operation of the Apache HTTP Server, such as the number of concurrent requests it can
handle and the locations of the various files. This section requires a large number of changes
and should be based on the Apache HTTP Server 2.0 configuration file, while migrating the old
settings into it.
2.2.1.1. Interface and Port Binding
The BindAddress and Port directives no longer exist; their functionality is now provided by a
more flexible Listen directive.
If Port 80 was set in the 1.3 version configuration file, change it to Listen 80 in the 2.0 configuration file. If Port was set to some value other than 80, then append the port number to the contents of the ServerName directive.
For example, the following is a sample Apache HTTP Server 1.3 directive:
Port 123 ServerName www.example.com
To migrate this setting to Apache HTTP Server 2.0, use the following structure:
Listen 123 ServerName www.example.com:123
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mpm_common.html#listen
•
http://httpd.apache.org/docs-2.0/mod/core.html#servername
2.2.1.2. Server-Pool Size Regulation
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When the Apache HTTP Server accepts requests, it dispatches child processes or threads to
handle them. This group of child processes or threads is known as a server-pool. Under Apache
HTTP Server 2.0, the responsibility for creating and maintaining these server-pools has been
abstracted to a group of modules called Multi-Processing Modules (MPMs). Unlike other modules, only one module from the MPM group can be loaded by the Apache HTTP Server. There
are three MPM modules that ship with 2.0: prefork, worker, and perchild. Currently only the
prefork and worker MPMs are available, although the perchild MPM may be available at a later
date.
The original Apache HTTP Server 1.3 behavior has been moved into the prefork MPM. The
prefork MPM accepts the same directives as Apache HTTP Server 1.3, so the following directives may be migrated directly:
•
StartServers
•
MinSpareServers
•
MaxSpareServers
•
MaxClients
•
MaxRequestsPerChild
The worker MPM implements a multi-process, multi-threaded server providing greater scalability. When using this MPM, requests are handled by threads, conserving system resources and
allowing large numbers of requests to be served efficiently. Although some of the directives accepted by the worker MPM are the same as those accepted by the prefork MPM, the values for
those directives should not be transfered directly from an Apache HTTP Server 1.3 installation.
It is best to instead use the default values as a guide, then experiment to determine what values
work best.
Important
To use the worker MPM, create the file /etc/sysconfig/httpd and add the following
directive:
HTTPD=/usr/sbin/httpd.worker
For more on the topic of MPMs, refer to the following documentation on the Apache Software
Foundation's website:
•
http://httpd.apache.org/docs-2.0/mpm.html
2.2.1.3. Dynamic Shared Object (DSO) Support
There are many changes required here, and it is highly recommended that anyone trying to
modify an Apache HTTP Server 1.3 configuration to suit version 2.0 (as opposed to migrating
the changes into the version 2.0 configuration) copy this section from the stock Apache HTTP
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
Server 2.0 configuration file.
Those who do not want to copy the section from the stock Apache HTTP Server 2.0 configuration should note the following:
•
The AddModule and ClearModuleList directives no longer exist. These directives where used
to ensure that modules could be enabled in the correct order. The Apache HTTP Server 2.0
API allows modules to specify their ordering, eliminating the need for these two directives.
•
The order of the LoadModule lines are no longer relevant in most cases.
•
Many modules have been added, removed, renamed, split up, or incorporated into others.
•
LoadModule
•
The various HAVE_XXX definitions are no longer defined.
lines for modules packaged in their own RPMs (mod_ssl, php, mod_perl, and the
like) are no longer necessary as they can be found in their relevant files within the /
etc/httpd/conf.d/ directory.
Important
If modifying the original file, note that it is of paramount importance that the httpd.conf contains the following directive:
Include conf.d/*.conf
Omission of this directive results in the failure of all modules packaged in their own
RPMs (such as mod_perl, php, and mod_ssl).
2.2.1.4. Other Global Environment Changes
The following directives have been removed from Apache HTTP Server 2.0's configuration:
•
ServerType
— The Apache HTTP Server can only be run as ServerType
this directive irrelevant.
•
AccessConfig
standalone
making
and ResourceConfig — These directives have been removed as they mirror the
functionality of the Include directive. If the AccessConfig and ResourceConfig directives are
set, replace them with Include directives.
To ensure that the files are read in the order implied by the older directives, the Include directives should be placed at the end of the httpd.conf, with the one corresponding to ResourceConfig preceding the one corresponding to AccessConfig. If using the default values,
include them explicitly as conf/srm.conf and conf/access.conf files.
2.2.2. Main Server Configuration
The main server configuration section of the configuration file sets up the main server, which responds to any requests that are not handled by a virtual host defined within a <VirtualHost>
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
container. Values here also provide defaults for any <VirtualHost> containers defined.
The directives used in this section have changed little between Apache HTTP Server 1.3 and
version 2.0. If the main server configuration is heavily customized, it may be easier to modify the
existing configuration file to suit Apache HTTP Server 2.0. Users with only lightly customized
main server sections should migrate their changes into the default 2.0 configuration.
2.2.2.1. UserDir Mapping
The UserDir directive is used to enable URLs such as http://example.com/~bob/ to map to a
subdirectory within the home directory of the user bob, such as /home/bob/public_html/. A sideeffect of this feature allows a potential attacker to determine whether a given username is
present on the system. For this reason, the default configuration for Apache HTTP Server 2.0
disables this directive.
To enable UserDir mapping, change the directive in httpd.conf from:
UserDir disable
to the following:
UserDir public_html
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mod_userdir.html#userdir
2.2.2.2. Logging
The following logging directives have been removed:
•
AgentLog
•
RefererLog
•
RefererIgnore
However, agent and referrer logs are still available using the CustomLog and LogFormat directives.
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mod_log_config.html#customlog
•
http://httpd.apache.org/docs-2.0/mod/mod_log_config.html#logformat
2.2.2.3. Directory Indexing
The deprecated FancyIndexing directive has now been removed. The same functionality is available through the FancyIndexingoption within the IndexOptions directive.
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
The VersionSort option to the IndexOptions directive causes files containing version numbers to
be sorted in a more natural way. For example, httpd-2.0.6.tar appears before httpd2.0.36.tar in a directory index page.
The defaults for the ReadmeName and HeaderName directives have changed from README and HEADER
to README.html and HEADER.html.
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mod_autoindex.html#indexoptions
•
http://httpd.apache.org/docs-2.0/mod/mod_autoindex.html#readmename
•
http://httpd.apache.org/docs-2.0/mod/mod_autoindex.html#headername
2.2.2.4. Content Negotiation
The CacheNegotiatedDocs directive now takes the argument on or off. Existing instances of
CacheNegotiatedDocs should be replaced with CacheNegotiatedDocs on.
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mod_negotiation.html#cachenegotiateddocs
2.2.2.5. Error Documents
To use a hard-coded message with the ErrorDocument directive, the message should be enclosed in a pair of double quotation marks ", rather than just preceded by a double quotation
mark as required in Apache HTTP Server 1.3.
For example, the following is a sample Apache HTTP Server 1.3 directive:
ErrorDocument 404 "The document was not found
To migrate an ErrorDocument setting to Apache HTTP Server 2.0, use the following structure:
ErrorDocument 404 "The document was not found"
Note the trailing double quote in the previous ErrorDocument directive example.
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/core.html#errordocument
2.2.3. Virtual Host Configuration
The contents of all <VirtualHost> containers should be migrated in the same way as the main
server section as described in Section 2.2.2, “Main Server Configuration”.
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
Important
Note that SSL/TLS virtual host configuration has been moved out of the main server configuration file and into /etc/httpd/conf.d/ssl.conf.
•
http://httpd.apache.org/docs-2.0/vhosts/
2.2.4. Modules and Apache HTTP Server 2.0
In Apache HTTP Server 2.0, the module system has been changed to allow modules to be
chained together or combined in new and interesting ways. Common Gateway Interface (CGI)
scripts, for example, can generate server-parsed HTML documents which can then be processed by mod_include. This opens up a tremendous number of possibilities with regards to how
modules can be combined to achieve a specific goal.
The way this works is that each request is served by exactly one handler module followed by
zero or more filter modules.
Under Apache HTTP Server 1.3, for example, a Perl script would be handled in its entirety by
the Perl module (mod_perl). Under Apache HTTP Server 2.0, the request is initially handled by
the core module — which serves static files — and is then filtered by mod_perl.
Exactly how to use this, and all other new features of Apache HTTP Server 2.0, is beyond the
scope of this document; however, the change has ramifications if the PATH_INFO directive is used
for a document which is handled by a module that is now implemented as a filter, as each contains trailing path information after the true file name. The core module, which initially handles
the request, does not by default understand PATH_INFO and returns 404 Not Found errors for requests that contain such information. As an alternative, use the AcceptPathInfo directive to coerce the core module into accepting requests with PATH_INFO.
The following is an example of this directive:
AcceptPathInfo on
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/core.html#acceptpathinfo
•
http://httpd.apache.org/docs-2.0/handler.html
•
http://httpd.apache.org/docs-2.0/filter.html
2.2.4.1. The suexec Module
In Apache HTTP Server 2.0, the mod_suexec module uses the SuexecUserGroup directive, rather
than the User and Group directives, which is used for configuring virtual hosts. The User and
Group directives can still be used in general, but are deprecated for configuring virtual hosts.
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
For example, the following is a sample Apache HTTP Server 1.3 directive:
<VirtualHost vhost.example.com:80> User someone Group somegroup </VirtualHost>
To migrate this setting to Apache HTTP Server 2.0, use the following structure:
<VirtualHost vhost.example.com:80> SuexecUserGroup someone somegroup </VirtualHost>
2.2.4.2. The mod_ssl Module
The configuration for mod_ssl has been moved from the httpd.conf file into the /
etc/httpd/conf.d/ssl.conf file. For this file to be loaded, and for mod_ssl to work, the statement
Include conf.d/*.conf must be in the httpd.conf file as described in Section 2.2.1.3, “Dynamic
Shared Object (DSO) Support”.
ServerName
directives in SSL virtual hosts must explicitly specify the port number.
For example, the following is a sample Apache HTTP Server 1.3 directive:
<VirtualHost _default_:443> # General setup for the virtual host ServerName ssl.example.name ... </VirtualH
To migrate this setting to Apache HTTP Server 2.0, use the following structure:
<VirtualHost _default_:443> # General setup for the virtual host ServerName ssl.host.name:443 ... </Virtual
It is also important to note that both the SSLLog and SSLLogLevel directives have been removed.
The mod_ssl module now obeys the ErrorLog and LogLevel directives. Refer to ErrorLog and
LogLevel for more information about these directives.
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mod_ssl.html
•
http://httpd.apache.org/docs-2.0/vhosts/
2.2.4.3. The mod_proxy Module
Proxy access control statements are now placed inside a <Proxy> block rather than a <Directory
proxy:>.
The caching functionality of the old mod_proxy has been split out into the following three modules:
•
mod_cache
•
mod_disk_cache
•
mod_mem_cache
These generally use directives similar to the older versions of the mod_proxy module, but it is advisable to verify each directive before migrating any cache settings.
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mod_proxy.html
2.2.4.4. The mod_include Module
The mod_include module is now implemented as a filter and is therefore enabled differently.
Refer to Section 2.2.4, “Modules and Apache HTTP Server 2.0” for more about filters.
For example, the following is a sample Apache HTTP Server 1.3 directive:
AddType text/html .shtml AddHandler server-parsed .shtml
To migrate this setting to Apache HTTP Server 2.0, use the following structure:
AddType text/html .shtml AddOutputFilter INCLUDES .shtml
Note that the Options
.htaccess file.
+Includes
directive is still required for the <Directory> container or in a
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mod_include.html
2.2.4.5. The mod_auth_dbm and mod_auth_db Modules
Apache HTTP Server 1.3 supported two authentication modules, mod_auth_db and mod_auth_dbm,
which used Berkeley Databases and DBM databases respectively. These modules have been
combined into a single module named mod_auth_dbm in Apache HTTP Server 2.0, which can access several different database formats. To migrate from mod_auth_db, configuration files should
be adjusted by replacing AuthDBUserFile and AuthDBGroupFile with the mod_auth_dbm equivalents, AuthDBMUserFile and AuthDBMGroupFile. Also, the directive AuthDBMType DB must be added
to indicate the type of database file in use.
The following example shows a sample mod_auth_db configuration for Apache HTTP Server 1.3:
<Location /private/> AuthType Basic AuthName "My Private Files" AuthDBUserFile /var/www/authdb require vali
To migrate this setting to version 2.0 of Apache HTTP Server, use the following structure:
<Location /private/> AuthType Basic AuthName "My Private Files" AuthDBMUserFile /var/www/authdb AuthDBMType
Note that the AuthDBMUserFile directive can also be used in .htaccess files.
The dbmmanage Perl script, used to manipulate username and password databases, has been replaced by htdbm in Apache HTTP Server 2.0. The htdbm program offers equivalent functionality
and, like mod_auth_dbm, can operate a variety of database formats; the -T option can be used on
the command line to specify the format to use.
Table 21.1, “Migrating from dbmmanage to htdbm” shows how to migrate from a DBM-format
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
database to htdbm format using dbmmanage.
Action
dbmmanage command (1.3) Equivalent htdbm command (2.0)
Add user to database (using
given password)
dbmmanage authdb add user-
htdbm -b -TDB authdb user-
name password
name password
Add user to database
(prompts for password)
dbmmanage authdb adduser
htdbm -TDB authdb username
Remove user from database
dbmmanage authdb delete
htdbm -x -TDB authdb user-
username
name
List users in database
dbmmanage authdb view
htdbm -l -TDB authdb
Verify a password
dbmmanage authdb check
htdbm -v -TDB authdb user-
username
name
username
Table 21.1. Migrating from dbmmanage to htdbm
The -m and -s options work with both dbmmanage and htdbm, enabling the use of the MD5 or
SHA1 algorithms for hashing passwords, respectively.
When creating a new database with htdbm, the -c option must be used.
For more on this topic, refer to the following documentation on the Apache Software Foundation's website:
•
http://httpd.apache.org/docs-2.0/mod/mod_auth_dbm.html
2.2.4.6. The mod_perl Module
The configuration for mod_perl has been moved from httpd.conf into the file /
etc/httpd/conf.d/perl.conf. For this file to be loaded, and hence for mod_perl to work, the
statement Include conf.d/*.conf must be included in httpd.conf as described in Section 2.2.1.3, “Dynamic Shared Object (DSO) Support”.
Occurrences of Apache:: in httpd.conf must be replaced with ModPerl::. Additionally, the manner in which handlers are registered has been changed.
This is a sample Apache HTTP Server 1.3 mod_perl configuration:
<Directory /var/www/perl> SetHandler perl-script PerlHandler Apache::Registry Options +ExecCGI </Directory>
This is the equivalent mod_perl for Apache HTTP Server 2.0:
<Directory /var/www/perl> SetHandler perl-script PerlResponseHandler ModPerl::Registry Options +ExecCGI </D
Most modules for mod_perl 1.x should work without modification with mod_perl 2.x. XS modules
require recompilation and may require minor Makefile modifications.
2.2.4.7. The mod_python Module
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2.2. Migrating Apache HTTP Server 1.3 Configuration Files to 2.0
Configuration for mod_python has moved from httpd.conf to the /etc/httpd/conf.d/python.conf
file. For this file to be loaded, and hence for mod_python to work, the statement Include
conf.d/*.conf must be in httpd.conf as described in Section 2.2.1.3, “Dynamic Shared Object
(DSO) Support”.
2.2.4.8. PHP
The configuration for PHP has been moved from httpd.conf into the file /
etc/httpd/conf.d/php.conf. For this file to be loaded, the statement Include conf.d/*.conf
must be in httpd.conf as described in Section 2.2.1.3, “Dynamic Shared Object (DSO)
Support”.
Note
Any PHP configuration directives used in Apache HTTP Server 1.3 are now fully
compatible, when migrating to Apache HTTP Server 2.0 on Red Hat Enterprise
Linux 5.
In PHP version 4.2.0 and later the default set of predefined variables which are available in the
global scope has changed. Individual input and server variables are, by default, no longer
placed directly into the global scope. This change may cause scripts to break. Revert to the old
behavior by setting register_globals to On in the file /etc/php.ini.
For more on this topic, refer to the following URL for details concerning the global scope
changes:
•
http://www.php.net/release_4_1_0.php
2.2.4.9. The mod_authz_ldap Module
Red Hat Enterprise Linux ships with the mod_authz_ldap module for the Apache HTTP Server.
This module uses the short form of the distinguished name for a subject and the issuer of the
client SSL certificate to determine the distinguished name of the user within an LDAP directory.
It is also capable of authorizing users based on attributes of that user's LDAP directory entry,
determining access to assets based on the user and group privileges of the asset, and denying
access for users with expired passwords. The mod_ssl module is required when using the
mod_authz_ldap module.
Important
The mod_authz_ldap module does not authenticate a user to an LDAP directory using an encrypted password hash. This functionality is provided by the experimental
mod_auth_ldap module. Refer to the mod_auth_ldap module documentation online at
http://httpd.apache.org/docs-2.0/mod/mod_auth_ldap.html for details on the status
of this module.
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3. Starting and Stopping httpd
The /etc/httpd/conf.d/authz_ldap.conf file configures the mod_authz_ldap module.
Refer to /usr/share/doc/mod_authz_ldap-<version>/index.html (replacing <version> with the
version number of the package) or http://authzldap.othello.ch/ for more information on configuring the mod_authz_ldap third party module.
3. Starting and Stopping
httpd
After installing the httpd package, review the Apache HTTP Server's documentation available
online at http://httpd.apache.org/docs/2.2/.
The httpd RPM installs the /etc/init.d/httpd script, which can be accessed using the /
sbin/service command.
Starting httpd using the apachectl control script sets the environmental variables in /
etc/sysconfig/httpd and starts httpd. You can also set the environment variables using the init
script.
To start the server using the apachectl control script as root type:
apachectl start
You can also start httpd using /sbin/service httpd start. This starts httpd but does not set
the environment variables. If you are using the default Listen directive in httpd.conf, which is
port 80, you will need to have root privileges to start the apache server.
To stop the server, as root type:
apachectl stop
You can also stop httpd using /sbin/service httpd stop. The restart option is a shorthand
way of stopping and then starting the Apache HTTP Server.
You can restart the server as root by typing:
apachectl restart
or:/sbin/service httpd restart
Apache will display a message on the console or in the ErrorLog if it encounters an error while
starting.
By default, the httpd service does not start automatically at boot time. If you would wish to have
Apache startup at boot time, you will need to add a call to apachectl in your startup files within
the rc.N directory. A typical file used is rc.local. As this starts Apache as root, it is recommended to properly configure your security and authentication before adding this call.
You can also configure the httpd service to start up at boot time, using an initscript utility, such
as /sbin/chkconfig, /usr/sbin/ntsysv, or the Services Configuration Tool program.
You can also display the status of your httpd server by typing:
apachectl status
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4. Apache HTTP Server Configuration
The status module mod_status however needs to be enabled in your httpd.conf configuration
file for this to work. For more details on mod_status can be found on http://httpd.apache.org/docs/2.2/mod/mod_status.html.
Note
If running the Apache HTTP Server as a secure server, the secure server's password is required after the machine boots when using an encrypted private SSL
key.
You can find more information on http://httpd.apache.org/docs/2.2/ssl
4. Apache HTTP Server Configuration
The HTTP Configuration Tool allows you to configure the /etc/httpd/conf/httpd.conf configuration file for the Apache HTTP Server. It does not use the old srm.conf or access.conf configuration files; leave them empty. Through the graphical interface, you can configure directives
such as virtual hosts, logging attributes, and maximum number of connections. To start the
HTTD Configuration Tool, click on System => Administration => Server Settings => HTTP.
Only modules provided with Red Hat Enterprise Linux can be configured with the HTTP Configuration Tool. If additional modules are installed, they can not be configured using this tool.
Caution
Do not edit the /etc/httpd/conf/httpd.conf configuration file by hand if you wish to
use this tool. The HTTP Configuration Tool generates this file after you save your
changes and exit the program. If you want to add additional modules or configuration options that are not available in HTTP Configuration Tool, you cannot use
this tool.
The general steps for configuring the Apache HTTP Server using the HTTP Configuration Tool
are as follows:
1.
Configure the basic settings under the Main tab.
2.
Click on the Virtual Hosts tab and configure the default settings.
3.
Under the Virtual Hosts tab, configure the Default Virtual Host.
4.
To serve more than one URL or virtual host, add any additional virtual hosts.
5.
Configure the server settings under the Server tab.
6.
Configure the connections settings under the Performance Tuning tab.
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4.1. Basic Settings
7.
Copy all necessary files to the DocumentRoot and cgi-bin directories.
8.
Exit the application and select to save your settings.
4.1. Basic Settings
Use the Main tab to configure the basic server settings.
Figure 21.1. Basic Settings
Enter a fully qualified domain name that you have the right to use in the Server Name text area.
This option corresponds to the ServerName
[http://httpd.apache.org/docs/2.2/mod/core.html#servername] directive in httpd.conf. The
ServerName directive sets the hostname of the Web server. It is used when creating redirection
URLs. If you do not define a server name, the Web server attempts to resolve it from the IP address of the system. The server name does not have to be the domain name resolved from the
IP address of the server. For example, you might set the server name to www.example.com
while the server's real DNS name is foo.example.com.
Enter the email address of the person who maintains the Web server in the Webmaster email
address text area. This option corresponds to the ServerAdmin
[http://httpd.apache.org/docs/2.2/mod/core.html#serveradmin] directive in httpd.conf. If you
configure the server's error pages to contain an email address, this email address is used so
that users can report a problem to the server's administrator. The default value is
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4.2. Default Settings
[email protected]
Use the Available Addresses area to define the ports on which the server accepts incoming requests. This option corresponds to the Listen
[http://httpd.apache.org/docs/2.2/mod/mpm_common.html#listen] directive in httpd.conf. By default, Red Hat configures the Apache HTTP Server to listen to port 80 for non-secure Web communications.
Click the Add button to define additional ports on which to accept requests. A window as shown
in Figure 21.2, “Available Addresses” appears. Either choose the Listen to all addresses option to listen to all IP addresses on the defined port or specify a particular IP address over which
the server accepts connections in the Address field. Only specify one IP address per port number. To specify more than one IP address with the same port number, create an entry for each
IP address. If at all possible, use an IP address instead of a domain name to prevent a DNS
lookup failure. Refer to http://httpd.apache.org/docs/2.2/dns-caveats.html for more information
about Issues Regarding DNS and Apache.
Entering an asterisk (*) in the Address field is the same as choosing Listen to all addresses.
Clicking the Edit button in the Available Addresses frame shows the same window as the Add
button except with the fields populated for the selected entry. To delete an entry, select it and
click the Delete button.
Tip
If you set the server to listen to a port under 1024, you must be root to start it. For
port 1024 and above, httpd can be started as a regular user.
Figure 21.2. Available Addresses
4.2. Default Settings
After defining the Server Name, Webmaster email address, and Available Addresses, click
the Virtual Hosts tab. The figure below illustrates the Virtual Hosts tab.
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4.2. Default Settings
Figure 21.3. Virtual Hosts Tab
Clicking on Edit will display the Virtual Host Properties window from which you can set your
preferred settings. To add new settings, click on the Add button which will also display the Virtual Host Properties window. Clicking on the Edit Default Settings button, displays the Virtual Host Properties window without the General Options tab.
In the General Options tab, you can change the hostname, the document root directory and
also set the webmaster's email address. In the Host information, you can set the Virtual Host's
IP Address and Host Name. The figure below illustrates the General Options tab.
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4.2. Default Settings
Figure 21.4. General Options
If you add a virtual host, the settings you configure for the virtual host take precedence for that
virtual host. For a directive not defined within the virtual host settings, the default value is used.
4.2.1. Site Configuration
The figure below illustrates the Page Optionstab from which you can configure the Directory
Page Search List and Error Pages. If you are unsure of these settings, do not modify them.
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4.2. Default Settings
Figure 21.5. Site Configuration
The entries listed in the Directory Page Search List define the DirectoryIndex
[http://httpd.apache.org/docs/2.2/mod/mod_dir.html#directoryindex] directive. The DirectoryIndex is the default page served by the server when a user requests an index of a directory by
specifying a forward slash (/) at the end of the directory name.
For example, when a user requests the page http://www.example.com/this_directory/, they
are going to get either the DirectoryIndex page, if it exists, or a server-generated directory list.
The server tries to find one of the files listed in the DirectoryIndex directive and returns the first
one it finds. If it does not find any of these files and if Options Indexes is set for that directory,
the server generates and returns a list, in HTML format, of the subdirectories and files in the directory.
Use the Error Code section to configure Apache HTTP Server to redirect the client to a local or
external URL in the event of a problem or error. This option corresponds to the ErrorDocument
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4.2. Default Settings
[http://httpd.apache.org/docs/2.2/mod/core.html#errordocument] directive. If a problem or error
occurs when a client tries to connect to the Apache HTTP Server, the default action is to display
the short error message shown in the Error Code column. To override this default configuration,
select the error code and click the Edit button. Choose Default to display the default short error
message. Choose URL to redirect the client to an external URL and enter a complete URL, including the http://, in the Location field. Choose File to redirect the client to an internal URL
and enter a file location under the document root for the Web server. The location must begin
the a slash (/) and be relative to the Document Root.
For example, to redirect a 404 Not Found error code to a webpage that you created in a file
called 404.html, copy 404.html to DocumentRoot/../error/404.html. In this case, DocumentRoot is
the Document Root directory that you have defined (the default is /var/www/html/). If the Document Root is left as the default location, the file should be copied to /var/www/error/404.html.
Then, choose File as the Behavior for 404 - Not Found error code and enter /error/404.html
as the Location.
From the Default Error Page Footer menu, you can choose one of the following options:
•
Show footer with email address — Display the default footer at the bottom of all error
pages along with the email address of the website maintainer specified by the ServerAdmin
[http://httpd.apache.org/docs/2.2/mod/core.html#serveradmin] directive.
•
Show footer — Display just the default footer at the bottom of error pages.
•
No footer — Do not display a footer at the bottom of error pages.
4.2.2. SSL Support
The mod_ssl enables encryption of the HTTP protocol over SSL. SSL (Secure Sockets Layer)
protocol is used for communication and encryption over TCP/IP networks. The SSL tab enables
you to configure SSL for your server. To configure SSL you need to provide the path to your:
•
Certificate file - equivalent to using the SSLCertificateFile directive which points the path to
the PEM (Privacy Enhanced Mail)-encoded server certificate file.
•
Key file - equivalent to using the SSLCertificateKeyFile directive which points the path to the
PEM-encoded server private key file.
•
Certificate chain file - equivalent to using the SSLCertificateChainFile directive which points
the path to the certificate file containing all the server's chain of certificates.
•
Certificate authority file - is an encrypted file used to confirm the authenticity or identity of
parties communicating with the server.
You can find out more about configuration directives for SSL on http://httpd.apache.org/docs/2.2/mod/directives.html#S
[http://httpd.apache.org/docs/2.2/mod/directives.html#S]. You also need to determine which
SSL options to enable. These are equivalent to using the SSLOptions with the following options:
•
FakeBasicAuth - enables standard authentication methods used by Apache. This means
that the Client X509 certificate's Subject Distinguished Name (DN) is translated into a basic
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4.2. Default Settings
HTTP username.
•
ExportCertData - creates CGI environment variables in SSL_SERVER_CERT, SSL_CLIENT_CERT
and SSL_CLIENT_CERT_CHAIN_n where n is a number 0,1,2,3,4... These files are used for more
certificate checks by CGI scripts.
•
CompatEnvVars - enables backward compatibility for Apache SSL by adding CGI environment variables.
•
StrictRequire - enables strict access which forces denial of access whenever the SSLRequireSSL and SSLRequire directives indicate access is forbiden.
•
OptRenegotiate - enables avoidance of unnecessary handshakes by mod_ssl which also performs safe parameter checks. It is recommended to enable OptRenegotiate on a per directory basis.
More information on the above SSL options can be found on http://httpd.apache.org/docs/2.2/mod/mod_ssl.html#ssloptions
[http://httpd.apache.org/docs/2.2/mod/mod_ssl.html#ssloptions]. The figure below illustrates the
SSL tab and the options discussed above.
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4.2. Default Settings
Figure 21.6. SSL
4.2.3. Logging
Use the Logging tab to configure options for specific transfer and error logs.
By default, the server writes the transfer log to the /var/log/httpd/access_log file and the error
log to the /var/log/httpd/error_log file.
The transfer log contains a list of all attempts to access the Web server. It records the IP address of the client that is attempting to connect, the date and time of the attempt, and the file on
the Web server that it is trying to retrieve. Enter the name of the path and file in which to store
this information. If the path and file name do not start with a slash (/), the path is relative to the
server root directory as configured. This option corresponds to the TransferLog
[http://httpd.apache.org/docs/2.2/mod/mod_log_config.html#transferlog] directive.
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4.2. Default Settings
Figure 21.7. Logging
You can configure a custom log format by checking Use custom logging facilities and entering a custom log string in the Custom Log String field. This configures the LogFormat
[http://httpd.apache.org/docs/2.2/mod/mod_log_config.html#logformat] directive. Refer to http://httpd.apache.org/docs/2.2/mod/mod_log_config.html#logformat
[http://httpd.apache.org/docs-2.0/mod/mod_log_config.html#formats] for details on the format of
this directive.
The error log contains a list of any server errors that occur. Enter the name of the path and file
in which to store this information. If the path and file name do not start with a slash (/), the path
is relative to the server root directory as configured. This option corresponds to the ErrorLog
[http://httpd.apache.org/docs/2.2/mod/core.html#errorlog] directive.
Use the Log Level menu to set the verbosity of the error messages in the error logs. It can be
set (from least verbose to most verbose) to emerg, alert, crit, error, warn, notice, info or debug.
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4.2. Default Settings
This option corresponds to the LogLevel
[http://httpd.apache.org/docs/2.2/mod/core.html#loglevel] directive.
The value chosen with the Reverse DNS Lookup menu defines the HostnameLookups
[http://httpd.apache.org/docs/2.2/mod/core.html#hostnamelookups] directive. Choosing No Reverse Lookup sets the value to off. Choosing Reverse Lookup sets the value to on. Choosing
Double Reverse Lookup sets the value to double.
If you choose Reverse Lookup, your server automatically resolves the IP address for each connection which requests a document from your Web server. Resolving the IP address means that
your server makes one or more connections to the DNS in order to find out the hostname that
corresponds to a particular IP address.
If you choose Double Reverse Lookup, your server performs a double-reverse DNS. In other
words, after a reverse lookup is performed, a forward lookup is performed on the result. At least
one of the IP addresses in the forward lookup must match the address from the first reverse
lookup.
Generally, you should leave this option set to No Reverse Lookup, because the DNS requests
add a load to your server and may slow it down. If your server is busy, the effects of trying to
perform these reverse lookups or double reverse lookups may be quite noticeable.
Reverse lookups and double reverse lookups are also an issue for the Internet as a whole.
Each individual connection made to look up each hostname adds up. Therefore, for your own
Web server's benefit, as well as for the Internet's benefit, you should leave this option set to No
Reverse Lookup.
4.2.4. Environment Variables
Use the Environment tab to configure options for specific variables to set, pass, or unset for
CGI scripts.
Sometimes it is necessary to modify environment variables for CGI scripts or server-side include
(SSI) pages. The Apache HTTP Server can use the mod_env module to configure the environment variables which are passed to CGI scripts and SSI pages. Use the Environment Variables page to configure the directives for this module.
Use the Set for CGI Scripts section to set an environment variable that is passed to CGI scripts
and SSI pages. For example, to set the environment variable MAXNUM to 50, click the Add button
inside the Set for CGI Script section, as shown in Figure 21.8, “Environment Variables”, and
type MAXNUM in the Environment Variable text field and 50 in the Value to set text field. Click
OK to add it to the list. The Set for CGI Scripts section configures the SetEnv
[http://httpd.apache.org/docs/2.2/mod/mod_env.html#setenv] directive.
Use the Pass to CGI Scripts section to pass the value of an environment variable when the
server is first started to CGI scripts. To see this environment variable, type the command env at
a shell prompt. Click the Add button inside the Pass to CGI Scripts section and enter the name
of the environment variable in the resulting dialog box. Click OK to add it to the list. The Pass to
CGI Scripts section configures the PassEnv
[http://httpd.apache.org/docs/2.2/mod/mod_env.html#passenv] directive.
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4.2. Default Settings
Figure 21.8. Environment Variables
To remove an environment variable so that the value is not passed to CGI scripts and SSI
pages, use the Unset for CGI Scripts section. Click Add in the Unset for CGI Scripts section,
and enter the name of the environment variable to unset. Click OK to add it to the list. This corresponds to the UnsetEnv [http://httpd.apache.org/docs/2.2/mod/mod_env.html#unsetenv] directive.
To edit any of these environment values, select it from the list and click the corresponding Edit
button. To delete any entry from the list, select it and click the corresponding Delete button.
To learn more about environment variables in the Apache HTTP Server, refer to the following:
http://httpd.apache.org/docs/2.2/env.html
4.2.5. Directories
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4.2. Default Settings
Use the Directories page in the Performance tab to configure options for specific directories.
This corresponds to the <Directory> [http://httpd.apache.org/docs/2.2/mod/core.html#directory]
directive.
Figure 21.9. Directories
Click the Edit button in the top right-hand corner to configure the Default Directory Options for
all directories that are not specified in the Directory list below it. The options that you choose
are listed as the Options [http://httpd.apache.org/docs/2.2/mod/core.html#options] directive within the <Directory> [http://httpd.apache.org/docs/2.2/mod/core.html#directory] directive. You can
configure the following options:
•
ExecCGI — Allow execution of CGI scripts. CGI scripts are not executed if this option is not
chosen.
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4.2. Default Settings
•
FollowSymLinks — Allow symbolic links to be followed.
•
Includes — Allow server-side includes.
•
IncludesNOEXEC — Allow server-side includes, but disable the #exec and #include commands in CGI scripts.
•
Indexes — Display a formatted list of the directory's contents, if no DirectoryIndex (such as
index.html) exists in the requested directory.
•
Multiview — Support content-negotiated multiviews; this option is disabled by default.
•
SymLinksIfOwnerMatch — Only follow symbolic links if the target file or directory has the
same owner as the link.
To specify options for specific directories, click the Add button beside the Directory list box. A
window as shown in Figure 21.10, “Directory Settings” appears. Enter the directory to configure
in the Directory text field at the bottom of the window. Select the options in the right-hand list
and configure the Order [http://httpd.apache.org/docs-2.0/mod/mod_access.html#order] directive with the left-hand side options. The Order directive controls the order in which allow and
deny directives are evaluated. In the Allow hosts from and Deny hosts from text field, you
can specify one of the following:
•
Allow all hosts — Type all to allow access to all hosts.
•
Partial domain name — Allow all hosts whose names match or end with the specified string.
•
Full IP address — Allow access to a specific IP address.
•
A subnet — Such as 192.168.1.0/255.255.255.0
•
A network CIDR specification — such as 10.3.0.0/16
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5. Configuration Directives in httpd.conf
Figure 21.10. Directory Settings
If you check the Let .htaccess files override directory options, the configuration directives in
the .htaccess file take precedence.
5. Configuration Directives in
httpd.conf
The Apache HTTP Server configuration file is /etc/httpd/conf/httpd.conf. The httpd.conf file
is well-commented and mostly self-explanatory. The default configuration works for most situations; however, it is a good idea to become familiar some of the more important configuration
options.
Warning
With the release of Apache HTTP Server 2.2, many configuration options have
changed. If migrating from version 1.3 to 2.2, please firstly read Section 2.2,
“Migrating Apache HTTP Server 1.3 Configuration Files to 2.0”.
5.1. General Configuration Tips
If configuring the Apache HTTP Server, edit /etc/httpd/conf/httpd.conf and then either reload,
restart, or stop and start the httpd process as outlined in Section 3, “Starting and Stopping httpd”.
Before editing httpd.conf, make a copy the original file. Creating a backup makes it easier to recover from mistakes made while editing the configuration file.
If a mistake is made and the Web server does not work correctly, first review recently edited
passages in httpd.conf to verify there are no typos.
Next look in the Web server's error log, /var/log/httpd/error_log. The error log may not be
easy to interpret, depending on your level of expertise. However, the last entries in the error log
should provide useful information.
The following subsections contain a list of short descriptions for many of the directives included
in httpd.conf. These descriptions are not exhaustive. For more information, refer to the Apache
documentation online at http://httpd.apache.org/docs/2.2/.
For more information about mod_ssl directives, refer to the documentation online at http://httpd.apache.org/docs/2.2/mod/mod_ssl.html.
AccessFileName. AccessFileName names the file which the server should use for access control information in each directory. The default is .htaccess.
Immediately after the AccessFileName directive, a set of Files tags apply access control to any
file beginning with a .ht. These directives deny Web access to any .htaccess files (or other files
which begin with .ht) for security reasons.
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5.1. General Configuration Tips
Action. Action specifies a MIME content type and CGI script pair, so that when a file of that
media type is requested, a particular CGI script is executed.
AddDescription. When using FancyIndexing as an IndexOptions parameter, the AddDescription directive can be used to display user-specified descriptions for certain files or file types in a
server generated directory listing. The AddDescription directive supports listing specific files,
wildcard expressions, or file extensions.
AddEncoding. AddEncoding names file name extensions which should specify a particular encoding type. AddEncoding can also be used to instruct some browsers to uncompress certain
files as they are downloaded.
AddHandler. AddHandler maps file extensions to specific handlers. For example, the cgiscript handler can be matched with the extension .cgi to automatically treat a file ending with
.cgi as a CGI script. The following is a sample AddHandler directive for the .cgi extension.
AddHandler cgi-script .cgi
This directive enables CGIs outside of the cgi-bin to function in any directory on the server
which has the ExecCGI option within the directories container. Refer to Directory for more information about setting the ExecCGI option for a directory.
In addition to CGI scripts, the AddHandler directive is used to process server-parsed HTML and
image-map files.
AddIcon. AddIcon specifies which icon to show in server generated directory listings for files
with certain extensions. For example, the Web server is set to show the icon binary.gif for files
with .bin or .exe extensions.
AddIconByEncoding. This directive names icons which are displayed by files with MIME encoding in server generated directory listings. For example, by default, the Web server shows the
compressed.gif icon next to MIME encoded x-compress and x-gzip files in server generated directory listings.
AddIconByType. This directive names icons which are displayed next to files with MIME types
in server generated directory listings. For example, the server shows the icon text.gif next to
files with a mime-type of text, in server generated directory listings.
AddLanguage. AddLanguage associates file name extensions with specific languages. This directive is useful for Apache HTTP Servers which serve content in multiple languages based on
the client Web browser's language settings.
AddType. Use the AddType directive to define or override a default MIME type and file extension pairs. The following example directive tells the Apache HTTP Server to recognize the .tgz
file extension:
AddType application/x-tar .tgz
Alias. The Alias setting allows directories outside the DocumentRoot directory to be accessible.
Any URL ending in the alias automatically resolves to the alias' path. By default, one alias for an
icons/ directory is already set up. An icons/ directory can be accessed by the Web server, but
the directory is not in the DocumentRoot.
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5.1. General Configuration Tips
Allow. Allow specifies which client can access a given directory. The client can be all, a domain name, an IP address, a partial IP address, a network/netmask pair, and so on. The DocumentRoot directory is configured to Allow requests from all, meaning everyone has access.
AllowOverride. The AllowOverride directive sets whether any Options can be overridden by
the declarations in an .htaccess file. By default, both the root directory and the DocumentRoot are
set to allow no .htaccess overrides.
BrowserMatch. The BrowserMatch directive allows the server to define environment variables
and take appropriate actions based on the User-Agent HTTP header field — which identifies the
client's Web browser type. By default, the Web server uses BrowserMatch to deny connections to
specific browsers with known problems and also to disable keepalives and HTTP header
flushes for browsers that are known to have problems with those actions.
Cache Directives. A number of commented cache directives are supplied by the default
Apache HTTP Server configuration file. In most cases, uncommenting these lines by removing
the hash mark (#) from the beginning of the line is sufficient. The following, however, is a list of
some of the more important cache-related directives.
•
CacheEnable
— Specifies whether the cache is a disk, memory, or file descriptor cache. By
default CacheEnable configures a disk cache for URLs at or below /.
•
CacheRoot
•
CacheSize
— Specifies the name of the directory containing cached files. The default CacheRoot is the /var/httpd/proxy/ directory.
ize
— Specifies how much space the cache can use in kilobytes. The default CacheSis 5 KB.
The following is a list of some of the other common cache-related directives.
•
CacheMaxExpire
— Specifies how long HTML documents are retained (without a reload from
the originating Web server) in the cache. The default is 24 hours (86400 seconds).
•
CacheLastModifiedFactor
•
CacheDefaultExpire
•
NoProxy
— Specifies the creation of an expiry (expiration) date for a document which did not come from its originating server with its own expiry set. The default
CacheLastModifiedFactor is set to 0.1, meaning that the expiry date for such documents
equals one-tenth of the amount of time since the document was last modified.
— Specifies the expiry time in hours for a document that was received
using a protocol that does not support expiry times. The default is set to 1 hour (3600
seconds).
— Specifies a space-separated list of subnets, IP addresses, domains, or hosts
whose content is not cached. This setting is most useful for Intranet sites.
CacheNegotiatedDocs. By default, the Web server asks proxy servers not to cache any documents which were negotiated on the basis of content (that is, they may change over time or because of the input from the requester). If CacheNegotiatedDocs is set to on, this function is disabled and proxy servers are allowed to cache such documents.
CustomLog.
CustomLog
identifies the log file and the log file format. By default, the access log
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5.1. General Configuration Tips
is recorded to the /var/log/httpd/access_log file while errors are recorded in the /
var/log/httpd/error_log file.
The default CustomLog format is the combined log file format, as illustrated here:
remotehost rfc931 user date "request" status bytes referrer user-agent
DefaultIcon. DefaultIcon specifies the icon displayed in server generated directory listings for
files which have no other icon specified. The unknown.gif image file is the default.
DefaultType. DefaultType sets a default content type for the Web server to use for documents
whose MIME types cannot be determined. The default is text/plain.
Deny. Deny works similar to Allow, except it specifies who is denied access. The DocumentRoot
is not configured to Deny requests from anyone by default.
Directory. <Directory /path/to/directory> and </Directory> tags create a container used to
enclose a group of configuration directives which apply only to a specific directory and its subdirectories. Any directive which is applicable to a directory may be used within Directory tags.
By default, very restrictive parameters are applied to the root directory (/), using the Options
(refer to Options) and AllowOverride (refer to AllowOverride) directives. Under this configuration, any directory on the system which needs more permissive settings has to be explicitly given those settings.
In the default configuration, another Directory container is configured for the DocumentRoot
which assigns less rigid parameters to the directory tree so that the Apache HTTP Server can
access the files residing there.
The Directory container can be also be used to configure additional cgi-bin directories for server-side applications outside of the directory specified in the ScriptAlias directive (refer to
ScriptAlias for more information).
To accomplish this, the Directory container must set the ExecCGI option for that directory.
For example, if CGI scripts are located in /home/my_cgi_directory, add the following Directory
container to the httpd.conf file:
<Directory /home/my_cgi_directory> Options +ExecCGI </Directory>
Next, the AddHandler directive must be uncommented to identify files with the .cgi extension as
CGI scripts. Refer to AddHandler for instructions on setting AddHandler.
For this to work, permissions for CGI scripts, and the entire path to the scripts, must be set to
0755.
DirectoryIndex. The DirectoryIndex is the default page served by the server when a user requests an index of a directory by specifying a forward slash (/) at the end of the directory name.
When a user requests the page http://example/this_directory/, they get either the DirectoryIndex page, if it exists, or a server-generated directory list. The default for DirectoryIndex is index.html and the index.html.var type map. The server tries to find either of these files and returns the first one it finds. If it does not find one of these files and Options Indexes is set for that
directory, the server generates and returns a listing, in HTML format, of the subdirectories and
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5.1. General Configuration Tips
files within the directory, unless the directory listing feature is turned off.
DocumentRoot. DocumentRoot is the directory which contains most of the HTML files which are
served in response to requests. The default DocumentRoot, for both the non-secure and secure
Web servers, is the /var/www/html directory. For example, the server might receive a request for
the following document:
http://example.com/foo.html
The server looks for the following file in the default directory:
/var/www/html/foo.html
To change the DocumentRoot so that it is not shared by the secure and the non-secure Web servers, refer to Section 7, “Virtual Hosts”.
ErrorDocument. The ErrorDocument directive associates an HTTP response code with a message or a URL to be sent back to the client. By default, the Web server outputs a simple and
usually cryptic error message when an error occurs. The ErrorDocument directive forces the Web
server to instead output a customized message or page.
Important
To be valid, the message must be enclosed in a pair of double quotes ".
ErrorLog. ErrorLog specifies the file where server errors are logged. By default, this directive is
set to /var/log/httpd/error_log.
ExtendedStatus. The ExtendedStatus directive controls whether Apache generates basic (off)
or detailed server status information (on), when the server-status handler is called. The serverstatus handler is called using Location tags. More information on calling server-status is included in Location.
Group. Specifies the group name of the Apache HTTP Server processes.
This directive has been deprecated for the configuration of virtual hosts.
By default, Group is set to apache.
HeaderName. HeaderName names the file which, if it exists in the directory, is prepended to the
start of server generated directory listings. Like ReadmeName, the server tries to include it as an
HTML document if possible or in plain text if not.
HostnameLookups. HostnameLookups can be set to on, off, or double. If HostnameLookups is set
to on, the server automatically resolves the IP address for each connection. Resolving the IP address means that the server makes one or more connections to a DNS server, adding processing overhead. If HostnameLookups is set to double, the server performs a double-reverse
DNS look up adding even more processing overhead.
To conserve resources on the server, HostnameLookups is set to off by default.
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5.1. General Configuration Tips
If hostnames are required in server log files, consider running one of the many log analyzer
tools that perform the DNS lookups more efficiently and in bulk when rotating the Web server
log files.
IfDefine. The IfDefine tags surround configuration directives that are applied if the "test" stated
in the IfDefine tag is true. The directives are ignored if the test is false.
The test in the IfDefine tags is a parameter name (for example, HAVE_PERL). If the parameter is
defined, meaning that it is provided as an argument to the server's start-up command, then the
test is true. In this case, when the Web server is started, the test is true and the directives contained in the IfDefine tags are applied.
IfModule. <IfModule> and </IfModule> tags create a conditional container which are only activated if the specified module is loaded. Directives within the IfModule container are processed
under one of two conditions. The directives are processed if the module contained within the
starting <IfModule> tag is loaded. Or, if an exclamation point ! appears before the module name,
the directives are processed only if the module specified in the <IfModule> tag is not loaded.
For more information about Apache HTTP Server modules, refer to Section 6, “Adding
Modules”.
Include.
Include
allows other configuration files to be included at runtime.
The path to these configuration files can be absolute or relative to the ServerRoot.
Important
For the server to use individually packaged modules, such as mod_ssl, mod_perl,
and php, the following directive must be included in Section 1: Global Environment
of httpd.conf:
Include conf.d/*.conf
IndexIgnore. IndexIgnore lists file extensions, partial file names, wildcard expressions, or full
file names. The Web server does not include any files which match any of those parameters in
server generated directory listings.
IndexOptions. IndexOptions controls the appearance of server generated directing listings, by
adding icons, file descriptions, and so on. If Options Indexes is set (refer to Options), the Web
server generates a directory listing when the Web server receives an HTTP request for a directory without an index.
First, the Web server looks in the requested directory for a file matching the names listed in the
DirectoryIndex directive (usually, index.html). If an index.html file is not found, Apache HTTP
Server creates an HTML directory listing of the requested directory. The appearance of this directory listing is controlled, in part, by the IndexOptions directive.
The default configuration turns on FancyIndexing. This means that a user can re-sort a directory
listing by clicking on column headers. Another click on the same header switches from ascend-
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5.1. General Configuration Tips
ing to descending order. FancyIndexing also shows different icons for different files, based upon
file extensions.
The AddDescription option, when used in conjunction with FancyIndexing, presents a short description for the file in server generated directory listings.
has a number of other parameters which can be set to control the appearance of
server generated directories. The IconHeight and IconWidth parameters require the server to include HTML HEIGHT and WIDTH tags for the icons in server generated webpages. The IconsAreLinks parameter combines the graphical icon with the HTML link anchor, which contains the
URL link target.
IndexOptions
KeepAlive. KeepAlive sets whether the server allows more than one request per connection
and can be used to prevent any one client from consuming too much of the server's resources.
By default Keepalive is set to off. If Keepalive is set to on and the server becomes very busy,
the server can quickly spawn the maximum number of child processes. In this situation, the
server slows down significantly. If Keepalive is enabled, it is a good idea to set the the KeepAliveTimeout low (refer to KeepAliveTimeout for more information about the KeepAliveTimeout directive) and monitor the /var/log/httpd/error_log log file on the server. This log reports when the
server is running out of child processes.
KeepAliveTimeout. KeepAliveTimeout sets the number of seconds the server waits after a request has been served before it closes the connection. Once the server receives a request, the
Timeout directive applies instead. The KeepAliveTimeout directive is set to 15 seconds by default.
LanguagePriority. LanguagePriority sets precedence for different languages in case the client
Web browser has no language preference set.
Listen. The Listen command identifies the ports on which the Web server accepts incoming requests. By default, the Apache HTTP Server is set to listen to port 80 for non-secure Web communications and (in the /etc/httpd/conf.d/ssl.conf file which defines any secure servers) to
port 443 for secure Web communications.
If the Apache HTTP Server is configured to listen to a port under 1024, only the root user can
start it. For port 1024 and above, httpd can be started as a regular user.
The Listen directive can also be used to specify particular IP addresses over which the server
accepts connections.
LoadModule. LoadModule is used to load Dynamic Shared Object (DSO) modules. More information on the Apache HTTP Server's DSO support, including instructions for using the LoadModule
directive, can be found in Section 6, “Adding Modules”. Note, the load order of the modules is
no longer important with Apache HTTP Server 2.0. Refer to Section 2.2.1.3, “Dynamic Shared
Object (DSO) Support” for more information about Apache HTTP Server 2.0 DSO support.
Location. The <Location> and </Location> tags create a container in which access control
based on URL can be specified.
For instance, to allow people connecting from within the server's domain to see status reports,
use the following directives:
<Location /server-status> SetHandler server-status Order deny,allow Deny from all Allow from <.example.com>
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5.1. General Configuration Tips
Replace <.example.com> with the second-level domain name for the Web server.
To provide server configuration reports (including installed modules and configuration directives)
to requests from inside the domain, use the following directives:
<Location /server-info> SetHandler server-info Order deny,allow Deny from all Allow from <.example.com> </L
Again, replace <.example.com> with the second-level domain name for the Web server.
LogFormat. The LogFormat directive configures the format of the various Web server log files.
The actual LogFormat used depends on the settings given in the CustomLog directive (refer to
CustomLog).
The following are the format options if the CustomLog directive is set to combined:
%h
(remote host's IP address or hostname)
Lists the remote IP address of the requesting client. If HostnameLookups is set to on, the client
hostname is recorded unless it is not available from DNS.
%l
(rfc931)
Not used. A hyphen - appears in the log file for this field.
%u
(authenticated user)
Lists the username of the user recorded if authentication was required. Usually, this is not
used, so a hyphen - appears in the log file for this field.
%t
(date)
Lists the date and time of the request.
%r
(request string)
Lists the request string exactly as it came from the browser or client.
%s
(status)
Lists the HTTP status code which was returned to the client host.
%b
(bytes)
Lists the size of the document.
(referrer)
Lists the URL of the webpage which referred the client host to Web server.
%\"%{Referer}i\"
(user-agent)
Lists the type of Web browser making the request.
%\"%{User-Agent}i\"
LogLevel. LogLevel sets how verbose the error messages in the error logs are. LogLevel can
be set (from least verbose to most verbose) to emerg, alert, crit, error, warn, notice, info, or
debug. The default LogLevel is warn.
MaxKeepAliveRequests. This directive sets the maximum number of requests allowed per
persistent connection. The Apache Project recommends a high setting, which improves the
server's performance. MaxKeepAliveRequests is set to 100 by default, which should be appropriate for most situations.
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5.1. General Configuration Tips
NameVirtualHost. The NameVirtualHost directive associates an IP address and port number, if
necessary, for any name-based virtual hosts. Name-based virtual hosting allows one Apache
HTTP Server to serve different domains without using multiple IP addresses.
Note
Name-based virtual hosts only work with non-secure HTTP connections. If using
virtual hosts with a secure server, use IP address-based virtual hosts instead.
To enable name-based virtual hosting, uncomment the NameVirtualHost configuration directive
and add the correct IP address. Then add additional VirtualHost containers for each virtual host
as is necessary for your configuration.
Options. The Options directive controls which server features are available in a particular directory. For example, under the restrictive parameters specified for the root directory, Options is
only set to the FollowSymLinks directive. No features are enabled, except that the server is allowed to follow symbolic links in the root directory.
By default, in the DocumentRoot directory, Options is set to include Indexes and FollowSymLinks.
Indexes permits the server to generate a directory listing for a directory if no DirectoryIndex (for
example, index.html) is specified. FollowSymLinks allows the server to follow symbolic links in
that directory.
Note
statements from the main server configuration section need to be replicated to each VirtualHost container individually. Refer to VirtualHost for more information.
Options
Order. The Order directive controls the order in which allow and deny directives are evaluated.
The server is configured to evaluate the Allow directives before the Deny directives for the DocumentRoot directory.
PidFile. PidFile names the file where the server records its process ID (PID). By default the
PID is listed in /var/run/httpd.pid.
Proxy. <Proxy *> and </Proxy> tags create a container which encloses a group of configuration
directives meant to apply only to the proxy server. Many directives which are allowed within a
<Directory> container may also be used within <Proxy> container.
ProxyRequests. To configure the Apache HTTP Server to function as a proxy server, remove
the hash mark (#) from the beginning of the <IfModule mod_proxy.c> line, the ProxyRequests,
and each line in the <Proxy> stanza. Set the ProxyRequests directive to On, and set which domains are allowed access to the server in the Allow from directive of the <Proxy> stanza.
ReadmeName.
ReadmeName
names the file which, if it exists in the directory, is appended to the
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5.1. General Configuration Tips
end of server generated directory listings. The Web server first tries to include the file as an
HTML document and then tries to include it as plain text. By default, ReadmeName is set to
README.html.
Redirect. When a webpage is moved, Redirect can be used to map the file location to a new
URL. The format is as follows:
Redirect /<old-path>/<file-name> http://<current-domain>/<current-path>/<file-name>
In this example, replace <old-path> with the old path information for <file-name> and
<current-domain> and <current-path> with the current domain and path information for
<file-name>.
In this example, any requests for <file-name> at the old location is automatically redirected to
the new location.
For more advanced redirection techniques, use the mod_rewrite module included with the
Apache HTTP Server. For more information about configuring the mod_rewrite module, refer to
the Apache Software Foundation documentation online at http://httpd.apache.org/docs/2.2/mod/mod_rewrite.html
[http://httpd.apache.org/docs/2.2/mod/mod_rewrite.html].
ScriptAlias. The ScriptAlias directive defines where CGI scripts are located. Generally, it is
not good practice to leave CGI scripts within the DocumentRoot, where they can potentially be
viewed as text documents. For this reason, a special directory outside of the DocumentRoot directory containing server-side executables and scripts is designated by the ScriptAlias directive.
This directory is known as a cgi-bin and is set to /var/www/cgi-bin/ by default.
It is possible to establish directories for storing executables outside of the cgi-bin/ directory.
For instructions on doing so, refer to AddHandler and Directory.
ServerAdmin. Sets the ServerAdmin directive to the email address of the Web server administrator. This email address shows up in error messages on server-generated Web pages, so
users can report a problem by sending email to the server administrator.
By default, ServerAdmin is set to [email protected]
A common way to set up ServerAdmin is to set it to [email protected] Once set, alias webmaster to the person responsible for the Web server in /etc/aliases and run /
usr/bin/newaliases.
ServerName. ServerName specifies a hostname and port number (matching the Listen directive) for the server. The ServerName does not need to match the machine's actual hostname. For
example, the Web server may be www.example.com, but the server's hostname is actually
foo.example.com. The value specified in ServerName must be a valid Domain Name Service
(DNS) name that can be resolved by the system — do not make something up.
The following is a sample ServerName directive:
ServerName www.example.com:80
When specifying a ServerName, be sure the IP address and server name pair are included in the
/etc/hosts file.
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5.1. General Configuration Tips
ServerRoot. The ServerRoot directive specifies the top-level directory containing website content. By default, ServerRoot is set to "/etc/httpd" for both secure and non-secure servers.
ServerSignature. The ServerSignature directive adds a line containing the Apache HTTP
Server server version and the ServerName to any server-generated documents, such as error
messages sent back to clients. ServerSignature is set to on by default.
can be set to EMail which adds a mailto:ServerAdmin HTML tag to the signature line of auto-generated responses. ServerSignature can also be set to Off to stop Apache
from sending out its version number and module information. Please also check the ServerTokens settings.
ServerSignature
ServerTokens. The ServerTokens directive determines if the Server response header field sent
back to clients should include details of the Operating System type and information about compiled-in modules. By default, ServerTokens is set to Full which sends information about the Operating System type and compiled-in modules. Setting the ServerTokens to Prod sends the
product name only and is recommended as many hackers check information in the Server
header when scanning for vulnerabilities. You can also set the ServerTokens to Min (minimal) or
to OS (operating system).
SuexecUserGroup. The SuexecUserGroup directive, which originates from the mod_suexec module, allows the specification of user and group execution privileges for CGI programs. Non-CGI
requests are still processed with the user and group specified in the User and Group directives.
Note
From version 2.0, the SuexecUserGroup directive replaced the Apache HTTP Server
1.3 configuration of using the User and Group directives inside the configuration of
VirtualHosts sections.
Timeout. Timeout defines, in seconds, the amount of time that the server waits for receipts and
transmissions during communications. Timeout is set to 300 seconds by default, which is appropriate for most situations.
TypesConfig. TypesConfig names the file which sets the default list of MIME type mappings
(file name extensions to content types). The default TypesConfig file is /etc/mime.types. Instead
of editing /etc/mime.types, the recommended way to add MIME type mappings is to use the AddType directive.
For more information about AddType, refer to AddType.
UseCanonicalName. When set to on, this directive configures the Apache HTTP Server to reference itself using the value specified in the ServerName and Port directives. When UseCanonicalName is set to off, the server instead uses the value used by the requesting client when referring to itself.
UseCanonicalName
is set to off by default.
User. The User directive sets the username of the server process and determines what files the
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5.2. Configuration Directives for SSL
server is allowed to access. Any files inaccessible to this user are also inaccessible to clients
connecting to the Apache HTTP Server.
By default User is set to apache.
This directive has been deprecated for the configuration of virtual hosts.
Note
For security reasons, the Apache HTTP Server does not run as the root user.
UserDir. UserDir is the subdirectory within each user's home directory where they should place
personal HTML files which are served by the Web server. This directive is set to disable by default.
The name for the subdirectory is set to public_html in the default configuration. For example,
the server might receive the following request:
http://example.com/~username/foo.html
The server would look for the file:
/home/username/public_html/foo.html
In the above example, /home/username/ is the user's home directory (note that the default path
to users' home directories may vary).
Make sure that the permissions on the users' home directories are set correctly. Users' home
directories must be set to 0711. The read (r) and execute (x) bits must be set on the users' public_html directories (0755 also works). Files that are served in a users' public_html directories
must be set to at least 0644.
VirtualHost. <VirtualHost> and </VirtualHost> tags create a container outlining the characteristics of a virtual host. The VirtualHost container accepts most configuration directives.
A commented VirtualHost container is provided in httpd.conf, which illustrates the minimum
set of configuration directives necessary for each virtual host. Refer to Section 7, “Virtual Hosts”
for more information about virtual hosts.
Note
The default SSL virtual host container now resides in the file /
etc/httpd/conf.d/ssl.conf.
5.2. Configuration Directives for SSL
The directives in /etc/httpd/conf.d/ssl.conf file can be configured to enable secure Web communications using SSL and TLS.
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5.3. MPM Specific Server-Pool Directives
SetEnvIf. SetEnvIf sets environment variables based on the headers of incoming connections.
It is not solely an SSL directive, though it is present in the supplied /etc/httpd/conf.d/ssl.conf
file. It's purpose in this context is to disable HTTP keepalive and to allow SSL to close the connection without a closing notification from the client browser. This setting is necessary for certain browsers that do not reliably shut down the SSL connection.
For more information on other directives within the SSL configuration file, refer to the following
URLs:
•
http://localhost/manual/mod/mod_ssl.html
•
http://httpd.apache.org/docs/2.2/mod/mod_ssl.html
Note
In most cases, SSL directives are configured appropriately during the installation of
Red Hat Enterprise Linux. Be careful when altering Apache HTTP Secure Server
directives, misconfiguration can lead to security vulnerabilities.
5.3. MPM Specific Server-Pool Directives
As explained in Section 2.2.1.2, “Server-Pool Size Regulation”, the responsibility for managing
characteristics of the server-pool falls to a module group called MPMs under Apache HTTP
Server 2.0. The characteristics of the server-pool differ depending upon which MPM is used. For
this reason, an IfModule container is necessary to define the server-pool for the MPM in use.
By default, Apache HTTP Server 2.0 defines the server-pool for both the prefork and worker
MPMs.
The following section list directives found within the MPM-specific server-pool containers.
MaxClients. MaxClients sets a limit on the total number of server processes, or simultaneously
connected clients, that can run at one time. The main purpose of this directive is to keep a runaway Apache HTTP Server from crashing the operating system. For busy servers this value
should be set to a high value. The server's default is set to 150 regardless of the MPM in use.
However, it is not recommended that the value for MaxClients exceeds 256 when using the prefork MPM.
MaxRequestsPerChild. MaxRequestsPerChild sets the total number of requests each child
server process serves before the child dies. The main reason for setting MaxRequestsPerChild is
to avoid long-lived process induced memory leaks. The default MaxRequestsPerChild for the prefork MPM is 4000 and for the worker MPM is 0.
MinSpareServers and MaxSpareServers. These values are only used with the prefork MPM.
They adjust how the Apache HTTP Server dynamically adapts to the perceived load by maintaining an appropriate number of spare server processes based on the number of incoming requests. The server checks the number of servers waiting for a request and kills some if there
are more than MaxSpareServers or creates some if the number of servers is less than MinSpare-
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6. Adding Modules
Servers.
The default MinSpareServers value is 5; the default MaxSpareServers value is 20. These default
settings should be appropriate for most situations. Be careful not to increase the MinSpareServers to a large number as doing so creates a heavy processing load on the server even when
traffic is light.
MinSpareThreads and MaxSpareThreads. These values are only used with the worker MPM.
They adjust how the Apache HTTP Server dynamically adapts to the perceived load by maintaining an appropriate number of spare server threads based on the number of incoming requests. The server checks the number of server threads waiting for a request and kills some if
there are more than MaxSpareThreads or creates some if the number of servers is less than MinSpareThreads.
The default MinSpareThreads value is 25; the default MaxSpareThreads value is 75. These default
settings should be appropriate for most situations. The value for MaxSpareThreads must be greater than or equal to the sum of MinSpareThreads and ThreadsPerChild, else the Apache HTTP
Server automatically corrects it.
StartServers. The StartServers directive sets how many server processes are created upon
startup. Since the Web server dynamically kills and creates server processes based on traffic
load, it is not necessary to change this parameter. The Web server is set to start 8 server processes at startup for the prefork MPM and 2 for the worker MPM.
ThreadsPerChild. This value is only used with the worker MPM. It sets the number of threads
within each child process. The default value for this directive is 25.
6. Adding Modules
The Apache HTTP Server is distributed with a number of modules. More information about
Apache HTTP modules can be found on http://httpd.apache.org/docs/2.2/mod/.
The Apache HTTP Server supports Dynamically Shared Objects (DSOs), or modules, which
can easily be loaded at runtime as necessary.
The Apache Project provides complete DSO documentation online at http://httpd.apache.org/docs/2.2/dso.html. Or, if the http-manual package is installed, documentation about DSOs can be found online at http://localhost/manual/mod/.
For the Apache HTTP Server to use a DSO, it must be specified in a LoadModule directive within
/etc/httpd/conf/httpd.conf. If the module is provided by a separate package, the line must appear within the modules configuration file in the /etc/httpd/conf.d/ directory. Refer to LoadModule for more information.
If adding or deleting modules from http.conf, Apache HTTP Server must be reloaded or restarted, as referred to in Section 3, “Starting and Stopping httpd”.
If creating a new module, first install the httpd-devel package which contains the include files,
the header files, as well as the APache eXtenSion (/usr/sbin/apxs) application, which uses the
include files and the header files to compile DSOs.
After writing a module, use /usr/sbin/apxs to compile the module sources outside the Apache
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7. Virtual Hosts
source tree. For more information about using the /usr/sbin/apxs command, refer to the the
Apache documentation online at http://httpd.apache.org/docs/2.2/dso.html as well as the apxs
man page.
Once compiled, put the module in the /usr/lib/httpd/modules/ directory. For RHEL platforms
using default-64-bit userspace (x86_64, ia64, ?) this path will be /usr/lib64/httpd/modules/.
Then add a LoadModule line to the httpd.conf, using the following structure:
LoadModule <module-name> <path/to/module.so>
Where <module-name> is the name of the module and <path/to/module.so> is the path to the
DSO.
7. Virtual Hosts
The Apache HTTP Server's built in virtual hosting allows the server to provide different information based on which IP address, hostname, or port is being requested. A complete guide to using virtual hosts is available online at http://httpd.apache.org/docs/2.2/vhosts/.
7.1. Setting Up Virtual Hosts
To create a name-based virtual host, it is best to use the virtual host container provided in httpd.conf as an example.
The virtual host example read as follows:
#NameVirtualHost *:80 # #<VirtualHost *:80> # ServerAdmin [email protected] # DocumentRoot /
To activate name-based virtual hosting, uncomment the NameVirtualHost line by removing the
hash mark (#) and replace the asterisk (*) with the IP address assigned to the machine.
Next, configure a virtual host by uncommenting and customizing the <VirtualHost> container.
On the <VirtualHost> line, change the asterisk (*) to the server's IP address. Change the
ServerName to a valid DNS name assigned to the machine, and configure the other directives as
necessary.
The <VirtualHost> container is highly customizable and accepts almost every directive available
within the main server configuration.
Tip
If configuring a virtual host to listen on a non-default port, that port must be added
to the Listen directive in the global settings section of /etc/httpd/conf/httpd.conf
file.
To activate a newly created virtual host, the Apache HTTP Server must be reloaded or restarted. Refer to Section 3, “Starting and Stopping httpd” for further instructions.
Comprehensive information about creating and configuring both name-based and IP address322
8. Apache HTTP Secure Server Configuration
based virtual hosts is provided online at http://httpd.apache.org/docs/2.2/vhosts/.
8. Apache HTTP Secure Server Configuration
This section provides basic information on the Apache HTTP Server with the mod_ssl security
module enabled to use the OpenSSL library and toolkit. The combination of these three components are referred to in this section as the secure Web server or just as the secure server.
The mod_ssl module is a security module for the Apache HTTP Server. The mod_ssl module
uses the tools provided by the OpenSSL Project to add a very important feature to the Apache
HTTP Server — the ability to encrypt communications. In contrast, regular HTTP communications between a browser and a Web server are sent in plain text, which could be intercepted
and read by someone along the route between the browser and the server.
This section is not meant to be complete and exclusive documentation for any of these programs. When possible, this guide points to appropriate places where you can find more in-depth
documentation on particular subjects.
This section shows you how to install these programs. You can also learn the steps necessary
to generate a private key and a certificate request, how to generate your own self-signed certificate, and how to install a certificate to use with your secure server.
The mod_ssl configuration file is located at /etc/httpd/conf.d/ssl.conf. For this file to be
loaded, and hence for mod_ssl to work, you must have the statement Include conf.d/*.conf in
the /etc/httpd/conf/httpd.conf file. This statement is included by default in the default Apache
HTTP Server configuration file.
8.1. An Overview of Security-Related Packages
To enable the secure server, you must have the following packages installed at a minimum:
httpd
The httpd package contains the httpd daemon and related utilities, configuration files,
icons, Apache HTTP Server modules, man pages, and other files used by the Apache HTTP Server.
mod_ssl
The mod_ssl package includes the mod_ssl module, which provides strong cryptography for
the Apache HTTP Server via the Secure Sockets Layer (SSL) and Transport Layer Security
(TLS) protocols.
openssl
The openssl package contains the OpenSSL toolkit. The OpenSSL toolkit implements the
SSL and TLS protocols, and also includes a general purpose cryptography library.
crypto-utils
The crypto-utils package provides a set of utilities to generate and manage SSL certificates and private keys. Among these utilities is genkey.
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8.2. An Overview of Certificates and Security
Additionally, other software packages provide certain security functionalities (but are not required by the secure server to function):
8.2. An Overview of Certificates and Security
Your secure server provides security using a combination of the Secure Sockets Layer (SSL)
protocol and (in most cases) a digital certificate from a Certificate Authority (CA). SSL handles
the encrypted communications as well as the mutual authentication between browsers and your
secure server. The CA-approved digital certificate provides authentication for your secure server
(the CA puts its reputation behind its certification of your organization's identity). When your
browser is communicating using SSL encryption, the https:// prefix is used at the beginning of
the Uniform Resource Locator (URL) in the navigation bar.
Encryption depends upon the use of keys (think of them as secret encoder/decoder rings in
data format). In conventional or symmetric cryptography, both ends of the transaction have the
same key, which they use to decode each other's transmissions. In public or asymmetric cryptography, two keys co-exist: a public key and a private key. A person or an organization keeps
their private key a secret and publishes their public key. Data encoded with the public key can
only be decoded with the private key; data encoded with the private key can only be decoded
with the public key.
To set up your secure server, use public cryptography to create a public and private key pair. In
most cases, you send your certificate request (including your public key), proof of your company's identity, and payment to a CA. The CA verifies the certificate request and your identity,
and then sends back a certificate for your secure server.
A secure server uses a certificate to identify itself to Web browsers. You can generate your own
certificate (called a "self-signed" certificate), or you can get a certificate from a CA. A certificate
from a reputable CA guarantees that a website is associated with a particular company or organization.
Alternatively, you can create your own self-signed certificate. Note, however, that self-signed
certificates should not be used in most production environments. Self-signed certificates are not
automatically accepted by a user's browser — users are prompted by the browser to accept the
certificate and create the secure connection. Refer to Section 8.4, “Types of Certificates” for
more information on the differences between self-signed and CA-signed certificates.
Once you have a self-signed certificate or a signed certificate from the CA of your choice, you
must install it on your secure server.
8.3. Using Pre-Existing Keys and Certificates
If you already have an existing key and certificate (for example, if you are installing the secure
server to replace another company's secure server product), you can probably use your existing
key and certificate with the secure server. The following two situations provide instances where
you are not able to use your existing key and certificate:
•
If you are changing your IP address or domain name — Certificates are issued for a particular IP address and domain name pair. You must get a new certificate if you are changing
your IP address or domain name.
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8.4. Types of Certificates
•
If you have a certificate from VeriSign and you are changing your server software — VeriSign is a widely used CA. If you already have a VeriSign certificate for another purpose, you
may have been considering using your existing VeriSign certificate with your new secure
server. However, you are not be allowed to because VeriSign issues certificates for one specific server software and IP address/domain name combination.
If you change either of those parameters (for example, if you previously used a different secure server product), the VeriSign certificate you obtained to use with the previous configuration will not work with the new configuration. You must obtain a new certificate.
If you have an existing key and certificate that you can use, you do not have to generate a new
key and obtain a new certificate. However, you may need to move and rename the files which
contain your key and certificate.
Move your existing key file to:
/etc/pki/tls/private/server.key
Move your existing certificate file to:
/etc/pki/tls/certs/server.crt
If you are upgrading from the Red Hat Secure Web Server, your old key (httpsd.key) and certificate (httpsd.crt) are located in /etc/httpd/conf/. Move and rename your key and certificate
so that the secure server can use them. Use the following two commands to move and rename
your key and certificate files:
mv /etc/httpd/conf/httpsd.key /etc/pki/tls/private/server.key mv /etc/httpd/conf/httpsd.crt /etc/pki/tls/ce
Then, start your secure server with the command:
/sbin/service httpd start
8.4. Types of Certificates
If you installed your secure server from the RPM package provided by Red Hat, a randomly
generated private key and a test certificate are generated and put into the appropriate directories. Before you begin using your secure server, however, you must generate your own key and
obtain a certificate which correctly identifies your server.
You need a key and a certificate to operate your secure server — which means that you can
either generate a self-signed certificate or purchase a CA-signed certificate from a CA. What are
the differences between the two?
A CA-signed certificate provides two important capabilities for your server:
•
Browsers (usually) automatically recognize the certificate and allow a secure connection to
be made, without prompting the user.
•
When a CA issues a signed certificate, they are guaranteeing the identity of the organization
that is providing the webpages to the browser.
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8.5. Generating a Key
If your secure server is being accessed by the public at large, your secure server needs a certificate signed by a CA so that people who visit your website know that the website is owned by
the organization who claims to own it. Before signing a certificate, a CA verifies that the organization requesting the certificate was actually who they claimed to be.
Most Web browsers that support SSL have a list of CAs whose certificates they automatically
accept. If a browser encounters a certificate whose authorizing CA is not in the list, the browser
asks the user to either accept or decline the connection.
You can generate a self-signed certificate for your secure server, but be aware that a selfsigned certificate does not provide the same functionality as a CA-signed certificate. A selfsigned certificate is not automatically recognized by most Web browsers and does not provide
any guarantee concerning the identity of the organization that is providing the website. A CAsigned certificate provides both of these important capabilities for a secure server. If your secure
server is to be used in a production environment, a CA-signed certificate is recommended.
The process of getting a certificate from a CA is fairly easy. A quick overview is as follows:
1.
Create an encryption private and public key pair.
2.
Create a certificate request based on the public key. The certificate request contains information about your server and the company hosting it.
3.
Send the certificate request, along with documents proving your identity, to a CA. Red Hat
does not make recommendations on which certificate authority to choose. Your decision
may be based on your past experiences, on the experiences of your friends or colleagues,
or purely on monetary factors.
Once you have decided upon a CA, you need to follow the instructions they provide on how
to obtain a certificate from them.
4.
When the CA is satisfied that you are indeed who you claim to be, they provide you with a
digital certificate.
5.
Install this certificate on your secure server and begin handling secure transactions.
Whether you are getting a certificate from a CA or generating your own self-signed certificate,
the first step is to generate a key. Refer to Section 8.5, “Generating a Key” for instructions.
8.5. Generating a Key
You must be root to generate a key.
First, use the cd command to change to the /etc/httpd/conf/ directory. Remove the fake key
and certificate that were generated during the installation with the following commands:
rm ssl.key/server.keyrm ssl.crt/server.crt
The crypto-utils package contains the genkey utility which you can use to generate keys as the
name implies. To create your own private key, please ensure the crypto-utils package is installed. You can view more options by typing man genkey in your terminal. Assuming you wish to
generate keys for www.example.com using the genkey utility, type in the following command in
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8.5. Generating a Key
your terminal:
genkey www.example.com
Please note that the make based process is no longer shipped with RHEL 5. This will start the
genkey graphical user interface. The figure below illustrates the first screen. To navigate, use the
keyboard arrow and tab keys. This windows indicates where your key will be stored and
prompts you to proceed or cancel the operation. To proceed to the next step, select Next and
press the Return (Enter) key.
Figure 21.11. Keypair generation
The next screen prompts you to choose the size of your key. As indicated, the smaller the size
of your key, the faster will the response from your server be and the lesser your level of security.
On selecting your preferred, key size using the arrow keys, select Next to proceed to the next
step. The figure below illustrates the key size selection screen.
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8.5. Generating a Key
Figure 21.12. Choose key size
Selecting the next step will initiate the random bits generation process which may take some
time depending on the size of your selected key. The larger the size of your key, the longer it
will take to generate it.
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8.5. Generating a Key
Figure 21.13. Generating random bits
On generating your key, you will be prompted to send a Certificate Request (CSR) to a Certificate Authority (CA).
Figure 21.14. Generate CSR
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8.5. Generating a Key
Selecting Yes will prompt you to select the Certificate Authority you wish to send your request
to. Selecting No will allow you to generate a self-signed certificate. The next step for this is illustrated in Figure 21.17, “Generating a self signed certificate for your server”.
Figure 21.15. Choose Certificate Authority (CA)
On Selecting your preferred option, select Next to proceed to the next step. The next screen allows you to enter the details of your certificate.
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8.5. Generating a Key
Figure 21.16. Enter details for your certificate
If you prefer to generate a self signed cert key pair, you should not generate a CSR. To do this,
select No as your preferred option in the Generate CSR screen. This will display the figure below from which you can enter your certificate details. Entering your certificate details and pressing the return key will display the Figure 21.19, “Protecting your private key” from which you can
choose to encrypt your private key or not.
Figure 21.17. Generating a self signed certificate for your server
On entering the details of your certificate, select Next to proceed. The figure below illustrates an
example of a the next screen displayed after completing the details for a certificate to be sent to
Equifax. Please note that if you are generating a self signed key, for your server, this screen is
not displayed.
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8.5. Generating a Key
Figure 21.18. Begin certificate request
Pressing the return key, will display the next screen from which you can enable or disable the
encryption of the private key. Use the spacebar to enable or disable this. When enabled, a [*]
character will be displayed. On selecting your preferred option, select Next to proceed to the
next step.
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8.5. Generating a Key
Figure 21.19. Protecting your private key
The next screen allows you to set your key passphase. Please do not lose this pass phase as
you will not be able to run the server without it. You will need to regenerate a new private or
public key pair and request a new certificate from your CA as indicated. For security, the passphase is not displayed as you type. On typing your preferred passphase, select Next to go back
to your terminal.
Figure 21.20. Set passphase
If you attempt to run genkey makeca on a server that has an existing key pair, an error message
will be displayed as illustrated below. You need to delete your existing key file as indicated to
generate a new key pair.
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8.6. How to configure the server to use the new key
Figure 21.21. genkey error
•
http://httpd.apache.org/docs/2.2/ssl/
•
http://httpd.apache.org/docs/2.2/vhosts/
8.6. How to configure the server to use the new key
The steps to configure the Apache HTTP Server to use the new key are:
•
Obtain the signed certificate from the CA after submitting the CSR.
•
Copy the certificate to the path, for example /etc/pki/tls/certs/www.example.com.crt
•
Edit /etc/httpd/conf.d/ssl.conf. Change the SSLCertificateFile and SSLCertificateKey
lines to be.
SSLCertificateFile /etc/pki/tls/certs/www.example.com.crt
SSLCertificateKeyFile /etc/pki/tls/private/www.example.com.key
where the "www.example.com" part should match the argument passed on the genkey command.
9. Additional Resources
To learn more about the Apache HTTP Server, refer to the following resources.
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9.1. Useful Websites
9.1. Useful Websites
•
http://httpd.apache.org/ — The official website for the Apache HTTP Server with documentation on all the directives and default modules.
•
http://www.modssl.org/ — The official website for mod_ssl.
•
http://www.apacheweek.com/ — A comprehensive online weekly newsletter about all things
Apache.
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Chapter 22. FTP
File Transfer Protocol (FTP) is one of the oldest and most commonly used protocols found on
the Internet today. Its purpose is to reliably transfer files between computer hosts on a network
without requiring the user to log directly into the remote host or have knowledge of how to use
the remote system. It allows users to access files on remote systems using a standard set of
simple commands.
This chapter outlines the basics of the FTP protocol, as well as configuration options for the
primary FTP server shipped with Red Hat Enterprise Linux, vsftpd.
1. The File Transport Protocol
However, because FTP is so prevalent on the Internet, it is often required to share files to the
public. System administrators, therefore, should be aware of the FTP protocol's unique characteristics.
1.1. Multiple Ports, Multiple Modes
Unlike most protocols used on the Internet, FTP requires multiple network ports to work properly. When an FTP client application initiates a connection to an FTP server, it opens port 21 on
the server — known as the command port. This port is used to issue all commands to the server. Any data requested from the server is returned to the client via a data port. The port number
for data connections, and the way in which data connections are initialized, vary depending
upon whether the client requests the data in active or passive mode.
The following defines these modes:
active mode
Active mode is the original method used by the FTP protocol for transferring data to the client application. When an active mode data transfer is initiated by the FTP client, the server
opens a connection from port 20 on the server to the IP address and a random, unprivileged
port (greater than 1024) specified by the client. This arrangement means that the client machine must be allowed to accept connections over any port above 1024. With the growth of
insecure networks, such as the Internet, the use of firewalls to protect client machines is
now prevalent. Because these client-side firewalls often deny incoming connections from
active mode FTP servers, passive mode was devised.
passive mode
Passive mode, like active mode, is initiated by the FTP client application. When requesting
data from the server, the FTP client indicates it wants to access the data in passive mode
and the server provides the IP address and a random, unprivileged port (greater than 1024)
on the server. The client then connects to that port on the server to download the requested
information.
While passive mode resolves issues for client-side firewall interference with data connections, it can complicate administration of the server-side firewall. You can reduce the number of open ports on a server by limiting the range of unprivileged ports on the FTP server.
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2. FTP Servers
This also simplifies the process of configuring firewall rules for the server. Refer to Section 5.8, “Network Options” for more about limiting passive ports.
2. FTP Servers
Red Hat Enterprise Linux ships with two different FTP servers:
•
Red Hat Content Accelerator — A kernel-based Web server that delivers high performance
Web server and FTP services. Since speed as its primary design goal, it has limited functionality and runs only as an anonymous FTP server. For more information about configuring
and administering Red Hat Content Accelerator, consult the documentation available online at http://www.redhat.com/docs/manuals/tux/.
•
vsftpd
— A fast, secure FTP daemon which is the preferred FTP server for Red Hat Enterprise Linux. The remainder of this chapter focuses on vsftpd.
2.1. vsftpd
The Very Secure FTP Daemon (vsftpd) is designed from the ground up to be fast, stable, and,
most importantly, secure. Its ability to handle large numbers of connections efficiently and securely is why vsftpd is the only stand-alone FTP distributed with Red Hat Enterprise Linux.
The security model used by vsftpd has three primary aspects:
•
Strong separation of privileged and non-privileged processes — Separate processes handle
different tasks, and each of these processes run with the minimal privileges required for the
task.
•
Tasks requiring elevated privileges are handled by processes with the minimal privilege necessary — By leveraging compatibilities found in the libcap library, tasks that usually require
full root privileges can be executed more safely from a less privileged process.
•
Most processes run in a chroot jail — Whenever possible, processes are change-rooted to
the directory being shared; this directory is then considered a chroot jail. For example, if the
directory /var/ftp/ is the primary shared directory, vsftpd reassigns /var/ftp/ to the new
root directory, known as /. This disallows any potential malicious hacker activities for any directories not contained below the new root directory.
Use of these security practices has the following effect on how vsftpd deals with requests:
•
The parent process runs with the least privileges required — The parent process dynamically calculates the level of privileges it requires to minimize the level of risk. Child processes
handle direct interaction with the FTP clients and run with as close to no privileges as possible.
•
All operations requiring elevated privileges are handled by a small parent process — Much
like the Apache HTTP Server, vsftpd launches unprivileged child processes to handle incoming connections. This allows the privileged, parent process to be as small as possible
and handle relatively few tasks.
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3. Files Installed with vsftpd
•
All requests from unprivileged child processes are distrusted by the parent process — Communication with child processes are received over a socket, and the validity of any information from child processes is checked before being acted on.
•
Most interaction with FTP clients is handled by unprivileged child processes in a chroot jail
— Because these child processes are unprivileged and only have access to the directory being shared, any crashed processes only allows the attacker access to the shared files.
3. Files Installed with
vsftpd
The vsftpd RPM installs the daemon (/usr/sbin/vsftpd), its configuration and related files, as
well as FTP directories onto the system. The following lists the files and directories related to
vsftpd configuration:
•
/etc/rc.d/init.d/vsftpd
— The initialization script (initscript) used by the /sbin/service
command to start, stop, or reload vsftpd. Refer to Section 4, “Starting and Stopping vsftpd”
for more information about using this script.
•
/etc/pam.d/vsftpd
— The Pluggable Authentication Modules (PAM) configuration file for
This file specifies the requirements a user must meet to login to the FTP server. For
more information, refer to Section 4, “Pluggable Authentication Modules (PAM)”.
vsftpd.
•
/etc/vsftpd/vsftpd.conf
— The configuration file for vsftpd. Refer to Section 5, “vsftpd
Configuration Options” for a list of important options contained within this file.
•
/etc/vsftpd.ftpusers
•
/etc/vsftpd.user_list
•
/var/ftp/
— A list of users not allowed to log into vsftpd. By default, this list includes the root, bin, and daemon users, among others.
— This file can be configured to either deny or allow access to the
users listed, depending on whether the userlist_deny directive is set to YES (default) or NO in
/etc/vsftpd/vsftpd.conf. If /etc/vsftpd.user_list is used to grant access to users, the
usernames listed must not appear in /etc/vsftpd.ftpusers.
— The directory containing files served by vsftpd. It also contains the /
directory for anonymous users. Both directories are world-readable, but writable only by the root user.
var/ftp/pub/
4. Starting and Stopping
vsftpd
The vsftpd RPM installs the /etc/rc.d/init.d/vsftpd script, which can be accessed using the /
sbin/service command.
To start the server, as root type:
/sbin/service vsftpd start
To stop the server, as root type:
/sbin/service vsftpd stop
The restart option is a shorthand way of stopping and then starting vsftpd. This is the most ef338
4.1. Starting Multiple Copies of vsftpd
ficient way to make configuration changes take effect after editing the configuration file for vsftpd.
To restart the server, as root type:
/sbin/service vsftpd restart
The condrestart (conditional restart) option only starts vsftpd if it is currently running. This option is useful for scripts, because it does not start the daemon if it is not running.
To conditionally restart the server, as root type:
/sbin/service vsftpd condrestart
By default, the vsftpd service does not start automatically at boot time. To configure the vsftpd
service to start at boot time, use an initscript utility, such as /sbin/chkconfig, /usr/sbin/ntsysv,
or the Services Configuration Tool program. Refer to Chapter 15, Controlling Access to Services for more information regarding these tools.
4.1. Starting Multiple Copies of vsftpd
Sometimes one computer is used to serve multiple FTP domains. This is a technique called
multihoming. One way to multihome using vsftpd is by running multiple copies of the daemon,
each with its own configuration file.
To do this, first assign all relevant IP addresses to network devices or alias network devices on
the system. Refer to Chapter 14, Network Configuration for more information about configuring
network devices and device aliases. Additional information can be found about network configuration scripts in Chapter 13, Network Interfaces.
Next, the DNS server for the FTP domains must be configured to reference the correct machine.
For information about BIND and its configuration files, refer to Chapter 16, Berkeley Internet
Name Domain (BIND).
For vsftpd to answer requests on different IP addresses, multiple copies of the daemon must be
running. The first copy must be run using the vsftpd initscripts, as outlined in Section 4,
“Starting and Stopping vsftpd”. This copy uses the standard configuration file, /
etc/vsftpd/vsftpd.conf.
Each additional FTP site must have a configuration file with a unique name in the /etc/vsftpd/
directory, such as /etc/vsftpd/vsftpd-site-2.conf. Each configuration file must be readable
and writable only by root. Within each configuration file for each FTP server listening on an IPv4
network, the following directive must be unique:
listen_address=N.N.N.N
Replace N.N.N.N with the unique IP address for the FTP site being served. If the site is using
IPv6, use the listen_address6 directive instead.
Once each additional server has a configuration file, the vsftpd daemon must be launched from
a root shell prompt using the following command:
vsftpd /etc/vsftpd/<configuration-file> [amp
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]
5. vsftpd Configuration Options
In the above command, replace <configuration-file> with the unique name for the server's
configuration file, such as /etc/vsftpd/vsftpd-site-2.conf.
Other directives to consider altering on a per-server basis are:
•
anon_root
•
local_root
•
vsftpd_log_file
•
xferlog_file
For a detailed list of directives available within vsftpd's configuration file, refer to Section 5,
“vsftpd Configuration Options”.
To configure any additional servers to start automatically at boot time, add the above command
to the end of the /etc/rc.local file.
5.
vsftpd
Configuration Options
Although vsftpd may not offer the level of customization other widely available FTP servers
have, it offers enough options to fill most administrator's needs. The fact that it is not overly feature-laden limits configuration and programmatic errors.
All configuration of vsftpd is handled by its configuration file, /etc/vsftpd/vsftpd.conf. Each
directive is on its own line within the file and follows the following format:
<directive>=<value>
For each directive, replace <directive> with a valid directive and <value> with a valid value.
Important
There must not be any spaces between the <directive>, equal symbol, and the
<value> in a directive.
Comment lines must be preceded by a hash mark (#) and are ignored by the daemon.
For a complete list of all directives available, refer to the man page for vsftpd.conf.
Important
For an overview of ways to secure vsftpd, refer to Section 2, “Server Security”.
The following is a list of some of the more important directives within /etc/vsftpd/vsftpd.conf.
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5.1. Daemon Options
All directives not explicitly found within vsftpd's configuration file are set to their default value.
5.1. Daemon Options
The following is a list of directives which control the overall behavior of the vsftpd daemon.
•
— When enabled, vsftpd runs in stand-alone mode. Red Hat Enterprise Linux sets
this value to YES. This directive cannot be used in conjunction with the listen_ipv6 directive.
listen
The default value is NO.
•
— When enabled, vsftpd runs in stand-alone mode, but listens only to IPv6
sockets. This directive cannot be used in conjunction with the listen directive.
listen_ipv6
The default value is NO.
•
— When enabled, vsftpd attempts to maintain login sessions for each user
through Pluggable Authentication Modules (PAM). Refer to Section 4, “Pluggable Authentication Modules (PAM)” for more information. If session logging is not necessary, disabling this
option allows vsftpd to run with less processes and lower privileges.
session_support
The default value is YES.
5.2. Log In Options and Access Controls
The following is a list of directives which control the login behavior and access control mechanisms.
•
— When enabled, anonymous users are allowed to log in. The usernames
anonymous and ftp are accepted.
anonymous_enable
The default value is YES.
Refer to Section 5.3, “Anonymous User Options” for a list of directives affecting anonymous
users.
•
— If the deny_email_enable directive is set to YES, this directive specifies
the file containing a list of anonymous email passwords which are not permitted access to
the server.
banned_email_file
The default value is /etc/vsftpd.banned_emails.
•
— Specifies the file containing text displayed when a connection is established
to the server. This option overrides any text specified in the ftpd_banner directive.
banner_file
There is no default value for this directive.
•
— Specifies a comma-delimited list of FTP commands allowed by the server.
All other commands are rejected.
cmds_allowed
There is no default value for this directive.
•
deny_email_enable
— When enabled, any anonymous user utilizing email passwords spe-
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5.3. Anonymous User Options
cified in the /etc/vsftpd.banned_emails are denied access to the server. The name of the file
referenced by this directive can be specified using the banned_email_file directive.
The default value is NO.
•
— When enabled, the string specified within this directive is displayed when a
connection is established to the server. This option can be overridden by the banner_file directive.
ftpd_banner
By default vsftpd displays its standard banner.
•
local_enable
— When enabled, local users are allowed to log into the system.
The default value is YES.
Refer to Section 5.4, “Local User Options” for a list of directives affecting local users.
•
pam_service_name
— Specifies the PAM service name for vsftpd.
The default value is ftp. Note, in Red Hat Enterprise Linux, the value is set to vsftpd.
•
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
•
userlist_deny
— When used in conjunction with the userlist_enable directive and set to NO,
all local users are denied access unless the username is listed in the file specified by the
userlist_file directive. Because access is denied before the client is asked for a password,
setting this directive to NO prevents local users from submitting unencrypted passwords over
the network.
The default value is YES.
•
— When enabled, the users listed in the file specified by the userlist_file
directive are denied access. Because access is denied before the client is asked for a password, users are prevented from submitting unencrypted passwords over the network.
userlist_enable
The default value is NO, however under Red Hat Enterprise Linux the value is set to YES.
•
userlist_file
— Specifies the file referenced by vsftpd when the userlist_enable directive
is enabled.
The default value is /etc/vsftpd.user_list and is created during installation.
•
— Specifies a comma separated list of FTP commands that the server allows.
Any other commands are rejected.
cmds_allowed
There is no default value for this directive.
5.3. Anonymous User Options
The following lists directives which control anonymous user access to the server. To use these
options, the anonymous_enable directive must be set to YES.
•
— When enabled in conjunction with the write_enable directive,
anonymous users are allowed to create new directories within a parent directory which has
anon_mkdir_write_enable
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5.4. Local User Options
write permissions.
The default value is NO.
•
anon_root
— Specifies the directory vsftpd changes to after an anonymous user logs in.
There is no default value for this directive.
•
— When enabled in conjunction with the write_enable directive, anonymous users are allowed to upload files within a parent directory which has write permissions.
anon_upload_enable
The default value is NO.
•
anon_world_readable_only
— When enabled, anonymous users are only allowed to down-
load world-readable files.
The default value is YES.
•
— Specifies the local user account (listed in /etc/passwd) used for the anonymous FTP user. The home directory specified in /etc/passwd for the user is the root directory
of the anonymous FTP user.
ftp_username
The default value is ftp.
•
no_anon_password
— When enabled, the anonymous user is not asked for a password.
The default value is NO.
•
— When enabled, only a specified list of email passwords for anonymous logins are accepted. This is a convenient way to offer limited security to public content without the need for virtual users.
secure_email_list_enable
Anonymous logins are prevented unless the password provided is listed in /
etc/vsftpd.email_passwords. The file format is one password per line, with no trailing white
spaces.
The default value is NO.
5.4. Local User Options
The following lists directives which characterize the way local users access the server. To use
these options, the local_enable directive must be set to YES.
•
— When enabled, the FTP command SITE CHMOD is allowed for local users.
This command allows the users to change the permissions on files.
chmod_enable
The default value is YES.
•
— When enabled, the local users listed in the file specified in the chroot_list_file directive are placed in a chroot jail upon log in.
chroot_list_enable
If enabled in conjunction with the chroot_local_user directive, the local users listed in the file
specified in the chroot_list_file directive are not placed in a chroot jail upon log in.
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5.5. Directory Options
The default value is NO.
•
chroot_list_file
root_list_enable
— Specifies the file containing a list of local users referenced when the chdirective is set to YES.
The default value is /etc/vsftpd.chroot_list.
•
chroot_local_user
— When enabled, local users are change-rooted to their home directories
after logging in.
The default value is NO.
Warning
Enabling chroot_local_user opens up a number of security issues, especially
for users with upload privileges. For this reason, it is not recommended.
•
— When enabled, all non-anonymous users are logged in as the user guest,
which is the local user specified in the guest_username directive.
guest_enable
The default value is NO.
•
guest_username
— Specifies the username the guest user is mapped to.
The default value is ftp.
•
local_root
— Specifies the directory vsftpd changes to after a local user logs in.
There is no default value for this directive.
•
— Specifies the umask value for file creation. Note that the default value is in
octal form (a numerical system with a base of eight), which includes a "0" prefix. Otherwise
the value is treated as a base-10 integer.
local_umask
The default value is 022.
•
— When enabled in conjunction with the chroot_local_user directive,
vsftpd change-roots local users based on the occurrence of the /./ in the home directory
field within /etc/passwd.
passwd_chroot_enable
The default value is NO.
•
— Specifies the path to a directory containing configuration files bearing the
name of local system users that contain specific setting for that user. Any directive in the
user's configuration file overrides those found in /etc/vsftpd/vsftpd.conf.
user_config_dir
There is no default value for this directive.
5.5. Directory Options
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5.6. File Transfer Options
The following lists directives which affect directories.
•
dirlist_enable
— When enabled, users are allowed to view directory lists.
The default value is YES.
•
— When enabled, a message is displayed whenever a user enters a directory with a message file. This message resides within the current directory. The name of
this file is specified in the message_file directive and is .message by default.
dirmessage_enable
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
•
— When enabled, files beginning with a dot (.) are listed in directory listings, with the exception of the . and .. files.
force_dot_files
The default value is NO.
•
hide_ids
— When enabled, all directory listings show ftp as the user and group for each file.
The default value is NO.
•
message_file
— Specifies the name of the message file when using the dirmessage_enable
directive.
The default value is .message.
•
— When enabled, test usernames and group names are used in place of
UID and GID entries. Enabling this option may slow performance of the server.
text_userdb_names
The default value is NO.
•
use_localtime
— When enabled, directory listings reveal the local time for the computer in-
stead of GMT.
The default value is NO.
5.6. File Transfer Options
The following lists directives which affect directories.
•
download_enable
— When enabled, file downloads are permitted.
The default value is YES.
•
— When enabled, all files uploaded by anonymous users are owned by the
user specified in the chown_username directive.
chown_uploads
The default value is NO.
•
— Specifies the ownership of anonymously uploaded files if the
chown_uploads directive is enabled.
chown_username
The default value is root.
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5.7. Logging Options
•
— When enabled, FTP commands which can change the file system are allowed, such as DELE, RNFR, and STOR.
write_enable
The default value is YES.
5.7. Logging Options
The following lists directives which affect vsftpd's logging behavior.
•
— When enabled in conjunction with xferlog_enable, vsftpd writes two files
simultaneously: a wu-ftpd-compatible log to the file specified in the xferlog_file directive (/
var/log/xferlog by default) and a standard vsftpd log file specified in the vsftpd_log_file
directive (/var/log/vsftpd.log by default).
dual_log_enable
The default value is NO.
•
— When enabled in conjunction with xferlog_enable and with xferset to NO, all FTP commands and responses are logged. This directive is
useful for debugging.
log_ftp_protocol
log_std_format
The default value is NO.
•
— When enabled in conjunction with xferlog_enable, all logging normally written to the standard vsftpd log file specified in the vsftpd_log_file directive (/
var/log/vsftpd.log by default) is sent to the system logger instead under the FTPD facility.
syslog_enable
The default value is NO.
•
— Specifies the vsftpd log file. For this file to be used, xferlog_enable must
be enabled and xferlog_std_format must either be set to NO or, if xferlog_std_format is set
to YES, dual_log_enable must be enabled. It is important to note that if syslog_enable is set to
YES, the system log is used instead of the file specified in this directive.
vsftpd_log_file
The default value is /var/log/vsftpd.log.
•
— When enabled, vsftpd logs connections (vsftpd format only) and file
transfer information to the log file specified in the vsftpd_log_file directive (/
var/log/vsftpd.log by default). If xferlog_std_format is set to YES, file transfer information is
logged but connections are not, and the log file specified in xferlog_file (/var/log/xferlog
by default) is used instead. It is important to note that both log files and log formats are used
if dual_log_enable is set to YES.
xferlog_enable
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
•
— Specifies the wu-ftpd-compatible log file. For this file to be used, xferlog_enable must be enabled and xferlog_std_format must be set to YES. It is also used if
dual_log_enable is set to YES.
xferlog_file
The default value is /var/log/xferlog.
•
— When enabled in conjunction with xferlog_enable, only a wu-ftpdcompatible file transfer log is written to the file specified in the xferlog_file directive (/
var/log/xferlog by default). It is important to note that this file only logs file transfers and
xferlog_std_format
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5.8. Network Options
does not log connections to the server.
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
Important
To maintain compatibility with log files written by the older wu-ftpd FTP server, the
xferlog_std_format directive is set to YES under Red Hat Enterprise Linux.
However, this setting means that connections to the server are not logged.
To both log connections in vsftpd format and maintain a wu-ftpd-compatible file
transfer log, set dual_log_enable to YES.
If maintaining a wu-ftpd-compatible file transfer log is not important, either set
xferlog_std_format to NO, comment the line with a hash mark (#), or delete the line
entirely.
5.8. Network Options
The following lists directives which affect how vsftpd interacts with the network.
•
accept_timeout
— Specifies the amount of time for a client using passive mode to establish
a connection.
The default value is 60.
•
anon_max_rate
— Specifies the maximum data transfer rate for anonymous users in bytes
per second.
The default value is 0, which does not limit the transfer rate.
•
When enabled, vsftpd runs with enough privileges to open port 20 on
the server during active mode data transfers. Disabling this option allows vsftpd to run with
less privileges, but may be incompatible with some FTP clients.
connect_from_port_20
The default value is NO. Note, in Red Hat Enterprise Linux, the value is set to YES.
•
— Specifies the maximum amount of time a client using active mode has to
respond to a data connection, in seconds.
connect_timeout
The default value is 60.
•
— Specifies maximum amount of time data transfers are allowed
to stall, in seconds. Once triggered, the connection to the remote client is closed.
data_connection_timeout
The default value is 300.
•
ftp_data_port
— Specifies the port used for active data connections when conis set to YES.
nect_from_port_20
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5.8. Network Options
The default value is 20.
•
— Specifies the maximum amount of time between commands from a
remote client. Once triggered, the connection to the remote client is closed.
idle_session_timeout
The default value is 300.
•
listen_address
— Specifies the IP address on which vsftpd listens for network connections.
There is no default value for this directive.
Tip
If running multiple copies of vsftpd serving different IP addresses, the configuration file for each copy of the vsftpd daemon must have a different value for
this directive. Refer to Section 4.1, “Starting Multiple Copies of vsftpd” for more
information about multihomed FTP servers.
•
— Specifies the IPv6 address on which vsftpd listens for network connections when listen_ipv6 is set to YES.
listen_address6
There is no default value for this directive.
Tip
If running multiple copies of vsftpd serving different IP addresses, the configuration file for each copy of the vsftpd daemon must have a different value for
this directive. Refer to Section 4.1, “Starting Multiple Copies of vsftpd” for more
information about multihomed FTP servers.
•
listen_port
— Specifies the port on which vsftpd listens for network connections.
The default value is 21.
•
— Specifies the maximum rate data is transferred for local users logged into
the server in bytes per second.
local_max_rate
The default value is 0, which does not limit the transfer rate.
•
— Specifies the maximum number of simultaneous clients allowed to connect to
the server when it is running in standalone mode. Any additional client connections would
result in an error message.
max_clients
The default value is 0, which does not limit connections.
•
max_per_ip
— Specifies the maximum of clients allowed to connected from the same source
IP address.
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6. Additional Resources
The default value is 0, which does not limit connections.
•
— Specifies the IP address for the public facing IP address of the server for
servers behind Network Address Translation (NAT) firewalls. This enables vsftpd to hand
out the correct return address for passive mode connections.
pasv_address
There is no default value for this directive.
•
pasv_enable
— When enabled, passive mode connects are allowed.
The default value is YES.
•
— Specifies the highest possible port sent to the FTP clients for passive
mode connections. This setting is used to limit the port range so that firewall rules are easier
to create.
pasv_max_port
The default value is 0, which does not limit the highest passive port range. The value must
not exceed 65535.
•
— Specifies the lowest possible port sent to the FTP clients for passive mode
connections. This setting is used to limit the port range so that firewall rules are easier to
create.
pasv_min_port
The default value is 0, which does not limit the lowest passive port range. The value must
not be lower 1024.
•
— When enabled, data connections are not checked to make sure they
are originating from the same IP address. This setting is only useful for certain types of tunneling.
pasv_promiscuous
Caution
Do not enable this option unless absolutely necessary as it disables an important security feature which verifies that passive mode connections originate from
the same IP address as the control connection that initiates the data transfer.
The default value is NO.
•
port_enable
— When enabled, active mode connects are allowed.
The default value is YES.
6. Additional Resources
For more information about vsftpd, refer to the following resources.
6.1. Installed Documentation
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6.2. Useful Websites
•
The /usr/share/doc/vsftpd-<version-number>/ directory — Replace <version-number> with
the installed version of the vsftpd package. This directory contains a README with basic information about the software. The TUNING file contains basic performance tuning tips and the
SECURITY/ directory contains information about the security model employed by vsftpd.
•
vsftpd
related man pages — There are a number of man pages for the daemon and configuration files. The following lists some of the more important man pages.
Server Applications
•
man vsftpd
— Describes available command line options for vsftpd.
Configuration Files
•
man vsftpd.conf
— Contains a detailed list of options available within the configuration
file for vsftpd.
•
— Describes the format and options available within the TCP wrappers configuration files: hosts.allow and hosts.deny.
man 5 hosts_access
6.2. Useful Websites
•
http://vsftpd.beasts.org/ — The vsftpd project page is a great place to locate the latest documentation and to contact the author of the software.
•
http://slacksite.com/other/ftp.html — This website provides a concise explanation of the differences between active and passive mode FTP.
•
http://www.ietf.org/rfc/rfc0959.txt — The original Request for Comments (RFC) of the FTP
protocol from the IETF.
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Chapter 23. Email
The birth of electronic mail (email) occurred in the early 1960s. The mailbox was a file in a
user's home directory that was readable only by that user. Primitive mail applications appended
new text messages to the bottom of the file, making the user wade through the constantly growing file to find any particular message. This system was only capable of sending messages to
users on the same system.
The first network transfer of an electronic mail message file took place in 1971 when a computer
engineer named Ray Tomlinson sent a test message between two machines via ARPANET —
the precursor to the Internet. Communication via email soon became very popular, comprising
75 percent of ARPANET's traffic in less than two years.
Today, email systems based on standardized network protocols have evolved into some of the
most widely used services on the Internet. Red Hat Enterprise Linux offers many advanced applications to serve and access email.
This chapter reviews modern email protocols in use today and some of the programs designed
to send and receive email.
1. Email Protocols
Today, email is delivered using a client/server architecture. An email message is created using
a mail client program. This program then sends the message to a server. The server then forwards the message to the recipient's email server, where the message is then supplied to the
recipient's email client.
To enable this process, a variety of standard network protocols allow different machines, often
running different operating systems and using different email programs, to send and receive
email.
The following protocols discussed are the most commonly used in the transfer of email.
1.1. Mail Transport Protocols
Mail delivery from a client application to the server, and from an originating server to the destination server, is handled by the Simple Mail Transfer Protocol (SMTP).
1.1.1. SMTP
The primary purpose of SMTP is to transfer email between mail servers. However, it is critical
for email clients as well. To send email, the client sends the message to an outgoing mail server, which in turn contacts the destination mail server for delivery. For this reason, it is necessary
to specify an SMTP server when configuring an email client.
Under Red Hat Enterprise Linux, a user can configure an SMTP server on the local machine to
handle mail delivery. However, it is also possible to configure remote SMTP servers for outgoing
mail.
One important point to make about the SMTP protocol is that it does not require authentication.
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1.2. Mail Access Protocols
This allows anyone on the Internet to send email to anyone else or even to large groups of
people. It is this characteristic of SMTP that makes junk email or spam possible. Imposing relay
restrictions limits random users on the Internet from sending email through your SMTP server,
to other servers on the internet. Servers that do not impose such restrictions are called open relay servers.
By default, Sendmail (/usr/sbin/sendmail) is the default SMTP program under Red Hat Enterprise Linux. However, a simpler mail server application called Postfix (/usr/sbin/postfix) is
also available.
1.2. Mail Access Protocols
There are two primary protocols used by email client applications to retrieve email from mail
servers: the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP).
1.2.1. POP
The default POP server under Red Hat Enterprise Linux is /usr/lib/cyrus-imapd/pop3d and is
provided by the cyrus-imapd package. When using a POP server, email messages are downloaded by email client applications. By default, most POP email clients are automatically configured to delete the message on the email server after it has been successfully transferred,
however this setting usually can be changed.
POP is fully compatible with important Internet messaging standards, such as Multipurpose Internet Mail Extensions (MIME), which allow for email attachments.
POP works best for users who have one system on which to read email. It also works well for
users who do not have a persistent connection to the Internet or the network containing the mail
server. Unfortunately for those with slow network connections, POP requires client programs
upon authentication to download the entire content of each message. This can take a long time
if any messages have large attachments.
The most current version of the standard POP protocol is POP3.
There are, however, a variety of lesser-used POP protocol variants:
•
APOP — POP3 with MDS authentication. An encoded hash of the user's password is sent
from the email client to the server rather then sending an unencrypted password.
•
KPOP — POP3 with Kerberos authentication. Refer to Section 6, “Kerberos” for more information.
•
RPOP — POP3 with RPOP authentication. This uses a per-user ID, similar to a password, to
authenticate POP requests. However, this ID is not encrypted, so RPOP is no more secure
than standard POP.
For added security, it is possible to use Secure Socket Layer (SSL) encryption for client authentication and data transfer sessions. This can be enabled by using the ipop3s service or by using
the /usr/sbin/stunnel program. Refer to Section 6.1, “Securing Communication” for more information.
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1.2. Mail Access Protocols
1.2.2. IMAP
The default IMAP server under Red Hat Enterprise Linux is /usr/lib/cyrus-imapd/imapd and is
provided by the cyrus-imapd package. When using an IMAP mail server, email messages remain on the server where users can read or delete them. IMAP also allows client applications to
create, rename, or delete mail directories on the server to organize and store email.
IMAP is particularly useful for those who access their email using multiple machines. The protocol is also convenient for users connecting to the mail server via a slow connection, because
only the email header information is downloaded for messages until opened, saving bandwidth.
The user also has the ability to delete messages without viewing or downloading them.
For convenience, IMAP client applications are capable of caching copies of messages locally,
so the user can browse previously read messages when not directly connected to the IMAP
server.
IMAP, like POP, is fully compatible with important Internet messaging standards, such as MIME,
which allow for email attachments.
For added security, it is possible to use SSL encryption for client authentication and data transfer sessions. This can be enabled by using the imaps service, or by using the /usr/sbin/stunnel
program. Refer to Section 6.1, “Securing Communication” for more information.
Other free, as well as commercial, IMAP clients and servers are available, many of which extend the IMAP protocol and provide additional functionality. A comprehensive list can be found
online at http://www.imap.org/products/longlist.htm.
1.2.3. Dovecot
The imap-login and pop3-login daemons which implement the IMAP and POP3 protocols are
included in the dovecot package. The use of IMAP and POP is configured through dovecot; by
default dovecot runs only IMAP. To configure dovecot to use POP:
1.
Edit /etc/dovecot.conf to have the line:
protocols = imap imaps pop3 pop3s
2.
Make that change operational for the current session by running the command:
/sbin/service dovecot restart
3.
Make that change operational after the next reboot by running the command:
chkconfig dovecot on
Please note that dovecot only reports that it started the IMAP server, but also starts the
POP3 server.
Unlike SMTP, both of these protocols require connecting clients to authenticate using a username and password. By default, passwords for both protocols are passed over the network unencrypted.
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2. Email Program Classifications
To configure SSL on dovecot:
•
Edit the dovecot configuration file /etc/pki/dovecot/dovecot-openssl.conf as you prefer.
However in a typical installation, this file does not require modification.
•
Rename, move or delete the files /etc/pki/dovecot/certs/dovecot.pem and /
etc/pki/dovecot/private/dovecot.pem.
•
Execute the /usr/share/doc/dovecot-1.0/examples/mkcert.sh script which creates the dovecot self signed certificates. The certificates are copied in the /etc/pki/dovecot/certs and /
etc/pki/dovecot/private directories. To implement the changes, restart dovecot (/
sbin/service dovecot restart).
More details on dovecot can be found online at http://www.dovecot.org.
2. Email Program Classifications
In general, all email applications fall into at least one of three classifications. Each classification
plays a specific role in the process of moving and managing email messages. While most users
are only aware of the specific email program they use to receive and send messages, each one
is important for ensuring that email arrives at the correct destination.
2.1. Mail Transport Agent
A Mail Transport Agent (MTA) transports email messages between hosts using SMTP. A message may involve several MTAs as it moves to its intended destination.
While the delivery of messages between machines may seem rather straightforward, the entire
process of deciding if a particular MTA can or should accept a message for delivery is quite
complicated. In addition, due to problems from spam, use of a particular MTA is usually restricted by the MTA's configuration or the access configuration for the network on which the MTA
resides.
Many modern email client programs can act as an MTA when sending email. However, this action should not be confused with the role of a true MTA. The sole reason email client programs
are capable of sending email like an MTA is because the host running the application does not
have its own MTA. This is particularly true for email client programs on non-UNIX-based operating systems. However, these client programs only send outbound messages to an MTA they are
authorized to use and do not directly deliver the message to the intended recipient's email server.
Since Red Hat Enterprise Linux installs two MTAs, Sendmail and Postfix, email client programs
are often not required to act as an MTA. Red Hat Enterprise Linux also includes a special purpose MTA called Fetchmail.
For more information on Sendmail, Postfix, and Fetchmail, refer to Section 3, “Mail Transport
Agents”.
2.2. Mail Delivery Agent
A Mail Delivery Agent (MDA) is invoked by the MTA to file incoming email in the proper user's
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2.3. Mail User Agent
mailbox. In many cases, the MDA is actually a Local Delivery Agent (LDA), such as mail or
Procmail.
Any program that actually handles a message for delivery to the point where it can be read by
an email client application can be considered an MDA. For this reason, some MTAs (such as
Sendmail and Postfix) can fill the role of an MDA when they append new email messages to a
local user's mail spool file. In general, MDAs do not transport messages between systems nor
do they provide a user interface; MDAs distribute and sort messages on the local machine for
an email client application to access.
2.3. Mail User Agent
A Mail User Agent (MUA) is synonymous with an email client application. An MUA is a program
that, at the very least, allows a user to read and compose email messages. Many MUAs are
capable of retrieving messages via the POP or IMAP protocols, setting up mailboxes to store
messages, and sending outbound messages to an MTA.
MUAs may be graphical, such as Evolution, or have a very simple, text-based interface, such as
mutt.
3. Mail Transport Agents
Red Hat Enterprise Linux includes two primary MTAs, Sendmail and Postfix. Sendmail is configured as the default MTA, although it is easy to switch the default MTA to Postfix.
3.1. Sendmail
Sendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually using
the SMTP protocol. However, Sendmail is highly configurable, allowing control over almost
every aspect of how email is handled, including the protocol used. Many system administrators
elect to use Sendmail as their MTA due to its power and scalability.
3.1.1. Purpose and Limitations
It is important to be aware of what Sendmail is and what it can do, as opposed to what it is not.
In these days of monolithic applications that fulfill multiple roles, Sendmail may seem like the
only application needed to run an email server within an organization. Technically, this is true,
as Sendmail can spool mail to each users' directory and deliver outbound mail for users.
However, most users actually require much more than simple email delivery. Users usually want
to interact with their email using an MUA, that uses POP or IMAP, to download their messages
to their local machine. Or, they may prefer a Web interface to gain access to their mailbox.
These other applications can work in conjunction with Sendmail, but they actually exist for different reasons and can operate separately from one another.
It is beyond the scope of this section to go into all that Sendmail should or could be configured
to do. With literally hundreds of different options and rule sets, entire volumes have been dedicated to helping explain everything that can be done and how to fix things that go wrong. Refer to
the Section 7, “Additional Resources” for a list of Sendmail resources.
This section reviews the files installed with Sendmail by default and reviews basic configuration
changes, including how to stop unwanted email (spam) and how to extend Sendmail with the
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3.1. Sendmail
Lightweight Directory Access Protocol (LDAP).
3.1.2. The Default Sendmail Installation
The Sendmail executable is /usr/sbin/sendmail.
Sendmail's lengthy and detailed configuration file is /etc/mail/sendmail.cf. Avoid editing the
sendmail.cf file directly. To make configuration changes to Sendmail, edit the /
etc/mail/sendmail.mc file, back up the original /etc/mail/sendmail.cf, and use the following alternatives to generate a new configuration file:
•
Use the included makefile in /etc/mail (make all -C /etc/mail) to create a new /
etc/mail/sendmail.cf configuration file. All other generated files in /etc/mail (db files) will
be regenerated if needed. The old makemap commands are still usable. The make command will automatically be used by service sendmail start | restart | reload if the make
package is installed.
•
Alternatively you may use the included m4 macro processor to create a new /
etc/mail/sendmail.cf.
More information on configuring Sendmail can be found in Section 3.1.3, “Common Sendmail
Configuration Changes”.
Various Sendmail configuration files are installed in the /etc/mail/ directory including:
•
access
— Specifies which systems can use Sendmail for outbound email.
•
domaintable
•
local-host-names
•
mailertable
•
virtusertable
— Specifies domain name mapping.
— Specifies aliases for the host.
— Specifies instructions that override routing for particular domains.
— Specifies a domain-specific form of aliasing, allowing multiple virtual domains to be hosted on one machine.
Several of the configuration files in /etc/mail/, such as access, domaintable, mailertable and
virtusertable, must actually store their information in database files before Sendmail can use
any configuration changes. To include any changes made to these configurations in their database files, run the command
makemap hash /etc/mail/<name> < /etc/mail/<name>
where <name> is replaced with the name of the configuration file to convert.
For example, to have all emails addressed to the example.com domain delivered to
<[email protected]> , add the following line to the virtusertable file:
@example.com [email protected]
To finalize the change, the virtusertable.db file must be updated using the following command
as root:
makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable
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3.1. Sendmail
This creates an updated virtusertable.db file containing the new configuration.
3.1.3. Common Sendmail Configuration Changes
When altering the Sendmail configuration file, it is best not to edit an existing file, but to generate an entirely new /etc/mail/sendmail.cf file.
Caution
Before changing the sendmail.cf file, it is a good idea to create a backup copy.
To add the desired functionality to Sendmail, edit the /etc/mail/sendmail.mc file as the root
user. When finished, use the m4 macro processor to generate a new sendmail.cf by executing
the following command:
m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
By default, the m4 macro processor is installed with Sendmail but is part of the m4 package.
After creating a new /etc/mail/sendmail.cf file, restart Sendmail for the changes to take effect.
The easiest way to do this is to type the following command:
/sbin/service sendmail restart
Important
The default sendmail.cf file does not allow Sendmail to accept network connections from any host other than the local computer. To configure Sendmail as a
server for other clients, edit the /etc/mail/sendmail.mc file, and either change the
address specified in the Addr= option of the DAEMON_OPTIONS directive from
127.0.0.1 to the IP address of an active network device or comment out the DAEMON_OPTIONS directive all together by placing dnl at the beginning of the line. When
finished, regenerate /etc/mail/sendmail.cf by executing the following command:
m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
The default configuration which ships with Red Hat Enterprise Linux works for most SMTP-only
sites. However, it does not work for UUCP (UNIX to UNIX Copy) sites. If using UUCP mail
transfers, the /etc/mail/sendmail.mc file must be reconfigured and a new /
etc/mail/sendmail.cf must be generated.
Consult the /usr/share/sendmail-cf/README file before editing any files in the directories under
the /usr/share/sendmail-cf directory, as they can affect the future configuration of /
etc/mail/sendmail.cf files.
3.1.4. Masquerading
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3.1. Sendmail
One common Sendmail configuration is to have a single machine act as a mail gateway for all
machines on the network. For instance, a company may want to have a machine called
mail.example.com that handles all of their email and assigns a consistent return address to all
outgoing mail.
In this situation, the Sendmail server must masquerade the machine names on the company
network so that their return address is [email protected] instead of [email protected]
To do this, add the following lines to /etc/mail/sendmail.mc:
FEATURE(always_add_domain)dnl FEATURE(`masquerade_entire_domain') dnl FEATURE(`masquerade_envelope') dnl FE
After generating a new sendmail.cf using m4, this configuration makes all mail from inside the
network appear as if it were sent from bigcorp.com.
3.1.5. Stopping Spam
Email spam can be defined as unnecessary and unwanted email received by a user who never
requested the communication. It is a disruptive, costly, and widespread abuse of Internet communication standards.
Sendmail makes it relatively easy to block new spamming techniques being employed to send
junk email. It even blocks many of the more usual spamming methods by default. Main antispam features available in sendmail are header checks, relaying denial (default from version
8.9), access database and sender information checks.
For example, forwarding of SMTP messages, also called relaying, has been disabled by default
since Sendmail version 8.9. Before this change occurred, Sendmail directed the mail host
(x.edu) to accept messages from one party (y.com) and sent them to a different party (z.net).
Now, however, Sendmail must be configured to permit any domain to relay mail through the
server. To configure relay domains, edit the /etc/mail/relay-domains file and restart Sendmail.
However, many times users are bombarded with spam from other servers throughout the Internet. In these instances, Sendmail's access control features available through the /
etc/mail/access file can be used to prevent connections from unwanted hosts. The following example illustrates how this file can be used to both block and specifically allow access to the
Sendmail server:
badspammer.com ERROR:550 "Go away and do not spam us anymore" tux.badspammer.com OK 10.0 RELAY
This example shows that any email sent from badspammer.com is blocked with a 550 RFC-821
compliant error code, with a message sent back to the spammer. Email sent from the
tux.badspammer.com sub-domain, is accepted. The last line shows that any email sent from the
10.0.*.* network can be relayed through the mail server.
Because /etc/mail/access.db is a database, use makemap to activate any changes. Do this using
the following command as root:
makemap hash /etc/mail/access < /etc/mail/access
Message header analysis allows you to reject mail based on header contents. SMTP servers
store information about an emails journey in the message header. As the message travels from
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3.2. Postfix
one MTA to another, each puts in a "Received" header above all the other Received headers. It
is however important to note that this information may be altered by spammers.
The above examples only represent a small part of what Sendmail can do in terms of allowing
or blocking access. Refer to the /usr/share/sendmail-cf/README for more information and examples.
Since Sendmail calls the Procmail MDA when delivering mail, it is also possible to use a spam
filtering program, such as SpamAssassin, to identify and file spam for users. Refer to Section 5.2.6, “Spam Filters” for more about using SpamAssassin.
3.1.6. Using Sendmail with LDAP
Using the Lightweight Directory Access Protocol (LDAP) is a very quick and powerful way to find
specific information about a particular user from a much larger group. For example, an LDAP
server can be used to look up a particular email address from a common corporate directory by
the user's last name. In this kind of implementation, LDAP is largely separate from Sendmail,
with LDAP storing the hierarchical user information and Sendmail only being given the result of
LDAP queries in pre-addressed email messages.
However, Sendmail supports a much greater integration with LDAP, where it uses LDAP to replace separately maintained files, such as aliases and virtusertables, on different mail servers
that work together to support a medium- to enterprise-level organization. In short, LDAP abstracts the mail routing level from Sendmail and its separate configuration files to a powerful
LDAP cluster that can be leveraged by many different applications.
The current version of Sendmail contains support for LDAP. To extend the Sendmail server using LDAP, first get an LDAP server, such as OpenLDAP, running and properly configured. Then
edit the /etc/mail/sendmail.mc to include the following:
LDAPROUTE_DOMAIN('yourdomain.com')dnl FEATURE('ldap_routing')dnl
Note
This is only for a very basic configuration of Sendmail with LDAP. The configuration can differ greatly from this depending on the implementation of LDAP, especially when configuring several Sendmail machines to use a common LDAP server.
Consult /usr/share/sendmail-cf/README for detailed LDAP routing configuration instructions and examples.
Next, recreate the /etc/mail/sendmail.cf file by running m4 and restarting Sendmail. Refer to
Section 3.1.3, “Common Sendmail Configuration Changes” for instructions.
For more information on LDAP, refer to Chapter 24, Lightweight Directory Access Protocol
(LDAP).
3.2. Postfix
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3.2. Postfix
Originally developed at IBM by security expert and programmer Wietse Venema, Postfix is a
Sendmail-compatible MTA that is designed to be secure, fast, and easy to configure.
To improve security, Postfix uses a modular design, where small processes with limited privileges are launched by a master daemon. The smaller, less privileged processes perform very
specific tasks related to the various stages of mail delivery and run in a change rooted environment to limit the effects of attacks.
Configuring Postfix to accept network connections from hosts other than the local computer
takes only a few minor changes in its configuration file. Yet for those with more complex needs,
Postfix provides a variety of configuration options, as well as third party add ons that make it a
very versatile and full-featured MTA.
The configuration files for Postfix are human readable and support upward of 250 directives.
Unlike Sendmail, no macro processing is required for changes to take effect and the majority of
the most commonly used options are described in the heavily commented files.
Important
Before using Postfix, the default MTA must be switched from Sendmail to Postfix.
3.2.1. The Default Postfix Installation
The Postfix executable is /usr/sbin/postfix. This daemon launches all related processes
needed to handle mail delivery.
Postfix stores its configuration files in the /etc/postfix/ directory. The following is a list of the
more commonly used files:
•
access
— Used for access control, this file specifies which hosts are allowed to connect to
Postfix.
•
aliases
•
main.cf
•
master.cf
— A configurable list required by the mail protocol.
— The global Postfix configuration file. The majority of configuration options are
specified in this file.
— Specifies how Postfix interacts with various processes to accomplish mail deliv-
ery.
•
transport
— Maps email addresses to relay hosts.
Important
The default /etc/postfix/main.cf file does not allow Postfix to accept network
connections from a host other than the local computer. For instructions on configuring Postfix as a server for other clients, refer to Section 3.2.2, “Basic Postfix
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3.3. Fetchmail
Configuration”.
When changing some options within files in the /etc/postfix/ directory, it may be necessary to
restart the postfix service for the changes to take effect. The easiest way to do this is to type
the following command:
/sbin/service postfix restart
3.2.2. Basic Postfix Configuration
By default, Postfix does not accept network connections from any host other than the local host.
Perform the following steps as root to enable mail delivery for other hosts on the network:
•
Edit the /etc/postfix/main.cf file with a text editor, such as vi.
•
Uncomment the mydomain line by removing the hash mark (#), and replace domain.tld with
the domain the mail server is servicing, such as example.com.
•
Uncomment the myorigin
•
Uncomment the myhostname line, and replace host.domain.tld with the hostname for the machine.
•
Uncomment the mydestination
•
Uncomment the mynetworks line, and replace 168.100.189.0/28 with a valid network setting
for hosts that can connect to the server.
•
Uncomment the inet_interfaces
•
Restart the postfix service.
= $mydomain
line.
= $myhostname, localhost.$mydomain
= all
line.
line.
Once these steps are complete, the host accepts outside emails for delivery.
Postfix has a large assortment of configuration options. One of the best ways to learn how to
configure Postfix is to read the comments within /etc/postfix/main.cf. Additional resources including information about LDAP and SpamAssassin integration are available online at http://www.postfix.org/.
3.3. Fetchmail
Fetchmail is an MTA which retrieves email from remote servers and delivers it to the local MTA.
Many users appreciate the ability to separate the process of downloading their messages located on a remote server from the process of reading and organizing their email in an MUA. Designed with the needs of dial-up users in mind, Fetchmail connects and quickly downloads all of
the email messages to the mail spool file using any number of protocols, including POP3 and
IMAP. It can even forward email messages to an SMTP server, if necessary.
Fetchmail is configured for each user through the use of a .fetchmailrc file in the user's home
directory.
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3.3. Fetchmail
Using preferences in the .fetchmailrc file, Fetchmail checks for email on a remote server and
downloads it. It then delivers it to port 25 on the local machine, using the local MTA to place the
email in the correct user's spool file. If Procmail is available, it is launched to filter the email and
place it in a mailbox so that it can be read by an MUA.
3.3.1. Fetchmail Configuration Options
Although it is possible to pass all necessary options on the command line to check for email on
a remote server when executing Fetchmail, using a .fetchmailrc file is much easier. Place any
desired configuration options in the .fetchmailrc file for those options to be used each time the
fetchmail command is issued. It is possible to override these at the time Fetchmail is run by
specifying that option on the command line.
A user's .fetchmailrc file contains three classes of configuration options:
•
global options — Gives Fetchmail instructions that control the operation of the program or
provide settings for every connection that checks for email.
•
server options — Specifies necessary information about the server being polled, such as the
hostname, as well as preferences for specific email servers, such as the port to check or
number of seconds to wait before timing out. These options affect every user using that server.
•
user options — Contains information, such as username and password, necessary to authenticate and check for email using a specified email server.
Global options appear at the top of the .fetchmailrc file, followed by one or more server options, each of which designate a different email server that Fetchmail should check. User options follow server options for each user account checking that email server. Like server options,
multiple user options may be specified for use with a particular server as well as to check multiple email accounts on the same server.
Server options are called into service in the .fetchmailrc file by the use of a special option verb,
poll or skip, that precedes any of the server information. The poll action tells Fetchmail to use
this server option when it is run, which checks for email using the specified user options. Any
server options after a skip action, however, are not checked unless this server's hostname is
specified when Fetchmail is invoked. The skip option is useful when testing configurations in
.fetchmailrc because it only checks skipped servers when specifically invoked, and does not
affect any currently working configurations.
A sample .fetchmailrc file looks similar to the following example:
set postmaster "user1" set bouncemail poll pop.domain.com proto pop3 user 'user1' there with password 'secr
In this example, the global options specify that the user is sent email as a last resort (postmaster
option) and all email errors are sent to the postmaster instead of the sender (bouncemail option).
The set action tells Fetchmail that this line contains a global option. Then, two email servers are
specified, one set to check using POP3, the other for trying various protocols to find one that
works. Two users are checked using the second server option, but all email found for any user
is sent to user1's mail spool. This allows multiple mailboxes to be checked on multiple servers,
while appearing in a single MUA inbox. Each user's specific information begins with the user ac-
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3.3. Fetchmail
tion.
Note
Users are not required to place their password in the .fetchmailrc file. Omitting
the with password '<password>' section causes Fetchmail to ask for a password
when it is launched.
Fetchmail has numerous global, server, and local options. Many of these options are rarely
used or only apply to very specific situations. The fetchmail man page explains each option in
detail, but the most common ones are listed here.
3.3.2. Global Options
Each global option should be placed on a single line after a set action.
•
daemon <seconds>
— Specifies daemon-mode, where Fetchmail stays in the background. Replace <seconds> with the number of seconds Fetchmail is to wait before polling the server.
•
postmaster
•
syslog
— Specifies a local user to send mail to in case of delivery problems.
— Specifies the log file for errors and status messages. By default, this is /
var/log/maillog.
3.3.3. Server Options
Server options must be placed on their own line in .fetchmailrc after a poll or skip action.
•
auth <auth-type>
— Replace <auth-type> with the type of authentication to be used. By default, password authentication is used, but some protocols support other types of authentication, including kerberos_v5, kerberos_v4, and ssh. If the any authentication type is used,
Fetchmail first tries methods that do not require a password, then methods that mask the
password, and finally attempts to send the password unencrypted to authenticate to the
server.
•
interval <number>
•
port <port-number>
•
proto <protocol>
•
timeout <seconds>
— Polls the specified server every <number> of times that it checks for
email on all configured servers. This option is generally used for email servers where the
user rarely receives messages.
— Replace <port-number> with the port number. This value overrides the
default port number for the specified protocol.
— Replace <protocol> with the protocol, such as pop3 or imap, to use
when checking for messages on the server.
— Replace <seconds> with the number of seconds of server inactivity
after which Fetchmail gives up on a connection attempt. If this value is not set, a default of
300 seconds is assumed.
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3.3. Fetchmail
3.3.4. User Options
User options may be placed on their own lines beneath a server option or on the same line as
the server option. In either case, the defined options must follow the user option (defined below).
•
fetchall
— Orders Fetchmail to download all messages in the queue, including messages
that have already been viewed. By default, Fetchmail only pulls down new messages.
•
fetchlimit <number>
— Replace <number> with the number of messages to be retrieved be-
fore stopping.
•
flush
— Deletes all previously viewed messages in the queue before retrieving new messages.
•
limit <max-number-bytes>
•
password '<password>'
•
preconnect "<command>"
•
postconnect "<command>"
•
ssl
•
user "<username>"
— Replace <max-number-bytes> with the maximum size in bytes
that messages are allowed to be when retrieved by Fetchmail. This option is useful with slow
network links, when a large message takes too long to download.
— Replace <password> with the user's password.
— Replace <command> with a command to be executed before retrieving messages for the user.
— Replace <command> with a command to be executed after retrieving messages for the user.
— Activates SSL encryption.
— Replace <username> with the username used by Fetchmail to retrieve
messages. This option must precede all other user options.
3.3.5. Fetchmail Command Options
Most Fetchmail options used on the command line when executing the fetchmail command mirror the .fetchmailrc configuration options. In this way, Fetchmail may be used with or without a
configuration file. These options are not used on the command line by most users because it is
easier to leave them in the .fetchmailrc file.
There may be times when it is desirable to run the fetchmail command with other options for a
particular purpose. It is possible to issue command options to temporarily override a
.fetchmailrc setting that is causing an error, as any options specified at the command line override configuration file options.
3.3.6. Informational or Debugging Options
Certain options used after the fetchmail command can supply important information.
•
--configdump
— Displays every possible option based on information from .fetchmailrc and
Fetchmail defaults. No email is retrieved for any users when using this option.
•
-s
— Executes Fetchmail in silent mode, preventing any messages, other than errors, from
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4. Mail Transport Agent (MTA) Configuration
appearing after the fetchmail command.
•
-v
— Executes Fetchmail in verbose mode, displaying every communication between Fetchmail and remote email servers.
•
-V
— Displays detailed version information, lists its global options, and shows settings to be
used with each user, including the email protocol and authentication method. No email is retrieved for any users when using this option.
3.3.7. Special Options
These options are occasionally useful for overriding defaults often found in the .fetchmailrc file.
•
-a
— Fetchmail downloads all messages from the remote email server, whether new or previously viewed. By default, Fetchmail only downloads new messages.
•
-k
•
-l <max-number-bytes>
•
--quit
— Fetchmail leaves the messages on the remote email server after downloading them.
This option overrides the default behavior of deleting messages after downloading them.
— Fetchmail does not download any messages over a particular size
and leaves them on the remote email server.
— Quits the Fetchmail daemon process.
More commands and .fetchmailrc options can be found in the fetchmail man page.
4. Mail Transport Agent (MTA) Configuration
A Mail Transport Agent (MTA) is essential for sending email. A Mail User Agent (MUA) such as
Evolution, Thunderbird, and Mutt, is used to read and compose email. When a user sends an
email from an MUA, the message is handed off to the MTA, which sends the message through
a series of MTAs until it reaches its destination.
Even if a user does not plan to send email from the system, some automated tasks or system
programs might use the /bin/mail command to send email containing log messages to the root
user of the local system.
Red Hat Enterprise Linux 5 provides three MTAs: Sendmail, Postfix, and Exim. If all three are
installed, sendmail is the default MTA. The Mail Transport Agent Switcher allows for the selection of either sendmail, postfix, or exim as the default MTA for the system.
The system-switch-mail RPM package must be installed to use the text-based version of the
Mail Transport Agent Switcher program. If you want to use the graphical version, the systemswitch-mail-gnome package must also be installed.
Note
For more information on installing RPM packages, refer to Part II, “Package Management”.
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5. Mail Delivery Agents
To start the Mail Transport Agent Switcher, select System (the main menu on the panel) =>
Administration => Mail Transport Agent Switcher, or type the command system-switch-mail
at a shell prompt (for example, in an XTerm or GNOME terminal).
The program automatically detects if the X Window System is running. If it is running, the program starts in graphical mode as shown in Figure 23.1, “Mail Transport Agent Switcher”. If X is
not detected, it starts in text-mode. To force Mail Transport Agent Switcher to run in textmode, use the command system-switch-mail-nox.
Figure 23.1. Mail Transport Agent Switcher
If you select OK to change the MTA, the selected mail daemon is enabled to start at boot time,
and the unselected mail daemons are disabled so that they do not start at boot time. The selected mail daemon is started, and any other mail daemon is stopped; thus making the changes
take place immediately.
5. Mail Delivery Agents
Red Hat Enterprise Linux includes two primary MDAs, Procmail and mail. Both of the applications are considered LDAs and both move email from the MTA's spool file into the user's mailbox. However, Procmail provides a robust filtering system.
This section details only Procmail. For information on the mail command, consult its man page.
Procmail delivers and filters email as it is placed in the mail spool file of the localhost. It is
powerful, gentle on system resources, and widely used. Procmail can play a critical role in delivering email to be read by email client applications.
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5.1. Procmail Configuration
Procmail can be invoked in several different ways. Whenever an MTA places an email into the
mail spool file, Procmail is launched. Procmail then filters and files the email for the MUA and
quits. Alternatively, the MUA can be configured to execute Procmail any time a message is received so that messages are moved into their correct mailboxes. By default, the presence of /
etc/procmailrc or of a .procmailrc file (also called an rc file) in the user's home directory invokes Procmail whenever an MTA receives a new message.
Whether Procmail acts upon an email message depends upon whether the message matches a
specified set of conditions or recipes in the rc file. If a message matches a recipe, then the
email is placed in a specified file, is deleted, or is otherwise processed.
When Procmail starts, it reads the email message and separates the body from the header information. Next, Procmail looks for /etc/procmailrc and rc files in the /etc/procmailrcs directory for default, system-wide, Procmail environmental variables and recipes. Procmail then
searches for a .procmailrc file in the user's home directory. Many users also create additional
rc files for Procmail that are referred to within the .procmailrc file in their home directory.
By default, no system-wide rc files exist in the /etc/ directory and no .procmailrc files exist in
any user's home directory. Therefore, to use Procmail, each user must construct a .procmailrc
file with specific environment variables and rules.
5.1. Procmail Configuration
The Procmail configuration file contains important environmental variables. These variables
specify things such as which messages to sort and what to do with the messages that do not
match any recipes.
These environmental variables usually appear at the beginning of .procmailrc in the following
format:
<env-variable>="<value>"
In this example, <env-variable> is the name of the variable and <value> defines the variable.
There are many environment variables not used by most Procmail users and many of the more
important environment variables are already defined by a default value. Most of the time, the following variables are used:
•
DEFAULT
— Sets the default mailbox where messages that do not match any recipes are
placed.
The default DEFAULT value is the same as $ORGMAIL.
•
— Specifies additional rc files containing more recipes for messages to be
checked against. This breaks up the Procmail recipe lists into individual files that fulfill different roles, such as blocking spam and managing email lists, that can then be turned off or on
by using comment characters in the user's .procmailrc file.
INCLUDERC
For example, lines in a user's .procmailrc file may look like this:
MAILDIR=$HOME/Msgs INCLUDERC=$MAILDIR/lists.rc INCLUDERC=$MAILDIR/spam.rc
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5.2. Procmail Recipes
If the user wants to turn off Procmail filtering of their email lists but leave spam control in
place, they would comment out the first INCLUDERC line with a hash mark character (#).
•
LOCKSLEEP
— Sets the amount of time, in seconds, between attempts by Procmail to use a
particular lockfile. The default is eight seconds.
•
LOCKTIMEOUT
•
LOGFILE
•
MAILDIR
•
ORGMAIL
— Sets the amount of time, in seconds, that must pass after a lockfile was last
modified before Procmail assumes that the lockfile is old and can be deleted. The default is
1024 seconds.
— The file to which any Procmail information or error messages are written.
— Sets the current working directory for Procmail. If set, all other Procmail paths are
relative to this directory.
— Specifies the original mailbox, or another place to put the messages if they cannot be placed in the default or recipe-required location.
By default, a value of /var/spool/mail/$LOGNAME is used.
•
SUSPEND
— Sets the amount of time, in seconds, that Procmail pauses if a necessary resource, such as swap space, is not available.
•
SWITCHRC
•
VERBOSE
— Allows a user to specify an external file containing additional Procmail recipes,
much like the INCLUDERC option, except that recipe checking is actually stopped on the referring configuration file and only the recipes on the SWITCHRC-specified file are used.
— Causes Procmail to log more information. This option is useful for debugging.
Other important environmental variables are pulled from the shell, such as LOGNAME, which is the
login name; HOME, which is the location of the home directory; and SHELL, which is the default
shell.
A comprehensive explanation of all environments variables, as well as their default values, is
available in the procmailrc man page.
5.2. Procmail Recipes
New users often find the construction of recipes the most difficult part of learning to use Procmail. To some extent, this is understandable, as recipes do their message matching using regular expressions, which is a particular format used to specify qualifications for a matching string.
However, regular expressions are not very difficult to construct and even less difficult to understand when read. Additionally, the consistency of the way Procmail recipes are written, regardless of regular expressions, makes it easy to learn by example. To see example Procmail recipes, refer to Section 5.2.5, “Recipe Examples”.
Procmail recipes take the following form:
:0<flags>: <lockfile-name> * <special-condition-character><condition-1> * <special-condition-character><con
The first two characters in a Procmail recipe are a colon and a zero. Various flags can be placed
after the zero to control how Procmail processes the recipe. A colon after the <flags> section
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5.2. Procmail Recipes
specifies that a lockfile is created for this message. If a lockfile is created, the name can be specified by replacing <lockfile-name>.
A recipe can contain several conditions to match against the message. If it has no conditions,
every message matches the recipe. Regular expressions are placed in some conditions to facilitate message matching. If multiple conditions are used, they must all match for the action to be
performed. Conditions are checked based on the flags set in the recipe's first line. Optional special characters placed after the * character can further control the condition.
The <action-to-perform> specifies the action taken when the message matches one of the conditions. There can only be one action per recipe. In many cases, the name of a mailbox is used
here to direct matching messages into that file, effectively sorting the email. Special action characters may also be used before the action is specified. Refer to Section 5.2.4, “Special Conditions and Actions” for more information.
5.2.1. Delivering vs. Non-Delivering Recipes
The action used if the recipe matches a particular message determines whether it is considered
a delivering or non-delivering recipe. A delivering recipe contains an action that writes the message to a file, sends the message to another program, or forwards the message to another
email address. A non-delivering recipe covers any other actions, such as a nesting block. A
nesting block is a set of actions, contained in braces {}, that are performed on messages which
match the recipe's conditions. Nesting blocks can be nested inside one another, providing greater control for identifying and performing actions on messages.
When messages match a delivering recipe, Procmail performs the specified action and stops
comparing the message against any other recipes. Messages that match non-delivering recipes
continue to be compared against other recipes.
5.2.2. Flags
Flags are essential to determine how or if a recipe's conditions are compared to a message.
The following flags are commonly used:
•
A
— Specifies that this recipe is only used if the previous recipe without an A or a flag also
matched this message.
•
a
•
B
•
b
•
c
•
D
•
E
— Specifies that this recipe is only used if the previous recipe with an A or a flag also
matched this message and was successfully completed.
— Parses the body of the message and looks for matching conditions.
— Uses the body in any resulting action, such as writing the message to a file or forwarding it. This is the default behavior.
— Generates a carbon copy of the email. This is useful with delivering recipes, since the
required action can be performed on the message and a copy of the message can continue
being processed in the rc files.
— Makes the egrep comparison case-sensitive. By default, the comparison process is not
case-sensitive.
— While similar to the A flag, the conditions in the recipe are only compared to the mes369
5.2. Procmail Recipes
sage if the immediately preceding the recipe without an E flag did not match. This is comparable to an else action.
•
e
— The recipe is compared to the message only if the action specified in the immediately
preceding recipe fails.
•
f
•
H
•
h
•
w
•
W
— Uses the pipe as a filter.
— Parses the header of the message and looks for matching conditions. This occurs by
default.
— Uses the header in a resulting action. This is the default behavior.
— Tells Procmail to wait for the specified filter or program to finish, and reports whether or
not it was successful before considering the message filtered.
— Is identical to w except that "Program failure" messages are suppressed.
For a detailed list of additional flags, refer to the procmailrc man page.
5.2.3. Specifying a Local Lockfile
Lockfiles are very useful with Procmail to ensure that more than one process does not try to alter a message simultaneously. Specify a local lockfile by placing a colon (:) after any flags on a
recipe's first line. This creates a local lockfile based on the destination file name plus whatever
has been set in the LOCKEXT global environment variable.
Alternatively, specify the name of the local lockfile to be used with this recipe after the colon.
5.2.4. Special Conditions and Actions
Special characters used before Procmail recipe conditions and actions change the way they are
interpreted.
The following characters may be used after the * character at the beginning of a recipe's condition line:
•
!
— In the condition line, this character inverts the condition, causing a match to occur only if
the condition does not match the message.
•
<
— Checks if the message is under a specified number of bytes.
•
>
— Checks if the message is over a specified number of bytes.
The following characters are used to perform special actions:
•
!
— In the action line, this character tells Procmail to forward the message to the specified
email addresses.
•
$
•
|
— Refers to a variable set earlier in the rc file. This is often used to set a common mailbox
that is referred to by various recipes.
— Starts a specified program to process the message.
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5.2. Procmail Recipes
•
and } — Constructs a nesting block, used to contain additional recipes to apply to matching messages.
{
If no special character is used at the beginning of the action line, Procmail assumes that the action line is specifying the mailbox in which to write the message.
5.2.5. Recipe Examples
Procmail is an extremely flexible program, but as a result of this flexibility, composing Procmail
recipes from scratch can be difficult for new users.
The best way to develop the skills to build Procmail recipe conditions stems from a strong understanding of regular expressions combined with looking at many examples built by others. A
thorough explanation of regular expressions is beyond the scope of this section. The structure
of Procmail recipes and useful sample Procmail recipes can be found at various places on the
Internet (such as http://www.iki.fi/era/procmail/links.html). The proper use and adaptation of regular expressions can be derived by viewing these recipe examples. In addition, introductory information about basic regular expression rules can be found in the grep man page.
The following simple examples demonstrate the basic structure of Procmail recipes and can
provide the foundation for more intricate constructions.
A basic recipe may not even contain conditions, as is illustrated in the following example:
:0: new-mail.spool
The first line specifies that a local lockfile is to be created but does not specify a name, so Procmail uses the destination file name and appends the value specified in the LOCKEXT environment
variable. No condition is specified, so every message matches this recipe and is placed in the
single spool file called new-mail.spool, located within the directory specified by the MAILDIR environment variable. An MUA can then view messages in this file.
A basic recipe, such as this, can be placed at the end of all rc files to direct messages to a default location.
The following example matched messages from a specific email address and throws them
away.
:0 * ^From: [email protected] /dev/null
With this example, any messages sent by [email protected] are sent to the /dev/null device,
deleting them.
Caution
Be certain that rules are working as intended before sending messages to /
dev/null for permanent deletion. If a recipe inadvertently catches unintended messages, and those messages disappear, it becomes difficult to troubleshoot the rule.
A better solution is to point the recipe's action to a special mailbox, which can be
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5.2. Procmail Recipes
checked from time to time to look for false positives. Once satisfied that no messages are accidentally being matched, delete the mailbox and direct the action to
send the messages to /dev/null.
The following recipe grabs email sent from a particular mailing list and places it in a specified
folder.
:0: * ^(From|CC|To).*tux-lug tuxlug
Any messages sent from the [email protected] mailing list are placed in the tuxlug mailbox
automatically for the MUA. Note that the condition in this example matches the message if it has
the mailing list's email address on the From, CC, or To lines.
Consult the many Procmail online resources available in Section 7, “Additional Resources” for
more detailed and powerful recipes.
5.2.6. Spam Filters
Because it is called by Sendmail, Postfix, and Fetchmail upon receiving new emails, Procmail
can be used as a powerful tool for combating spam.
This is particularly true when Procmail is used in conjunction with SpamAssassin. When used
together, these two applications can quickly identify spam emails, and sort or destroy them.
SpamAssassin uses header analysis, text analysis, blacklists, a spam-tracking database, and
self-learning Bayesian spam analysis to quickly and accurately identify and tag spam.
The easiest way for a local user to use SpamAssassin is to place the following line near the top
of the ~/.procmailrc file:
INCLUDERC=/etc/mail/spamassassin/spamassassin-default.rc
The /etc/mail/spamassassin/spamassassin-default.rc contains a simple Procmail rule that activates SpamAssassin for all incoming email. If an email is determined to be spam, it is tagged
in the header as such and the title is prepended with the following pattern:
*****SPAM*****
The message body of the email is also prepended with a running tally of what elements caused
it to be diagnosed as spam.
To file email tagged as spam, a rule similar to the following can be used:
:0 Hw * ^X-Spam-Status: Yes spam
This rule files all email tagged in the header as spam into a mailbox called spam.
Since SpamAssassin is a Perl script, it may be necessary on busy servers to use the binary
SpamAssassin daemon (spamd) and client application (spamc). Configuring SpamAssassin this
way, however, requires root access to the host.
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6. Mail User Agents
To start the spamd daemon, type the following command as root:
/sbin/service spamassassin start
To start the SpamAssassin daemon when the system is booted, use an initscript utility, such as
the Services Configuration Tool (system-config-services), to turn on the spamassassin service. Refer to for more information about initscript utilities.
To configure Procmail to use the SpamAssassin client application instead of the Perl script,
place the following line near the top of the ~/.procmailrc file. For a system-wide configuration,
place it in /etc/procmailrc:
INCLUDERC=/etc/mail/spamassassin/spamassassin-spamc.rc
6. Mail User Agents
There are scores of mail programs available under Red Hat Enterprise Linux. There are fullfeatured, graphical email client programs, such as Ximian Evolution, as well as text-based
email programs such as mutt.
The remainder of this section focuses on securing communication between the client and server.
6.1. Securing Communication
Popular MUAs included with Red Hat Enterprise Linux, such as Ximian Evolution and mutt offer SSL-encrypted email sessions.
Like any other service that flows over a network unencrypted, important email information, such
as usernames, passwords, and entire messages, may be intercepted and viewed by users on
the network. Additionally, since the standard POP and IMAP protocols pass authentication information unencrypted, it is possible for an attacker to gain access to user accounts by collecting usernames and passwords as they are passed over the network.
6.1.1. Secure Email Clients
Most Linux MUAs designed to check email on remote servers support SSL encryption. To use
SSL when retrieving email, it must be enabled on both the email client and server.
SSL is easy to enable on the client-side, often done with the click of a button in the MUA's configuration window or via an option in the MUA's configuration file. Secure IMAP and POP have
known port numbers (993 and 995, respectively) that the MUA uses to authenticate and download messages.
6.1.2. Securing Email Client Communications
Offering SSL encryption to IMAP and POP users on the email server is a simple matter.
First, create an SSL certificate. This can be done two ways: by applying to a Certificate Authority (CA) for an SSL certificate or by creating a self-signed certificate.
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6.1. Securing Communication
Caution
Self-signed certificates should be used for testing purposes only. Any server used
in a production environment should use an SSL certificate granted by a CA.
To create a self-signed SSL certificate for IMAP, change to the /etc/pki/tls/certs/ directory
and type the following commands as root:
rm -f cyrus-imapd.pem make cyrus-imapd.pem
Answer all of the questions to complete the process.
To create a self-signed SSL certificate for POP, change to the /etc/pki/tls/certs/ directory,
and type the following commands as root:
rm -f ipop3d.pem make ipop3d.pem
Again, answer all of the questions to complete the process.
Important
Please be sure to remove the default imapd.pem and ipop3d.pem files before issuing
each make command.
Once finished, execute the /sbin/service
mon which controls imapd and ipop3d.
xinetd restart
command to restart the xinetd dae-
Alternatively, the stunnel command can be used as an SSL encryption wrapper around the
standard, non-secure daemons, imapd or pop3d.
The stunnel program uses external OpenSSL libraries included with Red Hat Enterprise Linux
to provide strong cryptography and protect the connections. It is best to apply to a CA to obtain
an SSL certificate, but it is also possible to create a self-signed certificate.
To create a self-signed SSL certificate, change to the /etc/pki/tls/certs/ directory, and type
the following command:
make stunnel.pem
Again, answer all of the questions to complete the process.
Once the certificate is generated, it is possible to use the stunnel command to start the imapd
mail daemon using the following command:
/usr/sbin/stunnel -d 993 -l /usr/sbin/imapd imapd
Once this command is issued, it is possible to open an IMAP email client and connect to the
email server using SSL encryption.
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7. Additional Resources
To start the pop3d using the stunnel command, type the following command:
/usr/sbin/stunnel -d 995 -l /usr/sbin/pop3d pop3d
For more information about how to use stunnel, read the stunnel man page or refer to the documents in the /usr/share/doc/stunnel-<version-number>/ directory, where <version-number> is
the version number for stunnel.
7. Additional Resources
The following is a list of additional documentation about email applications.
7.1. Installed Documentation
•
Information on configuring Sendmail is included with the sendmail and sendmail-cf packages.
•
— Contains information on m4, file locations for Sendmail,
supported mailers, how to access enhanced features, and more.
/usr/share/sendmail-cf/README
In addition, the sendmail and aliases man pages contain helpful information covering various
Sendmail options and the proper configuration of the Sendmail /etc/mail/aliases file.
•
/usr/share/doc/postfix-<version-number>
— Contains a large amount of information about
ways to configure Postfix. Replace <version-number> with the version number of Postfix.
•
/usr/share/doc/fetchmail-<version-number>
•
/usr/share/doc/procmail-<version-number>
— Contains a full list of Fetchmail features in
the FEATURES file and an introductory FAQ document. Replace <version-number> with the version number of Fetchmail.
— Contains a README file that provides an overview of Procmail, a FEATURES file that explores every program feature, and an FAQ file with answers to many common configuration questions. Replace <version-number> with the version
number of Procmail.
When learning how Procmail works and creating new recipes, the following Procmail man
pages are invaluable:
•
procmail
— Provides an overview of how Procmail works and the steps involved with filtering email.
•
procmailrc
— Explains the rc file format used to construct recipes.
•
procmailex
— Gives a number of useful, real-world examples of Procmail recipes.
•
procmailsc
•
/usr/share/doc/spamassassin-<version-number>/
— Explains the weighted scoring technique used by Procmail to match a particular recipe to a message.
— Contains a large amount of information pertaining to SpamAssassin. Replace <version-number> with the version number of
the spamassassin package.
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7.3. Related Books
7.2. Useful Websites
•
http://www.sendmail.org/ — Offers a thorough technical breakdown of Sendmail features,
documentation and configuration examples.
•
http://www.sendmail.com/ — Contains news, interviews and articles concerning Sendmail,
including an expanded view of the many options available.
•
http://www.postfix.org/ — The Postfix project home page contains a wealth of information
about Postfix. The mailing list is a particularly good place to look for information.
•
http://fetchmail.berlios.de/ — The home page for Fetchmail, featuring an online manual, and
a thorough FAQ.
•
http://www.procmail.org/ — The home page for Procmail with links to assorted mailing lists
dedicated to Procmail as well as various FAQ documents.
•
http://partmaps.org/era/procmail/mini-faq.html — An excellent Procmail FAQ, offers
troubleshooting tips, details about file locking, and the use of wildcard characters.
•
http://www.uwasa.fi/~ts/info/proctips.html — Contains dozens of tips that make using Procmail much easier. Includes instructions on how to test .procmailrc files and use Procmail
scoring to decide if a particular action should be taken.
•
http://www.spamassassin.org/ — The official site of the SpamAssassin project.
7.3. Related Books
•
Sendmail Milters: A Guide for Fighting Spam by Bryan Costales and Marcia Flynt; AddisonWesley — A good Sendmail guide that can help you customise your mail filters.
•
Sendmail by Bryan Costales with Eric Allman et al; O'Reilly & Associates — A good Sendmail reference written with the assistance of the original creator of Delivermail and Sendmail.
•
Removing the Spam: Email Processing and Filtering by Geoff Mulligan; Addison-Wesley
Publishing Company — A volume that looks at various methods used by email administrators using established tools, such as Sendmail and Procmail, to manage spam problems.
•
Internet Email Protocols: A Developer's Guide by Kevin Johnson; Addison-Wesley Publishing Company — Provides a very thorough review of major email protocols and the security
they provide.
•
Managing IMAP by Dianna Mullet and Kevin Mullet; O'Reilly & Associates — Details the
steps required to configure an IMAP server.
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Chapter 24. Lightweight Directory
Access Protocol (LDAP)
The Lightweight Directory Access Protocol (LDAP) is a set of open protocols used to access
centrally stored information over a network. It is based on the X.500 standard for directory sharing, but is less complex and resource-intensive. For this reason, LDAP is sometimes referred to
as "X.500 Lite." The X.500 standard is a directory that contains hierarchical and categorized information, which could include information such as names, addresses, and phone numbers.
Like X.500, LDAP organizes information in a hierarchal manner using directories. These directories can store a variety of information and can even be used in a manner similar to the Network
Information Service (NIS), enabling anyone to access their account from any machine on the
LDAP enabled network.
In many cases, LDAP is used as a virtual phone directory, allowing users to easily access contact information for other users. But LDAP is more flexible than a traditional phone directory, as
it is capable of referring a querent to other LDAP servers throughout the world, providing an adhoc global repository of information. Currently, however, LDAP is more commonly used within
individual organizations, like universities, government departments, and private companies.
LDAP is a client/server system. The server can use a variety of databases to store a directory,
each optimized for quick and copious read operations. When an LDAP client application connects to an LDAP server, it can either query a directory or attempt to modify it. In the event of a
query, the server either answers the query locally, or it can refer the querent to an LDAP server
which does have the answer. If the client application is attempting to modify information within
an LDAP directory, the server verifies that the user has permission to make the change and
then adds or updates the information.
This chapter refers to the configuration and use of OpenLDAP 2.0, an open source implementation of the LDAPv2 and LDAPv3 protocols.
1. Why Use LDAP?
The main benefit of using LDAP is that information for an entire organization can be consolidated into a central repository. For example, rather than managing user lists for each group within an organization, LDAP can be used as a central directory accessible from anywhere on the
network. And because LDAP supports Secure Sockets Layer (SSL) and Transport Layer Security (TLS), sensitive data can be protected from prying eyes.
LDAP also supports a number of back-end databases in which to store directories. This allows
administrators the flexibility to deploy the database best suited for the type of information the
server is to disseminate. Because LDAP also has a well-defined client Application Programming
Interface (API), the number of LDAP-enabled applications are numerous and increasing in
quantity and quality.
1.1. OpenLDAP Features
OpenLDAP includes a number of important features.
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2. LDAP Terminology
•
LDAPv3 Support — OpenLDAP supports Simple Authentication and Security Layer (SASL),
Transport Layer Security (TLS), and Secure Sockets Layer (SSL), among other improvements. Many of the changes in the protocol since LDAPv2 are designed to make LDAP more
secure.
•
IPv6 Support — OpenLDAP supports the next generation Internet Protocol version 6.
•
LDAP Over IPC — OpenLDAP can communicate within a system using interprocess communication (IPC). This enhances security by eliminating the need to communicate over a
network.
•
Updated C API — Improves the way programmers can connect to and use LDAP directory
servers.
•
LDIFv1 Support — Provides full compliance with the LDAP Data Interchange Format (LDIF)
version 1.
•
Enhanced Stand-Alone LDAP Server — Includes an updated access control system, thread
pooling, better tools, and much more.
2. LDAP Terminology
Any discussion of LDAP requires a basic understanding of a set of LDAP-specific terms:
•
entry — A single unit within an LDAP directory. Each entry is identified by its unique Distinguished Name (DN).
•
attributes — Information directly associated with an entry. For example, an organization
could be represented as an LDAP entry. Attributes associated with the organization might include a fax number, an address, and so on. People can also be represented as entries in an
LDAP directory, with common attributes such as the person's telephone number and email
address.
Some attributes are required, while other attributes are optional. An objectclass definition
sets which attributes are required for each entry. Objectclass definitions are found in various
schema files, located in the /etc/openldap/schema/ directory. For more information, refer to
Section 5, “The /etc/openldap/schema/ Directory”.
The assertion of an attribute and its corresponding value is also referred to as a Relative
Distinguished Name (RDN). An RDN is only unique per entry, whereas a DN is globally
unique.
•
LDIF — The LDAP Data Interchange Format (LDIF) is an ASCII text representation of LDAP
entries. Files used for importing data to LDAP servers must be in LDIF format. An LDIF entry
looks similar to the following example:
[<id>] dn: <distinguished name> <attrtype>: <attrvalue> <attrtype>: <attrvalue> <attrtype>: <attrvalue>
Each entry can contain as many <attrtype>:
dicates the end of an entry.
<attrvalue>
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pairs as needed. A blank line in-
3. OpenLDAP Daemons and Utilities
Caution
All <attrtype> and <attrvalue> pairs must be defined in a corresponding
schema file to use this information.
Any value enclosed within a < and a > is a variable and can be set whenever a new LDAP
entry is created. This rule does not apply, however, to <id>. The <id> is a number determined by the application used to edit the entry.
3. OpenLDAP Daemons and Utilities
The suite of OpenLDAP libraries and tools are included within the following packages:
•
openldap
— Contains the libraries necessary to run the OpenLDAP server and client applica-
tions.
•
openldap-clients
— Contains command line tools for viewing and modifying directories on
an LDAP server.
•
openldap-servers
— Contains the servers and other utilities necessary to configure and run
an LDAP server.
There are two servers contained in the openldap-servers package: the Standalone LDAP Daemon (/usr/sbin/slapd) and the Standalone LDAP Update Replication Daemon (/
usr/sbin/slurpd).
The slapd daemon is the standalone LDAP server while the slurpd daemon is used to synchronize changes from one LDAP server to other LDAP servers on the network. The slurpd
daemon is only used when dealing with multiple LDAP servers.
To perform administrative tasks, the openldap-servers package installs the following utilities into
the /usr/sbin/ directory:
•
slapadd
— Adds entries from an LDIF file to an LDAP directory. For example, the command
reads in the LDIF file, ldif-input, containing the new
/usr/sbin/slapadd -l ldif-input
entries.
Important
Only the root user may use /usr/sbin/slapadd. However, the directory server
runs as the ldap user. Therefore the directory server is unable to modify any
files created by slapadd. To correct this issue, after using slapadd, type the following command:
chown -R ldap /var/lib/ldap
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3.1. NSS, PAM, and LDAP
•
slapcat
— Pulls entries from an LDAP directory in the default format, Sleepycat Software's
Berkeley DB system, and saves them in an LDIF file. For example, the command /
usr/sbin/slapcat -l ldif-output outputs an LDIF file called ldif-output containing the
entries from the LDAP directory.
•
slapindex
•
slappasswd
— Re-indexes the slapd directory based on the current content. This tool should
be run whenever indexing options within /etc/openldap/slapd.conf are changed.
— Generates an encrypted user password value for use with ldapmodify or the
rootpw value in the slapd configuration file, /etc/openldap/slapd.conf. Execute the /
usr/sbin/slappasswd command to create the password.
Warning
You must stop slapd by issuing the /sbin/service ldap stop command before using slapadd, slapcat or slapindex. Otherwise, the integrity of the LDAP directory is
at risk.
For more information on using these utilities, refer to their respective man pages.
The openldap-clients package installs tools into /usr/bin/ which are used to add, modify, and
delete entries in an LDAP directory. These tools include the following:
•
ldapadd
— Adds entries to an LDAP directory by accepting input via a file or standard input;
ldapadd is actually a hard link to ldapmodify -a.
•
ldapdelete
•
ldapmodify
— Deletes entries from an LDAP directory by accepting user input at a shell
prompt or via a file.
— Modifies entries in an LDAP directory, accepting input via a file or standard in-
put.
•
ldappasswd
— Sets the password for an LDAP user.
•
ldapsearch
— Searches for entries in an LDAP directory using a shell prompt.
•
ldapcompare
•
ldapwhoami
— Opens a connection to an LDAP server, binds, and performs a comparison
using specified parameters.
— Opens a connection to an LDAP server, binds, and performs a whoami opera-
tion.
•
ldapmodrdn
— Opens a connection to an LDAP server, binds, and modifies the RDNs of
entries.
With the exception of ldapsearch, each of these utilities is more easily used by referencing a file
containing the changes to be made rather than typing a command for each entry to be changed
within an LDAP directory. The format of such a file is outlined in the man page for each utility.
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3.2. PHP4, LDAP, and the Apache HTTP Server
3.1. NSS, PAM, and LDAP
In addition to the OpenLDAP packages, Red Hat Enterprise Linux includes a package called
nss_ldap, which enhances LDAP's ability to integrate into both Linux and other UNIX environments.
The nss_ldap package provides the following modules (where <version> refers to the version of
libnss_ldap in use):
•
/lib/libnss_ldap-<version>.so
•
/lib/security/pam_ldap.so
The nss_ldap package provides the following modules for Itanium or AMD64 architectures:
•
/lib64/libnss_ldap-<version>.so
•
/lib64/security/pam_ldap.so
The libnss_ldap-<version>.so module allows applications to look up users, groups, hosts, and
other information using an LDAP directory via the Nameservice Switch (NSS) interface of glibc.
NSS allows applications to authenticate using LDAP in conjunction with the NIS name service
and flat authentication files.
The pam_ldap module allows PAM-aware applications to authenticate users using information
stored in an LDAP directory. PAM-aware applications include console login, POP and IMAP
mail servers, and Samba. By deploying an LDAP server on a network, all of these applications
can authenticate using the same user ID and password combination, greatly simplifying administration.
For more about configuring PAM, refer to Section 4, “Pluggable Authentication Modules (PAM)”
and the PAM man pages.
3.2. PHP4, LDAP, and the Apache HTTP Server
Red Hat Enterprise Linux includes a package containing an LDAP module for the PHP serverside scripting language.
The php-ldap package adds LDAP support to the PHP4 HTML-embedded scripting language
via the /usr/lib/php4/ldap.so module. This module allows PHP4 scripts to access information
stored in an LDAP directory.
Red Hat Enterprise Linux ships with the mod_authz_ldap module for the Apache HTTP Server.
This module uses the short form of the distinguished name for a subject and the issuer of the
client SSL certificate to determine the distinguished name of the user within an LDAP directory.
It is also capable of authorizing users based on attributes of that user's LDAP directory entry,
determining access to assets based on the user and group privileges of the asset, and denying
access for users with expired passwords. The mod_ssl module is required when using the
mod_authz_ldap module.
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3.3. LDAP Client Applications
Important
The mod_authz_ldap module does not authenticate a user to an LDAP directory using an encrypted password hash. This functionality is provided by the experimental
mod_auth_ldap module, which is not included with Red Hat Enterprise Linux. Refer
to the Apache Software Foundation website online at http://www.apache.org/ for
details on the status of this module.
3.3. LDAP Client Applications
There are graphical LDAP clients available which support creating and modifying directories, but
they are not included with Red Hat Enterprise Linux. One such application is LDAP Browser/Editor — A Java-based tool available online at http://www.iit.edu/~gawojar/ldap/.
Other LDAP clients access directories as read-only, using them to reference, but not alter, organization-wide information. Some examples of such applications are Sendmail, Mozilla,
Gnome Meeting, and Evolution.
4. OpenLDAP Configuration Files
OpenLDAP configuration files are installed into the /etc/openldap/ directory. The following is a
brief list highlighting the most important directories and files:
•
/etc/openldap/ldap.conf
— This is the configuration file for all client applications which use
the OpenLDAP libraries such as ldapsearch, ldapadd, Sendmail, Evolution, and Gnome
Meeting.
•
/etc/openldap/slapd.conf
•
/etc/openldap/schema/
— This is the configuration file for the slapd daemon. Refer to
Section 6.1, “Editing /etc/openldap/slapd.conf” for more information.
directory — This subdirectory contains the schema used by the slapd
daemon. Refer to Section 5, “The /etc/openldap/schema/ Directory” for more information.
Note
If the nss_ldap package is installed, it creates a file named /etc/ldap.conf. This file
is used by the PAM and NSS modules supplied by the nss_ldap package. Refer to
Section 7, “Configuring a System to Authenticate Using OpenLDAP” for more information.
5. The
/etc/openldap/schema/
Directory
The /etc/openldap/schema/ directory holds LDAP definitions, previously located in the
slapd.at.conf and slapd.oc.conf files. The /etc/openldap/schema/redhat/ directory holds cus-
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6. OpenLDAP Setup Overview
tomized schemas distributed by Red Hat for Red Hat Enterprise Linux.
All attribute syntax definitions and objectclass definitions are now located in the different
schema files. The various schema files are referenced in /etc/openldap/slapd.conf using include lines, as shown in this example:
include /etc/openldap/schema/core.schema include /etc/openldap/schema/cosine.schema include /etc/openldap/
Caution
Do not modify schema items defined in the schema files installed by OpenLDAP.
It is possible to extend the schema used by OpenLDAP to support additional attribute types and
object classes using the default schema files as a guide. To do this, create a local.schema file in
the /etc/openldap/schema/ directory. Reference this new schema within slapd.conf by adding
the following line below the default include schema lines:
include /etc/openldap/schema/local.schema
Next, define new attribute types and object classes within the local.schema file. Many organizations use existing attribute types from the schema files installed by default and add new object
classes to the local.schema file.
Extending the schema to match certain specialized requirements is quite involved and beyond
the scope of this chapter. Refer to http://www.openldap.org/doc/admin/schema.html for information.
6. OpenLDAP Setup Overview
This section provides a quick overview for installing and configuring an OpenLDAP directory.
For more details, refer to the following URLs:
•
http://www.openldap.org/doc/admin/quickstart.html — The Quick-Start Guide on the OpenLDAP website.
•
http://www.tldp.org/HOWTO/LDAP-HOWTO/index.html — The LDAP Linux HOWTO from
the Linux Documentation Project.
The basic steps for creating an LDAP server are as follows:
1.
Install the openldap, openldap-servers, and openldap-clients RPMs.
2.
Edit the /etc/openldap/slapd.conf file to specify the LDAP domain and server. Refer to
Section 6.1, “Editing /etc/openldap/slapd.conf” for more information.
3.
Start slapd with the command:
/sbin/service ldap start
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6.1. Editing /etc/openldap/slapd.conf
After configuring LDAP, use chkconfig, /usr/sbin/ntsysv, or the Services Configuration
Tool to configure LDAP to start at boot time. For more information about configuring services, refer to Chapter 15, Controlling Access to Services.
4.
Add entries to an LDAP directory with ldapadd.
5.
Use ldapsearch to determine if slapd is accessing the information correctly.
6.
At this point, the LDAP directory should be functioning properly and can be configured with
LDAP-enabled applications.
6.1. Editing /etc/openldap/slapd.conf
To use the slapd LDAP server, modify its configuration file, /etc/openldap/slapd.conf, to specify the correct domain and server.
The suffix line names the domain for which the LDAP server provides information and should
be changed from:
suffix "dc=your-domain,dc=com"
Edit it accordingly so that it reflects a fully qualified domain name. For example:
suffix "dc=example,dc=com"
The rootdn entry is the Distinguished Name (DN) for a user who is unrestricted by access controls or administrative limit parameters set for operations on the LDAP directory. The rootdn
user can be thought of as the root user for the LDAP directory. In the configuration file, change
the rootdn line from its default value as in the following example:
rootdn "cn=root,dc=example,dc=com"
When populating an LDAP directory over a network, change the rootpw line — replacing the default value with an encrypted password string. To create an encrypted password string, type the
following command:
slappasswd
When prompted, type and then re-type a password. The program prints the resulting encrypted
password to the shell prompt.
Next, copy the newly created encrypted password into the /etc/openldap/slapd.conf on one of
the rootpw lines and remove the hash mark (#).
When finished, the line should look similar to the following example:
rootpw {SSHA}vv2y+i6V6esazrIv70xSSnNAJE18bb2u
Warning
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7. Configuring a System to Authenticate Using OpenLDAP
LDAP passwords, including the rootpw directive specified in /
etc/openldap/slapd.conf, are sent over the network unencrypted, unless TLS encryption is enabled.
To enable TLS encryption, review the comments in /etc/openldap/slapd.conf and
refer to the man page for slapd.conf.
For added security, the rootpw directive should be commented out after populating the LDAP
directory by preceding it with a hash mark (#).
When using the /usr/sbin/slapadd command line tool locally to populate the LDAP directory,
use of the rootpw directive is not necessary.
Important
Only the root user can use /usr/sbin/slapadd. However, the directory server runs
as the ldap user. Therefore, the directory server is unable to modify any files created by slapadd. To correct this issue, after using slapadd, type the following command:
chown -R ldap /var/lib/ldap
7. Configuring a System to Authenticate Using OpenLDAP
This section provides a brief overview of how to configure OpenLDAP user authentication. Unless you are an OpenLDAP expert, more documentation than is provided here is necessary.
Refer to the references provided in Section 9, “Additional Resources” for more information.
Install the Necessary LDAP Packages. First, make sure that the appropriate packages are installed on both the LDAP server and the LDAP client machines. The LDAP server needs the
openldap-servers package.
The openldap, openldap-clients, and nss_ldap packages need to be installed on all LDAP client
machines.
Edit the Configuration Files.
•
On the server, edit the /etc/openldap/slapd.conf file on the LDAP server to make sure it
matches the specifics of the organization. Refer to Section 6.1, “Editing /
etc/openldap/slapd.conf” for instructions about editing slapd.conf.
•
On the client machines, both /etc/ldap.conf and /etc/openldap/ldap.conf need to contain
the proper server and search base information for the organization.
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7.1. PAM and LDAP
To do this, run the graphical Authentication Configuration Tool (system-config-authentication) and select Enable LDAP Support under the User Information tab.
It is also possible to edit these files by hand.
•
On the client machines, the /etc/nsswitch.conf must be edited to use LDAP.
To do this, run the Authentication Configuration Tool (system-config-authentication) and
select Enable LDAP Support under the User Information tab.
If editing /etc/nsswitch.conf by hand, add ldap to the appropriate lines.
For example:
passwd: files ldap shadow: files ldap group: files ldap
7.1. PAM and LDAP
To have standard PAM-enabled applications use LDAP for authentication, run the Authentication Configuration Tool (system-config-authentication) and select Enable LDAP Support
under the the Authentication tab. For more about configuring PAM, refer to Section 4,
“Pluggable Authentication Modules (PAM)” and the PAM man pages.
7.2. Migrating Old Authentication Information to LDAP
Format
The /usr/share/openldap/migration/ directory contains a set of shell and Perl scripts for migrating authentication information into an LDAP format.
Note
Perl must be installed on the system to use these scripts.
First, modify the migrate_common.ph file so that it reflects the correct domain. The default DNS
domain should be changed from its default value to something like:
$DEFAULT_MAIL_DOMAIN = "example";
The default base should also be changed to something like:
$DEFAULT_BASE = "dc=example,dc=com";
The job of migrating a user database into a format that is LDAP readable falls to a group of migration scripts installed in the same directory. Using Table 24.1, “LDAP Migration Scripts”, decide which script to run to migrate the user database.
Run the appropriate script based on the existing name service.
The README and the migration-tools.txt files in the /usr/share/openldap/migration/ directory
386
8. Migrating Directories from Earlier Releases
provide more details on how to migrate the information.
Existing name service
Is LDAP running?
Script to Use
/etc
flat files
yes
migrate_all_online.sh
/etc
flat files
no
migrate_all_offline.sh
NetInfo
yes
migrate_all_netinfo_online.sh
NetInfo
no
migrate_all_netinfo_offline.sh
NIS (YP)
yes
migrate_all_nis_online.sh
NIS (YP)
no
migrate_all_nis_offline.sh
Table 24.1. LDAP Migration Scripts
8. Migrating Directories from Earlier Releases
With Red Hat Enterprise Linux, OpenLDAP uses Sleepycat Software's Berkeley DB system as
its on-disk storage format for directories. Earlier versions of OpenLDAP used GNU Database
Manager (gdbm). For this reason, before upgrading an LDAP implementation to Red Hat Enterprise Linux 5.0.0, original LDAP data should first be exported before the upgrade, and then reimported afterwards. This can be achieved by performing the following steps:
1.
Before upgrading the operating system, run the command /usr/sbin/slapcat -l ldifoutput. This outputs an LDIF file called ldif-output containing the entries from the LDAP
directory.
2.
Upgrade the operating system, being careful not to reformat the partition containing the
LDIF file.
3.
Re-import the LDAP directory to the upgraded Berkeley DB format by executing the command /usr/sbin/slapadd -l ldif-output.
9. Additional Resources
The following resources offer additional information on LDAP. It is highly recommended that you
review these, especially the OpenLDAP website and the LDAP HOWTO, before configuring
LDAP on your system(s).
9.1. Installed Documentation
•
/usr/share/docs/openldap-<versionnumber>/
directory — Contains a general README docu-
ment and miscellaneous information.
387
9.2. Useful Websites
•
LDAP related man pages — There are a number of man pages for the various applications
and configuration files involved with LDAP. The following is a list of some of the more important man pages.
Client Applications
•
man ldapadd
— Describes how to add entries to an LDAP directory.
•
man ldapdelete
— Describes how to delete entries within an LDAP directory.
•
man ldapmodify
— Describes how to modify entries within an LDAP directory.
•
man ldapsearch
— Describes how to search for entries within an LDAP directory.
•
man ldappasswd
— Describes how to set or change the password of an LDAP user.
•
man ldapcompare
•
man ldapwhoami
— Describes how to use the ldapwhoami tool.
•
man ldapmodrdn
— Describes how to modify the RDNs of entries.
— Describes how to use the ldapcompare tool.
Server Applications
•
man slapd
— Describes command line options for the LDAP server.
•
man slurpd
— Describes command line options for the LDAP replication server.
Administrative Applications
•
man slapadd
— Describes command line options used to add entries to a slapd data-
base.
•
man slapcat
— Describes command line options used to generate an LDIF file from a
slapd database.
•
man slapindex
•
man slappasswd
— Describes command line options used to regenerate an index based
upon the contents of a slapd database.
— Describes command line options used to generate user passwords
for LDAP directories.
Configuration Files
•
man ldap.conf
— Describes the format and options available within the configuration file
for LDAP clients.
•
man slapd.conf
— Describes the format and options available within the configuration
file referenced by both the LDAP server applications (slapd and slurpd) and the LDAP
administrative tools (slapadd, slapcat, and slapindex).
388
9.3. Related Books
9.2. Useful Websites
•
http://www.openldap.org/ [http://www.openldap.org] — Home of the OpenLDAP Project. This
website contains a wealth of information about configuring OpenLDAP as well as a future
roadmap and version changes.
•
http://www.padl.com/ [http://www.padl.com] — Developers of nss_ldap and pam_ldap, among
other useful LDAP tools.
•
http://www.kingsmountain.com/ldapRoadmap.shtml — Jeff Hodges' LDAP Road Map contains links to several useful FAQs and emerging news concerning the LDAP protocol.
•
http://www.ldapman.org/articles/ — Articles that offer a good introduction to LDAP, including
methods to design a directory tree and customizing directory structures.
9.3. Related Books
•
OpenLDAP by Example by John Terpstra and Benjamin Coles; Prentice Hall.
•
Implementing LDAP by Mark Wilcox; Wrox Press, Inc.
•
Understanding and Deploying LDAP Directory Services by Tim Howes et al.; Macmillan
Technical Publishing.
389
Chapter 25. Authentication
Configuration
When a user logs in to a Red Hat Enterprise Linux system, the username and password combination must be verified, or authenticated, as a valid and active user. Sometimes the information to verify the user is located on the local system, and other times the system defers the authentication to a user database on a remote system.
The Authentication Configuration Tool provides a graphical interface for configuring user information retrieval from NIS, LDAP, and Hesiod servers. This tool also allows you to configure
LDAP, Kerberos, and SMB as authentication protocols.
Note
If you configured a medium or high security level during installation (or with the Security Level Configuration Tool), then the firewall will prevent NIS (Network Information Service) authentication.
This chapter does not explain each of the different authentication types in detail. Instead, it explains how to use the Authentication Configuration Tool to configure them.
To start the graphical version of the Authentication Configuration Tool from the desktop, select the System (on the panel) => Administration => Authentication or type the command
system-config-authentication at a shell prompt (for example, in an XTerm or a GNOME terminal).
Important
After exiting the authentication program, the changes made take effect immediately.
1. User Information
The User Information tab allows you to configure how users should be authenticated, and has
several options. To enable an option, click the empty checkbox beside it. To disable an option,
click the checkbox beside it to clear the checkbox. Click OK to exit the program and apply the
changes.
390
1. User Information
Figure 25.1. User Information
The following list explains what each option configures:
NIS. The Enable NIS Support option configures the system to connect to an NIS server (as an
NIS client) for user and password authentication. Click the Configure NIS... button to specify
the NIS domain and NIS server. If the NIS server is not specified, the daemon attempts to find it
via broadcast.
The ypbind package must be installed for this option to work. If NIS support is enabled, the
portmap and ypbind services are started and are also enabled to start at boot time.
For more information about NIS, refer to Section 2.3, “Securing NIS”.
LDAP. The Enable LDAP Support option instructs the system to retrieve user information via
LDAP. Click the Configure LDAP... button to specify the following:
391
1. User Information
•
LDAP Search Base DN — Specifies that user information should be retrieved using the listed Distinguished Name (DN).
•
LDAP Server — Specifies the IP address of the LDAP server.
•
Use TLS to encrypt connections — When enabled, Transport Layer Security will be used
to encrypt passwords sent to the LDAP server. The Download CA Certificate option allows
you to specify a URL from which to download a valid CA (Certificate Authority) Certificate. A
valid CA Certificate must be in PEM (Privacy Enhanced Mail) format.
For more information about CA Certificates, refer to Section 8.2, “An Overview of Certificates
and Security”.
The openldap-clients package must be installed for this option to work.
For more information about LDAP, refer to Chapter 24, Lightweight Directory Access Protocol
(LDAP).
Hesiod. The Enable Hesiod Support option configures the system to retrieve information
(including user information) from a remote Hesiod database. Click the Configure Hesiod... button to specify the following:
•
Hesiod LHS — Specifies the domain prefix used for Hesiod queries.
•
Hesiod RHS — Specifies the default Hesiod domain.
The hesiod package must be installed for this option to work.
For more information about Hesiod, refer to its man page using the command man hesiod. You
can also refer to the hesiod.conf man page (man hesiod.conf) for more information on LHS and
RHS.
Winbind. The Enable Winbind Support option configures the system to connect to a Windows Active Directory or a Windows domain controller. User information from the specified directory or domain controller can then be accessed, and server authentication options can be
configured. Click the Configure Winbind... button to specify the following:
•
Winbind Domain — Specifies the Windows Active Directory or domain controller to connect
to.
•
Security Model — Allows you to select a security model, which configures how clients
should respond to Samba. The drop-down list allows you select any of the following:
•
user — This is the default mode. With this level of security, a client must first log in with a
valid username and password. Encrypted passwords can also be used in this security
mode.
•
server — In this mode, Samba will attempt to validate the username/password by authenticating it through another SMB server (for example, a Windows NT Server). If the attempt fails, the user mode will take effect instead.
•
domain — In this mode, Samba will attempt to validate the username/password by au-
392
2. Authentication
thenticating it through a Windows NT Primary or Backup Domain Controller, similar to
how a Windows NT Server would.
•
ads — This mode instructs Samba to act as a domain member in an Active Directory
Server (ADS) realm. To operate in this mode, the krb5-server package must be installed,
and Kerberos must be configured properly.
•
Winbind ADS Realm — When the ads Security Model is selected, this allows you to specify
the ADS Realm the Samba server should act as a domain member of.
•
Winbind Domain Controllers — Use this option to specify which domain controller winbind
should use. For more information about domain controllers, please refer to Section 6.3,
“Domain Controller”.
•
Template Shell — When filling out the user information for a Windows NT user, the winbindd daemon uses the value chosen here to to specify the login shell for that user.
For more information about the winbind service, refer to winbindd under Section 2, “Samba
Daemons and Related Services”.
2. Authentication
The Authentication tab allows for the configuration of network authentication methods. To enable an option, click the empty checkbox beside it. To disable an option, click the checkbox beside it to clear the checkbox.
393
2. Authentication
Figure 25.2. Authentication
The following explains what each option configures:
Kerberos. The Enable Kerberos Support option enables Kerberos authentication. Click the
Configure Kerberos... button to open the Kerberos Settings dialogue and configure the following:
•
Realm — Configures the realm for the Kerberos server. The realm is the network that uses
Kerberos, composed of one or more KDCs and a potentially large number of clients.
•
KDC — Defines the Key Distribution Center (KDC), which is the server that issues Kerberos
tickets.
•
Admin Servers — Specifies the administration server(s) running kadmind.
394
3. Options
The Kerberos Settings dialogue also allows you to use DNS to resolve hosts to realms and
locate KDCs for realms.
The krb5-libs and krb5-workstation packages must be installed for this option to work. For
more information about Kerberos, refer to Section 6, “Kerberos”.
LDAP. The Enable LDAP Support option instructs standard PAM-enabled applications to use
LDAP for authentication. The Configure LDAP... button allows you to configure LDAP support
with options identical to those present in Configure LDAP... under the User Information tab.
For more information about these options, refer to Section 1, “User Information”.
The openldap-clients package must be installed for this option to work.
Smart Card. The Enable Smart Card Support option enables Smart Card authentication. This
allows users to log in using a certificate and key associated stored on a smart card. Click the
Configure Smart Card... button for more options.
The pam_pkcs11 and coolkey packages must be installed for this option to work. For more information about smart cards, refer to Section 3.1.3, “Supported Smart Cards” under Section 3,
“Single Sign-on (SSO)”.
SMB. The Enable SMB Support option configures PAM to use a Server Message Block (SMB)
server to authenticate users. SMB refers to a client-server protocol used for cross-system communication; it is also the protocol used by Samba to appear as a Windows server to Windows
clients. Click the Configure SMB... button to specify the following:
•
Workgroup — Specifies the SMB workgroup to use.
•
Domain Controllers — Specifies the SMB domain controllers to use.
Winbind. The Enable Winbind Support option configures the system to connect to a Windows Active Directory or a Windows domain controller. User information from the specified directory or domain controller can then be accessed, and server authentication options can be
configured.
The Configure Winbind... options are identical to those in the Configure Winbind... button on
the User Information tab. Please refer to Winbind (under Section 1, “User Information”) for
more information.
3. Options
This tab allows other configuration options, as listed below.
395
3. Options
Figure 25.3. Options
Cache User Information. Select this option to enable the name service cache daemon (nscd)
and configure it to start at boot time.
The nscd package must be installed for this option to work. For more information about nscd,
refer to its man page using the command man nscd.
Use Shadow Passwords. Select this option to store passwords in shadow password format in
the /etc/shadow file instead of /etc/passwd. Shadow passwords are enabled by default during
installation and are highly recommended to increase the security of the system.
The shadow-utils package must be installed for this option to work. For more information about
shadow passwords, refer to Section 6, “Shadow Passwords”.
Use MD5 Passwords. Select this option to enable MD5 passwords, which allows passwords to
396
4. Command Line Version
be up to 256 characters instead of eight characters or less. It is selected by default during installation and is highly recommended for increased security.
Local authorization is sufficient for local users. When this option is enabled, the system will
not check authorization from network services (such as LDAP or Kerberos) for user accounts
maintained in its /etc/passwd file.
Authenticate system accounts by network services. Enabling this option configures the system to allow network services (such as LDAP or Kerberos) to authenticate system accounts
(including root) in the machine.
4. Command Line Version
The Authentication Configuration Tool can also be run as a command line tool with no interface. The command line version can be used in a configuration script or a kickstart script. The
authentication options are summarized in Table 25.1, “Command Line Options”.
Tip
These options can also be found in the authconfig man page or by typing authconfig --help at a shell prompt.
Option
Description
--enableshadow
Enable shadow passwords
--disableshadow
Disable shadow passwords
--enablemd5
Enable MD5 passwords
--disablemd5
Disable MD5 passwords
--enablenis
Enable NIS
--disablenis
Disable NIS
--nisdomain=<domain>
Specify NIS domain
--nisserver=<server>
Specify NIS server
--enableldap
Enable LDAP for user information
--disableldap
Disable LDAP for user information
--enableldaptls
Enable use of TLS with LDAP
--disableldaptls
Disable use of TLS with LDAP
--enableldapauth
Enable LDAP for authentication
--disableldapauth
Disable LDAP for authentication
--ldapserver=<server>
Specify LDAP server
397
4. Command Line Version
Option
Description
--ldapbasedn=<dn>
Specify LDAP base DN
--enablekrb5
Enable Kerberos
--disablekrb5
Disable Kerberos
--krb5kdc=<kdc>
Specify Kerberos KDC
--krb5adminserver=<server>
Specify Kerberos administration
server
--krb5realm=<realm>
Specify Kerberos realm
--enablekrb5kdcdns
Enable use of DNS to find Kerberos
KDCs
--disablekrb5kdcdns
Disable use of DNS to find Kerberos
KDCs
--enablekrb5realmdns
Enable use of DNS to find Kerberos
realms
--disablekrb5realmdns
Disable use of DNS to find Kerberos
realms
--enablesmbauth
Enable SMB
--disablesmbauth
Disable SMB
--smbworkgroup=<workgroup>
Specify SMB workgroup
--smbservers=<server>
Specify SMB servers
--enablewinbind
Enable winbind for user information
by default
--disablewinbind
Disable winbind for user information
by default
--enablewinbindauth
Enable winbindauth for authentication by default
--disablewinbindauth
Disable winbindauth for authentication by default
--smbsecurity=<user|server|domain|ads>
Security mode to use for Samba
and winbind
--smbrealm=<STRING>
Default realm for Samba and winbind when security=ads
--smbidmapuid=<lowest-highest>
UID range winbind assigns to domain or ADS users
--smbidmapgid=<lowest-highest>
GID range winbind assigns to domain or ADS users
398
4. Command Line Version
Option
Description
--winbindseparator=<\>
Character used to separate the domain and user part of winbind usernames if winbindusedefaultdomain is
not enabled
--winbindtemplatehomedir=</home/%D/%U>
Directory that winbind users have as
their home
--winbindtemplateprimarygroup=<nobody>
Group that winbind users have as
their primary group
--winbindtemplateshell=</bin/false>
Shell that winbind users have as
their default login shell
--enablewinbindusedefaultdomain
Configures winbind to assume that
users with no domain in their usernames are domain users
--disablewinbindusedefaultdomain
Configures winbind to assume that
users with no domain in their usernames are not domain users
--winbindjoin=<Administrator>
Joins the winbind domain or ADS
realm now as this administrator
--enablewins
Enable WINS for hostname resolution
--disablewins
Disable WINS for hostname resolution
--enablehesiod
Enable Hesiod
--disablehesiod
Disable Hesiod
--hesiodlhs=<lhs>
Specify Hesiod LHS
--hesiodrhs=<rhs>
Specify Hesiod RHS
--enablecache
Enable nscd
--disablecache
Disable nscd
--nostart
Do not start or stop the portmap, ypbind, or nscd services even if they
are configured
--kickstart
Do not display the user interface
--probe
Probe and display network defaults
Table 25.1. Command Line Options
399
Part IV. System Configuration
Part of a system administrator's job is configuring the system for various tasks, types of users,
and hardware configurations. This section explains how to configure a Red Hat Enterprise Linux
system.
Chapter 26. Console Access
When normal (non-root) users log into a computer locally, they are given two types of special
permissions:
1.
They can run certain programs that they would otherwise be unable to run.
2.
They can access certain files (normally special device files used to access diskettes, CDROMs, and so on) that they would otherwise be unable to access.
Since there are multiple consoles on a single computer and multiple users can be logged into
the computer locally at the same time, one of the users has to essentially win the race to access
the files. The first user to log in at the console owns those files. Once the first user logs out, the
next user who logs in owns the files.
In contrast, every user who logs in at the console is allowed to run programs that accomplish
tasks normally restricted to the root user. If X is running, these actions can be included as menu
items in a graphical user interface. As shipped, these console-accessible programs include
halt, poweroff, and reboot.
1. Disabling Shutdown Via Ctrl-Alt-Del
By default, /etc/inittab specifies that your system is set to shutdown and reboot in response to
a Ctrl-Alt-Del key combination used at the console. To completely disable this ability, comment
out the following line in /etc/inittab by putting a hash mark (#) in front of it:
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
Alternatively, you may want to allow certain non-root users the right to shutdown or reboot the
system from the console using Ctrl-Alt-Del . You can restrict this privilege to certain users, by
taking the following steps:
1.
Add the -a option to the /etc/inittab line shown above, so that it reads:
ca::ctrlaltdel:/sbin/shutdown -a -t3 -r now
The -a flag tells shutdown to look for the /etc/shutdown.allow file.
2.
Create a file named shutdown.allow in /etc. The shutdown.allow file should list the usernames of any users who are allowed to shutdown the system using Ctrl-Alt-Del . The
format of the shutdown.allow file is a list of usernames, one per line, like the following:
stephen jack sophie
According to this example shutdown.allow file, the users stephen, jack, and sophie are allowed
to shutdown the system from the console using Ctrl-Alt-Del . When that key combination is
used, the shutdown -a command in /etc/inittab checks to see if any of the users in /
etc/shutdown.allow (or root) are logged in on a virtual console. If one of them is, the shutdown
of the system continues; if not, an error message is written to the system console instead.
401
2. Disabling Console Program Access
For more information on shutdown.allow, refer to the shutdown man page.
2. Disabling Console Program Access
To disable access by users to console programs, run the following command as root:
rm -f /etc/security/console.apps/*
In environments where the console is otherwise secured (BIOS and boot loader passwords are
set, Ctrl-Alt-Delete is disabled, the power and reset switches are disabled, and so forth), you
may not want to allow any user at the console to run poweroff, halt, and reboot, which are accessible from the console by default.
To disable these abilities, run the following commands as root:
rm -f /etc/security/console.apps/poweroff rm -f /etc/security/console.apps/halt rm -f /etc/security/console
3. Defining the Console
The pam_console.so module uses the /etc/security/console.perms file to determine the permissions for users at the system console. The syntax of the file is very flexible; you can edit the file
so that these instructions no longer apply. However, the default file has a line that looks like this:
<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9]
When users log in, they are attached to some sort of named terminal, which can be either an X
server with a name like :0 or mymachine.example.com:1.0, or a device like /dev/ttyS0 or /
dev/pts/2. The default is to define that local virtual consoles and local X servers are considered
local, but if you want to consider the serial terminal next to you on port /dev/ttyS1 to also be
local, you can change that line to read:
<console>=tty[0-9][0-9]* vc/[0-9][0-9]* :[0-9]\.[0-9] :[0-9] /dev/ttyS1
4. Making Files Accessible From the Console
The default settings for individual device classes and permission definitions are defined in /
etc/security/console.perms.d/50-default.perms. To edit file and device permissions, it is advisable to create a new default file in /etc/security/console.perms.d/ containing your preferred
settings for a specified set of files or devices. The name of the new default file must begin with a
number higher than 50 (for example, 51-default.perms) in order to override 50-default.perms.
To do this, create a new file named 51-default.perms in /etc/security/console.perms.d/:
touch /etc/security/console.perms.d/51-default.perms
Open the original default perms file, 50-default.perms. The first section defines device classes,
with lines similar to the following:
<floppy>=/dev/fd[0-1]* \ /dev/floppy/* /mnt/floppy* <sound>=/dev/dsp* /dev/audio* /dev/midi* \ /dev/mixer*
402
5. Enabling Console Access for Other Applications
Items enclosed in brackets name the device; in the above example, <cdrom> refers to the CDROM drive. To add a new device, do not define it in the default 50-default.perms file; instead,
define it in 51-default.perms. For example, to define a scanner, add the following line to
51-default.perms:
<scanner>=/dev/scanner /dev/usb/scanner*
Of course, you must use the appropriate name for the device. Ensure that /dev/scanner is really
your scanner and not some other device, such as your hard drive.
Once you have properly defined a device or file, the second step is to specify its permission
definitions. The second section of /etc/security/console.perms.d/50-default.perms defines
this, with lines similar to the following:
<console> 0660 <floppy> 0660 root.floppy <console> 0600 <sound> 0640 root <console> 0600 <cdrom> 0600 root.
To define permissions for a scanner, add a line similar to the following in 51-default.perms:
<console> 0600 <scanner> 0600 root
Then, when you log in at the console, you are given ownership of the /dev/scanner device with
the permissions of 0600 (readable and writable by you only). When you log out, the device is
owned by root, and still has the permissions 0600 (now readable and writable by root only).
Warning
You must never edit the default 50-default.perms file. To edit permissions for a
device already defined in 50-default.perms, add the desired permission definition
for that device in 51-default.perms. This will override whatever permissions are
defined in 50-default.perms.
5. Enabling Console Access for Other Applications
To make other applications accessible to console users, a bit more work is required.
First of all, console access only works for applications which reside in /sbin/ or /usr/sbin/, so
the application that you wish to run must be there. After verifying that, perform the following
steps:
1.
Create a link from the name of your application, such as our sample foo program, to the /
usr/bin/consolehelper application:
cd /usr/bin ln -s consolehelper foo
2.
Create the file /etc/security/console.apps/foo:
touch /etc/security/console.apps/foo
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6. The floppy Group
3.
Create a PAM configuration file for the foo service in /etc/pam.d/. An easy way to do this is
to copy the PAM configuration file of the halt service, and then modify the copy if you want
to change the behavior:
cp /etc/pam.d/halt /etc/pam.d/foo
Now, when /usr/bin/foo is executed, consolehelper is called, which authenticates the user with
the help of /usr/sbin/userhelper. To authenticate the user, consolehelper asks for the user's
password if /etc/pam.d/foo is a copy of /etc/pam.d/halt (otherwise, it does precisely what is
specified in /etc/pam.d/foo) and then runs /usr/sbin/foo with root permissions.
In the PAM configuration file, an application can be configured to use the pam_timestamp module to remember (or cache) a successful authentication attempt. When an application is started
and proper authentication is provided (the root password), a timestamp file is created. By default, a successful authentication is cached for five minutes. During this time, any other application that is configured to use pam_timestamp and run from the same session is automatically authenticated for the user — the user does not have to enter the root password again.
This module is included in the pam package. To enable this feature, add the following lines to
your PAM configuration file in etc/pam.d/:
auth include config-util account include config-util session include config-util
These lines can be copied from any of the /etc/pam.d/system-config-* configuration files. Note
that these lines must be added below any other auth sufficientsession optional lines in your
PAM configuration file.
If an application configured to use pam_timestamp is successfully authenticated from the Applications (the main menu on the panel), the
icon is displayed in the notification area of the
panel if you are running the GNOME or KDE desktop environment. After the authentication expires (the default is five minutes), the icon disappears.
The user can select to forget the cached authentication by clicking on the icon and selecting the
option to forget authentication.
6. The
floppy
Group
If, for whatever reason, console access is not appropriate for you and your non-root users require access to your system's diskette drive, this can be done using the floppy group. Add the
user(s) to the floppy group using the tool of your choice. For example, the gpasswd command
can be used to add user fred to the floppy group:
gpasswd -a fred floppy
Now, user fred is able to access the system's diskette drive from the console.
404
Chapter 27. The
sysconfig
Directory
The /etc/sysconfig/ directory contains a variety of system configuration files for Red Hat Enterprise Linux.
This chapter outlines some of the files found in the /etc/sysconfig/ directory, their function, and
their contents. The information in this chapter is not intended to be complete, as many of these
files have a variety of options that are only used in very specific or rare circumstances.
1. Files in the
/etc/sysconfig/
Directory
The following sections offer descriptions of files normally found in the /etc/sysconfig/ directory.
Files not listed here, as well as extra file options, are found in the
/usr/share/doc/initscripts-<version-number>/sysconfig.txt file (replace <version-number>
with the version of the initscripts package). Alternatively, looking through the initscripts in the
/etc/rc.d/ directory can prove helpful.
Note
If some of the files listed here are not present in the /etc/sysconfig/ directory,
then the corresponding program may not be installed.
1.1. /etc/sysconfig/amd
The /etc/sysconfig/amd file contains various parameters used by amd; these parameters allow
for the automatic mounting and unmounting of file systems.
1.2. /etc/sysconfig/apmd
The /etc/sysconfig/apmd file is used by apmd to configure what power settings to start/
stop/change on suspend or resume. This file configures how apmd functions at boot time, depending on whether the hardware supports Advanced Power Management (APM) or whether
the user has configured the system to use it. The apm daemon is a monitoring program that
works with power management code within the Linux kernel. It is capable of alerting users to
low battery power on laptops and other power-related settings.
1.3. /etc/sysconfig/arpwatch
The /etc/sysconfig/arpwatch file is used to pass arguments to the arpwatch daemon at boot
time. The arpwatch daemon maintains a table of Ethernet MAC addresses and their IP address
pairings. By default, this file sets the owner of the arpwatch process to the user pcap and sends
any messages to the root mail queue. For more information regarding available parameters for
this file, refer to the arpwatch man page.
1.4. /etc/sysconfig/authconfig
405
1.5. /etc/sysconfig/autofs
The /etc/sysconfig/authconfig file sets the authorization to be used on the host. It contains
one or more of the following lines:
•
•
•
USEMD5=<value>,
•
yes
•
no
where <value> is one of the following:
— MD5 is used for authentication.
— MD5 is not used for authentication.
USEKERBEROS=<value>,
•
yes
•
no
where <value> is one of the following:
— Kerberos is used for authentication.
— Kerberos is not used for authentication.
USELDAPAUTH=<value>,
•
yes
•
no
where <value> is one of the following:
— LDAP is used for authentication.
— LDAP is not used for authentication.
1.5. /etc/sysconfig/autofs
The /etc/sysconfig/autofs file defines custom options for the automatic mounting of devices.
This file controls the operation of the automount daemons, which automatically mount file systems when you use them and unmount them after a period of inactivity. File systems can include network file systems, CD-ROMs, diskettes, and other media.
The /etc/sysconfig/autofs file may contain the following:
•
LOCALOPTIONS="<value>",
where <value> is a string for defining machine-specific automount
rules. The default value is an empty string ("").
•
DAEMONOPTIONS="<value>",
•
UNDERSCORETODOT=<value>,
•
DISABLE_DIRECT=<value>,
where <value> is the timeout length in seconds before unmounting
the device. The default value is 60 seconds ("--timeout=60").
where <value> is a binary value that controls whether to convert
underscores in file names into dots. For example, auto_home to auto.home and auto_mnt to
auto.mnt. The default value is 1 (true).
where <value> is a binary value that controls whether to disable direct mount support, as the Linux implementation does not conform to the Sun Microsystems'
automounter behavior. The default value is 1 (true), and allows for compatibility with the Sun
automounter options specification syntax.
1.6. /etc/sysconfig/clock
The /etc/sysconfig/clock file controls the interpretation of values read from the system hardware clock.
406
1.7. /etc/sysconfig/desktop
The correct values are:
•
•
UTC=<value>,
•
true
•
false
•
or no — The hardware clock is set to local time.
where <value> is the following:
or no — This value indicates that the normal UNIX epoch is in use. Other values
are used by systems not supported by Red Hat Enterprise Linux.
false
SRM=<value>,
•
•
or yes — The hardware clock is set to Universal Time.
ARC=<value>,
•
where <value> is one of the following boolean values:
where <value> is the following:
or no — This value indicates that the normal UNIX epoch is in use. Other values
are used by systems not supported by Red Hat Enterprise Linux.
false
— The time zone file under /usr/share/zoneinfo that /etc/localtime is a
copy of. The file contains information such as:
ZONE=<filename>
ZONE="America/New York"
Note that the ZONE parameter is read by the Time and Date Properties Tool (system-config-date), and manually editing it does not change the system timezone.
Earlier releases of Red Hat Enterprise Linux used the following values (which are deprecated):
•
CLOCKMODE=<value>,
where <value> is one of the following:
•
GMT
— The clock is set to Universal Time (Greenwich Mean Time).
•
ARC
— The ARC console's 42-year time offset is in effect (for Alpha-based systems only).
1.7. /etc/sysconfig/desktop
The /etc/sysconfig/desktop file specifies the desktop for new users and the display manager to
run when entering runlevel 5.
Correct values are:
•
•
DESKTOP="<value>",
•
GNOME
•
KDE
where "<value>" is one of the following:
— Selects the GNOME desktop environment.
— Selects the KDE desktop environment.
DISPLAYMANAGER="<value>",
where "<value>" is one of the following:
407
1.8. /etc/sysconfig/dhcpd
•
GNOME
— Selects the GNOME Display Manager.
•
KDE
— Selects the KDE Display Manager.
•
XDM
— Selects the X Display Manager.
For more information, refer to Chapter 30, The X Window System.
1.8. /etc/sysconfig/dhcpd
The /etc/sysconfig/dhcpd file is used to pass arguments to the dhcpd daemon at boot time. The
dhcpd daemon implements the Dynamic Host Configuration Protocol (DHCP) and the Internet
Bootstrap Protocol (BOOTP). DHCP and BOOTP assign hostnames to machines on the network. For more information about what parameters are available in this file, refer to the dhcpd
man page.
1.9. /etc/sysconfig/exim
The /etc/sysconfig/exim file allows messages to be sent to one or more clients, routing the
messages over whatever networks are necessary. The file sets the default values for exim to
run. Its default values are set to run as a background daemon and to check its queue each hour
in case something has backed up.
The values include:
•
•
DAEMON=<value>,
•
yes
•
no
where <value> is one of the following:
— exim should be configured to listen to port 25 for incoming mail. yes implies the use
of the Exim's -bd options.
— exim should not be configured to listen to port 25 for incoming mail.
which is given to exim as -q$QUEUE. The -q option is not given to exim if /
etc/sysconfig/exim exists and QUEUE is empty or undefined.
QUEUE=1h
1.10. /etc/sysconfig/firstboot
The first time the system boots, the /sbin/init program calls the etc/rc.d/init.d/firstboot
script, which in turn launches the Setup Agent. This application allows the user to install the
latest updates as well as additional applications and documentation.
The /etc/sysconfig/firstboot file tells the Setup Agent application not to run on subsequent
reboots. To run it the next time the system boots, remove /etc/sysconfig/firstboot and execute chkconfig --level 5 firstboot on.
1.11. /etc/sysconfig/gpm
The /etc/sysconfig/gpm file is used to pass arguments to the gpm daemon at boot time. The gpm
daemon is the mouse server which allows mouse acceleration and middle-click pasting. For
more information about what parameters are available for this file, refer to the gpm man page. By
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1.12. /etc/sysconfig/hwconf
default, the DEVICE directive is set to /dev/input/mice.
1.12. /etc/sysconfig/hwconf
The /etc/sysconfig/hwconf file lists all the hardware that kudzu detected on the system, as well
as the drivers used, vendor ID, and device ID information. The kudzu program detects and configures new and/or changed hardware on a system. The /etc/sysconfig/hwconf file is not meant
to be manually edited. If edited, devices could suddenly show up as being added or removed.
1.13. /etc/sysconfig/i18n
The /etc/sysconfig/i18n file sets the default language, any supported languages, and the default system font. For example:
LANG="en_US.UTF-8"
SUPPORTED="en_US.UTF-8:en_US:en"
SYSFONT="latarcyrheb-sun16"
1.14. /etc/sysconfig/init
The /etc/sysconfig/init file controls how the system appears and functions during the boot
process.
The following values may be used:
•
BOOTUP=<value>,
where <value> is one of the following:
•
color
— The standard color boot display, where the success or failure of devices and
services starting up is shown in different colors.
•
verbose
•
Anything else means a new display, but without ANSI-formatting.
— An old style display which provides more information than purely a message
of success or failure.
•
RES_COL=<value>,
where <value> is the number of the column of the screen to start status labels. The default is set to 60.
•
MOVE_TO_COL=<value>,
echo -en
where <value> moves the cursor to the value in the RES_COL line via the
command.
•
SETCOLOR_SUCCESS=<value>,
where <value> sets the success color via the echo
The default color is set to green.
•
SETCOLOR_FAILURE=<value>,
•
SETCOLOR_WARNING=<value>,
•
SETCOLOR_NORMAL=<value>,
•
LOGLEVEL=<value>,
where <value> sets the failure color via the echo
The default color is set to red.
-en
-en
where <value> sets the warning color via the echo
The default color is set to yellow.
command.
command.
-en
where <value> resets the color to "normal" via the echo
command.
-en.
where <value> sets the initial console logging level for the kernel. The de-
409
1.15. /etc/sysconfig/ip6tables-config
fault is 3; 8 means everything (including debugging), while 1 means only kernel panics. The
syslogd daemon overrides this setting once started.
•
PROMPT=<value>,
•
yes
•
no
where <value> is one of the following boolean values:
— Enables the key check for interactive mode.
— Disables the key check for interactive mode.
1.15. /etc/sysconfig/ip6tables-config
The /etc/sysconfig/ip6tables-config file stores information used by the kernel to set up IPv6
packet filtering at boot time or whenever the ip6tables service is started.
Do not modify this file by hand unless familiar with how to construct ip6tables rules. Rules also
can be created manually using the /sbin/ip6tables command. Once created, add the rules to
the /etc/sysconfig/ip6tables file by typing the following command:
/sbin/service ip6tables save
Once this file exists, any firewall rules saved in it persists through a system reboot or a service
restart.
For more information on ip6tables, refer to Section 9, “IPTables”.
1.16. /etc/sysconfig/iptables-config
The /etc/sysconfig/iptables-config file stores information used by the kernel to set up packet
filtering services at boot time or whenever the service is started.
Do not modify this file by hand unless you are familiar with constructing iptables rules. The
easiest way to add rules is to use the Security Level Configuration Tool (system-config-selinux) application to create a firewall. These applications automatically edit this file at the
end of the process.
Rules can also be created manually using the /sbin/iptables command. Once created, add the
rule(s) to the /etc/sysconfig/iptables file by typing the following command:
/sbin/service iptables save
Once this file exists, any firewall rules saved in it persists through a system reboot or a service
restart.
For more information on iptables, refer to Section 9, “IPTables”.
1.17. /etc/sysconfig/irda
The /etc/sysconfig/irda file controls how infrared devices on the system are configured at startup.
The following values may be used:
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1.18. /etc/sysconfig/keyboard
•
IRDA=<value>,
•
yes
•
no
where <value> is one of the following boolean values:
— irattach runs and periodically checks to see if anything is trying to connect to the
infrared port, such as another notebook computer trying to make a network connection.
For infrared devices to work on the system, this line must be set to yes.
— irattach does not run, preventing infrared device communication.
•
DEVICE=<value>,
where <value> is the device (usually a serial port) that handles infrared connections. A sample serial device entry could be /dev/ttyS2.
•
DONGLE=<value>,
•
DISCOVERY=<value>,
where <value> specifies the type of dongle being used for infrared communication. This setting exists for people who use serial dongles rather than real infrared ports.
A dongle is a device that is attached to a traditional serial port to communicate via infrared.
This line is commented out by default because notebooks with real infrared ports are far
more common than computers with add-on dongles. A sample dongle entry could be actisys+.
•
yes
•
no
where <value> is one of the following boolean values:
— Starts irattach in discovery mode, meaning it actively checks for other infrared
devices. This must be turned on for the machine to actively look for an infrared connection (meaning the peer that does not initiate the connection).
— Does not start irattach in discovery mode.
1.18. /etc/sysconfig/keyboard
The /etc/sysconfig/keyboard file controls the behavior of the keyboard. The following values
may be used:
•
KEYBOARDTYPE="sun|pc"
where sun means a Sun keyboard is attached on /dev/kbd, or pc
means a PS/2 keyboard connected to a PS/2 port.
•
KEYTABLE="<file>",
where <file> is the name of a keytable file.
For example: KEYTABLE="us". The files that can be used as keytables start in /
lib/kbd/keymaps/i386 and branch into different keyboard layouts from there, all labeled
<file>.kmap.gz. The first file found beneath /lib/kbd/keymaps/i386 that matches the KEYTABLE setting is used.
1.19. /etc/sysconfig/kudzu
The /etc/sysconfig/kuzdu file triggers a safe probe of the system hardware by kudzu at boot
time. A safe probe is one that disables serial port probing.
•
SAFE=<value>,
•
yes
where <value> is one of the following:
— kuzdu does a safe probe.
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1.20. /etc/sysconfig/named
•
no
— kuzdu does a normal probe.
1.20. /etc/sysconfig/named
The /etc/sysconfig/named file is used to pass arguments to the named daemon at boot time. The
named daemon is a Domain Name System (DNS) server which implements the Berkeley Internet
Name Domain (BIND) version 9 distribution. This server maintains a table of which hostnames
are associated with IP addresses on the network.
Currently, only the following values may be used:
•
ROOTDIR="</some/where>",
where </some/where> refers to the full directory path of a configured chroot environment under which named runs. This chroot environment must first be
configured. Type info chroot for more information.
•
OPTIONS="<value>",
where <value> is any option listed in the man page for named except -t. In
place of -t, use the ROOTDIR line above.
For more information about available parameters for this file, refer to the named man page. For
detailed information on how to configure a BIND DNS server, refer to Chapter 16, Berkeley Internet Name Domain (BIND). By default, the file contains no parameters.
1.21. /etc/sysconfig/netdump
The /etc/sysconfig/netdump file is the configuration file for the /etc/init.d/netdump service.
The netdump service sends both oops data and memory dumps over the network. In general,
netdump is not a required service; only run it if absolutely necessary. For more information about
what parameters are available for this file, refer to the netdump man page.
1.22. /etc/sysconfig/network
The /etc/sysconfig/network file is used to specify information about the desired network configuration. The following values may be used:
•
NETWORKING=<value>,
•
yes
•
no
where <value> is one of the following boolean values:
— Networking should be configured.
— Networking should not be configured.
•
HOSTNAME=<value>,
where <value> should be the Fully Qualified Domain Name (FQDN), such
as hostname.expample.com, but can be whatever hostname is necessary.
•
GATEWAY=<value>,
•
GATEWAYDEV=<value>,
•
NISDOMAIN=<value>,
where <value> is the IP address of the network's gateway.
where <value> is the gateway device, such as eth0.
where <value> is the NIS domain name.
412
1.24. /etc/sysconfig/radvd
1.23. /etc/sysconfig/ntpd
The /etc/sysconfig/ntpd file is used to pass arguments to the ntpd daemon at boot time. The
ntpd daemon sets and maintains the system clock to synchronize with an Internet standard time
server. It implements version 4 of the Network Time Protocol (NTP). For more information about
what parameters are available for this file, use a Web browser to view the following file: /
usr/share/doc/ntp-<version>/ntpd.htm (where <version> is the version number of ntpd). By default, this file sets the owner of the ntpd process to the user ntp.
1.24. /etc/sysconfig/radvd
The /etc/sysconfig/radvd file is used to pass arguments to the radvd daemon at boot time. The
radvd daemon listens for router requests and sends router advertisements for the IP version 6
protocol. This service allows hosts on a network to dynamically change their default routers
based on these router advertisements. For more information about available parameters for this
file, refer to the radvd man page. By default, this file sets the owner of the radvd process to the
user radvd.
1.25. /etc/sysconfig/samba
The /etc/sysconfig/samba file is used to pass arguments to the smbd and the nmbd daemons at
boot time. The smbd daemon offers file sharing connectivity for Windows clients on the network.
The nmbd daemon offers NetBIOS over IP naming services. For more information about what
parameters are available for this file, refer to the smbd man page. By default, this file sets smbd
and nmbd to run in daemon mode.
1.26. /etc/sysconfig/selinux
The /etc/sysconfig/selinux file contains the basic configuration options for SELinux. This file is
a symbolic link to /etc/selinux/config.
1.27. /etc/sysconfig/sendmail
The /etc/sysconfig/sendmail file allows messages to be sent to one or more clients, routing the
messages over whatever networks are necessary. The file sets the default values for the Sendmail application to run. Its default values are set to run as a background daemon and to check
its queue each hour in case something has backed up.
Values include:
•
•
DAEMON=<value>,
•
yes
•
no
where <value> is one of the following:
— Sendmail should be configured to listen to port 25 for incoming mail. yes implies
the use of Sendmail's -bd options.
— Sendmail should not be configured to listen to port 25 for incoming mail.
which is given to Sendmail as -q$QUEUE. The -q option is not given to Sendmail if /
etc/sysconfig/sendmail exists and QUEUE is empty or undefined.
QUEUE=1h
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1.29. /etc/sysconfig/squid
1.28. /etc/sysconfig/spamassassin
The /etc/sysconfig/spamassassin file is used to pass arguments to the spamd daemon (a daemonized version of Spamassassin) at boot time. Spamassassin is an email spam filter application. For a list of available options, refer to the spamd man page. By default, it configures spamd
to run in daemon mode, create user preferences, and auto-create whitelists (allowed bulk
senders).
For more information about Spamassassin, refer to Section 5.2.6, “Spam Filters”.
1.29. /etc/sysconfig/squid
The /etc/sysconfig/squid file is used to pass arguments to the squid daemon at boot time. The
squid daemon is a proxy caching server for Web client applications. For more information on
configuring a squid proxy server, use a Web browser to open the
/usr/share/doc/squid-<version>/ directory (replace <version> with the squid version number installed on the system). By default, this file sets squid to start in daemon mode and sets the
amount of time before it shuts itself down.
1.30. /etc/sysconfig/system-config-selinux
The /etc/sysconfig/system-config-selinux file contains all options chosen by the user the last
time the Security Level Configuration Tool (system-config-selinux) was run. Users should
not modify this file by hand. For more information about the Security Level Configuration
Tool, refer to Section 8.2, “Basic Firewall Configuration”.
1.31. /etc/sysconfig/system-config-users
The /etc/sysconfig/system-config-users file is the configuration file for the graphical application, User Manager. This file is used to filter out system users such as root, daemon, or lp. This
file is edited by the Preferences => Filter system users and groups pull-down menu in the
User Manager application and should never be edited by hand. For more information on using
this application, refer to Section 1, “User and Group Configuration”.
1.32. /etc/sysconfig/system-logviewer
The /etc/sysconfig/system-logviewer file is the configuration file for the graphical, interactive
log viewing application, Log Viewer. This file is edited by the Edit => Preferences pull-down
menu in the Log Viewer application and should not be edited by hand. For more information on
using this application, refer to Chapter 35, Log Files.
1.33. /etc/sysconfig/tux
The /etc/sysconfig/tux file is the configuration file for the Red Hat Content Accelerator
(formerly known as TUX), the kernel-based Web server. For more information on configuring the
Red Hat Content Accelerator, use a Web browser to open the /usr/share/doc/tux-<version>/
tux/index.html file (replace <version> with the version number of TUX installed on the system).
The parameters available for this file are listed in /usr/share/doc/tux-<version>/
tux/parameters.html.
414
1.35. /etc/sysconfig/xinetd
1.34. /etc/sysconfig/vncservers
The /etc/sysconfig/vncservers file configures the way the Virtual Network Computing (VNC)
server starts up.
VNC is a remote display system which allows users to view the desktop environment not only
on the machine where it is running but across different networks on a variety of architectures.
It may contain the following:
•
VNCSERVERS=<value>,
where <value> is set to something like "1:fred", to indicate that a VNC
server should be started for user fred on display :1. User fred must have set a VNC password using the vncpasswd command before attempting to connect to the remote VNC server.
1.35. /etc/sysconfig/xinetd
The /etc/sysconfig/xinetd file is used to pass arguments to the xinetd daemon at boot time.
The xinetd daemon starts programs that provide Internet services when a request to the port for
that service is received. For more information about available parameters for this file, refer to
the xinetd man page. For more information on the xinetd service, refer to Section 5.3, “xinetd”.
2. Directories in the
/etc/sysconfig/
Directory
The following directories are normally found in /etc/sysconfig/.
apm-scripts/
This directory contains the APM suspend/resume script. Do not edit the files directly. If customization is necessary, create a file called /etc/sysconfig/apm-scripts/apmcontinue which
is called at the end of the script. It is also possible to control the script by editing /
etc/sysconfig/apmd.
cbq/
This directory contains the configuration files needed to do Class Based Queuing for bandwidth management on network interfaces. CBQ divides user traffic into a hierarchy of
classes based on any combination of IP addresses, protocols, and application types.
networking/
This directory is used by the Network Administration Tool (system-config-network), and
its contents should not be edited manually. For more information about configuring network
interfaces using the Network Administration Tool, refer to Chapter 14, Network Configuration.
network-scripts/
This directory contains the following network-related configuration files:
•
Network configuration files for each configured network interface, such as ifcfg-eth0 for
the eth0 Ethernet interface.
•
Scripts used to bring network interfaces up and down, such as ifup and ifdown.
415
3. Additional Resources
•
Scripts used to bring ISDN interfaces up and down, such as ifup-isdn and ifdown-isdn.
•
Various shared network function scripts which should not be edited directly.
For more information on the network-scripts directory, refer to Chapter 13, Network Interfaces.
rhn/
This directory contains the configuration files and GPG keys for Red Hat Network. No files
in this directory should be edited by hand. For more information on Red Hat Network, refer
to the Red Hat Network website online at https://rhn.redhat.com/.
3. Additional Resources
This chapter is only intended as an introduction to the files in the /etc/sysconfig/ directory. The
following source contains more comprehensive information.
3.1. Installed Documentation
•
— This file contains a more
authoritative listing of the files found in the /etc/sysconfig/ directory and the configuration
options available for them. The <version-number> in the path to this file corresponds to the
version of the initscripts package installed.
/usr/share/doc/initscripts-<version-number>/sysconfig.txt
416
Chapter 28. Date and Time
Configuration
The Time and Date Properties Tool allows the user to change the system date and time, to
configure the time zone used by the system, and to setup the Network Time Protocol (NTP)
daemon to synchronize the system clock with a time server.
You must be running the X Window System and have root privileges to use the tool. There are
three ways to start the application:
•
From the desktop, go to Applications (the main menu on the panel) => System Settings =>
Date & Time
•
From the desktop, right-click on the time in the toolbar and select Adjust Date and Time.
•
Type the command system-config-date, system-config-time, or dateconfig at a shell prompt
(for example, in an XTerm or a GNOME terminal).
1. Time and Date Properties
As shown in Figure 28.1, “Time and Date Properties”, the first tabbed window that appears is for
configuring the system date and time.
417
2. Network Time Protocol (NTP) Properties
Figure 28.1. Time and Date Properties
To change the date, use the arrows to the left and right of the month to change the month, use
the arrows to the left and right of the year to change the year, and click on the day of the week
to change the day of the week.
To change the time, use the up and down arrow buttons beside the Hour, Minute, and Second
in the Time section.
Clicking the OK button applies any changes made to the date and time, the NTP daemon settings, and the time zone settings. It also exits the program.
2. Network Time Protocol (NTP) Properties
418
2. Network Time Protocol (NTP) Properties
As shown in Figure 28.2, “NTP Properties”, the second tabbed window that appears is for configuring NTP.
Figure 28.2. NTP Properties
The Network Time Protocol (NTP) daemon synchronizes the system clock with a remote time
server or time source. The application allows you to configure an NTP daemon to synchronize
your system clock with a remote server. To enable this feature, select Enable Network Time
Protocol. This enables the NTP Servers list and other options. You can choose one of the predefined servers, edit a predefined server by clicking the Edit or add a new server name by clicking Add. Your system does not start synchronizing with the NTP server until you click OK. After
clicking OK, the configuration is saved and the NTP daemon is started (or restarted if it is
already running).
Clicking the OK button applies any changes made to the date and time, the NTP daemon set419
3. Time Zone Configuration
tings, and the time zone settings. It also exits the program.
3. Time Zone Configuration
As shown in Figure 28.3, “Timezone Properties”, the third tabbed window that appears is for
configuring the system time zone.
To configure the system time zone, click the Time Zone tab. The time zone can be changed by
either using the interactive map or by choosing the desired time zone from the list below the
map. To use the map, click on the desired region. The map zooms into the region selected,
after which you may choose the city specific to your time zone. A red X appears and the time
zone selection changes in the list below the map.
Alternatively, you can also use the list below the map. In the same way that the map lets you
choose a region before choosing a city, the list of time zones is now a treelist, with cities and
countries grouped within their specific continents. Non-geographic time zones have also been
added to address needs in the scientific community.
Click OK to apply the changes and exit the program.
420
3. Time Zone Configuration
Figure 28.3. Timezone Properties
If your system clock is set to use UTC, select the System clock uses UTC option. UTC stands
for the Universal Time, Coordinated, also known as Greenwich Mean Time (GMT). Other time
zones are determined by adding or subtracting from the UTC time.
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Chapter 29. Keyboard Configuration
The installation program allows you to configure a keyboard layout for your system. To configure a different keyboard layout after installation, use the Keyboard Configuration Tool.
To start the Keyboard Configuration Tool, select System (on the panel) => Administration
=> Keyboard, or type the command system-config-keyboard at a shell prompt.
Figure 29.1. Keyboard Configuration Tool
Select a keyboard layout from the list (for example, U.S. English) and click OK.
Changes take effect immediately.
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Chapter 30. The X Window System
While the heart of Red Hat Enterprise Linux is the kernel, for many users, the face of the operating system is the graphical environment provided by the X Window System, also called X.
Other windowing environments have existed in the UNIX world, including some that predate the
release of the X Window System in June 1984. Nonetheless, X has been the default graphical
environment for most UNIX-like operating systems, including Red Hat Enterprise Linux, for
many years.
The graphical environment for Red Hat Enterprise Linux is supplied by the X.Org Foundation,
an open source organization created to manage development and strategy for the X Window
System and related technologies. X.Org is a large-scale, rapidly developing project with hundreds of developers around the world. It features a wide degree of support for a variety of hardware devices and architectures, and can run on a variety of different operating systems and
platforms. This release for Red Hat Enterprise Linux specifically includes the X11R7.1 release
of the X Window System.
The X Window System uses a client-server architecture. The X server (the Xorg binary) listens
for connections from X client applications via a network or local loopback interface. The server
communicates with the hardware, such as the video card, monitor, keyboard, and mouse. X client applications exist in the user-space, creating a graphical user interface (GUI) for the user
and passing user requests to the X server.
1. The X11R7.1 Release
Red Hat Enterprise Linux 5.0.0 now uses the X11R7.1 release as the base X Window System,
which includes several video driver, EXA, and platform support enhancements over the previous
release, among others. In addition, this release also includes several automatic configuration
features for the X server.
X11R7.1 is the first release to take specific advantage of the modularisation of the X Window
System. This modularisaton, which splits X into logically distinct modules, makes it easier for
open source developers to contribute code to the system.
Important
Red Hat Enterprise Linux no longer provides the XFree86™ server packages. Before upgrading a system to the latest version of Red Hat Enterprise Linux, be sure
that the system's video card is compatible with the X11R7.1 release by checking
the Red Hat Hardware Compatibility List located online at http://hardware.redhat.com/.
In the X11R7.1 release, all libraries, headers, and binaries now live under /usr/ instead of /
usr/X11R6. The /etc/X11/ directory contains configuration files for X client and server applications. This includes configuration files for the X server itself, the xfs font server, the X display
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2. Desktop Environments and Window Managers
managers, and many other base components.
The configuration file for the newer Fontconfig-based font architecture is still /
etc/fonts/fonts.conf. For more on configuring and adding fonts, refer to Section 4, “Fonts”.
Because the X server performs advanced tasks on a wide array of hardware, it requires detailed
information about the hardware it works on. The X server automatically detects some of this information; other details must be configured.
The installation program installs and configures X automatically, unless the X11R7.1 release
packages are not selected for installation. However, if there are any changes to the monitor,
video card or other devices managed by the X server, X must be reconfigured. The best way to
do this is to use the &RHXFREE86TOOL; (system-config-display), particularly for devices that
are not detected manually.
In Red Hat Enterprise Linux's default graphical environment, the &RHXFREE86TOOL; is available at System (on the panel) => Administration => Display.
Changes made with the &RHXFREE86TOOL; take effect after logging out and logging back in.
For more information about &RHXFREE86TOOL;, refer to Chapter 31, X Window System Configuration.
In some situations, reconfiguring the X server may require manually editing its configuration file,
/etc/X11/xorg.conf. For information about the structure of this file, refer to Section 3, “X Server
Configuration Files”.
2. Desktop Environments and Window Managers
Once an X server is running, X client applications can connect to it and create a GUI for the
user. A range of GUIs are possible with Red Hat Enterprise Linux, from the rudimentary Tab
Window Manager to the highly developed and interactive GNOME desktop environment that
most Red Hat Enterprise Linux users are familiar with.
To create the latter, more comprehensive GUI, two main classes of X client application must
connect to the X server: a desktop environment and a window manager.
2.1. Desktop Environments
A desktop environment integrates various X clients to create a common graphical user environment and development platform.
Desktop environments have advanced features allowing X clients and other running processes
to communicate with one another, while also allowing all applications written to work in that environment to perform advanced tasks, such as drag and drop operations.
Red Hat Enterprise Linux provides two desktop environments:
•
GNOME — The default desktop environment for Red Hat Enterprise Linux based on the
GTK+ 2 graphical toolkit.
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2.2. Window Managers
•
KDE — An alternative desktop environment based on the Qt 3 graphical toolkit.
Both GNOME and KDE have advanced productivity applications, such as word processors,
spreadsheets, and Web browsers; both also provide tools to customize the look and feel of the
GUI. Additionally, if both the GTK+ 2 and the Qt libraries are present, KDE applications can run
in GNOME and vice-versa.
2.2. Window Managers
Window managers are X client programs which are either part of a desktop environment or, in
some cases, stand-alone. Their primary purpose is to control the way graphical windows are positioned, resized, or moved. Window managers also control title bars, window focus behavior,
and user-specified key and mouse button bindings.
Four window managers are included with Red Hat Enterprise Linux:
kwin
The KWin window manager is the default window manager for KDE. It is an efficient window
manager which supports custom themes.
metacity
The Metacity window manager is the default window manager for GNOME. It is a simple
and efficient window manager which also supports custom themes. To run this window
manager, you need to install the metacity package.
mwm
The Motif Window Manager (mwm) is a basic, stand-alone window manager. Since it is designed to be a stand-alone window manager, it should not be used in conjunction with
GNOME or KDE. To run this window manager, you need to install the openmotif package.
twm
The minimalist Tab Window Manager (twm, which provides the most basic tool set of any of
the window managers, can be used either as a stand-alone or with a desktop environment.
It is installed as part of the X11R7.1 release.
To run any of the aforementioned window managers, you will first need to boot into Runlevel 3.
For instructions on how to do this, refer to Section 1, “Runlevels”.
Once you are logged in to Runlevel 3, you will be presented with a terminal prompt, not a graphical environment. To start a window manager, type xinit -e <path-to-window-manager> at the
prompt.
is the location of the window manager binary file. The binary file can
be located by typing which window-manager-name, where window-manager-name is the name of the
window manager you want to run.
<path-to-window-manager>
For example:
[email protected]# which twm/usr/bin/twm
[email protected]# xinit -e /usr/bin/twm
The first command above returns the absolute path to the twm window manager, the second
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3. X Server Configuration Files
command starts twm.
To exit a window manager, close the last window or press Ctrl-Alt-Backspace. Once you have
exited the window manager, you can log back into Runlevel 5 by typing startx at the prompt.
3. X Server Configuration Files
The X server is a single binary executable (/usr/bin/Xorg). Associated configuration files are
stored in the /etc/X11/ directory (as is a symbolic link — X — which points to /usr/bin/Xorg).
The configuration file for the X server is /etc/X11/xorg.conf.
The directory /usr/lib/xorg/modules/ contains X server modules that can be loaded dynamically at runtime. By default, only some modules in /usr/lib/xorg/modules/ are automatically
loaded by the X server.
To load optional modules, they must be specified in the X server configuration file, /
etc/X11/xorg.conf. For more information about loading modules, refer to Section 3.1.5,
“Module”.
When Red Hat Enterprise Linux 5.0.0 is installed, the configuration files for X are created using
information gathered about the system hardware during the installation process.
3.1. xorg.conf
While there is rarely a need to manually edit the /etc/X11/xorg.conf file, it is useful to understand the various sections and optional parameters available, especially when troubleshooting.
3.1.1. The Structure
The /etc/X11/xorg.conf file is comprised of many different sections which address specific aspects of the system hardware.
Each section begins with a Section "<section-name>" line (where <section-name> is the title for
the section) and ends with an EndSection line. Each section contains lines that include option
names and one or more option values. These are sometimes enclosed in double quotes (").
Lines beginning with a hash mark (#) are not read by the X server and are used for humanreadable comments.
Some options within the /etc/X11/xorg.conf file accept a boolean switch which turns the feature
on or off. Acceptable boolean values are:
•
1, on, true,
or yes — Turns the option on.
•
0, off, false,
or no — Turns the option off.
The following are some of the more important sections in the order in which they appear in a
typical /etc/X11/xorg.conf file. More detailed information about the X server configuration file
can be found in the xorg.conf man page.
3.1.2. ServerFlags
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3.1. xorg.conf
The optional ServerFlags section contains miscellaneous global X server settings. Any settings
in this section may be overridden by options placed in the ServerLayout section (refer to Section 3.1.3, “ServerLayout” for details).
Each entry within the ServerFlags section is on its own line and begins with the term Option followed by an option enclosed in double quotation marks (").
The following is a sample ServerFlags section:
Section "ServerFlags" Option "DontZap" "true" EndSection
The following lists some of the most useful options:
•
"DontZap" "<boolean>"
— When the value of <boolean> is set to true, this setting prevents
the use of the Ctrl-Alt-Backspace key combination to immediately terminate the X server.
•
"DontZoom" "<boolean>"
— When the value of <boolean> is set to true, this setting prevents
cycling through configured video resolutions using the Ctrl-Alt-Keypad-Plus and
Ctrl-Alt-Keypad-Minus key combinations.
3.1.3. ServerLayout
The ServerLayout section binds together the input and output devices controlled by the X server.
At a minimum, this section must specify one output device and one input device. By default, a
monitor (output device) and keyboard (input device) are specified.
The following example illustrates a typical ServerLayout section:
Section "ServerLayout" Identifier "Default Layout" Screen 0 "Screen0" 0 0 InputDevice "Mouse0" "CorePointer
The following entries are commonly used in the ServerLayout section:
•
Identifier
•
Screen
— Specifies a unique name for this ServerLayout section.
— Specifies the name of a Screen section to be used with the X server. More than
one Screen option may be present.
The following is an example of a typical Screen entry:
Screen 0 "Screen0" 0 0
The first number in this example Screen entry (0) indicates that the first monitor connector or
head on the video card uses the configuration specified in the Screen section with the identifier "Screen0".
An example of a Screen section with the identifier "Screen0" can be found in Section 3.1.9,
“Screen”.
If the video card has more than one head, another Screen entry with a different number and
a different Screen section identifier is necessary .
The numbers to the right of "Screen0" give the absolute X and Y coordinates for the upperleft corner of the screen (0 0 by default).
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3.1. xorg.conf
•
InputDevice
— Specifies the name of an InputDevice section to be used with the X server.
It is advisable that there be at least two InputDevice entries: one for the default mouse and
one for the default keyboard. The options CorePointer and CoreKeyboard indicate that these
are the primary mouse and keyboard.
•
— An optional entry which specifies extra parameters for the section. Any options listed here override those listed in the ServerFlags section.
Option "<option-name>"
Replace <option-name> with a valid option listed for this section in the xorg.conf man page.
It is possible to put more than one ServerLayout section in the /etc/X11/xorg.conf file. By default, the server only reads the first one it encounters, however.
If there is an alternative ServerLayout section, it can be specified as a command line argument
when starting an X session.
3.1.4. Files
The Files section sets paths for services vital to the X server, such as the font path. This is an
optional section, these paths are normally detected automatically. This section may be used to
override any automatically detected defaults.
The following example illustrates a typical Files section:
Section "Files" RgbPath "/usr/share/X11/rgb.txt" FontPath "unix/:7100" EndSection
The following entries are commonly used in the Files section:
•
RgbPath
— Specifies the location of the RGB color database. This database defines all valid
color names in X and ties them to specific RGB values.
•
FontPath
— Specifies where the X server must connect to obtain fonts from the xfs font serv-
er.
By default, the FontPath is unix/:7100. This tells the X server to obtain font information using
UNIX-domain sockets for inter-process communication (IPC) on port 7100.
Refer to Section 4, “Fonts” for more information concerning X and fonts.
•
— An optional parameter which specifies alternate directories which store X
server modules.
ModulePath
3.1.5. Module
By default, the X server automatically loads the following modules from the /
usr/lib/xorg/modules/ directory:
•
extmod
•
dbe
•
glx
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3.1. xorg.conf
•
freetype
•
type1
•
record
•
dri
The default directory for loading these modules can be changed by specifying a different directory with the optional ModulePath parameter in the Files section. Refer to Section 3.1.4, “Files”
for more information on this section.
Adding a Module section to /etc/X11/xorg.conf instructs the X server to load the modules listed
in this section instead of the default modules.
For example, the following typical Module section:
Section "Module" Load "fbdevhw" EndSection
instructs the X server to load the fbdevhw instead of the default modules.
As such, if you add a Module section to /etc/X11/xorg.conf, you will need to specify any default
modules you want to load as well as any extra modules.
3.1.6. InputDevice
Each InputDevice section configures one input device for the X server. Systems typically have
at least one InputDevice section for the keyboard. It is perfectly normal to have no entry for a
mouse, as most mouse settings are automatically detected.
The following example illustrates a typical InputDevice section for a keyboard:
Section "InputDevice" Identifier "Keyboard0" Driver "kbd" Option "XkbModel" "pc105" Option "XkbLayout" "us"
The following entries are commonly used in the InputDevice section:
•
Identifier
— Specifies a unique name for this InputDevice section. This is a required entry.
•
Driver
— Specifies the name of the device driver X must load for the device.
•
Option
— Specifies necessary options pertaining to the device.
A mouse may also be specified to override any autodetected defaults for the device. The following options are typically included when adding a mouse in the xorg.conf:
•
Protocol
— Specifies the protocol used by the mouse, such as IMPS/2.
•
Device
•
Emulate3Buttons
— Specifies the location of the physical device.
— Specifies whether to allow a two-button mouse to act like a threebutton mouse when both mouse buttons are pressed simultaneously.
Consult the xorg.conf man page for a list of valid options for this section.
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3.1. xorg.conf
3.1.7. Monitor
Each Monitor section configures one type of monitor used by the system. This is an optional
entry as well, as most monitors are now automatically detected.
The easiest way to configure a monitor is to configure X during the installation process or by using the &RHXFREE86TOOL;. For more information about using the &RHXFREE86TOOL;,
refer to Chapter 31, X Window System Configuration.
This example illustrates a typical Monitor section for a monitor:
Section "Monitor" Identifier "Monitor0" VendorName "Monitor Vendor" ModelName "DDC Probed Monitor - ViewSon
Warning
Be careful when manually editing values in the Monitor section of /
etc/X11/xorg.conf. Inappropriate values can damage or destroy a monitor. Consult
the monitor's documentation for a listing of safe operating parameters.
The following are commonly entries used in the Monitor section:
•
Identifier
— Specifies a unique name for this Monitor section. This is a required entry.
•
VendorName
— An optional parameter which specifies the vendor of the monitor.
•
ModelName
•
DisplaySize
•
HorizSync
•
VertRefresh
•
Modeline
•
Option "<option-name>"
— An optional parameter which specifies the monitor's model name.
— An optional parameter which specifies, in millimeters, the physical size of the
monitor's picture area.
— Specifies the range of horizontal sync frequencies compatible with the monitor
in kHz. These values help the X server determine the validity of built-in or specified Modeline
entries for the monitor.
— Specifies the range of vertical refresh frequencies supported by the monitor,
in kHz. These values help the X server determine the validity of built in or specified Modeline
entries for the monitor.
— An optional parameter which specifies additional video modes for the monitor at
particular resolutions, with certain horizontal sync and vertical refresh resolutions. Refer to
the xorg.conf man page for a more detailed explanation of Modeline entries.
— An optional entry which specifies extra parameters for the section. Replace <option-name> with a valid option listed for this section in the xorg.conf man
page.
3.1.8. Device
Each Device section configures one video card on the system. While one Device section is the
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3.1. xorg.conf
minimum, additional instances may occur for each video card installed on the machine.
The best way to configure a video card is to configure X during the installation process or by using the &RHXFREE86TOOL;. For more about using the &RHXFREE86TOOL;, refer to
Chapter 31, X Window System Configuration.
The following example illustrates a typical Device section for a video card:
Section "Device" Identifier "Videocard0" Driver "mga" VendorName "Videocard vendor" BoardName "Matrox Mille
The following entries are commonly used in the Device section:
•
Identifier
•
Driver
•
VendorName
•
BoardName
•
VideoRam
•
BusID
•
Screen
— Specifies a unique name for this Device section. This is a required entry.
— Specifies which driver the X server must load to utilize the video card. A list of
drivers can be found in /usr/share/hwdata/videodrivers, which is installed with the hwdata
package.
— An optional parameter which specifies the vendor of the video card.
— An optional parameter which specifies the name of the video card.
— An optional parameter which specifies the amount of RAM available on the
video card in kilobytes. This setting is only necessary for video cards the X server cannot
probe to detect the amount of video RAM.
— An entry which specifies the bus location of the video card. On systems with only
one video card a BusID entry is optional and may not even be present in the default /
etc/X11/xorg.conf file. On systems with more than one video card, however, a BusID entry
must be present.
— An optional entry which specifies which monitor connector or head on the video
card the Device section configures. This option is only useful for video cards with multiple
heads.
If multiple monitors are connected to different heads on the same video card, separate
Device sections must exist and each of these sections must have a different Screen value.
Values for the Screen entry must be an integer. The first head on the video card has a value
of 0. The value for each additional head increments this value by one.
•
— An optional entry which specifies extra parameters for the section. Replace <option-name> with a valid option listed for this section in the xorg.conf man
page.
Option "<option-name>"
One of the more common options is "dpms" (for Display Power Management Signaling, a
VESA standard), which activates the Service Star energy compliance setting for the monitor.
3.1.9. Screen
Each Screen section binds one video card (or video card head) to one monitor by referencing
the Device section and the Monitor section for each. While one Screen section is the minimum,
additional instances may occur for each video card and monitor combination present on the machine.
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4. Fonts
The following example illustrates a typical Screen section:
Section "Screen" Identifier "Screen0" Device "Videocard0" Monitor "Monitor0" DefaultDepth 16 SubSection "Di
The following entries are commonly used in the Screen section:
•
Identifier
•
Device
•
Monitor
— Specifies a unique name for this Screen section. This is a required entry.
— Specifies the unique name of a Device section. This is a required entry.
Monitor
— Specifies the unique name of a Monitor section. This is only required if a specific
section is defined in the xorg.conf file. Normally, monitors are automatically detec-
ted.
•
DefaultDepth
— Specifies the default color depth in bits. In the previous example, 16 (which
provides thousands of colors) is the default. Only one DefaultDepth is permitted, although
this can be overridden with the Xorg command line option -depth <n>,where <n> is any additional depth specified.
•
SubSection "Display"
— Specifies the screen modes available at a particular color depth.
The Screen section can have multiple Display subsections, which are entirely optional since
screen modes are automatically detected.
This subsection is normally used to override autodetected modes.
•
— An optional entry which specifies extra parameters for the section. Replace <option-name> with a valid option listed for this section in the xorg.conf man
page.
Option "<option-name>"
3.1.10. DRI
The optional DRI section specifies parameters for the Direct Rendering Infrastructure (DRI). DRI
is an interface which allows 3D software applications to take advantage of 3D hardware acceleration capabilities built into most modern video hardware. In addition, DRI can improve 2D performance via hardware acceleration, if supported by the video card driver.
This section rarely appears, as the DRI Group and Mode are automatically initialized to default
values. If a different Group or Mode is desired, then adding this section to the xorg.conf file will
override those defaults.
The following example illustrates a typical DRI section:
Section "DRI" Group 0 Mode 0666 EndSection
Since different video cards use DRI in different ways, do not add to this section without first referring to http://dri.sourceforge.net/.
4. Fonts
Red Hat Enterprise Linux uses two subsystems to manage and display fonts under X: Fontconfig and xfs.
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4.1. Fontconfig
The newer Fontconfig font subsystem simplifies font management and provides advanced display features, such as anti-aliasing. This system is used automatically for applications programmed using the Qt 3 or GTK+ 2 graphical toolkit.
For compatibility, Red Hat Enterprise Linux includes the original font subsystem, called the core
X font subsystem. This system, which is over 15 years old, is based around the X Font Server
(xfs).
This section discusses how to configure fonts for X using both systems.
4.1. Fontconfig
The Fontconfig font subsystem allows applications to directly access fonts on the system and
use Xft or other rendering mechanisms to render Fontconfig fonts with advanced anti-aliasing.
Graphical applications can use the Xft library with Fontconfig to draw text to the screen.
Over time, the Fontconfig/Xft font subsystem replaces the core X font subsystem.
Important
The Fontconfig font subsystem does not yet work for OpenOffice.org, which uses
its own font rendering technology.
It is important to note that Fontconfig uses the /etc/fonts/fonts.conf configuration file, which
should not be edited by hand.
Tip
Due to the transition to the new font system, GTK+ 1.2 applications are not affected by any changes made via the Font Preferences dialog (accessed by selecting System (on the panel) => Preferences => Fonts). For these applications, a
font can be configured by adding the following lines to the file ~/.gtkrc.mine:
style "user-font" { fontset = "<font-specification>" } widget_class "*" style "user-font"
Replace <font-specification> with a font specification in the style used by traditional X applications, such as adobe-helvetica-medium-r-normal--*-120-*-*-*-*-*-*. A full list of core fonts can
be obtained by running xlsfonts or created interactively using the xfontsel command.
4.1.1. Adding Fonts to Fontconfig
Adding new fonts to the Fontconfig subsystem is a straightforward process.
1.
To add fonts system-wide, copy the new fonts into the /usr/share/fonts/ directory. It is a
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4.2. Core X Font System
good idea to create a new subdirectory, such as local/ or similar, to help distinguish
between user-installed and default fonts.
To add fonts for an individual user, copy the new fonts into the .fonts/ directory in the
user's home directory.
2.
Use the fc-cache command to update the font information cache, as in the following example:
fc-cache <path-to-font-directory>
In this command, replace <path-to-font-directory> with the directory containing the new
fonts (either /usr/share/fonts/local/ or /home/<user>/.fonts/).
Tip
Individual users may also install fonts graphically, by typing fonts:/// into the
Nautilus address bar, and dragging the new font files there.
Important
If the font file name ends with a .gz extension, it is compressed and cannot be
used until uncompressed. To do this, use the gunzip command or double-click the
file and drag the font to a directory in Nautilus.
4.2. Core X Font System
For compatibility, Red Hat Enterprise Linux provides the core X font subsystem, which uses the
X Font Server (xfs) to provide fonts to X client applications.
The X server looks for a font server specified in the FontPath directive within the Files section of
the /etc/X11/xorg.conf configuration file. Refer to Section 3.1.4, “Files” for more information
about the FontPath entry.
The X server connects to the xfs server on a specified port to acquire font information. For this
reason, the xfs service must be running for X to start. For more about configuring services for a
particular runlevel, refer to Chapter 15, Controlling Access to Services.
4.2.1. xfs Configuration
The /etc/rc.d/init.d/xfs script starts the xfs server. Several options can be configured within
its configuration file, /etc/X11/fs/config.
The following lists common options:
•
alternate-servers
— Specifies a list of alternate font servers to be used if this font server is
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4.2. Core X Font System
not available. A comma must separate each font server in a list.
•
— Specifies an ordered list of font paths to use. A comma must separate each
font path in a list.
catalogue
Use the string :unscaled immediately after the font path to make the unscaled fonts in that
path load first. Then specify the entire path again, so that other scaled fonts are also loaded.
•
client-limit
— Specifies the maximum number of clients the font server services. The de-
fault is 10.
•
clone-self
— Allows the font server to clone a new version of itself when the client-limit is
hit. By default, this option is on.
•
default-point-size
•
default-resolutions
•
deferglyphs
•
error-file
•
no-listen
— Specifies the default point size for any font that does not specify this
value. The value for this option is set in decipoints. The default of 120 corresponds to a 12
point font.
— Specifies a list of resolutions supported by the X server. Each resolution in the list must be separated by a comma.
— Specifies whether to defer loading glyphs (the graphic used to visually represent a font). To disable this feature use none, to enable this feature for all fonts use all, or
to turn this feature on only for 16-bit fonts use 16.
— Specifies the path and file name of a location where xfs errors are logged.
— Prevents xfs from listening to particular protocols. By default, this option is set
to tcp to prevent xfs from listening on TCP ports for security reasons.
Tip
If xfs is used to serve fonts over the network, remove this line.
•
port
— Specifies the TCP port that xfs listens on if no-listen does not exist or is commented out.
•
use-syslog
— Specifies whether to use the system error log.
4.2.2. Adding Fonts to xfs
To add fonts to the core X font subsystem (xfs), follow these steps:
1.
If it does not already exist, create a directory called /usr/share/fonts/local/ using the following command as root:
mkdir /usr/share/fonts/local/
If creating the /usr/share/fonts/local/ directory is necessary, it must be added to the xfs
path using the following command as root:
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5. Runlevels and X
chkfontpath --add /usr/share/fonts/local/
2.
Copy the new font file into the /usr/share/fonts/local/ directory
3.
Update the font information by issuing the following command as root:
ttmkfdir -d /usr/share/fonts/local/ -o /usr/share/fonts/local/fonts.scale
4.
Reload the xfs font server configuration file by issuing the following command as root:
service xfs reload
5. Runlevels and X
In most cases, the Red Hat Enterprise Linux installer configures a machine to boot into a graphical login environment, known as Runlevel 5. It is possible, however, to boot into a text-only
multi-user mode called Runlevel 3 and begin an X session from there.
For more information about runlevels, refer to Section 1, “Runlevels”.
The following subsections review how X starts up in both runlevel 3 and runlevel 5.
5.1. Runlevel 3
When in runlevel 3, the best way to start an X session is to log in and type startx. The startx
command is a front-end to the xinit command, which launches the X server (Xorg) and connects X client applications to it. Because the user is already logged into the system at runlevel
3, startx does not launch a display manager or authenticate users. Refer to Section 5.2,
“Runlevel 5” for more information about display managers.
When the startx command is executed, it searches for the .xinitrc file in the user's home directory to define the desktop environment and possibly other X client applications to run. If no
.xinitrc file is present, it uses the system default /etc/X11/xinit/xinitrc file instead.
The default xinitrc script then searches for user-defined files and default system files, including
.Xresources, .Xmodmap, and .Xkbmap in the user's home directory, and Xresources, Xmodmap, and
Xkbmap in the /etc/X11/ directory. The Xmodmap and Xkbmap files, if they exist, are used by the
xmodmap utility to configure the keyboard. The Xresources file is read to assign specific preference values to applications.
After setting these options, the xinitrc script executes all scripts located in the /
etc/X11/xinit/xinitrc.d/ directory. One important script in this directory is xinput.sh, which
configures settings such as the default language.
Next, the xinitrc script attempts to execute .Xclients in the user's home directory and turns to
/etc/X11/xinit/Xclients if it cannot be found. The purpose of the Xclients file is to start the
desktop environment or, possibly, just a basic window manager. The .Xclients script in the
user's home directory starts the user-specified desktop environment in the .Xclients-default
file. If .Xclients does not exist in the user's home directory, the standard /
etc/X11/xinit/Xclients script attempts to start another desktop environment, trying GNOME
first and then KDE followed by twm.
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5.2. Runlevel 5
When in runlevel 3, the user is returned to a text mode user session after ending an X session.
5.2. Runlevel 5
When the system boots into runlevel 5, a special X client application called a display manager is
launched. A user must authenticate using the display manager before any desktop environment
or window managers are launched.
Depending on the desktop environments installed on the system, three different display managers are available to handle user authentication.
•
GNOME
— The default display manager for Red Hat Enterprise Linux, GNOME allows the user to
configure language settings, shutdown, restart or log in to the system.
•
KDE
•
xdm
— KDE's display manager which allows the user to shutdown, restart or log in to the system.
— A very basic display manager which only lets the user log in to the system.
When booting into runlevel 5, the prefdm script determines the preferred display manager by referencing the /etc/sysconfig/desktop file. A list of options for this file is available in this file:
/usr/share/doc/initscripts-<version-number>/sysconfig.txt
where <version-number> is the version number of the initscripts package.
Each of the display managers reference the /etc/X11/xdm/Xsetup_0 file to set up the login
screen. Once the user logs into the system, the /etc/X11/xdm/GiveConsole script runs to assign
ownership of the console to the user. Then, the /etc/X11/xdm/Xsession script runs to accomplish
many of the tasks normally performed by the xinitrc script when starting X from runlevel 3, including setting system and user resources, as well as running the scripts in the /
etc/X11/xinit/xinitrc.d/ directory.
Users can specify which desktop environment they want to utilize when they authenticate using
the GNOME or KDE display managers by selecting it from the Sessions menu item (accessed by
selecting System (on the panel) => Preferences => More Preferences => Sessions). If the
desktop environment is not specified in the display manager, the /etc/X11/xdm/Xsession script
checks the .xsession and .Xclients files in the user's home directory to decide which desktop
environment to load. As a last resort, the /etc/X11/xinit/Xclients file is used to select a
desktop environment or window manager to use in the same way as runlevel 3.
When the user finishes an X session on the default display (:0) and logs out, the /
etc/X11/xdm/TakeConsole script runs and reassigns ownership of the console to the root user.
The original display manager, which continues running after the user logged in, takes control by
spawning a new display manager. This restarts the X server, displays a new login window, and
starts the entire process over again.
The user is returned to the display manager after logging out of X from runlevel 5.
For more information on how display managers control user authentication, refer to the /
usr/share/doc/gdm-<version-number>/README (where <version-number> is the version number for
the gdm package installed) and the xdm man page.
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6.1. Installed Documentation
6. Additional Resources
There is a large amount of detailed information available about the X server, the clients that
connect to it, and the assorted desktop environments and window managers.
6.1. Installed Documentation
•
/usr/share/X11/doc/
— contains detailed documentation on the X Window System architecture, as well as how to get additional information about the Xorg project as a new user.
•
man xorg.conf
•
man Xorg
— Contains information about the xorg.conf configuration files, including the
meaning and syntax for the different sections within the files.
— Describes the Xorg display server.
6.2. Useful Websites
•
http://www.X.org/ — Home page of the X.Org Foundation, which produces the X11R7.1 release of the X Window System. The X11R7.1 release is bundled with Red Hat Enterprise
Linux to control the necessary hardware and provide a GUI environment.
•
http://dri.sourceforge.net/ — Home page of the DRI (Direct Rendering Infrastructure) project.
The DRI is the core hardware 3D acceleration component of X.
•
http://www.gnome.org/ [http://www.gnome.org] — Home of the GNOME project.
•
http://www.kde.org/ [http://www.kde.org] — Home of the KDE desktop environment.
438
Chapter 31. X Window System
Configuration
During installation, the system's monitor, video card, and display settings are configured. To
change any of these settings after installation, use the &RHXFREE86TOOL;.
To start the &RHXFREE86TOOL;, go to System (on the panel) => Administration => Display,
or type the command system-config-display at a shell prompt (for example, in an XTerm or
GNOME terminal). If the X Window System is not running, a small version of X is started to run
the program.
After changing any of the settings, log out of the graphical desktop and log back in to enable the
changes.
1. Display Settings
The Settings tab allows users to change the resolution and color depth. The display of a monitor consists of tiny dots called pixels. The number of pixels displayed at one time is called the
resolution. For example, the resolution 1024x768 means that 1024 horizontal pixels and 768
vertical pixels are used. The higher the resolution values, the more images the monitor can display at one time.
The color depth of the display determines how many possible colors are displayed. A higher color depth means more contrast between colors.
439
2. Display Hardware Settings
Figure 31.1. Display Settings
2. Display Hardware Settings
When the &RHXFREE86TOOL; is started, it probes the monitor and video card. If the hardware
is probed properly, the information for it is shown on the Hardware tab as shown in Figure 31.2,
“Display Hardware Settings”.
440
3. Dual Head Display Settings
Figure 31.2. Display Hardware Settings
To change the monitor type or any of its settings, click the corresponding Configure button. To
change the video card type or any of its settings, click the Configure button beside its settings.
3. Dual Head Display Settings
If multiple video cards are installed on the system, dual head monitor support is available and is
configured via the Dual head tab, as shown in Figure 31.3, “Dual Head Display Settings”.
441
3. Dual Head Display Settings
Figure 31.3. Dual Head Display Settings
To enable use of Dual head, check the Use dual head checkbox.
To configure the second monitor type, click the corresponding Configure button. You can also
configure the other Dual head settings by using the corresponding drop-down list.
For the Desktop layout option, selecting Spanning Desktops allows both monitors to use an
enlarged usable workspace. Selecting Individual Desktops shares the mouse and keyboard
among the displays, but restricts windows to a single display.
442
Chapter 32. Users and Groups
The control of users and groups is a core element of Red Hat Enterprise Linux system administration.
Users can be either people (meaning accounts tied to physical users) or accounts which exist
for specific applications to use.
Groups are logical expressions of organization, tying users together for a common purpose.
Users within a group can read, write, or execute files owned by that group.
Each user and group has a unique numerical identification number called a userid (UID) and a
groupid (GID), respectively.
A user who creates a file is also the owner and group owner of that file. The file is assigned separate read, write, and execute permissions for the owner, the group, and everyone else. The file
owner can be changed only by the root user, and access permissions can be changed by both
the root user and file owner.
Red Hat Enterprise Linux also supports access control lists (ACLs) for files and directories
which allow permissions for specific users outside of the owner to be set. For more information
about ACLs, refer to Chapter 8, Access Control Lists.
1. User and Group Configuration
The User Manager allows you to view, modify, add, and delete local users and groups.
To use the User Manager, you must be running the X Window System, have root privileges,
and have the system-config-users RPM package installed. To start the User Manager from the
desktop, go to System (on the panel) => Administration => Users & Groups. You can also
type the command system-config-users at a shell prompt (for example, in an XTerm or a
GNOME terminal).
443
1.1. Adding a New User
Figure 32.1. User Manager
To view a list of local users on the system, click the Users tab. To view a list of local groups on
the system, click the Groups tab.
To find a specific user or group, type the first few letters of the name in the Search filter field.
Press Enter or click the Apply filter button. The filtered list is displayed.
To sort the users or groups, click on the column name. The users or groups are sorted according to the value of that column.
Red Hat Enterprise Linux reserves user IDs below 500 for system users. By default, User Manager does not display system users. To view all users, including the system users, go to Edit =>
Preferences and uncheck Hide system users and groups from the dialog box.
1.1. Adding a New User
To add a new user, click the Add User button. A window as shown in Figure 32.2, “New User”
appears. Type the username and full name for the new user in the appropriate fields. Type the
user's password in the Password and Confirm Password fields. The password must be at
least six characters.
Tip
It is advisable to use a much longer password, as this makes it more difficult for an
444
1.1. Adding a New User
intruder to guess it and access the account without permission. It is also recommended that the password not be based on a dictionary term; use a combination of
letters, numbers and special characters.
Select a login shell. If you are not sure which shell to select, accept the default value of /
bin/bash. The default home directory is /home/<username>/. You can change the home directory
that is created for the user, or you can choose not to create the home directory by unselecting
Create home directory.
If you select to create the home directory, default configuration files are copied from the /
etc/skel/ directory into the new home directory.
Red Hat Enterprise Linux uses a user private group (UPG) scheme. The UPG scheme does not
add or change anything in the standard UNIX way of handling groups; it offers a new convention. Whenever you create a new user, by default, a unique group with the same name as the
user is created. If you do not want to create this group, unselect Create a private group for the
user.
To specify a user ID for the user, select Specify user ID manually. If the option is not selected,
the next available user ID above 500 is assigned to the new user. Because Red Hat Enterprise
Linux reserves user IDs below 500 for system users, it is not advisable to manually assign user
IDs 1-499.
Click OK to create the user.
445
1.2. Modifying User Properties
Figure 32.2. New User
To configure more advanced user properties, such as password expiration, modify the user's
properties after adding the user. Refer to Section 1.2, “Modifying User Properties” for more information.
1.2. Modifying User Properties
To view the properties of an existing user, click on the Users tab, select the user from the user
list, and click Properties from the menu (or choose File => Properties from the pulldown
menu). A window similar to Figure 32.3, “User Properties” appears.
446
1.3. Adding a New Group
Figure 32.3. User Properties
The User Properties window is divided into multiple tabbed pages:
•
User Data — Shows the basic user information configured when you added the user. Use
this tab to change the user's full name, password, home directory, or login shell.
•
Account Info — Select Enable account expiration if you want the account to expire on a
certain date. Enter the date in the provided fields. Select Local password is locked to lock
the user account and prevent the user from logging into the system.
•
Password Info — Displays the date that the user's password last changed. To force the
user to change passwords after a certain number of days, select Enable password expiration and enter a desired value in the Days before change required: field. The number of
days before the user's password expires, the number of days before the user is warned to
change passwords, and days before the account becomes inactive can also be changed.
•
Groups — Allows you to view and configure the Primary Group of the user, as well as other
groups that you want the user to be a member of.
1.3. Adding a New Group
447
1.4. Modifying Group Properties
To add a new user group, click the Add Group button. A window similar to Figure 32.4, “New
Group” appears. Type the name of the new group to create. To specify a group ID for the new
group, select Specify group ID manually and select the GID. Note that Red Hat Enterprise
Linux also reserves group IDs lower than 500 for system groups.
Figure 32.4. New Group
Click OK to create the group. The new group appears in the group list.
1.4. Modifying Group Properties
To view the properties of an existing group, select the group from the group list and click Properties from the menu (or choose File => Properties from the pulldown menu). A window similar
to Figure 32.5, “Group Properties” appears.
448
2. User and Group Management Tools
Figure 32.5. Group Properties
The Group Users tab displays which users are members of the group. Use this tab to add or remove users from the group. Click OK to save your changes.
2. User and Group Management Tools
Managing users and groups can be a tedious task; this is why Red Hat Enterprise Linux
provides tools and conventions to make them easier to manage.
The easiest way to manage users and groups is through the graphical application, User Manager (system-config-users). For more information on User Manager, refer to Section 1, “User
and Group Configuration”.
The following command line tools can also be used to manage users and groups:
•
useradd, usermod,
and userdel — Industry-standard methods of adding, deleting and modifying user accounts
•
groupadd, groupmod,
•
gpasswd
•
pwck, grpck
and groupdel — Industry-standard methods of adding, deleting, and
modifying user groups
— Industry-standard method of administering the /etc/group file
— Tools used for the verification of the password, group, and associated shadow
files
•
pwconv, pwunconv
— Tools used for the conversion of passwords to shadow passwords and
back to standard passwords
2.1. Command Line Configuration
If you prefer command line tools or do not have the X Window System installed, use this section
to configure users and groups.
2.2. Adding a User
To add a user to the system:
1.
Issue the useradd command to create a locked user account:
useradd <username>
2.
Unlock the account by issuing the passwd command to assign a password and set password
aging guidelines:
passwd <username>
Command line options for useradd are detailed in Table 32.1, “useradd Command Line Options”.
449
2.3. Adding a Group
Option
Description
-c '<comment>'
<comment>
-d<home-dir>
Home directory to be used instead of default /home/<username>/
-e<date>
Date for the account to be disabled in the format YYYY-MM-DD
-f<days>
Number of days after the password expires until the account is disabled. If 0 is specified, the account is disabled immediately after
the password expires. If -1 is specified, the account is not be disabled after the password expires.
-g<group-name>
Group name or group number for the user's default group. The
group must exist prior to being specified here.
-G<group-list>
List of additional (other than default) group names or group numbers, separated by commas, of which the user is a member. The
groups must exist prior to being specified here.
-m
Create the home directory if it does not exist.
-M
Do not create the home directory.
-n
Do not create a user private group for the user.
-r
Create a system account with a UID less than 500 and without a
home directory
-p<password>
The password encrypted with crypt
-s
User's login shell, which defaults to /bin/bash
-u<uid>
User ID for the user, which must be unique and greater than 499
can be replaced with any string. This option is generally
used to specify the full name of a user.
Table 32.1. useradd Command Line Options
2.3. Adding a Group
To add a group to the system, use the command groupadd:
groupadd <group-name>
Command line options for groupadd are detailed in Table 32.2, “groupadd Command Line Options”.
Option
Description
-g<gid>
Group ID for the group, which must be unique and greater than
499
-r
Create a system group with a GID less than 500
-f
When used with -g<gid> and <gid> already exists, groupadd will
450
2.4. Password Aging
Option
Description
choose another unique <gid> for the group.
Table 32.2. groupadd Command Line Options
2.4. Password Aging
For security reasons, it is advisable to require users to change their passwords periodically. This
can be done when adding or editing a user on the Password Info tab of the User Manager.
To configure password expiration for a user from a shell prompt, use the chage command, followed by an option from Table 32.3, “chage Command Line Options”, followed by the username
of the user.
Important
Shadow passwords must be enabled to use the chage command.
Option
Description
-m<days>
Specifies the minimum number of days between which the user
must change passwords. If the value is 0, the password does not
expire.
-M<days>
Specifies the maximum number of days for which the password is
valid. When the number of days specified by this option plus the
number of days specified with the -d option is less than the current
day, the user must change passwords before using the account.
-d<days>
Specifies the number of days since January 1, 1970 the password
was changed
-I<days>
Specifies the number of inactive days after the password expiration before locking the account. If the value is 0, the account is not
locked after the password expires.
-E<date>
Specifies the date on which the account is locked, in the format
YYYY-MM-DD. Instead of the date, the number of days since
January 1, 1970 can also be used.
-W<days>
Specifies the number of days before the password expiration date
to warn the user.
Table 32.3. chage Command Line Options
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2.4. Password Aging
Tip
If the chage command is followed directly by a username (with no options), it displays the current password aging values and allows them to be changed.
You can configure a password to expire the first time a user logs in. This forces users to change
passwords the first time they log in.
Note
This process will not work if the user logs in using the SSH protocol.
1.
Lock the user password — If the user does not exist, use the useradd command to create
the user account, but do not give it a password so that it remains locked.
If the password is already enabled, lock it with the command:
usermod -L username
2.
Force immediate password expiration — Type the following command:
chage -d 0 username
This command sets the value for the date the password was last changed to the epoch
(January 1, 1970). This value forces immediate password expiration no matter what password aging policy, if any, is in place.
3.
Unlock the account — There are two common approaches to this step. The administrator
can assign an initial password or assign a null password.
Warning
Do not use the passwd command to set the password as it disables the immediate password expiration just configured.
To assign an initial password, use the following steps:
•
Start the command line Python interpreter with the python command. It displays the following:
Python 2.4.3 (#1, Jul 21 2006, 08:46:09) [GCC 4.1.1 20060718 (Red Hat 4.1.1-9)] on linux2 Type "he
•
At the prompt, type the following commands. Replace <password> with the password to
encrypt and <salt> with a random combination of at least 2 of the following: any alpha452
2.5. Explaining the Process
numeric character, the slash (/) character or a dot (.):
import crypt; print crypt.crypt("<password>","<salt>")
The output is the encrypted password, similar to '12CsGd8FRcMSM'.
•
Press Ctrl-D to exit the Python interpreter.
•
At the shell, enter the following command (replacing <encrypted-password> with the encrypted output of the Python interpreter):
usermod -p "<encrypted-password>" <username>
Alternatively, you can assign a null password instead of an initial password. To do this, use
the following command:
usermod -p "" username
Caution
Using a null password, while convenient, is a highly unsecure practice, as any
third party can log in first an access the system using the unsecure username.
Always make sure that the user is ready to log in before unlocking an account
with a null password.
In either case, upon initial log in, the user is prompted for a new password.
2.5. Explaining the Process
The following steps illustrate what happens if the command useradd
that has shadow passwords enabled:
1.
juan
is issued on a system
A new line for juan is created in /etc/passwd. The line has the following characteristics:
•
It begins with the username juan.
•
There is an x for the password field indicating that the system is using shadow passwords.
•
A UID greater than 499 is created. (Under Red Hat Enterprise Linux, UIDs and GIDs below 500 are reserved for system use.)
•
A GID greater than 499 is created.
•
The optional GECOS information is left blank.
•
The home directory for juan is set to /home/juan/.
•
The default shell is set to /bin/bash.
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3. Standard Users
2.
A new line for juan is created in /etc/shadow. The line has the following characteristics:
•
It begins with the username juan.
•
Two exclamation points (!!) appear in the password field of the /etc/shadow file, which
locks the account.
Note
If an encrypted password is passed using the -p flag, it is placed in the /
etc/shadow file on the new line for the user.
•
3.
The password is set to never expire.
A new line for a group named juan is created in /etc/group. A group with the same name
as a user is called a user private group. For more information on user private groups, refer
to Section 1.1, “Adding a New User”.
The line created in /etc/group has the following characteristics:
4.
•
It begins with the group name juan.
•
An x appears in the password field indicating that the system is using shadow group
passwords.
•
The GID matches the one listed for user juan in /etc/passwd.
A new line for a group named juan is created in /etc/gshadow. The line has the following
characteristics:
•
It begins with the group name juan.
•
An exclamation point (!) appears in the password field of the /etc/gshadow file, which
locks the group.
•
All other fields are blank.
5.
A directory for user juan is created in the /home/ directory. This directory is owned by user
juan and group juan. However, it has read, write, and execute privileges only for the user
juan. All other permissions are denied.
6.
The files within the /etc/skel/ directory (which contain default user settings) are copied into the new /home/juan/ directory.
At this point, a locked account called juan exists on the system. To activate it, the administrator
must next assign a password to the account using the passwd command and, optionally, set
password aging guidelines.
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3. Standard Users
3. Standard Users
Table 32.4, “Standard Users” lists the standard users configured in the /etc/passwd file by an
Everything installation. The groupid (GID) in this table is the primary group for the user. See
Section 4, “Standard Groups” for a listing of standard groups.
User
UID
GID
Home Directory
Shell
root
0
0
/root
/bin/bash
bin
1
1
/bin
/sbin/nologin
daemon
2
2
/sbin
/sbin/nologin
adm
3
4
/var/adm
/sbin/nologin
lp
4
7
/var/spool/lpd
/sbin/nologin
sync
5
0
/sbin
/bin/sync
shutdown
6
0
/sbin
/sbin/shutdown
halt
7
0
/sbin
/sbin/halt
mail
8
12
/var/spool/mail
/sbin/nologin
news
9
13
/etc/news
uucp
10
14
/var/spool/uucp
/sbin/nologin
operator
11
0
/root
/sbin/nologin
games
12
100
/usr/games
/sbin/nologin
gopher
13
30
/var/gopher
/sbin/nologin
ftp
14
50
/var/ftp
/sbin/nologin
nobody
99
99
/
/sbin/nologin
rpm
37
37
/var/lib/rpm
/sbin/nologin
vcsa
69
69
/dev
/sbin/nologin
dbus
81
81
/
/sbin/nologin
ntp
38
38
/etc/ntp
/sbin/nologin
canna
39
39
/var/lib/canna
/sbin/nologin
nscd
28
28
/
/sbin/nologin
rpc
32
32
/
/sbin/nologin
postfix
89
89
/var/spool/postfix
/sbin/nologin
mailman
41
41
/var/mailman
/sbin/nologin
named
25
25
/var/named
/bin/false
amanda
33
6
var/lib/amanda/
/bin/bash
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4. Standard Groups
User
UID
GID
Home Directory
Shell
postgres
26
26
/var/lib/pgsql
/bin/bash
exim
93
93
/var/spool/exim
/sbin/nologin
sshd
74
74
/var/empty/sshd
/sbin/nologin
rpcuser
29
29
/var/lib/nfs
/sbin/nologin
nsfnobody
65534
65534
/var/lib/nfs
/sbin/nologin
pvm
24
24
/usr/share/pvm3
/bin/bash
apache
48
48
/var/www
/sbin/nologin
xfs
43
43
/etc/X11/fs
/sbin/nologin
gdm
42
42
/var/gdm
/sbin/nologin
htt
100
101
/usr/lib/im
/sbin/nologin
mysql
27
27
/var/lib/mysql
/bin/bash
webalizer
67
67
/var/www/usage
/sbin/nologin
mailnull
47
47
/var/spool/mqueue
/sbin/nologin
smmsp
51
51
/var/spool/mqueue
/sbin/nologin
squid
23
23
/var/spool/squid
/sbin/nologin
ldap
55
55
/var/lib/ldap
/bin/false
netdump
34
34
/var/crash
/bin/bash
pcap
77
77
/var/arpwatch
/sbin/nologin
radiusd
95
95
/
/bin/false
radvd
75
75
/
/sbin/nologin
quagga
92
92
/var/run/quagga
/sbin/login
wnn
49
49
/var/lib/wnn
/sbin/nologin
dovecot
97
97
/usr/libexec/dovecot
/sbin/nologin
Table 32.4. Standard Users
4. Standard Groups
Table 32.5, “Standard Groups” lists the standard groups configured by an Everything installation. Groups are stored in the /etc/group file.
456
4. Standard Groups
Group
GID
Members
root
0
root
bin
1
root, bin, daemon
daemon
2
root, bin, daemon
sys
3
root, bin, adm
adm
4
root, adm, daemon
tty
5
disk
6
root
lp
7
daemon, lp
mem
8
kmem
9
wheel
10
root
mail
12
mail, postfix, exim
news
13
news
uucp
14
uucp
man
15
games
20
gopher
30
dip
40
ftp
50
lock
54
nobody
99
users
100
rpm
37
utmp
22
floppy
19
vcsa
69
dbus
81
ntp
38
canna
39
nscd
28
457
4. Standard Groups
Group
GID
rpc
32
postdrop
90
postfix
89
mailman
41
exim
93
named
25
postgres
26
sshd
74
rpcuser
29
nfsnobody
65534
pvm
24
apache
48
xfs
43
gdm
42
htt
101
mysql
27
webalizer
67
mailnull
47
smmsp
51
squid
23
ldap
55
netdump
34
pcap
77
quaggavt
102
quagga
92
radvd
75
slocate
21
wnn
49
dovecot
97
radiusd
95
Members
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5. User Private Groups
Table 32.5. Standard Groups
5. User Private Groups
Red Hat Enterprise Linux uses a user private group (UPG) scheme, which makes UNIX groups
easier to manage.
A UPG is created whenever a new user is added to the system. A UPG has the same name as
the user for which it was created and that user is the only member of the UPG.
UPGs make it safe to set default permissions for a newly created file or directory, allowing both
the user and the group of that user to make modifications to the file or directory.
The setting which determines what permissions are applied to a newly created file or directory is
called a umask and is configured in the /etc/bashrc file. Traditionally on UNIX systems, the
umask is set to 022, which allows only the user who created the file or directory to make modifications. Under this scheme, all other users, including members of the creator's group, are not allowed to make any modifications. However, under the UPG scheme, this "group protection" is
not necessary since every user has their own private group.
5.1. Group Directories
Many IT organizations like to create a group for each major project and then assign people to
the group if they need to access that project's files. Using this traditional scheme, managing files
has been difficult; when someone creates a file, it is associated with the primary group to which
they belong. When a single person works on multiple projects, it is difficult to associate the right
files with the right group. Using the UPG scheme, however, groups are automatically assigned
to files created within a directory with the setgid bit set. The setgid bit makes managing group
projects that share a common directory very simple because any files a user creates within the
directory are owned by the group which owns the directory.
Let us say, for example, that a group of people need to work on files in the /
usr/share/emacs/site-lisp/ directory. Some people are trusted to modify the directory, but certainly not everyone is trusted. First create an emacs group, as in the following command:
/usr/sbin/groupadd emacs
To associate the contents of the directory with the emacs group, type:
chown -R root.emacs /usr/share/emacs/site-lisp
Now, it is possible to add the proper users to the group with the gpasswd command:
/usr/bin/gpasswd -a <username> emacs
To allow users to create files within the directory, use the following command:
chmod 775 /usr/share/emacs/site-lisp
When a user creates a new file, it is assigned the group of the user's default private group.
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6. Shadow Passwords
Next, set the setgid bit, which assigns everything created in the directory the same group permission as the directory itself (emacs). Use the following command:
chmod 2775 /usr/share/emacs/site-lisp
At this point, because the default umask of each user is 002, all members of the emacs group
can create and edit files in the /usr/share/emacs/site-lisp/ directory without the administrator
having to change file permissions every time users write new files.
6. Shadow Passwords
In multiuser environments it is very important to use shadow passwords (provided by the shadow-utils package). Doing so enhances the security of system authentication files. For this reason, the installation program enables shadow passwords by default.
The following lists the advantages pf shadow passwords have over the traditional way of storing
passwords on UNIX-based systems:
•
Improves system security by moving encrypted password hashes from the world-readable /
etc/passwd file to /etc/shadow, which is readable only by the root user.
•
Stores information about password aging.
•
Allows the use the /etc/login.defs file to enforce security policies.
Most utilities provided by the shadow-utils package work properly whether or not shadow passwords are enabled. However, since password aging information is stored exclusively in the /
etc/shadow file, any commands which create or modify password aging information do not work.
The following is a list of commands which do not work without first enabling shadow passwords:
•
chage
•
gpasswd
•
/usr/sbin/usermod-e
or -f options
•
/usr/sbin/useradd-e
or -f options
7. Additional Resources
For more information about users and groups, and tools to manage them, refer to the following
resources.
7.1. Installed Documentation
•
Related man pages — There are a number of man pages for the various applications and
configuration files involved with managing users and groups. Some of the more important
man pages have been listed here:
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7.1. Installed Documentation
User and Group Administrative Applications
•
man chage
— A command to modify password aging policies and account expiration.
•
man gpasswd
•
man groupadd
•
man grpck
•
man groupdel
— A command to remove groups.
•
man groupmod
— A command to modify group membership.
•
man pwck
•
man pwconv
•
man pwunconv
•
man useradd
— A command to add users.
•
man userdel
— A command to remove users.
•
man usermod
— A command to modify users.
— A command to administer the /etc/group file.
— A command to add groups.
— A command to verify the /etc/group file.
— A command to verify the /etc/passwd and /etc/shadow files.
— A tool to convert standard passwords to shadow passwords.
— A tool to convert shadow passwords to standard passwords.
Configuration Files
•
man 5 group
— The file containing group information for the system.
•
man 5 passwd
— The file containing user information for the system.
•
man 5 shadow
— The file containing passwords and account expiration information for the
system.
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Chapter 33. Printer Configuration
Printer Configuration Tool allows users to configure a printer. This tool helps maintain the
printer configuration file, print spool directories, print filters, and printer classes.
Red Hat Enterprise Linux 5.0.0 uses the Common Unix Printing System (CUPS). If a system
was upgraded from a previous Red Hat Enterprise Linux version that used CUPS, the upgrade
process preserves the configured queues.
Using Printer Configuration Tool requires root privileges. To start the application, select System (on the panel) => Administration => Printing, or type the command system-config-printer at a shell prompt.
Figure 33.1. Printer Configuration Tool
The following types of print queues can be configured:
•
AppSocket/HP JetDirect — a printer connected directly to the network through HP JetDirect or Appsocket interface instead of a computer.
•
Internet Printing Protocol (IPP) — a printer that can be accessed over a TCP/IP network
via the Internet Printing Protocol (for example, a printer attached to another Red Hat Enterprise Linux system running CUPS on the network).
•
LPD/LPR Host or Printer — a printer attached to a different UNIX system that can be accessed over a TCP/IP network (for example, a printer attached to another Red Hat Enterprise Linux system running LPD on the network).
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1. Adding a Local Printer
•
Networked Windows (SMB) — a printer attached to a different system which is sharing a
printer over an SMB network (for example, a printer attached to a Microsoft Windows™ machine).
•
Networked JetDirect — a printer connected directly to the network through HP JetDirect instead of a computer.
Important
If you add a new print queue or modify an existing one, you must apply the
changes for them to take effect.
Clicking the Apply button prompts the printer daemon to restart with the changes you have configured.
Clicking the Revert button discards unapplied changes.
1. Adding a Local Printer
To add a local printer, such as one attached through a parallel port or USB port on your computer, click the New Printer button in the main Printer Configuration Tool window to display
the window in Figure 33.2, “Adding a Printer”.
Figure 33.2. Adding a Printer
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2. Adding an IPP Printer
Click Forward to proceed.
Enter a unique name for the printer in the Printer Name field. The printer name can contain letters, numbers, dashes (-), and underscores (_); it must not contain any spaces.
You can also use the Description and Location fields to further distinguish this printer from
others that may be configured on your system. Both of these fields are optional, and may contain spaces.
Click Forward to open the New Printer dialogue (refer to Figure 33.3, “Adding a Local Printer”).
If the printer has been automatically detected, the printer model appears in Select Connection.
Select the printer model and click Forward to continue.
If the device does not automatically appear, select the device to which the printer is connected
(such as LPT #1 or Serial Port #1) in Select Connection.
Figure 33.3. Adding a Local Printer
Next, select the printer type. Refer to Section 5, “Selecting the Printer Model and Finishing” for
details.
2. Adding an IPP Printer
An IPP printer is a printer attached to a different system on the same TCP/IP network. The system this printer is attached to may either be running CUPS or simply configured to use IPP.
If a firewall is enabled on the printer server, then the firewall should be configured to allow send
/ receive connections on the incoming UDP port 631. If a firewall is enabled on on the client (the
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3. Adding a Samba (SMB) Printer
system sending the print request) then the firewall must be allowed to accept and create connections through port 631.
You can add a networked IPP printer by clicking the New Printer button in the main Printer
Configuration Tool window to display the window in Figure 33.2, “Adding a Printer”. Enter the
Printer Name (printer names cannot contain spaces and may contain letters, numbers, dashes
(-), and underscores (_)), Description, and Location to distinguish this printer from others that
you may configure on your system. Click Forward to proceed.
In the window shown in Figure 33.4, “Adding an IPP Printer”, enter the hostname of the IPP
printer in the Hostname field as well as a unique name for the printer in the Printername field.
Figure 33.4. Adding an IPP Printer
Click Forward to continue.
Next, select the printer type. Refer to Section 5, “Selecting the Printer Model and Finishing” for
details.
3. Adding a Samba (SMB) Printer
You can add a Samba (SMB) based printer share by clicking the New Printer button in the main
Printer Configuration Tool window to display the window in Figure 33.2, “Adding a Printer”.
Enter a unique name for the printer in the Printer Name field. The printer name can contain letters, numbers, dashes (-), and underscores (_); it must not contain any spaces.
You can also use the Description and Location fields to further distinguish this printer from
others that may be configured on your system. Both of these fields are optional, and may con-
465
3. Adding a Samba (SMB) Printer
tain spaces.
Figure 33.5. Adding a SMB Printer
As shown in Figure 33.5, “Adding a SMB Printer”, available SMB shares are automatically detected and listed in the Share column. Click the arrow (
) beside a Workgroup to expand it.
From the expanded list, select a printer.
If the printer you are looking for does not appear in the list, enter the SMB address in the smb://
field. Use the format computer name/printer share. In Figure 33.5, “Adding a SMB Printer”, the
computer name is dellbox, while the printer share is r2.
In the Username field, enter the username to access the printer. This user must exist on the
SMB system, and the user must have permission to access the printer. The default user name
is typically guest for Windows servers, or nobody for Samba servers.
Enter the Password (if required) for the user specified in the Username field.
You can then test the connection by clicking Verify. Upon successful verification, a dialog box
appears confirming printer share accessibility.
Next, select the printer type. Refer to Section 5, “Selecting the Printer Model and Finishing” for
details.
Warning
Samba printer usernames and passwords are stored in the printer server as unen-
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4. Adding a JetDirect Printer
crypted files readable by root and lpd. Thus, other users that have root access to
the printer server can view the username and password you use to access the
Samba printer.
As such, when you choose a username and password to access a Samba printer,
it is advisable that you choose a password that is different from what you use to
access your local Red Hat Enterprise Linux system.
If there are files shared on the Samba print server, it is recommended that they
also use a password different from what is used by the print queue.
4. Adding a JetDirect Printer
To add a JetDirect or AppSocket connected printer share, click the New Printer button in the
main Printer Configuration Tool window to display the window in Figure 33.2, “Adding a Printer”. Enter a unique name for the printer in the Printer Name field. The printer name can contain
letters, numbers, dashes (-), and underscores (_); it must not contain any spaces.
You can also use the Description and Location fields to further distinguish this printer from
others that may be configured on your system. Both of these fields are optional, and may contain spaces.
Figure 33.6. Adding a JetDirect Printer
Click Forward to continue.
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5. Selecting the Printer Model and Finishing
Text fields for the following options appear:
•
Hostname — The hostname or IP address of the JetDirect printer.
•
Port Number — The port on the JetDirect printer that is listening for print jobs. The default
port is 9100.
Next, select the printer type. Refer to Section 5, “Selecting the Printer Model and Finishing” for
details.
5. Selecting the Printer Model and Finishing
Once you have properly selected a printer queue type, you can choose either option:
•
Select a Printer from database - If you select this option, choose the make of your printer
from the list of Makes. If your printer make is not listed, choose Generic.
•
Provide PPD file - A PostScript Printer Description (PPD) file may also be provided with your
printer. This file is normally provided by the manufacturer. If you are provided with a PPD
file, you can choose this option and use the browser bar below the option description to select the PPD file.
Refer to Figure 33.7, “Selecting a Printer Model”.
Figure 33.7. Selecting a Printer Model
After choosing an option, click Forward to continue. Figure 33.7, “Selecting a Printer Model” appears. You now have to choose the corresponding model and driver for the printer.
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5.1. Confirming Printer Configuration
The recommended printed driver is automatically selected based on the printer model you
chose. The print driver processes the data that you want to print into a format the printer can understand. Since a local printer is attached directly to your computer, you need a printer driver to
process the data that is sent to the printer.
If you have a PPD file for the device (usually provided by the manufacturer), you can select it by
choosing Provide PPD file. You can then browse the filesystem for the PPD file by clicking
Browse.
5.1. Confirming Printer Configuration
The last step is to confirm your printer configuration. Click Apply to add the print queue if the
settings are correct. Click Back to modify the printer configuration.
After applying the changes, print a test page to ensure the configuration is correct. Refer to Section 6, “Printing a Test Page” for details.
6. Printing a Test Page
After you have configured your printer, you should print a test page to make sure the printer is
functioning properly. To print a test page, select the printer that you want to try out from the
printer list, then click Print Test Page from the printer's Settings tab.
If you change the print driver or modify the driver options, you should print a test page to test
the different configuration.
7. Modifying Existing Printers
To delete an existing printer, select the printer and click the Delete button on the toolbar. The
printer is removed from the printer list once you confirm deletion of the printer configuration.
To set the default printer, select the printer from the printer list and click the Make Default Printer button in the Settings tab.
7.1. The Settings Tab
To change printer driver configuration, click the corresponding name in the Printer list and click
the Settings tab.
You can modify printer settings such as make and model, make a printer the default, print a test
page, change the device location (URI), and more.
469
7.2. The Policies Tab
Figure 33.8. Settings Tab
7.2. The Policies Tab
To change settings in print output, click the Policies tab.
For example, to create a banner page (a page that describes aspects of the print job such as
the originating printer, the username from the which the job originated, and the security status of
the document being printed) click the Starting Banner or Ending Banner drop-menu and
choose the option that best describes the nature of the print jobs (such as topsecret,
classified, or confidential).
470
7.3. The Access Control Tab
Figure 33.9. Policies Tab
You can also configure the Error Policy of the printer, by choosing an option from the dropdown menu. You can choose to abort the print job, retry, or stop it.
7.3. The Access Control Tab
You can change user-level access to the configured printer by clicking the Access Control tab.
Add users using the text box and click the Add button beside it. You can then choose to only allow use of the printer to that subset of users or deny use to those users.
471
7.4. The Printer and Job OptionsTab
Figure 33.10. Access Control Tab
7.4. The Printer and Job OptionsTab
The Printer Options tab contains various configuration options for the printer media and output.
472
8. Managing Print Jobs
Figure 33.11. Printer Options Tab
•
Page Size — Allows the paper size to be selected. The options include US Letter, US Legal,
A3, and A4
•
Media Source — set to Automatic by default. Change this option to use paper from a different tray.
•
Media Type — Allows you to change paper type. Options include: Plain, thick, bond, and
transparency.
•
Resolution — Configure the quality and detail of the printout (default is 300 dots per inch
(dpi).
•
Toner Saving — Choose whether the printer uses less toner to conserve resources.
You can also configure printer job options using the Job Options tab. Use the drop-menu and
choose the job options you wish to use, such as Landscape modes (horizontal or vertical printout), copies, or scaling (increase or decrease the size of the printable area, which can be
used to fit an oversize print area onto a smaller physical sheet of print medium).
8. Managing Print Jobs
When you send a print job to the printer daemon, such as printing a text file from Emacs or
printing an image from The GIMP, the print job is added to the print spool queue. The print
spool queue is a list of print jobs that have been sent to the printer and information about each
print request, such as the status of the request, the the job number, and more.
During the printing process, the Printer Status icon appears in the Notification Area on the
panel. To check the status of a print job, double click the Printer Status, which displays a window similar to Figure 33.12, “GNOME Print Status”.
473
9. Additional Resources
Figure 33.12. GNOME Print Status
To cancel a specific print job listed in the GNOME Print Status, select it from the list and select
Edit => Cancel Documents from the pulldown menu.
To view the list of print jobs in the print spool from a shell prompt, type the command lpq. The
last few lines look similar to the following:
Rank
Owner/ID
active [email protected]+902
Class
A
Job Files
902 sample.txt
Size Time
2050 01:20:46
Example 33.1. Example of lpq output
If you want to cancel a print job, find the job number of the request with the command lpq and
then use the command lprm job number. For example, lprm 902 would cancel the print job in
Example 33.1, “Example of lpq output”. You must have proper permissions to cancel a print job.
You can not cancel print jobs that were started by other users unless you are logged in as root
on the machine to which the printer is attached.
You can also print a file directly from a shell prompt. For example, the command lpr sample.txt
prints the text file sample.txt. The print filter determines what type of file it is and converts it into
a format the printer can understand.
9. Additional Resources
To learn more about printing on Red Hat Enterprise Linux, refer to the following resources.
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9.2. Useful Websites
9.1. Installed Documentation
•
map lpr
— The manual page for the lpr command that allows you to print files from the command line.
•
man lprm
— The manual page for the command line utility to remove print jobs from the print
queue.
•
man mpage
— The manual page for the command line utility to print multiple pages on one
sheet of paper.
•
man cupsd
•
man cupsd.conf
•
man classes.conf
— The manual page for the CUPS printer daemon.
— The manual page for the CUPS printer daemon configuration file.
— The manual page for the class configuration file for CUPS.
9.2. Useful Websites
•
http://www.linuxprinting.org — GNU/Linux Printing contains a large amount of information
about printing in Linux.
•
http://www.cups.org/ — Documentation, FAQs, and newsgroups about CUPS.
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Chapter 34. Automated Tasks
In Linux, tasks can be configured to run automatically within a specified period of time, on a
specified date, or when the system load average is below a specified number. Red Hat Enterprise Linux is pre-configured to run important system tasks to keep the system updated. For example, the slocate database used by the locate command is updated daily. A system administrator can use automated tasks to perform periodic backups, monitor the system, run custom
scripts, and more.
Red Hat Enterprise Linux comes with several automated tasks utilities: cron, at, and batch.
1. Cron
Cron is a daemon that can be used to schedule the execution of recurring tasks according to a
combination of the time, day of the month, month, day of the week, and week.
Cron assumes that the system is on continuously. If the system is not on when a task is scheduled, it is not executed. To schedule one-time tasks, refer to Section 2, “At and Batch”.
To use the cron service, the vixie-cron RPM package must be installed and the crond service
must be running. To determine if the package is installed, use the rpm -q vixie-cron command.
To determine if the service is running, use the command /sbin/service crond status.
1.1. Configuring Cron Tasks
The main configuration file for cron, /etc/crontab, contains the following lines:
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root HOME=/
# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly
The first four lines are variables used to configure the environment in which the cron tasks are
run. The SHELL variable tells the system which shell environment to use (in this example the
bash shell), while the PATH variable defines the path used to execute commands. The output of
the cron tasks are emailed to the username defined with the MAILTO variable. If the MAILTO variable is defined as an empty string (MAILTO=""), email is not sent. The HOME variable can be used
to set the home directory to use when executing commands or scripts.
Each line in the /etc/crontab file represents a task and has the following format:
minute
hour
•
minute
•
hour
day
month
dayofweek
command
— any integer from 0 to 59
— any integer from 0 to 23
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1.1. Configuring Cron Tasks
•
day
— any integer from 1 to 31 (must be a valid day if a month is specified)
•
month
•
dayofweek
•
command
— any integer from 1 to 12 (or the short name of the month such as jan or feb)
— any integer from 0 to 7, where 0 or 7 represents Sunday (or the short name of
the week such as sun or mon)
— the command to execute (the command can either be a command such as ls
or the command to execute a custom script)
/
proc >> /tmp/proc
For any of the above values, an asterisk (*) can be used to specify all valid values. For example,
an asterisk for the month value means execute the command every month within the constraints
of the other values.
A hyphen (-) between integers specifies a range of integers. For example, 1-4 means the integers 1, 2, 3, and 4.
A list of values separated by commas (,) specifies a list. For example, 3,
those four specific integers.
4, 6, 8
indicates
The forward slash (/) can be used to specify step values. The value of an integer can be
skipped within a range by following the range with /<integer>. For example, 0-59/2 can be used
to define every other minute in the minute field. Step values can also be used with an asterisk.
For instance, the value */3 can be used in the month field to run the task every third month.
Any lines that begin with a hash mark (#) are comments and are not processed.
As shown in the /etc/crontab file, the run-parts script executes the scripts in the /
etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly/, and /etc/cron.monthly/ directories on
an hourly, daily, weekly, or monthly basis respectively. The files in these directories should be
shell scripts.
If a cron task is required to be executed on a schedule other than hourly, daily, weekly, or
monthly, it can be added to the /etc/cron.d/ directory. All files in this directory use the same
syntax as /etc/crontab. Refer to Example 34.1, “Crontab Examples” for examples.
# record the memory usage of the system every monday
# at 3:30AM in the file /tmp/meminfo
30 3 * * mon cat /proc/meminfo >> /tmp/meminfo
# run custom script the first day of every month at 4:10AM
10 4 1 * * /root/scripts/backup.sh
Example 34.1. Crontab Examples
Users other than root can configure cron tasks by using the crontab utility. All user-defined
crontabs are stored in the /var/spool/cron/ directory and are executed using the usernames of
the users that created them. To create a crontab as a user, login as that user and type the command crontab -e to edit the user's crontab using the editor specified by the VISUAL or EDITOR environment variable. The file uses the same format as /etc/crontab. When the changes to the
crontab are saved, the crontab is stored according to username and written to the file /
var/spool/cron/username.
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1.2. Controlling Access to Cron
The cron daemon checks the /etc/crontab file, the /etc/cron.d/ directory, and the /
var/spool/cron/ directory every minute for any changes. If any changes are found, they are
loaded into memory. Thus, the daemon does not need to be restarted if a crontab file is
changed.
1.2. Controlling Access to Cron
The /etc/cron.allow and /etc/cron.deny files are used to restrict access to cron. The format of
both access control files is one username on each line. Whitespace is not permitted in either
file. The cron daemon (crond) does not have to be restarted if the access control files are modified. The access control files are read each time a user tries to add or delete a cron task.
The root user can always use cron, regardless of the usernames listed in the access control
files.
If the file cron.allow exists, only users listed in it are allowed to use cron, and the cron.deny file
is ignored.
If cron.allow does not exist, users listed in cron.deny are not allowed to use cron.
1.3. Starting and Stopping the Service
To start the cron service, use the command /sbin/service crond start. To stop the service,
use the command /sbin/service crond stop. It is recommended that you start the service at
boot time. Refer to Chapter 15, Controlling Access to Services for details on starting the cron
service automatically at boot time.
2. At and Batch
While cron is used to schedule recurring tasks, the at command is used to schedule a one-time
task at a specific time and the batch command is used to schedule a one-time task to be executed when the systems load average drops below 0.8.
To use at or batch, the at RPM package must be installed, and the atd service must be running.
To determine if the package is installed, use the rpm -q at command. To determine if the service is running, use the command /sbin/service atd status.
2.1. Configuring At Jobs
To schedule a one-time job at a specific time, type the command at
to execute the command.
time,
where time is the time
The argument time can be one of the following:
•
HH:MM format — For example, 04:00 specifies 4:00 a.m. If the time is already past, it is executed at the specified time the next day.
•
midnight — Specifies 12:00 a.m.
•
noon — Specifies 12:00 p.m.
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2.2. Configuring Batch Jobs
•
teatime — Specifies 4:00 p.m.
•
month-name day year format — For example, January 15 2002 specifies the 15th day of
January in the year 2002. The year is optional.
•
MMDDYY, MM/DD/YY, or MM.DD.YY formats — For example, 011502 for the 15th day of
January in the year 2002.
•
now + time — time is in minutes, hours, days, or weeks. For example, now + 5 days specifies that the command should be executed at the same time five days from now.
The time must be specified first, followed by the optional date. For more information about the
time format, read the /usr/share/doc/at-<version>/timespec text file.
After typing the at command with the time argument, the at> prompt is displayed. Type the
command to execute, press Enter, and type Ctrl-D . Multiple commands can be specified by
typing each command followed by the Enter key. After typing all the commands, press Enter to
go to a blank line and type Ctrl-D . Alternatively, a shell script can be entered at the prompt,
pressing Enter after each line in the script, and typing Ctrl-D on a blank line to exit. If a script is
entered, the shell used is the shell set in the user's SHELL environment, the user's login shell, or
/bin/sh (whichever is found first).
If the set of commands or script tries to display information to standard out, the output is
emailed to the user.
Use the command atq to view pending jobs. Refer to Section 2.3, “Viewing Pending Jobs” for
more information.
Usage of the at command can be restricted. For more information, refer to Section 2.5,
“Controlling Access to At and Batch” for details.
2.2. Configuring Batch Jobs
To execute a one-time task when the load average is below 0.8, use the batch command.
After typing the batch command, the at> prompt is displayed. Type the command to execute,
press Enter, and type Ctrl-D . Multiple commands can be specified by typing each command
followed by the Enter key. After typing all the commands, press Enter to go to a blank line and
type Ctrl-D . Alternatively, a shell script can be entered at the prompt, pressing Enter after each
line in the script, and typing Ctrl-D on a blank line to exit. If a script is entered, the shell used is
the shell set in the user's SHELL environment, the user's login shell, or /bin/sh (whichever is
found first). As soon as the load average is below 0.8, the set of commands or script is executed.
If the set of commands or script tries to display information to standard out, the output is
emailed to the user.
Use the command atq to view pending jobs. Refer to Section 2.3, “Viewing Pending Jobs” for
more information.
Usage of the batch command can be restricted. For more information, refer to Section 2.5,
“Controlling Access to At and Batch” for details.
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2.4. Additional Command Line Options
2.3. Viewing Pending Jobs
To view pending at and batch jobs, use the atq command. The atq command displays a list of
pending jobs, with each job on a line. Each line follows the job number, date, hour, job class,
and username format. Users can only view their own jobs. If the root user executes the atq
command, all jobs for all users are displayed.
2.4. Additional Command Line Options
Additional command line options for at and batch include:
Option
Description
-f
Read the commands or shell script from a file instead of specifying
them at the prompt.
-m
Send email to the user when the job has been completed.
-v
Display the time that the job is executed.
Table 34.1. at and batch Command Line Options
2.5. Controlling Access to At and Batch
The /etc/at.allow and /etc/at.deny files can be used to restrict access to the at and batch
commands. The format of both access control files is one username on each line. Whitespace is
not permitted in either file. The at daemon (atd) does not have to be restarted if the access control files are modified. The access control files are read each time a user tries to execute the at
or batch commands.
The root user can always execute at and batch commands, regardless of the access control
files.
If the file at.allow exists, only users listed in it are allowed to use at or batch, and the at.deny
file is ignored.
If at.allow does not exist, users listed in at.deny are not allowed to use at or batch.
2.6. Starting and Stopping the Service
To start the at service, use the command /sbin/service atd start. To stop the service, use the
command /sbin/service atd stop. It is recommended that you start the service at boot time.
Refer to Chapter 15, Controlling Access to Services for details on starting the cron service automatically at boot time.
3. Additional Resources
To learn more about configuring automated tasks, refer to the following resources.
3.1. Installed Documentation
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3.1. Installed Documentation
•
cron
man page — overview of cron.
•
crontab
•
/usr/share/doc/at-<version>/timespec
man pages in sections 1 and 5 — The man page in section 1 contains an overview
of the crontab file. The man page in section 5 contains the format for the file and some example entries.
contains more detailed information about the times
that can be specified for cron jobs.
•
at
man page — description of at and batch and their command line options.
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Chapter 35. Log Files
Log files are files that contain messages about the system, including the kernel, services, and
applications running on it. There are different log files for different information. For example,
there is a default system log file, a log file just for security messages, and a log file for cron
tasks.
Log files can be very useful when trying to troubleshoot a problem with the system such as trying to load a kernel driver or when looking for unauthorized log in attempts to the system. This
chapter discusses where to find log files, how to view log files, and what to look for in log files.
Some log files are controlled by a daemon called syslogd. A list of log messages maintained by
syslogd can be found in the /etc/syslog.conf configuration file.
1. Locating Log Files
Most log files are located in the /var/log/ directory. Some applications such as httpd and samba
have a directory within /var/log/ for their log files.
You may notice multiple files in the log file directory with numbers after them. These are created
when the log files are rotated. Log files are rotated so their file sizes do not become too large.
The logrotate package contains a cron task that automatically rotates log files according to the
/etc/logrotate.conf configuration file and the configuration files in the /etc/logrotate.d/ directory. By default, it is configured to rotate every week and keep four weeks worth of previous
log files.
2. Viewing Log Files
Most log files are in plain text format. You can view them with any text editor such as Vi or
Emacs. Some log files are readable by all users on the system; however, root privileges are required to read most log files.
To view system log files in an interactive, real-time application, use the System Log Viewer. To
start the application, go to Applications (the main menu on the panel) => System => System
Logs, or type the command gnome-system-log at a shell prompt.
The application only displays log files that exist; thus, the list might differ from the one shown in
Figure 35.1, “System Log Viewer”.
482
2. Viewing Log Files
Figure 35.1. System Log Viewer
To filter the contents of the selected log file, click on View from the menu and select Filter as illustrated below.
483
3. Adding a Log File
Figure 35.2. System Log Viewer - View Menu
Selecting the Filter menu item will display the Filter text field where you can type the keywords
you wish to use for your filter. To clear your filter click on the Clear button.The figure below illustrates a sample filter.
Figure 35.3. System Log Viewer - Filter
3. Adding a Log File
To add a log file you wish to view in the list, select File => Open. This will display the Open Log
window where you can select the directory and filename of the log file you wish to view.The figure below illustrates the Open Log window.
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4. Monitoring Log Files
Figure 35.4. Adding a Log File
Click on the Open button to open the file. The file is immediately added to the viewing list where
you can select it and view the contents.
Please also note that the System Log Viewer also allows you to open zipped logs whose filenames end in ".gz".
4. Monitoring Log Files
System Log Viewer monitors all opened logs by default. If a new line is added to a monitored
log file, the log name appears in bold in the log list. If the log file is selected or displayed, the
new lines appear in bold at the bottom of the log file and after five seconds are displayed in normal format. This is illustrated in the figures below. The figure below illustrates a new alert in the
messages log file. The log file is listed in bold text.
485
4. Monitoring Log Files
Figure 35.5. Log File Alert
Clicking on the messages log file displays the logs in the file with the new lines in bold as illustrated below.
486
4. Monitoring Log Files
Figure 35.6. Log file contents
The new lines are displayed in bold for five seconds after which they are displayed in normal
font.
487
4. Monitoring Log Files
Figure 35.7. Log file contents after five seconds
488
Part V. System Monitoring
System administrators also monitor system performance. Red Hat Enterprise Linux contains
tools to assist administrators with these tasks.
Chapter 36. SystemTap
1. Introduction
SystemTap provides a simple command line interface and scripting language to simplify the
gathering of information about the running Linux kernel so that it can be further analyzed. Data
may be extracted, filtered, and summarized quickly and safely, to enable diagnoses of complex
performance or functional problems.
SystemTap allows scripts to be written in the SystemTap scripting language, which are then
compiled to C-code kernel modules and inserted into the kernel.
The essential idea behind a systemtap script is to name events, and to give them handlers.
Whenever a specified event occurs, the Linux kernel runs the handler as if it were a quick subroutine, then resumes. There are several kind of events, such as entering or exiting a function, a
timer expiring, or the entire systemtap session starting or stopping. A handler is a series of
script language statements that specify the work to be done whenever the event occurs. This
work normally includes extracting data from the event context, storing them into internal variables, or printing results.
2. Implementation
SystemTap takes a compiler-oriented approach to generating instrumentation. Refer to Figure 36.1, “Flow of Data in SystemTap” "Flow of data in SystemTap" for an overall diagram of
SystemTap used in this discussion. In the upper right hand corner of the diagram is the
probe.stp, the probe script the developer has written. This is parsed by the translator into parse
trees. During this time the input is checked for syntax errors. The translator then performs elaboration, pulling in additional code from the script library and determining locations of probe
points and variables from the debug information. After the elaboration is complete the translator
can generate the probe.c, the kernel module in C.
The probe.c file is compiled into a regular kernel module, probe.ko, using the GCC compiler.
The compilation may pull in support code from the runtime libraries. After GCC has generated
the probe.ko, the SystemTap daemon is started to collect the output of the instrumentation module. The instrumentation module is loaded into the kernel, and data collection is started. Data
from the instrumentation module is transferred to user-space via relayfs and displayed by the
daemon. When the user hits Control-C the daemon unloads the module, which also shuts down
the data collection process.
490
3. Using SystemTap
Figure 36.1. Flow of Data in SystemTap
3. Using SystemTap
Systemtap works by translating a SystemTap script to C, running the system C compiler to create a kernel module from that. When the module is loaded, it activates all the probed events by
hooking into the kernel. Then, as events occur on any processor, the compiled handlers run.
Eventually, the session stops, the hooks are disconnected, and the module removed. This entire process is driven from a single command-line program, stap.
3.1. Tracing
The simplest kind of probe is simply to trace an event. This is the effect of inserting strategically
located print statements into a program. This is often the first step of problem solving: explore
by seeing a history of what has happened.
This style of instrumentation is the simplest. It just asks systemtap to print something at each
event. To express this in the script language, you need to say where to probe and what to print
there.
3.1.1. Where to Probe
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3.1. Tracing
Systemtap supports a number of built-in events. The library of scripts that comes with systemtap, each called a "tapset", may define additional ones defined in terms of the built-in family.
See the stapprobes man page for details. All these events are named using a unified syntax that
looks like dot-separated parameterized identifiers:
Event
Description
begin
The startup of the systemtap session.
end
The end of the systemtap session.
kernel.function("sys_open")
The entry to the function named sys_open in
the kernel.
syscall.close.return
The return from the close system call..
module("ext3").statement(0xdeadbeef)
The addressed instruction in the ext3 filesystem driver.
timer.ms(200)
A timer that fires every 200 milliseconds.
Table 36.1. SystemTap Events
We will use as a demonstration case that you would like to trace all function entries and exits in
a source file, for example net/socket.c in the kernel. The kernel.function probe point lets you
express that easily, since systemtap examines the kernel's debugging information to relate object code to source code. It works like a debugger: if you can name or place it, you can probe it.
Use kernel.function("*@net/socket.c") for the function entries, and kernel.function("*@net/socket.c").return for the exits. Note the use of wildcards in the function
name part, and the subsequent @FILENAME part. You can also put wildcards into the file name,
and even add a colon (:) and a line number, if you want to restrict the search that precisely.
Since systemtap will put a separate probe in every place that matches a probe point, a few wildcards can expand to hundreds or thousands of probes, so be careful what you ask for.
Once you identify the probe points, the skeleton of the systemtap script appears. The probe
keyword introduces a probe point, or a comma-separated list of them. The following { and }
braces enclose the handler for all listed probe points.
You can run this script as is, though with empty handlers there will be no output. Put the two
lines into a new file. Run stap -v FILE. Terminate it any time with ^C. (The -v option tells systemtap to print more verbose messages during its processing. Try the -h option to see more options.)
3.1.2. What to Print
Since you are interested in each function that was entered and exited, a line should be printed
for each, containing the function name. In order to make that list easy to read, systemtap should
indent the lines so that functions called by other traced functions are nested deeper. To tell
each single process apart from any others that may be running concurrently, systemtap should
also print the process ID in the line.
492
Chapter 37. Gathering System
Information
Before you learn how to configure your system, you should learn how to gather essential system
information. For example, you should know how to find the amount of free memory, the amount
of available hard drive space, how your hard drive is partitioned, and what processes are running. This chapter discusses how to retrieve this type of information from your Red Hat Enterprise Linux system using simple commands and a few simple programs.
1. System Processes
The ps ax command displays a list of current system processes, including processes owned by
other users. To display the owner alongside each process, use the ps aux command. This list is
a static list; in other words, it is a snapshot of what was running when you invoked the command. If you want a constantly updated list of running processes, use top as described below.
The ps output can be long. To prevent it from scrolling off the screen, you can pipe it through
less:
ps aux | less
You can use the ps command in combination with the grep command to see if a process is running. For example, to determine if Emacs is running, use the following command:
ps ax | grep emacs
The top command displays currently running processes and important information about them
including their memory and CPU usage. The list is both real-time and interactive. An example of
output from the top command is provided as follows:
top - 15:02:46 up 35 min, 4 users, load average: 0.17, 0.65, 1.00 Tasks: 110 total, 1 running, 107 sleepin
To exit top, press the q key.
Table 37.1, “Interactive top commands” contains useful interactive commands that you can use
with top. For more information, refer to the top(1) manual page.
Command
Description
Space
Immediately refresh the display
h
Display a help screen
k
Kill a process. You are prompted for the process ID and the signal to send to it.
n
Change the number of processes displayed.
You are prompted to enter the number.
u
Sort by user.
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1. System Processes
Command
Description
M
Sort by memory usage.
P
Sort by CPU usage.
Table 37.1. Interactive top commands
If you prefer a graphical interface for top, you can use the GNOME System Monitor. To start it
from the desktop, select System => Administration => System Monitor or type gnome-system-monitor at a shell prompt (such as an XTerm). Select the Process Listing tab.
The GNOME System Monitor allows you to search for a process in the list of running processes. Using the Gnome System Monitor, you can also view all processes, your processes, or
active processes.
The Edit menu item allows you to:
•
Stop a process.
•
Continue or start a process.
•
End a processes.
•
Kill a process.
•
Change the priority of a selected process.
•
Edit the System Monitor preferences. These include changing the interval seconds to refresh
the list and selecting process fields to display in the System Monitor window.
The View menu item allows you to:
•
View only active processes.
•
View all processes.
•
View my processes.
•
View process dependencies.
•
Hide a process.
•
View hidden processes.
•
View memory maps.
•
View the files opened by the selected process.
To stop a process, select it and click End Process. Alternatively you can also stop a process by
selecting it, clicking Edit on your menu and selecting Stop Process.
To sort the information by a specific column, click on the name of the column. This sorts the in494
2. Memory Usage
formation by the selected column in ascending order. Click on the name of the column again to
toggle the sort between ascending and descending order.
Figure 37.1. GNOME System Monitor
2. Memory Usage
The free command displays the total amount of physical memory and swap space for the system as well as the amount of memory that is used, free, shared, in kernel buffers, and cached.
total used free shared buffers cached Mem: 645712 549720 95992 0 176248 224452 -/+ buffers/cache: 149020 4
The command free
-m
shows the same information in megabytes, which are easier to read.
total used free shared buffers cached Mem: 630 536 93 0 172 219 -/+ buffers/cache: 145 485 Swap: 1279 0 12
If you prefer a graphical interface for free, you can use the GNOME System Monitor. To start it
from the desktop, go to System => Administration => System Monitor or type gnome-system-monitor at a shell prompt (such as an XTerm). Click on the Resources tab.
495
3. File Systems
Figure 37.2. GNOME System Monitor - Resources tab
3. File Systems
The df command reports the system's disk space usage. If you type the command df at a shell
prompt, the output looks similar to the following:
Filesystem 1K-blocks Used Available Use% Mounted on /dev/mapper/VolGroup00-LogVol00 11675568 6272120 48103
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4. Hardware
By default, this utility shows the partition size in 1 kilobyte blocks and the amount of used and
available disk space in kilobytes. To view the information in megabytes and gigabytes, use the
command df -h. The -h argument stands for human-readable format. The output looks similar
to the following:
Filesystem Size Used Avail Use% Mounted on /dev/mapper/VolGroup00-LogVol00 12G 6.0G 4.6G 57% / /dev/sda1 9
In the list of mounted partitions, there is an entry for /dev/shm. This entry represents the system's virtual memory file system.
The du command displays the estimated amount of space being used by files in a directory. If
you type du at a shell prompt, the disk usage for each of the subdirectories is displayed in a list.
The grand total for the current directory and subdirectories are also shown as the last line in the
list. If you do not want to see the totals for all the subdirectories, use the command du -hs to
see only the grand total for the directory in human-readable format. Use the du --help command to see more options.
To view the system's partitions and disk space usage in a graphical format, use the Gnome
System Monitor by clicking on System => Administration => System Monitor or type gnomesystem-monitor at a shell prompt (such as an XTerm). Select the File Systems tab to view the
system's partitions. The figure below illustrates the File Systems tab.
Figure 37.3. GNOME System Monitor - File Systems
4. Hardware
497
4. Hardware
If you are having trouble configuring your hardware or just want to know what hardware is in
your system, you can use the Hardware Browser application to display the hardware that can
be probed. To start the program from the desktop, select System (the main menu on the panel)
=> Administration => Hardware or type hwbrowser at a shell prompt. As shown in Figure 37.4,
“Hardware Browser”, it displays your CD-ROM devices, diskette drives, hard drives and their
partitions, network devices, pointing devices, system devices, and video cards. Click on the category name in the left menu, and the information is displayed.
Figure 37.4. Hardware Browser
The Device Manager application can also be used to display your system hardware. This application can be started by selecting System (the main menu on the panel) => Administration
=> Hardware like the Hardware Browser. To start the application from a terminal, type haldevice-manager. Depending on your installation preferences, the graphical menu above may
start this application or the Hardware Browser when clicked. The figure below illustrates the
Device Manager window.
498
5. Additional Resources
Figure 37.5. Device Manager
You can also use the lspci command to list all PCI devices. Use the command lspci
more verbose information or lspci -vv for very verbose output.
-v
for
For example, lspci can be used to determine the manufacturer, model, and memory size of a
system's video card:
00:00.0 Host bridge: ServerWorks CNB20LE Host Bridge (rev 06) 00:00.1 Host bridge: ServerWorks CNB20LE Hos
The lspci is also useful to determine the network card in your system if you do not know the
manufacturer or model number.
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5.1. Installed Documentation
5. Additional Resources
To learn more about gathering system information, refer to the following resources.
5.1. Installed Documentation
•
ps --help
— Displays a list of options that can be used with ps.
•
top
•
free
•
df
manual page — Type man
df
to learn more about the df command and its many options.
•
du
manual page — Type man
du
to learn more about the du command and its many options.
•
lspci
•
/proc/
manual page — Type man
top
manual page — type man
to learn more about top and its many options.
free
manual page — Type man
options.
to learn more about free and its many options.
lspci
to learn more about the lspci command and its many
directory — The contents of the /proc/ directory can also be used to gather more detailed system information.
500
Chapter 38. OProfile
OProfile is a low overhead, system-wide performance monitoring tool. It uses the performance
monitoring hardware on the processor to retrieve information about the kernel and executables
on the system, such as when memory is referenced, the number of L2 cache requests, and the
number of hardware interrupts received. On a Red Hat Enterprise Linux system, the oprofile
RPM package must be installed to use this tool.
Many processors include dedicated performance monitoring hardware. This hardware makes it
possible to detect when certain events happen (such as the requested data not being in cache).
The hardware normally takes the form of one or more counters that are incremented each time
an event takes place. When the counter value, essentially rolls over, an interrupt is generated,
making it possible to control the amount of detail (and therefore, overhead) produced by performance monitoring.
OProfile uses this hardware (or a timer-based substitute in cases where performance monitoring
hardware is not present) to collect samples of performance-related data each time a counter
generates an interrupt. These samples are periodically written out to disk; later, the data contained in these samples can then be used to generate reports on system-level and applicationlevel performance.
OProfile is a useful tool, but be aware of some limitations when using it:
•
Use of shared libraries — Samples for code in shared libraries are not attributed to the particular application unless the --separate=library option is used.
•
Performance monitoring samples are inexact — When a performance monitoring register
triggers a sample, the interrupt handling is not precise like a divide by zero exception. Due to
the out-of-order execution of instructions by the processor, the sample may be recorded on a
nearby instruction.
•
opreport
•
OProfile accumulates data from multiple runs — OProfile is a system-wide profiler and expects processes to start up and shut down multiple times. Thus, samples from multiple runs
accumulate. Use the command opcontrol --reset to clear out the samples from previous
runs.
•
Non-CPU-limited performance problems — OProfile is oriented to finding problems with
CPU-limited processes. OProfile does not identify processes that are asleep because they
are waiting on locks or for some other event to occur (for example an I/O device to finish an
operation).
does not associate samples for inline functions' properly — opreport uses a simple
address range mechanism to determine which function an address is in. Inline function
samples are not attributed to the inline function but rather to the function the inline function
was inserted into.
1. Overview of Tools
Table 38.1, “OProfile Commands” provides a brief overview of the tools provided with the opro-
501
2. Configuring OProfile
file
package.
Command
Description
ophelp
Displays available events for the system's processor along with a
brief description of each.
opimport
Converts sample database files from a foreign binary format to the
native format for the system. Only use this option when analyzing
a sample database from a different architecture.
Creates annotated source for an executable if the application was
compiled with debugging symbols. Refer to Section 5.4, “Using
opannotate” for details.
opannotate
opcontrol
Configures what data is collected. Refer to Section 2, “Configuring
OProfile” for details.
opreport
Retrieves profile data. Refer to Section 5.1, “Using opreport” for
details.
oprofiled
Runs as a daemon to periodically write sample data to disk.
Table 38.1. OProfile Commands
2. Configuring OProfile
Before OProfile can be run, it must be configured. At a minimum, selecting to monitor the kernel
(or selecting not to monitor the kernel) is required. The following sections describe how to use
the opcontrol utility to configure OProfile. As the opcontrol commands are executed, the setup
options are saved to the /root/.oprofile/daemonrc file.
2.1. Specifying the Kernel
First, configure whether OProfile should monitor the kernel. This is the only configuration option
that is required before starting OProfile. All others are optional.
To monitor the kernel, execute the following command as root:
opcontrol --setup --vmlinux=/usr/lib/debug/lib/modules/`uname -r`/vmlinux
Note
The debuginfo package must be installed (which contains the uncompressed kernel) in order to monitor the kernel.
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2.2. Setting Events to Monitor
To configure OProfile not to monitor the kernel, execute the following command as root:
opcontrol --setup --no-vmlinux
This command also loads the oprofile kernel module, if it is not already loaded, and creates the
/dev/oprofile/ directory, if it does not already exist. Refer to Section 6, “Understanding /
dev/oprofile/” for details about this directory.
Note
Even if OProfile is configured not to profile the kernel, the SMP kernel still must be
running so that the oprofile module can be loaded from it.
Setting whether samples should be collected within the kernel only changes what data is collected, not how or where the collected data is stored. To generate different sample files for the kernel and application libraries, refer to Section 2.3, “Separating Kernel and User-space Profiles”.
2.2. Setting Events to Monitor
Most processors contain counters, which are used by OProfile to monitor specific events. As
shown in Table 38.2, “OProfile Processors and Counters”, the number of counters available depends on the processor.
Processor
cpu_type
Number of Counters
Pentium Pro
i386/ppro
2
Pentium II
i386/pii
2
Pentium III
i386/piii
2
Pentium 4 (non-hyper-threaded)
i386/p4
8
Pentium 4 (hyper-threaded)
i386/p4-ht
4
Athlon
i386/athlon
4
AMD64
x86-64/hammer
4
Itanium
ia64/itanium
4
Itanium 2
ia64/itanium2
4
TIMER_INT
timer
1
IBM eServer iSeries and pSeries
timer
1
ppc64/power4
8
ppc64/power5
6
ppc64/970
8
timer
1
IBM eServer S/390 and S/390x
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2.2. Setting Events to Monitor
Processor
cpu_type
Number of Counters
IBM eServer zSeries
timer
1
Table 38.2. OProfile Processors and Counters
Use Table 38.2, “OProfile Processors and Counters” to verify that the correct processor type
was detected and to determine the number of events that can be monitored simultaneously.
timer is used as the processor type if the processor does not have supported performance
monitoring hardware.
If timer is used, events cannot be set for any processor because the hardware does not have
support for hardware performance counters. Instead, the timer interrupt is used for profiling.
If timer is not used as the processor type, the events monitored can be changed, and counter 0
for the processor is set to a time-based event by default. If more than one counter exists on the
processor, the counters other than counter 0 are not set to an event by default. The default
events monitored are shown in Table 38.3, “Default Events”.
Processor
Default Event for Counter
Description
Pentium Pro, Pentium CPU_CLK_UNHALTED
II, Pentium III, Athlon,
AMD64
The processor's clock is not halted
Pentium 4 (HT and
non-HT)
GLOBAL_POWER_EVENTS
The time during which the processor
is not stopped
Itanium 2
CPU_CYCLES
CPU Cycles
TIMER_INT
(none)
Sample for each timer interrupt
ppc64/power4
CYCLES
Processor Cycles
ppc64/power5
CYCLES
Processor Cycles
ppc64/970
CYCLES
Processor Cycles
Table 38.3. Default Events
The number of events that can be monitored at one time is determined by the number of counters for the processor. However, it is not a one-to-one correlation; on some processors, certain
events must be mapped to specific counters. To determine the number of counters available,
execute the following command:
ls -d /dev/oprofile/[0-9]*
The events available vary depending on the processor type. To determine the events available
for profiling, execute the following command as root (the list is specific to the system's processor type):
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2.3. Separating Kernel and User-space Profiles
ophelp
The events for each counter can be configured via the command line or with a graphical interface. For more information on the graphical interface, refer to Section 8, “Graphical Interface”. If
the counter cannot be set to a specific event, an error message is displayed.
To set the event for each configurable counter via the command line, use opcontrol:
opcontrol --event=<event-name>:<sample-rate>
Replace <event-name> with the exact name of the event from ophelp, and replace <sample-rate>
with the number of events between samples.
2.2.1. Sampling Rate
By default, a time-based event set is selected. It creates a sample every 100,000 clock cycles
per processor. If the timer interrupt is used, the timer is set to whatever the jiffy rate is and is not
user-settable. If the cpu_type is not timer, each event can have a sampling rate set for it. The
sampling rate is the number of events between each sample snapshot.
When setting the event for the counter, a sample rate can also be specified:
opcontrol --event=<event-name>:<sample-rate>
Replace <sample-rate> with the number of events to wait before sampling again. The smaller
the count, the more frequent the samples. For events that do not happen frequently, a lower
count may be needed to capture the event instances.
Caution
Be extremely careful when setting sampling rates. Sampling too frequently can
overload the system, causing the system to appear as if it is frozen or causing the
system to actually freeze.
2.2.2. Unit Masks
Some user performance monitoring events may also require unit masks to further define the
event.
Unit masks for each event are listed with the ophelp command. The values for each unit mask
are listed in hexadecimal format. To specify more than one unit mask, the hexadecimal values
must be combined using a bitwise or operation.
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>
2.3. Separating Kernel and User-space Profiles
By default, kernel mode and user mode information is gathered for each event. To configure
OProfile to ignore events in kernel mode for a specific counter, execute the following command:
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3. Starting and Stopping OProfile
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:0
Execute the following command to start profiling kernel mode for the counter again:
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:1
To configure OProfile to ignore events in user mode for a specific counter, execute the following
command:
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:<kernel>:0
Execute the following command to start profiling user mode for the counter again:
opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:<kernel>:1
When the OProfile daemon writes the profile data to sample files, it can separate the kernel and
library profile data into separate sample files. To configure how the daemon writes to sample
files, execute the following command as root:
opcontrol --separate=<choice>
<choice>
can be one of the following:
•
none
— do not separate the profiles (default)
•
library
•
kernel
•
all
— generate per-application profiles for libraries
— generate per-application profiles for the kernel and kernel modules
— generate per-application profiles for libraries and per-application profiles for the kernel
and kernel modules
If --separate=library is used, the sample file name includes the name of the executable as well
as the name of the library.
Note
These configuration changes will take effect when oprofile is restarted.
3. Starting and Stopping OProfile
To start monitoring the system with OProfile, execute the following command as root:
opcontrol --start
Output similar to the following is displayed:
Using log file /var/lib/oprofile/oprofiled.log Daemon started. Profiler running.
The settings in /root/.oprofile/daemonrc are used.
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4. Saving Data
The OProfile daemon, oprofiled, is started; it periodically writes the sample data to the /
var/lib/oprofile/samples/ directory. The log file for the daemon is located at /
var/lib/oprofile/oprofiled.log.
To stop the profiler, execute the following command as root:
opcontrol --shutdown
4. Saving Data
Sometimes it is useful to save samples at a specific time. For example, when profiling an executable, it may be useful to gather different samples based on different input data sets. If the
number of events to be monitored exceeds the number of counters available for the processor,
multiple runs of OProfile can be used to collect data, saving the sample data to different files
each time.
To save the current set of sample files, execute the following command, replacing <name> with a
unique descriptive name for the current session.
opcontrol --save=<name>
The directory /var/lib/oprofile/samples/name/ is created and the current sample files are
copied to it.
5. Analyzing the Data
Periodically, the OProfile daemon, oprofiled, collects the samples and writes them to the /
var/lib/oprofile/samples/ directory. Before reading the data, make sure all data has been written to this directory by executing the following command as root:
opcontrol --dump
Each sample file name is based on the name of the executable. For example, the samples for
the default event on a Pentium III processor for /bin/bash becomes:
\{root\}/bin/bash/\{dep\}/\{root\}/bin/bash/CPU_CLK_UNHALTED.100000
The following tools are available to profile the sample data once it has been collected:
•
opreport
•
opannotate
Use these tools, along with the binaries profiled, to generate reports that can be further analyzed.
Warning
The executable being profiled must be used with these tools to analyze the data. If
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5.1. Using opreport
it must change after the data is collected, backup the executable used to create
the samples as well as the sample files. Please note that the sample file and the
binary have to agree. Making a backup isn't going to work if they do not match.
oparchive can be used to address this problem.
Samples for each executable are written to a single sample file. Samples from each dynamically
linked library are also written to a single sample file. While OProfile is running, if the executable
being monitored changes and a sample file for the executable exists, the existing sample file is
automatically deleted. Thus, if the existing sample file is needed, it must be backed up, along
with the executable used to create it before replacing the executable with a new version. The
oprofile analysis tools use the executable file that created the samples during analysis. If the executable changes the analysis tools will be unable to analyze the associated samples. Refer to
Section 4, “Saving Data” for details on how to backup the sample file.
5.1. Using opreport
The opreport tool provides an overview of all the executables being profiled.
The following is part of a sample output:
Profiling through timer interrupt TIMER:0| samples| %| ------------------ 25926 97.5212 no-vmlinux 359 1.3
Each executable is listed on its own line. The first column is the number of samples recorded for
the executable. The second column is the percentage of samples relative to the total number of
samples. The third column is the name of the executable.
Refer to the opreport man page for a list of available command line options, such as the -r option used to sort the output from the executable with the smallest number of samples to the one
with the largest number of samples.
5.2. Using opreport on a Single Executable
To retrieve more detailed profiled information about a specific executable, use opreport:
opreport <mode><executable>
<executable>
must be the full path to the executable to be analyzed. <mode> must be one of the
following:
-l
List sample data by symbols. For example, the following is part of the output from running
the command opreport -l /lib/tls/libc-<version>.so:
samples % symbol name 12 21.4286 __gconv_transform_utf8_internal 5 8.9286 _int_malloc 4 7.1429 malloc
The first column is the number of samples for the symbol, the second column is the percentage of samples for this symbol relative to the overall samples for the executable, and the
third column is the symbol name.
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5.3. Getting more detailed output on the modules
To sort the output from the largest number of samples to the smallest (reverse order), use r in conjunction with the -l option.
-i <symbol-name>
List sample data specific to a symbol name. For example, the following output is from the
command opreport -l -i __gconv_transform_utf8_internal /lib/tls/libc-<version>.so:
samples % symbol name 12 100.000 __gconv_transform_utf8_internal
The first line is a summary for the symbol/executable combination.
The first column is the number of samples for the memory symbol. The second column is
the percentage of samples for the memory address relative to the total number of samples
for the symbol. The third column is the symbol name.
-d
List sample data by symbols with more detail than -l. For example, the following output is
from the command opreport -l -d __gconv_transform_utf8_internal
/lib/tls/libc-<version>.so:
vma samples % symbol name 00a98640 12 100.000 __gconv_transform_utf8_internal 00a98640 1 8.3333 00a986
The data is the same as the -l option except that for each symbol, each virtual memory address used is shown. For each virtual memory address, the number of samples and percentage of samples relative to the number of samples for the symbol is displayed.
-x<symbol-name>
Exclude the comma-separated list of symbols from the output.
session:<name>
Specify the full path to the session or a directory relative to the /var/lib/oprofile/samples/
directory.
5.3. Getting more detailed output on the modules
OProfile collects data on a system-wide basis for kernel- and user-space code running on the
machine. However, once a module is loaded into the kernel, the information about the origin of
the kernel module is lost. The module could have come from the initrd file on boot up, the directory with the various kernel modules, or a locally created kernel module. As a result when OProfile records sample for a module, it just lists the samples for the modules for an executable in
the root directory, but this is unlikely to be the place with the actual code for the module. You
will need to take some steps to make sure that analysis tools get the executable.
For example on an AMD64 machine the sampling is set up to record "Data cache accesses"
and "Data cache misses" and assuming you would like to see the data for the ext3 module:
$ opreport /ext3
CPU: AMD64 processors, speed 797.948 MHz (estimated)
Counted DATA_CACHE_ACCESSES events (Data cache accesses) with a unit mask of 0x00 (No unit mask) count 5000
Counted DATA_CACHE_MISSES events (Data cache misses) with a unit mask of 0x00 (No unit mask) count 500000
DATA_CACHE_ACC...|DATA_CACHE_MIS...|
samples|
%| samples|
%|
-----------------------------------148721 100.000
1493 100.000 ext3
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5.4. Using opannotate
To get a more detailed view of the actions of the module, you will need to either have the module unstripped (e.g. installed from a custom build) or have the debuginfo RPM installed for the
kernel.
Find out which kernel is running, "uname -a", get the appropriate debuginfo rpm, and install on
the machine.
Then make a symbolic link so oprofile finds the code for the module in the correct place:
# ln -s /lib/modules/`uname -r`/kernel/fs/ext3/ext3.ko /ext3
Then the detailed information can be obtained with:
# opreport image:/ext3 -l|more
warning: could not check that the binary file /ext3 has not been modified since the profile was taken. Resu
CPU: AMD64 processors, speed 797.948 MHz (estimated)
Counted DATA_CACHE_ACCESSES events (Data cache accesses) with a unit mask of 0x00 (No unit mask) count 5000
Counted DATA_CACHE_MISSES events (Data cache misses) with a unit mask of 0x00 (No unit mask) count 500000
samples %
samples %
symbol name
16728
11.2479 7
0.4689 ext3_group_sparse
16454
11.0637 4
0.2679 ext3_count_free_blocks
14583
9.8056 51
3.4159 ext3_fill_super
8281
5.5681 129
8.6403 ext3_ioctl
7810
5.2514 62
4.1527 ext3_write_info
7286
4.8991 67
4.4876 ext3_ordered_writepage
6509
4.3767 130
8.7073 ext3_new_inode
6378
4.2886 156
10.4488 ext3_new_block
5932
3.9887 87
5.8272 ext3_xattr_block_list
...
5.4. Using opannotate
The opannotate tool tries to match the samples for particular instructions to the corresponding
lines in the source code. The resulting files generated should have the samples for the lines at
the left. It also puts in a comment at the beginning of each function listing the total samples for
the function.
For this utility to work, the executable must be compiled with GCC's -g option. By default, Red
Hat Enterprise Linux packages are not compiled with this option.
The general syntax for opannotate is as follows:
opannotate --search-dirs <src-dir> --source <executable>
The directory containing the source code and the executable to be analyzed must be specified.
Refer to the opannotate man page for a list of additional command line options.
6. Understanding
/dev/oprofile/
The /dev/oprofile/ directory contains the file system for OProfile. Use the cat command to display the values of the virtual files in this file system. For example, the following command displays the type of processor OProfile detected:
cat /dev/oprofile/cpu_type
A directory exists in /dev/oprofile/ for each counter. For example, if there are 2 counters, the
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7. Example Usage
directories /dev/oprofile/0/ and dev/oprofile/1/ exist.
Each directory for a counter contains the following files:
•
count
— The interval between samples.
•
enabled
•
event
•
kernel
•
unit_mask
•
user
— If 0, the counter is off and no samples are collected for it; if 1, the counter is on
and samples are being collected for it.
— The event to monitor.
— If 0, samples are not collected for this counter event when the processor is in kernel-space; if 1, samples are collected even if the processor is in kernel-space.
— Defines which unit masks are enabled for the counter.
— If 0, samples are not collected for the counter event when the processor is in userspace; if 1, samples are collected even if the processor is in user-space.
The values of these files can be retrieved with the cat command. For example:
cat /dev/oprofile/0/count
7. Example Usage
While OProfile can be used by developers to analyze application performance, it can also be
used by system administrators to perform system analysis. For example:
•
Determine which applications and services are used the most on a system — opreport can
be used to determine how much processor time an application or service uses. If the system
is used for multiple services but is under performing, the services consuming the most processor time can be moved to dedicated systems.
•
Determine processor usage — The CPU_CLK_UNHALTED event can be monitored to determine
the processor load over a given period of time. This data can then be used to determine if
additional processors or a faster processor might improve system performance.
8. Graphical Interface
Some OProfile preferences can be set with a graphical interface. To start it, execute the
oprof_start command as root at a shell prompt. To use the graphical interface, you will need to
have the oprofile-gui package installed.
After changing any of the options, save them by clicking the Save and quit button. The preferences are written to /root/.oprofile/daemonrc, and the application exits. Exiting the application
does not stop OProfile from sampling.
On the Setup tab, to set events for the processor counters as discussed in Section 2.2, “Setting
Events to Monitor”, select the counter from the pulldown menu and select the event from the list.
A brief description of the event appears in the text box below the list. Only events available for
the specific counter and the specific architecture are displayed. The interface also displays
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8. Graphical Interface
whether the profiler is running and some brief statistics about it.
Figure 38.1. OProfile Setup
On the right side of the tab, select the Profile kernel option to count events in kernel mode for
the currently selected event, as discussed in Section 2.3, “Separating Kernel and User-space
Profiles”. If this option is unselected, no samples are collected for the kernel.
Select the Profile user binaries option to count events in user mode for the currently selected
event, as discussed in Section 2.3, “Separating Kernel and User-space Profiles”. If this option is
unselected, no samples are collected for user applications.
Use the Count text field to set the sampling rate for the currently selected event as discussed in
Section 2.2.1, “Sampling Rate”.
If any unit masks are available for the currently selected event, as discussed in Section 2.2.2,
“Unit Masks”, they are displayed in the Unit Masks area on the right side of the Setup tab. Select the checkbox beside the unit mask to enable it for the event.
512
9. Additional Resources
On the Configuration tab, to profile the kernel, enter the name and location of the vmlinux file
for the kernel to monitor in the Kernel image file text field. To configure OProfile not to monitor
the kernel, select No kernel image.
Figure 38.2. OProfile Configuration
If the Verbose option is selected, the oprofiled daemon log includes more information.
If Per-application kernel samples files is selected, OProfile generates per-application profiles
for the kernel and kernel modules as discussed in Section 2.3, “Separating Kernel and Userspace Profiles”. This is equivalent to the opcontrol --separate=kernel command. If Perapplication shared libs samples files is selected, OProfile generates per-application profiles
for libraries. This is equivalent to the opcontrol --separate=library command.
To force data to be written to samples files as discussed in Section 5, “Analyzing the Data”, click
the Flush profiler data button. This is equivalent to the opcontrol --dump command.
To start OProfile from the graphical interface, click Start profiler. To stop the profiler, click Stop
profiler. Exiting the application does not stop OProfile from sampling.
9. Additional Resources
This chapter only highlights OProfile and how to configure and use it. To learn more, refer to the
following resources.
9.1. Installed Docs
•
/usr/share/doc/oprofile-<version>/oprofile.html
•
oprofile
— OProfile Manual
man page — Discusses opcontrol, opreport, opannotate, and ophelp
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9.2. Useful Websites
9.2. Useful Websites
•
http://oprofile.sourceforge.net/ — Contains the latest documentation, mailing lists, IRC channels, and more.
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Part VI. Kernel and Driver
Configuration
System administrators can learn about and customize their kernels. Red Hat Enterprise Linux
contains kernel tools to assist administrators with their customizations.
Chapter 39. Manually Upgrading the
Kernel
The Red Hat Enterprise Linux kernel is custom built by the Red Hat Enterprise Linux kernel
team to ensure its integrity and compatibility with supported hardware. Before Red Hat releases
a kernel, it must first pass a rigorous set of quality assurance tests.
Red Hat Enterprise Linux kernels are packaged in RPM format so that they are easy to upgrade
and verify using the Package Management Tool, or the yum command. The Package Management Tool automatically queries the Red Hat Enterprise Linux servers and determines which
packages need to be updated on your machine, including the kernel. This chapter is only useful
for those individuals that require manual updating of kernel packages, without using the yum
command.
Warning
Building a custom kernel is not supported by the Red Hat Global Services Support
team, and therefore is not explored in this manual.
Tip
The use of yum is highly recommended by Red Hat for installing upgraded kernels.
For more information on Red Hat Network, the Package Management Tool, and yum, refer to
Chapter 12, Red Hat Network.
1. Overview of Kernel Packages
Red Hat Enterprise Linux contains the following kernel packages (some may not apply to your
architecture):
•
kernel
— Contains the kernel for multi-processor systems. For x86 system, only the first
4GB of RAM is used. As such, x86 systems with over 4GB of RAM should use the kernelPAE.
•
kernel-devel
•
kernel-PAE
— Contains the kernel headers and makefiles sufficient to build modules
against the kernel package.
(only for i686 systems) — This package offers the following key configuration options (in addition to the options already enabled for the kernel package):
•
Support for over 4GB of RAM (up to 64GB for the x86)
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2. Preparing to Upgrade
•
PAE (Physical Address Extension) or 3-level paging on x86 processors that support PAE
•
4GB/4GB split: 4GB of virtual address space for the kernel and almost 4GB for each user
process on x86 systems
•
kernel-PAE-devel
— Contains the kernel headers and makefiles required to build modules
against the kernel-PAE package
•
kernel-doc
— Contains documentation files from the kernel source. Various portions of the
Linux kernel and the device drivers shipped with it are documented in these files. Installation
of this package provides a reference to the options that can be passed to Linux kernel modules at load time.
By default, these files are placed in the /usr/share/doc/kernel-doc-<version>/ directory.
•
kernel-headers
— Includes the C header files that specify the interface between the Linux
kernel and userspace libraries and programs. The header files define structures and constants that are needed for building most standard programs.
•
kernel-xen
•
kernel-xen-devel
— Includes a version of the Linux kernel which is needed to run Virtualization.
— Contains the kernel headers and makefiles required to build modules
against the kernel-xen package
Note
The kernel-source package has been removed and replaced with an RPM that can
only be retrieved from Red Hat Network. This *.src.rpm package must then be rebuilt locally using the rpmbuild command. For more information on obtaining and
installing the kernel source package, refer to the latest updated Release Notes
(including all updates) at http://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/index.html
2. Preparing to Upgrade
Before upgrading the kernel, it is recommended that you take some precautionary steps. The
first step is to make sure working boot media exists for the system in case a problem occurs. If
the boot loader is not configured properly to boot the new kernel, the system cannot be booted
into Red Hat Enterprise Linux without working boot media.
To create a boot diskette, login as root, and run the command /sbin/mkbootdisk
shell prompt.
`uname -r`
at a
Tip
Refer to the mkbootdisk man page for more options. You can create bootable me-
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3. Downloading the Upgraded Kernel
dia via CD-Rs, CD-RWs, and USB flash drives, provided that your system BIOS
also supports it.
Reboot the machine with the boot media and verify that it works before continuing.
To determine which kernel packages are installed, execute the command rpm
nel at a shell prompt:
-qa | grep ker-
The output contains some or all of the following packages, depending on the system's architecture (the version numbers and packages may differ):
kernel-2.6.9-5.EL
kernel-devel-2.6.9-5.EL
kernel-utils-2.6.9-5.EL
kernel-doc-2.6.9-5.EL
kernel-smp-2.6.9-5.EL
kernel-smp-devel-2.6.9-5.EL
kernel-hugemem-devel-2.6.9-5.EL
From the output, determine which packages need to be download for the kernel upgrade. For a
single processor system, the only required package is the kernel package. Refer to Section 1,
“Overview of Kernel Packages” for descriptions of the different packages.
In the file name, each kernel package contains the architecture for which the package was built.
The format is kernel-<variant>-<version>.<arch>.rpm, where <variant> is one of either PAE, xen,
and so forth. The <arch> is one of the following:
•
x86_64
•
ia64
•
ppc64
•
s390
•
s390x
•
i686
for the AMD64 and Intel EM64T architectures
for the Intel®Itanium™ architecture
for the IBM®eServer™pSeries™ architecture
for the IBM®S/390® architecture
for the IBM®eServer™System z® architecture
for Intel®Pentium® II, Intel®Pentium® III, Intel®Pentium® 4, AMD Athlon®, and AMD
Duron® systems
3. Downloading the Upgraded Kernel
There are several ways to determine if an updated kernel is available for the system.
•
Security Errata — Refer to http://www.redhat.com/security/updates/ for information on security errata, including kernel upgrades that fix security issues.
•
Via Red Hat Network — Download and install the kernel RPM packages. Red Hat Network
can download the latest kernel, upgrade the kernel on the system, create an initial RAM disk
image if needed, and configure the boot loader to boot the new kernel. For more information,
518
4. Performing the Upgrade
refer to http://www.redhat.com/docs/manuals/RHNetwork/
[http://www.redhat.com/docs/manuals/RHNetwork/].
If Red Hat Network was used to download and install the updated kernel, follow the instructions
in Section 5, “Verifying the Initial RAM Disk Image” and Section 6, “Verifying the Boot Loader”,
only do not change the kernel to boot by default. Red Hat Network automatically changes the
default kernel to the latest version. To install the kernel manually, continue to Section 4,
“Performing the Upgrade”.
4. Performing the Upgrade
After retrieving all of the necessary packages, it is time to upgrade the existing kernel.
Important
It is strongly recommended that you keep the old kernel in case there are problems
with the new kernel.
At a shell prompt, change to the directory that contains the kernel RPM packages. Use -i argument with the rpm command to keep the old kernel. Do not use the -U option, since it overwrites
the currently installed kernel, which creates boot loader problems. For example:
rpm -ivh kernel-<kernel version>.<arch>.rpm
The next step is to verify that the initial RAM disk image has been created. Refer to Section 5,
“Verifying the Initial RAM Disk Image” for details.
5. Verifying the Initial RAM Disk Image
If the system uses the ext3 file system, a SCSI controller, or labels to reference partitions in /
etc/fstab, an initial RAM disk is needed. The initial RAM disk allows a modular kernel to have
access to modules that it might need to boot from before the kernel has access to the device
where the modules normally reside.
On architectures other than IBM eServer iSeries, the initial RAM disk can be created with the
mkinitrd command. However, this step is performed automatically if the kernel and its associated packages are installed or upgraded from the RPM packages distributed by Red Hat; in
such cases, you do not need to create the initial RAM disk manually. To verify that an initial
RAM disk already exists, use the command ls -l /boot to make sure the initrd-<version>.img
file was created (the version should match the version of the kernel just installed).
On iSeries systems, the initial RAM disk file and vmlinux file are combined into one file, which is
created with the addRamDisk command