PythonQ 248-8XX Installation guide

Red Hat Linux 8.0
The Official Red Hat Linux
Reference Guide
Red Hat Linux 8.0: The Official Red Hat Linux Reference Guide
Copyright © 2002 by Red Hat, Inc.
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Table of Contents
Introduction....................................................................................................................................... vii
1. Changes To This Manual .................................................................................................... vii
2. Finding Appropriate Documentation .................................................................................viii
2.1. Documentation For First-Time Linux Users.......................................................viii
2.2. For the More Experienced ..................................................................................... x
2.3. Documentation for Linux Gurus............................................................................ x
3. Document Conventions......................................................................................................... x
4. Using the Mouse ................................................................................................................xiii
5. Copying and Pasting Text With X......................................................................................xiii
6. More to Come ....................................................................................................................xiii
6.1. We Need Feedback! ............................................................................................xiii
7. Sign Up for Support ........................................................................................................... xiv
I. System Reference........................................................................................................................... xv
1. File System Structure.......................................................................................................... 17
1.1. Why Share a Common Structure?........................................................................ 17
1.2. Overview of File System Hierarchy Standard (FHS) .......................................... 17
1.3. Special File Locations.......................................................................................... 21
2. The proc File System......................................................................................................... 23
2.1. A Virtual File System........................................................................................... 23
2.2. Top-level Files in the proc Filesystem................................................................ 24
2.3. Directories in /proc/ .......................................................................................... 37
2.4. Using sysctl ...................................................................................................... 52
2.5. Additional Resources ........................................................................................... 52
3. Boot Process, Init, and Shutdown ....................................................................................... 55
3.1. The Boot Process ................................................................................................. 55
3.2. A Detailed Look at the Boot Process................................................................... 55
3.3. Running Programs at Boot Time ......................................................................... 60
3.4. Differences in the Boot Process of Other Architectures ...................................... 60
3.5. SysV Init .............................................................................................................. 60
3.6. Init Runlevels ....................................................................................................... 61
3.7. The /etc/sysconfig/ Directory...................................................................... 62
3.8. Shutting Down ..................................................................................................... 74
4. Boot Loaders....................................................................................................................... 77
4.1. Boot Loaders and System Architecture ............................................................... 77
4.2. GRUB................................................................................................................... 77
4.3. Installing GRUB .................................................................................................. 78
4.4. GRUB Terminology ............................................................................................. 79
4.5. GRUB Interfaces.................................................................................................. 80
4.6. GRUB Commands ............................................................................................... 81
4.7. GRUB Menu Configuration File.......................................................................... 82
4.8. LILO .................................................................................................................... 84
4.9. Options in /etc/lilo.conf ............................................................................. 85
4.10. Changing Runlevels at Boot Time ..................................................................... 86
4.11. Additional Resources ......................................................................................... 87
5. Users and Groups................................................................................................................ 89
5.1. Tools for User and Group Creation...................................................................... 89
5.2. Standard Users ..................................................................................................... 89
5.3. Standard Groups................................................................................................... 91
5.4. User Private Groups ............................................................................................. 92
5.5. Shadow Utilities................................................................................................... 94
6. The X Window System ....................................................................................................... 97
6.1. The Power of X .................................................................................................... 97
6.2. XFree86................................................................................................................ 97
6.3. Desktop Environments and Window Managers................................................. 102
6.4. Runlevels............................................................................................................ 103
6.5. Fonts................................................................................................................... 105
6.6. Additional Resources ......................................................................................... 106
II. Security Reference ..................................................................................................................... 109
7. Pluggable Authentication Modules (PAM)....................................................................... 111
7.1. Advantages of PAM ........................................................................................... 111
7.2. PAM Configuration Files ................................................................................... 111
7.3. PAM Modules .................................................................................................... 112
7.4. PAM Module Control Flags............................................................................... 113
7.5. PAM Module Paths ............................................................................................ 114
7.6. PAM Module Arguments ................................................................................... 114
7.7. Sample PAM Configuration Files ...................................................................... 114
7.8. PAM and Device Ownership.............................................................................. 116
7.9. Additional Resources ......................................................................................... 117
8. TCP Wrappers and xinetd .............................................................................................. 119
8.1. What Are TCP Wrappers? ................................................................................. 119
8.2. Host-Based Access Control Lists....................................................................... 119
8.3. Access Control Using xinetd .......................................................................... 122
8.4. Additional Resources ......................................................................................... 126
9. SSH Protocol..................................................................................................................... 129
9.1. Features of SSH ................................................................................................. 129
9.2. Event Sequence of an SSH Connection ............................................................. 130
9.3. Layers of SSH Security...................................................................................... 130
9.4. OpenSSH Configuration Files ........................................................................... 132
9.5. More Than a Secure Shell.................................................................................. 133
9.6. Require SSH for Remote Connections .............................................................. 134
10. Kerberos .......................................................................................................................... 137
10.1. Advantages of Kerberos................................................................................... 137
10.2. Kerberos Disadvantages................................................................................... 137
10.3. Kerberos Terminology ..................................................................................... 137
10.4. How Kerberos Works ....................................................................................... 139
10.5. Kerberos and PAM........................................................................................... 140
10.6. Configuring a Kerberos 5 Server ..................................................................... 140
10.7. Configuring a Kerberos 5 Client ...................................................................... 142
10.8. Additional Resources ....................................................................................... 142
11. Tripwire........................................................................................................................... 145
11.1. How to Use Tripwire........................................................................................ 145
11.2. Installing the Tripwire RPM ............................................................................ 146
11.3. Customizing Tripwire ...................................................................................... 147
11.4. Initialize the Tripwire Database....................................................................... 149
11.5. Running an Integrity Check ............................................................................. 150
11.6. Examining Tripwire Reports............................................................................ 150
11.7. Updating the Tripwire Database ...................................................................... 152
11.8. Updating the Tripwire Policy File ................................................................... 153
11.9. Updating the Tripwire Configuration File ....................................................... 154
11.10. Tripwire File Location Reference .................................................................. 155
11.11. Additional Resources ..................................................................................... 156
III. Network Services Reference .................................................................................................... 157
12. Network Scripts .............................................................................................................. 159
12.1. Network Configuration Files............................................................................ 159
12.2. Interface Configuration Files ........................................................................... 159
12.3. Interface Control Scripts .................................................................................. 163
12.4. Network Functions........................................................................................... 164
12.5. Additional Resources ....................................................................................... 165
13. Firewalls and iptables ................................................................................................. 167
13.1. Packet Filtering ................................................................................................ 167
13.2. Differences between iptables and ipchains ............................................. 168
13.3. Options Used in iptables Commands .......................................................... 169
13.4. Storing iptables Information ....................................................................... 175
13.5. Additional Resources ....................................................................................... 176
14. Apache HTTP Server ...................................................................................................... 177
14.1. Apache HTTP Server 2.0 ................................................................................. 177
14.2. Migrating Apache HTTP Server 1.3 Configuration Files ................................ 178
14.3. After Installation .............................................................................................. 187
14.4. Starting and Stopping httpd ........................................................................... 188
14.5. Configuration Directives in httpd.conf ....................................................... 189
14.6. Default Modules............................................................................................... 204
14.7. Adding Modules to Your Server ...................................................................... 205
14.8. Using Virtual Hosts.......................................................................................... 206
14.9. Additional Resources ....................................................................................... 208
15. Email ............................................................................................................................... 209
15.1. Protocols .......................................................................................................... 209
15.2. Email Program Classifications......................................................................... 211
15.3. Sendmail .......................................................................................................... 212
15.4. Fetchmail.......................................................................................................... 216
15.5. Procmail ........................................................................................................... 220
15.6. Security ............................................................................................................ 225
15.7. Additional Resources ....................................................................................... 226
16. Berkeley Internet Name Domain (BIND)....................................................................... 229
16.1. Introduction to DNS and BIND ....................................................................... 229
16.2. BIND Configuration Files................................................................................ 230
16.3. Using rndc ...................................................................................................... 240
16.4. Advanced Features of BIND............................................................................ 242
16.5. Common Mistakes to Avoid ............................................................................ 243
16.6. Additional Resources ....................................................................................... 244
17. Network File System (NFS)............................................................................................ 247
17.1. Methodology .................................................................................................... 247
17.2. NFS Server Configuration Files....................................................................... 249
17.3. NFS Client Configuration Files ....................................................................... 251
17.4. Securing NFS ................................................................................................... 253
17.5. Additional Resources ....................................................................................... 254
18. Lightweight Directory Access Protocol (LDAP)............................................................ 257
18.1. Why Use LDAP? ............................................................................................. 257
18.2. OpenLDAP Daemons and Utilities.................................................................. 258
18.3. LDAP Terminology.......................................................................................... 259
18.4. OpenLDAP Configuration Files....................................................................... 260
18.5. OpenLDAP Setup Overview............................................................................ 262
18.6. Upgrading To OpenLDAP Version 2.0 ............................................................ 263
18.7. Configuring Your System to Authenticate Using OpenLDAP......................... 263
18.8. Additional Resources ....................................................................................... 265
IV. Appendixes ................................................................................................................................ 267
A. General Parameters and Modules .................................................................................... 269
A.1. Specifying Module Parameters ......................................................................... 269
A.2. CD-ROM Module Parameters........................................................................... 270
A.3. SCSI parameters................................................................................................ 272
A.4. Ethernet Parameters .......................................................................................... 275
Index................................................................................................................................................. 281
Colophon.......................................................................................................................................... 293
Welcome to the Official Red Hat Linux Reference Guide.
The Official Red Hat Linux Reference Guide contains useful information about your Red Hat Linux
system. From fundamental concepts, such as the structure of the Red Hat Linux file system, to the
finer points of system security and authentication control, we hope you will find this book to be a
valuable resource.
This guide is for you if you want to learn a bit more about how your Red Hat Linux system works.
Topics that you can explore within this manual include the following:
The file system structure
The boot process
The X Windowing System
Security issues
Network services
1. Changes To This Manual
This manual has been reorganized for clarity and updated for the latest features of Red Hat Linux 8.0.
Some of the changes include:
A New Boot Loaders Chapter
The GRUB chapter has been revised and expanded to include LILO.
An Updated Apache HTTP Server Chapter
There is now a guide for migrating from version 1.3 to version 2.0 of the Apache HTTP Server.
Also, the list of server configuration options has been updated. Special thanks to Gary Benson
and Joe Orton for his hard work on the Apache HTTP Server migration guide.
Updated LDAP Chapter
The LDAP chapter has been cleaned up and reorganized.
Updated iptables Chapter
The iptables chapter has been clarified and reorganized.
Updated Tripwire Chapter
The Tripwire chapter has been clarified and reorganized (and even has a fancy new illustration
Before reading this guide, you should be familiar with the contents of the Official Red Hat Linux
Installation Guide concerning installation issues, the Official Red Hat Linux Getting Started Guide for
basic Linux concepts and the Official Red Hat Linux Customization Guide for general customization
instructions. The Official Red Hat Linux Reference Guide contains information about advanced topics
that may not affect every user, depending upon how they use their Red Hat Linux system.
HTML and PDF versions of all Official Red Hat Linux manuals are available online at
Although this manual reflects the most current information possible, you should read the Red Hat
Linux Release Notes for information that may not have been available prior to our documentation
being finalized. The Release Notes can be found on the Red Hat Linux CD #1 and online at the
following URL:
2. Finding Appropriate Documentation
You need documentation that is appropriate to your level of Linux expertise. Otherwise, you might
feel overwhelmed or not find the necessary information to answer your questions. The Official Red
Hat Linux Reference Guide deals with the more technical aspects and options of your Red Hat Linux
system. This section will help you decide whether to look in this manual for the information you need
or consider other Red Hat Linux manuals, including online sources, in your search.
Three different categories of people use Red Hat Linux, and each of these categories require different sets of documentation and informative sources. To help you figure out where you should start,
determine your own experience level:
New to Linux
This type of user has never used any Linux (or Linux-like) operating system before or has had
only limited exposure to Linux. They may or may not have experience using other operating
systems (such as Windows). Is this you? If so, skip ahead to Section 2.1.
Some Linux Experience
This type of user has installed and successfully used Linux (but not Red Hat Linux) before or may
have equivalent experience with other Linux-like operating systems. Does this describe you? If
so, turn to Section 2.2.
Experienced User
This type of user has installed and successfully used Red Hat Linux before. If this describes you,
turn to Section 2.3.
2.1. Documentation For First-Time Linux Users
For someone new to Linux, the amount of information available on any particular subject, such as
printing, starting up the system or partitioning your hard drive, can be overwhelming. It helps to
initially step back and gain a decent base of information centered around how Linux works before
tackling these kinds of advanced issues.
Your first goal should be to obtain some useful documentation. This cannot be stressed enough. Without documentation, you will only become frustrated at your inability to get your Red Hat Linux system
working the way you want.
You should acquire the following types of Linux documentation:
A brief history of Linux — Many aspects of Linux are the way they are because of historical precedent. The Linux culture is also based on past events, needs or requirements. A basic understanding
of the history of Linux will help you figure out how to solve many potential problems before you
actually see them.
An explanation of how Linux works — While delving into the most arcane aspects of the Linux
kernel is not necessary, it is a good idea to know something about how Linux is put together. This
is particularly important if you have been working with other operating systems, as some of the
assumptions you currently hold about how computers work may not transfer from that operating
system to Linux.
An introductory command overview (with examples) — This is probably the most important thing
to look for in Linux documentation. The underlying design philosophy for Linux is that it’s better
to use many small commands connected together in different ways than it is to have a few large
(and complex) commands that do the whole job themselves. Without examples that illustrate this
approach to doing things, you may find yourself intimidated by the sheer number of commands
available on your Red Hat Linux system.
Keep in mind that you do not have to memorize all of the available Linux commands. Different
techniques exist to help you find the specific command you need to accomplish a task. You need
only know the general way in which Linux functions, what you need to accomplish, and how to
access the tool that will give you the exact instructions you need to execute the command.
The Official Red Hat Linux Installation Guide is a excellent reference for helping you get your Red
Hat Linux system successfully installed and initially configured. The Official Red Hat Linux Getting
Started Guide covers the history of Linux, basic system commands, GNOME, KDE, RPM, and many
other fundamental concepts. You should start with these two books and use them to build the base of
your Red Hat Linux knowledge. Before long, more complicated concepts will begin to make sense
because you already grasp the general ideas.
Beyond reading Red Hat Linux manuals, several other excellent documentation resources are available
for little or no cost:
2.1.1. Introduction to Linux Websites
• — On the Red Hat website, you will find links to the Linux Documentation
Project (LDP), online versions of the Red Hat Linux manuals, FAQs (Frequently Asked Questions),
a database which can help you find a Linux Users Group near you, technical information in the Red
Hat Support Knowledge Base, and more.
• — The Linux Headquarters website features easy to follow,
step-by-step guides for a variety of Linux tasks.
2.1.2. Introduction to Linux Newsgroups
You can participate in newsgroups by watching the discussions of others attempting to solve problems,
or by actively asking or answering questions. Experienced Linux users are known to be extremely
helpful when trying to assist new users with various Linux issues — especially if you are posing your
questions in the right venue. If you do not have access to a news reader application, you can access this
information via the web at Dozens of Linux-related newsgroups exist, including
the following:
• — A great place to get help from fellow Linux users.
linux.redhat — This newsgroup primarily covers Red Hat Linux-specific issues.
linux.redhat.install — Pose installation questions to this newsgroup or search it to see how others
solved similar problems.
linux.redhat.misc — Questions or requests for help that do not really fit into traditional categories
go here.
linux.redhat.rpm — A good place to go if you are having trouble using RPM to accomplish particular objectives.
2.1.3. Beginning Linux Books
Red Hat Linux for Dummies, 2nd Edition by Jon "maddog" Hall; IDG
Special Edition Using Red Hat Linux by Alan Simpson, John Ray and Neal Jamison; Que
Running Linux by Matt Welsh and Lar Kaufman; O’Reilly & Associates
Red Hat Linux 7 Unleashed by William Ball and David Pitts; Sams
The books suggested here are excellent primary sources of information for basic knowledge about
a Red Hat Linux system. For more in-depth information concerning the various topics discussed
throughout this book, many of the chapters list specific book titles, usually in an Additional Resources
2.2. For the More Experienced
If you have used other Linux distributions, you probably already have a basic grasp of the most frequently used commands. You may have installed your own Linux system, and maybe you have even
downloaded and built software you found on the Internet. After installing Linux, however, configuration issues can be very confusing.
The Official Red Hat Linux Customization Guide is designed to help explain the various ways your
Red Hat Linux system can be configured to meet specific objectives. Use this manual to learn about
specific configuration options and how to put them into effect.
When you are installing software that is not covered in the Official Red Hat Linux
Customization Guide, it is often helpful to see what other people in similar circumstances
have done. HOWTO documents from the Linux Documentation Project, available at, document particular
aspects of Linux, from low-level kernel esoteric changes to using Linux for amateur radio station
2.3. Documentation for Linux Gurus
If you are a long-time Red Hat Linux user, you probably already know that one of the best ways to
understand a particular program is to read its source code and/or configuration files. A major advantage
of Red Hat Linux is the availability of the source code for anyone to read.
Obviously, not everyone is a C programmer, so the source code may not be helpful for you. However,
if you have the knowledge and skills necessary to read it, the source code holds all of the answers.
3. Document Conventions
When you read this manual, you will see that 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:
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 filenames). In these cases, they are considered
to be part of the command, so the entire phrase will be 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.
Filenames, directory names, paths, and RPM package names are represented this way. This style
should indicate that a particular file or directory exists by that name on your Red Hat Linux
system. Examples:
The .bashrc file in your home directory contains bash shell definitions and aliases for your own
The /etc/fstab file contains information about different system devices and filesystems.
Install the webalizer RPM if you want to use a Web server log file analysis program.
This style should indicate to you that the program named is an end-user application (as opposed
to system software). For example:
Use Mozilla to browse the Web.
A key on the keyboard is shown in this style. For example:
To use [Tab] completion, type in a character and then press the [Tab] key. Your terminal will
display the list of files in the directory that start with that letter.
A combination of keystrokes is represented in this way. For example:
The [Ctrl]-[Alt]-[Backspace] key combination will exit your graphical session and return 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 will be shown in this style.
When you see text shown in this style, it is being used to identify 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
When you see a word in this style, it 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, you will see the New Tab option that allows you to open
multiple shell prompts in the same window.
If you need to type in a sequence of commands from a GUI menu, they will be shown like the
following example:
Go to Main Menu Button (on the Panel) => Programming => Emacs to start the Emacs text
button on a GUI screen or window
This style indicates that the text will 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
When you see text in this style, it indicates text displayed by the computer on the command line.
You will see responses to commands you typed in, error messages, and interactive prompts for
your input during scripts or programs shown this way. For example:
Use the ls command to display the contents of a directory:
$ ls
The output returned in response to the command (in this case, the contents of the directory) is
shown in this style.
A prompt, which is a computer’s way of signifying that it is ready for you to input something,
will be shown in this style. Examples:
[stephen@maturin stephen]$
leopard login:
user input
Text that the user has to type, 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 will need to type in the text
command at the boot: prompt.
Additionally, we use several different strategies to draw your attention to certain pieces of information.
In order of how critical the information is to your system, these items will be marked as note, tip,
important, caution, or a warning. For example:
Remember that Linux is case sensitive. In other words, a rose is not a ROSE is not a rOsE.
The directory /usr/share/doc contains additional documentation for packages installed on your
If you modify the DHCP configuration file, the changes will not take effect until you restart the DHCP
Do not perform routine tasks as root — use a regular user account unless you need to use the root
account for system administration tasks.
If you choose not to partition manually, a server installation will remove all existing partitions on all
installed hard drives. Do not choose this installation class unless you are sure you have no data you
need to save.
4. Using the Mouse
Red Hat Linux is designed to use a three-button mouse. If you have a two-button mouse, you should
have selected three-button emulation during the installation process. If you’re using three-button emulation, pressing both mouse buttons at the same time equates to pressing the missing third (middle)
In this document, if you are instructed to click with the mouse on something, that means click the left
mouse button. If you need to use the middle or right mouse button, that will be explicitly stated. (This
will be reversed if you’ve configured your mouse to be used by a left handed person.)
The phrase "drag and drop" may be familiar to you. If you’re instructed to drag and drop an item on
your GUI desktop, click on something and hold the mouse button down. While continuing to hold
down the mouse button, drag the item by moving the mouse to a new location. When you’ve reached
the desired location, release the mouse button to drop the item.
5. Copying and Pasting Text With X
Copying and pasting text is easy using your mouse and the X Window System. To copy text, simply
click and drag your mouse over the text to highlight it. To paste the text somewhere, click the middle
mouse button in the spot where the text should be placed.
6. More to Come
The Official Red Hat Linux Reference Guide is part of Red Hat’s commitment to provide useful and
timely support to Red Hat Linux users. Future editions will feature expanded information on changes
to system structure and organization, new and powerful security tools, and other resources to help you
extend the power of your Red Hat Linux system — and your ability to use it.
That’s where you can help.
6.1. We Need Feedback!
If you find an error in the Official Red Hat Linux Reference Guide, or if you have thought of a
way to make this manual better, we’d love to hear from you! Please submit a report in Bugzilla
( against the component rhl-rg.
Be sure to mention the manual’s identifier:
rhl-rg(EN)-8.0-Print-RHI (2002-08-14T22:29-0400)
If you mention the manual’s identifier, we will know exactly which version of the guide you have.
If you have a suggestion for improving the documentation, try to be as specific as possible when describing it. If you have found an error, please include the section number and some of the surrounding
text so we can find it easily.
7. Sign Up for Support
If you have an official edition of Red Hat Linux 8.0, please remember to sign up for the benefits you
are entitled to as a Red Hat customer.
You will be entitled to any or all of the following benefits, depending upon the Official Red Hat Linux
product you purchased:
Official Red Hat support — Get help with your installation questions from Red Hat, Inc.’s support
Red Hat Network — Easily update your packages and receive security notices that are customized
for your system. Go to for more details.
Under the Brim: The Official Red Hat E-Newsletter — Every month, get the latest news and product
information directly from Red Hat.
To sign up, go to You will find your Product ID on a black, red,
and white card in your Official Red Hat Linux box.
To read more about technical support for Official Red Hat Linux, refer to the Getting Technical Support Appendix in the Official Red Hat Linux Installation Guide.
Good luck, and thank you for choosing Red Hat Linux!
The Red Hat Documentation Team
System Reference
Chapter 1.
File System Structure
1.1. Why Share a Common Structure?
An operating system’s file system structure is its most basic level of organization. Almost all of the
ways an operating system interacts with its users, applications, and security model are dependent upon
the way it stores its files on a storage device. It is crucial for a variety of reasons that users, as well as
programs, be able to refer to a common guideline to know where to read and write files.
A file system can be seen in terms of two different logical categories of files:
Shareable vs. unshareable files
Variable vs. static files
Shareable files are those that can be accessed by various hosts; unshareable files are not available to
any other hosts. Variable files can change at any time without any intervention; static files, such as
read-only documentation and binaries, do not change without an action from the system administrator
or an agent that the system administrator has placed in motion to accomplish that task.
The reason for looking at files in this way is to help you understand the type of permissions given to
the directory that holds them. The way in which the operating system and its users need to use the files
determines the directory where those files should be placed, whether the directory is mounted readonly or read-write, and the level of access allowed on each file. The top level of this organization is
crucial, as the access to the underlying directories can be restricted or security problems may manifest
themselves if the top level is left disorganized or without a widely-used structure.
However, simply having a structure does not mean very much unless it is a standard. Competing
structures can actually cause more problems than they fix. Because of this, Red Hat has chosen the
the most widely-used file system structure and extended it only slightly to accommodate special files
used within Red Hat Linux.
1.2. Overview of File System Hierarchy Standard (FHS)
Red Hat is committed to the Filesystem Hierarchy Standard (FHS), a collaborative document that
defines the names and locations of many files and directories. We will continue to track and follow
the standard to keep Red Hat Linux FHS-compliant.
The current FHS document is the authoritative reference to any FHS-compliant file system, but the
standard leaves many areas undefined or extensible. In this section, we provide an overview of the
standard and a description of the parts of the file system not covered by the standard.
The complete standard is available at:
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 because it contains
common executables and is not meant to be changed by users. Since the /usr directory is mounted
read-only, it an be mounted from the CD-ROM or from another machine via a read-only NFS mount.
Chapter 1. File System Structure
1.2.1. FHS Organization
The directories and files noted here are a small subset of those specified by the FHS document. Check
the latest FHS document for the most complete information. The /dev Directory
The /dev directory contains file system entries which represent devices that are attached to the system.
These files are essential for the system to function properly. The /etc Directory
The /etc directory is reserved for configuration files that are local to your machine. No binaries are
to be put in /etc. Any binaries that were formerly put in /etc should now go into /sbin or possibly
The X11 and skel directories are subdirectories of the /etc directory:
|- X11
|- skel
The X11 directory is for X11 configuration files such as XF86Config. The skel directory is for
"skeleton" user files, which are used to populate a home directory when a user is first created. The /lib Directory
The /lib directory should contain only those libraries that are 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. The /mnt Directory
The /mnt directory is for temporarily mounted file systems, such as CD-ROMs and floppy disks. The /opt Directory
The /opt directory provides an area for large, static application software packages to be stored.
For packages that wish to avoid putting their files throughout the file system, /opt provides a logical
and predictable organizational system under that package’s directory. This gives 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 /opt, then all
of its files could be placed within directories inside /opt/sample, such as /opt/sample/bin for
binaries and /opt/sample/man for manual pages.
Large packages that encompass many different sub-packages, each of which accomplish a particular
task, also go within /opt, giving that large package a standardized 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.
Chapter 1. File System Structure
19 The /proc Directory
The /proc directory contains special "files" that either extract information from or send information
to the kernel.
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, please see Chapter 2. The /sbin Directory
The /sbin directory is for executables used only by the root user. The executables in /sbin are only
used to boot and mount /usr and perform system recovery operations. The FHS says:
"/sbin typically contains files essential for booting the system in addition to the binaries in /bin. Anything
executed after /usr is known to be mounted (when there are no problems) should be placed in /usr/sbin.
Local-only system administration binaries should be placed into /usr/local/sbin."
At a minimum, the following programs should be in /sbin:
arp, clock,
getty, halt,
init, fdisk,
fsck.*, grub,
ifconfig, lilo,
mkfs.*, mkswap,
reboot, route,
shutdown, swapoff,
swapon, update The /usr Directory
The /usr directory is for files that can be shared across a whole site. The /usr directory usually has
its own partition, and it should be mountable read-only. At minimum, the following directories should
be subdirectories of /usr:
tmp -> ../var/tmp
The bin directory contains executables, dict contains non-FHS compliant documentation pages,
etc contains system-wide configuration files, games is for games, include contains C header files,
kerberos contains binaries and much more for Kerberos, and lib contains object files and libraries
Chapter 1. File System Structure
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 /sbin), share contains files that are not architecture-specific, src is for source
code, and X11R6 is for the X Window System (XFree86 on Red Hat Linux). 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:
|- bin
|- doc
|- etc
|- games
|- include
|- lib
|- libexec
|- sbin
|- share
|- src The /var Directory
Since the FHS requires that you be able to mount /usr 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 which should be subdirectories of /var:
mail -> spool/mail
Chapter 1. File System Structure
|- anacron
|- at
|- cron
|- fax
|- lpd
|- mail
|- mqueue
|- news
|- rwho
|- samba
|- slrnpull
|- squid
|- up2date
|- uucp
|- uucppublic
|- vbox
|- voice
System log files such as messages and lastlog go in /var/log. The /var/lib/rpm directory also
contains the RPM system databases. Lock files go in /var/lock, usually in directories particular for
the program using the file. The /var/spool directory has subdirectories for various systems that
need to store data files.
1.2.2. /usr/local in Red Hat Linux
In Red Hat Linux, the intended use for /usr/local is slightly different from that specified by the
FHS. The FHS says that /usr/local should be where you store software that is to remain safe
from system software upgrades. Since system upgrades from Red Hat are done safely with the rpm
command and graphical Package Management Tool application, you do not need to protect files by
putting them in /usr/local. Instead, we recommend you use /usr/local for software that is local
to your machine.
For instance, say you have mounted /usr via read-only NFS from a host named jake. If there is a
package or program you would like to install, but you are not allowed to write to jake, you should
install it under /usr/local. Later perhaps, if you have managed to convince the system administrator
of jake to install the program on /usr, you can uninstall it from /usr/local.
1.3. Special File Locations
Red Hat extends the FHS structure slightly to accommodate special files used by Red Hat Linux.
Most files pertaining to the Red Hat Package Manager (RPM) are kept in the /var/lib/rpm/ directory. For more information on RPM see the chapter titled Package Management with RPM in the
Official Red Hat Linux Customization Guide.
Chapter 1. File System Structure
The /var/spool/up2date/ directory contains files used by Red Hat Update Agent, including
RPM header information for the system. This location may also be used to temporarily store RPMs
downloaded while updating your system. For more information on Red Hat Network, see the Red Hat
Network website at
Another location specific to Red Hat 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.
See Section 3.7 for more information on what is within this directory and the role these files play in
the boot process.
Finally, one more directory worth noting is the /initrd/ directory. It is empty, but is used as a critical
mount point during the boot process.
Do not remove the /initrd/ directory for any reason. Removing this directory will cause your system
to fail to boot with a kernel panic error message.
Chapter 2.
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 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 about 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.
2.1. 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 constantly changing.
Virtual files such as interrupts, /proc/meminfo, /proc/mounts, and /proc/partitions provide an up-to-the-moment glimpse of the system’s hardware. Others, like /proc/filesystems 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.
2.1.1. Viewing Virtual Files
By using the cat, more, or less commands on files within the /proc/ directory, you can immediately access an enormous amount of information about the system. For example, if you want to see
what sort of CPU your computer has, type cat /proc/cpuinfo and you will see something similar
to the following:
processor : 0
vendor_id : AuthenticAMD
cpu family : 5
model : 9
model name : AMD-K6(tm) 3D+ Processor
stepping : 1
cpu MHz : 400.919
cache size : 256 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr mce cx8 pge mmx syscall 3dnow k6_mtrr
bogomips : 799.53
Chapter 2. The proc File System
When viewing different virtual files in the /proc/ file system, you will notice 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. Some examples of such applications are lspci,
apm, free, and top.
Some of the virtual files in the /proc/ directory are only readable by the root user.
2.1.2. 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 symbol to redirect the new value
to the file. For instance, to change your hostname on the fly, you can type:
echo > /proc/sys/kernel/hostname
Other files act as binary or boolean switches. For instance, if you type cat
/proc/sys/net/ipv4/ip_forward, you will see either a 0 or a 1. A 0 indicates the kernel is not
forwarding network packets. By using the echo command to change the value of the ip_forward
file to 1, you can immediately turn packet forwarding on.
Another command used to alter settings in the /proc/sys/ subdirectory is /sbin/sysctl. For more
information on this command, see Section 2.4
For a listing of some of the kernel configuration files available in the /proc/sys/, see Section 2.3.9.
2.2. Top-level Files in the proc Filesystem
Below is a list of some of the more useful virtual files in the top-level of the /proc/ directory.
In most cases, the content of files listed in this section will not be the same on your machine. This is
because much of the information pertains to the hardware on which you are running Red Hat Linux.
2.2.1. /proc/apm
This file provides information about the state of the Advanced Power Management (APM) system and
is used by the apm command. If the system with no battery is connected to an AC power source, this
virtual file looks similar to this:
1.16 1.2 0x07 0x01 0xff 0x80 -1% -1 ?
Chapter 2. The proc File System
Running the apm -v command on such a system results in output similar to this:
APM BIOS 1.2 (kernel driver 1.16)
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 running Red Hat Linux 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 contents of the apm
file change to something like this:
1.16 1.2 0x03 0x00 0x00 0x01 99% 1792 min
The apm -v command will now yield 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)
2.2.2. /proc/cmdline
This file shows the parameters passed to the kernel at the time it is started. A sample /proc/cmdline
file looks like this:
ro root=/dev/hda2
This tell us the kernel is mounted read-only — signified by (ro) — off of the second partition on the
first IDE device (/dev/hda2).
2.2.3. /proc/cpuinfo
This virtual file identifies the type of processor used by your system. The following is an example of
the output you would typically see from /proc/cpuinfo:
cpu family
model name
cpu MHz
cache size
cpuid level
AMD-K6(tm) 3D+ Processor
256 KB
fpu vme de pse tsc msr mce cx8 pge mmx syscall 3dnow k6_mtrr
Chapter 2. The proc File System
• processor — Provides each processor with an identifying
you will only see a 0.
number. If you only have one processor,
— Authoritatively tells you the type of processor you have in the system. If your
computer is an Intel-based system, simply place the number in front of "86" to determine the value.
This is particularly helpful if you are wondering about the architecture of an older system such
as the 586, 486, or 386. Because some RPM packages are compiled for each of these particular
architectures, this value enables you determine which package to install.
• cpu family
• model name
• cpu MHz
— Gives you the common name of the processor, including its project name.
— Shows the precise speed in megahertz for the processor to the thousandth decimal
• cache size
— Tells you the amount of level 2 memory cache available to the processor.
— 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.
• flags
2.2.4. /proc/devices
This file displays the various character and block devices currently configured for use with the kernel.
It does not include devices whose modules are not loaded into the kernel. Below is a sample output
from this virtual file:
Character devices:
1 mem
2 pty
3 ttyp
4 ttyS
5 cua
7 vcs
10 misc
14 sound
29 fb
36 netlink
128 ptm
129 ptm
136 pts
137 pts
162 raw
254 iscsictl
Block devices:
1 ramdisk
2 fd
3 ide0
9 md
22 ide1
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.
Block devices have a buffer available, allowing them to order requests before dealing with 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 more efficient order.
Character devices do not require buffering.
Chapter 2. The proc File System
The other difference is that block devices can send and receive information in blocks of a size configured per device. Character devices send data with no preconfigured size.
For more information about devices see /usr/src/linux-2.4/Documentation/devices.txt.
2.2.5. /proc/dma
This file contains a list of the registered ISA direct memory access (DMA) channels in use. A sample
/proc/dma files looks like this:
4: cascade
2.2.6. /proc/execdomains
This file lists the execution domains currently supported by the Linux kernel, along with the range of
personalities they support.
Think of execution domains as the "personality" for a particular 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 particular 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.
2.2.7. /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 this:
2.2.8. /proc/filesystems
This file displays a list of the file system types currently supported by the kernel. Sample output from
a generic kernel’s /proc/filesystems file looks similar to this:
nodev rootfs
nodev bdev
nodev proc
nodev sockfs
nodev tmpfs
nodev shm
nodev pipefs
nodev ramfs
nodev devpts
nodev autofs
nodev binfmt_misc
Chapter 2. The proc File System
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 name of the file systems supported.
The mount command cycles through these file systems when one is not specified as an argument.
2.2.9. /proc/interrupts
This file records the number of interrupts per IRQ on the x86 architecture. A standard
/proc/interrupts looks similar to this:
PS/2 Mouse
For a multi-processor machine, this file may look slightly different:
0: 1366814704
PS/2 Mouse
Intel EtherExpress Pro 10/100 Ethernet
The first column refers to the IRQ number. Each CPU in the system has its own column and its 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 a little
different. For x86 machines, the following values are common:
— This is the old AT computer interrupts.
— 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
• IO-APIC-edge
• IO-APIC-level
low again.
— Generates interrupts when its voltage signal goes high until the signal goes
Chapter 2. The proc File System
2.2.10. /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 :
e0000000-e3ffffff :
e4000000-e7ffffff :
e8000000-e8ffffff :
ea000000-ea00007f :
ffff0000-ffffffff :
System RAM
Video RAM area
Video ROM
System ROM
System RAM
: Kernel code
: Kernel data
VIA Technologies, Inc. VT82C597 [Apollo VP3]
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
The first column displays the memory registers used by each of the different types of memory. The
second column tells the kind of memory located within those registers. In particular, this column
will tell you which memory registers are used by the kernel within the system RAM or, if you have
multiple Ethernet ports on your NIC, the memory registers assigned for each port.
2.2.11. /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, with a beginning similar to this:
0000-001f :
0020-003f :
0040-005f :
0060-006f :
0070-007f :
0080-008f :
00a0-00bf :
00c0-00df :
00f0-00ff :
0170-0177 :
01f0-01f7 :
02f8-02ff :
0376-0376 :
03c0-03df :
03f6-03f6 :
03f8-03ff :
0cf8-0cff :
d000-dfff :
e000-e00f :
e800-e87f :
dma page reg
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 IO port address range reserved for the device listed in the second column.
Chapter 2. The proc File System
2.2.12. /proc/isapnp
This file lists Plug and Play (PnP) cards in ISA slots on the system. This is most often seen with
sound cards but may include any number of devices. A /proc/isapnp file with Soundblaster entry
in it looks similar to this:
Card 1 ’CTL0070:Creative ViBRA16C PnP’ PnP version 1.0 Product version 1.0
Logical device 0 ’CTL0001:Audio’
Device is not active
Active port 0x220,0x330,0x388
Active IRQ 5 [0x2]
Active DMA 1,5
Resources 0
Priority preferred
Port 0x220-0x220, align 0x0, size 0x10, 16-bit address decoding
Port 0x330-0x330, align 0x0, size 0x2, 16-bit address decoding
Port 0x388-0x3f8, align 0x0, size 0x4, 16-bit address decoding
IRQ 5 High-Edge
DMA 1 8-bit byte-count compatible
DMA 5 16-bit word-count compatible
Alternate resources 0:1
Priority acceptable
Port 0x220-0x280, align 0x1f, size 0x10, 16-bit address decoding
Port 0x300-0x330, align 0x2f, size 0x2, 16-bit address decoding
Port 0x388-0x3f8, align 0x0, size 0x4, 16-bit address decoding
IRQ 5,7,2/9,10 High-Edge
DMA 1,3 8-bit byte-count compatible
DMA 5,7 16-bit word-count compatible
This file can be quite long, depending on the number of devices displayed and their resource requirements.
Each card lists its name, PnP version number, and product version number. If the device is active
and configured, this file will also reveal the port and IRQ numbers for the device. In addition, to
ensure better compatibility, the card will specify preferred and acceptable values for a number
of different parameters. The goal here is to allow the PnP cards to work around one another and avoid
IRQ and port conflicts.
2.2.13. /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
physical memory (RAM) used plus 4KB.
The contents of this file are designed to be examined by a debugger, such as gdb and is not human
Do not view the /proc/kcore virtual file. The contents of the file will scramble text output on the
terminal. If you accidentally view this file, press [Ctrl]-[C] to stop the process then type reset to bring
back the command line prompt.
Chapter 2. The proc File System
2.2.14. /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.
2.2.15. /proc/ksyms
This file holds the kernel exported symbol definitions used by the module tools to dynamically link
and bind loadable modules.
speedo_debug [eepro100]
eepro100_init [eepro100]
st_template [st]
RDINDOOR [megaraid]
callDone [megaraid]
megaraid_detect [megaraid]
The first column lists the memory address for the kernel function, the second column refers to the
name of the function, and the last column reveals the name of the loaded module.
2.2.16. /proc/loadavg
This file provides a look at load average on the processor over time and additional data used by
uptime and other commands. A sample /proc/loadavg file looks similar to this:
0.20 0.18 0.12 1/80 11206
The first three columns measure CPU utilization of the last 1, 5, 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.
2.2.17. /proc/locks
This files displays the files currently locked by the kernel. The content of this file contains 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 this:
WRITE 807 03:05:308731 0 EOF c2a260c0 c025aa48 c2a26120
WRITE 708 03:05:308720 0 EOF c2a2611c c2a260c4 c025aa48
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 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, and 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 MAJORDEVICE:MINOR-DEVICE:INODE-NUMBER. The seventh column shows the start and end of the file’s
locked region. The remaining columns point to internal kernel data structures used for specialized
debugging and can be ignored.
Chapter 2. The proc File System
2.2.18. /proc/mdstat
This file contains the current information for multiple-disk, RAID configurations. If your system does
not contain such a configuration, then your /proc/mdstat file will look similar to this:
Personalities :
read_ahead not set
unused devices: none
This file remains in the state above unless you create a software RAID or md device. In that case, you
can view /proc/mdstat to give you a picture of what is currently happening with your mdX RAID
The /proc/mdstat file below shows a system with its md0 configured as a RAID 1 device. 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
2.2.19. /proc/meminfo
This is one of the more commonly used files in the /proc/ directory, as it reports back plenty of
valuable information about the current RAM usage on the system. A system with 256MB of RAM
and 384MB of swap space will likely have a /proc/meminfo file similar to this one:
Mem: 261709824
Swap: 402997248
free: shared: buffers: cached:
253407232 8302592
0 120745984 48689152
8192 402989056
255576 kB
8108 kB
0 kB
117916 kB
47548 kB
135300 kB
29276 kB
888 kB
0 kB
0 kB
0 kB
255576 kB
8108 kB
393552 kB
393544 kB
Much of the information here is used by the free, top, and ps commands. In fact, the output of the
free command is even similar in appearance to the contents and structure of /proc/meminfo. But
by looking directly at /proc/meminfo, more details are revealed:
— Displays the current state of physical RAM in the system, including a full breakdown of
total, used, free, shared, buffered, and cached memory utilization in bytes.
• Mem
• Swap
— Displays the total, used, and free amounts of swap space, in bytes.
• MemTotal
— Total amount of physical RAM, in kilobytes.
Chapter 2. The proc File System
• MemFree
— The amount of physical RAM, in kilobytes, left unused by the system.
• MemShared
• Buffers
— Unused with 2.4 and higher kernels but left in for compatibility with earlier kernel
— The amount of physical RAM, in kilobytes, used for file buffers.
• Cached
— The amount of physical RAM, in kilobytes, used as cache memory.
• Active
— The total amount of buffer or page cache memory, in kilobytes, that is in active use.
• Inact_dirty
— The total amount of buffer or cache pages, in kilobytes, that might be free and
• Inact_clean — The total
• Inact_target
amount of buffer or cache pages in kilobytes that are definitely free and
— The net amount of allocations per second, in kilobytes, averaged over one
and HighFree — The total and free amount of memory, respectively, that is not directly mapped into kernel space. The HighTotal value can vary based on the type of kernel used.
• HighTotal
and LowFree — The total and free amount of memory, respectively, that is directly
mapped into kernel space. The LowTotal value can vary based on the type of kernel used.
• LowTotal
• SwapTotal
• SwapFree
— The total amount of swap available, in kilobytes.
— The total amount of swap free, in kilobytes.
2.2.20. /proc/misc
This file lists miscellaneous drivers registered on the miscellaneous major device, which is device
number 10:
135 rtc
1 psaux
134 apm_bios
The first column is the minor number of each device, and the second column shows the driver in use.
2.2.21. /proc/modules
This file displays a list of all modules loaded into the kernel. Its contents will 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:
(autoclean) [ide-cd]
(autoclean) (unused)
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 tells you whether the module is currently loaded (1) or unloaded
Chapter 2. The proc File System
(0). The final column states if the module can unload itself automatically after a period without use
(autoclean) or if it is not being utilized (unused). Any module with a line containing a name listed
in brackets ([ or ]) tells you that this module depends upon another module to be present in order to
2.2.22. /proc/mounts
This file provides a quick list of all mounts in use by the system:
rootfs / rootfs rw 0 0
/dev/hda2 / ext3 rw 0 0
/proc /proc proc rw 0 0
/dev/hda1 /boot ext3 rw 0 0
none /dev/pts devpts rw 0 0
none /dev/shm tmpfs rw 0 0
none /proc/sys/fs/binfmt_misc binfmt_misc rw 0 0
The output found here is similar to contents of /etc/mtab, except that /proc/mount can be more
The first column specifies the device that is mounted, with the second column revealing the mountpoint. 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.
2.2.23. /proc/mtrr
This file refers to the current Memory Type Range Registers (MTRRs) in use with the system. If your
system’s architecture supports MTRRs, your mtrr might look something like this:
reg00: base=0x00000000 (
0MB), size=
64MB: write-back, count=1
MTRRs are used with Intel P6 family of processors (Pentium II and higher), and they are used to
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 over 150%.
Most of the time, this value is properly configured for you. For more information on MTRRs and
manually configuring this file, please see
2.2.24. /proc/partitions
Most of the information here is of little importance to the user, except for the following columns:
— The major number of the device with this partition. The major number in our example
(3) corresponds with the block device ide0 in /proc/devices.
• major
— 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.
• minor
• #blocks
• name
— Lists the number of physical disk blocks contained in a particular partition.
— The name of the partition.
Chapter 2. The proc File System
2.2.25. /proc/pci
This file contains a full listing of every PCI device on your system. Depending on the number of PCI
devices you have, /proc/pci can get rather long. An example from this file on a basic system looks
similar to this:
Bus 0, device
0, function 0:
Host bridge: Intel Corporation 440BX/ZX - 82443BX/ZX Host bridge (rev 3).
Master Capable. Latency=64.
Prefetchable 32 bit memory at 0xe4000000 [0xe7ffffff].
Bus 0, device
1, function 0:
PCI bridge: Intel Corporation 440BX/ZX - 82443BX/ZX AGP bridge (rev 3).
Master Capable. Latency=64. Min Gnt=128.
Bus 0, device
4, function 0:
ISA bridge: Intel Corporation 82371AB PIIX4 ISA (rev 2).
Bus 0, device
4, function 1:
IDE interface: Intel Corporation 82371AB PIIX4 IDE (rev 1).
Master Capable. Latency=32.
I/O at 0xd800 [0xd80f].
Bus 0, device
4, function 2:
USB Controller: Intel Corporation 82371AB PIIX4 USB (rev 1).
IRQ 5.
Master Capable. Latency=32.
I/O at 0xd400 [0xd41f].
Bus 0, device
4, function 3:
Bridge: Intel Corporation 82371AB PIIX4 ACPI (rev 2).
IRQ 9.
Bus 0, device
9, function 0:
Ethernet controller: Lite-On Communications Inc LNE100TX (rev 33).
IRQ 5.
Master Capable. Latency=32.
I/O at 0xd000 [0xd0ff].
Non-prefetchable 32 bit memory at 0xe3000000 [0xe30000ff].
Bus 0, device 12, function 0:
VGA compatible controller: S3 Inc. ViRGE/DX or /GX (rev 1).
IRQ 11.
Master Capable. Latency=32. Min Gnt=4.Max Lat=255.
Non-prefetchable 32 bit memory at 0xdc000000 [0xdfffffff].
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 you detailed IRQ information so you
can quickly look for conflicts.
To get a more readable version of this informations, type:
lspci -vb
Chapter 2. The proc File System
2.2.26. /proc/slabinfo
This file gives information about memory usage on the slab level. Linux kernels greater than 2.2 use
slab pools to manage memory above the page level. Commonly used objects have their own slab
pools. The following is a portion of a typical /proc/slabinfo virtual file:
slabinfo - version: 1.1
The values in this file occur in the following order: cache name, number of active objects, number of
total objects, size of the object, number of active slabs (blocks) of the objects, total number of slabs
of the objects, and the number of pages per slab.
Note that active in this case means an object is in use. Therefore an active object is one that is in use,
and an active slab is one that contains one or more active object.
2.2.27. /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, begins something like this:
cpu 1139111 3689 234449 84378914
cpu0 1139111 3689 234449 84378914
page 2675248 8567956
swap 10022 19226
intr 93326523 85756163 174412 0 3 3 0 6 0 1 0 428620 0 60330 0 1368304 5538681
disk_io: (3,0):(1408049,445601,5349480,962448,17135856)
ctxt 27269477
btime 886490134
processes 206458
Some of the more popular statistics include:
— Measures the number of jiffies (1/100 of a second) that the system has been in user mode,
user mode with low priority (nice), system mode, and the idle task, respectively. The total for all
CPUs is given at the top, and each individual CPU is listed below with its own statistics.
• cpu
• 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.
— The boot time, measured in the number of seconds since January 1, 1970, otherwise
known as the epoch.
• btime
Chapter 2. The proc File System
2.2.28. /proc/swaps
This file measures swap space and its utilization. For a system with only one swap partition, the output
of /proc/swap may look similar to this:
While some of this information can be found in other files in the /proc/ directory, /proc/swap
provides a snapshot of every swap filename, type of swap space, the total size, and the amount of this
space that is 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.
2.2.29. /proc/uptime
This file contains information about how long the system has on since its last restart. The output of
/proc/uptime is quite minimal:
350735.47 234388.90
The first number tells you the total number of seconds the system has been up. The second number
tells you how much of that time the machine has spent idle in seconds.
2.2.30. /proc/version
This files tells you the versions of the Linux kernel and gcc, as well as the version of Red Hat Linux
installed on the system:
Linux version 2.4.18-0.40 ( version 2.96 20000731
(Red Hat Linux 7.2 2.96-102)) #1 Tue May 28 04:28:05 EDT 2002
This information is used for a variety of purposes, including the version data presented when a user
logs in.
2.3. Directories in /proc/
Common groups of information concerning the kernel are grouped into directories and subdirectories
within the /proc/ directory.
2.3.1. Process Directories
Every /proc/ directory contains a number of directories numerical names. A listing of them may
start off like this:
Chapter 2. The proc File System
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
— This file contains the command issued when starting the process.
— Provides specific information about the utilization of each of the system’s CPUs. A process
running on a dual CPU system produces output similar to this:
• cpu
cpu 11 3
cpu0 0 0
cpu1 11 3
• cwd
— A symlink to the current working directory for the process.
— Gives 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.
• environ
• exe
— A symlink to the executable of this process.
— A directory containing all of the file descriptors for a particular process. These are given in
numbered links:
• fd
total 0
— Contains 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 this:
• maps
• 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
Chapter 2. The proc File System
4. Number of pages are code
5. Number of pages of data/stack
6. Number of pages of library
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 this:
• status
Name: sshd
State: S (sleeping)
Tgid: 797
Pid: 797
PPid: 1
TracerPid: 0
Uid: 0 0 0 0
Gid: 0 0 0 0
FDSize: 32
3072 kB
0 kB
840 kB
104 kB
12 kB
300 kB
2528 kB
SigPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 8000000000001000
SigCgt: 0000000000014005
CapInh: 0000000000000000
CapPrm: 00000000fffffeff
CapEff: 00000000fffffeff
Other than the process’s name and ID, the state (such as S (sleeping) or R (running) and
user/group ID running the process is available, as well as much more detailed data regarding memory usage. /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.
2.3.2. /proc/bus/
This directory contains information specific to the various buses available on the system. So, for
example, on a standard system containing ISA, PCI, and USB busses, current data on each of these
buses is available in its directory under /proc/bus/.
The contents of the subdirectories and files available varies greatly on the precise configuration of
your system. However, each of the directories for each of the bus types has at least one directory for
each bus of that type. These individual bus directories, usually signified with numbers, such as 00,
contains binary files that refer to the various devices available on that bus.
Chapter 2. The proc File System
So, for example, a system with a USB bus but no USB devices connected to it has a /proc/bus/usb/
directory containing several files:
total 0
1 root
1 root
1 root
0 May
0 May
0 May
3 16:25 001
3 16:25 devices
3 16:25 drivers
The /proc/bus/usb/ directory contains files that track the various devices on any USB buses, as
well as the drivers required to use them. The /proc/bus/usb/001/ directory contains all devices
on the first USB bus. By looking at the contents of the devices file, you can identify the USB root
hub on the motherboard:
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
#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
2.3.3. /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 this:
rtc_time : 01:38:43
rtc_date : 1998-02-13
rtc_epoch : 1900
alarm : 00:00:00
DST_enable : no
BCD : yes
24hr : yes
square_wave : no
alarm_IRQ : no
update_IRQ : no
periodic_IRQ : no
periodic_freq : 1024
batt_status : okay
For more information about the RTC, review /usr/src/linux-2.4/Documentation/rtc.txt.
2.3.4. /proc/fs
This directory shows which file systems are exported. If you are running an NFS server, you can type
cat /proc/fs/nfs/exports to view the file systems being shared and the permissions granted for
the those file systems. For more on sharing filesystem with NFS, see Chapter 17.
Chapter 2. The proc File System
2.3.5. /proc/ide/
This directory holds 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 also available, providing the version number of the various drivers used on the IDE channels:
ide-cdrom version 4.59
ide-floppy version 0.97
ide-disk version 1.10
Many chipsets also provide an informational file in this directory that gives additional data concerning
the drives connected through the the channels. For example, a generic Intel PIIX4 Ultra 33 chipset
produces a /proc/ide/piix that will tell you whether DMA or UDMA is enabled for the devices
on the IDE channels:
Intel PIIX4 Ultra 33 Chipset.
------------- Primary Channel ---------------- Secondary Channel ------------enabled
------------- drive0 --------- drive1 -------- drive0 ---------- drive1 -----DMA enabled:
UDMA enabled:
UDMA enabled:
Navigating into the directory for an IDE channel, such as ide0, provides additional information. The
channel file provides the channel number, while the model tells you the bus type for the channel
(such as pci). The Device Directory
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.
There is a symlink 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’s cache.
• capacity
• driver
• geometry
• media
— 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.
— The type of device, such as a disk.
Chapter 2. The proc File System
• model
— The model name or number of the device.
— A collection of current parameters of the device. This file usually contains quite a
bit of useful, technical information. A sample settings file for a standard IDE hard disk looks
similar to this:
• settings
2.3.6. /proc/irq/
This directory is used to set IRQ to CPU affinity, which allows you to connect a particular IRQ to only
one CPU. Alternatively, you can exclude a CPU from handling any IRQs.
Each IRQ has its own directory, allowing for 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.
/usr/src/linux-2.4/Documentation/filesystems/proc.txt contains more information.
2.3.7. /proc/net/
This directory provides a comprehensive look at various networking parameters and statistics. Each of
the files covers a specific range of information related to networking on the system. Below is a partial
listing of these virtual files:
— Contains the kernel’s ARP table. This file is particularly useful for connecting hardware
address to an IP address on a system.
• arp
— A directory containing files with various Asynchronous Transfer Mode (ATM) settings and
statistics. This directory is primarily used with ATM networking and ADSL cards.
• atm
— Lists the various network devices configured on the system, complete with transmit and
receive statistics. This file will quickly tell you 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.
• dev
Chapter 2. The proc File System
• dev_mcast
• igmp
— Displays the various Layer2 multicast groups each device is listening to.
— Lists the IP multicast addresses which this system joined.
• ip_fwchains —
• ip_fwnames
If ipchains are in use, this virtual file reveals any current rule.
— If ipchains are in use, this virtual file lists all firewall chain names.
• ip_masquerade —
• ip_mr_cache —
• ip_mr_vif
Provides a table of masquerading information under ipchains.
List of the multicast routing cache.
— List of multicast virtual interfaces.
— Contains a broad yet detailed collection of networking statistics, including TCP timeouts, SYN cookies sent and received, and much more.
• netstat
• psched
• raw
— List of global packet scheduler parameters.
— List of raw device statistics.
• route
— Displays the kernel’s routing table.
• rt_cache
— Contains the current routing cache.
— List of Simple Network Management Protocol (SNMP) data for various networking protocols in use.
• snmp
• sockstat
• tcp
• tr_rif
• udp
— Provides socket statistics.
— Contains detailed TCP socket information.
— The token ring RIF routing table.
— Contains detailed UDP socket information.
• unix
— Lists UNIX domain sockets currently in use.
• wireless
— Lists wireless interface data.
2.3.8. /proc/scsi/
This directory is analogous to the /proc/ide/ directory only 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 devices, as well as the model name, vendor, SCSI channel
and ID data is available.
For example, if a system contains a SCSI CD-ROM, tape drive, hard drives, and RAID controller, this
file would look similar to this:
Attached devices:
Host: scsi1 Channel: 00 Id: 05 Lun: 00
Vendor: NEC
Model: CD-ROM DRIVE:466
Host: scsi1 Channel: 00 Id: 06 Lun: 00
Vendor: ARCHIVE Model: Python 04106-XXX
Host: scsi2 Channel: 00 Id: 06 Lun: 00
Vendor: DELL
Model: 1x6 U2W SCSI BP
Host: scsi2 Channel: 02 Id: 00 Lun: 00
Vendor: MegaRAID Model: LD0 RAID5 34556R
Rev: 1.06
ANSI SCSI revision: 02
Rev: 7350
ANSI SCSI revision: 02
Rev: 5.35
ANSI SCSI revision: 02
Rev: 1.01
ANSI SCSI revision: 02
Each SCSI driver used by the system has its own directory in /proc/scsi/, which contains files
specific to each SCSI controller using that driver. So, for the example system above, aic7xxx and
Chapter 2. The proc File System
megaraid directories are present, as those two drivers are being utilized. The files in each of the
directories typically contain IO address range, IRQ, and statistics for the particular 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
: Enabled
: 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
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:
Actual queue depth per device for aic7xxx host instance 1:
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
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
From this screen, you can see the transfer speed to the various 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 instance, from the output above you can see in the can see that this
Chapter 2. The proc File System
controller is communicating with the CD-ROM at 20 megabytes per second, while the tape drive is
only connected at 10 megabytes per second.
2.3.9. /proc/sys/
The /proc/sys/ directory is different from others in /proc/ because it not only provides information about the system but also allows you to make configuration changes to the kernel. This allows the
administrator of the machine to immediately enable and disable kernel features.
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 reboot of the system.
For this reason, be sure you know the valid options for that file and the expected outcome before
attempting to change a value in /proc/sys/.
A good way to determine if a particular file can configured or is only designed to provide information
is to list it with the -l flag in the terminal. If the file is writable, you may use it to configure the kernel.
For example, a partial listing of /proc/sys/fs looks like this:
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 will change the sysrq file’s value from 0 (off) to 1 (on).
The purpose of the System Request Key is to allow you to immediately instruct the kernel to do
a number of important activities by using a simple key combination, such as immediately shutting
down or restarting a system, syncing all mounted file systems, or dumping important information to
your console. This feature is most useful when using a development kernel or if you are experiencing
system freezes. However, it is considered a security risk for an unattended console and is therefore
turned off by default under Red Hat Linux.
Refer to /usr/src/linux-2.4/Documentation/sysrq.txt for more information on the System
Request Key.
A few /proc/sys/ configuration files contain more than one value. In order 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
Chapter 2. The proc File System
Any configuration changes you make using the echo command will disappear when the system is
restarted. To make your configuration changes take effect at the time the system is booted, see
Section 2.4.
The /proc/sys/ directory contains several subdirectories controlling different aspects of a running
kernel. /proc/sys/dev/
This directory provides parameters for particular devices on the system. Most systems have at least
two directories, cdrom and raid, but customized kernels can have others, 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
CD-ROM information, Id: cdrom.c 3.12 2000/10/18
drive name: hdc
drive speed: 32
drive # of slots: 1
Can close tray: 1
Can open tray: 1
Can lock tray: 1
Can change speed: 1
Can select disk: 0
Can read multisession: 1
Can read MCN: 1
Reports media changed: 1
Can play audio: 1
Can write CD-R: 0
Can write CD-RW: 0
Can read DVD: 0
Can write DVD-R: 0
Can write DVD-RAM: 0
This file can be quickly scanned to discover the qualities of an unknown CD-ROM, at least in the eyes
of the kernel. If multiple CD-ROMs are available on a system, each device is given its own column of
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 to disable these features.
If RAID support is compiled into the kernel, a /proc/sys/dev/raid/ directory will be available
with at least two files in it: speed_limit_min and speed_limit_max. These settings determine
how much to accelerate the RAID device for particularly I/O intensive tasks, such as resyncing the
disks. /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:
Chapter 2. The proc File System
• dentry-state —
Provides the status of the directory cache. The file looks similar to this:
57411 52939 45 0 0 0
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 currently display only zeros.
• dquot-nr
— Shows the maximum number of cached disk quota entries.
— Allows you to change the maximum number of file handles that the kernel will
allocate. Raising the value in this file can resolve errors caused by a lack of available file handles.
• file-max
— Displays the number of allocated file handles, used file handles, and the maximum
number of file handles.
• file-nr
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.
• overflowgid
• super-max
• super-nr
— Controls the maximum number of superblocks available.
— Displays the current number of superblocks in use. /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 this:
• acct
4 2 30
The second value sets the threshold percentage of free space when logging will be suspended, while
the first value dictates the percentage of free space required for logging to resume. The third value
sets the interval in seconds that the kernel polls the file system to see if logging should be suspended
or resumed.
• cap-bound — Controls the capability
bounding settings, which provide 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.
The various values that are possible here are beyond the scope of this manual, so consult the kernel
documentation for more information.
— Controls whether [Ctrl]-[Alt]-[Delete] will gracefully restart the computer using init (value 0) or force an immediate reboot without syncing the dirty buffers to disk (value
• ctrl-alt-del
• domainname
• hostname
— Allows you to configure the system’s domain name, such as
— Allows you to configure the system’s host name, such as
• hotplug — 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 you are testing a new program to fulfill this role.
— Sets the location of the program to be used to load kernel modules when necessary.
The default value of /sbin/modprobe signifies that kmod will call it to actually load the module
when a kernel thread calls kmod.
• modprobe
• msgmax
— Sets the maximum size of any message sent from one process to another is set to
8192 bytes by default. You should be careful about raising this value, as queued messages between
Chapter 2. The proc File System
processes are stored in non-swappable kernel memory. Any increase in msgmax would increase
RAM requirements for the system.
• msgmnb
— 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 —
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.
• ostype
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.
• overflowgid
— Defines the number of seconds the kernel will postpone rebooting when the system
experiences a kernel panic. By default, the value is set to 0, which disables automatic rebooting
after a panic.
• panic
• printk — 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.
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 that will be 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.
— Configures the maximum number of POSIX realtime signals that the system may
have queued at any one time. The default value is 1024.
• rtsig-max
• rtsig-nr
— The current number of POSIX realtime signals queued by the kernel.
— This file configures semaphore settings within the kernel. A semaphore is a System V IPC
object that is used to control utilization of a particular process.
• sem
— 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.
• shmall
• shmmax — 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. By default, this value is 4096
• shmmni
— Activates the System Request Key, if this value is set to anything other than the default
of 0. See Section 2.3.9 for details about the System Request Key.
• sysrq
Chapter 2. The proc File System
• threads-max
value of 2048.
— Sets the maximum number of threads to be used by the kernel, with a default
• version — 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.
The random directory stores a number of values related to generating random numbers for the kernel. /proc/sys/net/
This directory contains assorted directories concerning various networking topics. Various configurations at the time of kernel compilation make available different directories here, such as appletalk,
ethernet, ipv4, ipx, and ipv6. Within these directories, you can adjust the assorted networking
values for that configuration on a running system.
Given the wide variety of possible networking options available with Linux and the great amount of
space required to discuss them, only the most common /proc/sys/net/ directories will be discussed.
The core directory contains a variety of settings that control the interaction between the kernel and
networking layers. The most important files there are:
— The amount of time in tenths of a second required to write a new warning
message. This is used to prevent Denial of Service (DoS) attacks. The default setting is 50.
• message_burst
— Also used to prevent DoS attacks by placing a cost on every warning message.
The higher the value of this file (default of 5), the more likely the warning message will be ignored.
• message_cost
The idea of a DoS attack is to bombard your system with requests that generate errors and fill up
disk partitions with log files or require all of your system’s resources to handle the error logging.
The settings in message_burst and message_cost are designed to be modified based on your
system’s acceptable risk versus the need for comprehensive logging.
• 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
— Configures the maximum ancillary buffer size allowed per socket.
• rmem_default —
• rmem_max
• wmem_default —
• wmem_max
Sets the receive socket buffer’s default size in bytes.
— Sets the receive socket buffer’s maximum size in bytes.
Sets the send socket buffer’s default size in bytes.
— Sets the send socket buffer’s maximum size in bytes.
The /ipv4 directory contains additional networking settings. Many of these settings, used in conjunction with one another, are very useful in preventing attacks on the system or using the system to
act as a router.
An erroneous change to these files may affect your remote connectivity to the system.
Here are some of the most important files in the /proc/sys/net/ipv4/ directory:
• icmp_destunreach_rate,
icmp_timeexeed_rate — Set the maximum ICMP send packet rate, in hundredths of a second,
to hosts under certain conditions. A setting of 0 removes any delay and is not a good idea.
Chapter 2. The proc File System
• icmp_echo_ignore_all 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
• ip_default_ttl — 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 will enable network packet forwarding.
• ip_forward
— 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 the 32768 to 61000 range in this file.
• ip_local_port_range
• tcp_syn_retries — Provides
a limit on the number of times your system will re-transmit a SYN
packet when attempting to make a connection.
— Sets the number of permitted re-transmissions attempting to answer an incoming connection. Default of 3.
• tcp_retries1
• tcp_retries2 —
Sets the number of permitted re-transmissions of TCP packets. Default of 15.
The /usr/src/linux-2.4/Documentation/networking/ip-sysctl.txt file 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 cover specific topics. The conf directory allows each of the systems interfaces to be configured in different ways,
including the use of a default settings for unconfigured devices (in the default subdirectory) and
settings that override all special configurations (in the all subdirectory).
In order to control connections between direct neighbors, meaning any other system directly connected to your system, the neigh directory allows special configurations for each interface. This
would allow you to treat systems differently that you trust more due to their relatively proximity to
your system. At the same time, it also makes it easy to put strict rules in place for systems several
hops away.
Routing over IPV4 also has its own directory, route. Unlike conf and neigh, the 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, simply 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/src/linux-2.4/Documentation/filesystems/proc.txt. /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 called swap space.
The following files are commonly found in the /proc/sys/vm/ directory:
• bdflush
— Sets various values related to the bdflush kernel daemon.
• buffermem — Allows
you to control the percentage amount of total system memory to be used for
buffer memory. Typical output for this file looks like this:
The first and last values set the minimum and maximum percentage of memory to be used as buffer
memory, respectively. The middle value sets the percentage of system memory dedicated to buffer
memory where the memory management subsystem will begin to clear buffer cache more than other
kinds of memory to compensate for a general lack of free memory.
Chapter 2. The proc File System
— Sets various values concerned with the kernel swap-out daemon, kswapd. This file has
three values:
• kswapd
512 32 8
The first value sets the maximum number of pages that kswapd will attempt to free in a single
attempt. The larger this number, the more aggressively the kernel can move to free pages. The
second value sets the minimum number of times that kswapd attempts to free a page. The third
value sets the number of pages kswapd attempts to write in a single attempt. Proper tuning of this
final value can improve performance on a system using a lot of swap space by telling the kernel to
write pages in large chunks, minimizing the number of disk seeks.
— Configures the maximum number of memory map areas a process may have.
In most cases, the default value of 65536 is appropriate.
• max_map_count
— When set to the default value of 0 the kernel estimates the amount of
memory available and fails requests that are blatantly invalid. Unfortunately, since memory is allocated using a heuristic rather than a precise algorithm, it can sometimes overload the system.
• overcommit_memory
If overcommit_memory is set to 1, then the potential for system overload is increased, but so is
the performance for memory intensive tasks, such as those used by some scientific software.
For customers who need more less risk of over memory commitment, the following two option
have been added. Setting overcommit_memory to 2 fails a memory request adds up to more that
half of the physical RAM, plus swap. Setting it to 3 fails if a memory request adds up to more than
swap alone can hold.
— Controls the amount of memory used by the page cache. The values in pagecache are percentages, and they work in a similar way as buffermem to enforce minimums and
• pagecache
maximums of available page cache memory.
• page-cluster —
Sets the number of pages read in a single attempt. The default value of 4, which
actually relates to 16 pages, is appropriate for most systems.
• pagetable_cache — Controls
the number of page tables that are cached on a per-processor basis.
The first and second values relate to the minimum and maximum number of page tables to set aside,
The /usr/src/linux-2.4/Documentation/sysctl/vm.txt file contains additional information on these various files.
2.3.10. /proc/sysvipc/
This directory contain 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).
2.3.11. /proc/tty/
This directory contains information about the available and currently used tty devices on the system.
Originally called a teletype device, 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
X11. The drivers file is a list of the current tty devices in use:
64-127 serial:callout
64-127 serial
0-255 pty:slave
Chapter 2. The proc File System
The /proc/tty/driver/serial file lists the usage statistics and status of each of the serial tty
In order that tty devices can be used in a similar way as network devices, the Linux kernel will enforce
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 be
able to see that 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, with detailed information available in the
ldisc directory.
2.4. Using sysctl
The sysctl command is used to view, set, and automate kernel settings in the /proc/sys/ directory.
To get a quick overview of all settings configurable in the /proc/sys/ directory, type the sysctl
-a command as root. This will create a large, comprehensive list, a small portion of which looks
something like this:
net.ipv4.route.min_delay = 2
kernel.sysrq = 0
kernel.sem = 250
This is the same basic information you would see if you viewed each of the files individually. The
only difference is the file location. The /proc/sys/net/ipv4/route/min_delay file is signified
by net.ipv4.route.min_delay, with the directory slashes replaced by dots and the proc.sys
portion assumed.
The sysctl command can be use in place of echo to assign values to writable files in the
/proc/sys/ directory. For instance instead of using this command:
echo 1 > /proc/sys/kernel/sysrq
You can use this sysctl command:
sysctl -w kernel.sysrq="1"
kernel.sysrq = 1
While quickly setting single values like this in /proc/sys/ is helpful during testing, it does not work
as well on a production system, as all /proc/sys/ special settings are lost when the machine is
rebooted. To preserve the settings that you like to make permanently to your kernel, add them to the
/etc/sysctl.conf file.
Every 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 dictate the values passed to
the kernel. Any values added to /etc/sysctl.conf will take effect each time the system boots.
Chapter 2. The proc File System
2.5. Additional Resources
Below are additional sources of information about /proc/.
2.5.1. Installed Documentation
Most of the best /proc/ documentation is available on your system.
• /usr/src/linux-2.4/Documentation/filesystems/proc.txt
limited, information about all aspects of the /proc/ directory.
• /usr/src/linux-2.4/Documentation/sysrq.txt
— Contains assorted, but
— An overview of System Request Key
• /usr/src/linux-2.4/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/src/linux-2.4/Documentation/networking/ip-sysctl.txt —
IP networking options.
A look at various
• /usr/src/linux-2.4 — Some of the most authoritative
information on /proc/ can be found by
reading the kernel source code. Make sure the kernel-source RPM is installed on your system
and look in the /usr/src/linux-2.4 directory for the source code.
2.5.2. Useful Websites
• — This site maintain a complete database of source, patches, and documentation for various versions of the Linux kernel.
Chapter 2. The proc File System
Chapter 3.
Boot Process, Init, and Shutdown
An important and powerful aspect of Red Hat Linux is the open method it uses for starting and stopping the operating system. During boot time, Red Hat Linux loads programs in a specific and configurable order. Once booted, you are free to change the configuration files controlling the boot process
as well as the configuration files for the programs started at boot-time. Similarly, system shutdown
gracefully terminates processes in an organized and configurable way, although customization of this
process is rarely required.
Understanding how the boot and shutdown processes work will not only allow you to easily customize
Red Hat Linux to your needs, but will also make it easier to troubleshoot problems related to starting
or shutting down your system.
3.1. The Boot Process
Below are the basic stages of the boot process for an x86 system:
1. The system BIOS checks the system and launches the first stage boot loader on the MBR of the
primary hard disk.
2. The first stage boot loader loads itself into memory and launches the second stage boot loader
from the /boot/ partition.
3. The kernel is loaded into memory, which in turn loads any necessary modules and mounts the
root partition read-only.
4. The kernel hands control of the boot process to the /sbin/init program.
5. The /sbin/init program loads all services and user-space tools, and mounts all partitions
listed in /etc/fstab.
6. The user is presented with a login prompt for the freshly booted Linux system.
Because configuration of the boot process is more common than the customization of the shutdown
process, the remainder of this chapter will discuss in detail how the boot process works and how it
can be customized to suite your needs.
3.2. A Detailed Look at the Boot Process
The very beginning of the boot process varies depending the platform on which you are booting Red
Hat Linux. However, once the kernel is found and loaded by the system, the default boot process is
identical across all architectures. In the example outlined below, the system will be an x86 computer.
3.2.1. The BIOS
When an x86 computer is booted, the processor looks at the end of system memory for the Basic
Input/Output System or BIOS program and runs it. The BIOS controls not only the first step of the
boot process, but also provides the lowest level interface to peripheral devices. For this reason it is
written into read-only, permanent memory and is always available for use.
Other platforms use different programs to perform low-level tasks roughly equivalent to those of the
BIOS on an x86 system. For instance, Itanium-based computers use the Extensible Firmware Interface
(EFI) Shell, while Alpha systems use the SRM console.
Chapter 3. Boot Process, Init, and Shutdown
Once loaded, the BIOS tests the system, looks for and checks peripherals and then locates a valid
device with which to boot the system. Usually, it first checks any floppy drives and CD-ROM drives
present for bootable media, then it looks to the system’s hard drives. The order of the drives searched
for booting can often be controlled with a setting in BIOS. Often, the first hard drive set to boot is
the C drive or the master IDE device on the primary IDE bus. The BIOS loads whatever program is
residing in the first sector of this device, called the Master Boot Record or MBR, into memory. The
MBR is only 512 bytes in size and contains machine code instructions for booting the machine along
with the partition table. Once found and loaded the BIOS passes control whatever program is on the
3.2.2. The Boot Loader
This section looks at the boot process for the x86 platform. Depending on your system’s architecture,
the boot process may differ slightly. Please see Section 3.4 for a brief overview of non-x86 boot
Linux boot loaders for the x86 platform are broken into at least two stages. The first stage is a small
machine code binary on the MBR. Its sole job is to locate the second stage boot loader and load the
first part of it into memory. Under Red Hat Linux you can install one of two boot loaders: GRUB
or LILO. GRUB is the default boot loader, but LILO is available for those who require it for their
hardware setup or who prefer it. For more information about configuring and using GRUB or LILO,
see Chapter 4.
If you are using LILO under Red Hat Linux, the second stage boot loader uses information on the
MBR to determine what boot options are available to the user. This means that any time a configuration change is made or you upgrade your kernel manually, you must run the /sbin/lilo -v -v
command to write the appropriate information to the MBR. For details on doing this, see Section 4.8.
GRUB, on the other hand, can read ext2 partitions and therefore simply loads its configuration file —
/boot/grub/grub.conf — when the second stage loader is called. See Section 4.7 for information
on how to edit this file.
/boot/grub/grub.conf will be updated automatically. For more information on RHN, refer to the
following URL,
Once the second stage boot loader is in memory, it presents the user with the Red Hat Linux initial,
graphical screen showing the different operating systems or kernels it has been configured to boot. If
you have only Red Hat Linux installed and have not changed anything in the /etc/lilo.conf or
/boot/grub/grub.conf, you will only see one option for booting.
If you have configured the boot loader to boot other operating systems, this screen gives you the
opportunity to select it. Use the arrow keys to highlight the operating system and press [Enter]. If you
do nothing, the boot loader will load the default selection.
If you installed Symmetric Multi-Processor (SMP) kernel support, you will see more than one option
the first time you boot your system. Under LILO you will see linux and linux-up. Under GRUB
you will see Red Hat Linux (kernel version) and Red Hat Linux (kernel version-smp).
Thelinux or Red Hat Linux (kernel version-smp) option is the SMP kernel. If you experience
any problems with the SMP kernel, select the linux-up or non-SMP kernel upon rebooting.
Chapter 3. Boot Process, Init, and Shutdown
If you need to alter the command line arguments to the kernel, see Chapter 4. For information on
changing the runlevel at the GRUB or LILO prompt, see Section 3.5.
Once the second stage boot loader has determined which kernel to boot, it locates the corresponding
kernel binary in the /boot/ directory. The proper binary is the /boot/vmlinuz-2.4.x-xx file that
corresponds to the boot loader’s settings. Next the boot loader places the appropriate initial RAM
disk image, called an initrd, into memory. The initrd is used by the kernel to load any drivers not
compiled into it that are necessary to boot the system. This is particularly important if you have SCSI
hard drives or are using the ext3 file system 1.
Do not remove the /initrd/ directory from the file system for any reason. Removing this directory
will cause your system to fail with a kernel panic error message at boot time.
Once the kernel and the initrd image are loaded into memory, the boot loader hands control of the
boot process to the kernel.
3.2.3. The Kernel
When the kernel loads, it immediately initializes and configures the computer’s memory. Next it configures the various hardware attached to the system, including all processors and I/O subsystems, as
well as any storage devices. It then looks for the compressed initrd image in a predetermined location in memory, decompresses it, mounts it, and loads all necessary drivers. Next it initializes file
system-related virtual devices, such as LVM or software RAID before unmounting the initrd disk
image and freeing up all the memory it once occupied.
After the kernel has initialized all the devices on the system, it creates a root device, mounts the root
partition read-only, and frees unused memory.
At this point, with the kernel loaded into memory and operational. However, with no user applications
to give the user the ability to provide meaningful input to the system, not much can be done with it.
To set up the user environment, the kernel starts the /sbin/init command.
3.2.4. The /sbin/init Program
The init program coordinates the rest of the boot process and configures the environment for the
When the init command starts, it becomes the parent or grandparent of all of the processes that start
up automatically on a Red Hat Linux system. First, it runs the /etc/rc.d/rc.sysinit script, which
sets your environment path, starts swap, checks the file systems, and so on. Basically, rc.sysinit
takes care of everything that your system needs to have done at system initialization. For example,
most systems use a clock, so on them rc.sysinit reads the /etc/sysconfig/clock configuration
file to initialize the clock. Another example is if you have special serial port processes which must be
initialized, rc.sysinit will execute the /etc/rc.serial file.
The init command then runs the /etc/inittab script, which describes how the system should be
set up in each SysV init runlevel 2. Among other things, the /etc/inittab sets the default runlevel
and dictates that /sbin/update should be run whenever it starts a given runlevel 3.
For details on making an initrd, see the section about converting to an ext3 file system in the Official Red
Hat Linux Customization Guide.
2. For more information on SysV init, see Section 3.5. For more about init runlevels, see Section 3.6.
3. The update command is used to flush dirty buffers back to disk.
Chapter 3. Boot Process, Init, and Shutdown
Next, the init command sets the source function library, /etc/rc.d/init.d/functions, for the
system. This spells out how to start or kill a program and how to determine the PID of a program.
The init program starts all of the background processes by looking in the appropriate rc directory for
the runlevel specified as default in /etc/inittab. The rc directories are numbered to corresponds
to the runlevel they represent. For instance, /etc/rc.d/rc5.d/ is the directory for runlevel five.
When booting to runlevel 5, the init program looks in the /etc/rc.d/rc5.d/ directory to
determine which processes to start and stop. Below is an example listing for a runlevel 5,
/etc/rc.d/rc5.d/ directory:
K01pppoe -> ../init.d/pppoe
K05innd -> ../init.d/innd
K10ntpd -> ../init.d/ntpd
K15httpd -> ../init.d/httpd
K15mysqld -> ../init.d/mysqld
K15pvmd -> ../init.d/pvmd
K16rarpd -> ../init.d/rarpd
K20bootparamd -> ../init.d/bootparamd
K20nfs -> ../init.d/nfs
K20rstatd -> ../init.d/rstatd
K20rusersd -> ../init.d/rusersd
K20rwalld -> ../init.d/rwalld
K20rwhod -> ../init.d/rwhod
K25squid -> ../init.d/squid
K28amd -> ../init.d/amd
K30mcserv -> ../init.d/mcserv
K34yppasswdd -> ../init.d/yppasswdd
K35dhcpd -> ../init.d/dhcpd
K35smb -> ../init.d/smb
K35vncserver -> ../init.d/vncserver
K45arpwatch -> ../init.d/arpwatch
K45named -> ../init.d/named
K50snmpd -> ../init.d/snmpd
K54pxe -> ../init.d/pxe
K55routed -> ../init.d/routed
K60mars-nwe -> ../init.d/mars-nwe
K61ldap -> ../init.d/ldap
K65kadmin -> ../init.d/kadmin
K65kprop -> ../init.d/kprop
K65krb524 -> ../init.d/krb524
K65krb5kdc -> ../init.d/krb5kdc
K75gated -> ../init.d/gated
K80nscd -> ../init.d/nscd
K84ypserv -> ../init.d/ypserv
K90ups -> ../init.d/ups
K96irda -> ../init.d/irda
S05kudzu -> ../init.d/kudzu
S06reconfig -> ../init.d/reconfig
S08ipchains -> ../init.d/ipchains
S10network -> ../init.d/network
S12syslog -> ../init.d/syslog
S13portmap -> ../init.d/portmap
S14nfslock -> ../init.d/nfslock
S18autofs -> ../init.d/autofs
S20random -> ../init.d/random
S25netfs -> ../init.d/netfs
S26apmd -> ../init.d/apmd
S35identd -> ../init.d/identd
Chapter 3. Boot Process, Init, and Shutdown
S40atd -> ../init.d/atd
S45pcmcia -> ../init.d/pcmcia
S55sshd -> ../init.d/sshd
S56rawdevices -> ../init.d/rawdevices
S56xinetd -> ../init.d/xinetd
S60lpd -> ../init.d/lpd
S75keytable -> ../init.d/keytable
S80isdn -> ../init.d/isdn
S80sendmail -> ../init.d/sendmail
S85gpm -> ../init.d/gpm
S90canna -> ../init.d/canna
S90crond -> ../init.d/crond
S90FreeWnn -> ../init.d/FreeWnn
S90xfs -> ../init.d/xfs
S95anacron -> ../init.d/anacron
S95firstboot -> ../init.d/firstboot
S97rhnsd -> ../init.d/rhnsd
S99local -> ../rc.local
S99mdmonitor -> ../init.d/mdmonitor
As you can see, none of the scripts that actually start and stop the services are located in the
/etc/rc.d/rc5.d/ directory. Rather, all of the files in /etc/rc.d/rc5.d/ are symbolic links
pointing to scripts located in the /etc/rc.d/init.d/ directory. Symbolic links are used in each of
the rc directories so that the runlevels can be reconfigured by creating, modifying, and deleting the
symbolic links without affecting the actual scripts they reference.
The name of each symbolic link begin with either a K or an S. The K links are processes that are killed
on that runlevel, while those beginning with an S are started.
The init command first stops all of the K symbolic links in the directory by issuing the
/etc/rc.d/init.d/ command stop command, where command is the process to be
killed. It then starts all of the S symbolic links by issuing /etc/rc.d/init.d/ command start.
After the system is finished booting, you can log in as root and execute these same scripts to start
and stop services. For instance, the command /etc/rc.d/init.d/httpd stop will stop the Apache
Web server.
Each of the symbolic links are numbered to dictate start order. You can change the order in which the
services are started or stopped by changing this number. Those symbolic links with the same number
are started alphabetically.
In the example /etc/rc.d/rc5.d/ directory above, the init command will kill pppoe, innd,
ntpd, httpd, mysqld, pvmd, rarpd, bootparamd, nfs, rstatd, rusersd, rwalld, rwhod,
squid, amd, mcserv, yppasswdd, dhcpd, smb, vncserver, arpwatch, named, snmpd, pxe,
routed, mars-nwe, ldap, kadmin, kprop, krb524, krb5kdc, gated, nscd, ypserv, ups, and
After all processes are killed, init looks into the same directory and finds start scripts for kudzu,
reconfig, ipchains, portmap, nfslock, autofs, random, netfs, apmd, identd, atd, pcmcia,
sshd, rawdevices, xinetd, lpd, keytable, isdn, sendmail, gpm, canna, crond, FreeWnn,
xfs, anacron, firstboot, rhnsd, and mdmonitor.
The last thing the init program does is run any scripts located in /etc/rc.d/rc.local (see Section 3.3 for more on customizing the rc.local file). At this point, the system is considered to be
operating at runlevel 5.
Chapter 3. Boot Process, Init, and Shutdown
After the init command has progressed through the appropriate rc directory for the runlevel, the
/etc/inittab script forks a getty process for each virtual console (login prompts) allocated to the
runlevel. Runlevels 2 through 5 get all six virtual consoles, while runlevel 1 (single user mode) gets
only one and runlevels 0 and 6 get none. The getty process opens communication pathways to tty
devices 4, sets their modes, prints the login prompt, gets the user name, and initiates the login process
for the user.
In runlevel 5, /etc/inittab runs a script called /etc/X11/prefdm. The prefdm script runs the
preferred X display manager, gdm if you are running GNOME or kdm if you are running KDE, based
on the contents of the /etc/sysconfig/desktop/ directory.
Finally, the init command runs the /etc/rc.d/rc.local script.
At this point, you should be looking at a login prompt.
3.3. Running Programs at Boot Time
The file /etc/rc.d/rc.local script is run by the init command at boot time, or when changing
runlevels, after all other initialization is complete. You can use this file to add additional commands
necessary for your environment. For instance, you can start additional daemons or initialize a printer.
In addition, if you require serial ports setup at boot time, you can create and edit /etc/rc.serial.
This script runs setserial commands to configure the system’s serial ports. See the setserial
man page for more information.
3.4. Differences in the Boot Process of Other Architectures
Once the Red Hat Linux kernel loads and hands off the boot process to the init command, the same
sequence of events occurs on every architecture. So the main difference between each architecture’s
boot process is in the application used to find and load the kernel.
For example, the Alpha architecture uses the aboot boot loader, while the Itanium architecture uses
the ELILO boot loader.
Consult the Official Red Hat Linux Installation Guide specific to these platforms for information on
configuring their boot loaders.
3.5. SysV Init
The SysV init is a standard process used by Red Hat Linux to control which software the init command launches or shuts off on a given runlevel. SysV init chosen because it is easier to use and more
flexible than the traditional BSD style init process.
The configuration files for SysV init are in the /etc/rc.d/ directory. Within this directory, are the
rc, rc.local, and rc.sysinit scripts as well as the following directories:
See Section 2.3.11 for more information on tty devices.
Chapter 3. Boot Process, Init, and Shutdown
The init.d directory contains the scripts used by the init command when controlling services.
Each of the numbered directories represent the six default runlevels configured by default under Red
Hat Linux. For more information on runlevels, see Section 3.6.
The default runlevel is listed in /etc/inittab. To find out the default runlevel for your system, look
for the line similar to the one below near the top of /etc/inittab:
The default runlevel listed in the example above is three, as the number after the first colon indicates.
If you want to change it, edit /etc/inittab as root.
Be very careful when editing /etc/inittab. Simple typos can cause your system to become unbootable. If this happens, you will either need a boot diskette for your system or you will need to enter
rescue mode to boot your computer and fix the file.
For more information on rescue mode, see the chapter titled Rescue Mode in the Official Red Hat
Linux Customization Guide.
3.6. Init Runlevels
The idea behind operating different services at different runlevels revolves around the fact that different systems can be used in a different ways. Some services cannot be used until the system is in a
particular state, or mode, such as ready for more than one user or has networking available.
There are times in which you may want to operate the system at a lower mode, such as fixing disk
corruption problems in runlevel 1, when no other users can possibly be on the system or leaving a
server in runlevel 3 without an X session running. In these cases, running services that depend upon
a higher system mode to function does not make sense because they will not work correctly anyway.
By already having each service assigned to start when its particular runlevel is reached, you ensure
an orderly start up process and can quickly change the mode of the machine without worrying about
which services to manually start or stop.
Generally, Red Hat Linux operates in runlevel 3 or runlevel 5 — both full multi-user modes. The
following runlevels are defined in Red Hat Linux:
• 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
The default runlevel for a system to boot into and stop is configured in /etc/inittab. For more
information on /etc/inittab, see Section 3.5.
Feel free to configure runlevels 2 and 4 as you see fit. Many users configure those runlevels in a way
that makes the most sense for them while leaving the standard runlevels 3 and 5 alone. This allows
Chapter 3. Boot Process, Init, and Shutdown
them to quickly move in and out of their custom configuration without disturbing the normal set of
features at the standard runlevels.
If your machine gets into a state where it will not boot due to a bad /etc/inittab or will not let you
log in because you have a corrupted /etc/passwd or if you have simply forgotten your password,
you can boot into single-user mode.
If you are using LILO, you can enter single-user mode by typing linux single at the LILO boot:
If you are using GRUB as your boot loader, you can enter single-user mode using the following steps.
In the graphical GRUB boot loader screen, select the Red Hat Linux boot label and press [e] to
edit it.
Arrow down to the kernel line and press [e] to edit it.
At the prompt, type single and press [Enter].
You will be returned to the GRUB screen with the kernel information. Press the [b] key to boot the
system into single user mode.
A very bare system will boot, and you will have a command shell from which you can fix things.
If this does not work, you will need to boot by typing linux init=/bin/bash at the LILO
boot: prompt. This places you at a shell prompt; note that no file systems other than the root file
system are mounted, and the root file system is mounted in read-only mode. To mount it in read-write
mode (to allow editing of a broken /etc/inittab, for example) do:
mount -n /proc
mount -o rw,remount /
3.6.1. Initscript Utilities
/sbin/chkconfig utility is a simple command-line tool for maintaining the
/etc/rc.d/init.d directory hierarchy. It relieves system administrators from having to directly
manipulate the numerous symbolic links in the directories under /etc/rc.d.
In addition, there is /sbin/ntsysv that provides a text-based interface, which you may find easier to
use than chkconfig’s command-line interface.
If you prefer a graphical interface, use the Services Configuration Tool program, which can be invoked using the redhat-config-services command.
All of these utilities must be run as root.
Please refer to the chapter titled Controlling Access to Services in Official Red Hat Linux Customization Guide for more information regarding these tools.
Next, we will discuss information in the files within the /etc/sysconfig/ directory which define
the parameters used by some services when they start up.
3.7. The /etc/sysconfig/ Directory
The following information outlines some of the files found in the /etc/sysconfig/ directory, their
function, and their contents. This information 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.
The /usr/share/doc/initscripts- version-number /sysconfig.txt file contains a
more authoritative listing of the files found in the /etc/sysconfig directory and the configuration
options available.
Chapter 3. Boot Process, Init, and Shutdown
3.7.1. Files in the /etc/sysconfig/ Directory
The following files are normally found in the /etc/sysconfig/ directory:
• amd
• apmd
• arpwatch
• authconfig
• cipe
• clock
• desktop
• dhcpd
• firstboot
• gpm
• harddisks
• hwconf
• i18n
• identd
• init
• ipchains
• iptables
• irda
• keyboard
• kudzu
• mouse
• named
• netdump
• network
• ntpd
• pcmcia
• radvd
• rawdevices
• redhat-config-users
• redhat-logviewer
• samba
• sendmail
• soundcard
• squid
• tux
• ups
Chapter 3. Boot Process, Init, and Shutdown
• vncservers
• xinetd
It is possible that your system may be missing a few of them if the corresponding program that would
need that file is not installed.
Next, we will take a look at each one. /etc/sysconfig/amd
The /etc/sysconfig/amd file contains various parameters used by amd allowing for the automounting and automatic unmounting of file systems. /etc/sysconfig/apmd
The /etc/sysconfig/apmd file is used by apmd as a configuration for what things to
start/stop/change on suspend or resume. It is set up to turn on or off apmd during startup, depending
on whether your hardware supports Advanced Power Management (APM) or if you choose not to use
it. apm is a monitoring daemon that works with power management code within the Linux kernel. It
can alert you to a low battery if you are using Red Hat Linux on a laptop, among other things. /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. For more information about what parameters you can use in this file, type man arpwatch. By
default, this file sets the owner of the arpwatch process to the user pcap. /etc/sysconfig/authconfig
The /etc/sysconfig/authconfig file sets the kind of authorization to be used on the host. It
contains one or more of the following lines:
• USEMD5= value
, where
is one of the following:
yes — MD5 is used for authentication.
no — MD5 is not used for authentication.
, where
is one of the following:
yes — Kerberos is used for authentication.
no — Kerberos is not used for authentication.
, where
yes — LDAP is used for authentication.
no — LDAP is not used for authentication.
is one of the following:
Chapter 3. Boot Process, Init, and Shutdown
65 /etc/sysconfig/clock
The /etc/sysconfig/clock file controls the interpretation of values read from the system clock.
Earlier releases of Red Hat Linux used the following values (which are deprecated):
• CLOCKMODE= value
, where
is one of the following:
GMT — Indicates that the clock is set to Universal Time (Greenwich Mean Time).
ARC — Indicates the ARC console’s 42-year time offset is in effect (for Alpha-based systems
Currently, the correct values are:
is one of the following boolean values:
true — Indicates that the clock is set to Universal Time. Any other value indicates that it is set
to local time.
• ARC= value
, where
• UTC= value
, where
is the following:
true — Indicates the ARC console’s 42-year time offset is in effect. Any other value indicates
that the normal UNIX epoch is assumed (for Alpha-based systems only).
• ZONE= filename
— Indicates the timezone file under /usr/share/zoneinfo that
/etc/localtime is a copy of, such as:
ZONE="America/New York" /etc/sysconfig/desktop
The /etc/sysconfig/desktop file specifies the desktop manager to be run, such as:
DESKTOP="GNOME" /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 you can use in this file, type man dhcpd. /etc/sysconfig/firstboot
Beginning with Red Hat Linux 8.0, the first time you boot the system, the /sbin/init program calls
the etc/rc.d/init.d/firstboot script. This allows the user to install additional applications and
documentation before the boot process completes.
The /etc/sysconfig/firstboot file tells the firstboot command not to run on subsequent
reboots. If you want firstboot to run the next time you boot the system, simply remove
/etc/sysconfig/firstboot and execute chkconfig --level 5 firstboot on.
Chapter 3. Boot Process, Init, and Shutdown /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 you can use in this file, type man gpm. By default, it sets the
mouse device to /dev/mouse. /etc/sysconfig/harddisks
The /etc/sysconfig/harddisks file allows you to tune your hard drive(s). You can also use
/etc/sysconfig/hardiskhd[a-h], to configure parameters for specific drives.
Do not make changes to this file lightly. If you change the default values stored here, you could corrupt
all of the data on your hard drive(s).
The /etc/sysconfig/harddisks file may contain the following:
• USE_DMA=1,
where setting this to 1 enables DMA. However, with some chipsets and hard drive
combinations, DMA can cause data corruption. Check with your hard drive documentation or manufacturer before enabling this.
• Multiple_IO=16,
where a setting of 16 allows for multiple sectors per I/O interrupt. When enabled, this feature reduces operating system overhead by 30-50%. Use with caution.
• EIDE_32BIT=3 enables
• LOOKAHEAD=1 enables
(E)IDE 32-bit I/O support to an interface card.
drive read-lookahead.
• EXTRA_PARAMS= specifies
where extra parameters can be added. /etc/sysconfig/hwconf
The /etc/sysconfig/hwconf file lists all the hardware that kudzu detected on your 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 you do edit it, devices could suddenly show up as being added or removed. /etc/sysconfig/i18n
The /etc/sysconfig/i18n file sets the default language, such as:
LANG="en_US" /etc/sysconfig/identd
The /etc/sysconfig/identd file is used to pass arguments to the identd daemon at boot time.
The identd daemon returns the username of processes with open TCP/IP connections. Some services
on the network, such as FTP and IRC servers, will complain and cause slow responses if identd is
not running. But in general, identd is not a required service, so if security is a concern, you should
not run it. For more information about what parameters you can use in this file, type man identd.
By default, the file contains no parameters.
Chapter 3. Boot Process, Init, and Shutdown
67 /etc/sysconfig/init
The /etc/sysconfig/init file controls how the system will appear and function during the boot
The following values may be used:
, where
• BOOTUP= value
is one of the following:
BOOTUP=color means the standard color boot display, where the success or failure of devices
BOOTUP=verbose means an old style display, which provides more information than purely a
Anything else means a new display, but without ANSI-formatting.
and services starting up is shown in different colors.
message of success or failure.
, where
labels. Defaults to 60.
• RES_COL= value
is the number of the column of the screen to start status
, where value moves the cursor to the value in the RES_COL line.
Defaults to ANSI sequences output by echo -e.
• MOVE_TO_COL= value
, where value sets the color to a color indicating success.
Defaults to ANSI sequences output by echo -e, setting the color to green.
, where value sets the color to a color indicating failure. Defaults to ANSI sequences output by echo -e, setting the color to red.
, where value sets the color to a color indicating warning.
Defaults to ANSI sequences output by echo -e, setting the color to yellow.
sequences output by echo -e.
, where
sets the color to ’normal’. Defaults to ANSI
, where value sets the initial console logging level for the kernel. The
default is 7; 8 means everything (including debugging); 1 means nothing except kernel panics.
syslogd will override this once it starts.
• LOGLEVEL= value
• PROMPT= value
, where
is one of the following boolean values:
yes — Enables the key check for interactive mode.
no — Disables the key check for interactive mode. /etc/sysconfig/ipchains
The /etc/sysconfig/ipchains file contains information used by the kernel to set up ipchains
packet filtering rules at boot time or whenever the service is started.
This file is modified by typing the command /sbin/service ipchains save when
valid ipchains rules are in place. You should not manually edit this file. Instead, use the
/sbin/ipchains command to configure the necessary packet filtering rules and then save the rules
to this file using /sbin/service ipchains save.
Use of ipchains to set up firewall rules is not recommended as it is deprecated and may disappear
from future releases of Red Hat Linux. If you need a firewall, you should use iptables instead.
Chapter 3. Boot Process, Init, and Shutdown /etc/sysconfig/iptables
Like /etc/sysconfig/ipchains, the /etc/sysconfig/iptables file stores information used
by the kernel to set up packet filtering services at boot time or whenever the service is started.
You should not modify this file by hand unless you are familiar with how to construct iptables
rules. The simplest way to add rules is to use the /usr/sbin/lokkit command or the gnomelokkit graphical application to create your firewall. Using these applications will automatically edit
this file at the end of the process.
If you wish, you can manually create rules using /sbin/iptables and then type /sbin/service
iptables save to add the rules to the /etc/sysconfig/iptables file.
Once this file exists, any firewall rules saved there will persist through a system reboot or a service
For more information on iptables see Chapter 13. /etc/sysconfig/irda
The /etc/sysconfig/irda file controls how infrared devices on your system are configured at
The following values may be used:
• IRDA= value
, where
is one of the following boolean values:
yes — irattach will be run, which periodically checks to see if anything is trying to connect
no — irattach will not be run, preventing infrared device communication.
to the infrared port, such as another notebook computer trying to make a network connection.
For infrared devices to work on your system, this line must be set to yes.
• DEVICE= value
, where
is the device (usually a serial port) that handles infrared
, 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.
• DONGLE= value
• DISCOVERY= value
, where
is one of the following boolean values:d
yes — Starts irattach in discovery mode, meaning it actively checks for other infrared de-
no — Does not start irattach in discovery mode.
vices. This needs to be turned on for the machine to be actively looking for an infrared connection
(meaning the peer that does not initiate the connection). /etc/sysconfig/keyboard
The /etc/sysconfig/keyboard file controls the behavior of the keyboard. The following values
may be used:
which is used on SPARCs only. sun means a Sun keyboard is attached
on /dev/kbd, and pc means a PS/2 keyboard connected to a PS/2 port.
• KEYTABLE= file
, where
is the name of a keytable file.
Chapter 3. Boot Process, Init, and Shutdown
For example: KEYTABLE="us". The files that can be used as keytables start in
/usr/lib/kbd/keymaps/i386 and branch into different keyboard layouts from there, all
labeled file .kmap.gz. The first file found beneath /usr/lib/kbd/keymaps/i386that
matches the KEYTABLE setting is used. /etc/sysconfig/kudzu
The /etc/sysconfig/kuzdu allows you to specify a safe probe of your system’s hardware by
kudzu at boot time. A safe probe is one that disables serial port probing.
• SAFE= value
, where
is one of the following:
yes — kuzdu does a safe probe.
no — kuzdu does a normal probe. /etc/sysconfig/mouse
The /etc/sysconfig/mouse file is used to specify information about the available mouse. The
following values may be used:
• FULLNAME= value
• MOUSETYPE= value
, where
, where
refers to the full name of the kind of mouse being used.
is one of the following:
microsoft — A Microsoft™ mouse.
mouseman — A MouseMan™ mouse.
mousesystems — A Mouse Systems™ mouse.
ps/2 — A PS/2 mouse.
msbm — A Microsoft™ bus mouse.
logibm — A Logitech™ bus mouse.
atibm — An ATI™ bus mouse.
logitech — A Logitech™ mouse.
mmseries — An older MouseMan™ mouse.
mmhittab — An mmhittab mouse.
• XEMU3= value
, where
is one of the following boolean values:
yes — The mouse only has two buttons, but three mouse buttons should be emulated.
no — The mouse already has three buttons.
, where value refers to the kind of mouse used when X is running.
The options here are the same as the MOUSETYPE setting in this same file.
• DEVICE= value
, where
value is the mouse device.
In addition, /dev/mouse is a symbolic link that points to the actual mouse device.
Chapter 3. Boot Process, Init, and Shutdown /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:
/some/where refers to the full directory path of a configured chroot environment under which named will run. This chroot environment must first be
configured. Type info chroot for more information on how to do this.
• ROOTDIR=" /some/where ", where
value any option listed in the man page for named except -t.
In place of -t, use the ROOTDIR line above instead.
• OPTIONS=" value ", where
For more information about what parameters you can use in this file, type man named. For detailed
information on how to configure a BIND DNS server, see Chapter 16. By default, the file contains no
parameters. /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, so you should only run it if you absolutely need to. For more
information about what parameters you can use in this file, type man netdump. /etc/sysconfig/network
The /etc/sysconfig/network file is used to specify information about the desired network configuration. The following values may be used:
, where
is one of the following boolean values:
yes — Networking should be configured.
no — Networking should not be configured.
, where value should be the Fully Qualified Domain Name (FQDN),
such as, but can be whatever hostname you want.
• HOSTNAME= value
For compatibility with older software that people might install (such as trn), the /etc/HOSTNAME
file should contain the same value as here.
• GATEWAY= value
, where
• NISDOMAIN= value
, where
, where
is the IP address of the network’s gateway.
is the gateway device, such as eth0.
is the NIS domain name.
Chapter 3. Boot Process, Init, and Shutdown
71 /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 you can use in this file, point a browser at 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. /etc/sysconfig/pcmcia
The /etc/sysconfig/pcmcia file is used to specify PCMCIA configuration information. The following values may be used:
• PCMCIA= value
, where
is one of the following:
yes — PCMCIA support should be enabled.
no — PCMCIA support should not be enabled.
, where
is one of the following:
i82365 — The computer has an i82365-style PCMCIA socket chipset.
tcic — The computer has a tcic-style PCMCIA socket chipset.
value is the socket driver (i82365 or tcic) timing parameters.
value , where value is the list of pcmcia_core options.
CARDMGR_OPTS= value , where value is the list of options for the PCMCIA cardmgr
• PCIC_OPTS= value
• PCIC= value
, where
(such as -q for quiet mode; -m to look for loadable kernel modules in the specified directory, and
so on). Read the cardmgr man page for more information. /etc/sysconfig/radvd
The /etc/sysconfig/radvd file is used to pass arguments to the radvd daemon at boot time.
The radvd daemon listens to 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 what parameters you can use in this file,
type man radvd. By default, this file sets the owner of the radvd process to the user radvd. /etc/sysconfig/rawdevices
The /etc/sysconfig/rawdevices file is used to configure raw device bindings, such as:
/dev/raw/raw1 /dev/sda1
/dev/raw/raw2 8 5
Chapter 3. Boot Process, Init, and Shutdown /etc/sysconfig/redhat-config-users
The /etc/sysconfig/redhat-config-users file is the configuration file for the graphical application, User Manager. Under Red Hat Linux 8.0 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 pulldown menu in the User Manager application and should not be edited by hand. For more information
on using this application, see the chapter called User and Group Configuration in the Official Red Hat
Linux Customization Guide. /etc/sysconfig/redhat-logviewer
The /etc/sysconfig/redhat-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, see the chapter called Log Files in the Official Red Hat Linux Customization Guide. /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
you can use in this file, type man smbd. By default, this file sets smbd and nmbd to run in daemon
mode. /etc/sysconfig/sendmail
The /etc/sysconfig/sendmail file allows messages to be sent to one or more recipients, routing
the message over whatever networks are necessary. The file sets the default values for the Sendmail
application to run. Its default values are to run as a background daemon, and to check its queue once
an hour in case something has backed up.
The following values may be used:
• DAEMON= value
, where
is one of the following boolean values:
yes — Sendmail should be configured to listen to port 25 for incoming mail. yes implies the
use of Sendmail’s -bd options.
no — Sendmail should not be configured to listen to port 25 for incoming mail.
QUEUE=1h 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. /etc/sysconfig/soundcard
The /etc/sysconfig/soundcard file is generated by sndconfig and should not be modified. The
sole use of this file is to determine what card entry in the menu to pop up by default the next time
sndconfig is run. Sound card configuration information is located in the /etc/modules.conf file.
It may contain the following:
• CARDTYPE= value
, where
is set to, for example, SB16 for a Soundblaster 16 sound
Chapter 3. Boot Process, Init, and Shutdown
73 /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 your
system). By default, this file sets squid top start in daemon mode and sets the amount of time before
it shuts itself down. /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 (replace # version $ with the version
number of TUX installed on your system). The parameters available for this file are listed in
/usr/share/doc/tux- version " /tux/parameters.html. /etc/sysconfig/ups
The /etc/sysconfig/ups file is used to specify information about any Uninterruptible Power Supplies (UPS) connected to your system. A UPS can be very valuable for a Red Hat Linux system
because it gives you time to correctly shut down the system in the case of power interruption. The
following values may be used:
is one of the following:
yes — A UPS device is connected to your system.
no — A UPS device is not connected to your system.
" , where
connected to the system:
• MODEL= value
must be one of the following or set to NONE if no UPS is
apcsmart — For a APC SmartUPS™ or similar device.
fentonups — For a Fenton UPS™.
optiups — For an OPTI-UPS™ device.
bestups — For a Best Power™ UPS.
genericups — For a generic brand UPS.
ups-trust425+625 — For a Trust™ UPS.
• DEVICE= value
, where
• SERVER= value
, where
value , where
specifies where the UPS is connected, such as /dev/ttyS0.
is a special command that needs to be passed to the UPS. /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 you to view a desktop environment not only on the
machine where it is running but across different networks on a variety of architectures.
Chapter 3. Boot Process, Init, and Shutdown
It may contain the following:
& , 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 vncpasswd before attempting to connect to the remote VNC server.
Note that when you use a VNC server, your communication with it is unencrypted, and
so it should not be used on an untrusted network. For specific instructions concerning
the use of SSH to secure the VNC communication, please read the information found at To find out more about SSH, see Chapter 9 or
Official Red Hat Linux Customization Guide. /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 what parameters you can use in this file, type man
xinetd. For more information on the xinetd service, see Section 8.3.
3.7.2. Directories in the /etc/sysconfig/ Directory
The following directories are normally found in /etc/sysconfig/ and a basic description of what
they contain:
— This contains the Red Hat APM suspend/resume script. You should not edit
this file directly. If you need customization, simple create a file called /etc/sysconfig/apmscripts/apmcontinue and it will be called at the end of the script. Also, you can control the
script by editing /etc/sysconfig/apmd.
• apm-scripts
— This directory contains the configuration files needed to do Class Based Queuing for bandwidth management on network interfaces.
• cbq
— This directory is used by the Network Administration Tool (redhat-confignetwork) and its contents should not be edited manually. For more information about configuring
• networking
network interfaces using the Network Administration Tool, see the chapter called Network Configuration in the Official Red Hat Linux Customization Guide.
• 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 up and down network interfaces, such as ifup and ifdown.
Scripts used to bring up and down ISDN interfaces, 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, see Chapter 12
— This directory contains the configuration files and GPG keys for the Red Hat Network. No
files in this directory should be edited by hand. For more information on the Red Hat Network, see
the Red Hat Network website at the following URL:
• rhn
Chapter 3. Boot Process, Init, and Shutdown
3.8. Shutting Down
To shut down Red Hat Linux, issue the shutdown command. You can read the shutdown man page
for complete details, but the two most common uses are:
/sbin/shutdown -h now
/sbin/shutdown -r now
You must run shutdown as root. After shutting everything down, the -h option will halt the machine,
and the -r option will reboot.
Non-root users can use the reboot and halt commands to shutdown the system while in runlevels 1
through 5. However, not all Linux operating systems support this feature.
If your computer does not power itself down, be careful not turn off the computer until you see a
message indicating that the system is halted.
Failure to wait for this message means you may be turning off the machine before the hard drive
partitions are unmounted, which can cause file system corruption
Chapter 3. Boot Process, Init, and Shutdown
Chapter 4.
Boot Loaders
Before Red Hat Linux can run on a system, it must be started by special program called a boot loader.
The boot loader program usually exists on the system’s primary hard drive or other media device and
is responsible for loading the Linux kernel its required files, or, in some cases, other operating systems
into memory.
4.1. Boot Loaders and System Architecture
Each system architecture which can run Red Hat Linux uses a different boot loader. For example, the
Alpha architecture uses the aboot boot loader, while the Itanium architecture uses the ELILO boot
This chapter discuss commands and configuration options for the two boot loaders supplied with Red
Hat Linux 8.0 for x86 architecture: GRUB and LILO.
4.2. GRUB
GNU GRand Unified Boot loader or GRUB is a program which enable the user to select which installed operating system or kernel to load at system boot time. It also allows the user to pass arguments
to the kernel.
4.2.1. GRUB and the x86 Boot Process
This section will discuss in more detail the specific role GRUB plays when booting an x86 system.
For a detailed look at the overall boot process, see Section 3.2.
GRUB loads itself into memory in the following stages:
1. The Stage 1 or primary boot loader is read into memory by the BIOS from the MBR 1. The
primary boot loader exists on less than 512 bytes of disk space within the MBR. The only thing
it does is load the Stage 1.5 or Stage 2 boot loader.
2. The Stage 1.5 boot loader is read into memory by the Stage 1 boot loader only if necessary. Some
hardware requires an intermediate step to get to the Stage 2 boot loader. This is sometimes true
when the /boot partition is above the 1024 cylinder head of the hard drive or when using LBA
mode. The Stage 1.5 boot loader is found either on the /boot partition or on a small part of the
MBR and the /boot partition.
3. The Stage 2 or secondary boot loader is read into memory. The secondary boot loader displays
the GRUB menu and command environment. This interface allows you to select which operating
system or Linux kernel to boot, pass arguments to the kernel, or look at system parameters, such
as available RAM.
4. The secondary boot loader reads the operating system or kernel and initrd into memory. Once
GRUB determines which operating system to start, it loads it into memory and hands control of
the machine to that operating system.
The boot method used to boot Red Hat Linux is called the direct loading method because the boot
loader loads the operating system directly. There is no intermediary between the boot loader and the
For more on the system BIOS and the MBR, see Section 3.2.1.
Chapter 4. Boot Loaders
The boot process used by other operating systems may differ. For example, Microsoft’s DOS and
Windows operating systems, as well as various other proprietary operating systems, are loaded using
a chain loading boot method. Under this method, the MBR simply points to the first sector of the
partition holding the operating system. There it finds the files necessary to actually boot that operating
GRUB supports both direct and chain-loading boot methods, allowing it to boot almost any operating
During installation, Microsoft’s DOS and Windows installer completely overwrites the MBR, destroying
any existing boot loader. If creating a dual boot system, it is best to install the Microsoft operating
system first. For instructions on how to do this, see the appendix titled Installing Red Hat Linux in a
Dual-Boot Environment in the Official Red Hat Linux Installation Guide.
4.2.2. Features of GRUB
GRUB contains a number of features that make it preferable to other boot loaders available for the
x86 architecture. Below is a list of some of the more important features:
GRUB provides a true command-based, pre-OS environment on x86 machines. This affords the user
maximum flexibility in loading operating systems with certain options or gathering information
about the system. For years many non-x86 architectures have employed pre-OS environments that
allow system booting from a command line. While some command features are available with LILO
and other x86 boot loaders, GRUB is more feature rich.
GRUB supports Logical Block Addressing (LBA) mode. LBA places the addressing conversion used
to find files in the hard driver’s firmware, and it is used on many IDE and all SCSI hard devices.
Before LBA, boot loaders could encounter the 1024-cylinder BIOS limitation, where the BIOS
could not find a file after that cylinder head of the disk. LBA support allows GRUB to boot operating
systems from partitions beyond the 1024-cylinder limit, so long as the system BIOS supports LBA
mode. Most modern BIOS revisions support LBA mode.
GRUB can read ext2 partitions. This allows GRUB to access its configuration file,
/boot/grub/grub.conf, every time the system boots, obviating the need for the user to write a
new version of the first stage boot loader to MBR when configuration changes are made. The only
time a user would need to reinstall GRUB on the MBR is if the physical location of the /boot
partition is moved on the disk. For details on installing GRUB to the MBR, see Section 4.3.
4.3. Installing GRUB
If GRUB was not installed during the Red Hat Linux installation process, you can install it afterward
and it will automatically become the default boot loader.
Before installing GRUB, you should make sure you have the latest GRUB package available or you
can use the GRUB package from the Red Hat Linux installation CD-ROMs. For instructions on installing packages, see the chapter titled Package Management with RPM in the Official Red Hat Linux
Customization Guide.
Once the GRUB package is installed, open a root shell prompt and run the command /sbin/grubinstall ' location ( , where ) location * is the location GRUB Stage 1 boot loader should
be installed.
Chapter 4. Boot Loaders
The following command installs GRUB to the MBR of the master IDE device on the primary IDE
bus, alos known as the C drive:
/sbin/grub-install /dev/hda
The next time you boot the system, you should see the GRUB graphical boot loader menu before the
kernel loads.
4.4. GRUB Terminology
One of the most important things to understand before using GRUB is how the program refers to devices, such as hard drives and partitions. This information is particularly important when configuring
GRUB to boot multiple operating systems.
4.4.1. Device Names
The first hard drive of a system is called (hd0) by GRUB. The first partition on that drive is called
(hd0,0), and the fifth partition on the second hard drive is called (hd1,4). In general, the naming
convention for file systems when using GRUB breaks down in this way:
( type-of-device
, +
bios-device-number , partition-number )
The parentheses and comma are very important to the device naming conventions. The
device , refers to whether a hard disk (hd) or floppy disk (fd) is being specified.
The + bios-device-number , is the number of the device according to the system’s BIOS, starting
with 0. The primary IDE hard drive is numbered 0, while the secondary IDE hard drive is numbered 1.
The ordering is roughly equivalent to the way the Linux kernel arranges the devices by letters, where
the a in hda relates to 0, the b in hdb relates to 1, and so on.
GRUB’s numbering system for devices starts with 0, not 1. Failing to make this distinction is one of
the most common mistakes made by new GRUB users.
partition-number relates to the number of a specific partition on a disk device. Like the
bios-device-number , the partition numbering starts at 0. While most partitions are specified
by numbers, if a system uses BSD partitions, they are signified by letters, such as a or c.
GRUB uses the following rules when naming devices and partitions:
It does not matter if system hard drives are IDE or SCSI. All hard drives start with hd. Floppy disks
start with fd.
To specify an entire device without respect to its partitions, simply leave off the comma and the
partition number. This is important when telling GRUB to configure the MBR for a particular disk.
For example, (hd0) specifies the MBR on the first device and (hd3) specifies the MBR on the
fourth device.
If a system has multiple drive devices, it is very important to know the drive boot order set in the
BIOS. This is rather simple to do if a system has only IDE or SCSI drives, but if there is a mix of
devices, it can become confusing.
Chapter 4. Boot Loaders
4.4.2. File Names
When typing commands to GRUB involving a file, such as a menu list to use when allowing the
booting of multiple operating systems, it is necessary to include the file immediately after specifying
the device and partition. A sample file specification to an absolute filename is organized as follows:
( type-of-device
/ .
bios-device-number , partition-number )/path/to/file
Most of the time, a user will specify files by the directory path on that partition, plus the filename.
It is also possible to specify files to GRUB that do not actually appear in the filesystem, such as a
chain loader that appears in the first few blocks of a partition. To specify these files, you must provide
a blocklist, which tells GRUB, block by block, where the file is located in the partition. Since a file
can be comprised of several different sets of blocks, there is a specific way to write blocklists. Each
file’s section location is described by an offset number of blocks and then a number of blocks from
that offset point, and the sections are put together in a comma-delimited order.
Consider the following blocklist:
This blocklist tells GRUB to use a file that starts at the first block on the partition and uses blocks 0
through 49, 99 through 124, and 199.
Knowing how to write blocklists is useful when using GRUB to load operating systems that use chain
loading, such as Microsoft Windows. It is possible to leave off the offset number of blocks if starting
at block 0. As an example, the chain loading file in the first partition of the first hard drive would have
the following name:
The following shows the chainloader command with a similar blocklist designation at the GRUB
command line after setting the correct device and partition as root:
chainloader +1
4.4.3. GRUB’s Root Filesystem
Some users are confused by the use of the term "root filesystem" with GRUB. It is important to
remember that GRUB’s root filesystem has nothing to do with the Linux root filesystem.
The GRUB root filesystem is the root partition for a particular device. GRUB uses this information to
mount the device and load files from it.
With Red Hat Linux, once GRUB has loaded its root partition, which equates to the /boot partition
and contains the Linux kernel, the kernel command can be executed with the location of the kernel
file as an option. Once the Linux kernel boots, it sets its own root filesystem. The original GRUB root
filesystem and its mounts are forgotten; they only existed to boot the kernel file.
Refer to the root and kernel commands in Section 4.6 for more information.
4.5. GRUB Interfaces
GRUB features three interfaces, which provide different levels of functionality. Each of these interfaces allows users to boot operating systems, and move between interfaces within the GRUB environment.
Chapter 4. Boot Loaders
4.5.1. Menu Interface
If GRUB was automatically configured by the Red Hat Linux installation program, this is the interface shown by default. A menu of operating systems or kernels preconfigured with their own boot
commands are displayed as a list, ordered by name. Use the arrow keys to select an option other than
the default selection and press the [Enter] key to boot it. Alternatively, a timeout period is set, so that
GRUB will start loading the default option.
From the menu interface, press the [e] key to enter the entry editor interface or the [c] key to load a
command line interface.
See Section 4.7 for more information on configuring this interface.
4.5.2. Menu Entry Editor Interface
To access the menu entry editor, press the [e] key from the boot loader menu. The GRUB commands
for that entry are displayed here, and users may alter these command lines before booting the operating
system by adding a command line ([o] inserts the new line after the current line and [O] before it),
editing one ([e]), or deleting one ([d]).
After all changes are made, hit the [b] key to execute the commands and boot the operating system.
The [Esc] key discards any changes and reloads the standard menu interface. The [c] key will load the
command line interface.
4.5.3. Command Line Interface
The command line is the most basic GRUB interface, but it is also the one that grants the most control.
The command line makes it possible to type any relevant GRUB commands followed by the [Enter]
key to execute them. This interface features some advanced shell-like features, including [Tab] key
completion, based on context, and [Ctrl] key combinations when typing commands, such as [Ctrl]-[a]
to move to the beginning of a line, and [Ctrl]-[e] to move to the end of a line. In addition, the arrow,
[Home], [End], and [Delete] keys work as they do in the bash shell.
See Section 4.6, for a list of common commands.
4.5.4. Order of Interface Use
When the GRUB environment loads the second stage boot loader, it looks for its configuration file.
When found, it uses the configuration file to build the menu list and displays the boot menu interface.
If the configuration file cannot be found, or if the configuration file is unreadable, GRUB will load the
command line interface to allow users to manually type the commands necessary to boot an operating
If the configuration file is not valid, GRUB will print out the error and ask for input. This can be
very helpful, because users will then be able to see precisely where the problem occurred and fix it
in the file. Pressing any key will reload the menu interface, where it is then possible to edit the menu
option and correct the problem based on the error reported by GRUB. If the correction fails, the error
is reported and GRUB will begin again.
4.6. GRUB Commands
GRUB allows a number of useful commands in its command line interface. Some of the commands
accept options after their name, and these options should be separated from the command and other
options on that line by space characters.
Chapter 4. Boot Loaders
The following is a list useful commands:
• boot
— Boots the operating system or chain loader that has been previously specified and loaded.
0 file-name 1 — Loads the specified file as a chain loader. To grab the file at the
first sector of the specified partition, use +1 as the file’s name.
• chainloader
• displaymem —
Displays the current use of memory, based on information from the BIOS. This is
useful to determine how much RAM a system has prior to booting it.
file-name — Enables users to specify an initial RAM disk to use when booting. An
initrd is necessary when the kernel needs certain modules in order to boot properly, such as when
• initrd
the root partition is formated with the ext3 file system.
2 stage-1 342 install-disk 352 stage-2 3
GRUB to the system MBR.
• install
config-file 3
— Installs
The install command will overwrite any other information in the MBR. If executed, any information (other than GRUB information) that is used to boot other operating systems, will be lost.
Make sure you know what you are doing before executing this command.
This command can be configured in several different ways. However, it is required to specify a
0 stage-1 1 , which signifies a device, partition, and file where the first boot loader image can be
found, such as (hd0,0)/grub/stage1. In addition, specify the disk where the stage 1 boot loader
should be installed, such as (hd0).
The 0 stage-2 1 section tells the stage 1 boot loader where the stage 2 boot loader is located, such
as (hd0,0)/grub/stage2. The p option tells the install command that a menu configuration
file is being specified in the 0 config-file 1 section, such as (hd0,0)/grub/grub.conf.
0 kernel-file-name 160 option-1 160 option-N 1 — Specifies the kernel file to
load from GRUB’s root filesystem when using direct loading to boot the operating system. Options
can follow the kernel command and will be passed to the kernel when it is loaded.
• kernel
For Red Hat Linux, an example kernel command looks like the following:
kernel /vmlinuz root=/dev/hda5
This line specifies that the vmlinuz file is loaded from GRUB’s root filesystem, such as (hd0,0).
An option is also passed to the kernel specifying that the root filesystem for the Linux kernel when
it loads should be on hda5, the fifth partition on the first IDE hard drive. Multiple options may be
placed after this option, if needed.
0 device-and-partition 1 — Configures GRUB’s root partition to be the particular device and partition, such as (hd0,0), and mounts the partition so that files can be read.
• root
• rootnoverify
does not mount the partition.
— Does the same thing as the root command but
Commands other than these are available. Type info grub for a full list of commands.
4.7. GRUB Menu Configuration File
The configuration file, which is used to create the list in GRUB’s menu interface of operating systems
to boot, essentially allows the user to select a pre-set group of commands to execute. The commands
given in Section 4.6 can be used, as well as some special commands that are only available in the
configuration file.
Chapter 4. Boot Loaders
4.7.1. Special Configuration File Commands
The following commands can only be used in the GRUB menu configuration file:
7 normal-color 897 selected-color 8 — Allows for the set up specific colors to be
used in the menu, where two colors are configured as the foreground and background. Use simple
color names, such as red/black. For example:
• color
color red/black green/blue
• default
times out.
• fallback
— The default entry title name that will be loaded if the menu interface
— If used, the entry title name to try if first attempt fails.
— If used, prevents the GRUB menu interface from being displayed, loading the
default entry when the timeout period expires. The user can see the standard GRUB menu by
pressing the [Esc] key.
• hiddenmenu
7 password 8 — If used, prevents the user who does not know the password from
editing the entries for this menu option.
• password
Optionally, it is possible to specify an alternate menu configuration file after the 7 password 8 , so
that, if the password is known, GRUB will restart the second stage of the boot loader and use this
alternate configuration file to build the menu. If this alternate file is left out of the command, then a
user who knows the password would be able to edit the current configuration file.
• timeout
— If used, sets the interval, in seconds, before GRUB loads the entry designated by the
default command.
• splashimage —
Specifies the location of the splash screen image to be used when GRUB boots.
title — Sets a title to be used with a particular group of commands used to load an operating
The # character can be used to place comments in the menu configuration file.
4.7.2. Configuration File Structure
GRUB menu interface’s configuration file is /boot/grub/grub.conf. The commands to set the
global preferences for the menu interface are placed at the top of the file, followed by the different
entries for each of the operating systems or kernels listed in the menu.
A very basic GRUB menu configuration file designed to boot either Red Hat Linux or Microsoft
Windows 2000 might look as follows:
# section to load linux
title Red Hat Linux (2.4.18-5.47)
root (hd0,0)
kernel /vmlinuz-2.4.18-5.47 ro root=/dev/sda2
initrd /initrd-2.4.18-5.47.img
# section to load Windows 2000
title windows
rootnoverify (hd0,0)
chainloader +1
Chapter 4. Boot Loaders
This file would tell GRUB to build a menu with Red Hat Linux as the default operating system, set
to autoboot it after 10 seconds. Two sections are given, one for each operating system entry, with
commands specific to this system’s disk partition table.
Note the default is specified as a number. This refers to the first title line GRUB comes across. If
you want windows to be the default, change the default= value to 1.
Configuring a GRUB menu configuration file to boot multiple operating systems is beyond the scope
of this chapter. Please consult Section 4.11 for detailed information about configuring GRUB to boot
different operating systems.
4.8. LILO
LILO is an acronym for LInux LOader and has been used to boot Linux on x86 systems for many years.
Although GRUB is now the default boot loader, some prefer to use LILO because it is more familiar
to them and others use it out of necessity, since GRUB may have trouble booting some hardware.
4.8.1. LILO and the x86 Boot Process
This section will discuss in more detail the specific role LILO plays when booting an x86 system. For
a detailed look at the overall boot process, see Section 3.2.
LILO loads itself into memory almost identically to GRUB, except it is only a two stage loader.
1. The Stage 1 or primary boot loader is read into memory by the BIOS from the MBR 2. The
primary boot loader exists on less than 512 bytes of disk space within the MBR. The only thing
it does is load the Stage 2 boot loader and pass to it disk geometry information.
2. The Stage 2 or secondary boot loader is read into memory. The secondary boot loader displays
the Red Hat Linux initial screen. This screen allows you to select which operating system or
Linux kernel to boot.
3. The Stage 2 boot loader reads the operating system or kernel and initrd into memory. Once
LILO determines which operating system to start, it loads it into memory and hands control of
the machine to that operating system.
Once the Stage 2 boot loader is in memory, LILO displays the initial Red Hat Linux screen with the
different operating systems or kernels it has been configured to boot. If you only have Red Hat Linux
installed and have not changed anything in LILO’s configuration file, you will see only linux as an
option. If you install SMP kernel support, you will see linux-up as an option. If you have set up LILO
to boot other operating systems as well, this screen is your chance to select what operating system
will boot. Use your arrow keys to highlight the operating system and press [Enter]
If you would like to have a command prompt to enter a command to LILO, press [Ctrl]-[X]. LILO
displays a LILO: prompt on the screen and waits for input from the user.
For more on the system BIOS and the MBR, see Section 3.2.1.
Chapter 4. Boot Loaders
4.8.2. LILO vs. GRUB
In general, LILO works similarly to GRUB except for three major differences:
It has no interactive command interface.
It stores information about the location of the kernel or other operating system it is to load on the
It cannot read ext2 partitions.
The first point means the command prompt for LILO is not interactive and only allows one command
with arguments.
The last two points mean that if you change LILO’s configuration file or install a new kernel, you must
rewrite the Stage 1 LILO boot loader to the MBR by issuing the /sbin/lilo -v -v command. This
is more risky than GRUB’s method, because a misconfigured MBR leaves the system unbootable.
With GRUB, if the configuration file is erroneously configured, it will simply default to its command
line interface.
If you upgrade the kernel using the Red Hat Update Agent, the MBR will be updated automatically.
For more information on RHN, refer to the following URL,
4.9. Options in /etc/lilo.conf
The /sbin/lilo -v -v command accesses the LILO configuration file, /etc/lilo.conf, to determine what to write to the MBR. If you are planning on using the lilo command, you need to know
how to edit this file.
If you are planning to edit /etc/lilo.conf, be sure to make a backup copy of the file before making
any changes. Also, be sure that you have a working boot floppy available so that you will be able
to boot the system and make changes to the MBR if there is a problem. See the man pages for
mkbootdisk for more information on creating a boot disk.
The file /etc/lilo.conf is used by lilo to determine which operating system or kernel to start,
as well as to know where to install itself (for example, /dev/hda for the first MBR of the first IDE
hard drive). A sample /etc/lilo.conf file looks like this (your /etc/lilo.conf may look a little
Chapter 4. Boot Loaders
This example shows a system configured to boot two operating systems: Red Hat Linux and DOS.
Here is a deeper look at a few of the lines of this file:
• boot=/dev/hda tells
LILO to install itself on the first hard disk on the first IDE controller.
• map=/boot/map locates
the map file. In normal use, this should not be modified.
• install=/boot/boot.b tells
LILO to install the specified file as the new boot sector. In normal
use, this should not be altered. If the install line is missing, LILO will assume a default of
/boot/boot.b as the file to be used.
The existence of prompt tells LILO to show you whatever is referenced in the message line. While
it is not recommended that you remove the prompt line, if you do remove it, you can still get a
prompt by holding down the [Shift] key while your machine starts to boot.
sets the amount of time that LILO will wait for user input before proceeding with
booting the default line entry. This is measured in tenths of a second, with 50 as the default.
• timeout=50
• message=/boot/message
system or kernel to boot.
refers to the screen that LILO displays to let you select the operating
describes the hard disk geometry to LILO. Another common entry here is linear. You
should not change this line unless you are very aware of what you are doing. Otherwise, you could
put your system in an unbootable state.
• lba32
refers to the default operating system for LILO to boot from the options listed
below this line. The name linux refers to the label line below in each of the boot options.
• default=linux
• image=/boot/vmlinuz-2.4.0-0.43.6
boot option.
specifies the linux kernel to boot with this particular
names the operating system option in the LILO screen. In this case, it is also the
name referred to by the default line.
• label=linux
• initrd=/boot/initrd-2.4.0-0.43.6.img refers
to the initial ram disk image that is used at
boot time to actually initialize and start the devices that makes booting the kernel possible. The
initial ram disk is a collection of machine-specific drivers necessary to operate a SCSI card, hard
drive, or any other device needed to load the kernel. You should never try to share initial ram disks
between machines.
read-only specifies that the root partition (see the root line below) is read-only and cannot be
altered during the boot process.
• root=/dev/hda5 tells
LILO what disk partition to use as the root partition.
4.10. Changing Runlevels at Boot Time
Under Red Hat Linux, it is possible to change your default runlevel at boot time.
If you use LILO as your boot loader, access the boot: prompt by typing [Ctrl]-[X]. Then type:
linux number
Chapter 4. Boot Loaders
In this command, replace number with either the number of the runlevel you wish to boot into (1
through 5), or the word single.
If you are using GRUB as your boot loader, follow these steps:
In the graphical GRUB boot loader screen, select the Red Hat Linux boot label and press [e] to
edit it.
Arrow down to the kernel line and press [e] to edit it.
At the prompt, type the number of the runlevel you wish to boot into (1 through 5), or the word
single and press [Enter].
You will be returned to the GRUB screen with the kernel information. Press the [b] key to boot the
For more information about runlevels, see Section 3.6.
4.11. Additional Resources
This chapter is only intended as an introduction to GRUB and LILO. Consult the following resources
to discover more about how GRUB and LILO work.
4.11.1. Installed Documentation
• /usr/share/doc/grub-0.92 —
figuring GRUB.
This directory contains good information about using and con-
The GRUB info page, accessible by typing the info grub command, contains a tutorial, a user
reference manual, a programmer reference manual, and an FAQ document about GRUB and its
— This directory contains a wealth of information about using and configuring LILO. In particular, the doc subdirectory contains a postscript file called that is highly informative.
• /usr/share/doc/lilo-21.4.4
4.11.2. Useful Websites
• — The home page of the GNU GRUB project. This site contains
information concerning the state of GRUB development and a FAQ.
• — The original GRUB documentation before the project was
handed off to the Free Software Foundation for further development.
• — Investigates
various uses for GRUB, including booting operating systems other than Linux.
• — An introductory article discussing the configuration of GRUB on a system from scratch, including an overview of GRUB command line options.
• — This mini-HOWTO discusses various uses for
LILO, including booting operating systems other than Linux.
Chapter 4. Boot Loaders
Chapter 5.
Users and Groups
Control of users and groups is a core element of Red Hat Linux system administration.
Users can be either people, meaning accounts tied to physical users, or logical users, meaning accounts
which exist for specific applications to use. Both types of users have a unique User ID (UID) and
Group ID (GID).
Groups are logical expressions of organization. Groups tie users together, giving them permissions to
read, write, or execute files.
When a file is created it is assigned a user and group. It is also assigned separate read, write, and
execute permissions for the file’s owner, group, and everyone else. The user and group of a particular
file, as well as the access permissions on the file, can be changed by root or, in most cases, by the
creator of the file.
Proper management of users and groups and effective management of file permissions, are among the
most important tasks a system administrator undertakes in the course of his duties.
5.1. Tools for User and Group Creation
Managing users and groups can be a tedious task, but Red Hat Linux provides a few tools and conventions to make management easier for administrators.
You can use tools like useradd or groupadd to create new users and groups from the shell prompt.
But an easier way to manage users and groups is through the graphical application, User Manager.
For more information on User Manager, refer to the Official Red Hat Linux Customization Guide.
In Section 5.4 further discuss file permission concepts and will walk through command line steps for
managing users.
5.2. Standard Users
In Table 5-1, you will find the standard users set up by the installation process in the /etc/passwd
file. The Group ID (GID) in this table is the primary group for the user. See Section 5.3 for a listing
of standard groups.
Home Directory
Chapter 5. Users and Groups
Home Directory
Chapter 5. Users and Groups
Home Directory
Table 5-1. Standard Users
5.3. Standard Groups
In Table 5-2, you will find the standard groups set up by the installation program. Groups are stored
on Red Hat Linux in the /etc/group file.
root, bin, daemon
root, bin, daemon
root, bin, adm
root, adm, daemon
daemon, lp
Chapter 5. Users and Groups
Table 5-2. Standard Groups
5.4. User Private Groups
Red Hat Linux uses a user private group (UPG) scheme, which makes UNIX groups easier to use.
The UPG scheme does not add or change anything in the standard UNIX way of handling groups;
it simply offers a new convention. Whenever you create a new user, by default, they have a unique
group. The scheme works as follows:
Chapter 5. Users and Groups
User Private Group
Every user has a primary group; the user is the only member of that group.
umask = 002
Traditionally, on UNIX systems the umask is 022, which prevents other users and other members
of a user’s primary group from modifying a user’s files. Since every user has their own private
group in the UPG scheme, this "group protection" is not needed. A umask of 002 will prevent
users from modifying other users’ private files. The umask is set in /etc/profile.
setgid bit on Directories
If you set the setgid bit on a directory (with chmod g+s directory ), files created in that directory will have their group set to the directory’s group.
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 group’s files. Using this traditional scheme, managing files has been
difficult because 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, which makes managing group projects that share a common
directory very simple.
For example, say you have a big project called devel, with many people editing the devel files in a
devel directory. Make a group called devel, chgrp the devel directory to devel, and add all of
the devel users to the devel group.
You can add a user to a group using User Manager (see the Official Red Hat Linux Customization Guide), or if you prefer to use the command line, use the /usr/sbin/groupadd groupname
command to create a group. The /usr/bin/gpasswd -a loginname groupname command will
add a user loginname to a group. (See the groupadd and gpasswd man pages if you need more
information on their options.) The /etc/group file contains the group information for your system.
If you created the devel group, added users to the devel group, changed the group for devel
directory to the devel group, and set the setgid bit for the devel directory, all devel users will be
able to edit the devel files and create new files in the devel directory. The files they create will always
retain their devel group status, so other devel users will always be able to edit them.
If you have multiple projects like devel and users who are working on multiple projects, these users
will never have to change their umask or group when they move from project to project. If set correctly,
the setgid bit on each project’s main directory "selects" the proper group for all files created in that
Since each user’s home directory is owned by the user and their private group, it is safe to set the
setgid bit on the home directory. However, by default, files are created with the primary group of the
user, so the setgid bit would be redundant.
5.4.1. User Private Group Rationale
Although the User Private Group (UPG) has existed in Red Hat Linux for quite some time, many
people still have questions about it, such as why UPG is necessary. To illustrate its use, consider the
following scenario.
You would like to have a group of people work on a set of files in the /usr/lib/emacs/sitelisp/ directory. You trust a few people to modify the directory but certainly not everyone. So first
create an emacs group:
/usr/sbin/groupadd emacs
In order to associate the contents of the directory with the emacs group, type:
Chapter 5. Users and Groups
chown -R root.emacs /usr/lib/emacs/site-lisp
Now, it is possible to add the proper users to the group with gpasswd:
/usr/bin/gpasswd -a
Allow the users to actually create files in the directory with the following command:
chmod 775 /usr/lib/emacs/site-lisp
When a user creates a new file, it is assigned the group of the user’s default private group. To prevent
this, perform the following command, which causes everything in the directory to be created with the
emacs group:
chmod 2775 /usr/lib/emacs/site-lisp
If the new file needs to be mode 664 for another user in the emacs group to be able to edit it, make the
default umask 002.
At this point, by making the default umask 002, you can easily set up groups that users can take
advantage of without any extra work every time users write files to the group’s common directory.
Just create the group, add the users, and do the above chown and chmod on the group’s directories.
5.5. Shadow Utilities
If you are in a multiuser environment and not using a networked authentication scheme such as Kerberos, you should consider using Shadow Utilities (also known as shadow passwords) for the enhanced protection offered for your system’s authentication files. During the installation of Red Hat
Linux, shadow password protection for your system is enabled by default, as are MD5 passwords (an
alternative and arguably more secure method of encrypting passwords for storage on your system).
Shadow passwords offer a few distinct advantages over the previous standard of storing passwords on
UNIX and Linux systems, including:
Improved system security by moving the encrypted passwords (normally found in /etc/passwd)
to /etc/shadow which is readable only by root
Information concerning password aging (how long it has been since a password was last changed)
Control over how long a password can remain unchanged before the user is required to change it
The ability to use the /etc/login.defs file to enforce a security policy, especially concerning
password aging
The shadow-utils package contains a number of utilities that support:
Conversion from normal to shadow passwords and back (pwconv, pwunconv)
Verification of the password, group, and associated shadow files (pwck, grpck)
Industry-standard methods of adding, deleting and modifying user accounts (useradd, usermod,
and userdel)
Industry-standard methods of adding, deleting, and modifying user groups (groupadd, groupmod,
and groupdel)
Industry-standard method of administering the /etc/group file using gpasswd
There are some additional points of interest concerning these utilities:
Chapter 5. Users and Groups
The utilities will work properly whether shadowing is enabled or not.
The utilities have been slightly modified to support Red Hat’s user private group scheme. For a
description of the modifications, see the useradd man page. For more information on user private
groups, turn to Section 5.4.
The adduser script has been replaced with a symbolic link to /usr/sbin/useradd.
The tools in the shadow-utils package are not Kerberos, NIS, hesiod, or LDAP enabled. New
users will be local only. For more information on Kerberos and LDAP, see Chapter 10 and Chapter
Chapter 5. Users and Groups
Chapter 6.
The X Window System
While the heart of Red Hat 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 simply X.
This chapter is an introduction to the behind-the-scenes world of XFree86, the open-source implementation of X provided with Red Hat Linux.
6.1. The Power of X
Linux began as a powerful, server-based operating system, excelling at efficiently processing complicated programs requiring high CPU utilization and handling requests from hundreds or thousands
of clients through network connections. However, because of its open nature and stability, Linux has
quickly developed into a popular GUI-based operating system for workstations, both in the home or
in the workplace.
In the UNIX™ world, windowing environments have existed for decades, predating many of the
current mainstream operating systems. The X Window System is now the dominant graphical user
interface (GUI) for UNIX-like operating systems.
To create this GUI for the user, X uses a client-server architecture. An X server process is started
and X client processes can connect to it via a network or local loopback interface. The server process
handles the communication with the hardware, such as the video card, monitor, keyboard, and mouse.
The X client exists in the user-space, issuing requests to the X server.
On Red Hat Linux systems, the XFree86 server fills the role of the X server. As a large scope open
source software project, with hundreds of developers around the world, XFree86 features rapid development, a wide degree of support for various hardware devices and architectures, and the ability to
run on different operating systems and platforms.
Most Red Hat Linux desktop users are unaware of the XFree86 server running on their system. They
are much more concerned with the particular desktop environment in which they spend most of their
time. The Red Hat Linux installation program does an excellent job of configuring your XFree86
server during the installation process, ensuring that X performs optimally when first started.
The X server performs many difficult tasks using a wide array of hardware, requiring detailed configuration. If some aspect of your system changes, such as the monitor or video card, XFree86 will need
to be reconfigured. In addition, if you are troubleshooting a problem with XFree86 that cannot be
solved using a configuration utility, such as the X Configuration Tool (redhat-config-xfree86),
you may need to access its configuration file directly.
X Configuration Tool is capable of configuring XFree86 while the X server is active. To activate the
configuration program from the command-line, type redhat-config-xfree86. To start X Configuration Tool while in an active X session, go to the Main Menu Button (on the Panel) => System
Tools => Display. After using X Configuration Tool during an X session, you will need to log out of
the current X session, then log back in for the changes to take effect.
Chapter 6. The X Window System
6.2. XFree86
Red Hat Linux 8.0 uses XFree86 version 4.2 as the base X Window System, which includes the
various necessary X libraries, fonts, utilities, documentation, and development tools.
Red Hat no longer provides the older XFree86 version 3 server packages. Before upgrading
to the latest version of Red Hat Linux be sure that your video card is compatible with
XFree86 version 4 by checking the Red Hat Hardware Compatibility List located at this
The X server includes many cutting edge XFree86 technology enhancements such as hardware 3D
acceleration support, the XRender extension for anti-aliased fonts, a modular driver based design,
support for modern video hardware and input devices, and many other features.
The Red Hat Linux installation program installs the base components of XFree86. You may choose to
install any optional XFree86 packages.
The X Window System resides primarily in two locations in the file system:
/usr/X11R6/ directory
A directory containing X client binaries (the bin directory), assorted header files (the include
directory), libraries (the lib directory), and manual pages (the man directory), and various other
X documentation (the /usr/X11R6/lib/X11/doc/ directory).
/etc/X11/ directory
The /etc/X11/ directory hierarchy contains all of the configuration files for the various components that make up the X Window System. This includes configuration files for the X server
itself, the X font server (xfs), the X Display Manager (xdm), and many other base components.
Display managers such as gdm and kdm, as well as various window managers, and other X tools
also store their configuration in this hierarchy.
XFree86 version 4 server is a single binary executable — /usr/X11R6/bin/XFree86. This server
dynamically loads various X server modules at runtime from the /usr/X11R6/lib/modules/ directory including video drivers, font engine drivers, and other modules as needed. Some of these modules
are automatically loaded by the server, whereas some are optional features that you must specify in
the XFree86 server’s configuration file, /etc/X11/XF86Config, before they can be used. The video
drivers are located in the /usr/X11R6/lib/modules/drivers/ directory. The DRI hardware accelerated 3D drivers are located in the /usr/X11R6/lib/modules/dri/ directory.
6.2.1. XFree86 Server Configuration Files
The XFree86 server configuration files are stored in the /etc/X11/ directory. The XFree86 version
4 server uses /etc/X11/XF86Config. When Red Hat Linux is installed, the configuration files for
XFree86 are created using information gathered during the installation process.
While there is rarely a need to manually edit these files, it is useful to know about the various sections
and optional parameters found in them.
Each section begins with a Section " < section-name = " line and ends with an EndSection line.
Within each of the sections, you will find several lines containing an option name and at least one
option value, occasionally seen in quotes. The following list explores the most useful sections of an
XFree86 version 4 file and the roles of various popular settings.
Chapter 6. The X Window System
Specifies information about the video card used by the system. You must have at least one Device section in your configuration file. You may have multiple Device sections in the case of
multiple video cards or multiple settings that can run a single card. The following options are
required or widely used:
— Specifies the bus location of the video card. This option is only necessary for systems with multiple cards and must be set so that the Device section will use the proper settings
for the correct card.
• BusID
• Driver
— Tells XFree86 which driver to load in order to use the video card.
— Provides a unique name for this video card. Usually, this name is set to the
exact name of the video card used in the Device section.
• Identifier
• Screen —
An optional setting used when a video card has more than one head, or connector,
to go out to a separate monitor. If you have multiple monitors connected to one video card,
separate Device sections must exist for each of them with a different Screen value for each
Device section. The value accepted by this option is a number starting at 0 and increasing by
one for each head on the video card.
• VideoRam — The
amount of RAM available on the video card in kilobytes. This setting is not
normally necessary since the XFree86 server can usually probe the video card to autodetect
the amount of video RAM. But since there are some video cards XFree86 cannot correctly
autodetect, this option allows you to specify the amount of video RAM.
Direct Rendering Infrastructure (DRI) is an interface which primarily allows 3D software applications to take advantage of the 3D hardware acceleration capabilities on modern supported
video hardware. In addition, DRI can improve 2D hardware acceleration performance when using drivers that have been enhanced to use the DRI for 2D operations. This section is ignored
unless DRI is enabled in the Module section.
Since different video cards use DRI in different ways. Before changing any DRI values, read the
/usr/X11R6/lib/X11/doc/README.DRI file for specific information about your particular
video card.
This section sets paths for services vital to the XFree86 server, such as the font path. Common
options include:
• FontPath — Sets the
locations where the XFree86 server can find fonts. Different fixed paths
to directories holding font files can be placed here, separated by commas. By default, Red
Hat Linux uses xfs as the font server and points FontPath to unix/:7100. This tells the
XFree86 server to obtain font information by using UNIX-domain sockets for inter-process
communication (IPC).
See Section 6.5 for more information concerning XFree86 and fonts.
— Allows you to set up multiple directories to use for storing modules loaded
by the XFree86 server.
• ModulePath
• RgbPath —
Tells the XFree86 server where the RGB color database is located on the system.
This database file defines all valid color names in XFree86 and ties them to specific RGB
Chapter 6. The X Window System
Configures an input device such as a mouse or keyboard used to submit information into the system using the XFree86 server. Most systems have at least two InputDevice sections, keyboard
and mouse. Each section includes these two lines:
• Driver
— Tells XFree86 the name of the driver to load to use the device.
— Sets the name of the device, usually the name of the device followed by a
number, starting with 0 for the first device. For example, the first keyboard InputDevice
would have an Identifier of Keyboard0.
• Identifier
Most InputDevice sections contain lines assigning specific options to that device. Each of
these lines start with Option and contain the name of the option in quotes, followed by the
value to assign to that option. Mice usually receive options such as Protocol, such as PS/2,
and Device, which designates the mouse to use for this section. The InputDevice section is
well commented, allowing you to configure additional options for your particular devices by
uncommenting certain lines.
Tells the XFree86 server which modules from the /usr/X11R6/lib/modules/ directory to
load. Modules provide the XFree86 server with additional functionality.
You should not edit these values.
Refers to the type of monitor used by the system. While one Monitor section is the minimum,
there may be several Monitor sections, one for each monitor in use with the machine.
Be careful when manually editing values in the options of the Monitor section. Inappropriate
values in this section could damage or destroy your monitor. Consult the documentation that
came with your monitor for the safe operating parameters available.
The following options are usually configured during installation or when using X Configuration
— Tells XFree86 the range of horizontal sync frequencies compatible with the
monitor in kHz. These values are used as a guide by the XFree86 server so that it will know
whether to use a particular Modeline entry’s values with this monitor.
• HorizSync
— Provides a unique name for this monitor, usually numbering each monitor
starting at 0. The first monitor would be named Monitor0, the second Monitor1, and so on.
• Identifier
— Used to specify the video modes used by the monitor at particular resolutions,
with certain horizontal sync and vertical refresh resolutions. Modeline entries are usually
preceded by a comment that explains what the mode line specifies.
• Modeline
If your configuration file does not include comments for the various mode lines, you can scan
over the values (also called mode descriptions) to uncover what the mode line is attempting to
do. See the XF86Config man page for detailed explanations of each mode description section.
• ModelName
— An optional parameter that displays the model name of the monitor.
• VendorName
— An optional parameter that displays the vendor that manufactured the moni-
Chapter 6. The X Window System
— Lists the vertical refresh range frequencies supported by the monitor, in
kHz. These values are used as a guide by the XFree86 server so that it will know whether to
use a particular Modeline entry’s values with this monitor.
• VertRefresh
Binds together a particular Device and Monitor that can be utilized as a pair and contain certain
settings. You must have at least one Screen section in your configuration file. The following
options are common:
— Tells the Screen section the default color depth to try in bits. 8 is the
default, 16 provides thousands of colors, and 32 displays millions of colors.
• DefaultDepth
• Device
— Signifies the name of the Device section to use with this Screen section.
• Identifier — Identifies the Screen section so that it can be referred to by a ServerLayout
section and be utilized.
• Monitor
— Tells the name of the Monitor section to be used with this Screen section.
You may also have a Display subsection within the Screen section which tells the XFree86
server the color depth (Depth) and resolution (Mode) to try first when using this particular monitor and video card.
Contains miscellaneous global XFree86 server settings. These settings may be overridden by
options placed in the ServerLayout section. Among the most useful settings:
• DontZap —
Prevents the use of the [Ctrl]-[Alt]-[Backspace] key combination to immediately
terminate the XFree86 server.
— Prevents cycling through configured video resolutions using the [Ctrl]-[Alt][Keypad-Plus] and [Ctrl]-[Alt]-[Keypad-Minus] key combinations.
• DontZoom
Binds together a Screen section with the necessary InputDevice sections and various options
to create a unified collection of preferences used by the XFree86 server as it starts. If you have
more than one ServerLayout section, and the one to use is not specified on the command line
when bringing up the XFree86 server, the first ServerLayout section in the configuration file
is used.
The following options are used in a ServerLayout section:
• Identifier —
A unique name used to describe this ServerLayout section.
— The names of any InputDevice sections to be used with the XFree86
server. Most users will only have two lines here, Keyboard0 and Mouse0, the first keyboard
and mouse configured for the system. The options CoreKeyboard and CorePointer refer to
the fact that these are the preferred keyboard and mouse, respectively, to use with the XFree86
• InputDevice
— The name of the Screen section to use. The number to the left of the name of the
Screen section refers to the particular screen number to use in a multi-head configuration.
For standard single-head video cards, this value is 0. The numbers to the right give the X and
Y absolute coordinates for the upper-left corner of the screen, by default 0 0.
• Screen
Below is an example of a typical screen entry:
"Screen0" 0 0
Chapter 6. The X Window System
For more information, refer to the XF86Config man page.
To review the current configuration of your XFree86 server, type the xset -q command. This provides you with information about the keyboard, pointer, screen saver, and font paths.
6.3. Desktop Environments and Window Managers
The configuration of an XFree86 server is useless until accessed by an X client that will use it to
display a program using the hardware controlled by the X server. X clients are programs designed to
take advantage of the X server’s hardware, usually to provide interactivity with a user.
You do not have to run a complicated window manager in conjunction with a particular desktop
environment to use X client applications. Assuming that you are not already in an X environment
and do not have an .xinitrc file in your home directory, type the xinit command to start X with
a basic terminal window (the default xterm application). You will see that this basic environment
utilizes your keyboard, mouse, video card, and monitor with the XFree86 server, using the server’s
hardware preferences. Type exit at the xterm prompt to leave this basic X environment.
Of course, most computer users require more features and utility from their GUI. Developers have
added layers of features to create highly developed and interactive environments that utilize the full
power of the XFree86 server. These layers break into two fundamental groups based on their purpose.
6.3.1. Window Managers
Window managers are X client programs that control the way other X clients are positioned, resized,
or moved. Window managers can also provide titlebars to windows, keyboard focus by keyboard or
mouse, and user-specified key and mouse button bindings. Window managers work with a collection
of different X clients, wrapping around the program, making it look a certain way and appear on the
screen in a particular place.
Four window managers are included with Red Hat Linux 8.0:
— The minimalist Tab Window Manager, which provides the most basic toolset of any of the
window managers.
• twm
• mwm —
The default window manager for the GNOME desktop environment, mwm stands for Metacity Window Manager. It is a simple and efficient window manager which supports custom themes.
— This full featured window manager for was was the default until the release of Red
Hat Linux 8.0. It can be used either with or without the GNOME desktop environment.
• sawfish
— WindowMaker is a full-featured GNU window manager designed to emulate the look
and feel of the NEXTSTEP environment.
• wmaker
These window managers can be run as individual X clients to gain a better sense of their differences. Type the xinit > path-to-window-manager ? command, where > path-to-windowmanager ? is the location of the window manager binary file. The binary file can be found by typing
which > window-manager-name ? or by looking for the name of the window manager in a bin
6.3.2. Desktop Environments
A desktop environment brings together assorted X clients that can be run together using similar methods, utilizing a common development environment.
Desktop environments are different from window managers, which only control the appearance and
placement of X client windows. Desktop environments contain advanced features that allow X clients
and other running processes to communicate with one another. This allows all applications written
Chapter 6. The X Window System
to work in that environment to commonly integrate and be used in new ways, such as permitting
drag-and-drop behavior with text.
GNOME is the default desktop environment for Red Hat Linux, using the GTK2 base widget toolkit
and miscellaneous other widgets that extend the base functionality. KDE, another desktop environment, uses a different toolkit called Qt. GNOME and KDE both contain advanced productivity applications, such as word processors, spreadsheets, and control panel devices that allow you to have
complete control of the look-and-feel of your user experience. Both environments can run standard X
client applications, and most KDE applications can run in GNOME, if the Qt libraries are installed.
When you start X using the startx command, a pre-specified desktop environment is utilized. To
change the default desktop environment used when X starts, open a terminal and type the switchdesk
command. This brings up a graphical utility that allows you to select the desktop environment or
window manager to use the next time X starts.
Desktop environments utilize window managers to provide the consistency in appearance between
different applications. KDE contains its own window manager, called kwm, specifically for this functionality.
For information on the customization of the GNOME and KDE desktop environments, see the Official
Red Hat Linux Getting Started Guide.
6.4. Runlevels
Most users run X from runlevels 3 or 5. Runlevel 3 places your system in multi-user mode with
full networking capabilities. The machine will boot to a text-based login prompt with all necessary
preconfigured services started. Most servers are run in runlevel 3, as X is not necessary to provide any
services utilized by most users. Runlevel 5 is similar to 3, except that it automatically starts X and
provides a graphical login screen. Many workstation users prefer this method, because it never forces
them to see a command prompt.
The default runlevel used when your system boots can be found in the /etc/inittab file. If you
have a line in that file that looks like id:3:initdefault:, then your system will boot to runlevel
3. If you have a line that looks like id:5:initdefault:, your system is set to boot into runlevel 5.
As root, change the runlevel number in this file to set a different default. Save the file and restart your
system to verify that it boots to the correct runlevel. More information on runlevels can be found in
Section 3.6.
6.4.1. Runlevel 3: startx
When in runlevel 3, the preferred way to start an X session is to type the startx command. startx, a
front-end to the xinit program, launches the XFree86 server and connects the X clients to it. Because
you must already be logged into the system at runlevel 3 to be able to type commands, startx is only
designed to bring up certain X clients, such as a desktop environment, in a particular manner. It does
not provide any user authentication.
When startx begins, it looks for a .xinitrc file in the user’s home directory to define the X clients
to run. If it does not find that file present, it will run the system default /etc/X11/xinit/xinitrc
script instead. The startx script then does the same thing with the .xserverrc file, looking for
it in the user’s home directory and then running the default /etc/X11/xinit/xserverrc script if
it cannot be found. Since many different X clients exist, the xinitrc files are very important. The
xserverrc script is less important. It only sets the X server to connect to the X clients. Since the
default X server is already configured with the /etc/X11/X link, Red Hat Linux does not install a
default xserverrc.
The default xinitrc script then looks 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
Chapter 6. The X Window System
xmodmap utility to configure the keyboard. The Xresources files are read to assign specific preference values to particular 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, which
configures settings such as the default language to use and the desktop environment to start from
Next, the xinitrc script tries 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 or window manager in the
.Xclients-default file. If .Xclients does not exist in the user’s home directory, the standard
/etc/X11/init/Xclients script attempts to start another desktop environment, trying GNOME
first and then KDE. If a desktop environment cannot be found by this point, Xclients attempts the
default window manager listed in the .wm_style file in the user’s home directory. If this fails, it
cycles through a predefined list of window managers.
The preferred X client applications should now have started by this point, along with the XFree86
server. If you need more details concerning starting X in runlevel 3, refer to the startx and xinit
man pages and read through the scripts named above.
6.4.2. Runlevel 5: prefdm
Runlevel 5 uses a slightly different method to start X. When the system starts, no one is logged into
the system by default. In order for a user to start a session, they must log in to the system. In runlevel
5, users authenticating themselves at the console use a display manager, a special X client that allows
the user to submit their login name and password.
Depending on the desktop environments installed on your specific Red Hat Linux system, three different display managers are available to handle the user authentication. The xdm display manager is
the original X authentication tool. xdm only allows you to log in and start an X session, nothing more.
The gdm display manager, designed to work with the GNOME desktop environment, and kdm display
manager, used with the KDE desktop environment, allow you to set the desktop environment, or session, you would like to use after authentication. Additionally, you can restart or halt the system from
the login screen. The gdm display manager also allows you to configure the language you would like
to use.
When the system enters runlevel 5, a line in the /etc/inittab file specifies that the prefdm script is
executed in order to determine the preferred display manager to bring up for user authentication. The
prefdm script uses the preferences stated in the /etc/sysconfig/desktop file to find the proper
display manager. If no desktop environment is specified, prefdm cycles through the gdm, kdm, and
xdm display managers to find one to use. Once one is found, prefdm launches it to handle the user
Each of the display managers look to 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 done by the xinitrc script when starting X in runlevel 3, including setting system and user resources, as well as running the scripts in the /etc/X11/xinit/xinitrc.d/
The user can specify which desktop environment they want to utilize when they authenticate using the
gdm or kdm display managers by selecting it from the Session menu. If the desktop environment is not
specified in the display manager, the /etc/X11/xdm/Xsession script will check 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.
Chapter 6. The X Window System
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 continued running after the user logged in, takes control by
spawning a new display manager. This restarts the XFree86 server, displays a new login window, and
starts the entire process over again.
For more information about how display managers control user authentication, read the xdm man page.
6.5. Fonts
Red Hat Linux uses xfs (X Font Server) to provide fonts to the XFree86 server and the X client
applications that connect to it. While it is possible to not use xfs and place the paths to font directories
in your XF86Config configuration file, xfs has several advantages:
It is easier to add and remove fonts, including editing the font path. The font path is a collection of
paths in the file system where font files are stored. The xfs service keeps the font path out of the
XFree86 configuration files, making it easier to edit.
Fonts may be stored on one machine acting as a networked font server and can be shared among
multiple X servers over the network. A common set of fonts can be maintained in one place and
easily shared between all users.
More types of fonts are supported. xfs can handle TrueType, Type1, and bitmap fonts.
The XFree86 configuration files know whether to use xfs or hard coded font paths because of the
FontPath setting in their Files sections. By default, the FontPath is set to unix/:7100. This
tells the XFree86 server to connect to port 7100 using an inner-machine communication link. The
xfs server listening on this port will respond with font information when queried by the XFree86
The xfs service must be running when X is started. If it is not, you will be returned to a command
prompt with an error similar to failed to set default font path ’unix/:7100’. Check to
see if xfs is running using the ps aux | grep xfs command. By default, xfs is set to start in
runlevels 2, 3, 4, and 5, covering all runlevels where you would run X. If xfs is not running on
your system, you can start it as root using the /sbin/service xfs start command. Use the
/usr/sbin/ntsysv, serviceconf, or /sbin/chkconfig utilities to force it to start at the correct runlevels. For more on configuring services for a particular runlevel, refer to the chapter titled
Controlling Access to Services in the Official Red Hat Linux Customization Guide
6.5.1. xfs Configuration
The /etc/rc.d/init.d/xfs script starts the xfs server. Several options can be configured in the
/etc/X11/fs/config file:
— Sets a list of alternate font servers to be used if this font server is not
available. A comma must separate every font server in the list.
• alternate-servers
• catalogue — An ordered
list of font paths to use that contain the font files. A comma must follow
every font path before a new font path can be started in the list.
You can use the string :unscaled immediately after the font path to make the unscaled fonts in
that path load first. Then, you can specify the entire path again, so that other scaled fonts will also
be loaded.
• client-limit — Sets
the number of clients this font server will service before refusing to handle
any more. The default is 10.
Chapter 6. The X Window System
• clone-self
— Decides if the font server will clone a new version of itself when the client-
limit is hit. By default, this option is on. Set it to off to disable this feature.
• default-point-size —
Sets 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 12 point fonts.
— Specifies a list of resolutions supported by the XFree86 server. Each
resolution in the list must be separated by a comma.
• default-resolutions
— Tells xfs whether to defer the loading of glyphs, which is an image used to
visually represent a font. You can disable this feature (none), enable this feature for all fonts (all),
or turn this this feature on only for 16-bit fonts (16), which are largely used with Asian languages.
• deferglyphs
• error-file
be logged.
— Allows you to specify the path and file name of a locations where xfs errors can
— Tells xfs not to listen using a particular protocol. By default, this option is set to
tcp to prevent xfs from listening on TCP ports, primarily for security reasons. If you plan on using
xfs to serve fonts to networked workstations on a LAN, you need to remove the tcp from this line.
• no-listen
• port — Specifies the TCP port that xfs will listen on if no-listen does not exist or is commented
• use-syslog
— Tells xfs to use the system error log if set to on.
6.5.2. Adding Fonts
When using xfs, adding fonts to your system is rather straightforward. Use the chkfontpath -list command to see the font paths currently configured on your system. To add new fonts in a new
directory, follow these instructions as the root user:
1. Create a font directory, such as /usr/share/fonts and place the fonts inside that directory.
Be sure to set the permissions correctly; it is only necessary that the files can be read, no other
permissions are necessary.
2. Type the chkfontpath --add @ font-directory-path A command, where the @ fontdirectory-path A is the full path to the directory holding the fonts. This will add this font
path to the xfs configuration file.
You must have a fonts.dir file in your new font directory for the chkfontpath command to
work correctly. The creation of the fonts.dir file, as well as any other files used by xfs with
these fonts, is beyond the scope of this document.
Many font collections available for Linux include these files for you, it may not be necessary to
create them by hand.
3. Restart xfs using the /sbin/service xfs restart command. You will also need to restart
your X session.
4. Typing the chkfontpath --list command will show the new font path. Any fonts you added
will be available for use.
The Red Hat Support website contains more information on this subject, see: for additional help documents.
Chapter 6. The X Window System
6.6. Additional Resources
Much more can be said about the XFree86 server, the clients that connect to it, and the assorted
desktop environments and window managers. Advanced users interested in tweaking their XFree86
configuration will find these additional sources of information useful.
6.6.1. Installed Documentation
— Briefly describes the XFree86 architecture and how to
get additional information about the XFree86 project as a new user.
• /usr/X11R6/lib/X11/doc/README
• /usr/X11R6/lib/X11/doc/README.Config
open to XFree86 version 3 users.
• /usr/X11R6/lib/X11/doc/RELNOTES —
features available in XFree86.
— Explains the advanced configuration options
For advanced users that want to read about the latest
— Contains information about the XFree86 configuration files, including the
meaning and syntax for the different sections within the files.
• man XF86Config
— The primary man page for all XFree86 information, details the difference between local and network X server connections, explores common environmental variables, lists
command line options, and provides helpful administrative key combinations.
• man XFree86
• man Xserver —
Discribes the X display server.
6.6.2. Useful Websites
• — Home page of the XFree86 project, which produces the XFree86 open
source version of the X Window System. XFree86 is bundled with Red Hat Linux to control the
necessary hardware and provide a GUI environment.
• — Home page of the DRI (Direct Rendering Infrastructure) project. The
DRI is the core hardware 3D acceleration component of XFree86.
• — A HOWTO document detailing the manual installation and custom configuration of XFree86.
• — The home of the GNOME project.
• — The home of the KDE desktop environment.
6.6.3. Related Books
The Concise Guide to XFree86 for Linux by Aron Hsiao; Que — Provides an expert’s view of the
operation of XFree86 on Linux systems.
The New XFree86 by Bill Ball; Prima Publishing — Discuses XFree86 and its relationship with the
popular desktop environments, such as GNOME and KDE.
Beginning GTK+ and GNOME by Peter Wright; Wrox Press, Inc. — Introduces programmers to
the GNOME architecture, showing them how to get started with GTK+.
GTK+/GNOME Application Development by Havoc Pennington; New Riders Publishing — An
advanced look into the heart of GTK+ programming, focusing on sample code and a thorough look
at the available APIs.
Chapter 6. The X Window System
KDE 2.0 Development by David Sweet and Matthias Ettrich; Sams Publishing — Instructs beginning and advanced developers in how to take advantage of the many environment guidelines
required to built QT applications for KDE.
Security Reference
Chapter 7.
Pluggable Authentication Modules (PAM)
Programs which give privileges to users must properly authenticate each user. For instance, when you
log into a system, you provide your username and password, and the log in process uses this username
and password to verify your identity.
Pluggable Authentication Modules (PAM) allows the system administrator to set authentication policies for PAM-aware applications without having to recompile authentication programs. PAM does this
by utilizing a pluggable, modular architecture. Which modules PAM calls for a particular application
is determined by looking at that application’s PAM configuration file in the /etc/pam.d/ directory.
In most situations, you will never need to alter the default PAM configuration files for a PAM-aware
application. Whenever you use RPM to install programs that require authentication, they automatically
make the changes necessary to do normal password authentication using PAM. However, if you need
to customize the PAM configuration file, you must understand the structure of this file (see Section
7.2 for more information).
7.1. Advantages of PAM
When used correctly, PAM provides the following advantages for a system administrator:
It provides a common authentication scheme that can be used with a wide variety of applications.
It allows great flexibility and control over authentication for both the system administrator and
application developer.
It allows application developers to develop their program without implementing a particular authentication scheme. Instead, they can focus purely on the details of their program.
7.2. PAM Configuration Files
The directory /etc/pam.d/ contains the PAM configuration files for PAM-aware applications. In
earlier versions of PAM, the file /etc/pam.conf was used, but this file is now deprecated. The
pam.conf file is only read if the /etc/pam.d/ directory does not exist.
Each PAM-aware application or service — as applications designed to be used by many users are
commonly known — has its own file within the /etc/pam.d/ directory.
These files have a specific layout containing calls to modules usually located in the /lib/security/
directory. Additionally, each line within a PAM configuration file specifies a module type, a control
flag, a path to the module, and, sometimes, module arguments.
7.2.1. PAM Service Names
Each PAM configuration file in the /etc/pam.d/ directory is named after the service for which it
controls access. It is up to the PAM-aware program to define its service name and install its PAM
configuration file in the pam.d directory. For example, the login program defines its service name
as /etc/pam.d/login.
In general, the service name is the name of the program used to access the service, not the program used to provide the service. This is why the service wu-ftpd, defines its service name as
Chapter 7. Pluggable Authentication Modules (PAM)
The next four sections will describe the basic format of PAM configuration files and how they use
PAM modules to perform authentication for PAM-aware applications.
7.3. PAM Modules
There are four types of PAM modules used to control access to services. These types correlate to
different aspects of the authorization process:
— These modules are used to authenticate the user by, for example, asking for and checking
a password. It can also set credentials, such as group membership or Kerberos tickets.
• auth
— These modules are used to make sure access is allowed. For example, it can check if
the account is expired, or it can check if the user is allowed to log in at a particular time of day.
• account
• password
— These modules are used to set passwords.
— These modules are used after a user has been authenticated to manage the user’s
session. This module type can also perform additional tasks which are needed to allow access, like
mounting a user’s home directory or making his mailbox available.
• session
An individual module can address more than one of the above module types. For instance has components which address all four module types.
In a PAM configuration file, the module type is the first aspect defined. For example a typical line in
a configuration may look like this:
This instructs PAM to look at the auth component of the module.
7.3.1. Stacking Modules
Modules can be stacked, or placed upon one another, so that multiple modules are used together for
a particular purpose. Therefore the order in which the modules are listed is very important to the
authentication process.
Stacking makes it very easy for an administrator to require several conditions to exist before allowing
user authentication. For example, rlogin normally uses five stacked auth modules, as seen in its
PAM configuration file:
/lib/security/ service=system-auth
Before someone is allowed to use rlogin, PAM verifies that the /etc/nologin file does not exist,
that they are not trying to log in remotely as a root user over an unencrypted network connection,
and that any environmental variables can be loaded. Then, a successful rhosts authentication is
performed before the connection is allowed. If rhosts authentication fails, then standard password
authentication is performed.
Chapter 7. Pluggable Authentication Modules (PAM)
7.3.2. Creating Modules
New PAM modules can be added at any time, and PAM-aware applications can then use them. For
example, if you create a one-time-password creation method and write a PAM module to support
it, PAM-aware programs can immediately use the new module and password method without being
recompiled or otherwise modified. This is very beneficial because it lets you mix-and-match, as well
as test, authentication methods for different programs without recompiling them.
Documentation on writing modules is included with the system in the /usr/share/doc/pamversion-number/ directory.
7.4. PAM Module Control Flags
All PAM modules generate a success or failure result when checked. Control flags tell PAM what do
with the result. Since modules can be stacked in a particular order, control flags give you the ability
to set the importance of a module in respect to the modules that follow it.
Again, consider the rlogin PAM configuration file:
/lib/security/ service=system-auth
The order in which required modules are called is not critical. The sufficient and requisite
control flags is what causes order to become important. See below for an explanation of each type
of control flag.
After the module type is specified, the control flags decide how important the success or failure of that
particular module should be in the overall goal of allowing access to the service.
Four types of control flags are defined by the PAM standard:
— the module must be successfully checked in order to allow authentication. If a required module check fails, the user is not notified until all other modules of the same module type
• required
have been checked.
— the module must be successfully checked in order for the authentication to be successful. However, if a requisite module check fails, the user is notified immediately with a
message reflecting the first failed required or requisite module.
• requisite
— the module checks are ignored if it fails. But, if a sufficient flagged module
is successfully checked and no required flagged modules above it have failed, then no other
modules of this module type are checked and the user is authenticated.
• sufficient
— the module checks are ignored if it fails. If the module check is successful, it does
not play a role in the overall success or failure for that module type. The only time a module flagged
as optional is necessary for successful authentication is when no other modules of that type have
succeeded or failed. In this case, an optional module determines the overall PAM authentication
for that module type.
• optional
Chapter 7. Pluggable Authentication Modules (PAM)
A newer control flag syntax allowing for even more control is now available for PAM. Please see the
PAM docs located in the /usr/share/doc/pam-version-number/ directory for information on
this new syntax.
7.5. PAM Module Paths
Module paths tell PAM where to find the pluggable module to be used with the module type specified.
Usually, it is provided as the full path to the module, such as /lib/security/
However, if the full path is not given (in other words, the path does not start with a /), then the
module indicated is assumed to be in the /lib/security/ directory — the default location for
PAM modules.
7.6. PAM Module Arguments
PAM uses arguments to pass information to a pluggable module during authentication for a particular
module type. These arguments allow the PAM configuration files for particular programs to use a
common PAM module but in different ways.
For example, the module uses secrets stored in a Berkeley DB file to authenticate the
user. Berkeley DB is an open source database system designed to be embedded in many applications
to track information. The module takes a db argument, specifying the Berkeley DB filename to use,
which can be different for different services.
So, the line in a PAM configuration file look like this:
/lib/security/ db=path/to/file
Invalid arguments are ignored and do not otherwise affect the success or failure of the PAM module. When an invalid argument is passed, an error is usually written to /var/log/messages file.
However, since the reporting method is controlled by the PAM module, the module must be written
correctly to log the error to this file.
7.7. Sample PAM Configuration Files
Below is a sample PAM application configuration file:
/lib/security/ shadow nullok
/lib/security/ retry=3
/lib/security/ shadow nullok use_authtok
The first line is a comment as denoted by the # character — the comment symbol in PAM configuration
files. Lines two through four stack three modules for login authentication.
This line makes sure that if the user is trying to log in as root, the tty on which they are logging in is
listed in the /etc/securetty file, if that file exists.
/lib/security/ nullok
Chapter 7. Pluggable Authentication Modules (PAM)
This line causes the user to be asked for a password and then checks the password using the information stored in /etc/passwd and, if it exists, /etc/shadow. The module automatically detects and utilizes shadow passwords, stored in /etc/shadow, to authenticate users. Please
refer to the Section 5.5 for more information on shadow passwords.
The argument nullok instructs the module to allow a blank password.
This is the final authentication step. It checks to see if the file /etc/nologin exists. If nologin does
exist and the user is not root, authentication fails.
In this example, all three auth modules are checked, even if the first auth module fails. This prevents
the user from knowing at what stage their authentication failed. Such knowledge in the hands of an
attacker could allow them to more easily deduce how to crack the system.
This line causes any necessary account verification to be done. For example, if shadow passwords have
been enabled, the account component of the module will check to see if the account
has expired or if the user has not changed their password within the grace period allowed.
/lib/security/ retry=3
If a password has expired, the password component of the module prompts for a
new password. It then tests the newly created password to see whether the it can easily be determined
by a dictionary-based password cracking program. If it fails this test the first time, it gives the user
two more chances to create a strong password, due to the retry=3 argument.
/lib/security/ shadow nullok use_authtok
This line specifies that if the program changes the user’s password, it should use the password component of the module to do so. This will happen only if the auth portion of the module has determined that the password needs to be changed — for example, if a
shadow password has expired.
The argument shadow tells the module to create shadow passwords when updating a user’s password.
The argument nullok instructs the module to allow the user to change their password from a blank
password, otherwise a null password is treated as an account lock.
The final argument on this line, use_authtok, provides a good example of how one can stack PAM
modules. This argument tells the module not to prompt the user for a new password. Instead it is
to accept any password that passes through previous password module. This way all new passwords
must pass the test for secure passwords before being accepted.
session required /lib/security/
The final line specifies that the session component of the module will manage the
session. This module logs the username and the service type to /var/log/messages at the the
beginning and end of each session. It can be supplemented by stacking it with other session modules
if you need more functionality.
The next sample configuration file will illustrate auth module stacking for the rlogin program — a
program that allows users to log in remotely.
Chapter 7. Pluggable Authentication Modules (PAM)
/lib/security/ service=system-auth
First, checks to see if /etc/nologin exists. If is does, no one can log in except
for root.
The module then prevents root logins from occurring on insecure terminals.
This effectively disallows all root rlogin attempts for security reasons.
If you need to log in as root, use OpenSSH instead. For more information on the SSH protocol, see
Chapter 9.
This line loads the module, which sets the environmental variables specified in
The modules then authenticates the user using .rhosts in the user’s
home directory. If this succeeds, PAM immediately authenticates the rlogin session. If fails to authenticate the user, this failed authentication attempt is ignored.
/lib/security/ service=system-auth
If the module fails to authenticate the user, the module performs normal password authentication.
The argument service=system-auth means the user must now pass through the PAM configuration
for system authorization found in /etc/pam.d/system-auth.
If you do not want to prompt for a password when the securetty check fails and determines that
the user is trying to login as root remotely, you can change the module from
required to requisite.
7.8. PAM and Device Ownership
Red Hat Linux allows the first privileged user to log in on the physical console of the machine the
ability to manipulate devices and perform tasks normally reserved for root. This is done through a
PAM module called
Chapter 7. Pluggable Authentication Modules (PAM)
7.8.1. Device Ownership
When a user logs into a machine under Red Hat Linux, the module is called by
login or the graphical login programs, gdm and kdm. If this user is the first user to log in at the
physical console — called the console user — the module grants ownership of a variety of devices
normally owned by root. The console user owns these devices until the last local session for that user
ends. Once the user has logged out, ownership of the devices reverts back to their default values.
The devices affected include, but are not limited to, sound cards, floppy drives, and CD-ROM drives.
This allows a local user to manipulate these devices without attaining root, thus simplifying common
tasks for the console user.
In the file /etc/security/console.perms, you can edit the list of devices controlled by
7.8.2. Application Access
The console user is also allowed access to any program with a file bearing the command name in the
/etc/security/console.apps/ directory. These files do not need to contain any data, but must
have the exact name of the command to which they correspond.
One notable group of applications the console user has access to are three programs which shut off or
reboot the system. These are:
• /sbin/halt
• /sbin/reboot
• /sbin/poweroff
Because these are PAM-aware applications, they call the as a requirement for use.
For more information see the man pages for pam_console, console.perms, and console.apps.
7.9. Additional Resources
Below is a list of information sources for using and configuring PAM on your system. In addition to
these sources, you should read the PAM configuration files on your system to better understand how
they are structured.
7.9.1. Installed Documentation
man page — Good introductory information on PAM, including the structure and purpose of
the PAM configuration files.
• pam
— Contains a System Administrators’ Guide, a Module Writers’ Manual, and an Application Developers’ Manual. Also contains a copy of the PAM
standard, DCE-RFC 86.0.
• /usr/share/doc/pam-version-number
7.9.2. Useful Websites
• — The primary distribution website for the Linux-PAM
project, containing information on various PAM modules, a FAQ, and additional PAM documentation.
Chapter 7. Pluggable Authentication Modules (PAM)
Chapter 8.
TCP Wrappers and xinetd
Controlling access to network services can be a challenge. Firewalls are useful for controlling access
in and out of a particular network, but they can be difficult to configure. TCP wrappers and xinetd
control access to services by hostname and IP addresses. In addition, these tools also include logging
and utilization management capabilities that are easy to configure.
8.1. What Are TCP Wrappers?
TCP wrappers is installed by default with a server-class installation of Red Hat Linux 8.0, and provides
access control to a variety of services. Most modern network services, such as SSH, Telnet, and FTP,
make use of TCP wrappers, a program that is designed to stand guard between an incoming request
and the requested service.
The idea behind TCP wrappers is that client requests to server applications are "wrapped" by an
authenticating service, allowing a greater degree of access control and logging for anyone attempting
to use the service.
The functionality behind TCP wrappers is provided by libwrap.a, a library that network services,
such as xinetd, sshd, and portmap, are compiled against. Additional network services, even networking programs you may write, can be compiled against libwrap.a to provide this functionality.
Red Hat Linux bundles the necessary TCP wrapper programs and library in the tcp_wrappersB version C RPM file.
8.1.1. TCP Wrapper Advantages
When a user attempts to gain client access to a network service that is using TCP wrappers, a small
wrapper program reports the name of the service requested and the client’s host information. The
wrapper program does not directly send any information back to the client, and after the access control
directives are satisfied, the wrapper is unloaded and frees up its resources. The client and the server
can then resume actions without further wrapper intervention.
TCP wrappers provide two basic advantages over other network service control techniques:
The connecting client is unaware that TCP wrappers are in use. — Legitimate users will not notice
anything different, and attackers never receive any additional information about why their attempted
connections have failed.
TCP wrappers operate separately from the applications the wrapper program protects. — This
allows many server applications to share a common set of configuration files for simpler management.
8.2. Host-Based Access Control Lists
Host-based access for services that use TCP wrappers is controlled by two files: /etc/hosts.allow
and /etc/hosts.deny. These file use a simple format to control access to services on a server.
If no rules are specified in either hosts.allow or hosts.deny, then the default rule is to allow
anyone to access to the services.
Order is important since rules in hosts.allow take precedence over rules specified in hosts.deny.
Even if a rule specifically denying all access to a particular service is defined in hosts.deny, hosts
Chapter 8. TCP Wrappers and xinetd
specifically given access to the service in hosts.allow are allowed to access the service. In addition,
all rules in each file take effect from the top down.
Any changes to these files take effect immediately, so restarting services is not required.
8.2.1. Formatting Rules
All access control rules are placed on lines within hosts.allow and hosts.deny, and any blank
lines or lines that start with the comment character (#) are ignored. Each rule needs to be on its own
The rules must be formatted in the following manner:
daemon_list :
client_list [: spawn
Each of these options refer to a different part of the rule:
• daemon_list
— A collection of one or more process names or special wildcards, separated by
— One or more hostnames, host addresses, patterns, or wildcards, separated by
whitespace, to use when a particular process name matches a requested service.
• client_list
• shell_command —
is utilized.
An optional component that specifies something to be done in the event a rule
Patterns are particularly helpful when specifying groups of clients that may or may not access a
certain service. By placing a "." character at the beginning of a string, all hosts that share the end of
that string are applied to that rule. So, would catch both and The "." character at the end of a string has the same effect, except going the
other direction. This is primarily used for IP addresses, as a rule pertaining to 192.168.0. would
apply to the entire class C block of IP addresses. Netmask expressions can also be used as a pattern to
control access to a particular group of IP addresses. You can even use asterisks (*) or question marks
(?) to select entire groups of hostnames or IP addresses, so long as you do not use them in the same
string as the other types of patterns.
If a list of hostnames with access a service is too long or is difficult to control within host.allow
or hosts.deny, you can also specify the full path to a file (such as /etc/telnet.hosts.deny).
This file contains hostnames, host addresses, or patterns, separated by whitespace, that you want to
allow or deny access to that service. This method also works well to share access control lists between
various services, as changes would only need to be made in one file per service.
The following wildcards may be used in the access control rules instead of using specific hosts or
groups of hosts:
— Matches every client with a service. To allow a client access to all services, use the ALL in
the daemons section.
— Matches any host that does not contain a "." character.
— Matches any host where the hostname and host address are known or where the user is
— Matches any host where the hostname or host address are unknown or where the user
is unknown.
— Matches any host where the hostname does not match the host address.
Chapter 8. TCP Wrappers and xinetd
The KNOWN, UNKNOWN, and PARANOID wildcards should be used very carefully, as a disruption in name
resolution may make prevent legitimate users from gaining access to a service.
The access control language also contains a powerful operator, EXCEPT, which allows separate lists to
be combined within the same rule line. When EXCEPT is used between two lists, the first list applies
unless an entry from the second list matches an entity covered by the first list. EXCEPT can be used
with daemon or client lists. Consider the following hosts.allow example:
# all hosts are allowed to connect
# to all services except
# 123.123.123.* addresses can use all services except FTP
ALL EXCEPT in.ftpd: 123.123.123.
Organizationally, it usually makes more sense to use EXCEPT operators sparingly, choosing instead
to place the exceptions to the rule in the other access control file. This allows all administrators to
quickly scan the appropriate files to see what hosts should be allowed or denied access to which
services, without having to sort through the various EXCEPT operators.
The best way to manage access control with hosts.allow and hosts.deny is to use the two files
together to achieve the desired results.
Users that wish to prevent any hosts other than specific ones from accessing services usually place
ALL: ALL in hosts.deny. Then, they place lines in hosts.allow, such as:
in.ftpd: 10.0.1. EXCEPT
Alternatively, if you wish to allow anyone to use network services except for specific hosts, leave
hosts.allow blank and add any necessary restrictions to hosts.deny such as:
Be very careful about using hostnames and domain names in both access control files, especially
hosts.deny. Various tricks could be used by an attacker to circumvent rules specifying a hostname or
domain name. In addition, if your system selectively allows access based on hostname and domain
name information, any disruption in DNS service would prevent even authorized users from using
network services.
Using IP addresses whenever possible can prevent many problems when constructing access control
rules, especially those that deny access.
Beyond simply allowing or denying access to services for certain hosts, the TCP wrappers also supports the use of shell commands. These shell commands are most commonly used with deny rules
to set up booby traps, which usually trigger actions that log information about failed attempts to a
Chapter 8. TCP Wrappers and xinetd
special file or email an administrator. Below is an example of a booby trap in the hosts.deny file
which will write a log line containing the date and client information every time a host from the the
IP range to attempts to connect via Telnet:
in.telnetd: 10.0.1.: spawn (/bin/echo ‘date‘ %c >> /var/log/telnet.log) &
Another feature of using shell commands is support for expansions. Expansions provide the command with information about the client, server, and process involved. Below is a list of supported
• %a
— The client’s IP address.
• %A
— The server’s IP address.
• %c — Supplies
and IP address.
a variety of client information, such as the username and hostname, or the username
• %d
— The daemon process name.
• %h
— The client’s hostname (or IP address, if the hostname is unavailable).
• %H
— The server’s hostname (or IP address, if the hostname is unavailable).
— The client’s hostname. If unavailable, unknown is printed. If the client’s hostname and host
address do not match, paranoid is printed.
• %n
— The server’s hostname. If unavailable, unknown is printed. If the server’s hostname and host
address do not match, paranoid is printed.
• %N
• %p
— The daemon process ID.
— Various types of server information, such as the daemon process and the host or IP address
of the server.
• %s
• %u
— The client’s username. If unavailable, unknown is printed.
For a full explanation of available shell commands, as well as some additional access control examples, review see the man page for hosts_access.
Special attention must be given to portmap when host access control lists. Only IP addresses or
the ALL option should be used when specifying hosts to allow or deny, as host names are not supported. In addition, changes to the host access control lists that concern portmap may not take affect
As widely used services, such as NIS and NFS, depend on portmap to operate, be aware of these
limitations before depending on hosts.allow and hosts.deny to control access.
8.3. Access Control Using xinetd
The benefits offered by TCP wrappers are enhanced when the libwrap.a library is used in conjunction with xinetd, a super-daemon that provides additional access, logging, binding, redirection and
resource utilization control.
Red Hat Linux configures a variety of popular network services to be used with xinetd, including
FTP, IMAP, POP, and Telnet. When any of these services are accessed via their port numbers in
/etc/services, the xinetd daemon handles the request. Before bringing up the requested network
service, xinetd ensures that the client host information meets the access control rules, the number of
Chapter 8. TCP Wrappers and xinetd
instances of this service is under a particular threshold, and any other rules specified for that service
or all xinetd services are followed. Once the target service is brought up for the connecting client,
xinetd goes back to sleep, waiting for additional requests for the services it manages.
8.3.1. xinetd Configuration Files
The xinetd service is controlled by the /etc/xinetd.conf file, as well as the various servicespecific files in the /etc/xinetd.d/ directory. /etc/xinetd.conf
The xinetd.conf file is the parent of all xinetd-controlled service configuration files, as the
service-specific files are also parsed every time xinetd starts. By default, xinetd.conf contains
some basic configuration settings that apply to every service. Below is an example of a typical
SYSLOG authpriv
25 30
includedir /etc/xinetd.d
These lines control various aspects of xinetd:
• instances
— Sets the maximum number of requests a particular service can handle at once.
— Tells xinetd to use the authpriv log, specified in /etc/syslog.conf
and set to /var/log/secure by default, rather than using another specific file. Using
FILE /var/log/xinetdlog here instead would move xinetd logging to a separate
/var/log/xinetdlog file.
• log_type
— Lets xinetd know what to log if the connection is successful. By default,
the remote host’s IP address and the process ID of server processing the request are recorded.
• log_on_success
• log_on_failure —
Tells xinetd what to log if the connection fails or is not allowed.
— Tells xinetd to allow no more than 25 connections per second to a given service. If this
limit is reached, the service is retired for 30 seconds.
• cps
Both the log_on_success and log_on_failure settings in /etc/xinetd.conf are often modified
by each service, meaning that successful and failed connections will usually log more information
than is indicated in /etc/xinetd.conf.
Various logging options are available in /etc/xinetd.conf and the service-specific xinetd configuration files:
— Logs the fact that a failed attempt was made. (log_on_failure)
— Logs the length of time the service is used by a remote system. (log_on_success)
Chapter 8. TCP Wrappers and xinetd
— Logs the exit status or termination signal of the service. (log_on_success)
— Logs the remote host’s IP address. (log_on_failure and log_on_success)
— Logs the process ID of the server receiving the request. (log_on_success)
— Records information about the remote system in the case the service cannot be started.
Only particular services, such as login and finger, may use this option. (log_on_failure)
— Logs the remote user using the method defined in RFC 1413 for all multi-threaded
stream services. (log_on_failure and log_on_success)
Other options for /etc/xinetd.conf are available, such as per_source, which limits the maximum number of connections from a particular IP address to a specific service. See the man page for
xinetd for more information. Files in the /etc/xinetd.d/ Directory
The files in the /etc/xinetd.d/ directory are read every time xinetd starts, due to the includedir /etc/xinetd.d/ statement at the bottom of /etc/xinetd.conf. These files, with
names such as finger, ipop3, and rlogin, correlate to the services controlled by xinetd.
The files in /etc/xinetd.d/ use the same conventions as /etc/xinetd.conf. The primary reason
they are stored in separate configuration files is to make it easier to add and remove a service from
xinetd without affecting other services.
To get an idea of how these files are structured, consider the wu-ftp file:
service ftp
= stream
= no
= root
= /usr/sbin/in.ftpd
= -l -a
= 10
= yes
The first line defines the service’s name. The lines within the brackets contain settings that define how
this service is supposed to be started and used. The wu-ftp file states that the FTP service uses a
stream socket type (rather than dgram), the binary executable file to use, the arguments to pass to
the binary, the information to log in addition to the /etc/xinetd.conf settings, the priority with
which to run the service, and more.
The use of xinetd with a service also can serve as a basic level of protection from a Denial of
Service (DoS) attack. The max_load option takes a floating point value to set a CPU usage threshold
when no more connections for a particular service will be accepted, preventing certain services from
overwhelming the system. The cps option accepts an integer value to set a rate limit on the number
of connections available per second. Configuring this value to something low, such as 3, will help
prevent attackers from being able to flood your system with too many simultaneous requests for a
particular service.
Chapter 8. TCP Wrappers and xinetd
125 Access Control within xinetd
Users of xinetd services can choose to use the TCP wrapper host access control files
(/etc/hosts.allow and /etc/hosts.deny), provide access control via the xinetd
configuration files, or a mixture of both. Information concerning the use of TCP wrapper host access
control files can be found in Section 8.2. This section will discuss using xinetd to control access to
Unlike TCP wrapper host access control files, any changes to xinetd configuration files require a
restart of the xinetd service to go into affect.
The xinetd host access control available through its various configuration files is different from the
method used by TCP wrappers. While TCP wrappers places all of the access configuration within
two files, /etc/hosts.allow and /etc/hosts.deny, each service’s file in /etc/xinetd.d can
contain access control rules based on the hosts that will be allowed to use that service.
The following options are supported in the xinetd files to control host access:
• only_from
— Allows the hosts specified to use the service.
• no_access
— Blocks these hosts from using this service.
• access_times —
Specifies the time range when a particular service may be used. The time range
must be stated in a HH:MM-HH:MM format using 24-hour notation.
The only_from and no_access options can use a list of IP addresses or host names, or can specify
an entire network. Like TCP wrappers, combining xinetd access control with the proper logging
configuration for that service, you can not only block the request but also record every attempt to
access it.
For example, the following /etc/xinetd.d/telnet file can be used to block telnet access to a
system by a particular network group and restrict the overall time range that even legitimate users can
log in:
service telnet
= no
= stream
= no
= root
= /usr/sbin/in.telnetd
= 09:45-16:15
In this example, when any system from the subnet, such as, tries to telnet
into the server, they will receive a message stating Connection closed by foreign host. In
addition, their login attempt is logged in /var/log/secure:
START: telnet pid=16191 from=
START: telnet pid=16256 from=
FAIL: telnet address from=
EXIT: telnet status=0 pid=16256
Chapter 8. TCP Wrappers and xinetd Binding and Port Redirection
The service configuration files for xinetd also support binding the service to an IP address and
redirecting incoming requests for that service to another IP address, hostname, or port.
Binding is controlled with the bind option in the service configuration files and links the service to
one IP address on the system. When used, the bind option only allows requests for the proper IP
address to access the service. Each service can be bound to different network interfaces based on your
needs. This is particularly useful for systems with multiple network adapters or using multiple IP
addresses. For instance, you can configure telnet to listen only on the interface connected to a private
network and not to the interface connected with the Internet.
The redirect option accepts an IP address or hostname followed by a port number. It tells the service
to redirect any requests for this service to the specified host and port number. This feature can be used
to point to another port number on the same system, redirect the request to different IP address on the
same machine, shift the request to a totally different system and port number, or any combination of
these options. In this way, a user connecting to certain service on a system may be rerouted to another
system with no disruption.
The xinetd daemon is able to accomplish this redirection by spawning a process that stays alive for
the duration of the connection between the requesting client machine and the host actually providing
the service, transferring data between the two systems.
The real strength of the bind and redirect options can be seen when they are used together. By
binding a service to a particular IP address on a system and then redirecting requests for this service
to a second machine that only the first machine can see, you can use an internal system to provide
services for a totally different network. Alternatively, these options can be used to limit the exposure of
a particular service on a multi-homed machine to a known IP address, as well as redirect any requests
for that service to another machine specially configured for that purpose.
For example, consider a system that is used as a firewall with this setting for its telnet service:
service telnet
socket_type = stream
= no
= /usr/sbin/in.telnetd
log_on_success += DURATION USERID
log_on_failure += USERID
= 21 23
The bind and redirect options in this file will ensure that the telnet service on the machine is bound
to the external IP address (, the one facing the Internet. In addition, any requests for
telnet service sent to will be redirected via a second network adapter to an internal IP
address ( that only the firewall and internal systems can access. The firewall will then send
the communication between the two systems, and the connecting system will think it is connected to when it is actually connected to a different machine.
This feature is particularly useful for users with broadband connections and only one fixed IP address.
When using Network Address Translation (NAT), the systems behind the gateway machine, which
are using internal-only IP addresses, are not available from outside the gateway system. However,
when certain services controlled by xinetd are configured with the bind and redirect options, the
gateway machine can act as a type of proxy between outside systems and a particular internal machine
configured to provide the service. In addition, the various xinetd access control and logging options
are also available for additional protection, such as limiting the number of simultaneous connections
for the redirected service.
Chapter 8. TCP Wrappers and xinetd
8.4. Additional Resources
Additional information concerning TCP wrappers and xinetd is available on system documentation
and on the Web.
8.4.1. Installed Documentation
The bundled documentation on your system is a good place to start looking for additional TCP Wrappers, xinetd, and access control configuration options.
G — Contains a README file that discusses how
TCP wrappers work and the various hostname and host address spoofing risks that exist.
• /usr/share/doc/tcp_wrappers- version
G — Includes a README file that discusses aspects of access control and a sample.conf file with various ideas for modifying /etc/xinetd.d service
• /usr/share/doc/xinetd- version
For detailed information concerning the creation of TCP wrapper access control rules, read the
hosts_access(5)and hosts_options(5) man pages.
The xinetd(8) and xinetd.conf(5) man pages contain additional information for creating xinetd
configuration files and a description of how xinetd works.
8.4.2. Useful Websites
• — The home of xinetd, containing sample configuration files, a full listing
of features, and an informative FAQ.
• — A thorough tutorial that discusses
many different ways to tweak default xinetd configuration files to meet specific security goals.
Chapter 8. TCP Wrappers and xinetd
Chapter 9.
SSH Protocol
SSH™ allows users to log into host systems remotely. Unlike rlogin or telnet SSH encrypts the
login session, making it impossible for intruders to collect clear-text passwords.
SSH is designed to replace older, less secure terminal applications used to log into remote systems,
such as telnet or rsh. A related program called scp replaces older programs designed to copy files
between hosts, such as ftp or rcp. Because these older applications do not encrypt passwords between
the client and the server, you should avoid them whenever possible. Using secure methods to remotely
log into other systems will decrease the security risks for both your system and the system you are
logging into.
9.1. Features of SSH
SSH (or Secure SHell) is a protocol for creating a secure connection between two systems using a
client server architecture. Under the SSH protocol, the client machine initiates all connections to a
The SSH protocol provides the following safeguards:
After an initial connection, the client verifies it is connecting to the same server during subsequent
The client transmits its authentication information to the server, such as a username and password,
in an encrypted format.
All data sent and received during the connection is transferred using strong, 128 bit encryption,
making intercepted transmissions extremely difficult to decrypt and read.
The client has the ability to use X11 1 applications launched from the shell prompt. 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
secure insecure protocols, like POP, increasing overall system and data security.
Red Hat Linux 8.0 includes the general OpenSSH package (openssh), the OpenSSH server
(openssh-server) and client (openssh-clients) packages. Please see the chapter titled
OpenSSH in the Official Red Hat Linux Customization Guide for instructions on installing and
deploying OpenSSH. Also note that the OpenSSH packages require the OpenSSL package
(openssl). OpenSSL installs several important cryptographic libraries that help OpenSSH provide
encrypted communications.
A large number of client and server programs can use the SSH protocol. Several different SSH client
versions are available for almost every major operating system in use today. Even if the users connecting to your system are not running Red Hat Linux, they can still find and use an SSH client native
for their operating system.
X11 refers to the X11R6 windowing display system, traditionally referred to as X. Red Hat Linux includes
XFree86, a widely used, open source X Window System, which is based on X11R6.
Chapter 9. SSH Protocol
9.1.1. Why Use SSH?
Nefarious computer users have a variety of tools at their disposal to disrupt, intercept, and re-route
network traffic in an effort to gain access to your 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 entities, 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.
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 will remain
unaware it is communicating with the wrong host.
This can be attack can be mounted through the use of a packet sniffer — a common network utility.
This can be attack can be mounted through techniques known as DNS poisoning 2 or IP spoofing 3.
Both techniques intercept potentially sensitive information, and if the interception is 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 will not work, since each packet is encrypted using a key known
only by the local and remote systems.
9.2. Event Sequence of an SSH Connection
The following series of events help protect the integrity of SSH communication between two hosts.
First, a secure transport layer is created so that the client knows it is communicating with the correct
server. Then, the communication is encrypted between the client and server using a symmetric cipher.
With a encrypted connection to the server in place, the client safely authenticates itself to the server
without sending information in plain text.
Finally, with the client authenticated to the server, several different services can be safely and securely
used through the connection, such as an interactive shell session, X11 applications, and tunneled
TCP/IP ports.
9.3. Layers of SSH Security
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 currently exist. SSH version 1 contains several patented encryption algorithms
(however, several of these patents have expired) and a security hole that potentially allows for data to
be inserted into the data stream. The OpenSSH suite under Red Hat Linux 8.0 uses SSH version 2.0 by
default, although it also supports version 1. It is recommended that you use SSH version 2-compatible
servers and clients whenever possible.
DNS poisoning occurs when an intruder cracks a DNS server, pointing client systems to a maliciously du-
plicated host.
3. IP spoofing occurs when an intruder sends network packets which falsely appear to be from a trusted host on
the network.
Chapter 9. SSH Protocol
Both SSH protocol versions 1 and 2 add layers of security with each of these layers providing its own
type of protection.
9.3.1. Transport Layer
The primary role of the transport layer is to facilitate safe and secure communication between the
two hosts at the time of and after authentication. Usually running over TCP/IP, the transport layer
accomplishes this by handling the encryption and decryption of data and providing integrity protection
of data packets as they are sent and received. In addition, the transport layer 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:
Key exchange
The public key algorithm to be used
The symmetric encryption algorithm to be used
The message authentication algorithm to be used
The hash algorithm to be used
During the key exchange, the server identifies itself to the client with a host key. If the client has never
communicated with this particular server before, the server’s key will be unknown to the client and it
will not connect. OpenSSH gets around this problem by accepting the server’s host key after the user
is notified and verifies that he will accept 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.
It is possible for an attacker to masquerade as the 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 you should verify the integrity of a new SSH server by contacting the
server administrator before connecting for the first time or after a host key has changed.
SSH is designed to work with almost any kind of public key algorithm or encoding format. After
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, which generates another set of
hash values and a new shared secret value. Even if an attack is able to determine the hash and shared
secret value, he would have to determine this information each time a new key exchange is made in
order to monitor the communication.
9.3.2. 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 keyencoded signature or typing a password. The client will then try to authenticate itself to the server
using any of the supported methods.
Chapter 9. SSH Protocol
Servers 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 will support based on its
security model, and the client can choose the order of authentication methods to attempt from among
the available options. Thanks to the secure nature of the SSH transport layer, even seemingly insecure
authentication methods, such as a host-based authentication, are safe to use.
Most users requiring a secure shell will authenticate using a password. Since the password is encrypted
when moving over the the transport layer, it can be safely sent across any network.
9.3.3. Connection
After a successful authentication over the SSH transport layer, multiple channels are opened by multiplexing4 the single connection between the two systems. Each of these channels handles communication for different terminal sessions, forwarded X11 sessions, or other services seeking to use the SSH
Both clients and servers can create a new channel. Each channel is then assigned a different number
for 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 will 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
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.
9.4. 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).
System-wide SSH configuration information is stored in the /etc/ssh/ directory:
— 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.
• moduli
— The system-wide default SSH client configuration file. It is overridden if one is
also present in the user’s home directory (~/.ssh/config).
• ssh_config
• sshd_config —
The configuration file for the sshd daemon.
• ssh_host_dsa_key —
The DSA private key used by the sshd daemon.
• —
• ssh_host_key
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-
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.
Chapter 9. SSH Protocol
— The RSA public key used by the sshd daemon for version 1 of the SSH
• ssh_host_rsa_key
— The RSA private key used by the sshd daemon for version 2 of the SSH
— The RSA public key used by the sshd for version 2 of the SSH
User-specific SSH configuration information is stored in the user’s home directory within the
~/.ssh/ directory:
— 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. This is an optional authentication method.
• authorized_keys
• id_dsa
— Contains the DSA authentication identity of the user.
• id_rsa
— The DSA public key of the user.
— The RSA public key used by sshd for version 2 of the SSH protocol.
• identity
— The RSA private key used by sshd for version 1 of the SSH protocol.
• known_hosts — 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. If a host’s key
has changed, and you are not absolutely certain why, you should contact the system administrator
of the SSH server to make sure that the server has not been compromised. If a server’s host keys
are legitimately altered by a re-installation of Red Hat Linux the next time you log into that server
you will be notified that the host key stored in the known_hosts file does not match. To connect
to the server, the user must open the known_hosts file in a text editor and delete the key for that
host. This allows the SSH client to create a new host key.
See the man pages for ssh and sshd for information concerning the various directives available in
the SSH configuration files.
9.5. 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
9.5.1. X11 Forwarding
Opening an X11 session over an established SSH connection is as easy as running an X program on
the local machine. 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 your client machine
X11 forwarding can be very useful. For example, you can use X11 forwarding to create a secure,
interactive session with up2date on the server to update packages. To do this, connect to the server
using ssh and type:
up2date &
You will be asked to supply the root password for the server. Then, Red Hat Update Agent will
appear and you can update your packages on the server as though you were sitting in front of the
Chapter 9. SSH Protocol
9.5.2. Port Forwarding
With SSH you 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 allows
you to map any port from the server to any port on the client; the port numbers do not need to match
for it 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:remote-port username@hostname
Setting up port forwarding to listen on ports below 1024 requires root access.
So if you want to check your email on a server called using POP through an encrypted connection, you can use the following command:
ssh -L
Once the port forwarding channel is in place between your machine and the mail server, you can direct
a POP mail client to use port 1100 on localhost to check for new mail. Any requests sent to port 1100
on your system will be directed securely to the server.
If is not running an SSH server daemon, but you can log in via SSH to a machine on
the same network, you can still use SSH to secure the part of the POP connection. However, a slightly
different command is necessary:
ssh -L
In this example, POP requests from port 1100 on your machine are forwarded through the SSH connection on port 22 to the ssh server, Then, connects to port
110 on to allow you to check for new mail. Note that by using this technique,
only the connection between your system and 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 block 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.
Using port forwarding to forward connections in this manner allows any user on the client system
to connect to the service to which you are forwarding connections. If the client system becomes
compromised, the attacker will also have 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.
Chapter 9. SSH Protocol
9.6. Require SSH for Remote Connections
For SSH to be truly effective in protecting your network connections, you must stop using all insecure
connection protocols, such as telnet and rsh. Otherwise, a user’s password may be protected using
ssh for one log in only to be captured when he logs in again using telnet.
To disable insecure connection methods to your system, use the command line program chkconfig, the ncurses-based program ntsysv, or the graphical application Services Configuration Tool
(redhat-config-services). All of these tools require root access.
Some services to disable include:
• telnet
• rsh
• ftp
• rlogin
• wu-ftpd
• vsftpd
For more information on runlevels and configuring services with chkconfig, ntsysv, and Services
Configuration Tool, refer to the chapter titled Controlling Access to Services in the Official Red Hat
Linux Customization Guide.
Chapter 9. SSH Protocol
Chapter 10.
Kerberos is a network authentication protocol created by MIT which uses secret-key cryptography —
obviating the need to send passwords over the network. By authenticating using Kerberos, unauthorized users trying to intercept passwords on the network are effectively thwarted.
10.1. Advantages of Kerberos
Most conventional network systems use password-based authentication schemes. When a user authenticates to a network server, he must supply a user name and password for each service requiring
authentication. This information is sent over the network, and the server verifies their identity using
this information.
However, the transmission of authentication information for many services is in plain text. An attacker
with access to the network and a packet analyzer, also known as a packet sniffer, can intercept any
passwords sent in this manner.
The primary design goal of Kerberos is to eliminate the transmission across the network of authentication information. The proper use of Kerberos effectively eliminates the threat packet sniffers would
otherwise pose on a network.
10.2. Kerberos Disadvantages
Although Kerberos removes a common and severe security threat, it may be difficult to implement for
various reasons:
Migrating user passwords from a standard UNIX password database, such as /etc/passwd or
/etc/shadow, to a Kerberos password database can be tedious as there is no automated mechanism
to perform this task. For more information, refer to question number 2.23 in the Kerberos FAQ at
the following URL:
Kerberos has only partial compatibility with the Pluggable Authentication Modules (PAM) system
used by most servers running Red Hat Linux. For more information on this issue, see Section 10.5.
For an application to use Kerberos, its source must be modified to make the appropriate calls into
the Kerberos libraries. For some applications, this can be quite problematic due to size or frequency
that krb libraries must be called. For other applications, changes must be made to the way in which
the server and client side communicate. Again, this may require extensive programming. Closedsource applications that do not have Kerberos support by default are often the most problematic.
Kerberos assumes that you are using trusted hosts on an untrusted network. Its primary goal is to
prevent plain text passwords from being sent across that network. However, if anyone other than
the proper user has physical access to any of the hosts, especially the one that issues tickets used
for authentication, the entire Kerberos authentication system is at risk of being compromised.
Kerberos is an all or nothing solution. If you decide to use Kerberos on your network, you must
remember any passwords transferred to a service which does not use Kerberos for authentication
run the risk of being captured by packet sniffers. Thus, your network gains no benefit from the
use of Kerberos. To secure your network with Kerberos, you must either kerberize all applications
which send plain text passwords or do not use those applications on your network at all.
Chapter 10. Kerberos
10.3. Kerberos Terminology
Like any other system, Kerberos has its own terminology to define various aspects of the service.
Before learning how the service works, it is important to learn the following terms.
Encrypted data.
plain text
Unencrypted, human-readable data.
An entity on the network (a user, a host, or an application) that can get a ticket from Kerberos.
credential cache or ticket file
A file which contains the keys for encrypting communications between a user and various network services. Kerberos 5 supports a framework for using other cache types, such as shared
memory, but files are more thoroughly supported.
crypt hash
A one way hash used to authenticate users. While more secure than plain text, it is fairly easy to
decrypt for an experienced cracker.
Data used when encrypting or decrypting other data. Encrypted data cannot be decrypted without
the proper key or extremely good guessing.
Key Distribution Center (KDC)
A service that issues Kerberos tickets, usually run on the same host as the Ticket Granting Server
key table or keytab
A file that includes an unencrypted list of principals and their keys. Servers retrieve the keys they
need from keytab files instead of using kinit. The default keytab file is /etc/krb5.keytab.
The /usr/kerberos/sbin/kadmind command is the only service that uses any other file (it
uses /var/kerberos/krb5kdc/kadm5.keytab).
A user or service that can authenticate using Kerberos. A principal’s name is in the form
root[/instance]@REALM. For a typical user, the root is the same as their login ID. The
instance is optional. If the principal has an instance, it is separated from the root with a
forward slash ("/"). An empty string ("") is considered a valid instance (which differs from the
default NULL instance), but using it can be confusing. All principals in a realm have their own
key, which is derived from their password or randomly set for services.
A network that uses Kerberos, composed of one or more servers called KDCs and a potentially
large number of clients.
A program accessed over the network.
Chapter 10. Kerberos
A temporary set of electronic credentials that verify the identity of a client for a particular service.
Ticket Granting Service (TGS)
A server that issues tickets for a desired service which are in turn given to users for access to the
service. The TGS usually runs on the same host as the KDC
Ticket Granting Ticket (TGT)
A special ticket that allows the client to obtain additional tickets without applying for them from
the KDC.
10.4. How Kerberos Works
Now that the Kerberos terminology has been defined, the following is an overview of how the Kerberos
authentication system works.
Rather than authentication occurring between each client machine and each server, Kerberos uses
symmetric encryption and a trusted third party — known as the Key Distribution Center or KDC —
to authenticate users on a network to a suite of services on a network. Once authenticated, Kerberos
stores a ticket specific to that session on the user’s machine and any kerberized service will look for
this ticket rather than asking the user to authenticate using a password.
When a user on a kerberized network logs in to their workstation, their principal is sent to the Key
Distribution Center as a request for a Ticket Granting Ticket (TGT). This request can be sent by the
login program so that it is transparent to the user or can be sent by the kinit program after the user
logs in.
The KDC checks for the principal in its database. If the principal is found, the KDC creates a TGT,
encrypts it using the user’s key and sends it back to the user.
The login program on the client machine or kinit decrypts the TGT using the user’s key (which it
computes from the user’s password). The TGT is set to expire after a certain period of time and stored
in the client machine’s credentials cache. The expiration time is set so a compromised TGT can only
be used for a certain period of time (usually eight hours). This is safer than tradition password model
because the password is never passed over the network. Once the TGT is issued, the user will not have
to re-enter their password to the KDC until the TGT expires or they logout and login again.
When the user needs access to a network service, the client uses the TGT to request a ticket for the
service from the Ticket Granting Service (TGS), which runs on the KDC. The TGS issues a ticket for
the desired service, which is then used to authenticate the user.
The Kerberos system can be compromised anytime any user on the network authenticates against a
non-kerberized service by sending a password in plain text. Therefore use of non-kerberized services
should be discouraged. Such services include telnet and ftp. Use of other secure protocols, such as
SSH or SSL secured services, however, is acceptable.
This, of course, is a broad overview of how Kerberos authentication on a network would typically
work. For a more in-depth look at Kerberos authentication, refer to Section 10.8.
Chapter 10. Kerberos
Kerberos depends on certain network services to work correctly. First, Kerberos requires approximate
clock synchronization between the machines on the network. Therefore, a clock synchronization
program should be set up for the network, such as ntpd.
Also, since certain aspects of Kerberos rely on the Domain Name Service (DNS), be sure that the
DNS entries and hosts on the network are all properly configured. See the Kerberos V5 System
Administrator’s Guide, provided in PostScript and HTML formats in /usr/share/doc/krb5-serverversion-number , (where version-number is the version installed on the system) for more information.
10.5. Kerberos and PAM
Currently, kerberized services do not make use of Pluggable Authentication Modules (PAM) — kerberized servers bypass PAM completely. However, applications that use PAM can make use of Kerberos for authentication if the pam_krb5 module (provided in the pam_krb5 package) is installed.
The pam_krb5 package contains sample configuration files that allow services like login and gdm
to authenticate users and obtain initial credentials using their passwords. If access to network servers
is always done using kerberized services or services that use GSS-API, like IMAP, the network can
be considered reasonably safe.
Careful administrators will not add Kerberos password checking to all network services because most
of the protocols used by these services do not encrypt the password before sending it over the network.
The next section will describe how to set up a basic Kerberos server.
10.6. Configuring a Kerberos 5 Server
When you are setting up Kerberos, install the server first. If you need to set up slave servers, the details
of setting up relationships between master and slave servers are covered in the Kerberos 5 Installation
Guide located in the /usr/share/doc/krb5-server- H version-number I directory.
To configure a basic Kerberos server, follow these steps:
1. Be sure that you have clock synchronization and DNS working on your server before configuring Kerberos 5. Pay particular attention to time synchronization between the Kerberos server
and its various clients. If the server and client clocks are different by more than five minutes (this
default amount is configurable in Kerberos 5), Kerberos clients will not be able to authenticate
to the server. This clock synchronization is necessary to prevent an attacker from using an old
Kerberos ticket to masquerade as a valid user.
You should set up a Network Time Protocol (NTP) compatible client/server network under, even
if you are not using Kerberos. Red Hat Linux 8.0 includes the ntp package for easy installation.
See for additional information on NTP.
2. Install the krb5-libs, krb5-server, and krb5-workstation packages on the dedicated
machine which will run your KDC. This machine needs to be very secure — if possible, it
should not run any services other than the KDC.
If you would like to use a Graphical User Interface utility to administrate Kerberos, you should
also install the gnome-kerberos package. It contains krb5, a GUI tool for managing tickets.
3. Edit the /etc/krb5.conf and /var/kerberos/krb5kdc/kdc.conf configuration files to
reflect your realm name and domain-to-realm mappings. A simple realm can be constructed by
replacing instances of EXAMPLE.COM and with your domain name — being
certain to keep uppercase and lowercase names in the correct format — and by changing the
Chapter 10. Kerberos
KDC from to the name of your Kerberos server. By convention,
all realm names are uppercase and all DNS hostnames and domain names are lowercase. For
full details on the formats of these files, see their respective man pages.
4. Create the database using the kdb5_util utility from a shell prompt:
/usr/kerberos/sbin/kdb5_util create -s
The create command creates the database that will be used to store keys for your Kerberos
realm. The -s switch forces creation of a stash file in which the master server key is stored. If
no stash file is present from which to read the key, the Kerberos server (krb5kdc) will prompt
the user for the master server password (which can be used to regenerate the key) every time it
5. Edit the /var/kerberos/krb5kdc/kadm5.acl file. This file is used by kadmind to determine which principals have access to the Kerberos database and their level of access. Most
organizations will be able to get by with a single line:
Most users will be represented in the database by a single principal (with a NULL, or empty,
instance, such as joe@EXAMPLE.COM). With this configuration, users with a second principal
with an instance of admin (for example, joe/admin@EXAMPLE.COM) will be able to wield full
power over the realm’s Kerberos database.
Once kadmind is started on the server, any user will be able to access its services by running kadmin on any of the clients or servers in the realm. However, only users listed in the
kadm5.acl file will be able to modify the database in any way, except for changing their own
The kadmin utility communicates with the kadmind server over the network, and they use
Kerberos to handle authentication. Of course, you need to create the first principal before you
can connect to the server over the network to administer it. Create the first principal with the
kadmin.local command, which is specifically designed to be used on the same host as the
KDC and does not use Kerberos for authentication.
Type the following kadmin.local command at the KDC terminal to create the first principal:
/usr/kerberos/sbin/kadmin.local -q "addprinc username/admin"
6. Start Kerberos using the following commands:
/sbin/service krb5kdc start
/sbin/service kadmin start
/sbin/service krb524 start
7. Add principals for your users using the addprinc command with kadmin. kadmin and kadmin.local on the master KDC are a command line interfaces to the Kerberos administration
system. As such, many commands are available after launching the kadmin program. Please
see the kadmin man page for more information.
8. Verify that your server will issue tickets. First, run kinit to obtain a ticket and store it in
a credential cache file. Then use klist to view the list of credentials in your cache and use
kdestroy to destroy the cache and the credentials it contains.
By default, kinit attempts to authenticate using the login user name of the account you used
when you first logged into your system (not the Kerberos server). If that system user name does
not correspond to a principal in the Kerberos database, you will receive an error message. If
that happens, supply kinit with the name of your principal as an argument on the command
line (kinit principal).
Chapter 10. Kerberos
Once you have completed the steps listed above, the Kerberos server should be up and running. Next,
we will set up a Kerberos client.
10.7. Configuring a Kerberos 5 Client
Setting up a Kerberos 5 client is less involved than setting up a server. At minimum, install the client
packages and provide each client with a valid krb5.conf configuration file. Kerberized versions of
rsh and rlogin will also require some configuration changes.
1. Be sure that you have time synchronization in place between the Kerberos client and KDC.
See Section 10.6 for more information. In addition, verify that DNS is working properly on the
Kerberos client before configuring the Kerberos client programs.
2. Install the krb5-libs and krb5-workstation packages on all of the client machines. You
must supply a version of /etc/krb5.conf for each client; usually this can be the same
krb5.conf used by the KDC.
3. Before a workstation in the realm can allow users to connect using kerberized rsh and rlogin,
that workstation will need to have the xinetd package installed and have its own host principal
in the Kerberos database. The kshd and klogind server programs will also need access to the
keys for their service’s principal.
Using kadmin, add a host principal for the workstation on the KDC. The instance in this case
will be the hostname of the workstation. You can use the -randkey option to kadmin’s addprinc command on the KDC to create the principal and assign it a random key:
addprinc -randkey host/
Now that you have created the principal, you can extract the keys for the workstation by running
kadmin on the workstation itself , and using the ktadd command within kadmin:
ktadd -k /etc/krb5.keytab host/
In order to use the kerberized versions of rsh and rlogin, you must enable klogin, eklogin,
and kshell. 1
4. Other kerberized network services will need to be started. To use kerberized telnet, you must
enable krb5-telnet.
To provide FTP access, create and extract a key for a principal with a root of ftp, with the
instance set to the hostname of the FTP server. Then enable gssftp.
The IMAP server included in the imap package will use GSS-API authentication using Kerberos
5 if it finds the proper key in /etc/krb5.keytab. The root for the principal should be imap.
The CVS gserver uses a principal with a root of cvs and is otherwise identical to a pserver.
10.8. Additional Resources
For more information on Kerberos, refer to the following resources.
Refer to the chapter titled Controlling Access to Services in the Official Red Hat Linux Customization Guide
for details on enabling services.
Chapter 10. Kerberos
10.8.1. Installed Documentation
K — The Kerberos V5 Installation Guide
and the Kerberos V5 System Administrator’s Guide in PostScript and HTML formats. You must
have the krb5-server RPM package installed.
• /usr/share/doc/krb5-server- version-number
K — The Kerberos V5 UNIX
User’s Guide in PostScript and HTML formats. You must have the krb5-workstation RPM
package installed.
• /usr/share/doc/krb5-workstation- version-number
10.8.2. Useful Websites
• — Kerberos: The Network Authentication Protocol webpage
from MIT.
• — The Kerberos Frequently Asked
Questions (FAQ).
• — The PostScript version of Kerberos: An
Authentication Service for Open Network Systems by Jennifer G. Steiner, Clifford Neuman, and
Jeffrey I. Schiller. This document is the original paper describing Kerberos.
• — Designing an Authentication System: a Dialogue in Four Scenes originally by Bill Bryant in 1988, modified by Theodore Ts’o in 1997. This
document is a conversation between two developers who are thinking through the creation of a
Kerberos-style authentication system. The conversational style of the discussion make this a good
starting place for people who are completely unfamiliar with Kerberos.
• — How to Kerberize your Site.
• — Kerberos Network Design Manual is
a thorough overview of the Kerberos system.
Chapter 10. Kerberos
Chapter 11.
Tripwire data integrity assurance software monitors the reliability of critical system files and directories by identifying changes made to them. Tripwire configuration options include the ability to receive
alerts via email if particular files are altered and automated integrity checking via a cron job. Using
Tripwire for intrusion detection and damage assessment helps you keep track of system changes. Because Tripwire can positively identify files that have been added, modified, or deleted, it can speed
recovery from a break-in by keeping the number of files which must be restored to a minimum.
Tripwire compares files and directories against a database of file locations, dates modified, and other
data. The database contains baselines, which are snapshots of specified files and directories at a specific point in time. The contents of the baseline database should be generated before the system is at
risk of intrusion. After creating the baseline database, Tripwire then compares the current system to
the baseline and reports any modifications, additions, or deletions.
While Tripwire is a valuable tool for auditing the security state of Red Hat Linux systems, Tripwire is
not supported by Red Hat, Inc. Refer to the Tripwire project’s website ( for
more information about Tripwire.
11.1. How to Use Tripwire
The following flowchart illustrates how Tripwire works:
Figure 11-1. Using Tripwire
The following describes in more detail the numbered blocks shown in Figure 11-1
Chapter 11. Tripwire
1. Install Tripwire and customize the policy file.
Install the tripwire RPM (Section 11.2). Then, customize the sample configuration and policy
files (/etc/tripwire/twcfg.txt and /etc/tripwire/twpol.txt respectively) and run
the configuration script, /etc/tripwire/ For more information, see Section
2. Initialize the Tripwire database.
Build a database of critical system files to monitor based on the contents of the new, signed
Tripwire policy file, /etc/tripwire/tw.pol. For more information, see Section 11.4.
3. Run a Tripwire integrity check.
Compare the newly-created Tripwire database with the actual system files, looking for missing
or altered files. For more information, see Section 11.5.
4. Examine the Tripwire report file.
View the Tripwire report file using /usr/sbin/twprint to note integrity violations. For more
information, see Section 11.6.1.
5. If unauthorized integrity violations occur, take appropriate security measures.
If monitored files have been altered inappropriately, you can either replace the original files from
backup copies reinstall the program, or completely reinstall the operating system.
6. If the file alterations were valid, verify and update the Tripwire database file.
If the changes made to monitored files are intentional, edit Tripwire’s database file to ignore
those changes in subsequent reports. For more information, see Section 11.7.
7. If the policy file fails verification, update the Tripwire policy file.
To change the list of files Tripwire monitors or how it treats integrity violations, update
the supplied policy file (/etc/tripwire/twpol.txt), regenerate a signed copy
(/etc/tripwire/tw.pol), and update the Tripwire database. For more information, see
Section 11.8.
Refer to the appropriate sections within this chapter for detailed instructions on each step.
11.2. Installing the Tripwire RPM
The easiest way to install Tripwire is to select the tripwire RPM during the Red Hat Linux 8.0
installation process. However, if you have already installed Red Hat Linux 8.0, you can use rpm or
Package Management Tool (redhat-config-packages) to install the Tripwire RPM from the
Red Hat Linux 8.0 CD-ROMs.
If you are not sure whether or not Tripwire is installed, type the following command at a shell prompt:
rpm -q tripwire
If Tripwire is installed, this command will return the following:
tripwire- version-number
The following steps outline how to find and install Tripwire from CD-ROM using the RPM command
line application:
1. Insert CD 2 of the Red Hat Linux 8.0 installation CD-ROMs.
Chapter 11. Tripwire
2. If the CD-ROM does not automatically mount, type the following command:
mount /mnt/cdrom
3. Verify that the tripwire RPM is on the CD-ROM by typing:
ls /mnt/cdrom/RedHat/RPMS/ | grep tripwire
If the RPM is on the CD-ROM, this command will display the package name.
If the RPM is not on the CD-ROM, the shell prompt will return. In this case, you will need
to check CD 3 and, possibly, CD 1 of the Red Hat Linux 8.0 installation CD-ROMs by first
unmounting the CD-ROM then repeating steps one through three.
Unmount the CD-ROM by right-clicking on the CD-ROM icon and selecting Eject or by typing
the following command at the shell prompt:
umount /mnt/cdrom
4. Once you have located the tripwire RPM, install it by typing the following command as the
root user:
rpm -Uvh /mnt/cdrom/RedHat/RPMS/tripwire*.rpm
In the /usr/share/doc/tripwire- N version-number O / directory, you will find release notes
and README files for Tripwire. These documents contain important information about the default
policy file and other topics.
11.3. Customizing Tripwire
After you have installed the Tripwire RPM, you need to complete the following steps to initialize the
11.3.1. Edit /etc/tripwire/twcfg.txt
Although you are not required to edit this sample Tripwire configuration file, you may find it necessary
for your situation. For instance you may want to alter the location of Tripwire files, customize email
settings, or customize the level of detail for reports.
Below is a list of required user configurable variables in the /etc/tripwire/twcfg.txt file:
• POLFILE — Specifies the location
of the policy file; /etc/tripwire/tw.pol is the default value.
— Specifies the location of the database file; /var/lib/tripwire/$(HOSTNAME).twd
is the default value.
— Specifies the location of the report files. By default this value is set to
default value.
— Specifies the location of the site key file; /etc/tripwire/site.key is the
— Specifies the location of the local key file; /etc/tripwire/$(HOSTNAME)-
local.key is the default value.
If you edit the configuration file and leave any of the above variables undefined, the configuration file
will be invalid. If this occurs, when you execute the tripwire command it will report an error and exit.
The rest of the configurable variables in the sample /etc/tripwire/twcfg.txt file are optional.
These include the following:
Chapter 11. Tripwire
— Specifies the text editor called by Tripwire. The default value is /bin/vi.
— If set to true this variable configures Tripwire to wait as long as possible
before prompting the user for a password, thereby minimizing the amount of time the password is
in memory. The default value is false.
— If set to true this variable configures Tripwire to report if a file
within a watched directory changes and not to report the change for the directory itself. This limits
redundancy in Tripwire reports. The default value is false.
If set to true, this variable configures Tripwire to report information to the
syslog daemon via the "user" facility. The log level is set to notice. See the syslogd man page
for more information. The default value is false.
— If set to true this variable configures Tripwire to email a report at a
regular interval regardless of whether or not any violations have occurred. The default value is
Specifies the level detail for emailed reports. Valid values for this variable
are 0 through 4. The default value is 3.
— Specifies the level detail for reports generated by the twprint command. This
value can be overridden on the command line, but is set to 3 by default.
— Specifies which mail protocol Tripwire should use. Valid values are SMTP and
SENDMAIL. The default value is SENDMAIL.
— Specifies which mail program Tripwire should use. The default value is
/usr/sbin/sendmail -oi -t.
After editing the sample configuration file, you will need to configure the sample policy file.
For security purposes, you should either delete or store in a secure location any copies of the plain
text /etc/tripwire/twcfg.txt file after running the installation script or regenerating a signed configuration file. Alternatively, you can change the permissions so that it is not world readable.
11.3.2. Edit /etc/tripwire/twpol.txt
Although it is not required, you should edit this heavily commented sample Tripwire policy file to take
into account the specific applications, files, and directories on your system. Relying on the unaltered
sample configuration from the RPM may not adequately protect your system.
Modifying the policy file also increases the usefulness of Tripwire reports by minimizing false alerts
for files and programs you are not using and by adding functionality, such as email notification.
Notification via email is not configured by default. See Section 11.8.1 for more on configuring this
If you modify the sample policy file after running the configuration script, see Section 11.8 for instructions on regenerating a signed policy file.
Chapter 11. Tripwire
For security purposes, you should either delete or store in a secure location any copies of the plain
text /etc/tripwire/twpol.txt file after running the installation script or regenerating a signed configuration file. Alternatively, you can change the permissions so that it is not world readable.
11.3.3. Run the Script
As the root user, type /etc/tripwire/ at the shell prompt to run the configuration
script. The script will ask you for site and local passwords. These passwords are used
to generate cryptographic keys for protecting Tripwire files. The script then creates and signs these
When selecting the site and local passwords, you should consider the following guidelines:
Use at least eight alphanumeric and symbolic characters, but for each password do not exceed 1023.
Do not use quotes in a password.
Make the Tripwire passwords completely different from the root or any other password for the
Use unique passwords for both the site key and the local key.
The site key password protects the Tripwire configuration and policy files. The local key password
protects the Tripwire database and report files.
There is no way to decrypt a signed file if you forget your password. If you forget the passwords, the
files are unusable and you will have to run the configuration script again.
By encrypting its configuration, policy, database, and report files, Tripwire protects them from being
viewed by anyone who does not know the site and local passwords. This means that, even if an intruder
obtains root access to your system, they will not be able to alter the Tripwire files to hide their tracks.
Once encrypted and signed, the configuration and policy files generated by running the script should not be renamed or moved.
11.4. Initialize the Tripwire Database
Initialize the Tripwire database file by issuing the /usr/sbin/tripwire --init command at the
command line.
When initializing its database, Tripwire builds a collection of file system objects based on the rules in
the policy file. This database serves as the baseline for integrity checks.
To initialize the Tripwire database, use the following command:
/usr/sbin/tripwire --init
This command can take several minutes to run.
Once you finish these steps successfully, Tripwire has the baseline snapshot of your file system necessary to check for changes in critical files. After initializing the Tripwire database, you should run
Chapter 11. Tripwire
an initial integrity check. This check should be done prior to connecting the computer to the network,
and putting it into production. For instructions on doing this see Section 11.5.
Once Tripwire is configured to your satisfaction, you are free to place the system into production.
11.5. Running an Integrity Check
By default the Tripwire RPM adds a shell script called tripwire-check to the /etc/cron.daily/
directory. This will automatically run an integrity check once per day.
You can, however, run a Tripwire integrity check at any time by typing the following command:
/usr/sbin/tripwire --check
During an integrity check, Tripwire compares the current state of file system objects with the properties recorded in its database. Violations are printed to the screen and an encrypted copy of the report
is created in /var/lib/tripwire/report/. You can view the report using the twprint command
as outlined in Section 11.6.1.
If you would like to receive an email when certain types of integrity violations occur, you can configure this in the policy file. See Section 11.8.1 for instructions on how to set up and test this feature.
11.6. Examining Tripwire Reports
The /usr/sbin/twprint command is used to view encrypted Tripwire reports and databases.
11.6.1. Viewing Tripwire Reports
The twprint -m r command will display the contents of a Tripwire report in clear text. You must,
however, tell twprint which report file to display.
A twprint command for printing Tripwire reports looks similar to the following:
/usr/sbin/twprint -m r --twrfile /var/lib/tripwire/report/ name .twr
The -m r option in the command directs twprint to decode a Tripwire report. The --twrfile
option directs twprint to use a specific Tripwire report file.
The name of the Tripwire report that you want to see includes the name of the host that Tripwire
checked to generate the report, plus the creation date and time. You can review previously saved
reports at any time. Simply type ls /var/lib/tripwire/report to see a list of Tripwire reports.
Tripwire reports can be rather lengthy, depending upon the number of violations found or errors
generated. A sample report starts off like this:
Tripwire(R) 2.3.0 Integrity Check Report
Report generated by:
Report created on:
Database last updated on:
Fri Jan 12 04:04:42 2001
Tue Jan 9 16:19:34 2001
Report Summary:
Host name:
Host IP address:
Host ID:
Policy file used:
Configuration file used:
Chapter 11. Tripwire
Database file used:
Command line used:
/usr/sbin/tripwire --check
Rule Summary:
----------------------------------------------------------------------Section: Unix File System
----------------------------------------------------------------------Rule Name
Severity Level
Removed Modified
-------------------------------- -------Invariant Directories
Temporary directories
* Tripwire Data Files
Critical devices
User binaries
Tripwire Binaries
11.6.2. View Tripwire Databases
You can also use twprint to view the entire database or information about selected files in the
Tripwire database. This is useful for seeing just how much information Tripwire is tracking on your
To view the entire Tripwire database, type this command:
/usr/sbin/twprint -m d --print-dbfile | less
This command will generate a large amount of output, with the first few lines appearing similar to
Tripwire(R) 2.3.0 Database
Database generated by:
Database generated on:
Database last updated on:
Tue Jan
Tue Jan
9 13:56:42 2001
9 16:19:34 2001
Database Summary:
Host name:
Host IP address:
Host ID:
Policy file used:
Configuration file used:
Database file used:
Command line used:
/usr/sbin/tripwire --init
Object Summary:
----------------------------------------------------------------# Section: Unix File System
Modify Time
------------------------ ---------/
drwxr-xr-x root (0)
drwxr-xr-x root (0)
Mon Jan 8 08:20:45 2001
Chapter 11. Tripwire
-rwxr-xr-x root (0)
-rwxr-xr-x root (0)
-rwxr-xr-x root (0)
Tue Dec 12 05:51:35 2000
Thu Dec
7 22:35:05 2000
Thu Dec
7 22:35:05 2000
To see information about a particular file that Tripwire is tracking, such as /etc/hosts, use the
following command:
/usr/sbin/twprint -m d --print-dbfile /etc/hosts
The result will look similar to this:
Object name:
------------Object Type
Device Number
Inode Number
Num Links
----------Regular File
root (0)
root (0)
See man page for twprint for more options.
11.7. Updating the Tripwire Database
If you run an integrity check and Tripwire finds violations, you will first need to determine whether
the violations discovered are actual security breaches or the product of authorized modifications. If
you recently installed an application or edited critical system files, Tripwire will correctly report
integrity check violations. In this case, you should update your Tripwire database so those changes
are no longer reported as violations. However, if unauthorized changes are made to system files that
generate integrity check violations, then you should restore the original file from a backup, reinstall
the program, or, if the breach is severe enough, completely reinstall the operating system.
To update the Tripwire database so it accepts valid policy violations, Tripwire first cross-references
a report file against the database, then integrates into it valid violations from the report file. When
updating the database, be sure to use the most recent report.
Use the following command to update the Tripwire database, where name is the name of the most
recent report file:
/usr/sbin/tripwire --update --twrfile /var/lib/tripwire/report/ name .twr
Tripwire will display the report file using the default text editor specified on the EDITOR line of the
Tripwire configuration file. This give you an opportunity to deselect files you do not wish to update in
the Tripwire database.
Chapter 11. Tripwire
It is important that you change only authorized integrity violations in the database.
All proposed updates to the Tripwire database start with an [x] before the file name, similar to the
following example:
[x] "/usr/sbin/longrun"
[x] "/usr/sbin"
[x] "/usr/sbin/cpqarrayd"
If you want to specifically exclude a valid violation from being added to the Tripwire database, remove
the x. To accept any files with an x beside them as changes.
To edit files in the default text editor, vi, type i and press [Enter] to enter insert mode and make any
necessary changes. When finished press the [Esc] key, type :wq, and press [Enter].
After the editor closes, enter your local password and the database will be rebuilt and signed.
After a new Tripwire database is written, the newly authorized integrity violations will no longer show
up as warnings.
11.8. Updating the Tripwire Policy File
If you want to change the files Tripwire records in its database, change email configuration, or modify
the severity at which certain violations are reported, you need to edit your Tripwire policy file.
First, make whatever changes are necessary to the sample policy file /etc/tripwire/twpol.txt.
If you deleted this file (as you should whenever you are finished configuring Tripwire), you can regenerate it by issuing the following command:
twadmin --print-polfile > /etc/tripwire/twpol.txt
A common change to this policy file is to comment out any files that do not exist on your system
so that they will not generate a file not found error in your Tripwire reports. For example, if
your system does not have a /etc/smb.conf file, you can tell Tripwire not to try to look for it by
commenting out its line in twpol.txt with the # character as in the following example:
-> $(SEC_CONFIG) ;
Next, you must generate a new, signed /etc/tripwire/tw.pol file and generate an updated
database file based on this policy information. Assuming /etc/tripwire/twpol.txt is the edited
policy file, use this command:
/usr/sbin/twadmin --create-polfile -S site.key /etc/tripwire/twpol.txt
You will be asked for the site password. Then, the twpol.txt file will be encrypted and signed.
It is important that you update the Tripwire database after creating a new /etc/tripwire/tw.pol
file. The most reliable way to accomplish this is to delete your current Tripwire database and create a
new database using the new policy file.
If your Tripwire database file is named, type this command:
rm /var/lib/tripwire/
Chapter 11. Tripwire
Then type the following command to create a new database using the updated policy file:
/usr/sbin/tripwire --init
To make sure the database was correctly changed, run the first integrity check manually and view the
contents of the resulting report. See Section 11.5 and Section 11.6.1 for more on doing these tasks.
11.8.1. Tripwire and Email
You can configure Tripwire to send an email to one or more accounts if a specific type of policy
is violated. In order to do this, you need to figure out what policy rules should be monitored and
who should get the email when those rules are broken. Note that on large systems with multiple
administrators, you can have different sets of people notified depending on the types of violations.
Once you have determined who to notify and what rule violations to report to them, edit the
/etc/tripwire/twpol.txt file, adding an emailto= line to the rule directive section for each
appropriate rule. Do this by adding a comma after the severity= line and putting emailto= on
the next line, followed by one or more email addresses. More than one email address can be specified
if the addresses are separated by a semi-colon.
For example, if two administrators, Johnray and Bob, need to be notified if a networking program is
modified, change the Networking Programs rule directive in the policy file to look like this:
rulename = "Networking Programs",
severity = $(SIG_HI),
emailto =;
After changing the policy file, follow the instructions in Section 11.8 to generate an updated, encrypted
and signed copy of the Tripwire policy file. Sending Test Email Messages
To test Tripwire’s email notification configuration, use the following command:
/usr/sbin/tripwire --test --email your@email.address
A test email will immediately be sent to the email address by the tripwire program.
11.9. Updating the Tripwire Configuration File
If you want to change Tripwire’s configuration file, you should first edit the sample configuration
file /etc/tripwire/twcfg.txt. If you deleted this file (as you should whenever you are finished
configuring Tripwire), you can regenerate it by issuing the following command:
twadmin --print-cfgfile > /etc/tripwire/twcfg.txt
Tripwire will not recognize any configuration changes until the configuration text file is correctly
signed and converted to /etc/tripwire/tw.pol with the twadmin command.
Use the following command to regenerate a configuration file from the /etc/tripwire/twcfg.txt
text file:
/usr/sbin/twadmin --create-cfgfile -S site.key /etc/tripwire/twcfg.txt
Chapter 11. Tripwire
Since the configuration file does not not alter any Tripwire policies or files tracked by the application,
it is not necessary to regenerate the Tripwire database.
11.10. Tripwire File Location Reference
Before working with Tripwire, you should know where important files for the application are located.
Tripwire stores its files in a variety of places depending on their role.
Within the /usr/sbin/ directory you will find the following programs:
Within the /etc/tripwire/ directory you will find the following files:
• — The initialization script for Tripwire.
twcfg.txt — The sample configuration file supplied by the Tripwire RPM.
tw.cfg — The signed configuration file created by the script.
twpol.txt — The sample policy file supplied by the Tripwire RPM.
tw.pol — The signed policy file created by the script.
Key Files — The local and site keys created by the script which end with a .key
file extension.
After running the installation script you will find the following files in the
/var/lib/tripwire/ directory:
The Tripwire Database — The database of your system’s files which has a .twd file extension.
Tripwire Reports — The report/ directory is where Tripwire reports are stored.
The next section explains more about the roles these files play in the Tripwire system.
11.10.1. Tripwire Components
The following describes in more detail the roles the listed in the previous section play in the Tripwire
This is the encrypted Tripwire configuration file which stores system-specific information,
such as the location of Tripwire data files. The installer script and twadmin
command generates this file using the information in the text version of the configuration file,
After running the the installation script, the system administrator can change parameters by editing /etc/tripwire/twcfg.txt and regenerating a signed copy of the tw.cfg file using the
twadmin command. See Section 11.9 for more information on how to do this.
Chapter 11. Tripwire
The active Tripwire policy file is an encrypted file containing comments, rules, directives, and
variables. This file dictates the way Tripwire checks your system. Each rule in the policy file
specifies a system object to be monitored. Rules also describe which changes to the object to
report and which to ignore.
System objects are the files and directories you wish to monitor. Each object is identified by
an object name. A property refers to a single characteristic of an object that Tripwire software
can monitor. Directives control conditional processing of sets of rules in a policy file. During
installation, the sample text policy file , /etc/tripwire/twpol.txt, is used to generate the
active Tripwire policy file.
After running the the installation script, the system administrator can update the Tripwire policy
file by editing /etc/tripwire/twpol.txt and regenerating a signed copy of the tw.pol file
using the twadmin command. See Section 11.8 for more information on how to do this.
When first initialized, Tripwire uses the signed policy file rules to create this database file. The
Tripwire database is a baseline snapshot of the system in a known secure state. Tripwire compares this baseline against the current system to determine what changes have occurred. This
comparison is called an integrity check.
/var/lib/tripwire/report/ directory. The report files summarize any file changes that
violated the policy file rules during the integrity check. Tripwire reports are named using the
following convention: host_name-date_of_report-time_of_report.twr. These reports
detail the differences between the Tripwire database and your actual system files.
11.11. Additional Resources
Tripwire can do more than what is covered in this chapter. Refer to these additional sources for more
information about Tripwire.
11.11.1. Installed Documentation
U — An excellent starting point for learning
about how to customize the configuration and policy files in the /etc/tripwire/ directory.
• /usr/share/doc/tripwire- version-number
Also, refer to the man pages for tripwire, twadmin and twprint for help using those utilities.
11.11.2. Useful Websites
• — The home of the Tripwire Open Source Project, where you can find the
latest news on the application, including a helpful FAQ.
• — This links to the latest official documentation from the Tripwire project.
Network Services Reference
Chapter 12.
Network Scripts
Using Red Hat Linux, all network communications occur between configured interfaces and physical
networking devices connected to the system. The different types of interfaces that exist are as varied
as the physical devices they support.
The configuration files for network interfaces and the scripts to activate and deactivate them are located
in the /etc/sysconfig/network-scripts/ directory. While the existence of interface files can
differ from system to system, the three different types of files that exist in this directory, interface
configuration files, interface control scripts, and network function files, work together to enable Red
Hat Linux to use various network devices.
This chapter will explore the relationship between these files and how they are used.
12.1. Network Configuration Files
Before we review the interface configuration files themselves, let us itemize the primary configuration
files used by Red Hat Linux to configure networking. Understanding the role these files play in setting
up the network stack can be helpful when customizing your system.
The primary network configuration files are as follows:
— The main purpose of this file is to resolve hostnames that cannot be resolved
any other way. It can also be used on resolve hostnames on small networks with no DNS serer.
Regardless of the type of network the computer is on, this file should contain a line specifying the IP
address of the loopback device ( as localhost.localdomain. For more information
see the hosts man page.
• /etc/hosts
• /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 on this file see the resolv.conf man page.
• /etc/sysconfig/network —
Specifies routing and host information for all network interfaces.
For more information on this file and what directives it accepts, see Section
W — For each network interface on a Red Hat Linux system, there is a corresponding interface configuration script. Each of
these files provide information specific to a particular network interface. See Section 12.2 for more
information on this type of file and what directives it accepts.
• /etc/sysconfig/network-scripts/ifcfg- interface-name
The /etc/sysconfig/networking/ directory is used by the Network Administration Tool
(redhat-config-network) and its contents should not be edited manually. For more information
about configuring network interfaces using the Network Administration Tool, see the chapter
called Network Configuration in the Official Red Hat Linux Customization Guide.
12.2. Interface Configuration Files
Interface configuration files control the operation of individual network interface device. As your
Red Hat Linux system boots, it uses these files to determine what interfaces to bring up and how to
Chapter 12. Network Scripts
configure them. These files are usually named ifcfg- X name Y , where
of the device that the configuration file controls.
refers to the name
12.2.1. Ethernet Interfaces
One of the most common interface files is ifcfg-eth0, which controls the first network interface
card or NIC in the system. In a system with multiple NICs, you will also have multiple ifcfg-eth
files, each one with a unique number at the end of the file name. Because each device has its own
configuration file, you can control how each interface functions individually.
Below is a sample ifcfg-eth0 file for a system using a fixed IP address:
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 quite a bit different, because IP information
is provided by the DHCP server:
Most of the time you will probably want to use a GUI utility, such as Network Administration Tool
(redhat-config-network) to make changes to the various interface configuration files. See the
Official Red Hat Linux Customization Guide for instructions on using this tool.
You can also edit the configuration file for a given network interface by hand. Below is a listing of the
parameters one can expect to configure in an interface configuration file.
Within each of the interface configuration files, the following values are common:
• BOOTPROTO= protocol
, where
is one of the following:
none — No boot-time protocol should be used.
bootp — The BOOTP protocol should be used.
dhcp — The DHCP protocol should be used.
• BROADCAST= address
, where
is the broadcast address. This directive is depre-
Y , where X name Y is the name of the
dynamically-allocated PPP devices where it is the logical name).
• DEVICE= name
, where address is a name server address to be placed in
/etc/resolv.conf if the PEERDNS directive is set to yes.
• DNS{1,2}= address
• IPADDR= address
• NETMASK= mask
, where
, where
• NETWORK= address
• ONBOOT= answer
, where
, where
is the IP address.
is the netmask value.
is the network address. This directive is deprecated.
is one of the following:
Chapter 12. Network Scripts
yes — This device should be activated at boot-time.
no — This device should not be activated at boot-time.
• PEERDNS= answer
, where
is one of the following:
yes — Modify /etc/resolv.conf if the DNS directive is set. If you are using DCHP, then
yes is the default.
no — Do not modify /etc/resolv.conf.
SRCADDR= address , where
USERCTL= answer , where
is the specified source IP address for outgoing
is one of the following:
yes — Non-root users are allowed to control this device.
no — Non-root users are not allowed to control this device.
12.2.2. Dialup Interfaces
If you connect to the Internet via PPP dialup connection, you need a configuration file for that interface
as well.
This file is created automatically for you when you use wvdial, Network Administration Tool or
Kppp to create a dialup account. In addition, any changes to dialup account settings are reflected
in these interface configuration files. The Official Red Hat Linux Getting Started Guide contains instructions for using these GUI-based dialup connection tools. You can also create and edit this file
manually. A typical ifcfg-ppp0 files looks like this:
Serial Line Internet Protocol (SLIP) is another dialup interface, although it is less commonly used.
SLIP files have interface configuration file names such as ifcfg-sl0.
Among the options not already discussed that may be used in these files:
• DEFROUTE= answer
, where
is one of the following:
yes — Set this interface as the default route.
no — Do not set this interface as the default route.
• DEMAND= answer
, where
is one of the following:
Chapter 12. Network Scripts
yes — This interface will allow pppd to initiate a connection when someone attempts to use it.
no — A connection must be manually established for this interface.
] , where
interface will disconnect itself.
is the number of seconds of idle activity before the
] , where \ string ] is the initilization string passed to the modem device. This option is primarily used with SLIP interfaces.
• INITSTRING= string
] , where \ value ] is the baud rate of the device. Possible standard values
here include 57600, 38400, 19200, and 9600.
• LINESPEED= value
] , where \ device ] is the name of the serial device that is used to establish the connection for the interface.
• MODEMPORT= device
] , 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 will result in fewer dropped
packets and slightly improve the throughput for a connection.
• MTU= value
] , where
tion configurations.
• NAME= name
is the reference to the title given to a collection of dialup connec-
] , where \ name ] is the username given during the Password Authentication
Protocol (PAP) exchange that occurs to allow you to connect to a remote system.
• PAPNAME= name
, where
• PEERDNS= answer
is one of the following:
yes — Modify thedd system’s /etc/resolv.conf file entries to use the DNS servers provided
no — The /etc/resolv.conf file will not be modified.
by the remote system when a connection is established.
• PERSIST= answer
, where
is one of the following:
yes — This interface should be kept active at all times, even if deactivated after a modem hang
no — This interface should not be kept active at all times.
• REMIP= address
, where
is the remote system’s IP address. This is usually left
, where name associates this interface with a dialer configuration in
/etc/wvdial.conf, which contains the phone number to be dialed and other important informa-
tion for the interface.
12.2.3. Other Interfaces
Other common interface configuration files that use these options include ifcfg-lo, which controls
the IP protocol’s local loopback device, ifcfg-irlan0, which arranges settings for the first infrared
device, ifcfg-plip0, which controls the first PLIP device, and ifcfg-tr0, used with the first
Token Ring device.
A local loopback interface is often used in testing, as well as 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.
Chapter 12. Network Scripts
Never edit the loopback interface script, /etc/sysconfig/network-scripts/ifcfg-lo, by hand.
Doing so can prevent the system from operating correctly.
An infrared interface allows information between devices, such as a laptop and a printer, to flow over
an infrared link which works in a similar way to an Ethernet device except that it commonly occurs
over a peer-to-peer connection.
A Parallel Line Interface Protocol (PLIP) connection works much the same way, except that it utilizes
a parallel port.
Token Ring topologies are not as common on Local Area Networks as they once were, having been
eclipsed by Ethernet.
12.2.4. Alias and Clone Files
Two lesser-used types of interface configuration files found in the /etc/sysconfig/networkscripts/ directory are alias and clone files.
_ ^
Alias interface configuration files take names in the format of ifcfg- if-name : aliasvalue , and they allow an alias to point to an interface. For example, a ifcfg-eth0:0 file could
be configured to specify DEVICE=eth0:0 and a static IP address of, serving as an alias of
an Ethernet interface already configured to receive its IP information via DHCP in ifcfg-eth0. At
that point, the eth0 device is bound to a dynamic IP address, but it can always be referred to on that
system via the fixed IP address.
A clone interface configuration file should follow this naming convention, ifcfg- ^ if-name _ ^ clone-name _ . While an alias file is another way to refer to an existing interface configuration file,
a clone file is used to specify additional options when specifying an interface. For example, if you
have a standard DHCP Ethernet interface called eth0, it may look similar to this:
Since USERCTL is set to no if it is not specified, users cannot bring this interface up and down. To
give users this ability, create a clone by copying ifcfg-eth0 to ifcfg-eth0-user and add the
following line:
When a user brings up the eth0 interface with the ifup eth0-user command, the configuration options from ifcfg-eth0 and ifcfg-eth0-user are conmbined. 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
configuration tool, Network Administration Tool. For more on using this tool, see the chapter called
Network Configuration in the Official Red Hat Linux Customization Guide.
12.3. Interface Control Scripts
The interface control scripts activate and deactivated system interfaces. There are two primary interface control scripts, /sbin/ifdown and /sbin/ifup, that call on control scripts located in the
/etc/sysconfig/network-scripts/ directory.
Chapter 12. Network Scripts
The two interface control scripts are ifdown and ifup and are symbolic links to scripts in the /sbin/
directory. When either of these scripts are called, they require a value of the interface to be specified,
such as:
ifup eth0
Determining IP information for eth0... done.
that point, the /etc/rc.d/init.d/functions and /etc/sysconfig/networkscripts/network-functions files are used to perform a variety of tasks. See Section 12.4 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 for the type of interface device brings the interface up or
down. The following interface control scripts are the most common:
• ifup-aliases —
Configures IP aliases from interface configuration files when more than one IP
address is associated with an interface.
• ifdown-cipcb
up and down.
and ifup-cipcb — Used to bring Crypto IP Encapsulation (CIPE) connections
and ifup-ipv6 — Contains IPv6-related function calls using environment variables in various interface configuration files and /etc/sysconfig/network.
• ifdown-ipv6
• ifup-ipx
— Used to bring up an IPX interface.
• ifup-plip
— Used to bring up a PLIP interface.
• ifup-plusb
— Used to bring up a USB interface for network connections.
• ifdown-post and ifup-post —
up or down.
• ifdown-ppp
Contains commands to be executed after an interface is brought
and ifup-ppp — Used to bring a PPP interface up or down.
• 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
• ifdown-sl
and ifup-sl — Used to bring an SLIP interface up or down.
Be aware that removing or modifying any scripts in the /etc/sysconfig/network-scripts/
directory can cause interface connections to act strangely or fail, as these scripts. Only advanced users
should modify scripts related to a network interface.
You can also use the init script /etc/rc.d/init.d/network to activate and deactive all network
interfaces configured to start at boot time with the command:
/sbin/service network action
Where action is either start, stop, or restart. You can also use the command
/sbin/service/network status to view a list of configured devices and currently active
12.4. Network Functions
Red Hat Linux makes use of several files that contain important functions that are used in various
ways to bring interfaces up and down. Rather than forcing each interface control file to contain the
same functions as another, these functions are grouped together in a few files that can be sourced when
Chapter 12. Network Scripts
most common network functions file is network-functions, located in the
/etc/sysconfig/network-scripts/ directory. This file contains a variety of common IPv4
functions useful to many interface control scripts, such as contacting running programs that have
requested information about changes in an interface’s status, setting host names, finding a gateway
device, seeing if a particular device is down or not, and adding a default route.
As the functions required for IPv6 interfaces are different than IPv4 interfaces, a
network-functions-ipv6 file exists specifically to hold this information. IPv6 support must be
enabled in the kernel in order to communicate via that protocol. A function is present in this file that
checks for the presence of IPv6 support. Additionally, functions that 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 can also be found in this file.
12.5. Additional Resources
More information concerning network scripts can be found in the following places.
A comprehensive guide to
available options for network configuration files, including IPv6 options not covered in this chapter.
• /usr/share/doc/initscripts- version /sysconfig.txt —
— This Postscript™ file contains a
wealth of information about the ip command, which can be used to manipulate routing tables,
among other things. Use ghostview or kghostview to look at this file.
• /usr/share/doc/iproute- version /
Chapter 12. Network Scripts
Chapter 13.
Firewalls and iptables
Linux comes with advanced tools for packet filtering — the process of controlling network packets
as they enter, move through, and exit the network stack within the kernel. Pre-2.4 kernels relied on
ipchains for packet filtering and used lists of rules applied to packets at each step of the filtering
process. The introduction of the 2.4 kernel brought with it iptables (also called netfilter), which
is similar to ipchains but greatly expands on the scope and control available for filtering network
This chapter focuses on packet filtering basics, defines the differences between ipchains and iptables, explains various options available with iptables commands, and shows how filtering rules
can be preserved between system reboots.
If you require instructions for constructing iptables rules or setting up a firewall based on these
rules, please see Section 13.5 for more information.
The default firewall mechanism under the 2.4 kernel is iptables, but iptables cannot be used if
ipchains are already running. If ipchains are present at boot time, the kernel will issue an error
and fail to start iptables.
The functionality of ipchains is not affected by these errors.
13.1. Packet Filtering
Traffic moves through a network in packets. A network packet is collection of data in a specific size
and format. In order to transmit a file over a network, the sending computer must first break the file
into packets using the rules of the network protocol. Each of these packets holds a small part of the
file data. Upon receiving the transmission, the target computer reassembles the packets into the file.
Every packet contains information which helps it navigate the network and move toward its destination. The packet can tell computers along the way, as well as the destination machine, where it came
from, where it is going, and what type of packet it is, among other things. Most packets are designed to
carry data, although some protocols use packets in special ways. For example, the Transmission Control Protocol (TCP) uses a SYN packet, which contains no data, to initiate communication between
two systems.
The Linux kernel contains the built-in ability to filter packets, allowing some of them into the system
while stopping others. The 2.4 kernel’s netfilter has three built-in tables or rules lists. They are as
• filter
• nat
— This is the default table for handling network packets.
— This table used to alter packets that create a new connection.
• mangle
— This table is used for specific types of packet alteration.
Each of these tables in turn have a group of built-in chains which correspond to the actions performed
on the packet by the netfilter.
The built-in chains for the filter table are as follows:
INPUT — This chain applies to packets received via a network interface.
Chapter 13. Firewalls and iptables
OUTPUT — This chain applies to packets sent out via the same network interface which received
the packets.
FORWARD — This chain applies to packets received on one network interface and sent out on
The built-in chains for the nat table are as follows:
PREROUTING — This chain alters packets received via a network interface when they arrive.
OUTPUT — This chain alters locally-generated packets before they are routed via a network interface.
POSTROUTING — This chain alters packets before they are sent out via a network interface.
The built-in chains for the mangle table are as follows:
PREROUTING — This chain alters packets received via a network interface before they are routed.
OUTPUT — This chain alters locally-generated packets before they are routed via a network interface.
Every network packet received by or sent out of a Linux system is subject to at least one table.
A packet may be checked against multiple rules within each rules list before emerging at the end of
the chain. The structure and purpose of these rules may vary, but they usually seek to identify a packet
coming from or going to a particular IP address or set of addresses when using a particular protocol
and network service.
Regardless of their destination, when packets match a particular rule on one of the tables, they are
designated for a particular target or action to be applied to them. If the rule specifies an ACCEPT
target for a matching packet, the packet skips the rest of the rule checks and is allowed to continue
to its destination. If a rule specifies a DROP target, that packet is refused access to the system and
nothing is sent back to the host that sent the packet. If a rule specifies a REJECT target, the packet is
dropped, but an error packet is sent to the packet’s originator.
Every chain has a default policy to ACCEPT, DROP, REJECT, or QUEUE the packet to be passed
to user-space. If none of the rules in the chain apply to the packet, then the packet is dealt with in
accordance with the default policy.
The iptables command allows you to configure these rule lists, as well as set up new tables to be
used for your particular situation.
13.2. Differences between iptables and ipchains
At first glance, ipchains and iptables appear to be quite similar. Both methods of packet filtering
use chains of rules operating within the Linux kernel to decide not only which packets to let in or
out, but also what to do with packets that match certain rules. However, iptables offers a much
more extensible way of filtering packets, giving the administrator a greater amount of control without
building too much complexity into the entire system.
Specifically, users comfortable with ipchains should be aware of the following significant differences between ipchains and iptables before attempting to use iptables:
Under iptables, each filtered packet is processed using rules from only one chain rather than
multiple chains. For instance, a FORWARD packet coming into a system using ipchains would
have to go through the INPUT, FORWARD, and OUTPUT chains in order to move along to its
destination. However, iptables only sends packets to the INPUT chain if they are destined for
the local system and only sends them to the OUTPUT chain if the local system generated the
Chapter 13. Firewalls and iptables
packets. For this reason, you must be sure to place the rule designed to catch a particular packet in
the rule that will actually see the packet.
The advantage is that you now have more control over the disposition of each packet. If you are
attempting to block access to a particular website, it is now possible to block access attempts from
clients running on hosts which use your host as a gateway. An OUTPUT rule which denies access
will no longer prevent access for hosts which use your host as a gateway.
The DENY target has been changed to DROP. In ipchains, packets that matched a rule in a
chain could be directed to the DENY target, which silently dropped the packet. This target must be
changed to DROP in iptables to have the same effect.
Order matters when placing options in a rule. Previously, with ipchains, it did not matter very
much how you ordered the rule options. The iptables command is a bit pickier about where some
options may go. For example, you must now specify the source or destination port after the protocol
(ICMP, TCP, or UDP) to be used in a rule.
When specifying network interfaces to be used with a rule, you must only use incoming interfaces
(-i option) with INPUT or FORWARD chains and outgoing interfaces (-o option) with FORWARD
or OUTPUT chains. This is necessary because OUTPUT chains are no longer used by incoming
interfaces, and INPUT chains are not seen by packets moving through outgoing interfaces.
This is by no means a comprehensive list of the changes, given that iptables represents a fundamentally rewritten network filter. For more specific information, consult the Linux 2.4 Packet Filtering
HOWTO and the sources found in Section 13.5.
13.3. Options Used in iptables Commands
Rules that allow packets to be filtered by the kernel are put in place by running the iptables command. When using the iptables command, you must specify the following options:
Packet Type — This dictates what type of packets the command filters.
Packet Source or Destination — This dictates what packets the command filters based on the source
or destination of the packet.
Target — This dictates what action is taken on packets matching the above criteria.
The options used with given iptables rule must be grouped logically, based on the purpose and
conditions of the overall rule, in order for the rule to be valid.
13.3.1. Tables
A powerful aspect of iptables is that multiple tables can be used to decide the fate of a particular packet, depending upon the type of packet being monitored and what is to be done with the
packet. Thanks to the extensible nature of iptables, specialized tables can be created and stored
in the /etc/modules/ b kernel-version c /kernel/net/ipv4/netfilter/ directory to meet
specific goals. Think of iptables as being able to run multiple sets of ipchains rules in defined
chains, with each set fulfilling a particular role.
The default table, named filter, contains the standard built-in INPUT, OUTPUT, and FORWARD
chains. This is somewhat similar to the standard chains in use with ipchains. However, by default,
iptables also includes two additional tables that perform specific packet filtering jobs. The nat
table can be used to modify the source and destination addresses recorded in packets, and the mangle
table allows you to alter packets in specialized ways.
Each table contains default chains that perform necessary tasks based on the purpose of the table, but
you can easily set up new chains in each of the tables.
Chapter 13. Firewalls and iptables
13.3.2. Structure
Many iptables commands have the following structure:
iptables [-t table-name ] command
In this example, the d table-name e option allows the user to select a table other than the default
filter table to use with the command. The d command e option is the center of the command, dictating a specific action to perform, such as appending or deleting a rule from a particular chain, which is
specified by the d chain-name e option. Following the d chain-name e are pairs of parameters and
options that actually define the way the rule will work and what will happen when a packet matches
the rule.
When looking at the structure of an iptables command, it is important to remember that, unlike
most other commands, the length and complexity of an iptables command can change based on
its purpose. A simple command to remove a rule from a chain can be very short, while a command
designed to filter packets from a particular subnet using a variety of specific parameters and options
can be rather lengthy. When creating iptables commands it is helpful to recognize that some parameters and options may create the need for other parameters and options to further specify the previous
option’s request. In order to construct a valid rule, this must continue until every parameter and option
that requires another set of options is satisfied.
Type iptables -h to see a comprehensive list of iptables command structures.
13.3.3. Commands
Commands tell iptables to perform a specific action. Only one command is allowed per iptables
command string. With the exception of the help command, all commands are written in upper-case
The iptables commands are as follows:
— Appends the iptables rule to the end of the specified chain. This is the command used to
simply add a rule when rule order in the chain does not matter.
• -A
— Checks a particular rule before adding it to the user-specified chain. This command can
help you construct complicated iptables rules by prompting you for additional parameters and
• -C
— Deletes a rule in a particular chain by number (such as 5 for the fifth rule in a chain). You
can also type the entire rule, and iptables will delete the rule in the chain that matches it.
• -D
— Renames a user-defined chain. This does not affect the structure of the table. Rather, it just
saves you the trouble of deleting the chain, creating it under the new name, and reconfiguring all of
your rules for that chain.
• -E
— Flushes the selected chain, which effectively deletes every rule in the the chain. If no chain
is specified, this command flushes every rule from every chain.
• -F
— Provides a list of helpful command structures, as well as a quick summary of command
parameters and options.
• -h
• -I
— Inserts a rule in a chain at a particular point. Assign a number to the rule to be inserted and
iptables will put it there. If no number is specified, iptables will place your command at the
top of the rule list.
Chapter 13. Firewalls and iptables
Be aware of which option (-A or -I) you are using when adding a rule. The order of the rules can
be very important when determining if a particular packet applies to one rule or another. Make
sure when adding a rule to the beginning or end of the chain that it does not affect other rules in
that chain.
— Lists all of the rules in the chain specified after the command. To list all rules in all chains in
the default filter table, do not specify a chain or table. Otherwise, the following syntax should
be used to list the rules in a specific chain in a particular table:
• -L
iptables -L
Powerful options for the -L command that provide rule numbers and allow more verbose rule
descriptions, among others, are described in Section 13.3.7.
• -N
— Creates a new chain with a user-specified name.
— Sets the default policy for a particular chain, so that when packets traverse an entire chain
without matching a rule, they will be sent on to a particular target, such as ACCEPT or DROP.
• -P
— Replaces a rule in a particular chain. You must use a rule’s number after the chain’s name to
replace that rule. The first rule in a chain relates to rule number 1.
• -R
• -X
— Deletes a user-specified chain. Deleting a built-in chain for any table is not allowed.
• -Z
— Zeros the byte and packet counters in all chains for a particular table.
13.3.4. Parameters
Once certain iptables commands are specified, including those used to add, append, delete, insert,
or replace rules within a particular chain, parameters are required to begin the construction of the
packet filtering rule.
• -c —
Resets the counters for a particular rule. This parameter accepts the PKTS and BYTES options
to specify what counter to reset.
— Sets the destination hostname, IP address, or network of a packet that will match the rule.
When matching a network, you can use two different methods for signifying the netmask, such as or
• -d
• -f
— Applies this rule only to fragmented packets.
By using the ! option after this parameter, only unfragmented packets will be matched.
— Sets the incoming network interface, such as eth0 or ppp0, to use with a particular rule.
With iptables, this optional parameter may only be used with the INPUT and FORWARD chains
when used with the filter table and the PREROUTING chain with the nat and mangle tables.
• -i
This parameter features several useful options that may be used before specifying the name of an
! — Tells this parameter not to match, meaning that any specified interfaces are specifically
excluded from this rule.
+ — A wildcard character used to match all interfaces which match a particular string. For
example, the parameter -i eth+ would apply this rule to any Ethernet interfaces on your system
but exclude any other interfaces, such as ppp0.
If the -i parameter is used but no interface is specified, then every interface is affected by the rule.
— Tells iptables to jump to a particular target when a packet matches a particular rule. Valid
targets to be used after the -j option include the standard options, ACCEPT, DROP, QUEUE, and
RETURN, as well as extended options that are available through modules loaded by default with the
Red Hat Linux iptables RPM package, such as LOG, MARK, and REJECT, among others. See the
• -j
Chapter 13. Firewalls and iptables
iptables man page for more information on these and other targets, including rules regarding
their use.
You may also direct a packet matching this rule to a user-defined chain outside of the current
chain. This allows you to apply other rules against this packet, further filtering it with more specific
If no target is specified, the packet moves past the rule with no action taken. However, the counter
for this rule is still increased by one, as the packet matched the specified rule.
• -o — Sets the outgoing
network interface for a particular rule, and may only be used with OUTPUT
and FORWARD chains in the filter table and the POSTROUTING chain in the nat and mangle
tables. This parameter’s options are the same as those of the incoming network interface parameter
— Sets the IP protocol for the rule, which can be either icmp, tcp, udp, or all, to match every
supported protocol. In addition, lesser used protocols listed in /etc/protocols may also be used.
If this option is omitted when creating a rule, the all option is the default.
• -p
— Sets the source for a particular packet, using the same syntax as the destination (-d) parameter.
• -s
13.3.5. Match Options
Different network protocols provide specialized matching options which may be set in specific ways
to match a particular packet using that protocol. Of course, the protocol must first be specified in
the iptables command, such as using -p tcp l protocol-name m , to make the options for that
protocol available. TCP Protocol
These match options are available for the TCP protocol (-p tcp):
--dport — Sets the destination port for the packet. You can use either a network service name
(such as www or smtp), port number, or range of port numbers to configure this option. To browse
the names and aliases of network services and the port numbers they use, view the /etc/services
file. You can also use --destination-port to specify this match option.
To specify a specific range of port numbers, separate the two numbers with a colon (:), such as -p
tcp --dport 3000:3200. The largest valid range is 0:65535.
You may also use an exclamation point character (!) as a flag after the --dport option to tell
iptables to match all packets which do not use that network service or port.
--sport — Sets the source port of the packet, using the same options as --dport. You can also
use --source-port to specify this match option.
--syn — Applies to all TCP packets designed to initiate communication, commonly called SYN
packets. Any packets that carry a data payload are not touched. Placing an exclamation point character (!) as a flag after the --syn option causes all non-SYN packets to be matched.
--tcp-flags — Allows TCP packets with specific bits, or flags, set to be matched with a rule.
The --tcp-flags match option accepts two parameters after it, which are flags for the various
bits arranged in a comma-separated list. The first parameter is the mask, which sets the flags to be
examined on the packet. The second parameter refers to the flags that must be set in the packet to
make a match. The possible flags are ACK, FIN, PSH, RST, SYN, and URG. In addition, ALL and NONE
can also be used to match every flag or none of them.
For example, an iptables rule which contains -p tcp --tcp-flags ACK,FIN,SYN SYN will
only match TCP packets that have the SYN flag set and the ACK and FIN flags unset.
Chapter 13. Firewalls and iptables
Like many other options, using the exclamation point character (!) after --tcp-flags reverses the
effect of the match option, so that the second parameter’s flags must not be set in order to match.
--tcp-option — Attempts to match with TCP-specific options that can be set within a particular
packet. This match option can also be reversed with the exclamation point character (!). UDP Protocol
These match options are available for the UDP protocol (-p udp):
--dport — Specifies the destination port of the UDP packet, using the service name, port number,
or range of port numbers. The --destination-port match option may be used instead of -dport. See the --dport match option in Section for various ways to use this option.
--sport — Specifies the source port of the UDP packet, using the service name, port number, or
range of port numbers. The --source-port match option may be used instead of --sport. See
the --dport match option in Section for various ways to use this option. ICMP Protocol
Packets using the Internet Control Message Protocol (ICMP) can be matched using the following
option when -p icmp is specified:
--icmp-type — Sets the name or number of the ICMP type to match with the rule. A list of valid
ICMP names can be seen by typing the iptables -p icmp -h command. Modules with Additional Match Options
Additional match options are also available through modules loaded when the iptables command
calls them. To use a match option module, you must load the module by name by including -m
n module-name o in the iptables command.
A large number of modules are available by default. It is even possible to create your own modules
to provide additional match option functionality. Many modules exist, but only the most popular ones
are discussed here.
The limit module allows you to place a limit on how many packets will be matched to a particular
rule. This is especially beneficial when logging rule matches so that a flood of matching packets will
not fill up your logs with repetitive messages or use too many system resources.
--limit — Sets the number of matches for a particular range of time, specified with a number and
time modifier arranged in a number o / time o format. For example, using --limit 5/hour
only lets a rule match five times in a single hour.
If a number and time modifier are not used, the default value of 3/hour is assumed.
--limit-burst — Sets a limit on the number of packets able to match a rule at one time. This
option should be used in conjunction with the --limit option, and it accepts a number to set the
burst threshold.
If no number is specified, only five packets are initially able to match the rule.
The state module, which uses the --state match option, can match a packet with these particular
connection states:
ESTABLISHED — The matching packet is associated with other packets in an established connec-
Chapter 13. Firewalls and iptables
INVALID — The matching packet cannot be tied to a known connection.
NEW — The matching packet is either creating a new connection or is part of a two-way connection
not previously seen.
RELATED — The matching packet is starting a new connection related in some way to an existing
These connection states can be used in combination with one another by separating them with commas, such as -m state --state INVALID,NEW.
To specifically match a hardware MAC address of an Ethernet device, use the mac module, which
accepts --mac-source plus a MAC address as an option. To exclude a MAC address from a rule,
place an exclamation point (!) after the --mac-source match option.
To view other match options available through modules, see the iptables man page.
13.3.6. Target Options
Once a packet has matched a particular rule, the rule can direct the packet to a number of different
targets that decide its fate and, possibly, take additional actions, such as logging the action. Additionally, each chain has a default target, which is used if none of the rules on that chain match a packet or
if none of the rules which match the packet specify a target.
There are only a few standard targets available to decide what happens with the packet:
p user-defined-chain q — The name of a previously created and defined chain within this
table with rules that will be checked against this packet, in addition to any other rules in any other
chains that must be checked against this packet.
— Allows the packet to successfully move on to its destination or another chain.
— Drops the packet without responding to the requester. The system that sent the packet
is not notified of the failure. The packet is simply removed from the rule checking the chain and
— The packet is queued for handling by a user-space application.
• RETURN — Stops checking
the packet against rules in the current chain. If the packet with a RETURN
target matches a rule in a chain called from another chain, the packet is returned to the first chain to
resume rule checking where it left off. If the RETURN rule is used on a built-in chain and the packet
cannot move up to its previous chain, the default target for the current chain decides what action to
In addition to these standard targets, various other targets may be used with extensions called target
modules. For more information about match option modules, see Section
There are many extended target modules, most of which only apply to specific tables or situations. A
couple of the most popular target modules included by default in Red Hat Linux are:
Logs all packets that match this rule. Since the packets are logged by the kernel, the
/etc/syslog.conf file determines where these log entries are written. By default, they are
placed in the /var/log/messages file.
Various options can be used after the LOG target to specify the way in which logging occurs:
--log-level — Sets the priority level a of logging event. A list of priority levels can be found
in the syslog.conf man page.
--log-ip-options — Any options set in the header of a IP packet is logged.
Chapter 13. Firewalls and iptables
--log-prefix — Places a string before the log line when it is written. Accepts up to 29
characters after the --log-prefix option. This is useful for writing syslog filters for use in
conjunction with packet logging.
--log-tcp-options — Any options set in the header of a TCP packet is logged
--log-tcp-sequence — Writes the TCP sequence number for the packet in the log.
— Sends an error packet back to the system which sent the packet, and then drops the
packet. This target is useful if you would like to notify the system sending the matching packet of
the problem.
The REJECT target accepts a --reject-with r type s option which allows more detailed information to be sent with the error packet. The message port-unreachable is the default r type s
error given if no other option is used. For a full list of r type s options that can be used, see the
iptables man page.
Other target extensions, including several that are useful with masquerading using the nat table or
with packet alteration using the mangle table, can be found in the iptables man page.
13.3.7. Listing Options
The default list command, iptables -L, provides a very basic overview of the default filter table’s
current chains. Additional options provide more information and arrange that information in specific
— Display verbose output, such as the number of packets and bytes each chain has seen, the
number of packets and bytes each rule has matched, and which interfaces apply to a particular rule.
• -v
— Expands numbers into their exact values. On a busy system, the number of packets and
bytes seen by a particular chain or rule may be abbreviated using K (thousands), M (millions), and G
(billions) at the end of the number. This option forces the full number to be displayed.
• -x
— Displays IP addresses and port numbers in numeric format, rather than the default hostname
and network service format.
• -n
• --line-numbers —
Lists rules in each chain next to their numeric order in the chain. This option
is useful when attempting to delete a specific rule in a chain, or to locate where to insert a rule
within a chain.
13.4. Storing iptables Information
Rules created with the iptables command are stored in RAM only. If you restart your system after
setting up iptables rules, they will be lost. So in order for netfilter rules to persist through system
reboot, you need to save them to the /etc/sysconfig/iptables file.
To do this, type the /sbin/service iptables save command as the root user. This causes the
iptables init script to run the /sbin/iptables-save program and write the current iptables
configuration to the /etc/sysconfig/iptables file. This file should only be readable by root, so
your packet filtering rules are not viewable by average users.
The next time the system boots, the iptables init script will reapply the rules saved in
/etc/sysconfig/iptables by using the /sbin/iptables-restore command.
While it is always a good idea to test a new iptables rule before committing it to the
/etc/sysconfig/iptables file, it is possible to copy iptables rules into this file from another
Chapter 13. Firewalls and iptables
system’s version of this file. This allows you to quickly distribute sets of iptables rules to many
different machines.
If you distribute the /etc/sysconfig/iptables file to other machines, you must type
/sbin/service iptables restart for the new rules take effect.
13.5. Additional Resources
See the sources below for additional information on packet filtering with iptables.
13.5.1. Installed Documentation
The iptables man page contains a comprehensive description of various commands, parameters,
and other options.
13.5.2. Useful Websites
• — Contains assorted information about iptables, including an FAQ addressing specific problems you may experience and various helpful guides by Rusty Russell, the
Linux IP firewall maintainer. The HOWTO documents here cover subjects such as basic networking
concepts, 2.4 kernel packet filtering and NAT configurations.
• — A basic and general look at the
way packets move through the Linux kernel, plus an introduction to constructing simple iptables
• — This webpage contains
update-to-date links to a variety of packet filter resources.
Chapter 14.
Apache HTTP Server
The Apache HTTP Server is a robust, commercial-grade open source Web server developed by the
Apache Software Foundation ( Red Hat Linux 8.0 includes the Apache HTTP
Server version 2.0 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, however, outlines how to customize the Apache HTTP Server configuration file (/etc/httpd/conf/httpd.conf) for situations where the default configuration does not
suit your needs.
If you plan to use the graphical HTTP Configuration Tool (redhat-config-httpd), do not edit
the Apache HTTP Server’s configuration file. The HTTP Configuration Tool regenerates this file
whenever it is used.
If you need more information on the HTTP Configuration Tool, please see the chapter titled Apache
HTTP Server Configuration in the Official Red Hat Linux Customization Guide.
14.1. Apache HTTP Server 2.0
Red Hat Linux 8.0 ships with version 2.0 of the Apache HTTP Server. There are important differences
between version 2.0 and version 1.3 — which shipped with earlier releases of Red Hat Linux. This
section reviews some of the new features of Apache HTTP Server 2.0 and outlines important changes.
If you need to migrate a version 1.3 configuration file to the new format, refer to Section 14.2.
14.1.1. Features of Apache HTTP Server 2.0
The arrival of Apache HTTP Server 2.0 brings with it a number of new features. Among them are the
New Apache API — The Apache HTTP Server has a new, more powerful set of Application Programing Interfaces (APIs) for modules.
Modules built for Apache HTTP Server 1.3 will not work without being ported to the new API. If
you are unsure whether or not a particular module has been ported, consult with the package
maintainer before upgrading.
Filtering — Modules for Apache HTTP Server 2.0 have the ability to act as content filters. See
Section 14.2.4 for more on how filtering works.
IPv6 Support — Apache HTTP Server 2.0 supports next generation IP addressing.
Simplified Directives — A number of confusing directives have been removed while others have
been simplified. See Section 14.5 for more information about specific directives.
Multilingual Error Responses — When using Server Side Include (SSI) documents, customizable
error response pages can be delivered in multiple languages.
Multiprotocol Support — Apache HTTP Server 2.0 has the ability to serve multiple protocols.
Chapter 14. Apache HTTP Server
A more complete list complete list of changes can be found online at
14.1.2. Packaging Changes in Apache HTTP Server 2.0
Under Red Hat Linux 8.0 the Apache HTTP Server package has been renamed. Also, some related
packages have been renamed, deprecated, or incorporated into other packages.
Below is a list of the packaging changes:
The apache, apache-devel and apache-manual packages have been renamed as httpd,
httpd-devel and httpd-manual respectively.
The mod_dav package has been incorporated into the httpd package.
The mod_put and mod_roaming packages have been removed, since their functionality is a subset
of that provided by mod_dav.
The mod_auth_any and mod_bandwidth packages have been removed.
The version number for the mod_ssl package is now synchronized with the httpd package. This
means that the mod_ssl package for Apache HTTP Server 2.0 has a lower version number than
mod_ssl package for Apache HTTP Server 1.3.
14.1.3. File System Changes in Apache HTTP Server 2.0
The following changes to the file system layout occur when upgrading to Apache HTTP Server 2.0:
A new configuration directory, /etc/httpd/conf.d/, has been added. — This new directory
is used to store configuration files for individually packaged modules, such as mod_ssl,
mod_perl, and php. The server is instructed to load configuration files from this location by
the directive Include conf.d/*.conf within the Apache HTTP Server configuration file,
It is vital that this line be inserted when migrating an existing configuration.
The ab and logresolve programs have been moved. — These utility programs have been moved
from the /usr/sbin/ directory and into the /usr/bin/ directory. This will cause scripts with
absolute paths for these binaries to fail.
The dbmmanage command has been replaced. — The dbmmanage command has been replaced by
htdbm. See Section for more information.
The logrotate configuration file has has been renamed. — The logrotate configuration file
has been renamed from /etc/logrotate.d/apache to /etc/logrotate.d/httpd.
The next section will outline how to migrate an Apache HTTP Server 1.3 configuration to the new 2.0
14.2. Migrating Apache HTTP Server 1.3 Configuration Files
If you have upgraded your server from a previous version of Red Hat Linux upon which the Apache
HTTP Server was already installed, then the new stock configuration file for the Apache HTTP Server
2.0 package will be installed as /etc/httpd/conf/httpd.conf.rpmnew and your original version
1.3 httpd.conf will not be touched. It is, of course, entirely up to you whether you use the new
configuration file and migrate your old settings to it, or use your existing file as a base and modify
Chapter 14. Apache HTTP Server
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 version 2.0 are divided into
three sections. The goal of this guide is to suggest what is hopefully the easiest route.
If your httpd.conf is a modified version of the default Red Hat version and you have saved a copy
of the original then you may find it easiest to invoke the diff command, as in the following example:
diff -u httpd.conf.orig httpd.conf | less
This command will highlight the modifications you have made. If you do not have a copy of the
original file, extract it from an RPM package using the rpm2cpio and cpio commands, as in the
following example:
rpm2cpio apache-1.3.23-11.i386.rpm | cpio -i --make
Finally, it is useful to know that the Apache HTTP Server has a testing mode to check your configuration for errors. To use access it, type the following command:
apachectl configtest
14.2.1. 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 it uses. This section requires a large number of changes compared with
the others and it is therefore recommended that you base this section on the Apache HTTP Server 2.0
configuration file and migrate your old settings into it. Selecting Which Interfaces and Ports To Bind To
The BindAddress and Port directives no longer exist; their functionality is now provided by a more
flexible Listen directive.
If you had set Port 80 in your 1.3 version configuration file, you should change it to Listen 80
instead. If you had set Port to some value other than 80 then you must also append the port number
to the contents of your ServerName directive.
For example, the following is a sample Apache HTTP Server 1.3 directive:
Port 123
To migrate this setting to Apache HTTP Server 2.0, use the following structure:
Listen 123
For more on this topic, refer to the following documentation on the Apache Software Foundation’s
Chapter 14. Apache HTTP Server Server-pool Size Regulation
In Apache HTTP Server 2.0, the responsibility for accepting requests and dispatching child-processes
to handle them has been abstracted into 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 because an MPM module is responsible for basic request handling and dispatching. There are
three MPM modules that ship with version 2.0: prefork, worker, and perchild.
The original Apache HTTP Server 1.3 behavior has been moved into the prefork MPM. Currently
only the prefork MPM is available on Red Hat Linux, although other MPMs may be made available
at some later date.
The MPM supplied by default on Red Hat Linux is prefork which accepts the same directives as
Apache HTTP Server 1.3. The following directives may be migrated directly:
• StartServers
• MinSpareServers
• MaxSpareServers
• MaxClients
• MaxRequestsPerChild
For more on this topic, refer to the following documentation on the Apache Software Foundation’s
• Dynamic Shared Object (DSO) Support
There are many changes required here and it is highly recommended that anyone trying to modify an
Apache 1.3 configuration to suit Apache 2.0 (as opposed to migrating your changes into the Apache
2.0 configuration) copy this section from the stock Red Hat Linux Apache HTTP Server 2.0 configuration file.
If you do decide to try and modify your original file, please note that it is of paramount importance
that your httpd.conf contains the following directive:
Include conf.d/*.conf
Omission of this directive will result in the failure of all modules packaged in their own RPMs (such
as mod_perl, php, and mod_ssl).
Those who still 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 2.0 API allows modules
to specify their ordering, eliminating the need for these two directives.
The order of the LoadModule lines is no longer relevant.
Many modules have been added, removed, renamed, split up, or incorporated with each other.
Chapter 14. Apache HTTP Server
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 the relevant file in the /etc/httpd/conf.d/
• LoadModule
The various HAVE_XXX definitions are no longer defined. Other Global Environment Changes
The following directives have been removed from Apache HTTP Server 2.0’s configuration:
• ServerType — The Apache
this directive irrelevant.
HTTP Server can only be run as ServerType standalone making
and ResourceConfig — These directives have been removed since they mirror
the functionality of the Include directive. If you have AccessConfig and ResourceConfig
directives set then you need to replace these with Include directives.
• AccessConfig
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 httpd.conf, with the one corresponding to ResourceConfig
preceding the one corresponding to AccessConfig. If you were making use of the default values
you will need to include them explicitly as conf/srm.conf and conf/access.conf.
14.2.2. 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 t VirtualHost u definition. Values here also provide defaults for any t VirtualHost u containers you may define.
The directives used in this section have changed little between Apache HTTP Server 1.3 and version
2.0. If your main server configuration is heavily customized you may find it easier to modify your
existing configuration to suit Apache 2.0. Users with only lightly customized main server sections
should migrate their changes into the stock Apache 2.0 configuration. UserDir Mapping
The UserDir directive is used to enable URLs such as to map to a
subdirectory within the home directory of the user jim, such as /home/jim/public_html. A sideeffect of this feature allows a potential attacker to determine whether a given username is present on
the system, so 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
Chapter 14. Apache HTTP Server 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
• Directory Indexing
The deprecated FancyIndexing directive has now been removed. The same functionality is available
through the FancyIndexing option within the IndexOptions directive.
The new VersionSort option to the IndexOptions directive causes files containing version
numbers to be sorted in the natural way, so that httpd-2.0.6.tar would appear before
httpd-2.0.36.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
• 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
• Error Documents
To use a hard-coded message with the ErrorDocument directive, the message should be enclosed in
a pair of double quotes, rather than just preceded by a double quote as required in Apache 1.3.
To migrate an ErrorDocument setting to Apache HTTP Server 2.0, use the following structure:
ErrorDocument 404 "The document was not found"
Note that there is a trailing double quote in the example ErrorDocument directive above.
Chapter 14. Apache HTTP Server
For more on this topic, refer to the following documentation on the Apache Software Foundation’s
14.2.3. Virtual Hosts Configuration
The contents of all v VirtualHost w containers should be migrated in the same way as the main
server section as described in Section 14.2.2.
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.
For more on this topic, refer to the following documentation on the Apache Software Foundation’s
14.2.4. 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. CGI scripts, for example, can generate serverparsed 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 1.3, for example, a PHP script would be handled in its entirety by the PHP module.
Under Apache 2.0, the request is initially handled by the core module — which serves static files —
and is then filtered by the PHP module.
Exactly how to use this and all the other new features of Apache 2.0 for that matter is beyond the
scope of this document; however, the change has ramifications if you have used PATH_INFO, which
contains trailing path information after the true filename, in a document which is handled by a module
that is now implemented as a filter. The core module, which initially handles the request, does not by
default understand PATH_INFO and will return 404 Not Found errors for requests that contain such
information. You can use the AcceptPathInfo directive to coerce the core module into accepting
requests with PATH_INFO. Below is an example of this directive:
AcceptPathInfo on
For more on this topic, refer to the following documentation on the Apache Software Foundation’s
Chapter 14. Apache HTTP Server The mod_ssl Module
configuration for mod_ssl has been moved from httpd.conf into the file
/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 your httpd.conf as described in Section
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:
## SSL Virtual Host Context
VirtualHost _default_:443
# General setup for the virtual host
To migrate this setting to Apache HTTP Server 2.0, use the following structure:
## SSL Virtual Host Context
VirtualHost _default_:443
# General setup for the virtual host
For more on this topic, refer to the following documentation on the Apache Software Foundation’s
• The mod_proxy Module
Proxy access control statements are now placed inside a
proxy: .
block rather than a
The caching functionality of the old mod_proxy has been split out into the following three modules:
• mod_cache
• mod_disk_cache
• mod_file_cache
These generally use the same or similar directives as the older versions of the mod_proxy module.
For more on this topic, refer to the following documentation on the Apache Software Foundation’s
Chapter 14. Apache HTTP Server
185 The mod_include Module
The mod_include module is now implemented as a filter (see Section 14.2.4 for more on filters) and
is therefore enabled differently.
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 just as before, the Options +Includes directive is still required for the
section or in a .htaccess file.
Directory }
For more on this topic, refer to the following documentation on the Apache Software Foundation’s
• 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 in version 1.3, configuration files should
be adjusted by replacing AuthDBUserFile and AuthDBGroupFile with the mod_auth_dbm equivalents: AuthDBMUserFile and AuthDBMGroupFile. Also, you must add the directive AuthDBMType DB to indicate the type of database file in use.
The following example shows a sample mod_auth_db configuration for Apache 1.3:
Location /private/>
AuthType Basic
AuthName "My Private Files"
AuthDBUserFile /var/www/authdb
require valid-user
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 DB
require valid-user
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 14-1 shows how to migrate from a DBM-format database to htdbm format using dbmmanage.
Chapter 14. Apache HTTP Server
dbmmanage command
(Apache 1.3)
Equivalent htdbm
command (Apache 2.0)
Add user to database (using
given password)
dbmmanage authdb add
username password
htdbm -b -TDB authdb
username password
Add user to database (prompts
for password)
dbmmanage authdb
adduser username
htdbm -TDB authdb
Remove user from database
dbmmanage authdb delete
htdbm -x -TDB authdb
List users in database
dbmmanage authdb view
htdbm -l -TDB authdb
Verify a password
dbmmanage authdb check
htdbm -v -TDB authdb
Table 14-1. 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
• 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_perlto work, you
must have the statement Include conf.d/*.conf in your httpd.conf as described in Section
Occurances of Apache:: in your 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
This is the equivalent mod_perl for Apache HTTP Server 2.0:

Directory /var/www/perl
SetHandler perl-script
PerlModule ModPerl::Registry
PerlHandler ModPerl::Registry::handler
Options +ExecCGI
Most modules for mod_perl 1.x should work without modification with mod_perl 2.x. XS modules
will require recompilation and may possibly require minor Makefile modifications.
Chapter 14. Apache HTTP Server
187 The mod_python Module
The configuration for mod_python; has been moved from httpd.conf into the file
/etc/httpd/conf.d/python.conf. For this file to be loaded, and hence for mod_python; to
work, you must have the statement Include conf.d/*.conf in your httpd.conf as described in
Section PHP
configuration for PHP has been moved from httpd.conf into the file
/etc/httpd/conf.d/php.conf. For this file to be loaded, you must have the statement Include
conf.d/*.conf in your httpd.conf as described in Section
The PHP is now implemented as a filter and must therefore be enabled in a different manor. See
Section 14.2.4 for more about filters.
Under Apache HTTP Server 1.3, PHP was implemented using the following directives:
AddType application/x-httpd-php .php
AddType application/x-httpd-php-source .phps
Under Apache HTTP Server 2.0, use the following directives instead:
Files *.php
SetOutputFilter PHP
SetInputFilter PHP
In PHP 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. You may 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:
14.3. After Installation
After you have installed the httpd package, the Apache HTTP Server’s documentation is available
by installing the httpd-manual package and pointing a Web browser to http://localhost/manual/ or
you can browse the Apache documentation available on the Web at
The Apache HTTP Server’s documentation contains a full list and complete descriptions of all configuration options. For your convenience, this chapter provides short descriptions of the configuration
directives used by Apache HTTP Server 2.0.
The version of the Apache HTTP Server included with Red Hat Linux includes the ability to set
up secure Web servers using the strong SSL encryption provided by the mod_ssl and openssl
packages. As you look through the configuration files, be aware that it includes both a non-secure
and a secure Web server. The secure Web server runs as a virtual host, which is configured in the
/etc/httpd/conf.d/ssl.conf file. For more information about virtual hosts, see Section 14.8.
For information on configuring a secure server virtual host, Section 14.8.2. For information on setting
up an Apache HTTP Secure Server see the chapter titled Apache HTTP Secure Server Configuration
in the Official Red Hat Linux Customization Guide.
Chapter 14. Apache HTTP Server
Red Hat, Inc. does not ship FrontPage extensions as the Microsoft™ license prohibits the inclusion
of these extensions in a third party product. To find out more about FrontPage extensions and the
Apache HTTP Server, refer to
14.4. Starting and Stopping httpd
The the httpd RPM installs the /etc/rc.d/init.d/httpd Bourne script, which is accessed using
the /sbin/service command.
To start your server, as root type:
/sbin/service httpd start
If you are running the Apache HTTP Server as a secure server, you will be prompted to type your
To stop your server, type the command:
/sbin/service httpd stop
The command restart is a shorthand way of stopping and then starting your server. The restart
command explicitly stops and then starts your server. You will be prompted for your password if
you are running the Apache HTTP Server as a secure server. The restart command looks like the
/sbin/service httpd restart
If you just finished editing something in your httpd.conf file, you do not need to explicitly stop and
start your server. Instead, you can use the reload command.
If you are running the Apache HTTP Server as a secure server, you will not need to type your
password when using the reload option as the password will remain cached across reloads.
The reload command looks like the following example:
/sbin/service httpd reload
By default, the httpd process will not start automatically when your machine boots. You will
need to configure the httpd service to start up at boot time using an initscript utility, such as
/sbin/chkconfig, /sbin/ntsysv, or the Services Configuration Tool program.
Please refer to the chapter titled Controlling Access to Services in Official Red Hat Linux Customization Guide for more information regarding these tools.
Chapter 14. Apache HTTP Server
If you are running the Apache HTTP Server as a secure server, you will be prompted for the secure
server’s password after the machine boots, unless you generated a specific type of server key file.
For information about setting up an Apache HTTP Secure Server see the chapter titled Apache HTTP
Secure Server Configuration in the Official Red Hat Linux Customization Guide.
14.5. 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. Its default configuration will work for most situations, however you should become familiar some of the more important configuration options.
With the release of Apache HTTP Server 2.0, many configuration options have changed. If you need
to migrate a version 1.3 configuration file to the new format, refer to Section 14.2.
If you need to configure the Apache HTTP Server, edit httpd.conf and then either reload, restart,
or stop and start the httpd process. How to reload, stop and start the Apache HTTP Server is covered
in Section 14.4.
Before you edit httpd.conf, you should first copy the original file to something like httpd.confold, for example. By creating a backup, you can recover potential mistakes made while editing the
new configuration file.
If you do make a mistake and your Web server does not work correctly, first review what you have
recently edited in httpd.conf. Make sure 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 experience. If you have just experienced a problem,
however, the last entries in the error log should provide information about what happened.
The next sections provide short descriptions of the directives included in httpd.conf. These
descriptions are not exhaustive. If you need more information, refer to the Apache documentation
provided in HTML format at http://localhoast/manual/ or the Apache group documentation at
For more information about mod_ssl directives, refer to the documentation included in HTML
format at http://localhost/mod/mod_ssl.html or the Apache group mod_ssl documentation at
14.5.1. ServerRoot
The ServerRoot is the top-level directory which contains the server’s files. Both your secure and
non-secure servers are set to use a ServerRoot of "/etc/httpd".
14.5.2. LockFile
LockFile sets the path to the lockfile used when httpd is compiled with either
left at its default value.
Chapter 14. Apache HTTP Server
14.5.3. PidFile
PidFile names the file where the server records its process ID (pid). Your Web server is set to record
its pid in /var/run/
14.5.4. ScoreBoardFile
The ScoreBoardFile stores internal server process information, which is used for communication
between the parent server process and its child processes. Red Hat Linux uses shared memory to store
the ScoreBoardFile, the default of /etc/httpd/logs/apache_runtime_status is only used
as a fallback.
14.5.5. Timeout
Timeout defines, in seconds, the amount of time that your server will wait for receipts and transmissions during communications. Specifically, Timeout defines how long your server will wait to
receive a GET request, how long it will wait to receive TCP packets on a POST or PUT request and
how long it will wait between ACKs responding to TCP packets. Timeout is set to 300 seconds,
which is appropriate for most situations.
14.5.6. KeepAlive
KeepAlive sets whether your server will allow more than one request per connection (in other words,
it will allow persistent connections). KeepAlive can be used to prevent any one client from consum-
ing 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 will
slow down significantly. If Keepalive is enabled, it is a good idea to set the the KeepAliveTimeout low
(see Section 14.5.8) and monitor the servers /var/log/httpd/error_log. This log will report
when the server is running out of child processes.
14.5.7. MaxKeepAliveRequests
This directive sets the maximum number of requests allowed per persistent connection. The Apache
Group recommends a high setting, which will improve your server’s performance. MaxKeepAliveRequests is set to 100 by default, which should be appropriate for most situations.
14.5.8. KeepAliveTimeout
KeepAliveTimeout sets the number of seconds your server will wait after a request has been served
before it closes the connection. Once the server receives a request, the Timeout directive applies
14.5.9. MinSpareServers and MaxSpareServers
The Apache HTTP Server dynamically adapts to the perceived load by maintaining an appropriate
number of spare server processes based on the traffic. 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 MinSpareServers.
Chapter 14. Apache HTTP Server
Your server’s default MinSpareServers is 5; your server’s default MaxSpareServers is 20. These
default settings should be appropriate in most situations. You should not increase the MinSpareServers to a large number. Doing so will create a heavy processing load on your server even when
traffic is light.
14.5.10. StartServers
StartServers sets how many server processes are created upon startup. Since your Web server
dynamically kills and creates server processes based on traffic load, you will not need to change this
parameter. Your Web server is set to start eight server processes at startup.
14.5.11. MaxClients
MaxClients sets a limit on the total number of server processes, or simultaneously connected clients,
that can run at one time. You should keep MaxClients at a high number (your server’s default is set
to 150), because no one else will be allowed to connect once that number of simultaneously connected
clients is reached. You can not set MaxClients to higher than 256 without recompiling Apache. The
main reason for having MaxClients is to keep a runaway Web server from crashing your operating
14.5.12. 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 your server is 1000.
14.5.13. Listen
The Listen command identifies the ports on which your Web server will accept incoming requests.
Your Web server is set to listen to port 80 for non-secure Web communications and (in the virtual host
tags that define the secure server) to port 443 for secure Web communications.
If you set the Apache HTTP 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.
Listen can also be used to specify particular IP addresses over which the server will accept connec-
14.5.14. 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.
In order for the server to use individually packaged modules, such as mod_ssl, mod_perl, and php,
the following directive must be in Section 1: Global Environment of http.conf:
Include conf.d/*.conf
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14.5.15. LoadModule
LoadModule is used to load in Dynamic Shared Object (DSO) modules. More information on the the
Apache HTTP Server’s DSO support, including exactly how to use the LoadModule directive, can be
found in Section 14.7. Note, the load order of the modules is no longer important with Apache HTTP
Server 2.0. See Section for more information about Apache HTTP Server 2.0 DSO support.
14.5.16. IfDefine
The ƒ IfDefine „ and ƒ /IfDefine „ tags surround configuration directives that are applied if the
"test" stated in the ƒ IfDefine „ tag is true. The directives are ignored if the test is false.
The test in the ƒ 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 your Web server is started, the test is true and the directives contained in the
IfDefine tags are applied.
By default,
IfDefine HAVE_SSL tags surround the virtual host tags for your secure server.
IfDefine HAVE_SSL tags also surround the LoadModule and AddModule directives for the
14.5.17. ExtendedStatus
The ExtendedStatus directives controls whether Apache generates basic (off) or detailed server
status information (on), when the server-status handler is called. Server-status is called using
Location tags. More information on calling server-status is included in Section 14.5.65.
14.5.18. User
The User directive sets the userid used by the server to answer requests. User’s setting determines the
server’s access. Any files inaccessible to this user will also be inaccessible to your website’s visitors.
The default for User is apache.
The User should only have privileges so that it can access files which are supposed to be visible to
the outside world. The User is also the owner of any CGI processes spawned by the server. The User
should not be allowed to execute any code which is not intended to be in response to HTTP requests.
For security reasons, the Apache HTTP Server will refuse to run as User root. Using root as the
User will create large security holes for your Web server.
The parent httpd process first runs as root during normal operations but is then immediately handed
off to the apache user. The server must start as root because it needs to bind to a port below 1024
(the default port for secure Web communications is port 443; the default port for non-secure Web
communications is port 80). Ports below 1024 are reserved for system use, so they can not be used by
anyone but root. Once the server has attached itself to its port, however, it hands the process off to the
User before it accepts any connection requests.
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14.5.19. Group
The Group directive is similar to the User. The Group sets the group under which the server will
answer requests. The default Group is apache.
14.5.20. ServerAdmin
ServerAdmin should be the email address of the Web server’s administrator. This email address will
show up in error messages on server-generated webpages, so users can report a problem by sending
email to the server administrator. ServerAdmin is set by default to root@localhost.
Typically, a good way to set up ServerAdmin is to set it to Then
alias webmaster to the person responsible for the Web server in /etc/aliases. Finally, run
/usr/bin/newaliases to add the new alias.
14.5.21. ServerName
Use ServerName to set 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, you might want
to use when your server’s real name is actually Note
that the ServerName must be a valid Domain Name Service (DNS) name that you have the right to
use — do not just make something up. For example:
If you do specify a ServerName, be sure its IP address and server name pair are included in your
/etc/hosts file.
14.5.22. DocumentRoot
The DocumentRoot is the directory which contains most of the HTML files which will be 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
The server will look for the following file in the default directory:
If you want to change the DocumentRoot so that it is not shared by the secure and the non-secure
Web servers, see Section 14.8.
14.5.23. Directory
Directory /path/to/directory † and /Directory † tags are used to enclose a group of
configuration directives that are meant to apply only to that directory and all of its subdirectories. Any
directive which is applicable to a directory may be used within Directory † tags. File † tags
can be used in the same way, to apply to a specific file or files.
By default, very restrictive parameters are applied to the root directory, using the Options (see Section 14.5.24) and AllowOverride (see Section 14.5.25) directives. Under this configuration, any
directory on your system which needs more permissive settings has to be explicitly given those settings.
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Using Directory tags, the DocumentRoot is defined to have less rigid parameters, so that HTTP
requests can be served from it.
The cgi-bin directory is set up to allow the execution of CGI scripts, with the ExecCGI option. If
you need to execute a CGI script in another directory, you will need to set ExecCGI for that directory.
For example, if your cgi-bin is /var/www/cgi-bin, but you want to execute CGI scripts from
within /home/my_cgi_directory, add an ExecCGI directive to a set of Directory directives like
the following to your httpd.conf file:
Directory /home/my_cgi_directory
Options +ExecCGI
To allow CGI script execution in /home/my_cgi_directory, you will need to take a few extra
steps besides setting ExecCGI. You will also need to have the AddHandler directive uncommented
to identify files with the .cgi extension as CGI scripts. See Section 14.5.59 for instructions on setting
AddHandler. Permissions for CGI scripts, and the entire path to the scripts, must be set to 0755.
14.5.24. 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 set to only FollowSymLinks. No features are enabled, except that the server is allowed to follow symbolic links in
the root directory.
By default, in your DocumentRoot directory, Options is set to include Indexes, Includes and
FollowSymLinks. Indexes permits the server to generate a directory listing for a directory if no
DirectoryIndex (for example, index.html) is specified. Includes means that server-side includes are permitted. FollowSymLinks allows the server to follow symbolic links in that directory.
You will also need to include Options statements for directories within virtual hosts directives, if
you want your virtual hosts to recognize those Options.
For example, server side includes are already enabled inside the /var/www/html directory, because
of the Options Includes line within the Directory "/var/www/html" ˆ directives section.
However, if you want a virtual host to recognize server side includes, you will need to include a section
like the following within your virtual host’s tags:
Directory /var/www/html
Options Includes
14.5.25. AllowOverride
The AllowOverride directive sets whether or not 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.
14.5.26. Order
The Order directive simply controls the order in which allow and deny directives are evaluated.
Your server is configured to evaluate the Allow directives before the Deny directives for your DocumentRoot directory.
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14.5.27. Allow
Allow specifies which requester can access a given directory. The requester can be all, a domain
name, an IP address, a partial IP address, a network/netmask pair, and so on. Your DocumentRoot
directory is configured to Allow requests from all meaning everyone has access.
14.5.28. Deny
Deny works just like Allow, but you are specifying who is denied access. Your DocumentRoot is not
configured to Deny requests from anyone by default.
14.5.29. UserDir
UserDir is the name of the subdirectory within each user’s home directory where they should place
personal HTML files which are to be served by the Web server.
By default, the subdirectory is public_html. For example, the server might receive the following
The server would look for the file:
In the above example, /home/username/ is the user’s home directory (note that the default path to
users’ home directories may be different on your system).
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 will also work). Files that will be served in users’ public_html directories must
be set to at least 0644.
This directive is set to disable by default.
14.5.30. 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://your_domain/this_directory/, they will get either the
DirectoryIndex page if it exists, or a server-generated directory list. The default for DirectoryIndex is index.html index.htm index.shtml index.php index.php4 index.php3 index.cgi. The server will try to find any one of these files, and will return the first one it finds. If
it does not find any of these files and Options Indexes is set for that directory, the server will
generate and return a listing, in HTML format, of the subdirectories and files in the directory.
14.5.31. AccessFileName
AccessFileName names the file which the server should use for access control information in each
directory. By default, your Web server is set to use .htaccess, if it exists, for access control information in each directory.
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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14.5.32. CacheNegotiatedDocs
By default, your 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 you set CacheNegotiatedDocs to on, you are disabling that function and proxy servers will be
allowed to cache documents.
14.5.33. UseCanonicalName
UseCanonicalName is set by default to on. UseCanonicalName allows the server to construct an
URL that references itself, using ServerName and Port. When the server refers to itself in response
to requests from clients, it uses this URL. If you set UseCanonicalName to off, the server will
instead use the value that came in the request from the client to refer to itself.
14.5.34. TypesConfig
TypesConfig names the file which sets the default list of MIME type mappings (filename 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
For more information about AddType, refer to Section 14.5.58.
14.5.35. DefaultType
DefaultType sets a default content type for the Web server to use for documents whose MIME types
can not be determined. Your Web server defaults to assume a plain text content type for any file with
an indeterminate content type.
14.5.36. IfModule
IfModule Š and /IfModule Š tags surround directives that are conditional. The directives contained within the IfModule tags are processed under one of two conditions. The directives are pro‰
cessed if the module contained within the starting IfModule Š tag is loaded in to httpd. Or, if an
"!" (an exclamation point) is included before the module name, the directives are processed only if the
module in the starting IfModule Š tag is not compiled in.
The mod_mime_magic.c file is included in these IfModule tags. The mod_mime_magic module
can be compared to the UNIX file command, which looks at a few bytes of a file’s contents, then
uses "magic numbers" and other hints in order to figure out the MIME type of the file.
If the mod_mime_magic module is compiled in to the Apache HTTP Server, these IfModule tags
tell the mod_mime_magic module where the hints definition file is: /usr/share/magic in this case.
The mod_mime_magic module is not compiled in by default. If you would like to use it, see Section
14.7, for instructions on how to add modules to your server.
14.5.37. HostnameLookups
HostnameLookups can be set to on, off or double. If you allow HostnameLookups (by setting
it to on), your server will automatically resolve the IP address for each connection which requests
a document from your Web server. Resolving the IP address means that your server will make one
or more connections to the DNS in order to find out the hostname that corresponds to a particular IP
address. If you set HostnameLookups to double, your server will perform a double-reverse DNS. In
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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 HostnameLookups set to off, because the DNS requests add a load to
your server and may slow it down. If your server is busy, the effects of HostnameLookups will be
HostnameLookups are also an issue for the Internet as a whole. All of the individual connections
made to look up each hostname add up. Therefore, for your own Web server’s benefit, as well as for
the good of the Internet as a whole, you should leave HostnameLookups set to off.
If you like to see the hostnames in your log files, consider running one of the many log analyzer tools
that perform the DNS lookups more efficiently and in bulk when you come to rotate your log files.
14.5.38. ErrorLog
ErrorLog names the file where server errors are logged. As this directive indicates, the error log file
for your Web server is /var/log/httpd/error_log.
The error log is a good place to look if your Web server generates any errors or fails, and you are not
sure what happened.
14.5.39. LogLevel
LogLevel sets how verbose the error messages in the error logs will be. 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.
14.5.40. LogFormat
The LogFormat directives in your httpd.conf file set up a format for the messages in your access
log. The actual LogFormat used will depend on the settings given in the CustomLog directive (see
Section 14.5.41).
14.5.41. CustomLog
CustomLog identifies the log file and the log file format. In your Web server’s default
configuration, CustomLog defines the log file in which accesses to your Web server are recorded:
/var/log/httpd/access_log. You will need to know the location of this file if you want to
generate any access-based server performance statistics for your Web server.
CustomLog also sets the log file format to combined. The common logfile format looks like this:
remotehost rfc931 authuser [date] "request" status bytes referer user-agent
The remote hostname. If the hostname is not available from DNS, or if HostnameLookups is
set to Off, then remotehost will be the IP address of the remote host.
Not used. You will see a - in the log file in its place.
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If authentication was required, this is the username with which the user identified herself. Usually, this is not used, so you will see a - in its place.
The date and time of the request.
The request string exactly as it came from the browser or client.
The HTTP status code which was returned to the browser or client.
The size of the document.
This can give the URL of the webpage which linked to the the current request.
This gives the name of the browser or client making the request.
14.5.42. ServerSignature
The ServerSignature directive adds a line containing the Apache server version and the ServerName of the serving host to any server-generated documents (for example, error messages sent back
to clients). ServerSignature is set to on by default. You can change it to off, so no signature line
will be added, or you can change it to EMail. EMail will add a mailto:ServerAdmin HTML tag
to the signature line.
14.5.43. Alias
The Alias setting allows directories to be outside the DocumentRoot directory and yet still accessible to the Web server. Any URL ending in the alias will automatically resolve to the alias’ path. By
default, one alias is already set up. An icons directory can be accessed by the Web server, but the
directory is not in the DocumentRoot. The icons directory, an alias, is actually /var/www/icons/,
not /var/www/html/icons/.
14.5.44. ScriptAlias
The ScriptAlias setting defines where CGI scripts (or other types of scripts) can be found. Generally, you do not want to leave CGI scripts within the DocumentRoot. If CGI scripts are in DocumentRoot, they could potentially be viewed as text documents. Even if you do not care if people can see
and then use your CGI scripts, revealing how they work creates opportunities for unscrupulous people
to exploit any security holes in the script and may create a security risk for your server. By default,
the cgi-bin directory is a ScriptAlias of /cgi-bin/ and is actually located in /var/www/cgibin/.
Your /var/www/cgi-bin directory has Options ExecCGI set, meaning that execution of CGI
scripts is permitted within that directory.
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See Section 14.5.59 and Section 14.5.23 for instructions on how to execute CGI scripts in directories
other than the cgi-bin.
14.5.45. Redirect
When a webpage is moved, Redirect can be used to map the old URL to a new URL. The format is
as follows:
Redirect /path/foo.html http://new_domain/path/foo.html
So, if an HTTP request is received for a page which used to be found at
http://your_domain/path/foo.html, the server will send back the new URL
(http://new_domain/path/foo.html) to the client, which should attempt to fetch the document
from the new URL.
For more advanced Redirection you can use the mod_rewrite module included with the server.
14.5.46. IndexOptions
IndexOptions controls the appearance of server generated directing listings, by adding icons, file
descriptions, and so on. If Options Indexes is set (see Section 14.5.24), your Web server may
generate a directory listening when your Web server receives an HTTP request like the following:
First, your Web server looks in that directory for a file from the list after the DirectoryIndex
directive (usually, index.html). If your Web server does not find one of those files, it creates an
HTML directory listing of the subdirectories and files in the directory. You can modify the appearance
of this directory listing using certain directives in httpd.conf, including IndexOptions.
Your default configuration sets FancyIndexing on. If FancyIndexing is turned on, clicking on the
column headers in the directory listing will sort the order of the display by that header. Another click
on the same header will switch from ascending to descending order and back. FancyIndexing also
shows different icons for different files, depending upon file extensions. If you use the AddDescription directive and turn FancyIndexing on, then a short description of a file will be included in the
server generated directory listing.
IndexOptions has a number of other parameters which can be set to control the appearance of server
generated directories. Parameters include IconHeight and IconWidth, to make the server include
HTML HEIGHT and WIDTH tags for the icons in server generated webpages; IconsAreLinks, for
making the icons act as part of the HTML link anchor along with the filename and others.
14.5.47. AddIconByEncoding
This directive names icons which will be displayed by files with MIME encoding, in server generated
directory listings. For example, by default, your Web server shows the compressed.gif icon next
to MIME encoded x-compress and x-gzip files in server generated directory listings.
14.5.48. AddIconByType
This directive names icons which will be displayed next to files with MIME types in server generated
directory listings. For example, your server is set to show the icon text.gif next to files with a
mime-type of "text," in server generated directory listings.
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14.5.49. AddIcon
AddIcon tells the server which icon to show in server generated directory listings for certain file types
or for files with certain extensions. For example, your Web server is set to show the icon binary.gif
for files with .bin or .exe extensions.
14.5.50. DefaultIcon
DefaultIcon names the icon to show in server generated directory listings for files which have no
other icon specified. The unknown.gif image file is the DefaultIcon for those files by default.
14.5.51. AddDescription
You can use AddDescription to show text that you specify for certain files, in server generated
directory listings (you will also need to enable FancyIndexing as an IndexOptions). You can
name specific files, wildcard expressions or file extensions to specify the files which this directive
should apply to. For example, you could use the following line:
AddDescription "A file that ends in .ni" .ni
In server generated directory listings, all files with extensions of .ni would have the description A
file that ends in .ni after the filename. Note that you will also need FancyIndexing turned
14.5.52. ReadmeName
ReadmeName names the file which (if it exists in the directory) will be appended to the end of server
generated directory listings. The Web server will first try to include the file as an HTML document
and then try to include it as plain text. By default, ReadmeName is set to README.
14.5.53. HeaderName
HeaderName names the file which (if it exists in the directory) will be prepended to the start of server
generated directory listings. Like ReadmeName, the server will try to include it as an HTML document
if possible, or in plain text if not.
14.5.54. IndexIgnore
IndexIgnore lists file extensions, partial filenames, wildcard expressions or full filenames. The Web
server will not include any files which match any of those parameters in server generated directory
14.5.55. AddEncoding
AddEncoding names filename extensions which should specify a particular encoding type. AddEncoding can also be used to instruct some browsers (not all) to uncompress certain files as they are
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14.5.56. AddLanguage
AddLanguage associates filename extensions with specific content languages. This directive is mostly
useful for content negotiation, when the server returns one of several documents based on the client’s
language preference as set in their browser.
14.5.57. LanguagePriority
LanguagePriority allows you to set precedence for different languages in which to serve files,
which will be in effect if the client expressed no preference for language in their browser.
14.5.58. AddType
Use the AddType directive to define MIME type and file extension pairs. For example, if you are
using PHP4, your Web server is using the AddType directive to make your Web server recognize files
with PHP extensions (.php4, .php3 .phtml .php) as PHP MIME types. The following directive
tells the Apache HTTP Server to recognize the .shtml file extension:
AddType text/html .shtml
AddHandler server-parsed .shtml
You will need to include the above line within the virtual host tags for any virtual hosts which should
allow server side includes.
14.5.59. AddHandler
AddHandler maps file extensions to specific handlers. For example, the cgi-script handler can
be used matched with the extension .cgi to automatically treat a file ending with .cgi as a CGI
script. This will work, even for files outside of the ScriptAlias directory, as long as you follow the
instructions provided here.
You have a CGI AddHandler line in your httpd.conf file:
AddHandler cgi-script .cgi
You will have to uncomment the line. Then Apache will execute CGI scripts for files ending in .cgi,
even if they are outside of the ScriptAlias, which is set by default to locate your /cgi-bin/
directory in /var/www/cgi-bin/.
You will also need to set ExecCGI as an Options for any directory containing a CGI script. See
Section 14.5.23 for more information about setting ExecCGI for a directory. Additionally, you will
need to make sure the permissions are set correctly for the CGI scripts and the directories containing
CGI scripts. CGI scripts and the entire directory path to the scripts must be set to 0755.
You will need to add the same AddHandler line to your VirtualHost setup, if you are using virtual
hosts and you want them to also recognize CGI scripts outside the ScriptAlias.
In addition to CGI scripts, your Web server also uses AddHandler to process server-parsed HTML
and imagemap files.
14.5.60. Action
Action allows you to specify a MIME content type and CGI script pair, so that whenever a file of
that media type is requested, a particular CGI script will be executed.
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14.5.61. MetaDir
MetaDir specifies the name of a directory where your Web server should look for files containing
meta information (extra HTTP headers) to include when serving documents.
14.5.62. MetaSuffix
MetaSuffix specifies the filename suffix for the file that contains meta information (extra HTTP
headers), which should be located in the MetaDir directory.
14.5.63. ErrorDocument
By default, in the event of a problem or error, your Web server outputs a simple and usually cryptic
error message back to the requesting client. Instead of using the default, you can use ErrorDocument
to configure your Web server so that it outputs a customized message or redirects the client to a local
or external URL. The ErrorDocument directive simply associates a HTTP response code with a
message or a URL which will be sent back to the client.
You must enclose the error message in a pair of double quotes for it to be valid.
14.5.64. BrowserMatch
The BrowserMatch directive allows your server to define environment variables and take appropriate actions based on the User-Agent HTTP header field — which identifies the client’s browser. By
default, your 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.
14.5.65. Location
tags allow you to specify access control based on URL.
If you want to allow people connecting from your domain to see server status reports, you should
uncomment the next section of directives:
# Location /server-status
SetHandler server-status
Order deny,allow
Deny from all
Allow from
# /Location
You must replace with your second level domain name.
If you want to provide server configuration reports (including installed modules and configuration
directives) to requests from inside your domain, you will need to uncomment the following lines:
# Location /server-info
SetHandler server-info
Order deny,allow
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Deny from all
Allow from
# /Location
Again, you must fill in
14.5.66. ProxyRequests
If you uncomment the IfModule tags surrounding the ProxyRequests directives, your Apache
HTTP Server will also function as a proxy server. You will also need to load the mod_proxy module.
For instructions on how to load in modules, see Section 14.7.
14.5.67. ProxyVia
The ProxyVia command controls whether or not an HTTP Via: header line is sent along with requests
or replies which go through the Apache proxy server. The Via: header will show the hostname if
ProxyVia is set to On, the hostname and the Apache HTTP Server version for Full, any Via: lines
will be passed along unchanged for Off, and Via: lines will be removed for Block.
14.5.68. Cache Directives
A number of cache directives are commented out in the proxy IfModule tags mentioned above. If
you are using the proxy server functionality and you want to also enable the proxy cache, you should
uncomment the cache directives as described. The default settings for your cache directives should be
appropriate for most configurations.
CacheRoot sets the name of the directory which will contain cached files. The default CacheRoot is
CacheSize sets how much space the cache can use, in KB. The default CacheSize is 5 KB.
CacheGcInterval sets a number of hours. After that number of hours, files in the cache will be
deleted if the cache is using more space than allowed by CacheSize. The default for CacheGcInterval is four hours.
Cached HTML documents will be retained (without a reload from the originating Web server) in the
cache for a maximum number of hours set by CacheMaxExpire. The default is 24 hours.
The CacheLastModifiedFactor affects 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.
CacheDefaultExpire is 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 one hour.
Any document that is retrieved from a host or domain that matches one set in NoCache will not be
cached. If you know of hosts or domains from which you do not want to cache documents, uncomment
NoCache and set their domains or hostnames here.
14.5.69. NameVirtualHost
You will need to use the NameVirtualHost directive for the IP address and port number, if necessary,
of any name-based virtual hosts you are setting up. The name-based virtual hosts configuration is used
when you want to set up different virtual hosts for different domains, but you do not have or do not
want to use different IP addresses for all of the different domain names for which your Web server
serves documents.
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Any name-based virtual hosts you set up will only work with non-secure HTTP connections as you
cannot use name-based virtual hosts with a secure server. If you need to use virtual hosts with a
secure server, you will need to use IP address-based virtual hosts.
If you are using name-based virtual hosts, uncomment the NameVirtualHost configuration directive
and add the correct IP address for your server after NameVirtualHost. Then add more information
about the different domains using the VirtualHost tags which surround the ServerName for each
virtual host, plus any other configuration directives which are only applicable to that virtual host.
14.5.70. VirtualHost
VirtualHost  and /VirtualHost  tags surround any configuration directives which are intended to apply to a virtual host. Most configuration directives can be used within virtual host tags,
and then they only apply to that particular virtual host.
A set of commented out VirtualHost tags surround some example configuration directives and
placeholders for the information you would need to fill in to set up a virtual host. Please see Section
14.8, for more information about virtual hosts.
All SSL virtual hosts contexts have been moved into the file /etc/httpd/conf.d/ssl.conf.
14.5.71. SetEnvIf
The Apache configuration directive SetEnvIf can be used to set environment variables based on
headers in the request. In the supplied httpd.conf file, it is used to disable HTTP keepalive and to
allow SSL to close the connection without a close notify alert from the client browser. This setting is
necessary for certain browsers that do not reliably shut down the SSL connection.
14.5.72. SSL Configuration Directives
The SSL directives in /etc/httpd/conf.d/ssl.conf file can be configured to enable secure Web
communications using SSL and TLS.
http://localhost/manual/mod/mod_ssl.html or the Apache group mod_ssl documentation at
For information about setting up an Apache HTTP Secure Server see the chapter titled Apache HTTP
Secure Server Configuration in the Official Red Hat Linux Customization Guide.
Do not modify your SSL directives unless you are absolutely sure about what you are doing. In most
cases, the SSL directives are configured appropriately as installed.
Chapter 14. Apache HTTP Server
14.6. Default Modules
The Apache HTTP Server is distributed with a number of modules. By default the following modules
are installed and enabled with the httpd package on Red Hat Linux:
Additionally, the following modules are available by installing additional packages:
14.7. Adding Modules to Your Server
Since the Apache HTTP Server 2.0 supports DSOs, you can easily load Apache modules or compile
in your own modules. DSO support also means modules are dynamically loaded at runtime only as
necessary, conserving memory resources on the server.
The Apache Group provides complete DSO Documentation at After installation of the http-manual package, you can also check
http://localhost/manual/mod/ for documentation on Apache modules.
For the Apache HTTP Server to use a dynamically shared module, that module must have a LoadModule line in httpd.conf.
Chapter 14. Apache HTTP Server
A sample LoadModule line looks like this:
LoadModule access_module modules/
If you add or delete modules from http.conf, you must reload or restart Apache, as covered in
Section 14.4.
If you have your own module, you can add it to the httpd.conf file so that it is compiled in and
loaded as a DSO. For this you need have the httpd-devel package installed because it contains the
include files, the header files, and the APache eXtenSion (apxs) application. The apxs command uses
the include files and the header files to compile Apaches modules.
If you have written your own module or are using a third party module, you should be able to use
apxs to compile your module sources outside the Apache source tree, without needing to tweak any
compiler or linker flags. If you need more information on apxs, please see the Apache documentation
at and the apxs, man page.
Once you have compiled the module, put your module in the /usr/lib/httpd/ directory. Then add
a LoadModule line to httpd.conf. After the LoadModule list in httpd.conf, add a line for the
shared object file for your module like the following:
Note that you will need to change the name of
foo_module ”
“ ”
as appro-
Once you have completed these steps, stop and start your Web server as outlined in Section 14.4. If
you have done everything correctly and your module is correctly coded, the Web server should find
your module and load it.
14.8. Using Virtual Hosts
You can use the Apache HTTP Server’s virtual hosts capability to run different servers for different
IP addresses, different host names, or different ports on the same server. If you are interested in using
virtual hosts, complete information is provided in the Apache documentation on your machine or on
the Web at
You cannot use name-based virtual hosts with your Red Hat Linux Advanced Server, because the
SSL handshake occurs before the HTTP request which identifies the appropriate name-based virtual
host. If you want to use name-based virtual hosts, they will only work with your non-secure Web
Virtual hosts are configured within the httpd.conf file, as described in Section 14.5. Please review
that section before you start to change the virtual hosts configuration on your machine.
14.8.1. The Secure Web Server Virtual Host
The default configuration of your Web server runs a non-secure and a secure server. Both servers use
the same IP address and host name, but they listen on different ports, and the secure server is a virtual
host configured. This configuration enables you to serve both secure and non-secure documents in an
manner. Setting up the secure HTTP transmission is very resource intensive, so generally you will be
able to serve far fewer pages per second with a secure server. You need to consider this when you
decide what information to include on the secure server and the non-secure server.
Chapter 14. Apache HTTP Server
The configuration directives for your secure server are contained within virtual host tags in the
/etc/httpd/conf.d/ssl.conf file. If you need to change anything about the configuration of
your secure server, you will need to change the configuration directives inside the virtual host tags.
By default, both the secure and the non-secure Web servers share the same DocumentRoot. To change
the DocumentRoot so that it is no longer shared by both the secure server and the non-secure server,
change one of the DocumentRoot directives. The DocumentRoot either inside or outside of the
virtual host tags in httpd.conf defines the DocumentRoot for the non-secure Web server. The
DocumentRoot within the virtual host tags in conf.d/ssl.conf define the document root for the
secure server.
The secure the Apache HTTP Server server listens on port 443, while your non-secure Web server
listens on port 80. To stop the non-secure Web server from accepting connections find the line which
Then comment out any line in httpd.conf which reads Listen 80.
14.8.2. Setting Up Virtual Hosts
To create a virtual host, you will need to alter the virtual host lines, provided as an example in
httpd.conf or create your own virtual host section.
The virtual host example lines read as follows:
# VirtualHost *
DocumentRoot /www/docs/
ErrorLog logs/
CustomLog logs/ common
# /VirtualHost
Uncomment all of the lines, and add the correct information for the virtual host.
In the first line, change * to your server’s IP address. Change the ServerName to a valid DNS name
to use for the virtual host.
You will also need to uncomment one of the NameVirtualHost lines below:
NameVirtualHost *
Next change the IP address to the IP address, and port if necessary, for the virtual host. When finished
it will look similar to the following example:
If you set up a virtual host and want it to listen on a non-default port, you will need to set up a virtual
host for that port and add a Listen directive for corresponding to that port.
Then add the port number to the first line of the virtual host configuration as in the following example:
VirtualHost ip_address_of_your_server:12331
This line would create a virtual host that listens on port 12331.
You must restart httpd to start a new virtual host. See Section 14.4 for instructions on how to start
and stop httpd.
Comprehensive information about creating and configuring both name-based and IP address-based
virtual hosts is provided on the Web at
Chapter 14. Apache HTTP Server
14.9. Additional Resources
To learn more about the Apache HTTP Server, refer to the following resources.
14.9.1. Useful Websites
• — The official website for the Apache Web server with documentation on
all the directives and default modules.
• — The official website for mod_ssl.
• — A comprehensive online online weekly about all things Apache.
14.9.2. Related Books
Apache Desktop Reference by Ralf S. Engelschall; Addison Wesley
Written by ASF member and mod_ssl author Ralf Engelschall, the Apache Desktop Reference provides a concise but comprehensive reference guide to using Apache at compilation, configuration,
and run time. This book is available online at
Professional Apache by Peter Wainwright; Wrox Press Ltd
Professional Apache is from Wrox Press Ltd’s "Programmer to Programmer" series and is aimed at
both experienced and novice Web server administrators.
Administering Apache by Mark Allan Arnold; Osborne Media Group
Apache Server Unleashed by Richard Bowen, et al; SAMS BOOKS
Apache Pocket Reference by Andrew Ford, Gigi Estabrook; O’Reilly
This book is targeted at Internet Service Providers who aim to provide more secure services.
An encyclopedic source for Apache.
This is the latest addition to the O’Reilly Pocket Reference series.
Chapter 15.
Email is one of the most widely used services on the Internet. Red Hat Linux offers many ways to
serve and access email, whether you are a desktop user or a system administrator.
This chapter looks at popular email protocols that are in use today and some of the programs designed
to deal with email.
15.1. Protocols
Email, like other network services, uses a variety of protocols. These protocols allow different machines, often running different operating systems and using different email programs, to communicate
with one another via email.
The following protocols are the most commonly used to transfer email from system to system.
15.1.1. IMAP
The Internet Message Access Protocol (IMAP) is a method used by email client applications to access
remotely stored messages. When using IMAP email messages remain on the remote mail server where
users can read or delete them and create, rename, or delete mailboxes to store the email.
In addition, IMAP is fully compatible with important Internet messaging standards, such as the Multipurpose Internet Mail Extensions (MIME), to allow for receiving attachments. Many email clients
that use IMAP can also be configured to cache a copy of the messages locally, so that you can browse
previously read messages when you are not directly connected to the IMAP server.
IMAP is primarily used by those who access their email using multiple machines. Also, users that
connect to the Internet or a private network via a low-bandwidth connection often use IMAP because
only the email header information is pulled off at first. This allows them to defer the downloading of
messages containing large attachments until a time when their limited bandwidth is not in use. In the
same way, email that the user does not want can be deleted without viewing the message body, saving
the need to even download it through their network connection.
The Request for Comment (RFC) documents that cover IMAP contain the assorted details and specifics
about how the protocol is designed to work. RFC-1730 first defined the way IMAP is used in version
4, but RFC-2060 discusses the IMAP implementation used with many IMAP servers, called version
The imap package in Red Hat Linux allows users to connect to your system and receive
their email using IMAP. Secure IMAP connections are supported through Secure Socket
Layer (SSL) technology built into the /usr/sbin/imapd daemon, allowing it to use the
/usr/share/ssl/certs/imapd.pem certificate file. The stunnel program is not required to
provide SSL-encryption for IMAP connections, though it can be used. See Section 15.6.2 for more
information concerning these two encryption options.
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 at
15.1.2. POP
The Post Office Protocol (POP) allows email clients to download email from remote servers and save
those messages on their local machine. Most POP email clients are automatically configured to delete
Chapter 15. Email
the message on the email server after it has been successfully transferred to the client’s system, though
this can usually be changed.
To connect to a POP server, the email client opens a TCP connection to port 110 on the server.
At the time the connection is made, the POP server sends the POP client a greeting, after which
the two machines send each other commands and responses specified in the protocol. As part of this
communication, the POP client is asked to authenticate itself in what is called the Authentication State,
where the user’s username and password are sent to the POP server. If authentication is successful,
then the POP client moves on to the Transaction State, where commands like LIST, RETR, and DELE
can be used to list, download, and delete the messages from the server. Messages set to be deleted
are not actually removed from the server until the POP client sends the QUIT command to end the
session. At this point, the POP server enters the Update State, where it deletes the flagged messages
and cleans up any resources remaining from the session.
POP is a much simpler protocol than IMAP, since fewer commands are sent between the client and
the server.
POP works best for users who only have one system to read email with since they download their
messages to that machine. POP also works well if you do not have a constant connection to the
Internet or the network containing your mail server.
Several RFCs cover the POP protocol, but RFC-1939 defines the basic outline of POP3, the current
Occasionally, you may run into lesser-used POP protocol variants:
APOP — POP3 with MDS authentication, where an encoded hash of your password is sent from
the email client to the server rather then sending the password in plain text.
KPOP — POP3 with Kerberos authentication. See Chapter 10 for more information concerning
Kerberos authentication.
RPOP — POP3 with RPOP authentication, which uses an ID issued per user, similar to a password,
to authenticate POP requests. However, this ID is not encrypted, so RPOP is no more secure than
standard POP.
Many POP servers, clients, and assorted other applications are available with Red Hat Linux. If you
prefer a graphical email client, either Mozilla Mail or Ximian Evolution is a good choice. In addition,
other email utilities, such as Fetchmail, can retrieve email via POP. If you are using your Red Hat
Linux system as a mail server, the imap package installs POP2 (ipop2) and POP3 (ipop3) daemons
in the /usr/sbin/ directory.
15.1.3. SMTP
While the IMAP and POP protocols involve allowing a user to be able to receive their email, the Simple
Mail Transfer Protocol (SMTP) is used to send email. Outgoing email uses SMTP to move from the
client’s machine to the server, on its way toward its final destination. Email servers attempting to move
a message between one another also use SMTP to communicate.
SMTP uses port 25 on the server for communication. A basic SMTP exchange begins with the connecting system issuing a MAIL From: ™ email-address š command to initiate exchange. The receiving system responds with a 250 message to acknowledge receipt of the first command. Then, the
connecting system hands the email addresses to receive the message to the receiving system, followed
by a DATA message. This tells the receiving system that the next part of the communication will be the
actual body of the email message. When the connecting system is finished with the email message, it
places a single dot (.) on a line. At that point, the message is considered sent.
SMTP also handles cases where email needs to be forwarded between systems, when the receiving
system knows where to send the message. The protocol can verify that certain users are served by a
Chapter 15. Email
particular mail server using the VRFY command or expand a mailing list using the EXPN command.
Email can also be relayed between two SMTP servers, if both systems permit such activity.
Unlike IMAP and POP, the SMTP protocol does not require authentication. This means that SMTP
servers can allow anyone on the Internet to use your system to send or relay mail to large lists of
recipients. It is this characteristic of SMTP that makes spam possible. Modern SMTP applications
attempt to minimize this behavior by restricting relaying and allowing only known hosts to send
RFC-821 outlines the basic behavior of SMTP, but several SMTP extensions, made possible by RFC1869, have added additional functionality to SMTP over the years by making new commands available. By initiating a conversation with an SMTP server with an EHLO command rather than HELO,
the connecting server can identify itself as one that supports SMTP extensions. The receiving server
answers with a 250 line containing the various SMTP extensions it supports. Then, the connecting
server can use the supported extensions as it wishes to accomplish the goals of the communication.
One extension adds SMTP Authentication through the AUTH command as outlined in RFC-2554.
Another widely used SMTP extension, detailed in RFC-2034, discusses the use of dot-separated,
standardized error codes for use between SMTP applications. Reading the various RFCs that involve
SMTP provides a background to the way email moves around the Internet. In addition, you can connect to an SMTP server via telnet by specifying port 25, such as telnet localhost 25. Executing
a few commands and sending a mail manually is a good way to understand how SMTP communications work.
Red Hat Linux 8.0 uses Sendmail (/usr/sbin/sendmail)as its SMTP program by default. However, an easier to use application called Postfix (/usr/sbin/postfix) is also available for use.
Section 15.2.2 for more on this topic.
15.2. Email Program Classifications
In general, all email applications fall into at least one of three classifications. Each of these classifications 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 of these
types is important for making sure email arrives at the correct destination.
15.2.1. Mail User Agent
A Mail User Agent (MUA) is a program that, at the very least, allows a user to read and compose email
messages. An MUA is often referred to as an email client. Of course, many MUAs help users do more
than that, including retrieving messages via the POP or IMAP protocols, setting up mailboxes to store
messages, or helping present new messages to a Mail Transfer Agent that will deliver them to their
final destination.
MUA programs may be graphical, such as Mozilla Mail, or have a very simple, text-based interface,
such as mutt.
15.2.2. Mail Transfer Agent
A Mail Transfer Agent (MTA) transfers email messages between machines using SMTP. A message
may involve several MTAs as it moves to its intended destination. Most users are totally unaware of
the presence of MTAs, even though every email message is sent through at least one MTA.
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 own
configuration or network access to the system running it.
Chapter 15. Email
Many of the larger and more complex MUAs can also be used to send email. However, this action
should not be confused with the actions of a true MTA. In order for users not running their own MTA
to move outbound messages off of their machine and onto a remote machine for delivery, they must
use a capacity in the MUA that transfers the message to an MTA they are authorized to use. However,
the MUA does not directly deliver the message to the intended recipient’s email server — that role is
reserved for the MTA.
Red Hat Linux uses Sendmail as its default MTA, though others may be used in its place.
For information about how to switch the default MTA from Sendmail to Postfix, see the chapter called
Mail Transport Agent (MTA) Configuration in the Official Red Hat Linux Customization Guide.
15.2.3. Mail Delivery Agent
A Mail Delivery Agent (MDA) is used by the MTA to deliver email to a particular user’s mailbox. In
many cases, an MDA is actually a Local Delivery Agent (LDA), such as /bin/mail or procmail.
However, Sendmail can also play the role of an MDA, such as when it accepts a message for a local
user and appends it to their email spool file. Any program that actually handles a message for delivery
to the point where it can be read by an MUA can be considered an MDA. Note that MDAs do not
transport messages between systems or interface with the end user.
Many users do not directly use MDAs, because only MTAs and MUAs are necessary to send and
receive email. However, some MDAs may be used to sort messages before they are read by a user,
which is a big help if you receive a lot of email.
15.3. Sendmail
Red Hat Linux uses Sendmail as its default MTA, whether they are intended for users on the same
system or for remote destinations. Other MTAs do exist, but many system administrators elect to use
Sendmail as their MTA due to its power, scalability, and compliance to important Internet standards,
such as SMTP.
Sendmail’s core duty, like other MTAs, is to safely move email between hosts, usually using the SMTP
protocol. However, Sendmail is highly configurable, allowing you to control almost every aspect of
how email is handled, including the protocol used.
15.3.1. History
Sendmail’s roots can be traced to the birth of email, occurring in the decade before the birth of
ARPANET, the precursor to the Internet. In those days, every user’s mailbox was a file that only
they had rights to read, and mail applications simply added text to that file. Every user had to wade
through their mail file to find any old mail, and reading new mail was a equally difficult. The first actual transfer of a mail message file from one host to another did not take place until 1972, when email
began to be moved by FTP over the NCP network protocol. This easier method of communication
quickly became popular, even to the point where it made up most of ARPANET’s traffic in less than a
However, a lack of standardization between competing protocols made email much harder to send
from some systems. This continued until the ARPANET standardized on TCP/IP in 1982. Soon after,
a new protocol, SMTP, materialized for transporting email messages. These developments, combined
with HOSTS files being replaced with DNS, allowed full-featured MTAs to materialize. Sendmail,
Chapter 15. Email
which grew out of an earlier email delivery system called Delivermail, quickly became the standard
as the email began to expand and become widely used.
15.3.2. Purpose and Limitations
It is important to be aware of what Sendmail is and what it can do for you as opposed to what it is
not. In these days of monolithic applications that fulfill multiple roles, you might initially think that
Sendmail is the only application you need to run an email server within your organization. Technically,
this is true, as Sendmail can spool mail to your users’ directories and accepts new email via the
command line. But, most users actually require much more than simple email delivery. They 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 and SMTP, 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 are dedicated to helping
explain everything that can be done and how to fix things that go wrong. You should consult the many
excellent resources on Sendmail in order to shape it to fit your exact specifications.
However, you should understand what files are installed with Sendmail by default on your system and
know how to make basic configuration changes. You should also be aware of how to stop unwanted
email (spam) and how to extend Sendmail with the Lightweight Directory Access Protocol (LDAP).
15.3.3. The Default Sendmail Installation
While you can download the source code for Sendmail and build your own copy, many users prefer
to use the version of Sendmail installed by default with their Red Hat Linux system. You can also use
the Red Hat Linux CD-ROMs to reinstall the sendmail RPM at a later point. Be aware that you must
alter the default configuration file for Sendmail to use it as a mail server for more the localhost. See
Section 15.3.4 for details.
After installation, the sendmail executable is placed in the /usr/sbin/ directory.
Sendmail’s lengthy and detailed configuration file,, is installed in the /etc/mail/
directory. You should avoid editing the file directly. Instead, to make
configuration changes to Sendmail, edit the /etc/mail/ file, back up the original
/etc/mail/, and then use the included m4 macro processor to create a new
/etc/mail/ More information on configuring Sendmail can be found in Section
Various Sendmail configuration files are installed in the /etc/mail/ directory including:
• access
— Specifies which systems can use Sendmail for relaying email.
• domaintable —
Allows you to provide domain name mapping.
• local-host-names —
• mailertable —
The place where you include all aliases for your machine.
Specifies instructions that override routing for particular domains.
• virtusertable — Permits you to do a domain-specific
domains to be hosted on one machine.
form of aliasing, allowing multiple virtual
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 you make to these configurations in their database
files, you must run the makemap hash /etc/mail/ › name œ› /etc/mail/ › name œ command,
where ž name Ÿ is the name of the configuration file to convert.
Chapter 15. Email
For example, if you want all email addressed to any account to be delivered to
<>, you need to add a line similar to the one below to the virtusertable
Then, to add this new information to the virtusertable.db file, execute makemap hash
/etc/mail/virtusertable as root. This will create a new
virtusertable.db containing the new configuration.
15.3.4. Common Sendmail Configuration Changes
Although a default file is installed in /etc/mail/ during the Red Hat Linux installation process, you will need to alter it to use some of the program’s more advanced features.
When altering
file, it is best generate
/etc/mail/ file instead of editing an existing one.
a entirely
Before changing the file, it is a good idea to backup the default version.
To add the desired functionality Sendmail edit the /etc/mail/ file. When you
are finished, use the m4 macro processor to generate a new by executing the m4
/etc/mail/ ¡ /etc/mail/ command. After creating a new
/etc/mail/, you must restart Sendmail to make it take effect. The easiest way to do
this is to type the /sbin/service sendmail restart command as root.
By default, the m4 macro processor is installed with Sendmail and is part of the sendmail-cf package.
The default does not allow sendmail to accept network connections from any host
other than the local computer. If you want to configure sendmail as a server for other clients, please
edit /etc/mail/ and change DAEMON_OPTIONS to also listen on network devices or
comment out this option all together. Then regenerate /etc/mail/ by running:
m4 /etc/mail/
This configuration should work for most SMTP-only sites. It will not work for UUCP (UNIX to UNIX
Copy) sites; you will need to generate a new if you must use UUCP mail transfers.
You should consult the /usr/share/sendmail-cf/README file before you edit any of the files
in the directories under the /usr/share/sendmail-cf directory, as they can affect how future
/etc/mail/ files are configured. Masquerading
One common Sendmail configuration is to have a single machine act as a mail gateway for all
the machines on the network. For instance, a company may want to have a machine called that handles all their email and assigns a consistant return address to all out
going mail.
Chapter 15. Email
In this situation, the sendmail server needs to masquerade the machine names on the company network
so that their return address is instead of
To do this, add the following lines to /etc/mail/
After generating a new using m4, this configuration will make all mail from inside the
network appear as if it were sent from
15.3.5. Stopping Spam with Sendmail
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 has made it relatively easy to block new spamming techniques being employed to send
junk email using your system. It even blocks many of the more usual spamming methods by default.
You would need to consciously activate them by changing your /etc/mail/ file in a
particular way to make your system susceptible.
For example, forwarding of SMTP messages, also referred to as SMTP relaying, has been disabled
by default since Sendmail version 8.9. Before this change occurred, Sendmail would direct your mail
host ( to accept messages from one party ( and send them to a different party (
Now, however, you have to specifically tell Sendmail to permit a domain to relay mail through your
domain. Simply edit the /etc/mail/relay-domains file and restart Sendmail by typing the service sendmail restart command as root to activate the changes.
However, many times, your users may be bombarded with spam from other servers throughout the Internet that are beyond your control. In these instances, you can use Sendmail’s access control features
available through the /etc/mail/access file. As root, add the domains that you would like to block
or specifically allow access, such as in this example:
550 Go away and do not spam us anymore
Because /etc/mail/access.db is a database, you need to use makemap to activate your
changes by recreating the database map. This is easily done by running the makemap hash
/etc/mail/access £ /etc/mail/access command as root.
This example shows that any email sent from would be blocked with a 550
RFC-821 compliant error code and message back to the spammer, except for email sent from the sub-domain, which would be accepted. The last line shows that any email
sent from the 10.0.*.* network can be relayed through your mail server.
As you might expect, this example only scratches the surface of what Sendmail can do in terms of
allowing or blocking access. See the /usr/share/doc/sendmail/ for more detailed
information and examples.
Chapter 15. Email
15.3.6. 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, you could use an
LDAP server to look up a particular email address from a common corporate directory by a 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 preaddressed 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, you can use LDAP to
abstract the mail routing level from Sendmail and its separate configuration files to a powerful LDAP
cluster that is being leveraged by many different applications.
The current version of Sendmail contains support for LDAP. To extend your Sendmail server using
LDAP, first get an LDAP server, such as OpenLDAP, running and properly configured. Then, you
need to edit your /etc/mail/ to include the following:
This is only for a very basic configuration of Sendmail with LDAP. Your configuration should differ
greatly from this depending on your implementation of LDAP, especially if you wish to configure
several Sendmail machines to use a common LDAP server.
Consult /usr/share/doc/sendmail/ for detailed LDAP routing configuration instructions
and examples.
Next, recreate your /etc/mail/ file by running m4 and restarting Sendmail. See Section 15.3.4 for instructions on doing this.
For more information on LDAP, see Chapter 18.
15.4. Fetchmail
Fetchmail is a program that can retrieve email from remote servers for on-demand TCP/IP connections. 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 will connect and quickly download all of your email
messages to your mail spool file using any number of protocols, including the POP3 and IMAP. It can
even forward your email messages to an SMTP server, if necessary.
Before attempting to use Fetchmail, be sure that it is installed on your system. If it is not, you can
install it using the fetchmail RPM on the Red Hat Linux CD-ROMs.
Fetchmail is configured for each user through the use of a .fetchmailrc file in the user’s home
directory. A program bundled with Fetchmail, called fetchmailconf, is very helpful for configuring
a basic .fetchmailrc file that you can customize at will.
Using preferences in the .fetchmailrc file, Fetchmail will check for email on a remote server and
pull it off, attempting to deliver 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 can then be used to filter the email and
place it in a mailbox so that it can be read by an MUA.
Chapter 15. Email
15.4.1. Fetchmail Configuration Options
Although it is possible to pass all options on the command line necessary to check for email on a
remote server when executing Fetchmail, using a .fetchmailrc file is much easier. All of your
configuration options go in the .fetchmailrc file, but you can override them at the time Fetchmail
is run by specifying that option on the command line.
A user’s .fetchmailrc file is divided into three particular types 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 you would like to see enforced with a particular email server, such as
the port to check or number of seconds to wait before timing out. These options affect every user
option used with that server.
user options — Contains information, such as username and password, necessary to authenticate
and check for email using a particular email server.
Global options go 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 you would like to check on that email server. Like server options,
multiple user options may be specified for use with a particular server, such as when you would like
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 actually checks it for email using the various user options.
Any server options after a skip action, however, are not checked unless you specify this server’s
hostname when Fetchmail is invoked. The skip option allows you to set up test configurations in
.fetchmailrc and only check using that server when specifically desired, without affecting any
currently working configurations.
A sample .fetchmailrc file looks like this:
set postmaster "user1"
set bouncemail
poll proto pop3
user ’user1’ there with password ’secret’ is user1 here
user ’user5’ there with password ’secret2’ is user1 here
user ’user7’ there with password ’secret3’ is user1 here
In this example, the global are options set so 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, and 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 users 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 action.
You do not have to place your password in the .fetchmailrc file. You can omit the with password ’ password ’ section. Fetchmail will then ask for your password when it is started with the
fetchmail command.
Chapter 15. Email
While you can set up your .fetchmailrc file manually, it is much easier to let the included fetchmailconf program do it for you. However, when testing new configurations, it is usually easier to
edit the .fetchmailrc file directly.
As expected with a program that services such a mature network service as email and uses so many
protocols, Fetchmail contains many different 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 of
these options in detail, but the most common ones are listed here. Global Options
Each global option should be placed on a single line after a set action.
¦ seconds § — Tells Fetchmail to automatically use daemon-mode, where it will stay in
the background and poll for mail at the interval specified.
• daemon
• postmaster
— Gives Fetchmail a local user to send mail to in case of delivery problems.
— Tells Fetchmail to start logging error and status messages in the system’s log file. By
default, this is /var/log/maillog.
• syslog Server Options
Place server options on their own line in .fetchmailrc after a poll or skip action.
¦ auth-type § — Specifies 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 will first try
methods that do not require a password, then methods that mask your password, and finally attempt
to send your password in the clear to authenticate to the server.
• auth
¦ number § — Tells Fetchmail to only poll this server every ¦ number § of times that
it checks for email on all configured servers. This option might be used with email servers where
you rarely receive messages.
• interval
• port
— Overrides the default port number for a specified protocol.
— Tells Fetchmail to use a specific protocol, such as pop3 or imap, to check
for messages on this server.
• proto
¦ seconds § — Configures Fetchmail to give up after a certain interval of server inactivity. If this value is not set, a default of 300 seconds is assumed.
• timeout 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 user options follow the user option (defined below).
— Orders Fetchmail to download all messages in the queue, including messages that
have already been viewed. By default, Fetchmail only pulls down new messages.
• fetchall
• fetchlimit
— Only allows a certain number of messages to be retrieved before
— Tells Fetchmail to delete all previously viewed messages in the queue before retrieving
new messages.
• flush
Chapter 15. Email
¨ max-number-bytes © — Allows you to specify that only messages below a particular
size may be retrieved. This option is useful with slow network links, when a large message will take
too long to download.
• limit
• password ’ password ’ —
Specifies the password to be used for this user.
• preconnect " command " — Tells Fetchmail
ing messages for this user.
• postconnect " command " — Tells
ing messages for this user.
to execute the specified command before retriev-
Fetchmail to execute the specified command after retriev-
— Allows Fetchmail to collect the message via an encrypted SSL connection, if the server
supports this.
• ssl
Sets the username used by Fetchmail to retrieve messages. This option
should be listed before any other user options.
• user " username " —
15.4.2. Fetchmail Command Options
Most of the Fetchmail options that may be used on the command line, when executing the fetchmail
command, mirror the .fetchmailrc configuration options. This is done so that Fetchmail may be
used with or without a configuration file. Most users will not use these options on the command line,
as it is easier to leave them in the .fetchmailrc file to be used whenever Fetchmail is run.
However, there are times when you may want to run the fetchmail command with other options for
a particular purpose. As any options specified at the command line override configuration file options,
you can also issue command options to temporarily override a .fetchmailrc setting that is causing
an error. Informational or Debugging Options
Certain options used after the fetchmail command can provide you with 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.
— Executes Fetchmail in silent mode, preventing any messages, other than errors, from appearing after the fetchmail command.
• -s
— Executes Fetchmail in verbose mode, displaying every communication between Fetchmail
and the remote email servers.
• -v
— Causes Fetchmail to display detailed version information, list its global options, and show
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.
• -V Special Options
These options are occasionally useful for overriding defaults often found in the .fetchmailrc file.
— Tells Fetchmail to download all messages from the remote email server, whether new or
previously viewed. By default, Fetchmail will only download new messages.
• -a
— Causes Fetchmail to leave the messages on the remote email server after downloading them.
This option overrides the default behavior of deleting messages after downloading them.
• -k
¨ max-number-bytes © — Tells Fetchmail to not download any messages over a particular
size and leave them on the remote email server.
• -l
• --quit
Chapter 15. Email
— Quits the Fetchmail daemon process.
More commands and .fetchmailrc options can be found on the fetchmail man page.
15.5. Procmail
Procmail allows you to filter email as it is received from a remote email server, or placed in your spool
file on a local or remote email server. It is powerful, gentle on system resources, and widely used.
Procmail, commonly referred to as a Local Delivery Agent (LDA), plays a small role in delivering
email to be read by an MUA.
In order to use Procmail, it must first be installed. Type the rpm -q procmail command to see if
the procmail package is installed. If, for some reason, Procmail is not on your system, install it from
the Red Hat Linux installation CD-ROMs.
Procmail can be invoked in several different ways. As email is placed on your email spool file, Procmail can be configured to start up, filter the email to locations configured for use with your MUA, and
quit. Or, your MUA could be configured to bring up Procmail any time a message is received so that
messages are moved into their correct mailboxes. In many cases, the presence of a .procmailrc file
in the user’s home directory will invoke Procmail, if Sendmail is being used.
The actions Procmail takes with an email are dependent upon instructions from particular recipes, or
rules, that messages are matched against by the program. If a message matches the recipe, then the
email will be placed in a certain file, deleted, or otherwise processed.
When Procmail starts, it reads the email message and separates the body from the header information.
Next, Procmail looks for the /etc/procmailrc file and rc files in the /etc/procmailrcs directory for default, system-wide, Procmail environmental variables and recipes. Then, Procmail looks for
a .procmailrc file in the user’s home directory to find rules specific to that user. Many users also
create additional rc files of their own for Procmail that are referred to by their .procmailrc file but
may be turned on or off quickly if a mail filtering problem develops.
By default, no system-wide rc files exist in the /etc directory, and no user .procmailrc files exist.
To begin using Procmail, you will need to construct a .procmailrc file with particular environment
variables and recipes explaining what you would like to do with certain messages.
In most configurations, the decision as to whether Procmail starts and attempts to filter your email is
based the existence of a user’s .procmailrc file. To disable Procmail, but save your work on the
.procmailrc file, move it to a similar file’s name using the mv ~/.procmailrc ~/.procmailrcSAVE command. When you are ready to begin testing Procmail again, change the name of the file
back to .procmailrc. Procmail will begin working again immediately.
15.5.1. Procmail Configuration
Procmail configuration files, most notably the user’s .procmailrc, contain important environmental
variables. These variables tell Procmail which messages to sort, what to do with the messages that do
not match any recipes, and so on.
These environmental variables usually appear at the beginning of .procmailrc in the following
env-variable =" value "
In this example, the
the variable.
is the name of the variable, and the
section defines
Chapter 15. Email
Many environment variables are not used by most Procmail users, and many of the more important
environment variables are already defined a default value. Most of the time, you will be dealing with
the following variables:
Sets the default mailbox where messages that do not match any recipes will be placed.
The default DEFAULT value is the same as $ORGMAIL.
— Specifies additional rc files containing more recipes for messages to be checked
against. This allows you to break up your Procmail recipe lists into individual files that fulfill
different roles, such as blocking spam and managing email lists, that can then be turned off or on
by using comment characters in the user’s .procmailrc file.
For example, two lines in a user’s .procmailrc file may look like this:
If the user would like to turn off the Procmail filtering of their email lists but leave spam control in
place, they could simple comment out the first INCLUDERC line with a # character.
— Sets the amount of time, in seconds, between attempts by Procmail to use a particular lockfile. The default is eight seconds.
— Sets the amount of time, in seconds, that must pass after a lockfile was last
modified before Procmail will assume that the lockfile is simply old and can be deleted. The default
is 1024 seconds.
— The location and file to contain any Procmail informational or error messages.
— Sets the current working directory for Procmail. If set, all other Procmail paths are
relative to this directory.
— Specifies the original mailbox, or another place to put the messages if they cannot be
placed in the default or recipe-required location.
By default, a value of /var/spool/mail/$LOGNAME is used.
— Sets the amount of time, in seconds, that Procmail will pause if a necessary resource,
such as swap space, is not available.
— 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 much more information. This option is useful for debugging.
Other important environmental variables are pulled from your shell, such as LOGNAME, which is your
login name, HOME, which is the location of your home directory, and SHELL, which is your default
A comprehensive explanation of all environments variables, as well as their default values, is available
on the procmailrc man page.
15.5.2. 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 figure out what is going on.
Chapter 15. Email
A thorough explanation of regular expressions is beyond the scope of this chapter. The structure of
Procmail recipes is more important, and useful sample Procmail recipes can be found at various places
on the Internet, including The proper use and adaptation of
the regular expressions found in these recipe examples depends upon an understanding of Procmail
recipe structure. Introductory information specific to basic regular expression rules can be found on
the grep man page.
A Procmail recipe takes the following form:
:0 flags :
The first two characters in a Procmail recipe are a colon and a zero. Various flags can optionally
be placed after the zero to control what Procmail does when processing this recipe. A colon after
the ¬ flags ­ section specifies that a lockfile will be created for this message. If a lockfile is to be
created, you specify its name in the ¬ lockfile-name ­ space.
A recipe can contain several conditions to match against the message. If it has no conditions, every
message will match the recipe. Regular expressions are placed in some conditions in order to facilitate
a match with a message. If multiple conditions are used, they must all match in order for an 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 what is to happen to a message if it 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. Delivering vs. Non-Delivering Recipes
The action used if the recipe matches a particular message determines whether recipe is considered
delivering or non-delivering. 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 nondelivering recipe covers any other actions, such as when a nesting block is used. A nesting block is an
action contained in braces { } that designates additional actions to perform on messages that match
the recipe’s conditions. Nesting blocks can be nested, providing greater control for identifying and
performing actions on messages.
Delivering recipes that match messages causes Procmail to perform the action specified and stop
comparing the message against any other recipes. Messages that match conditions in non-delivering
recipes will continue to be compared against other recipes in the current and following rc files. In
other words, non-delivering recipes cause the message to continue through the recipes after the specified action is taken on it. Flags
Flags are very important in determining how or if a recipe’s conditions are compared to a message.
The following flags are commonly used:
— Specifies that this recipe will only be used if the last previous recipe without an A or a flag
also matched this message.
• A
Chapter 15. Email
To ensure that the action on this last previous matching recipe was successfully completed before
allowing a match on the current recipe, use the a flag instead.
• B
— Parse the body of the message and look for matching conditions.
— Use the body in any resulting action, such as writing the message to a file or forwarding it.
This is the default behavior.
• b
— Generate 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.
• c
— Makes the egrep comparison case-sensitive. By default, the comparison process is not casesensitive.
• D
• E — Similar
to the A flag, except that the conditions in this recipe are only compared to the message
if the immediately preceding recipe without an E flag did not match. This is comparable to an else
Use the e flag instead if you only want this recipe checked if the preceding recipe matched but the
action failed.
• f
— Uses the pipe as a filter.
• H
— Parses the header of the message and looks for matching conditions. This occurs by default.
• h
— 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 report whether or not it
was successful before considering the message filtered.
• w
If you would like to ignore "Program failure" messages when deciding whether a filter or action
succeeded, use the W option instead.
Additional flags can be found in the procmailrc man page. Specifying a Local Lockfile
Lockfiles are very useful with Procmail to ensure that more than one process does not try to alter a
certain message at the same time. You can specify a local lockfile by placing a colon (:) after any
flags on a recipe’s first line. This will create a local lockfile based on the destination filename plus
whatever has been set in the LOCKEXT global environment variable.
Alternatively, you can specify the name of the local lockfile to be used with this recipe after the colon. Special Conditions and Actions
Particular characters used before Procmail recipe conditions and actions change the way they are
The following characters may be used after the * character at the beginning of a recipe’s condition
— Inverts the condition, causing a match to occur only if the condition does not match the
• !
— Checks to see if the message is under the specified number of bytes.
— Checks to see if the message is over a particular number of bytes.
The following characters are used to perform special actions:
• !
— Tells Procmail to forward the message to the specified email addresses
Chapter 15. Email
• $—
Refers to a variable set earlier in the rc file. This is usually used to set a common mailbox that
will be referred to by various recipes.
• |
— The pipe character tells Procmail to start a specific program to deal with this message.
and } — Constructs a nesting block, used to contain additional recipes to apply to matching
• {
If no special character is used at the beginning of the action line, then Procmail assumes that the action
line is specifying a mailbox where the message should be written. Recipe Examples
Procmail is an extremely flexible program, allowing you to match messages with very specific conditions and then perform detailed actions on them. As a result of this flexibility, however, composing a
Procmail recipe from scratch to achieve a certain goal 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. The following
very basic examples exist to serve as a demonstration of the structure of Procmail recipes and can
provide the foundation for more intricate constructions.
The most basic recipes does not even contain conditions, as is illustrated in the following example.
The first line starts the recipe by specifying that a local lockfile is to be created but does not specify
a name, leaving Procmail to use the destination filename and the LOCKEXT to name it. No condition
is specified, so every message will match this recipe and, therefore, will be 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 the messages in this file.
This basic recipe could go at the end of all rc files to direct messages to a default location. A more
complicated example might grab messages from a particular email address and throw them away, as
can be seen in this example.
* ^From:
With this example, any messages sent by are immediately moved to
/dev/null, deleting them.
Be very careful that a rule is working correctly before moving messages matching it to /dev/null,
which is a permanent deletion. If your recipe conditions inadvertently catch unintended messages,
you will not even know you are missing those messages unless the sender tells you.
A better solution is to point the recipe’s action to a special mailbox that you can check from time to
time in order to look for false positives, or messages that inadvertently matched the conditions. Once
you are satisfied that no messages are accidentally being matched, you can delete the mailbox and
direct the action to send the messages to /dev/null.
Procmail is primarily used as a filter for email, automatically placing it in the right place so that you
do not have to sort it manually. The following recipe in grabs email sent from a particular mailing list
and puts in the correct folder for you.
Chapter 15. Email
* ^(From|CC|To).*tux-lug
Any messages sent from the mailing list will be placed in the tuxlug mailbox automatically for your MUA. Note that the condition in this example will match the message if it
has the mailing list’s email address on the From, CC, or To lines.
Procmail can also be used to block spam, although this is not a good long-term solution for junk mail.
The following example illustrates a temporary spam filtering solution, where multiple recipes are set
to use a common mailbox to store the junk.
* To??^$
* ^(To|CC):.*,.*,.*,.*,.*,.*,.*,.*,.*,.*,.*,
* ^Message-Id:.* [^@]*
In this example, the junk mailbox is associated with the SPAM variable, so that you can change the
mailbox that holds your spam in one place. Then, three recipes look for messages to send to the junk
The first recipe looks for messages that have no recipient in the To line. The second recipe matches
any messages with 12 or more recipients. The third recipe looks for messages with a bad message ID.
These simple examples are provided to help get you started creating recipes. Consult the many Procmail online resources available from Section 15.7 to see more detailed and powerful recipes.
15.6. Security
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, all without the knowledge of the email server or client. When using standard POP and IMAP protocols, all authentication
information is sent "in the clear," meaning that someone on a network between the client and the
remote server can easily view it.
15.6.1. Secure Email Clients
Thankfully, most Linux MUAs designed to check email on remote servers support SSL to encrypt
messages as they are sent back and forth over the network. In order to use SSL when retrieving email,
it must be enabled on 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 area. Secure IMAP and POP have known port numbers (993 and 995, respectively) that the MUA
will use to authenticate and download messages.
Popular MUAs included with Red Hat Linux, such as Mozilla Mail, mutt, and pine, offer SSLencrypted email sessions.
Chapter 15. Email
15.6.2. Secure Email Servers
Offering SSL encryption to IMAP and POP users on the email server is almost as easy. Red Hat Linux
also includes the stunnel package, which is an SSL encryption wrapper that wraps around standard,
non-secure network traffic for certain services and prevents interceptors from being able to "sniff" the
communication between client and server.
The stunnel program uses external SSL libraries, such as the OpenSSL libraries included with Red
Hat Linux, to provide strong cryptography and protect your connections. You can apply to a Certificate
Authority (CA) for an SSL certificate, or you can create a self-signed certificate to provide the benefit
of the SSL encrypted communication.
To create a self-signed SSL certificate, change to the /usr/share/ssl/certs/ directory, type the
make stunnel.pem command, and answer the questions. Then, use stunnel to start the mail daemon that you wish to use.
For example, the following command could be used to start the IMAP server included with Red Hat
/usr/sbin/stunnel -d 993 -l /usr/sbin/imapd imapd
You should now be able to open an IMAP email client and connect to your email server using SSL
encryption. Of course, you will probably want to go a step further and configure your stunnelwrapped IMAP server to automatically start up at the correct runlevels.
For more information about how to use stunnel, read the stunnel man page or refer to the documents in the /usr/share/doc/stunnel- ³ version-number ´ directory.
Alternatively, the imap package bundled with Red Hat Linux contains the ability to provide SSL
encryption on its own without stunnel. For secure IMAP connections, create the SSL certificate by
changing to the /usr/share/ssl/certs/ directory and running the make imapd.pem command.
Then, set the imapd service to start at the proper runlevels.
You can also use the ipop3 package bundled with Red Hat Linux to provide SSL encryption on its
own without stunnel.
15.7. Additional Resources
Many users initially find email programs difficult to configure, primarily due to the large number of
options available. Below is a list of additional documentation to help you properly configure your mail
15.7.1. Installed Documentation
Information about how to configure Sendmail is included with the sendmail and sendmail-cf
/usr/share/doc/sendmail/ — Contains information on m4, file locations for
/usr/share/doc/sendmail/README — Contains information on the Sendmail directory
Sendmail, supported mailers, how to access enhanced features, and more.
structure, IDENT protocol support, details on directory permissions, and the common problems
these permissions can cause if misconfigured.
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, respectively.
Chapter 15. Email
¶ — Contains a full list of Fetchmail features in the FEATURES file and an introductory FAQ document.
• /usr/share/doc/fetchmail- version-number
¶ — 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.
• /usr/share/doc/procmail- version-number
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
procmailrc — Explains the rc file format used to construct recipes.
procmailex — Gives a number of useful, real-world examples of Procmail recipes.
procmailsc — Explains the weighted scoring technique used by Procmail to see if a particular
recipe matches a certain message.
15.7.2. Useful Websites
• — Provides an
overview of how email works and examines possible email solutions and configurations on the
client and server sides.
• — Looks at email from the
user’s perspective, investigates various popular email client applications and gives an introduction
to topics such as aliases, forwarding, auto-replying, mailing lists, mail filters, and spam.
• — Demonstrates a
way to retrieve POP email using SSH with port forwarding, so that your email passwords and
messages will be transferred securely.
• — Contains news, interviews, and articles concerning Sendmail, including
an expanded view of the many options available.
• — Offers a thorough technical breakdown of Sendmail features and configuration examples.
• — The home page for Fetchmail, featuring an online manual, and
a thorough FAQ.
• — The home page for Procmail, with links to assorted mailing lists dedicated to Procmail as well as various FAQ documents.
• — An excellent Procmail FAQ,
with troubleshooting tips and details about file locking and the use of wildcard characters.
• — Points out dozens of tips that make using Procmail in
various situations much easier, including how to test .procmailrc files and use Procmail scoring
to decide if a particular action should be taken.
15.7.3. Related Books
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.
Chapter 15. Email
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 that use
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.
Chapter 16.
Berkeley Internet Name Domain (BIND)
Today, the Internet and almost all local networks depend upon a working and reliable Domain Name
Service (DNS), which is used to resolve names of systems into IP addresses and vice versa.
In order to facilitate DNS on your network, a nameserver is required to translate these names into the
IP addresses necessary to make the connection. In addition, a nameserver can translate IP addresses
back into a system’s name, commonly called a reverse lookup.
This chapter discusses BIND, the structure of its configuration files, and how it may be locally or
remotely administered.
For instructions on configuring BIND using the graphical Bind Configuration Tool
(redhat-config-bind), please see the chapter called BIND Configuration in the Official Red Hat
Linux Customization Guide.
If you use the Bind Configuration Tool, you should not manually edit any BIND configuration files
because all changes will be overwritten the next time you use the Bind Configuration Tool.
16.1. Introduction to DNS and BIND
Systems using IP networks must know the IP address of a remote machine in order to connect to
it. However, most users prefer to use the name of a machine, called a hostname or a fully qualified
domain name (FQDN), when connecting to it.
Use of fully qualified domain names also have advantages for system administrators. They allow
administrators to flexibility in changing the IP addresses for individual machines without effecting
name-based queries to the machines. Conversely, administrators can shuffle which machines handle a
name-based query in a way transparent to the user.
The service that facilitates this is caused DNS, and it is normally implemented using centralized
servers that are authoritative for some domains and refer to other DNS servers for other domains.
DNS under Linux is made possible through the use of a nameserver daemon that performs the
IP/hostname translation. A client application will request information from the nameserver, usually
connecting to it on the server’s port 53. The nameserver will attempt to resolve the FQDN based on
its resolver library, which may contain authoritative information about the host requested or cached
data about that name from an earlier query. If the nameserver does not already have the answer in
its resolver library, it will turn to other nameservers, called root nameservers, to determine which
nameservers are authoritative for the FQDN in question. Then, with that information, it will query the
authoritative nameservers for that name to determine the IP address. If performing a reverse lookup,
the same procedure is used, except the query is made with an unknown IP address rather than a name.
16.1.1. Zones
On the Internet, the FQDN of a host can be broken down into different sections, and these sections are
organized in a hierarchy much like a tree, with a main trunk, primary branches, secondary branches,
and so forth. Consider the following FQDN:
Chapter 16. Berkeley Internet Name Domain (BIND)
When looking at how a FQDN is resolved to find the IP address that relates to a particular system,
you must read the name from right to left, with each level of the hierarchy divided by dots (.). In this
example, the com defines the top level domain for this FQDN. The domain name is a sub-domain
under com, with sales as a sub-domain under domain. The name furthest left in a FQDN is the
hostname, identifying a particular machine.
Except for the hostname, every section is a called a zone, which defines a particular namespace. A
namespace controls the naming of the sub-domains to its left. While this example only contains two
sub-domains, a 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 the nameserver’s configuration.
16.1.2. Types of Nameservers
There are four primary nameserver configuration types:
master — Stores original and authoritative zone records for a certain namespace, answering questions from other nameservers searching for answers concerning that namespace.
slave — Also answers queries from other nameservers concerning namespaces for which it is considered an authority. However, slave nameservers get their namespace information from master
nameservers via a zone transfer, where the slave sends the master a NOTIFY request for a particular
zone and the master responds with the information, if the slave is authorized to receive the transfer.
caching-only — Offers name to IP resolution services but is not authoritative for any zones. Answers for all resolutions are usually cached in a database stored in memory for a fixed period of
time, usually specified by the retrieved zone record, for quicker resolution for other DNS clients
after the first resolution.
forwarding — Forwards requests to a specific list of nameservers to be resolved. If none of the
specified nameservers can perform the resolution, the process stops and 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 resolution.
16.1.3. BIND as a Nameserver
Red Hat Linux includes BIND, which is a very popular, powerful, open source nameserver. BIND
uses the named daemon to provide name resolution services.
BIND version 9 also includes a utility called /usr/sbin/rndc which allows the administration of
the running named daemon. More information about rndc can be found in Section 16.3.
16.2. BIND Configuration Files
The BIND nameserver named server configuration is /etc/named.conf. All zone files are placed
in the /var/named/ directory.
Chapter 16. Berkeley Internet Name Domain (BIND)
The /etc/named.conf file must be free of errors in order for named to start. While some erroneous
options are not considered critical enough to stop the server, any errors in the statements themselves
will prevent the named service from starting.
Do not manually edit the /etc/named.conf file or any files in the /var/named/ directory if you are
using the Bind Configuration Tool. Any manual changes to those files will be overwritten the next
time the Bind Configuration Tool is used.
16.2.1. /etc/named.conf
The /etc/named.conf file is a collection of statements using nested options placed in ellipses { }.
A sample /etc/named.conf file is organized similar to this:
[" statement-1-name "] [ statement-1-class ] {
option-1 ;
option-2 ;
option-N ;
[" statement-2-name "] [ statement-2-class ] {
option-1 ;
option-2 ;
option-N ;
[" statement-N-name "] [ statement-N-class ] {
option-1 ;
option-2 ;
option-N ;
The ¹ statement-N-name º is only needed with acl, include, server, view, and zone statements. The ¹ statement-N-class º may only be specified with the zone statement.
The following is a list of valid comment tags you may use within /etc/named.conf:
• //
• #
— Use at the begining of a line comment that line.
— Use at the begining of a line comment that line.
/* and */ — Enclose text in these tags to create a block comment.
The following statements may be used in /etc/named.conf:
» acl-name ¼ 4 — Configures an access control list of IP addresses to be allowed or disallowed certain named services. Most of the time, individual IP addresses or IP network notation
(such as is used identify the exact IP addresses.
• acl
A few access control lists are already defined, so you do not have to configure an acl statement to
define them:
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.
Chapter 16. Berkeley Internet Name Domain (BIND)
When used with other /etc/named.conf statements and their options, acl statements can be
very useful in ensuring the proper use of your BIND nameserver as in this example:
acl black-hats {;;
acl red-hats {;
options {
blackhole { black-hats; };
allow-query { red-hats; };
allow-recursion { red-hats; };
This named.conf contains two access control lists black-hats and red-hats.
— Configures various security requirements necessary to use the rndc command to
administer the named service.
• controls
See Section to see how the controls statement should look, including various options
that may only be used with it.
¾ — Includes the specified file within the current configuration file,
allowing sensitive configuration data (such as keys) to be placed in a separate file with permissions
that prevent non-privileged users from reading it.
• include " file-name "
¾ — 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:
• key " key-name "
— The type of algorithm used, such as dsa or hmac-md5.
secret " key-value " — The encrypted key.
See Section for instruction on how to write a key statement.
— 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, with its own file name (file),
size limit (size), versioning (version), and level of importance (severity), can be constructed.
Once a customized channel has been defined, a category option is used the categorize the channel
and begin logging when named is restarted.
• logging
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 one that handles informational logging
messages (default_syslog) and another that specifically handles debugging messages
(default_debug). 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, see the BIND 9 Administrator Reference
• options —
Assigns values to many assorted options, including the use of forwarders, the location
of the named working directory, the names of the various files, and much more.
The following options are among the most commonly used:
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 only allow particular hosts to query the nameserver.
Chapter 16. Berkeley Internet Name Domain (BIND)
allow-recursion — Similar to allow-query, except it applies to recursive queries. By de-
directory — Changes the named working directory to something other than the default,
forward — Controls how forwarding occurs, if the forwarders option contains valid IP ad-
fault, all hosts are allowed to perform recursive queries on the nameserver.
dresses designating where to send requests.
If the first option is used, then the nameservers specified in the forwarders option are queried
first for the information, and if they do not have it, named will attempt the resolution itself.
If the only option is used, named will not attempt the resolution itself if the forwarders are not
forwarders — Specifies a list of nameservers where requests should be forwarded for resolu-
listen-on — Specifies the network interface that named will use to listen for queries. By
default, all interfaces are used.
This option is useful if you have more than one network interface and would like to limit the
systems that can make requests of your nameserver. For example, if you have a machine serving
as a gateway and a nameserver, and you would like to block any requests except those that
originate from your private network, your listen-on option might look like this:
options {
listen-on {; };
In this way, only requests that arrive from the network interface serving the private network
( will be accepted.
notify — Controls whether named notifies the slave servers when a zone is updated. The default
is yes, but you can set this to no, to prevent slaves from being notified, or explicit, to only
notify servers in an also-notify list.
pid-file — Allows you to specify the location of the process ID file created by named when
statistics-file — Allows you to specify the location of where the statistics file is written.
By default, named statistics are saved in /var/named/named.stats.
it starts.
Dozens of other options are also available, many of which rely upon one another to work properly.
See the BIND 9 Administrator Reference Manual for more details.
• server — Defines particular
options that affect how named should act toward remote nameservers,
especially regarding 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.
— Contains assorted public keys used for DNSSEC. See Section 16.4.3 for an
introduction to BIND security.
• trusted-keys
À — Creates special views that respond with a particular type of information depending upon the host contacting the nameserver. This allows some hosts to receive one
answer regarding a particular 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.
• view " view-name "
Multiple views may be used, so long as their names are unique. The match-clients option specifies the IP addresses that apply to a particular view. Any option statements 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
Chapter 16. Berkeley Internet Name Domain (BIND)
statements are listed is important, as the first view statement that matches a particular client’s IP
address is used.
See Section 16.4.2 for more information about the view statement.
 — Specifies particular zones for which this nameserver is authoritative.
The zone statement is primarily used to specify the file containing the zone’s configuration and
pass certain options about that zone to named that override other global option statements used in
• zone " zone-name "
The name of the zone is important, as it is the default value assigned to the $ORIGIN directive used
in the zone file and is appended to non-FQDNs. So, for example, if this zone statement defines the
namespace for, you should use as the à zone-name Ä so it will be
placed at the end of hostnames used in the zone file.
The most common zone statement options include:
allow-query — Specifies the clients that are allowed to request information about this zone.
allow-transfer — Specifies the slave servers that are allowed to request a transfer of the
allow-update — Specifies the hosts that are allowed to dynamically update information in
The default is to allow all query requests.
zone’s information. The default is to allow all transfer requests.
their zone. The default is to deny all dynamic update requests.
Be very careful about allowing hosts to update information about their zone. Do not enable this
option unless the host specified is completely trusted. It is generally better to have an administrator manually update the zone’s records and reload the named service, if possible.
file — Specifies the name of the file in the named working directory that contains the zone’s
configuration data. The default is /var/named.
masters — Used if the zone is defined as a slave type. The masters option tells a slave’s
named the IP addresses from which to request authoritative zone information.
notify — Works in a similar manner to the notify option used with the option statement.
type — Defines the type of zone. The following types may be used:
forward — Tells the nameserver to forward all requests for information about this zone to
hint — A special type of zone that is used to point to the root nameservers, which are used to
master — Designates this nameserver as authoritative for this zone. A zone should be set as
the master type if you have the zone’s configuration files on this system.
slave — Designates this nameserver as a slave server for this zone, telling named to request
other nameservers.
resolve queries when a zone is not otherwise known. You should not need to configure a hint
zone beyond the default in /etc/named.conf.
the zone’s configuration files from the master nameserver’s IP address for that zone.
zone-statistics — Tells named to keep statistics concerning this zone, writing them to either the default location (/var/named/named.stats) or the place specially designated by the
statistics-file option in the server statement, if it exists. 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
Chapter 16. Berkeley Internet Name Domain (BIND)
nameservers use only few of them. The following zone statements are very basic examples that can
be used in a master-slave nameserver relationship.
The following is an example of a zone statement for the primary nameserver hosting
zone "" IN {
type master;
file "";
allow-update { none; };
In the statement, the zone is identified as, the type is set to master, and the named
service is instructed to read the /var/named/ file. It also tells named not to
allow by any other hosts to update.
A slave server’s zone statement for looks slightly different form 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.
A slave server’s zone statement for may look like this:
zone "" {
type slave;
file "";
masters {; };
This zone statement tells named on the slave server to look for the master server at the
IP address to find information for the zone. The information the slave server receives
from the master server is saved in the /var/named/ file.
16.2.2. Zone Files
Zone files, which contain information about a particular namespace, are stored in the named working
directory. By default, this is /var/named. 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
Each zone file may contain directives and resource records. Directives tell the nameserver to do a
certain thing or apply a special setting to the zone. Resource records define the parameters of the
zone, assigning an identity within the zone’s namespace to particular systems. Directives are optional,
but resource records are required to provide nameservice to that zone. All directives and resource
records should go on their own lines.
Comments can be placed after semicolon characters (;) in zone files. Zone File Directives
Directives are identified by the leading $ character before the name of the directive and usually placed
at the top of the zone file.
The following directives are the most commonly used:
— Tells named to include another zone file in this zone file at the place where the
directive is used. This allows additional zone settings to be stored apart from the main zone file.
— Sets the domain name to be appended to any unqualified records, such as those that
only specify the host and nothing more.
Chapter 16. Berkeley Internet Name Domain (BIND)
For example, a zone file may contains the following line:
At this point, any names that are used in resource records and do not end in a trailing dot (.) will
have this domain name added to them. So, in other words, when the zone record is read by the
nameserver, the first line below will be interpreted as the second line:
The use of the $ORIGIN directive is unnecessary if you name the zone in /etc/named.conf the
same as the value you would assign to $ORIGIN. The zone’s name is used as the $ORIGIN directive’s value by default.
— Sets the default Time to Live (TTL) value for the zone. This is the number, in seconds,
given to nameservers that tells how long the zone’s resource records should continue to be valid. A
resource record can contains its own TTL value, which would override this directive.
• $TTL
Increasing this value tells remote nameservers to cache this zone’s information for a longer time.
This reduces the number of queries made concerning this zone, but it also lengthens the amount of
time required to proliferate resource record changes. Zone File Resource Records
Zone file resource records contain columns of data, separated by whitespace, that define the record.
All zone file resource records are assigned a particular type, which designates the record’s purpose.
The following types of resource records are the most commonly used:
• A
— Address record, which specifies an IP address to assign to a name, as in this example:
If the Ç host È value is omitted, then an A record points to a default IP address for the top of the
namespace. This system will be the target of all non-FQDN requests.
Consider the following A record examples for the zone file:
Requests for are pointed to, while requests for are
pointed to
— Canonical name record, maps one name to another: an alias.
The next example tells named that any requests sent to the Ç alias-name È will point to the host,
Ç real-name È . CNAME records are most commonly used to point services that use a common
naming scheme to the correct host.
Consider the following example. In this set up an A record binds a hostname to an IP address, while
a CNAME record points the commonly used www hostname to it.
— Mail eXchange record, which tells where mail sent to a particular namespace controlled by
this zone should go.
• MX
In this example, the Ç preference-value È allows you to numerically rank the email servers
you would prefer to receive email for this namespace, giving preference to some email systems
Chapter 16. Berkeley Internet Name Domain (BIND)
over others. The MX resource record with the lowest É preference-value Ê is preferred over
the others, but you can set multiple email servers with the same value to distribute email traffic
between them.
The É email-server-name Ê may be a hostname or FQDN, as long as it points to the correct
In this example, the first email server is preferred to the
email server when receiving email destined for the domain.
• NS
— NameServer record, which announces the authoritative nameservers for a particular zone.
This is an example of an NS record:
nameserver-name Ê should be a 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.
— PoinTeR record, designed to point to another part of the namespace.
PTR records are primarily used for reverse name resolution, as they point IP addresses back to a
particular name. See Section for more examples of PTR records in use.
• SOA — Start
Of Authority record, 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 example shows the basic structure of an SOA record:
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 primary nameserver that is
authoritative for this domain is used for the É primary-name-server Ê , and the email of the
person to contact about this namespace is substituted for the É hostmaster-email Ê .
The É serial-number Ê is incremented every time you change the zone file so that named will
know that it should reload this zone. The É time-to-refresh Ê tells any slave servers how
long to wait before asking the master nameserver if any changes have been made to the zone. The
É serial-number Ê value is used by the slave to determine if it is using outdated zone data and
should refresh it.
The É time-to-retry Ê tells the slave nameserver the interval to wait before issuing another
refresh request, if the master nameserver is not answering. If the master has not replied to a refresh
request before the É time-to-expire Ê elapses, the slave stops responding as an authority for
requests concerning that namespace.
The É minimum-TTL Ê requests that other nameservers cache the zone’s information for at least
this amount of time (in seconds).
With BIND, all times refer to seconds. However, you can also use abbreviations for other units of
time other than seconds, such as minutes (M), hours (H), days (D), and weeks (W). The table in Table
16-1 shows an amount of time in seconds and the equivalent time in another format.
Chapter 16. Berkeley Internet Name Domain (BIND)
Other Time Units
Table 16-1. Seconds compared to other time units
The following example illustrates the form an SOA resource record might take when it is configured
with real values.
2001062501 ; serial
; refresh after 6 hours
; retry after 1 hour
; expire after 1 week
86400 )
; minimum TTL of 1 day Zone File Examples
Seen individually, the directives and resource records can be difficult to grasp. However, everything
makes much more sense when it is placed together in a common file. The next example shows a very
basic zone file.
$TTL 86400
2001062501 ; serial
; refresh after 6 hours
; retry after 1 hour
; expire after 1 week
86400 )
; minimum TTL of 1 day
Chapter 16. Berkeley Internet Name Domain (BIND)
In this example, standard directives and SOA values are used. The authoritative nameservers are set
to be and, which have A records that tie them to
and, 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 dot (.), the $ORIGIN domain is
placed after them, expanding them to and Through
the related A resource records, their IP addresses can be determined.
The popular FTP and Web services, available at the standard and names, are pointed toward machines providing the appropriate services for those
names using CNAME records. Reverse Name Resolution Zone Files
A reverse name resolution zone file is used to translate an IP address in a particular namespace into a
FQDN. It looks very similar to a standard zone file, except that PTR resource records are used to link
the IP addresses to a certain system’s name.
A PTR record looks similar to this:
The Ï last-IP-digit Ð relates to the last number in an IP address that should point to a particular
system’s FQDN.
In the follow example, IP addresses through are pointed to corresponding
$TTL 86400
2001062501 ; serial
; refresh after 6 hours
; retry after 1 hour
; expire after 1 week
86400 )
; minimum TTL of 1 day
This zone file would be called into service with a zone statement in the /etc/named.conf file that
looks similar to this:
zone "" IN {
type master;
file "";
allow-update { none; };
There is very little difference between this example an a standard zone statement, except for how
the zone is named. Note that a reverse name resolution zone requires the first three blocks of the IP
Chapter 16. Berkeley Internet Name Domain (BIND)
address to be reversed and "" to be included after them. This allows the single block
of IP numbers used in the reverse name resolution zone file to be correctly attached with this zone.
16.3. Using rndc
BIND includes a utility called rndc which allows you to use command line statements to administer
the named daemon, locally, or remotely. The rndc program uses the /etc/rndc.conf file for its
configuration options, which can be overridden with command line options.
In order to prevent unauthorized users on other systems from controlling BIND on your server, a
shared secret key method is used to explicitly grant privileges to particular hosts. In order for rndc
to issue commands to any named, even on a local machine, the keys used in /etc/named.conf and
/etc/rndc.conf must match.
16.3.1. Configuring rndc
Before attempting to use the rndc command, verify that the proper configuration lines are in place in
the necessary files. Most likely, your configuration files are not properly set if you run rndc and see a
message that states:
rndc: connect: connection refused rndc and /etc/named.conf
In order for rndc to be allowed to connect to your named service, you must have a controls statement in your /etc/named.conf file when named starts. The sample controls statement shown in
the next example will allow you to execute rndc commands locally.
controls {
inet 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 Ó key-name Ô relates to
the key statement, which is also in the /etc/named.conf file. The next example illustrates a sample
key statement.
key " key-name " {
algorithm hmac-md5;
secret " key-value ";
In this case, the Ó key-value Ô is a HMAC-MD5 key. You can generate your own HMAC-MD5
keys with the following command:
dnssec-keygen -a hmac-md5 -b
A key with at least a 256-bit length is good idea. The actual key that should be placed in the
value Ô area can found in the Õ key-file-name Ö .
The name of the key used in /etc/named.conf should be something other than key.
Chapter 16. Berkeley Internet Name Domain (BIND)
241 /etc/rndc.conf
You need to add the following lines to /etc/rndc.conf if rndc is to automatically use the keys
specified in /etc/named.conf. This is done with an options statement:
options {
" key-name ";
This command sets a global default key, but the rndc command can also use different keys for particular servers, as in the following example:
server localhost {
key " key-name ";
However, this server statement is only really helpful if you connect to multiple servers with rndc.
The key is the most important statement in /etc/rndc.conf.
key " key-name " {
algorithm hmac-md5;
secret " key-value ";
key-name Ú
key-value Ú
should be exactly the same as their settings in
To test all of the settings, try the rndc reload command. You should see response similar to this:
rndc: reload command successful
If the command was not successful, carefully look over the /etc/named.conf and
/etc/rndc.conf files and look for errors.
16.3.2. rndc Command Line Options
An rndc command takes the following form:
Øi× command Øf× command-options Ø
The Ù options Ú area is not required, and you do not have to use Ù
the command requires them.
command-options Ú unless
When executing rndc on a properly configured localhost, the following commands are available:
• halt
— Stops the named service immediately.
• querylog
• refresh
— Turns on logging of all queries made by clients to this nameserver.
— Refreshes the nameserver’s database.
— Tells the nameserver to reload the zone files but keep all other previously cached responses. This allows you to make changes to zone files and have them take effect on your master
and slave servers without losing all stored name resolutions.
• reload
If your changes only affected a particular zone, you can tell named to only reload that one zone.
Type the name of the zone after the reload command.
Chapter 16. Berkeley Internet Name Domain (BIND)
• stats
• stop
— Dumps the current named stats to the /var/named/named.stats file.
— Stops the server gracefully, saving any dynamic update and IXFR data before exiting.
Occasionally, you may want to override the default settings in the /etc/rndc.conf file. The following options are available:
• -c
• -p
than the default 953.
— Tells rndc to use a configuration file other than the default
— Specifies a different port number to use for the rndc connection other
Û server Ü — Tells rndc to send the command to a server other than the default-server
option in /etc/rndc.conf file.
• -s
In order for this to work, you must have configured the other named service to accept commands
from your host and have the key for that nameservice.
key-name — Allows you to specify a key other than the default-key option in
/etc/rndc.conf file.
• -y
Additional information about these options can be found in the rndc man page.
16.4. 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, when properly configured, allow for a more secure and efficient DNS service.
Some of these advanced features, such as DNSSEC, TSIG, and IXFR, should only be used in network environments with nameservers that support the features. If your network environment includes
non-BIND or older BIND nameservers, check to see if a particular advanced feature is available
before attempting to use it.
Do not assume another type of nameserver supports all of these features, as many do not.
All of the features discussed here are discussed in greater detail in the BIND 9 Administrator Reference
Manual. See Section 16.6 for places to find this manual.
16.4.1. DNS Protocol Enhancements
BIND supports Incremental Zone Transfers (IXFR), where slave nameserver will only download the
updated portions of a zone modified on a master nameserver. The standard transfer AXFR 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 if you are also using dynamic updating to make changes to master
zone records. If you are manually editing zone files to make changes, AXFR will be used. More
information on dynamic updating is available in the BIND 9 Administrator Reference Manual.
Chapter 16. Berkeley Internet Name Domain (BIND)
16.4.2. Multiple Views
Through the use of the view statement in /etc/named.conf, BIND allows you to configure a nameserver to answer queries for some clients in a different way than it answers them for others.
This is primarily used to deny particular types of DNS queries from clients outside of your network,
while allowing those same queries from clients on 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.
16.4.3. 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, a shared secret key exists on the master and slave server,
verifying that a transfer from master to slave is authorized.
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.
BIND version 9 also support TKEY, which is another shared secret key method of authorizing zone
16.4.4. IP version 6
BIND version 9 can provide nameservice in IP version 6 (IPv6) environments, through the use of A6
zone records.
If your network environment includes both IPv4 and IPv6 hosts, you should use the lwresd
lightweight resolver daemon on your network clients. This daemon is essentially a very efficient,
caching-only nameserver, which understands the new A6 and DNAME records used with IPv6. See the
lwresd man page for more information.
16.5. Common Mistakes to Avoid
It is very common for beginners to make mistakes when editing BIND configuration files or run in to
initial difficulties using named. Be sure to avoid the following problems:
Make sure you increment the serial number when editing a zone file.
If the serial number is not incremented, your master nameserver may have the correct, new information, but your slave nameservers will never be notified of the change or 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 will cause named to refuse to start.
Chapter 16. Berkeley Internet Name Domain (BIND)
Remember to place dots (.) in zone files after all FQDNs and omit them on hostnames.
The dot denotes a fully qualified domain name. If the dot is omitted, then named will place the
name of the zone or the $ORIGIN value after the name to complete it.
If you are having problems with your firewall blocking connections from your named program to
other nameservers, you may need to 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. You can force this
behavior by adding the following line to an options statement in /etc/named.conf:
query-source address * port 53;
16.6. Additional Resources
The following sources of information will provide additional resources regarding BIND.
16.6.1. Installed Documentation
BIND features a full-range of installed documentation covering many different topics, each placed
in its own subject directory:
— Contains a README file with a list of the
/usr/share/doc/bind- version-number
/usr/share/doc/bind- version-number /arm — Contains HTML and SGML of the
/usr/share/doc/bind- version-number /draft — Contains assorted technical docu-
/usr/share/doc/bind- version-number /misc — 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 — Every RFC document related to
most recent features.
BIND 9 Administrator Reference Manual, which details BIND resource requirements, how to
configure different types of nameservers, perform load balancing, and other advanced topics. For
most new users of BIND, this is the best place to start.
ments that look at issues related to DNS service and some methods proposed to address them.
BIND is in this directory.
The following man pages are also useful:
named — Explores assorted arguments that can be used to control the BIND nameserver daemon,
rndc — Explains the different options available when using the rndc command to control a
such as the use of an alternative configuration file and running on a different port number or as a
different user.
BIND nameserver.
16.6.2. Useful Websites
• — The home page of the BIND project, where you can find
information concerning current releases and download a PDF version of the BIND 9 Administrator
Reference Manual.
Chapter 16. Berkeley Internet Name Domain (BIND)
245 — Covers the use of BIND as
a resolving, caching nameserver or the configuration of various zone files necessary to serve as the
primary nameserver for a domain.
16.6.3. 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 to
secure your 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.
Chapter 16. Berkeley Internet Name Domain (BIND)
Chapter 17.
Network File System (NFS)
NFS (Network File System) exists to allow hosts to mount partitions on a remote system and use them
as though they were local file systems. This allows files to be organized in a central location, while
providing the functionality of allowing authorized users continuous access to them.
Two versions of NFS are currently in use. NFS version 2 (NFSv2), which has been around for several
years, is widely supported by various operating systems. NFS version 3 (NFSv3) has several more
features, including a variable file handle size and better error reporting. Red Hat Linux supports both
NFSv2 and NFSv3, and uses NFSv3 by default when connecting with a server that supports it.
This chapter will focus on NFS version 2, though many of the concepts discussed also apply to version
3. Additionally, only fundamental NFS concepts and supplemental information will be provided. For
specific instructions regarding the configuration and operation of NFS on client or server machines,
see the chapter titled Network File System (NFS) in the Official Red Hat Linux Customization Guide.
17.1. Methodology
Linux uses a combination of kernel-level support and continuously running daemon processes to
provide NFS file sharing, however, NFS support must be enabled in the Linux kernel to function. NFS
uses Remote Procedure Calls (RPC) to route requests between clients and servers, meaning that the
portmap service must be enabled and active at the proper runlevels for NFS communication to occur.
Working with portmap, various other processes ensure that a particular NFS connection is allowed
and may proceed without error:
— The running process that receives the mount request from an NFS client and
checks to see if it matches with a currently exported file system.
• rpc.mountd
— The process that implements the user-level part of the NFS service. It works with
the Linux kernel to meet the dynamic demands of NFS clients, such as providing additional server
threads for NFS clients to uses.
• rpc.nfsd
• rpc.lockd —
A daemon that is not necessary with modern kernels. NFS file locking is now done
by the kernel. It is included with the nfs-utils package for users of older kernels that do not
include this functionality by default.
• rpc.statd — Implements
the Network Status Monitor (NSM) RPC protocol. This provides reboot
notification when an NFS server is restarted without being gracefully brought down.
• rpc.rquotad —
An RPC server that provides user quota information for remote users.
Not all of these programs are required for NFS service. The only services that must be enabled are
rpc.mountd, rpc.nfsd, and portmap. The other daemons provide additional functionality and
should only be used if your server environment requires them.
NFS version 2 uses the User Datagram Protocol (UDP) to provide a stateless network connection
between the client and server. NFS version 3 can use UDP or TCP running over an IP. The stateless
UDP connection minimizes network traffic, as the NFS server sends the client a cookie after the client
is authorized to access the shared volume. This cookie is a random value stored on the server’s side
and is passed with along with RPC requests from the client. The NFS server can be restarted without
affecting the clients and the cookie will remain intact.
NFS only performs authentication when a client system attempts to mount a remote file
system. To limit access, the NFS server first employs TCP wrappers. TCP wrappers reads the
/etc/hosts.allow and /etc/hosts.deny files to determine if a particular client should be
Chapter 17. Network File System (NFS)
permitted or prevented access to the NFS server. For more information on configuring access
controls with TCP wrappers, see Chapter 8.
After the client is allowed past TCP wrappers, the NFS server refers to its configuration file,
/etc/exports, to determine whether the client has enough privileges to mount any of the exported
file systems. After granting access, any file and directory operations are sent to the server using
remote procedure calls.
NFS mount privileges are granted specifically to a client, not a user. If you grant a client machine
access to an exported file system, any users of that machine will have access to the data.
When configuring the /etc/exports file, be extremely careful about granting read-write permissions
(rw) to a remote host.
17.1.1. NFS and portmap
NFS relies upon remote procedure calls (RPC) to function. portmap is required to map RPC requests
to the correct services. RPC processes notify portmap when they start, revealing the port number they
are monitoring and the RPC program numbers they expect to serve. The client system then contacts
portmap on the server with a particular RPC program number. portmap then redirects the client to
the proper port number to communicate with its intended 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. If, for some reason, the portmap service
unexpectedly quits, restart portmap and any services running when it was started.
The portmap service can be used with the host access files (/etc/hosts.allow and
/etc/hosts.deny) to control which remote systems are permitted to use RPC-based services on
your machine. See Chapter 8 for more information. Access control rules for portmap will affect all
RPC-based services. Alternatively, you can specify each of the NFS RPC daemons to be affected
by a particular access control rule. The man pages for rpc.mountd and rpc.statd contain
information regarding the precise syntax of these rules. portmap Status
As portmap provides the coordination between RPC services and the port numbers used to communicate with them, it is useful to be able to get a picture of the current RPC services using portmap
when troubleshooting. The rpcinfo command shows each RPC-based service with its port number,
RPC program number, version, and IP protocol type (TCP or UDP).
To make sure the proper NFS RPC-based services are enabled for portmap, rpcinfo -p can be
program vers proto
Chapter 17. Network File System (NFS)
The -p option probes the portmapper on the specified host or defaults to localhost if no specific host
is listed. Other options are available from the rpcinfo man page.
From the output above, various NFS services can be seen running. If one of the NFS services does
not start up correctly, portmap will be unable to map RPC requests from clients for that service to
the correct port. In many cases, restarting NFS as root (/sbin/service nfs restart) will cause
those service to correctly register with portmap and begin working.
17.2. NFS Server Configuration Files
Configuring a system to share files and directories using NFS is straightforward. Every file system
being exported to remote users via NFS, as well as the access rights relating to those file systems, is
located in the /etc/exports file. This file is read by the exportfs command to give rpc.mountd
and rpc.nfsd the information necessary to allow the remote mounting of a file system by an authorized host.
The exportfs command allows you to selectively export or unexport directories without restarting
the various NFS services. When exportfs is passed the proper options, the file systems to be exported are written 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.
Various options are available when using exportfs:
— 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.
• -r
— Causes all directories to be exported or unexported, depending on the other options passed
to exportfs.
• -a
• -o options
— Allows the user to specify directories to be exported that are not listed in
/etc/exports. These additional file system shares must be written in the same way they are
specified in /etc/exports. This option is used to test an exported file system before adding it
permanently to the list of file systems to be exported.
— Tells exportfs to ignore /etc/exports; only options given from the command line are
used to define exported file systems.
• -i
— Unexports directories from being mounted by remote users. The command exportfs -ua
effectively suspends NFS file sharing while keeping the various NFS daemons up. To allow NFS
sharing to continue, type exportfs -r.
• -u
— Verbose operation, where the file systems being exported or unexported are displayed in
greater detail when the exportfs command is executed.
• -v
If no options are passed to the exportfs command, it displays a list of currently exported file systems.
Changes to /etc/exports can also be read by reloading the NFS service with the service nfs
reload command. This keeps the NFS daemons running while re-exporting the /etc/exports file.
Chapter 17. Network File System (NFS)
17.2.1. /etc/exports
The /etc/exports file is the standard for controlling which file systems are exported to which hosts,
as well as specifying particular options that control everything. Blank lines are ignored, comments can
be made using #, and long lines can be wrapped with a backslash (\). Each exported file system should
be on its own line. 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 directly after the host
identifier, without any spaces separating the host and the first parenthesis.
In its simplest form, /etc/exports only needs to know the directory to be exported and the hosts
permitted to use it:
After re-exporting /etc/exports with the /sbin/service nfs reload command, the host will be able to mount /some/directory and can mount
/another/exported/directory. Because no options are specified in this example, several
default NFS preferences take effect:
— Read-only. Hosts mounting this file system will not be able to change it. To allow hosts to
make changes to the file system, you must specify rw (read-write).
• ro
— Allows the server to write data to the disk when it sees fit. While this is not important
if the host is accessing data as read-only, if a host is making changes to a read-write file system
and the server crashes, data could be lost. By specifying the sync option, all file writes must be
committed to the disk before the write request by the client is actually completed. This may lower
• async
— 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. Use no_wdelay to turn this feature
off, which only works if you are using the sync option.
• wdelay
— Makes any client accesses to the exported file system, made as the root user on
the client machine, take place as the nobody user ID. This effectively "squashes" the power of the
remote root user to the lowest local user, preventing remote root users from acting as though they
were the root user on the local system. 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 way, you can create a special user account for remote NFS
users to share and specify (anonuid= ß uid-value à ,anongid= ß gid-value à ), where ß uidvalue à is the user ID number and ß gid-value à is the group ID number.
• root_squash
In order to override these defaults, you must specify an option that takes its place. For example, if
you do not specify rw, then that export will only be shared read-only. Each default for every exported
file system must be explicitly overridden. Additionally, other options are available where no default
value is in place. 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). See the
exports man page for details on these lesser used options.
When specifying hostnames, you can use the following methods:
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 or IP addresses or those that match a particular string of letters.
Chapter 17. Network File System (NFS)
However, be careful when using wildcards with fully qualified domain names, as they tend to be
more exact than you would expect. For example, the use of * as wildcard will allow to access the exported file system, but not To match
both possibilities, as well as, you would have to provide *
IP networks — Allows the matching of hosts based on their IP addresses within a larger network. For example, will allow the first 16 IP addresses, from to, to access the exported file system but not and higher.
netgroups — Permits an NIS netgroup name, written as @ á group-name â , 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.
The way in which the /etc/exports file is formatted is very important, particularly concerning the
use of space characters. 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 unless they are used in comment lines.
For example, the following two lines do not mean the same thing:
/home (rw)
The first line allows only users from read-write access to the /home directory.
The second line allows users from to mount the directory read-only (the default),
but the rest of the world can mount it read-write. Be careful where space characters are used in
17.3. NFS Client Configuration Files
Any NFS share made available by a server can be mounted using various methods. Of course, the
share can be manually mounted, using themount command, to acquire the exported file system at a
particular mount point. However, this requires that the root user type the mount command every time
the system restarts. In addition, the root user must remember to unmount the file system when shutting
down the machine. Two methods of configuring NFS mounts include modifying the /etc/fstab or
using the autofs service.
17.3.1. /etc/fstab
Placing a properly formatted line in the /etc/fstab file has the same effect as manually mounting
the exported file system. The /etc/fstab file is read by the /etc/rc.d/init.d/netfs script at
system startup. The proper file system mounts, including NFS, are put into place.
A sample /etc/fstab line to mount an NFS export looks like the following:
â á
server : /path/of/dir
0 0
The á server-host â relates to the hostname, IP address, or fully qualified domain name of the
server exporting the file system. The á /path/to/shared/directory â tells the server what export to mount. The á /local/mount/point â specifies where on the local file system to mount the
exported directory. This mount point must exist before /etc/fstab is read or the mount will fail.
The nfs option specifies the type of file system being mounted.
Chapter 17. Network File System (NFS)
The ã options ä area specifies how the file system is to be mounted. For example, if the options area
states rw,suid on a particular mount, the exported file system will be mounted read-write and the
user and group ID set by the server will be used. Note, parentheses are not to be used here. For more
mount options, see Section 17.3.3.
17.3.2. autofs
One drawback to using /etc/fstab is that, regardless of how much you use that mounted file system,
your system must dedicate resources to keep that mount in place. This is not a problem with one or
two mounts, but when your system is maintaining mounts to a dozen systems at one time, overall
system performance can suffer. An alternative to /etc/fstab is to use the kernel-based automount
utility, which will mount and unmount NFS file systems automatically, saving resources.
The autofs script, located in /etc/rc.d/init.d, is used to control automount through the
/etc/auto.master primary configuration file. While automount can be specified on the
command line, it is more convenient to specify the mount points, hostname, exported directory, and
options in a set of files rather than typing them all by hand. By running autofs as a service that starts
and stops in designated runlevels, the mount configurations in the various files can be automatically
implemented. In order to use autofs, you must have the autofs RPM installed on your system.
The autofs configuration files are arranged in a parent-child relationship. A main configuration file
(/etc/auto.master) refers mount points on your system that are linked to a particular map type,
which take the form of other configuration files, programs, NIS maps, and other less common mount
methods. The auto.master file contains lines referring to each of these mount points, organized like
The ã mount-point ä indicates where the device or exported file system should mount on your local
file system. The ã map-type ä relates to the way in which the mount point will be mounted. The
most common method for auto mounting NFS exports is to use a file as the map type for the particular
mount point. The map file, usually named auto. ã mount-point ä , where ã mount-point ä is the
mount point designated in auto.master, contains lines that look like this:
ä ã
host : exported-file-system
The ã directory ä refers to the directory within the mount point where the exported file system
should be mounted. Much like a standard mount command, the host exporting the file system, as
well as the file system being exported, are required in the ã host ä : ã exported-file system ä
section. To specify particular options to be used when mounting the exported file system, place them
in the ã mount-options ä section, separated by commas. For NFS mounts that use autofs, you
should definitely place -fstype=nfs in the ã mount-options ä section, at a minimum.
While autofs configuration files can be used for a variety of mounts to many types of devices and file
systems, they are particularly useful in creating NFS mounts. For example, some organizations store a
user’s /home directory on a central server via an NFS share. Then, they configure the auto.master
file on each of the workstations to point to an auto.home file containing the specifics for how to
mount the /home directory via NFS. This allows the user to access personal data and configuration
files in their /home directory by logging in anywhere on the internal network. The auto.master file
in this situation would look similar to this:
This sets up the /home mount point on the local system to be configured by the /etc/auto.home
file, which may look similar to this:
Chapter 17. Network File System (NFS)
This line states that any directory a user tries to access under the local /home directory (due to the
asterisk character) should result in an NFS mount on the system within its exported
/home file system. The mount options specify that each /home directory NFS mounts should use a
particular collection of settings. For more information on mount options, including the ones used in
this example, see Section 17.3.3.
17.3.3. Common NFS Mount Options
Beyond mounting a file system via NFS on a remote host, a number of different options may be
specified at the time of the mount that can make it easier to use. These options can be used with
manual mount commands, /etc/fstab settings, and autofs, and other mounting methods.
The following options are the most popular for NFS mounts:
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).
• hard
If you specify hard, you will not be able to terminate the process waiting for the NFS communication to resume unless you also specify the intr option.
If you specify soft, you can set an additional timeo= ç value è option, where ç value è specifies
the number of seconds to pass before the error is reported.
• intr
— allows NFS requests to be interrupted if the server goes down or cannot be reached.
— is occasionally required when connecting to older NFS server. To require locking, use
the lock option.
• nolock
— does not permit the execution of binaries on the mounted file system. This is useful if
your Red Hat Linux system is mounting a non-Linux file system via NFS that contains binaries that
will not execute on your machine.
• noexec
• nosuid
— does not allow set-user-identifier or set-group-identifier bits to take effect.
• rsize=8192 and wsize=8192 — may 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 may not work well with larger
block sizes.
• nfsvers=2
or nfsvers=3 — specify which version of the NFS protocol to use.
Many more options are listed on the mount man page, including options for mounting non-NFS file
17.4. Securing NFS
NFS works well for sharing entire file systems with a large number of known hosts in a largely
transparent manner. Many users accessing files over an NFS mount may not be aware that the file
system they are using is not local to their system. 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 will minimize NFS security risks and better protect your data.
Chapter 17. Network File System (NFS)
17.4.1. Host Access
NFS controls who can mount an exported file system based on the host making the mount request, not
the user that will actually use the file system. Hosts must be given explicit rights to mount the exported
file system. Access control is not possible for users, other than file and directory permissions. In other
words, when you export a file system via NFS to a remote host, you are not only trusting the host you
are allowing to mount the file system. You are also allowing any user with access to that host to use
your file system as well. The risks of doing this can be controlled, such as requiring read-only mounts
and squashing users to a common user and group ID, but these solutions may prevent the mount from
being used in the way 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. The same risks hold true to compromised NIS servers, if NIS netgroups
are used to allow certain hosts to mount an NFS share. By using IP addresses in /etc/exports, this
kind of attack is more difficult.
Wildcards should be used sparingly when granting host access to an NFS share. The scope of the
wildcard may encompass systems that you may not know exist and should not be allowed to mount
the file system.
17.4.2. File Permissions
Once the NFS file system is mounted read-write by a remote host, protection for each shared file
involves its permissions, and its user and group ID ownership. If two users that share the same user
ID value mount the same NFS file system, they will be able to 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.
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
nobody account. You should never turn off root squashing unless multiple users with root access to
your server does not bother you.
If you are only allowing users to read files via your NFS share, consider using the all_squash
option, which makes every user accessing your exported file system to take the user ID of the nobody
17.5. 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 NFS file systems or mounting them as a client. Consult
these sources of information for more details.
17.5.1. Installed Documentation
ê — Covers the way NFS is implemented
in Linux, including a look at various NFS configurations and their impact on file transfer performance.
• /usr/share/doc/nfs-utilrs- version-number
The following man pages are very helpful:
mount — Contains a comprehensive look at mount options for both NFS server and client con-
Chapter 17. Network File System (NFS)
fstab — Gives details for the format of the /etc/fstab file used to mount file systems at
nfs — Provides detail on NFS-specific file system export and mount options.
exports — Shows common options used in the /etc/exports file when exporting NFS file
system boot.
17.5.2. 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.
Chapter 17. Network File System (NFS)
Chapter 18.
Lightweight Directory Access Protocol (LDAP)
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.
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 Network Information Service (NIS), enabling anyone to access their account from any machine on the LDAP enabled
In most cases, however, LDAP is used simply as a virtual phone directory, allowing users to easily
access contact information for other users. But LDAP goes beyond a traditional phone directory, because it is capable of propagating its directories to other LDAP servers throughout the world, providing global access to 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 upload information to it. In the event of a query, the
server either answers the query or, if it can not answer locally, it can refer the query upstream to a
higher level LDAP server which does have the answer. If the client application is attempting to upload
information to an LDAP directory, the server verifies that the user has permission to make the change
and then adds or updates the information.
This chapter will refer to the configuration and use of OpenLDAP 2.0, an open source implementation
of the LDAPv2 and LDAPv3 protocol.
18.1. Why Use LDAP?
The main benefit of 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, you can use LDAP as a central directory accessible from anywhere on the network. And since
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. Also, because LDAP has a well-defined Application Programming Interface (API), the
number of LDAP-enabled applications are numerous and increasing in quantity and quality.
On the negative side, LDAP can be non-trivial to configure.
18.1.1. OpenLDAP 2.0 Feature Enhancements
OpenLDAP 2.0 includes a number of important features.
LDAPv3 Support — OpenLDAP 2.0 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 obviating the need to communicate over a network.
Chapter 18. Lightweight Directory Access Protocol (LDAP)
Updated C API — Improves the way programmers can connect to and use the application.
LDIFv1 Support — 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.
18.2. OpenLDAP Daemons and Utilities
The suite of OpenLDAP libraries and tools is spread out over the following packages:
• openldap
— Contains the libraries necessary to run the openldap server and client applications.
• openldap-clients
an LDAP server.
• openldap-server
LDAP server.
— Contains command-line tools for viewing and modifying directories on
— Contains the servers and other utilities necessary to configure and run an
There are two servers contained in the openldap-servers package: the Standalone
LDAP Daemon (/usr/sbin/slapd) and the Standalone LDAP Update Replication Daemon
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
necessary when dealing with multiple LDAP servers.
To perform administrative tasks, the openldap-server package installs the following utilities into
the /usr/sbin/ directory:
• slapadd
— Adds entries from an LDIF file to an LDAP directory. For example,
/usr/sbin/slapadd -l ldif-input will read in the LDIF file, ldif-input, containing the
new entries.
— Pulls entries out of an LDAP directory in the default format — Berkeley DB — and
saves them in an LDIF file. For example, the command /usr/sbin/slapcat -l ldif-output
will output an LDIF file called ldif-output containing the entries from the LDAP directory.
• slapcat
• slapindex
— Re-indexes the slapd directory based on the current content.
— Generates an encrypted user password value for use with ldapmodify or
the rootpw value in the slapd configuration file, /etc/openldap/slapd.conf. Execute
/usr/sbin/slappasswd to create the password.
• slappasswd
Be sure to stop slapd by issuing /usr/sbin/service slapd stop before using slapadd, slapcat
or slapindex. Otherwise, the consistency of the LDAP directory is at risk.
See the man pages for each of these utilities for more information about how to use them.
The openldap-clients package installs tools used to add, modify, and delete entries in an LDAP
directory into /usr/bin/. These tools include the following:
• ldapmodify — Modifies
• ldapadd — Adds entries
entries in an LDAP directory, accepting input via a file or standard input.
to your directory by accepting input via a file or standard input; ldapadd
is actually a hard link to ldapmodify -a.
Chapter 18. Lightweight Directory Access Protocol (LDAP)
• ldapsearch
— Searches for entries in the LDAP directory using a shell prompt.
— Deletes entries from an LDAP directory by accepting input via user input at the
terminal or via a file.
• ldapdelete
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 you wish to change
in an LDAP directory. The format of such a file is outlined in each application’s man page.
18.2.1. NSS, PAM, and LDAP
In addition to the OpenLDAP packages, Red Hat 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:
• /lib/libnss_ldap- glibc-version .so
• /lib/security/
The libnss_ldap- ë glibc-version ì .so module allows applications to look up user, group,
hosts, and other information using an LDAP directory via glibc’s Nameservice Switch (NSS) interface. NSS allows applications to authenticate using LDAP in conjunction with Network Information
Service (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 your network, all of these login situations can authenticate against one user ID and password combination, greatly simplifying administration.
18.2.2. PHP4, the Apache HTTP Server, and LDAP
Red Hat Linux includes a package containing LDAP module for the PHP server-side scripting language.
The php-ldap package adds LDAP support to the PHP4 HTML-embedded scripting language via the
/usr/lib/php4/ module. This module allows PHP4 scripts to access information stored
in an LDAP directory.
Red Hat Linux 8.0 no longer ships with the auth_ldap package. This package provided LDAP support
for versions 1.3 and earlier of the Apache HTTP Server. See the Apache Software Foundation website
at for details on the status of this module.
18.2.3. LDAP Client Applications
There are graphical LDAP clients available which support creating and modifying directories, but they
do not ship with Red Hat Linux 8.0. One such application is LDAP Browser/Editor — A Java-based
tool available at
Most other LDAP clients access directories as read-only, using them to reference, but not alter,
organization-wide information. Some examples of such applications are Mozilla-based Web browsers,
Sendmail Balsa, Pine, Evolution, Gnome Meeting.
Chapter 18. Lightweight Directory Access Protocol (LDAP)
18.3. LDAP Terminology
An entry is one unit in an LDAP directory. Each entry is identified by its unique Distinguished Name
Each entry has attributes, which are pieces of information directly associated with the entry. For
example, an organization could be an LDAP entry. Attributes associated with the organization might
be its fax number, its address, and so on. People can also be entries in the LDAP directory. Common
attributes for people include the person’s telephone numbers and email addresses.
Some attributes are required, while other attributes are optional. An objectclass definition sets which
attributes are required and which are not for each entry. Objectclass definitions are found in various
schema files, located in the /etc/openldap/schema/ directory.
The LDAP Data Interchange Format (LDIF) is an ASCII text format for LDAP entries. Files that
import or export data to and from 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
An entry can contain as many í attrtype î :
the end of an entry.
All attrtype and
this information.
pairs as needed. A blank line indicates
pairs must be defined in a corresponding schema file to use
Any value enclosed within a í and a î is a variable and can be set whenever a new LDAP entry is
created, except for í id î . The í id î is a number determined by the application you use to edit the
You should never need to edit an LDIF entry by hand. Instead use an LDAP client application, such
as the ones listed in Section 18.2.
18.4. 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/schema/ directory
— This subdirectory contains the schema used by the slapd
daemon. See Section 18.4.2 for more information about this directory.
• /etc/openldap/ldap.conf — This is
the configuration file for all client applications which use
the OpenLDAP libraries. These include, but are not limited to, Sendmail, Pine, Balsa, Evolution,
and Gnome Meeting.
Chapter 18. Lightweight Directory Access Protocol (LDAP)
— This is the configuration file for the slapd daemon. See Section 18.4.1 for more information about this file.
• /etc/openldap/slapd.conf
If the nss_ldap package is installed, it will create a file named /etc/ldap.conf. This file is used by
the PAM and NSS modules supplied by the nss_ldap package. See Section 18.7 for more information
about this configuration file.
18.4.1. slapd.conf
In order to use the slapd LDAP server, you will need to modify its configuration file,
/etc/openldap/slapd.conf. You must to edit this file to make it specific to your domain and
The suffix line names the domain for which the LDAP server will provide information. The suffix
line should be changed from:
so that it reflects your domain name. For example:
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 to something like the example below:
Change the rootpw line to something like the example below:
In the rootpw example, you are using an encrypted root password, which is a much better idea than
leaving a plain text root password in the slapd.conf file. To make this encrypted string, type the
following command:
You will be prompted to type and then re-type a password. The program prints the resulting encrypted
password to the terminal.
LDAP passwords, including the rootpw directive specified in /etc/openldap/slapd.conf, are sent
over the network in plain text unless you enable TLS encryption.
For added security, the rootpw directive should only be used if the initial configuration and population
of the LDAP directory occurs over a network. After the task is completed, it is best to comment out
the rootpw directive by preceding it with a pound sign (#).
Chapter 18. Lightweight Directory Access Protocol (LDAP)
If you are using the slapadd command-line tool locally to populate the LDAP directory, using the
rootpw directive is not necessary.
18.4.2. The /etc/openldap/schema/ Directory
The /etc/openldap/schema/ directory holds LDAP definitions, previously located in the and slapd.oc.conf files. 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:
You should not modify any of the schema items defined in the schema files installed by OpenLDAP.
You can 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 your default include schema lines:
Next, go about defining your new attribute types and object classes within the local.schema file.
Many organizations use existing attribute types and object classes from the schema files installed by
default and modify them for use in the local.schema file. This can help you to learn the schema
syntax while meeting the immediate needs of your organization.
Extending schema to match certain specialized requirements is quite involved and beyond the scope
of this chapter. Visit for information on writing new
schema files.
18.5. OpenLDAP Setup Overview
This section provides a quick overview for installing and configuring an OpenLDAP directory. For
more details, refer to the following URLs:
• — The Quick-Start Guide on the OpenLDAP
• — The LDAP Linux HOWTO
from the Linux Documentation Project, mirrored on Red Hat’s website
Chapter 18. Lightweight Directory Access Protocol (LDAP)
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 reference your LDAP domain and server. Refer
to Section 18.4.1 for more information on how to edit this file.
3. Start slapd with the command:
/sbin/service/ldap start
After you have configured LDAP correctly, you can use chkconfig, ntsysv, or Services Configuration Tool to configure LDAP to start at boot time. For more information about configuring
services, see to the chapter titled Controlling Access to Services in the Official Red Hat Linux
Customization Guide.
4. Add entries to your LDAP directory with ldapadd.
5. Use ldapsearch to see if slapd is accessing the information correctly.
6. At this point, your LDAP directory should be functioning properly and you can configure any
LDAP-enabled applications to use the LDAP directory.
18.6. Upgrading To OpenLDAP Version 2.0
In OpenLDAP Version 2.0, the on-disk storage format used by the slapd LDAP server has changed.
If you are upgrading LDAP from Red Hat Linux 7.0 or earlier, you will need to extract the existing
LDAP directories to an LDIF file using the following command:
ldbmcat -n
In the above command, change ô ldif_file õ to the name of the output file. Next type the following command to import this file into OpenLDAP 2.0:
slapadd -l
18.7. Configuring Your System to Authenticate Using OpenLDAP
This section provides a brief overview of how to configure a Red Hat Linux system to authenticate
using OpenLDAP. Unless you are an OpenLDAP expert, you will probably need more documentation
than is provided here. Please refer to the references provided in Section 18.8 for more information.
18.7.1. Install the Necessary LDAP Packages
First, you should make sure that the appropriate packages are installed on both the LDAP server and
the LDAP client machines. The LDAP server needs the openldap-server package.
The LDAP client machines need the following packages installed: openldap, openldap-clients,
and nss_ldap.
18.7.2. Edit the Configuration Files On the Server, Edit slapd.conf
Edit the /etc/openldap/slapd.conf file on the LDAP server to make sure it matches the specifics
of your organization. Please refer to Section 18.4.1 for instructions on editing slapd.conf.
Chapter 18. Lightweight Directory Access Protocol (LDAP) On the Clients, Edit /etc/ldap.conf and /etc/openldap/ldap.conf
On all client machines, both /etc/ldap.conf and /etc/openldap/ldap.conf need to contain
the proper server and search base information for your organization.
The simplest way to do this is to run the authconfig application and select Use LDAP on the the User
Information Configuration screen.
You can also edit these files by hand. On the Clients, Edit /etc/nsswitch.conf
On all client machines, the /etc/nsswitch.conf must be edited to use LDAP.
The simplest way to do this is to run the authconfig application and select Use LDAP on the the User
Information Configuration screen.
If editing /etc/nsswitch.conf by hand, add ldap to the appropriate fields.
For example:
passwd: files ldap
shadow: files ldap
group: files ldap PAM and LDAP
To have standard PAM-enabled applications use LDAP for authentication, run authconfig and select
Use LDAP Authentication on the the Authentication Configuration screen. For more on configuring PAM consult, Chapter 7 and the PAM man pages.
18.7.3. 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 LDAP format.
You must have Perl installed on your system to use these scripts.
First, you will need to modify the file so that it reflects your domain. The
default DNS domain should be changed from its default value to something like:
$DEFAULT_MAIL_DOMAIN = "your_company";
The default base should also be changed, to something like:
$DEFAULT_BASE = "dc=your_company,dc=com";
The job of migrating a user database into a format LDAP can read falls to a group of migration scripts
installed with the nss_ldap package. Using Table 18-1, decide which script to run in order to migrate
your user database.
Chapter 18. Lightweight Directory Access Protocol (LDAP)
Existing name service
Script to Use
/etc flat files
/etc flat files
Table 18-1. LDAP Migration Scripts
Run the appropriate script based on your existing name service.
The README and the migration-tools.txt files in the /usr/share/openldap/migration directory provide more details on how to migrate the information.
18.8. Additional Resources
More information concerning LDAP is available. Please review these sources, especially the OpenLDAP website and the LDAP HOWTO, before configuring LDAP on your system.
18.8.1. Installed Documentation
LDAP man pages — The ldap man page is a good place to get started for an introduction to LDAP.
Man pages also exist for the various LDAP daemons and utilities.
• /usr/share/docs/openldap- versionnumber
and miscellaneous information.
— Contains a general README document
18.8.2. Useful Websites
• — Home of the OpenLDAP Project. This website contains a wealth of
information about configuring OpenLDAP.
• — An older, but still relevant
• — Developers of nss_ldap and pam_ldap, among other useful LDAP tools.
• — Jeff Hodges’ LDAP Road Map contains
links to several useful FAQs and emerging news concerning the LDAP protocol.
• — A useful look at managing groups in
• — Articles that offer a good introduction to LDAP, including
methods to design an directory tree and customizing directory structures.
Chapter 18. Lightweight Directory Access Protocol (LDAP)
18.8.3. Related Books
Implementing LDAP by Mark Wilcox; Wrox Press, Inc.
Understanding and Deploying LDAP Directory Services by Tim Howes et al.; Macmillan Technical
Appendix A.
General Parameters and Modules
This appendix is provided to illustrate some of the possible parameters available for some common
hardware device drivers1. In most cases, these additional parameters are unnecessary, since the kernel
may already be able to use the device without them. You should only use the settings provided in this
appendix if you are having trouble getting Red Hat Linux to use a particular device or you need to
override the system’s default parameters for the device.
During the installation of Red Hat Linux, some limits are placed on file systems and some device
drivers supported by the kernel. After installation, however, support exists for all file systems available under Linux. At the time of installation, the modularized kernel has support for (E)IDE devices
(including ATAPI CD-ROM drives), SCSI adapters, and network cards.
Because Red Hat Linux supports installation on many different types of hardware, some drivers
(including those for SCSI adapters, network cards, and many CD-ROMs) are not built into the Linux
kernel used by the installation program. Rather, they are available as modules and are loaded as you
need them during the installation process. If necessary, you will have the chance to specify options
for these modules when they are loaded from the driver disk.
To specify module parameters when a driver is loaded, type linux expert at the boot: prompt
and insert the driver disk when prompted to do so by the installation program. After reading the
driver disk, the installation program will ask you to select the type of device you are configuring. On
that screen, you can elect to specify a module parameter. Then, the installation program will display
a screen where you can type the correct parameters based on the particular type of device you are
After the installation is complete, you may want to rebuild a kernel that includes support for your
specific hardware configuration. Note that in most cases, a custom-built kernel is not necessary. See
the Official Red Hat Linux Customization Guide for more information about rebuilding your kernel.
A.1. Specifying Module Parameters
If you are providing parameters upon loading a module, you can usually specify them using one of
two different methods:
Specify a full set of parameters in one statement. For example, the parameter cdu31=0x340,0
could be used with a Sony CDU 31 or 33 at port 340 with no IRQ.
Specify the parameters individually. This method is used when one or more parameters in the
first set are not needed. For example, cdu31_port=0x340 cdu31a_irq=0 can be used as the
parameter for the same CD-ROM used as an example for the first method. An OR is used in the
CD-ROM, SCSI, and Ethernet tables in this appendix to show where the first parameter method
stops and the second method begins.
A driver is software enabling your system to use a particular hardware device. Without the driver, the kernel
may not know how to correctly access the device.
Appendix A. General Parameters and Modules
Only use one method, and not both, when loading a module with particular parameters.
When a parameter has commas, make sure you do not put a space after a comma.
A.2. CD-ROM Module Parameters
Not all of the CD-ROM drives that are listed are supported. Please check the Hardware Compatibility List on Red Hat’s website at to make sure your CD-ROM drive is
Even though parameters are specified after loading the driver disk and specifying the device, one
of the more commonly used parameters (hdX=cdrom) can be entered at the boot prompt (boot:)
during installation. This exception to the rule is allowed since it deals with support for IDE/ATAPI
CD-ROMs, which is already part of the kernel.
In the following tables, most modules listed without any parameters can either be auto-probed to
find the hardware or they require you to manually change settings in the module source code and
Aztech CD268-01A, Orchid
CD-3110, Okano/Wearnes
CDD110, Conrad TXC,
CyCDROM CR540 (non-IDE)
cdu31a=io_port,IRQ OR
Philips/LMS CDROM drive
206 with cm260 host adapter
Goldstar R420 CD-ROM
Appendix A. General Parameters and Modules
ISP16, MAD16, or Mozart
sound card CD-ROM interface
(OPTi 82C928 and OPTi
82C929) with
Sanyo/Panasonic, Sony, or
Mitsumi drives
drive_type OR
Mitsumi CD-ROM, Standard
Mitsumi CD-ROM,
Optics storage 8000 AT
"Dolphin" drive, Lasermate
Parallel-Port IDE CD-ROM
SB Pro 16 Compatible
Sanyo CDR-H94A
sjcd=io_port OR
Sony CDU-535 & 531 (some
Procomm drives)
Table A-1. Hardware Parameters
Here are some examples of these modules in use:
ATAPI CD-ROM, jumpered as master on the
second IDE channel
non-IDE Mitsumi CD-ROM on port 340, IRQ
Three non-IDE Mitsumi CD-ROM drives using
the experimental driver, io ports 300, 304, and
320 with IRQs 5, 10 and 11
Sony CDU 31 or 33 at port 340, no IRQ
cdu31=0x340,0 OR cdu31_port=0x340
Aztech CD-ROM at port 220
Panasonic-type CD-ROM on a SoundBlaster
interface at port 230
Phillips/LMS cm206 and cm260 at IO 340 and
IRQ 11
Goldstar R420 at IO 300
Mitsumi drive on a MAD16 soundcard at IO
Addr 330 and IRQ 1, probing DMA
Sony CDU 531 at IO address 320
Table A-2. Hardware Parameters Configuration Examples
Appendix A. General Parameters and Modules
Most newer Sound Blaster cards come with IDE interfaces. For these cards, you do not need to use
sbpcd parameters; only use hdX parameters.
A.3. SCSI parameters
Adaptec 28xx, R9xx, 39xx
3ware Storage Controller
AM53/79C974 (PC-SCSI) Driver AM53C974.o
Most Buslogic (now Mylex)
cards with "BT" part number
Mylex DAC960 RAID Controller DAC960.o
MCR53c406a-based SCSI
Initio INI-A100U2W
Advansys SCSI Cards
Adaptec AHA-152x
Adaptec AHA 154x amd
Adaptec AHA 1740
Adaptec AHA-274x, AHA-284x, aic7xxx.o
AHA-29xx, AHA-394x,
AHA-398x, AHA-274x,
AHA-274xT, AHA-2842,
AHA-2910B, AHA-2920C,
U2W/U2/U2B/, U2BOEM,
AHA-3985/U/W/UW, AIC-777x,
AIC-785x, AIC-786x, AIC-787x,
AIC-788x , AIC-789x, AIC-3860
Appendix A. General Parameters and Modules
Compaq Smart Array 5300
Compaq Smart/2 RAID
Compaq FibreChannel Controller cpqfc.o
Domex DMX3191D
Data Technology Corp
DTP SCSI host adapters
ISA, PM2021A/9X ISA,
PM2012A, PM2012B,
PM2022A/9X EISA,
PM2122A/9X, PM2322A/9X,
SmartRAID PM3021, PM3222,
DTP SCSI Adapters PM2011,
PM2021, PM2041, PM3021,
PM2012B, PM2022, PM2122,
PM2322, PM2042, PM3122,
PM3222, PM3332, PM2024,
PM2124, PM2044, PM2144,
PM3224, PM3334
Sun Enterprise Network Array
Future Domain TMC-16xx SCSI fdomain.o
NCR5380 (generic driver)
ICP RAID Controller
I2O Block Driver
IOMEGA MatchMaker parallel
port SCSI adapter
Always IN2000 ISA SCSI card
Initio INI-9X00U/UW SCSI host
AMI MegaRAID 418, 428, 438,
466, 762
OR in2000
Appendix A. General Parameters and Modules
NCR SCSI controllers with
option2:value2,... OR
Pro Audio Spectrum/Studio 16
PCI-2000 IntelliCache
IOMEGA PPA3 parallel port
SCSI host adapter
Perceptive Solutions PSI-240I
Qlogic 1280
Qlogic 2x00
QLogic ISP1020 Intelligent SCSI qlogicisp.o
cards IQ-PCI, IQ-PCI-10,
Qlogic ISP1020 SCSI SBUS
Future Domain TMC-885,
TMC-950 Seagate ST-01/02,
Future Domain TMC-8xx
Cards with the sym53c416
OR sym53c416 io=PORTBASE
Trantor T128/T128F/T228 SCSI
Host Adapter
Tekram DC-390(T) PCI
UltraStor 14F/34F (not 24F)
UltraStor 14F, 24F, and 34F
WD7000 Series
Table A-3. SCSI Parameters
Here are some examples of these modules in use:
Adaptec AHA1522 at port 330, IRQ 11, SCSI
ID 7
Adaptec AHA1542 at port 330
Appendix A. General Parameters and Modules
Future Domain TMC-800 at CA000, IRQ 10
controller_type=2 base_address=0xca000
Table A-4. SCSI Parameters Configuration Examples
A.4. Ethernet Parameters
3Com 3c501
3Com 3c503 and 3c503/16
3c503=io_port,IRQ OR 3c503
3Com EtherLink Plus
3c505=io_port,IRQ OR 3c505
3Com EtherLink 16
3c507=io_port,IRQ OR 3c507
io=io_port irq=IRQ
3Com EtherLink III
3Com ISA EtherLink XL
3Com EtherLink PCI
III/XL Vortex (3c590,
3c592, 3c595, 3c597)
Boomerang (3c900, 3c905,
RTL8139, SMC EZ Card
Fast Ethernet
RealTek cards using
RTL8129 or RTL8139 Fast
Ethernet chipsets
Apricot 82596
Ansel Communications
Model 3200
Alteon AceNIC Gigabit
Aironet Arlan 655
Allied Telesis AT1700
Broadcom BCM5700
10/100/1000 ethernet
0 is off
1 is on
ac3200=io_port,IRQ OR ac3200
at1700=io_port,IRQ OR at1700
io=io_port irq=IRQ
Appendix A. General Parameters and Modules
EtherWORKS DE425
TP PCI, DE435/450
10/100 PCI Kingston,
LinkSys, SMC8432,
SMC9332, Znyx31[45],
and Znyx346 10/100 cards
with DC21040 (no SROM),
DC21140[A], DC21142,
DC21143 chipsets
de4x5=io_port OR de4x5 io=io_port
de4x5 args=’ethX[fdx]
D-Link DE-600 Ethernet
Pocket Adapter
D-Link DE-620 Ethernet
Pocket Adapter
DE100, DE101, DE200
Turbo, DE201Turbo
DE202 Turbo TP/BNC,
DE210, DE422 EISA
Digi Intl. RightSwitch
DM9801 Fast Ethernet
Intel Ether Express/100
Intel EtherExpress/1000
Cabletron E2100
e2100=io_port,IRQ,mem OR e2100
io=io_port irq=IRQ mem=mem
Intel EtherExpress Pro10
eepro=io_port,IRQ OR eepro
io=io_port irq=IRQ
Intel i82557/i82558 PCI
EtherExpressPro driver
depca=io_port,IRQ OR depca
io=io_port irq=IRQ
If X =
0 = autodetect speed and duplex
1 = 10Mbps, half duplex
2 = 10Mbps, full duplex
3 = 100Mbps, half duplex
4 = 100Mbps, full duplex
Appendix A. General Parameters and Modules
Intel EtherExpress 16
eexpress=io_port,IRQ OR eexpress
io=io_port irq=IRQ
0x10 10base T half duplex
0x20 10base T full duplex
0x100 100base T half duplex
0x200 100baseT full duplex
SMC EtherPower II 9432
PCI (83c170/175 EPIC
Racal-Interlan ES3210
ICL EtherTeam 16i/32
eth16i=io_port,IRQ OR eth16i
ioaddr=io_port IRQ=IRQ
EtherWORKS 3 (DE203,
DE204 and DE205)
ewrk=io_port,IRQ OR ewrk
io=io_port irq=IRQ
A Packet Engines GNIC-II
hp-plus=io_port,IRQ OR hp-plus
io=io_port irq=IRQ
HP LAN Ethernet
hp=io_port,IRQ OR hp io=io_port
100VG-AnyLan Network
Adapters HP J2585B,
J2585A, J2970, J2973,
J2573 Compex ReadyLink
FreedomLine 100/VG
hp100=io_port,name OR hp100
hp100_port=io_port hp100_name=name
IBM Token Ring 16/4,
Shared-Memory IBM
Token Ring 16/4
ibmtr=io_port OR io=io_port
AT1500, HP J2405A, most
Mylex LNE390 EISA
NatSemi DP83815 Fast
NE1000 / NE2000
PCI NE2000 cards
RealTEk RTL-8029,
Winbond 89C940, Compex
RL2000, PCI NE2000
clones, NetVin,
NV5000SC, Via 82C926,
SureCom NE34
Novell NE3210 EISA
ne=io_port,IRQ OR ne io=io_port
Appendix A. General Parameters and Modules
MiCom-Interlan NI5010
NI5210 card (i82586
Ethernet chip)
NI6510 Ethernet
IBM Olympic-based PCI
token ring
AMD PCnet32 and AMD
SIS 900/701G PCI Fast
SysKonnect SK-98XX
SMC Ultra and SMC
EtherEZ ISA ethercard
(8K, 83c790)
SMC Ultra32 EISA
Ethernet card (32K)
Sun BigMac Ethernet
Sundance ST201 Alta
Sun Happy Meal Ethernet
Sun Quad Ethernet
Digital 21x4x Tulip PCI
Ethernet cards SMC
EtherPower 10
PCI(8432T/8432BT) SMC
EtherPower 10/100
EtherWorks 100/10
EtherWorks 10
QSILVER’s, Znyx 312
etherarray Allied Telesis
LA100PCI-T Danpex
EN-9400, Cogent EM110
VIA Rhine PCI Fast
Ethernet cards with either
the VIA VT86c100A
Rhine-II PCI or 3043
Rhine-I D-Link
DFE-930-TX PCI 10/100
AT&T GIS (nee NCR)
WaveLan ISA Card
ni52=io_port,IRQ OR ni52 io=io_port
smc-ultra=io_port,IRQ OR smc-ultra
io=io_port irq=IRQ
wavelan=[IRQ,0],io_port, NWID
Appendix A. General Parameters and Modules
WD8003 and
Ethernet cards
wd=io_port,IRQ,mem, mem_end OR wd
io=io_port irq=IRQ mem=mem
Compex RL100ATX-PCI
Packet Engines Yellowfin
Table A-5. Ethernet Module Parameters
Here are some examples of these modules in use:
NE2000 ISA card at IO address 300 and IRQ 11
ne=0x300,11 ether=0x300,11,eth0
Wavelan card at IO 390, autoprobe for IRQ, and
use the NWID to 0x4321
Table A-6. Ethernet Parameter Configuration Examples
A.4.1. Using Multiple Ethernet Cards
You can use multiple Ethernet cards in one machine. If each card uses a different driver (for example,
a 3c509 and a DE425), you simply need to add alias (and possibly options) lines for each card to
/etc/modules.conf. See the Official Red Hat Linux Customization Guide for more information.
If any two Ethernet cards use the same driver (such as two 3c509 cards or a 3c595 and a 3c905), you
will need to either give the two card addresses on the driver’s options line (for ISA cards) or simply
add one alias line for each card (for PCI cards).
For additional information about using more than one Ethernet card, see the Linux Ethernet-HOWTO
Appendix A. General Parameters and Modules
Apache configuration directive, 200
.fetchmailrc, 217
global options, 218
server options, 218
user options, 218
.procmailrc, 220
/dev directory, 18
/etc directory, 18
/etc/exports, 250
/etc/fstab, 251
(See BIND)
/etc/pam.conf, 111
(See Also PAM)
/etc/pam.d, 111
(See Also PAM)
/etc/sysconfig directory, 21
/etc/sysconfig/ directory
(See sysconfig directory)
/lib directory, 18
/lib/security/, 111
(See Also PAM)
/mnt directory, 18
/opt directory, 18
/proc directory, 19
/proc/ directory
(See proc file system)
/sbin directory, 19
/usr directory, 19
/usr/local directory, 20, 21
/var directory, 20
/var/lib/rpm directory, 21
/var/spool/up2date directory, 21
(See kernel modules)
aboot, 60, 77
access control
(See TCP wrappers)
Apache configuration directive, 195
Apache configuration directive, 201
Apache configuration directive, 200
Apache configuration directive, 200
Apache configuration directive, 201
Apache configuration directive, 199
Apache configuration directive, 199
Apache configuration directive, 201
Apache configuration directive, 201
Apache configuration directive, 198
Apache configuration directive, 195
Apache configuration directive, 194
(See Apache HTTP Server)
Apache HTTP Server
1.3 version
migration to 2.0, 178
2.0 version
features of, 177
file system changes, 178
migration from 1.3, 178
packaging changes, 178
additional resources, 208
related books, 208
useful websites, 208
configuration, 189
introducing, 177
log files, 189
reloading, 188
restarting, 188
running without security, 206
server status reports, 202
starting, 188
stopping, 188
troubleshooting, 189
Apache HTTP Server modules, 205
APXS Apache utility, 205
and LDAP, 264, 264
autofs, 252
Basic Input/Output System
(See BIOS)
Berkeley Internet Name Domain
(See BIND)
additional resources, 244
installed documentation, 244
related books, 245
useful websites, 244
common mistakes, 243
configuration files, 230
/etc/named.conf, 231
/var/named/ directory, 230
zone files, 235
configuration of
reverse name resolution, 239
zone file directives, 235
zone file examples, 238
zone file resource records, 236
zone statements, 234
features, 242
DNS enhancements, 242
IPv6, 243
multiple views, 243
security, 243
introducing, 229, 229
named daemon, 230
definition of, 229
nameserver types
caching-only, 230
forwarding, 230
master, 230
slave, 230
rndc program, 240
/etc/rndc.conf, 241
command line options, 241
configuring keys, 241
configuring named to use, 240
root nameserver
definition of, 229
definition of, 229
definition of, 55
(See Also boot process)
block devices, 26
(See Also /proc/devices)
definition of, 26
boot loaders, 77, 77, 77, 84
(See Also ELILO)
(See Also GRUB)
(See Also aboot)
(See Also LILO)
definition of, 77
types of, 77
boot process, 55, 55
(See Also boot loaders)
chain loading, 77
direct loading, 77
for x86, 55
stages of, 55, 55
/sbin/init command, 57
BIOS, 55
boot loader, 56
EFI shell, 55
kernel, 57
Apache configuration directive, 202
cache directives for Apache, 203
Apache configuration directive, 196
caching-only nameserver
(See BIND)
CD-ROM modules
(See kernel modules)
CGI scripts
allowing execution outside cgi-bin, 194
outside the ScriptAlias, 201
character devices, 26
(See Also /proc/devices)
definition of, 26
chkconfig, 62
(See Also services)
common logfile format, 197
Apache, 189
SSL, 204
virtual hosts, 206
configuration directives, Apache, 189
AccessFileName, 195
Action, 201
AddDescription, 200
AddEncoding, 200
AddHandler, 201
AddIcon, 200
AddIconByEncoding, 199
AddIconByType, 199
AddLanguage, 201
AddType, 201
Alias, 198
Allow, 195
AllowOverride, 194
BrowserMatch, 202
CacheNegotiatedDocs, 196
CustomLog, 197
DefaultIcon, 200
DefaultType, 196
Deny, 195
Directory, 193
DirectoryIndex, 195
DocumentRoot, 193
ErrorDocument, 202
ErrorLog, 197
ExtendedStatus, 192
for cache functionality, 203
for SSL functionality, 204
Group, 193
HeaderName, 200
HostnameLookups, 196
IfDefine, 192
IfModule, 196
Include, 191
IndexIgnore, 200
IndexOptions, 199
KeepAlive, 190
KeepAliveTimeout, 190
LanguagePriority, 201
Listen, 191
LoadModule, 192
Location, 202
LockFile, 189
LogFormat, 197
LogLevel, 197
MaxClients, 191
MaxKeepAliveRequests, 190
MaxRequestsPerChild, 191
MaxSpareServers, 190
MetaDir, 202
MetaSuffix, 202
MinSpareServers, 190
NameVirtualHost, 203
Options, 194
Order, 194
PidFile, 190
ProxyRequests, 203
ProxyVia, 203
ReadmeName, 200
Redirect, 199
ScoreBoardFile, 190
ScriptAlias, 198
ServerAdmin, 193
ServerName, 193
ServerRoot, 189
ServerSignature, 198
SetEnvIf, 204
StartServers, 191
Timeout, 190
TypesConfig, 196
UseCanonicalName, 196
User, 192
UserDir, 195
VirtualHost, 204
document, x
copying and pasting text
when using X, xiii
Apache configuration directive, 197
Apache configuration directive, 200
Apache configuration directive, 196
Denial odf Service attack, 49
(See Also /proc/sys/net/ directory)
definition of, 49
Denial of Service, 124
(See Also xinetd)
Apache configuration directive, 195
desktop environments
(See XFree86)
devices, local
ownership of, 116
(See Also PAM)
/dev, 18
/etc, 18
/lib, 18
/mnt, 18
/opt, 18
/proc, 19
/sbin, 19
/usr, 19
/usr/local, 20, 21
/var, 20
Apache configuration directive, 193
Apache configuration directive, 195
display managers
(See XFree86)
DNS, 229
(See Also BIND)
experienced user, x
finding appropriate, viii
first-time users, viii
books, x
newsgroups, ix
websites, ix
guru, x
Apache configuration directive, 193
changing, 206
changing shared, 207
(See Denial of Service)
DoS attack
(See Denial odf Service attack)
drag and drop, xiii
(See kernel modules)
loading, 205
(See email)
EFI shell
definition of, 55
(See Also boot process)
ELILO, 60, 77
email, 209
additional resources, 226
installed documentation, 226
related books, 227
useful websites, 227
Fetchmail, 216
Procmail, 220
program classifications, 211
protocols, 209
IMAP, 209
POP, 209
SMTP, 210
security, 225
clients, 225
servers, 226
Sendmail, 212
Mail Delivery Agent, 212
Mail Transfer Agent, 211
Mail User Agent, 211
epoch, 36
(See Also /proc/stat)
definition of, 36
Apache configuration directive, 202
Apache configuration directive, 197
(See network)
Ethernet modules
(See kernel modules)
execution domains, 27
(See Also /proc/execdomains)
definition of, 27
Apache configuration directive, 192
Extensible Firmware Interface shell
(See EFI shell)
contact information, xiv
Fetchmail, 216
additional resources, 226
command options, 219
informational, 219
special, 219
configuration options, 217
global options, 218
server options, 218
user options, 218
FHS, 18, 17
(See Also file system)
(See Also file system)
file system
FHS standard, 18
hierarchy, 17
organization, 18
structure, 17
(See proc file system)
files, proc file system
changing, 24, 52
viewing, 23, 52
forwarding nameserver
(See BIND)
frame buffer device, 27
(See Also /proc/fb)
FrontPage, 188
GNOME, 102
(See Also XFree86)
Apache configuration directive, 193
groups, 89
definition of, 89
GID, 89
standard, 91
tools for management of
groupadd, 89, 92
redhat-config-users, 89, 92
user private, 89, 92
rationale behind, 93
GRUB, 56
(See Also boot loaders)
additional resources, 87
installed documentation, 87
useful websites, 87
boot process, 77
changing runlevels with, 81, 86
commands, 81
configuration file
/boot/grub/grub.conf, 83
structure, 83
definition of, 77
features, 78
installing, 78
interfaces, 80
command line, 81
menu, 81
menu entry editor, 81
order of use, 81
menu configuration file, 82
commands, 83
role in boot process, 56
terminology, 79
devices, 79
files, 80
root filesystem, 80
grub.conf, 83
(See Also GRUB)
(See Also boot process)
directories used by, 60
role in boot process, 57
(See Also boot process)
runlevels accessed by, 61
SysV init
definition of, 60
initrd directory, 21
initscript utilities, 62
(See Also services)
introduction, vii
(See iptables)
additional resources, 176
installed documentation, 176
useful websites, 176
halt, 75
Apache configuration directive, 200
hierarchy, file system, 17
Apache configuration directive, 196
(See TCP wrappers)
(See TCP wrappers)
(See configuration directives, Apache)
target, 167
compared with ipchains, 168
match options, 172
modules, 173
options, 169
commands, 170
listing, 175
parameters, 171
structure, 170
tables, 169
target, 174
Apache configuration directive, 192
ifdown, 163
Apache configuration directive, 196
ifup, 163
Apache configuration directive, 191
Apache configuration directive, 200
Apache configuration directive, 199
init command, 57
overview of, 167
packet filtering basics, 167
ICMP, 173
TCP, 172
UDP, 173
rules list, 167
saving rules, 175
tables, 167
KDE, 102
(See Also XFree86)
Apache configuration directive, 190
Apache configuration directive, 190
additional resources, 142
installed documentation, 143
useful websites, 143
advantages of, 137
and PAM, 140
clients set up, 142
definition of, 137
disadvantages of, 137
how it works, 139
Key Distribution Center (KDC), 139
server set up, 140
terminology, 138
Ticket Granting Service (TGS), 139
Ticket Granting Ticket (TGT), 139
role in boot process, 57
kernel modules
CD-ROM modules
examples, 271
parameters, 270
Ethernet modules
examples, 279
parameters, 275
supporting multiple cards, 279
introducing, 269
module parameters
specifying, 269
SCSI modules
examples, 274
parameters, 272
types of, 269
Apache configuration directive, 201
additional resources, 265
installed documentation, 265
related books, 266
useful websites, 265
advantages of, 257
applications, 259
ldapadd, 258
ldapdelete, 258
ldapmodify, 258
ldapsearch, 258
OpenLDAP suite, 258
slapadd, 258
slapcat, 258
slapd, 258
slapindex, 258
slappasswd, 258
slurpd, 258
utilities, 258
authentication using, 263
authconfig, 264
editing /etc/ldap.conf, 264
editing /etc/nsswitch.conf, 264
editing /etc/openldap/ldap.conf, 264
editing slapd.conf, 263
packages, 263
PAM, 264
setting up clients, 264
configuration files
/etc/ldap.conf, 260
/etc/openldap/ldap.conf, 260
/etc/openldap/schema/ directory, 260, 262
/etc/openldap/slapd.conf, 260, 261
daemons, 258
definition of, 257
LDAPv2, 257
LDAPv3, 257
OpenLDAP features, 257
setting up, 262
migrating 1.x directories, 263
terminology, 260
using with Apache HTTP Server, 259
using with NSS, 259
using with PAM, 259
using with PHP4, 259
ldapadd command, 258
(See Also LDAP)
ldapdelete command, 258
(See Also LDAP)
ldapmodify command, 258
(See Also LDAP)
ldapsearch command, 258
(See Also LDAP)
Lightweight Directory Access Protocol
(See LDAP)
LILO, 56
(See Also boot loaders)
additional resources, 87
installed documentation, 87
useful websites, 87
boot process, 84
changing runlevels with, 86
configuration file
/etc/lilo.conf, 85
definition of, 84
role in boot process, 56
lilo.conf, 85
(See Also LILO)
Apache configuration directive, 191
Apache configuration directive, 192
Apache configuration directive, 202
Apache configuration directive, 189
log files
common logfile format, 197
Apache configuration directive, 197
Apache configuration directive, 197
lspci, 35
Mail Delivery Agent, 212
Mail Transfer Agent, 211
Mail User Agent, 211
Master Boot Record
(See MBR)
(See MBR)
master nameserver
(See BIND)
Apache configuration directive, 191
Apache configuration directive, 190
Apache configuration directive, 191
Apache configuration directive, 190
definition of, 55, 55
(See Also boot loader)
(See Also boot process)
(See Mail Delivery Agent)
Metacity, 102
(See Also XFree86)
Apache configuration directive, 202
Apache configuration directive, 202
(See Mail User Agent)
Apache configuration directive, 190
module parameters
(See kernel modules)
(See kernel modules)
loading, 205
your own, 205
default, 205
how to use, xiii
(See Mail Transfer Agent)
named daemon
(See BIND)
(See BIND)
(See BIND)
Apache configuration directive, 203
(See iptables)
additional resources, 165
/sbin/ifdown, 163
/sbin/ifup, 163
/sbin/service network, 163
configuration, 159
functions, 164
interfaces, 159
alias, 163
clone, 163
dialup, 161
Ethernet, 160
scripts, 159
Network File System
(See NFS)
additional resources, 254
installed documentation, 254
related books, 255
/etc/fstab, 251
autofs, 252
configuration, 251
mount options, 253
introducing, 247
methodology, 247
portmap, 248
security, 253
file permissions, 254
host access, 254
configuration files, 249
NIC modules
(See kernel modules)
non-secure Web server
disabling, 207
ntsysv, 62
(See Also services)
objects, dynamically shared
(See DSOs)
(See LDAP)
OpenSSH, 129
(See Also SSH)
configuration files for, 132
Apache configuration directive, 194
Apache configuration directive, 194
packet filtering
(See iptables)
additional resources, 117
installed documentation, 117
useful websites, 117
advantages of, 111
configuration files, 111
control flags, 113
definition of, 111
Kerberos and, 140
modules, 112
arguments, 114
components, 112
creating, 113
location of, 114
stacking, 112, 114
types, 112
definition of, 116
sample configuration files, 114
service names, 111
shadow passwords, 114
(See PAM)
password, 114
(See Also PAM)
shadow passwords, 114
Apache configuration directive, 190
Pluggable Authentication Modules
(See PAM)
portmap, 248
rpcinfo, 248
(See XFree86)
proc file system
/proc/apm, 24
/proc/bus/ directory, 39
/proc/cmdline, 25
/proc/cpuinfo, 25
block devices, 26
character devices, 26
/proc/dma, 27
/proc/driver/ directory, 40
/proc/execdomains, 27
/proc/fb, 27
/proc/filesystems, 27
/proc/fs/ directory, 40
/proc/ide directory
device directories, 41
/proc/ide/ directory, 41
/proc/interrupts, 28
/proc/iomem, 29
/proc/ioports, 29
/proc/irq/ directory, 42
/proc/isapnp, 30
/proc/kcore, 30
/proc/kmsg, 31
/proc/ksyms, 31
/proc/loadavg, 31
/proc/locks, 31
/proc/mdstat, 32
/proc/meminfo, 32
/proc/misc, 33
/proc/modules, 33
/proc/mounts, 34
/proc/mtrr, 34
/proc/net/ directory, 42
/proc/partitions, 34
viewing using lspci, 35
/proc/scsi/ directory, 43
/proc/self/ directory, 39
/proc/slabinfo, 36
/proc/stat, 36
/proc/swaps, 37
/proc/sys/ directory, 45, 52
(See Also sysctl)
/proc/sys/dev/ directory, 46
/proc/sys/fs/ directory, 46
/proc/sys/kernel/ directory, 47
(See system request key)
/proc/sys/net/ directory, 49
/proc/sys/vm/ directory, 50
/proc/sysvipc/ directory, 51
/proc/tty/ directory, 51
/proc/uptime, 37
/proc/version, 37
additional resources, 53
installed documentation, 53
useful websites, 53
changing files within, 24, 45, 52
files within, top-level, 24
introduced, 23
process directories, 37
subdirectories within, 37
viewing files within, 23
Procmail, 220
additional resources, 226
configuration, 220
recipes, 221
delivering, 222
examples, 224
flags, 222
local lockfiles, 223
non-delivering, 222
special actions, 223
special conditions, 223
running at boot time, 60
proxy server, 203, 203
Apache configuration directive, 203
Apache configuration directive, 203
public_html directories, 195
modifying, 60
Apache configuration directive, 200
Red Hat Linux-specific file locations
/etc/sysconfig/, 21
(See Also sysconfig directory)
/var/lib/rpm, 21
/var/spool/up2date, 21
Apache configuration directive, 199
root nameserver
(See BIND)
rpcinfo, 248
runlevels, 61
changing at boot time, 86
changing with GRUB, 81
sawfish, 102
(See Also XFree86)
Apache configuration directive, 190
Apache configuration directive, 198
SCSI modules
(See kernel modules)
configuring, 204
running Apache without, 206
Sendmail, 212
additional resources, 226
aliases, 214
common configuration changes, 214
default installation, 213
history, 212
LDAP and, 216
limitations, 213
masquerading, 214
purpose, 213
spam, 215
with UUCP, 214
server side includes, 194, 201
virtual hosts, 194
Apache configuration directive, 193
Apache configuration directive, 193
Apache configuration directive, 189
Apache configuration directive, 198
configuring with chkconfig, 62
configuring with ntsysv, 62
configuring with Services Configuration Tool, 62
Services Configuration Tool, 62
(See Also services)
Apache configuration directive, 204
(See password)
utilities, 94
shutdown, 75
slab pools
(See /proc/slabinfo)
slapadd command, 258
(See Also LDAP)
slapcat command, 258
(See Also LDAP)
slapd command, 258
(See Also LDAP)
slapindex command, 258
(See Also LDAP)
slappasswd command, 258
(See Also LDAP)
slave nameserver
(See BIND)
slurpd command, 258
(See Also LDAP)
SSH protocol, 129
authentication, 131
configuration files, 132
connection sequence, 130
features of, 129
insecure protocols and, 135
layers of, 130
channels, 132
transport layer, 131
port forwarding, 134
requiring for remote login, 135
security risks, 130
X11 forwarding, 133
SSL directives, 204
Apache configuration directive, 191
startx, 103
(See Also XFree86)
common, 17
stunnel, 226
sysconfig directory
/etc/sysconfig/amd, 64
/etc/sysconfig/apm-scripts/ directory, 74
/etc/sysconfig/apmd, 64
/etc/sysconfig/arpwatch, 64
/etc/sysconfig/authconfig, 64
/etc/sysconfig/cbq/ directory, 74
/etc/sysconfig/clock, 65
/etc/sysconfig/desktop, 65
/etc/sysconfig/dhcpd, 65
/etc/sysconfig/firstboot, 65
/etc/sysconfig/gpm, 66
/etc/sysconfig/harddisks, 66
/etc/sysconfig/hwconf, 66
/etc/sysconfig/identd, 66
/etc/sysconfig/init, 67
/etc/sysconfig/ipchains, 67
/etc/sysconfig/iptables, 68, 175
/etc/sysconfig/irda, 68
/etc/sysconfig/keyboard, 68
/etc/sysconfig/kudzu, 69
/etc/sysconfig/mouse, 69
/etc/sysconfig/named, 70
/etc/sysconfig/netdump, 70
/etc/sysconfig/network, 70
/etc/sysconfig/network-scripts/ directory, 159, 74
(See Also network)
/etc/sysconfig/networking/ directory, 74
/etc/sysconfig/ntpd, 71
/etc/sysconfig/pcmcia, 71
/etc/sysconfig/radvd, 71
/etc/sysconfig/rawdevices, 71
/etc/sysconfig/redhat-config-users, 72
/etc/sysconfig/redhat-logviewer, 72
/etc/sysconfig/rhn/ directory, 74
/etc/sysconfig/samba, 72
/etc/sysconfig/sendmail, 72
/etc/sysconfig/soundcard, 72
/etc/sysconfig/squid, 73
/etc/sysconfig/tux, 73
/etc/sysconfig/ups, 73
/etc/sysconfig/vncservers, 73
/etc/sysconfig/xinetd, 74
additional information about, 62
directories in, 74
files found in, 63
configuring with /etc/sysctl.conf, 52
controlling /proc/sys/, 52
(See system request key)
(See system request key)
shutdown, 75
system request key
definition of, 45
enabling, 45
SysV init
(See init command)
TCP wrappers
access control, 119
/etc/hosts.allow, 119
/etc/hosts.deny, 119
expansions, 121
formatting rule, 120
operators, 121
patterns, 120
shell commands, 121
wildcards, 120
additional resources, 127
installed documentation, 127
useful websites, 127
advantages of, 119
definition of, 119
uses for, 119
xinetd, 122
Apache configuration directive, 190
additional resources, 156
installed documentation, 156
useful websites, 156
applications, 155
tripwire, 155
tripwire-check, 150
twadmin, 153, 154, 155, 155
twprint, 150, 151, 155
configuration files, 155
database file, 155, 155
key files, 155
modifying, 147
report files, 155, 155
signing of, 154
tw.cfg, 155, 155
tw.pol, 155, 155
twcfg.txt, 155
twpol.txt, 155
updating, 154
definition of, 155
initialization of, 149
updating, 152
email functions, 154
testing, 154
flowchart of, 145
installation of
customizing configuration, 147
initializing Tripwire database, 149
installing the RPM, 146
setting passwords, 149
tripwire --init command, 149 script, 149
integrity check
tripwire --check command, 150
introducing, 145
policy file
modifying, 148
updating, 153
definition of, 155
generating, 150
viewing, 150
error log, 197
Apache configuration directive, 196
Apache configuration directive, 196
Apache configuration directive, 192
user private groups
(See groups)
rationale behind, 93
Apache configuration directive, 195
users, 89
/etc/passwd, 89
definition of, 89
personal HTML directories, 195
standard, 89
tools for management of
redhat-config-users, 89
useradd, 89
types of, 89
UID, 89
shadow, 94
virtual file system
(See proc file system)
virtual files
(See proc file system)
virtual hosts
configuring, 206
Listen command, 207
name-based, 206
Options, 194
server side includes, 194, 201
Apache configuration directive, 204
email address for, 193
window managers, 102
(See Also XFree86)
(See wmaker)
wmaker, 102
(See Also XFree86)
(See XFree86)
X Window System
(See XFree86)
(See LDAP)
X.500 Lite
(See LDAP)
additional resources, 107
installed documentation, 107
related books, 107
useful websites, 107
configuration files
/etc/X11/ directory, 98
/etc/X11/XF86Config, 98
options within, 98
desktop environments, 102
fonts, 105
introducing, 97
runlevels, 103
3, 103
5, 104
X Configuration Tool, 97
X clients, 97
desktop environments, 102
GNOME, 102
KDE, 102
mwm (Metacity), 102
sawfish, 102
twm, 102
window managers, 102, 102
wmaker, 102
xinit command, 102
X display managers
definition of, 104
gdm, 104
kdm, 104
prefdm script, 104
xdm, 104
X font server
adding fonts, 106
configuration of, 105
xfs, 98, 105
X server, 97
features of, 98
XFree86, 98
(See XFree86)
xinetd, 122
(See Also TCP wrappers)
access control with, 125
binding, 126
configuration files, 123
/etc/xinetd.conf, 123
/etc/xinetd.d/ directory, 124
port redirection, 126
to prevent DoS attacks, 124
(See XFree86)
The Official Red Hat Linux manuals are written in DocBook SGML v4.1 format. The HTML and
PDF formats are produced using custom DSSSL stylesheets and custom jade wrapper scripts.
Marianne Pecci <> created the admonition graphics (note, tip, important, caution, and warning). They may be redistributed with written permission from Marianne Pecci and Red
Hat, Inc..
The Red Hat Linux Product Documentation Team consists of the following people:
Sandra A. Moore — Primary Writer/Maintainer of the Official Red Hat Linux x86 Installation Guide;
Contributing Writer to the Official Red Hat Linux Getting Started Guide
Tammy Fox — Primary Writer/Maintainer of the Official Red Hat Linux Customization Guide; Contributing Writer to the Official Red Hat Linux Getting Started Guide; Writer/Maintainer of custom
DocBook stylesheets and scripts
Edward C. Bailey — Contributing Writer to the Official Red Hat Linux x86 Installation Guide
Johnray Fuller — Primary Writer/Maintainer of the Official Red Hat Linux Reference Guide; Cowriter/Co-maintainer of the Official Red Hat Linux Security Guide
John Ha — Primary Writer/Maintainer to the Official Red Hat Linux Getting Started Guide; Cowriter/Co-maintainer of the Official Red Hat Linux Security Guide