Nmap Scripting Engine Documentation

Nmap Scripting Engine Documentation
Nmap Scripting Engine
Documentation
Table of Contents
1. Introduction ....................................................................................................................... 2
2. Usage and Examples ........................................................................................................... 4
2.1. Script Categories ...................................................................................................... 4
2.2. Command-line Arguments .......................................................................................... 7
2.3. Script Selection ........................................................................................................ 8
2.4. Arguments to Scripts ................................................................................................. 9
2.5. Complete Examples ................................................................................................ 10
3. Script Format ................................................................................................................... 10
3.1. description Field .............................................................................................. 10
3.2. categories Field ................................................................................................ 10
3.3. author Field ....................................................................................................... 10
3.4. license Field ..................................................................................................... 11
3.5. dependencies Field ............................................................................................ 11
3.6. Port and Host Rules ................................................................................................ 12
3.7. Action .................................................................................................................. 12
4. Script Language ................................................................................................................ 12
4.1. Lua Base Language ................................................................................................. 12
5. NSE Scripts ..................................................................................................................... 13
6. NSE Libraries ................................................................................................................... 13
6.1. List of All Libraries ................................................................................................. 13
6.2. Hacking NSE Libraries ............................................................................................ 17
6.3. Adding C Modules to Nselib ..................................................................................... 17
7. Nmap API ........................................................................................................................ 18
7.1. Information Passed to a Script ................................................................................... 18
7.2. Network I/O API .................................................................................................... 20
Connect-style network I/O ...................................................................................... 21
Raw packet network I/O ......................................................................................... 21
7.3. Exception Handling ................................................................................................ 22
7.4. The Registry .......................................................................................................... 23
8. Script Writing Tutorial ....................................................................................................... 23
8.1. The Head .............................................................................................................. 24
8.2. The Rule ............................................................................................................... 25
8.3. The Mechanism ...................................................................................................... 25
9. Writing Script Documentation (NSEDoc) .............................................................................. 27
9.1. NSE Documentation Tags ......................................................................................... 29
10. Script Parallelism in NSE .................................................................................................. 30
10.1. Worker Threads .................................................................................................... 31
10.2. Thread Mutexes .................................................................................................... 32
10.3. Condition Variables ............................................................................................... 34
1
10.4. Collaborative Multithreading ...................................................................................
The Base Thread ..................................................................................................
11. Version Detection Using NSE ............................................................................................
12. Example Script: finger.nse ..........................................................................................
13. Implementation Details .....................................................................................................
13.1. Initialization Phase ................................................................................................
13.2. Matching Scripts with Targets ..................................................................................
13.3. Script Execution ...................................................................................................
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41
1. Introduction
Note
This PDF version of the NSE documentation was prepared for the presentation by Fyodor and
David Fifield at the Black Hat Briefings Las Vegas 2010. While reading this will certainly help
you master the Nmap Scripting Engine, we aim to make our talk useful, informative, and
entertaining even for folks who haven't. You can read the latest version of this online at
http://nmap.org/book/nse.html.
The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and flexible features. It allows users to
write (and share) simple scripts to automate a wide variety of networking tasks. Those scripts are then
executed in parallel with the speed and efficiency you expect from Nmap. Users can rely on the growing
and diverse set of scripts distributed with Nmap, or write their own to meet custom needs.
We designed NSE to be versatile, with the following tasks in mind:
Network discovery
This is Nmap's bread and butter. Examples include looking up whois data based on the target domain,
querying ARIN, RIPE, or APNIC for the target IP to determine ownership, performing identd lookups
on open ports, SNMP queries, and listing available NFS/SMB/RPC shares and services.
More sophisticated version detection
The Nmap version detection system (http://nmap.org/book/vscan.html) is able to recognize thousands
of different services through its probe and regular expression signature based matching system, but it
cannot recognize everything. For example, identifying the Skype v2 service requires two independent
probes, which version detection isn't flexible enough to handle. Nmap could also recognize more SNMP
services if it tried a few hundred different community names by brute force. Neither of these tasks are
well suited to traditional Nmap version detection, but both are easily accomplished with NSE. For these
reasons, version detection now calls NSE by default to handle some tricky services. This is described
in Section 11, “Version Detection Using NSE” [37].
Vulnerability detection
When a new vulnerability is discovered, you often want to scan your networks quickly to identify
vulnerable systems before the bad guys do. While Nmap isn't a comprehensive vulnerability scanner,
NSE is powerful enough to handle even demanding vulnerability checks. Many vulnerability detection
scripts are already available and we plan to distribute more as they are written.
2
1. Introduction
Backdoor detection
Many attackers and some automated worms leave backdoors to enable later reentry. Some of these can
be detected by Nmap's regular expression based version detection. For example, within hours of the
MyDoom worm hitting the Internet, Jay Moran posted an Nmap version detection probe and signature
so that others could quickly scan their networks for MyDoom infections. NSE is needed to reliably
detect more complex worms and backdoors.
Vulnerability exploitation
As a general scripting language, NSE can even be used to exploit vulnerabilities rather than just find
them. The capability to add custom exploit scripts may be valuable for some people (particularly
penetration testers), though we aren't planning to turn Nmap into an exploitation framework such as
Metasploit1.
These listed items were our initial goals, and we expect Nmap users to come up with even more inventive
uses for NSE.
Scripts are written in the embedded Lua programming language2. The language itself is well documented in
the books Programming in Lua, Second Edition and Lua 5.1 Reference Manual. The reference manual is
also freely available online3, as is the first edition of Programming in Lua4. Given the availability of these
excellent general Lua programming references, this document only covers aspects and extensions specific
to Nmap's scripting engine.
NSE is activated with the -sC option (or --script if you wish to specify a custom set of scripts) and
results are integrated into Nmap normal and XML output. Two types of scripts are supported: service and
host scripts. Service scripts relate to a certain open port (service) on the target host, and any results they
produce are included next to that port in the Nmap output port table. Host scripts, on the other hand, run no
more than once against each target IP and produce results below the port table. Example 1 shows a typical
script scan. Service scripts producing output in this example are ssh-hostkey, which provides the system's
RSA and DSA SSH keys, and rpcinfo, which queries portmapper to enumerate available services. The
only host script producing output in this example is smb-os-discovery, which collects a variety of
information from SMB servers. Nmap discovered all of this information in a third of a second.
1
http://www.metasploit.com
http://www.lua.org/
3
http://www.lua.org/manual/5.1/
4
http://www.lua.org/pil/
2
1. Introduction
3
Example 1. Typical NSE output
# nmap -sC -p22,111,139 -T4 localhost
Starting Nmap ( http://nmap.org )
Interesting ports on flog (127.0.0.1):
PORT
STATE SERVICE
22/tcp
open ssh
| ssh-hostkey: 1024 b1:36:0d:3f:50:dc:13:96:b2:6e:34:39:0d:9b:1a:38 (DSA)
|_ 2048 77:d0:20:1c:44:1f:87:a0:30:aa:85:cf:e8:ca:4c:11 (RSA)
111/tcp open rpcbind
| rpcinfo:
| 100000 2,3,4
111/udp rpcbind
| 100024 1
56454/udp status
|_ 100000 2,3,4
111/tcp rpcbind
139/tcp open netbios-ssn
Host script results:
| smb-os-discovery: Unix
| LAN Manager: Samba 3.0.31-0.fc8
|_ Name: WORKGROUP
Nmap done: 1 IP address (1 host up) scanned in 0.33 seconds
2. Usage and Examples
While NSE has a complex implementation for efficiency, it is strikingly easy to use. Simply specify -sC to
enable the most common scripts. Or specify the --script option to choose your own scripts to execute
by providing categories, script file names, or the name of directories full of scripts you wish to execute. You
can customize some scripts by providing arguments to them via the --script-args option. The two
remaining options, --script-trace and --script-updatedb, are generally only used for script
debugging and development. Script scanning is also included as part of the -A (aggressive scan) option.
Script scanning is normally done in combination with a port scan, because scripts may be run or not run
depending on the port states found by the scan. With the -sn option it is possible to run a script scan without
a port scan, only host discovery. In this case only host scripts will be eligible to run. To run a script scan
with neither a host discovery nor a port scan, use the -Pn -sn options together with -sC or --script.
Every host will be assumed up and still only host scripts will be run. This technique is useful for scripts like
whois.nse that only use the remote system's address and don't require it to be up.
Scripts are not run in a sandbox and thus could accidentally or maliciously damage your system or invade
your privacy. Never run scripts from third parties unless you trust the authors or have carefully audited the
scripts yourself.
2.1. Script Categories
NSE scripts define a list of categories they belong to. Currently defined categories are auth, default,
discovery, external, fuzzer, intrusive, malware, safe, version, and vuln. Category
names are not case sensitive. The following list describes each category.
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2. Usage and Examples
auth
These scripts try to determine authentication credentials on the target system, often through a brute-force
attack. Examples include snmp-brute, http-auth, and ftp-anon.
default
These scripts are the default set and are run when using the -sC or -A options rather than listing scripts
with --script. This category can also be specified explicitly like any other using
--script=default. Many factors are considered in deciding whether a script should be run by
default:
Speed
A default scan must finish quickly, which excludes brute force authentication crackers, web spiders,
and any other scripts which can take minutes or hours to scan a single service.
Usefulness
Default scans need to produce valuable and actionable information. If even the script author has
trouble explaining why an average networking or security professional would find the output
valuable, the script should not run by default. The script may still be worth including in Nmap so
that administrators can run for those occasions when they do need the extra information.
Verbosity
Nmap output is used for a wide variety of purposes and needs to be readable and concise. A script
which frequently produces pages full of output should not be added to the default category.
When there is no important information to report, NSE scripts (particularly default ones) should
return nothing. Checking for an obscure vulnerability may be OK by default as long as it only
produces output when that vulnerability discovered.
Reliability
Many scripts use heuristics and fuzzy signature matching to reach conclusions about the target host
or service. Examples include sniffer-detect and sql-injection. If the script is often
wrong, it doesn't belong in the default category where it may confuse or mislead casual users.
Users who specify a script or category directly are generally more advanced and likely know how
the script works or at least where to find its documentation.
Intrusiveness
Some scripts are very intrusive because they use significant resources on the remote system, are
likely to crash the system or service, or are likely to be perceived as an attack by the remote
administrators. The more intrusive a script is, the less suitable it is for the default category.
Default scripts are almost always in the safe category too, though we occasionally allow
intrusive scripts by default when they are only mildly intrusive and score well in the other
factors.
Privacy
Some scripts, particularly those in the external category described later, divulge information to
third parties by their very nature. For example, the whois script must divulge the target IP address
to regional whois registries. We have also considered (and decided against) adding scripts which
check target SSH and SSL key fingerprints against Internet weak key databases. The more
privacy-invasive a script is, the less suitable it is for default category inclusion.
2. Usage and Examples
5
We don't have exact thresholds for each of these criteria, and many of them are subjective. All of these
factors are considered together when making a decision whether to promote a script into the default
category. A few default scripts are identd-owners (determines the username running remote services
using identd), http-auth (obtains authentication scheme and realm of web sites requiring
authentication), and ftp-anon (tests whether an FTP server allows anonymous access).
discovery
These scripts try to actively discover more about the network by querying public registries, SNMP-enabled
devices, directory services, and the like. Examples include html-title (obtains the title of the root
path of web sites), smb-enum-shares (enumerates Windows shares), and snmp-sysdescr (extracts
system details via SNMP).
external
Scripts in this category may send data to a third-party database or other network resource. An example
of this is whois, which makes a connection to whois servers to learn about the address of the target.
There is always the possibility that operators of the third-party database will record anything you send
to them, which in many cases will include your IP address and the address of the target. Most scripts
involve traffic strictly between the scanning computer and the client; any that do not are placed in this
category.
fuzzer
This category contains scripts which are designed to send server software unexpected or randomized
fields in each packet. While this technique can useful for finding undiscovered bugs and vulnerabilities
in software, it is both a slow process and bandwidth intensive. An example of a script in this category
is dns-fuzz, which bombards a DNS server with slightly flawed domain requests until either the
server crashes or a user specified time limit elapses.
intrusive
These are scripts that cannot be classified in the safe category because the risks are too high that they
will crash the target system, use up significant resources on the target host (such as bandwidth or CPU
time), or otherwise be perceived as malicious by the target's system administrators. Examples are
http-open-proxy (which attempts to use the target server as an HTTP proxy) and snmp-brute
(which tries to guess a device's SNMP community string by sending common values such as public,
private, and cisco). Unless a script is in the special version category, it should be categorized
as either safe or intrusive.
malware
These scripts test whether the target platform is infected by malware or backdoors. Examples include
smtp-strangeport, which watches for SMTP servers running on unusual port numbers, and
auth-spoof, which detects identd spoofing daemons which provide a fake answer before even
receiving a query. Both of these behaviors are commonly associated with malware infections.
safe
Scripts which weren't designed to crash services, use large amounts of network bandwidth or other
resources, or exploit security holes are categorized as safe. These are less likely to offend remote
administrators, though (as with all other Nmap features) we cannot guarantee that they won't ever cause
adverse reactions. Most of these perform general network discovery. Examples are ssh-hostkey
(retrieves an SSH host key) and html-title (grabs the title from a web page). Scripts in the version
category are not categorized by safety, but any other scripts which aren't in safe should be placed in
intrusive.
6
2. Usage and Examples
version
The scripts in this special category are an extension to the version detection feature and cannot be selected
explicitly. They are selected to run only if version detection (-sV) was requested. Their output cannot
be distinguished from version detection output and they do not produce service or host script results.
Examples are skypev2-version, pptp-version, and iax2-version.
vuln
These scripts check for specific known vulnerabilities and generally only report results if they are found.
Examples include realvnc-auth-bypass and xampp-default-auth.
2.2. Command-line Arguments
These are the five command line arguments specific to script-scanning:
-sC
Performs a script scan using the default set of scripts. It is equivalent to --script=default. Some
of the scripts in this default category are considered intrusive and should not be run against a target
network without permission.
--script <filename>|<category>|<directory>|<expression>|all[,...]
Runs a script scan using the comma-separated list of filenames, script categories, and directories. Each
element in the list may also be a Boolean expression describing a more complex set of scripts. Each
element is interpreted first as an expression, then as a category, and finally as a file or directory name.
The special argument all makes every script in Nmap's script database eligible to run. The all argument
should be used with caution as NSE may contain dangerous scripts including exploits, brute force
authentication crackers, and denial of service attacks.
File and directory names may be relative or absolute. Absolute names are used directly. Relative paths
are looked for in the following places until found:
--datadir
$NMAPDIR
~/.nmap (not searched on Windows)
NMAPDATADIR
the current directory
A scripts subdirectory is also tried in each of these.
When a directory name is given, Nmap loads every file in the directory whose name ends with .nse.
All other files are ignored and directories are not searched recursively. When a filename is given, it does
not have to have the .nse extension; it will be added automatically if necessary.
See Section 2.3, “Script Selection” [8] for examples and a full explanation of the --script option.
Nmap scripts are stored in a scripts subdirectory of the Nmap data directory by default (see
http://nmap.org/book/data-files.html). For efficiency, scripts are indexed in a database stored in
scripts/script.db, which lists the category or categories in which each script belongs. The
argument all will execute all scripts in the Nmap script database, but should be used cautiously since
Nmap may contain exploits, denial of service attacks, and other dangerous scripts.
2. Usage and Examples
7
--script-args <args>
Provides arguments to the scripts. See Section 2.4, “Arguments to Scripts” [9] for a detailed explanation.
--script-trace
This option is similar to --packet-trace, but works at the application level rather than packet by
packet. If this option is specified, all incoming and outgoing communication performed by scripts is
printed. The displayed information includes the communication protocol, source and target addresses,
and the transmitted data. If more than 5% of transmitted data is unprintable, hex dumps are given instead.
Specifying --packet-trace enables script tracing too.
--script-updatedb
This option updates the script database found in scripts/script.db which is used by Nmap to
determine the available default scripts and categories. It is only necessary to update the database if you
have added or removed NSE scripts from the default scripts directory or if you have changed the
categories of any script. This option is used by itself without arguments: nmap --script-updatedb.
Some other Nmap options have effects on script scans. The most prominent of these is -sV. A version scan
automatically executes the scripts in the version category. The scripts in this category are slightly different
than other scripts because their output blends in with the version scan results and they do not produce any
script scan output.
Another option which affects the scripting engine is -A. The aggressive Nmap mode implies the -sC option.
2.3. Script Selection
The --script option takes a comma-separated list of categories, filenames, and directory names. Some
simple examples of its use:
nmap --script default,safe
Loads all scripts in the default and safe categories.
nmap --script smb-os-discovery
Loads only the smb-os-discovery.nse script. Note that the .nse extension is optional.
nmap --script default,banner,/home/user/customscripts
Loads the script in the default category, the banner.nse script, and all .nse files in the directory
/home/user/customscripts.
When referring to scripts from script.db by name, you can use a shell-style ‘*’ wildcard.
nmap --script "http-*"
Loads all scripts whose name starts with http-, such as http-auth.nse and
http-open-proxy.nse. The argument to --script had to be in quotes to protect the wildcard
from the shell.
More complicated script selection can be done using the and, or, and not operators to build Boolean
expressions. The operators have the same precedence as in Lua: not is the highest, followed by and and
then or. You can alter precedence by using parentheses. Because expressions contain space characters it is
necessary to quote them.
8
2. Usage and Examples
nmap --script "not intrusive"
Loads every script except for those in the intrusive category.
nmap --script "default or safe"
This is functionally equivalent to nmap --script "default,safe". It loads all scripts that are in the
default category or the safe category or both.
nmap --script "default and safe"
Loads those scripts that are in both the default and safe categories.
nmap --script "(default or safe or intrusive) and not http-*"
Loads scripts in the default, safe, or intrusive categories, except for those whose names start
with http-.
Names in a Boolean expression may be a category, a filename from script.db, or all. A name is any
sequence of characters not containing ‘ ’, ‘,’, ‘(’, ‘)’, or ‘;’, except for the sequences and, or, and not,
which are operators.
2.4. Arguments to Scripts
Arguments may be passed to NSE scripts using the --script-args option. The arguments describe a
table of key-value pairs and possibly array values. The arguments are provided to scripts as a table in the
registry called nmap.registry.args.
The syntax for script arguments is similar to Lua's table constructor syntax. Arguments are a comma-separated
list of name=value pairs. Names and values may be strings not containing whitespace or the characters
‘{’, ‘}’, ‘=’, or ‘,’. To include one of these characters in a string, enclose the string in single or double
quotes. Within a quoted string, ‘\’ escapes a quote. A backslash is only used to escape quotation marks in
this special case; in all other cases a backslash is interpreted literally.
Values may also be tables enclosed in {}, just as in Lua. A table may contain simple string values, for
example a list of proxy hosts; or more name-value pairs, including nested tables. Nested subtables are
commonly used to pass arguments specific to one script, in a table named after the script. That is what is
happening with the whois table in the example below.
Here is a typical Nmap invocation with script arguments:
nmap -sC --script-args 'user=foo,pass=",{}=bar",whois={whodb=nofollow+ripe},userdb=C:\Some\Path\To\Fi
Notice that the script arguments are surrounded in single quotes. This prevents the shell from interpreting
the double quotes and doing automatic string concatenation. The command results in this Lua table:
{user="foo",pass=",{}=bar",whois={whodb="nofollow+ripe"},userdb="C:\\Some\\Path\\To\\F
You could then access the username "foo" inside your script with this statement:
local username = nmap.registry.args.user
The online NSE Documentation Portal at http://nmap.org/nsedoc/ lists the arguments that each script accepts.
2. Usage and Examples
9
2.5. Complete Examples
nmap -sC example.com
A simple script scan using the default set of scripts.
nmap -sn -sC example.com
A script scan without a port scan; only host scripts are eligible to run.
nmap -Pn -sn -sC example.com
A script scan without host discovery or a port scan. All hosts are assumed up and only host scripts are
eligible to run.
nmap --script smb-os-discovery --script-trace example.com
Execute a specific script with script tracing.
nmap --script snmp-sysdescr --script-args snmpcommunity=admin example.com
Run an individual script that takes a script argument.
nmap --script mycustomscripts,safe example.com
Execute all scripts in the mycustomscripts directory as well as all scripts in the safe category.
3. Script Format
NSE scripts consist of two–five descriptive fields along with either a port or host rule defining when the
script should be executed and an action block containing the actual script instructions. Values can be assigned
to the descriptive fields just as you would assign any other Lua variables. Their names must be lowercase
as shown in this section.
3.1. description Field
The description field describes what a script is testing for and any important notes the user should be
aware of. Depending on script complexity, the description may vary from a few sentences to a few paragraphs.
The first paragraph should be a brief synopsis of the script function suitable for stand-alone presentation to
the user. Further paragraphs may provide much more script detail.
3.2. categories Field
The categories field defines one or more categories to which a script belongs (see Section 2.1, “Script
Categories” [4]). The categories are case-insensitive and may be specified in any order. They are listed in
an array-style Lua table as in this example:
categories = {"default", "discovery", "safe"}
3.3. author Field
The author field contains the script authors' names and can also contain contact information (such as home
page URLs). We no longer recommend including email addresses because spammers might scrape them
10
3. Script Format
from the nsedoc web site. This optional field is not used by NSE, but gives script authors their due credit or
blame.
3.4. license Field
Nmap is a community project and we welcome all sorts of code contributions, including NSE scripts. So if
you write a valuable script, don't keep it to yourself! The optional license field helps ensure that we have
legal permission to distribute all the scripts which come with Nmap. All of those scripts currently use the
standard Nmap license (described in http://nmap.org/book/man-legal.html). They include the following line:
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
The Nmap license is similar to the GNU GPL. Script authors may use a BSD-style license (no advertising
clause) instead if they prefer that.
3.5. dependencies Field
The dependencies field is an array containing the names of scripts that should run before this script.
This is used when one script can make use of the results of another. For example, most of the smb-* scripts
depend on smb-brute, because the accounts found by smb-brute may allow the other scripts to get
more information.
When we say “depend on”, we mean it in a loose sense. That is, a script will still run despite missing
dependencies. Given the dependencies, the script will run after all the scripts listed in the dependencies array.
This is an example of the dependencies table from smb-os-discovery:
dependencies = {"smb-brute"}
The dependencies table is optional. NSE will assume the script has no dependencies if the field is omitted.
Dependencies establish an internal ordering of scripts, assigning each one a number called a “runlevel”5.
When running your scripts you will see the runlevel (along with the total number of runlevels) of each
grouping of scripts run in NSE's output:
NSE: Script scanning 127.0.0.1.
NSE: Starting runlevel 1 (of 3) scan.
Initiating NSE at 17:38
Completed NSE at 17:38, 0.00s elapsed
NSE: Starting runlevel 2 (of 3) scan.
Initiating NSE at 17:38
Completed NSE at 17:38, 0.00s elapsed
NSE: Starting runlevel 3 (of 3) scan.
Initiating NSE at 17:38
Completed NSE at 17:38, 0.00s elapsed
NSE: Script Scanning completed.
5
Up through Nmap version 5.10BETA2, dependencies didn't exist and script authors had to set a runlevel field manually.
3. Script Format
11
3.6. Port and Host Rules
Nmap uses the script rules to determine whether a script should be run against a target. A script contains
either a port rule, which governs which ports of a target the scripts may run against, or a host rule, which
specifies that the script should be run only once against a target IP and only if the given conditions are met.
A rule is a Lua function that returns either true or false. The script action is only performed if its rule
evaluates to true. Host rules accept a host table as their argument and may test, for example, the IP address
or hostname of the target. A port rule accepts both host and port tables as arguments for any TCP or UDP
port in the open, open|filtered, or unfiltered port states. Port rules generally test factors such as
the port number, port state, or listening service name in deciding whether to run against a port. Example
rules are shown in Section 8.2, “The Rule” [25].
3.7. Action
The action is the heart of an NSE script. It contains all of the instructions to be executed when the script's
port or host rule triggers. It is a Lua function which accepts the same arguments as the rule and can return
either nil or a string. If a string is returned by a service script, the string and script's filename are printed
in the Nmap port table output. A string returned by a host script is printed below the port table. No output
is produced if the script returns nil. For an example of an NSE action refer to Section 8.3, “The
Mechanism” [25].
4. Script Language
The core of the Nmap Scripting Engine is an embeddable Lua interpreter. Lua is a lightweight language
designed for extensibility. It offers a powerful and well documented API for interfacing with other software
such as Nmap.
The second part of the Nmap Scripting Engine is the NSE Library, which connects Lua and Nmap. This
layer handles issues such as initialization of the Lua interpreter, scheduling of parallel script execution, script
retrieval and more. It is also the heart of the NSE network I/O framework and the exception handling
mechanism. It also includes utility libraries to make scripts more powerful and convenient. The utility library
modules and extensions are described in Section 6, “NSE Libraries” [13].
4.1. Lua Base Language
The Nmap scripting language is an embedded Lua6 interpreter which was extended with libraries for interfacing
with Nmap. The Nmap API is in the Lua namespace nmap. This means that all calls to resources provided
by Nmap have an nmap prefix. nmap.new_socket(), for example, returns a new socket wrapper object.
The Nmap library layer also takes care of initializing the Lua context, scheduling parallel scripts and collecting
the output produced by completed scripts.
During the planning stages, we considered several programming languages as the base for Nmap scripting.
Another option was to implement a completely new programming language. Our criteria were strict: NSE
had to be easy to use, small in size, compatible with the Nmap license, scalable, fast and parallelizable.
Several previous efforts (by other projects) to design their own security auditing language from scratch
6
http://www.lua.org/
12
4. Script Language
resulted in awkward solutions, so we decided early not to follow that route. First the Guile Scheme interpreter
was considered, but the preference drifted towards the Elk interpreter due to its more favorable license. But
parallelizing Elk scripts would have been difficult. In addition, we expect that most Nmap users prefer
procedural programming over functional languages such as Scheme. Larger interpreters such as Perl, Python,
and Ruby are well-known and loved, but are difficult to embed efficiently. In the end, Lua excelled in all of
our criteria. It is small, distributed under the liberal MIT open source license, has coroutines for efficient
parallel script execution, was designed with embeddability in mind, has excellent documentation, and is
actively developed by a large and committed community. Lua is now even embedded in other popular open
source security tools including the Wireshark sniffer and Snort IDS.
5. NSE Scripts
This section (a long list of NSE scripts with brief summaries) is omitted since we already provide a better
online interface to the information at the NSE Documentation Portal7.
6. NSE Libraries
In addition to the significant built-in capabilities of Lua, we have written or integrated many extension
libraries which make script writing more powerful and convenient. These libraries (sometimes called modules)
are compiled if necessary and installed along with Nmap. They have their own directory, nselib, which
is installed in the configured Nmap data directory. Scripts need only require 8 the default libraries in
order to use them.
6.1. List of All Libraries
This list is just an overview to give an idea of what libraries are available. Developers will want to consult
the complete documentation at http://nmap.org/nsedoc/.
afp
This library was written by Patrik Karlsson <patrik@cqure.net> to facilitate communication with the
Apple AFP Service. It is not feature complete and still missing several functions.
asn1
ASN1 functions.
backdoor
This config file is designed for adding a backdoor to the system. It has a few options by default, only
one enabled by default. I suggest
base64
Base64 encoding and decoding. Follows RFC 4648.
bin
Pack and unpack binary data.
7
8
http://nmap.org/nsedoc/
http://www.lua.org/manual/5.1/manual.html#pdf-require
5. NSE Scripts
13
bit
Bitwise operations on integers.
citrixxml
This module was written by Patrik Karlsson and facilitates communication with the Citrix XML Service.
It is not feature complete and is missing several functions and parameters.
comm
Common communication functions for network discovery tasks like banner grabbing and data exchange.
datafiles
Read and parse some of Nmap's data files: nmap-protocols, nmap-rpc, nmap-services, and
nmap-mac-prefixes.
db2
DB2 Library supporting a very limited subset of operations
default
More verbose network scripts
dns
Simple DNS library supporting packet creation, encoding, decoding, and querying.
drive
This configuration file pulls info about a given harddrive
experimental
This is the configuration file for modules that aren't quite ready for prime time yet.
http
Client-side HTTP library.
imap
IMAP functions.
ipOps
Utility functions for manipulating and comparing IP addresses.
json
Library methods for handling Json data. It handles json encoding and decoding
ldap
Library methods for handling LDAP.
listop
Functional-style list operations.
match
Buffered network I/O helper functions.
14
6. NSE Libraries
mongodb
Library methods for handling MongoDB, creating and parsing packets
msrpc
By making heavy use of the 'smb' library, this library will call various MSRPC functions. The functions
used here can be accessed over TCP ports 445 and 139, with an established session. A NULL session
(the default) will work for some functions and operating systems (or configurations), but not for others.
msrpcperformance
This module is designed to parse the PERF_DATA_BLOCK structure, which is stored in the registry
under HKEY_PERFORMANCE_DATA. By querying this structure, you can get a whole lot of
information about what's going on.
msrpctypes
This module was written to marshall parameters for Microsoft RPC (MSRPC) calls. The values passed
in and out are based on structs defined by the protocol, and documented by Samba developers. For
detailed breakdowns of the types, take a look at Samba 4.0's .idl files.
mssql
MSSQL Library supporting a very limited subset of operations
mysql
Simple
MySQL
Library
supporting
a
very
limited
http://forge.mysql.com/wiki/MySQL_Internals_ClientServer_Protocol
subset
of
operations
netbios
Creates and parses NetBIOS traffic. The primary use for this is to send NetBIOS name requests.
network
This is the default configuration file. It simply runs some built-in Window programs to gather information
about the remote system. It's intended to be simple, demonstrate some of the concepts, and not break/alte
anything.
nmap
Interface with Nmap internals.
nsedebug
Converts an arbitrary data type into a string. Will recursively convert tables. This can be very useful for
debugging.
openssl
OpenSSL bindings.
packet
Facilities for manipulating raw packets.
pcre
Perl Compatible Regular Expressions.
6. NSE Libraries
15
pgsql
PostgreSQL library supporting both version 2 and version 3 of the protocol The library currently contains
the bare minimum to perform authentication Authentication is supported with or without SSL enabled
and using the plain-text or MD5 authentication mechanisms
pop3
POP3 functions.
proxy
Functions for proxy testing
pwdump
This config file is designed for running password-dumping scripts. So far, it supports pwdump6 2.0.0
and fgdump.
rpc
RPC Library supporting a very limited subset of operations
shortport
Functions for building short portrules.
smb
Implements functionality related to Server Message Block (SMB, also known as CIFS) traffic, which
is a Windows protocol.
smbauth
This module takes care of the authentication used in SMB (LM, NTLM, LMv2, NTLMv2). There is a
lot to this functionality, so if you're interested in how it works, read on.
snmp
SNMP functions.
ssh1
Functions for the SSH-1 protocol.
ssh2
Functions for the SSH-2 protocol.
stdnse
Standard Nmap Scripting Engine functions.
strbuf
String buffer facilities.
strict
Strict Declared Global library.
tab
Arrange output into tables.
16
6. NSE Libraries
unpwdb
Username/password database library.
url
URI parsing, composition, and relative URL resolution.
6.2. Hacking NSE Libraries
Libraries often accidentally make use of globals variables when local scope was intended. Two or more
scripts that make use of library functions which unintentionally use the same global variable will find that
variable constantly rewritten. This is a serious bug that can cause NSE to stall or a correct script to
spectacularly fail, and, because Lua uses global-by-default scope assignment when it encounters a variable,
this is also a common bug.
Consider a global variable being used by two different scripts, within the library, to hold sockets or data.
When one script is yielded after storing data in the variable, another script awakens only to replace that data.
In contrast, a local variable would store the information on the stack of the running script separate from
others.
To help correct this problem, NSE now uses an adapted library from the standard Lua distribution called
strict.lua. The library will raise a runtime error on any access or modification of a global variable
which was undeclared in the file scope. A global variable is considered declared if the library makes an
assignment to the global name (even nil) in the file scope.
6.3. Adding C Modules to Nselib
A few of the modules included in nselib are written in C or C++ rather than Lua. Two examples are bit
and pcre. We recommend that modules be written in Lua if possible, but C and C++ may be more appropriate
if performance is critical or (as with the pcre and openssl modules) you are linking to an existing C
library. This section describes how to write your own compiled extensions to nselib.
The Lua C API is described at length in Programming in Lua, Second Edition, so this is a short summary.
C modules consist of functions that follow the protocol of the lua_CFunction9 type. The functions are
registered with Lua and assembled into a library by calling the luaL_register function. A special
initialization function provides the interface between the module and the rest of the NSE code. By convention
the initialization function is named in the form luaopen_<module>.
The smallest compiled module that comes with NSE is bit, and one of the most straightforward is openssl.
These modules serve as good examples for a beginning module writer. The source code for bit is found in
nse_bit.cc and nse_bit.h, while the openssl source is in nse_openssl.cc and
nse_openssl.h. Most of the other compiled modules follow this nse_<module name>.cc naming
convention.
Reviewing the openssl module shows that one of the functions in nse_openssl.cc is l_md5, which
calculates an MD5 digest. Its function prototype is:
static int l_md5(lua_State *L);
9
http://www.lua.org/manual/5.1/manual.html#lua_CFunction
6. NSE Libraries
17
The prototype shows that l_md5 matches the lua_CFunction type. The function is static because it does not
have to be visible to other compiled code. Only an address is required to register it with Lua. Later in the
file, l_md5 is entered into an array of type luaL_reg and associated with the name md5:
static const struct luaL_reg openssllib[] = {
{ "md5", l_md5 },
{ NULL, NULL }
};
This function will now be known as md5 to NSE. Next the library is registered with a call to
luaL_register inside the initialization function luaopen_openssl, as shown next. Some lines
relating to the registration of OpenSSL BIGNUM types have been omitted:
LUALIB_API int luaopen_openssl(lua_State *L) {
luaL_register(L, OPENSSLLIBNAME, openssllib);
return 1;
}
The function luaopen_openssl is the only function in the file that is exposed in nse_openssl.h.
OPENSSLLIBNAME is simply the string "openssl".
After a compiled module is written, it must be added to NSE by including it in the list of standard libraries
in nse_main.cc. Then the module's source file names must be added to Makefile.in in the appropriate
places. For both these tasks you can simply follow the example of the other C modules. For the Windows
build, the new source files must be added to the mswin32/nmap.vcproj project file using MS Visual
Studio (see http://nmap.org/book/inst-windows.html#inst-win-source).
7. Nmap API
NSE scripts have access to several Nmap facilities for writing flexible and elegant scripts. The API provides
target host details such as port states and version detection results. It also offers an interface to the Nsock
library for efficient network I/O.
7.1. Information Passed to a Script
An effective Nmap scripting engine requires more than just a Lua interpreter. Users need easy access to the
information Nmap has learned about the target hosts. This data is passed as arguments to the NSE script's
action method. The arguments, host and port, are Lua tables which contain information on the target
against which the script is executed. If a script matched a hostrule, it gets only the host table, and if it
matched a portrule it gets both host and port. The following list describes each variable in these two
tables.
host
This table is passed as a parameter to the rule and action functions. It contains information on the
operating system run by the host (if the -O switch was supplied), the IP address and the host name of
the scanned target.
host.os
The os entry in the host table is an array of strings. The strings (as many as eight) are the names of the
operating systems the target is possibly running. Strings are only entered in this array if the target machine
18
7. Nmap API
is a perfect match for one or more OS database entries. If Nmap was run without the -O option, then
host.os is nil.
host.ip
Contains a string representation of the IP address of the target host. If the scan was run against a host
name and the reverse DNS query returned more than one IP addresses then the same IP address is used
as the one chosen for the scan.
host.name
Contains the reverse DNS entry of the scanned target host represented as a string. If the host has no
reverse DNS entry, the value of the field is an empty string.
host.targetname
Contains the name of the host as specified on the command line. If the target given on the command
line contains a netmask or is an IP address the value of the field is nil.
host.directly_connected
A Boolean value indicating whether or not the target host is directly connected to (i.e. on the same
network segment as) the host running Nmap.
host.mac_addr
MAC address of the destination host (six-byte long binary string) or nil, if the host is not directly
connected.
host.mac_addr_next_hop
MAC address of the first hop in the route to the host, or nil if not available.
host.mac_addr_src
Our own MAC address, which was used to connect to the host (either our network card's, or (with
--spoof-mac) the spoofed address).
host.interface
A string containing the interface name (dnet-style) through which packets to the host are sent.
host.bin_ip
The target host's IPv4 address as a 32-bit binary value.
host.bin_ip_src
Our host's (running Nmap) source IPv4 address as a 32-bit binary value.
port
The port table is passed to an NSE service script (i.e. only those with a portrule rather than a hostrule)
in the same fashion as the host table. It contains information about the port against which the script is
running. While this table is not passed to host scripts, port states on the target can still be requested from
Nmap using the nmap.get_port_state() and nmap.get_ports() calls.
port.number
Contains the port number of the target port.
port.protocol
Defines the protocol of the target port. Valid values are "tcp" and "udp".
7. Nmap API
19
port.service
Contains a string representation of the service running on port.number as detected by the Nmap
service detection. If the port.version field is nil, Nmap has guessed the service based on the port
number. Otherwise version detection was able to determine the listening service and this field is equal
to port.version.name.
port.version
This entry is a table which contains information retrieved by the Nmap version scanning engine. Some
of the values (such as service name, service type confidence, and the RPC-related values) may be
retrieved by Nmap even if a version scan was not performed. Values which were not determined default
to nil. The meaning of each value is given in the following table:
Table 1. port.version values
Name
Description
name
Contains the service name Nmap decided on for the port.
name_confidence
Evaluates how confident Nmap is about the accuracy of name, from 1
(least confident) to 10.
product,
version, These five variables are the same as those described under
i n
extrainfo, hostname, < v e r s i o n i n f o >
http://nmap.org/book/vscan-fileformat.html#vscan-db-match.
ostype, devicetype
service_tunnel
Contains the string "none" or "ssl" based on whether or not Nmap
used SSL tunneling to detect the service.
service_fp
The service fingerprint, if any, is provided in this value. This is described
in http://nmap.org/book/vscan-community.html.
rpc_status
Contains a string value of good_prog if we were able to determine the
program number of an RPC service listening on the port, unknown if
the port appears to be RPC but we couldn't determine the program number,
not_rpc if the port doesn't appear be RPC, or untested if we haven't
checked for RPC status.
r p c _ p r o g r a m , The detected RPC program number and the range of version numbers
r p c _ l o w v e r , supported by that program. These will be nil if rpc_status is anything
other than good_prog.
rpc_highver
port.state
Contains information on the state of the port. Service scripts are only run against ports in the open or
open|filtered states, so port.state generally contains one of those values. Other values might
appear if the port table is a result of the get_port_state or get_ports functions. You can adjust
the port state using the nmap.set_port_state() call. This is normally done when an
open|filtered port is determined to be open.
7.2. Network I/O API
To allow for efficient and parallelizable network I/O, NSE provides an interface to Nsock, the Nmap socket
library. The smart callback mechanism Nsock uses is fully transparent to NSE scripts. The main benefit of
NSE's sockets is that they never block on I/O operations, allowing many scripts to be run in parallel. The
20
7. Nmap API
I/O parallelism is fully transparent to authors of NSE scripts. In NSE you can either program as if you were
using a single non-blocking socket or you can program as if your connection is blocking. Even blocking I/O
calls return once a specified timeout has been exceeded. Two flavors of Network I/O are supported:
connect-style and raw packet.
Connect-style network I/O
This part of the network API should be suitable for most classical network uses: Users create a socket, connect
it to a remote address, send and receive data and finally close the socket. Everything up to the Transport
layer (which is either TCP, UDP or SSL) is handled by the library.
An NSE socket is created by calling nmap.new_socket, which returns a socket object. The socket object
supports the usual connect, send, receive, and close methods. Additionally the functions
receive_bytes, receive_lines, and receive_buf allow greater control over data reception.
Example 2 shows the use of connect-style network operations. The try function is used for error handling,
as described in Section 7.3, “Exception Handling” [22].
Example 2. Connect-style I/O
require("nmap")
local socket = nmap.new_socket()
socket:set_timeout(1000)
try = nmap.new_try(function() socket:close() end)
try(socket:connect(host.ip, port.number))
try(socket:send("login"))
response = try(socket:receive())
socket:close()
Raw packet network I/O
For those cases where the connection-oriented approach is too high-level, NSE provides script developers
with the option of raw packet network I/O.
Raw packet reception is handled through a Libpcap wrapper inside the Nsock library. The steps are to open
a capture device, register listeners with the device, and then process packets as they are received.
The pcap_open method creates a handle for raw socket reads from an ordinary socket object. This method
takes a callback function, which computes a packet hash from a packet (including its headers). This hash
can return any binary string, which is later compared to the strings registered with the pcap_register
function. The packet hash callback will normally extract some portion of the packet, such as its source
address.
The pcap reader is instructed to listen for certain packets using the pcap_register function. The function
takes a binary string which is compared against the hash value of every packet received. Those packets whose
hashes match any registered strings will be returned by the pcap_receive method. Register the empty
string to receive all packets.
A script receives all packets for which a listener has been registered by calling the pcap_receive method.
The method blocks until a packet is received or a timeout occurs.
7. Nmap API
21
The more general the packet hash computing function is kept, the more scripts may receive the packet and
proceed with their execution. To handle packet capture inside your script you first have to create a socket
with nmap.new_socket and later close the socket with socket_object:close—just like with the
connection-based network I/O.
While receiving packets is important, sending them is certainly a key feature as well. To accomplish this,
NSE provides access to sending at the IP and Ethernet layers. Raw packet writes do not use the same socket
object as raw packet reads, so the nmap.new_dnet function is called to create the required object for
sending. After this, a raw socket or Ethernet interface handle can be opened for use.
Once the dnet object is created, the function ip_open can be called to initialize the object for IP sending.
ip_send sends the actual raw packet, which must start with the IPv4 header. The dnet object places no
restrictions on which IP hosts may be sent to, so the same object may be used to send to many different hosts
while it is open. To close the raw socket, call ip_close.
For sending at a lower level than IP, NSE provides functions for writing Ethernet frames. ethernet_open
initializes the dnet object for sending by opening an Ethernet interface. The raw frame is sent with
ethernet_send. To close the handle, call ethernet_close.
Sometimes the easiest ways to understand complex APIs is by example. The ipidseq.nse script included
with Nmap uses raw IP packets to test hosts for suitability for Nmap's Idle Scan (-sI). The
sniffer-detect.nse script also included with Nmap uses raw Ethernet frames in an attempt to detect
promiscuous-mode machines on the network (those running sniffers).
7.3. Exception Handling
NSE provides an exception handling mechanism which is not present in the base Lua language. It is tailored
specifically for network I/O operations, and follows a functional programming paradigm rather than an object
oriented one. The nmap.new_try API method is used to create an exception handler. This method returns
a function which takes a variable number of arguments that are assumed to be the return values of another
function. If an exception is detected in the return values (the first return value is false), then the script execution
is aborted and no output is produced. Optionally, you can pass a function to new_try which will be called
if an exception is caught. The function would generally perform any required cleanup operations.
Example 3 shows cleanup exception handling at work. A new function named catch is defined to simply
close the newly created socket in case of an error. It is then used to protect connection and communication
attempts on that socket. If no catch function is specified, execution of the script aborts without further
ado—open sockets will remain open until the next run of Lua's garbage collector. If the verbosity level is at
least one or if the scan is performed in debugging mode a description of the uncaught error condition is
printed on standard output. Note that it is currently not easily possible to group several statements in one try
block.
22
7. Nmap API
Example 3. Exception handling example
local result, socket, try, catch
result = ""
socket = nmap.new_socket()
catch = function()
socket:close()
end
try = nmap.new_try(catch)
try(socket:connect(host.ip, port.number))
result = try(socket:receive_lines(1))
try(socket:send(result))
Writing a function which is treated properly by the try/catch mechanism is straightforward. The function
should return multiple values. The first value should be a Boolean which is true upon successful completion
of the function and false otherwise. If the function completed successfully, the try construct consumes
the indicator value and returns the remaining values. If the function failed then the second returned value
must be a string describing the error condition. Note that if the value is not nil or false it is treated as
true so you can return your value in the normal case and return nil, <error description> if an
error occurs.
7.4. The Registry
The registry is a Lua table (accessible as nmap.registry) with the special property that it is visible by
all scripts and retains its state between script executions. The registry is transient—it is not stored between
Nmap executions. Every script can read and write to the registry. Scripts commonly use it to save information
for other instances of the same script. For example, the whois and asn-query scripts may query one IP
address, but receive information which may apply to tens of thousands of IPs on that network. Saving the
information in the registry may prevent other script threads from having to repeat the query.
The registry may also be used to hand information to completely different scripts. For example, the
snmp-brute script saves a discovered community name in the registry where it may be used by other
SNMP scripts. Script which use the results of another script must declare it using the dependencies
variable to make sure that the earlier script runs first.
Because every script can write to the registry table, it is important to avoid conflicts by choosing keys wisely
(uniquely).
8. Script Writing Tutorial
Suppose that you are convinced of the power of NSE. How do you go about writing your own script? Let's
say that you want to extract information from an identification server to determine the owner of the process
listening on a TCP port. This is not really the purpose of identd (it is meant for querying the owner of outgoing
connections, not listening daemons), but many identd servers allow it anyway. Nmap used to have this
functionality (called ident scan), but it was removed while transitioning to a new scan engine architecture.
The protocol identd uses is pretty simple, but still too complicated to handle with Nmap's version detection
language. First, you connect to the identification server and send a query of the form <port-on-server>,
8. Script Writing Tutorial
23
<port-on-client> and terminated with a newline character. The server should then respond with a
string containing the server port, client port, response type, and address information. The address information
is omitted if there is an error. More details are available in RFC 1413, but this description is sufficient for
our purposes. The protocol cannot be modeled in Nmap's version detection language for two reasons. The
first is that you need to know both the local and the remote port of a connection. Version detection does not
provide this data. The second, more severe obstacle, is that you need two open connections to the target—one
to the identification server and one to the listening port you wish to query. Both obstacles are easily overcome
with NSE.
The anatomy of a script is described in Section 3, “Script Format” [10]. In this section we will show how
the described structure is utilized.
8.1. The Head
The head of the script is essentially its meta information. This includes the fields: description,
categories, dependencies, author, and license as well as initial NSEDoc information such as
usage, args, and output tags (see Section 9, “Writing Script Documentation (NSEDoc)” [27]).
The description field should contain a paragraph or more describing what the script does. If anything about
the script results might confuse or mislead users, and you can't eliminate the issue by improving the script
or results text, it should be documented in the description. If there are multiple paragraphs, the first is
used as a short summary where necessary. Make sure that first paragraph can serve as a stand alone abstract.
This description is short because it is such a simple script:
description = [[
Attempts to find the owner of an open TCP port by querying an auth
(identd - port 113) daemon which must also be open on the target system.
]]
Next comes NSEDoc information. This script is missing the common @usage and @args tags since it is
so simple, but it does have an NSEDoc @output tag:
----@output
-- 21/tcp
open
-- |_ auth-owners:
-- 22/tcp
open
-- |_ auth-owners:
-- 25/tcp
open
-- |_ auth-owners:
-- 80/tcp
open
-- |_ auth-owners:
-- 113/tcp open
-- |_ auth-owners:
-- 587/tcp open
-- |_ auth-owners:
-- 5666/tcp open
-- |_ auth-owners:
24
ftp
ProFTPD 1.3.1
nobody
ssh
OpenSSH 4.3p2 Debian 9etch2 (protocol 2.0)
root
smtp
Postfix smtpd
postfix
http
Apache httpd 2.0.61 ((Unix) PHP/4.4.7 ...)
dhapache
auth?
nobody
submission Postfix smtpd
postfix
unknown
root
8. Script Writing Tutorial
Next come the author, license, and categories tags. This script belongs to the safe because we
are not using the service for anything it was not intended for. Because this script is one that should run by
default it is also in the default category. Here are the variables in context:
author = "Diman Todorov"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
categories = {"default", "safe"}
8.2. The Rule
The rule section is a Lua method which decides whether to skip or execute the script's action method against
a particular service or host. This decision is usually based on the host and port information passed to the rule
function. In the case of the identification script, it is slightly more complicated than that. To decide whether
to run the identification script against a given port we need to know if there is an auth server running on the
target machine. In other words, the script should be run only if the currently scanned TCP port is open and
TCP port 113 is also open. For now we will rely on the fact that identification servers listen on TCP port
113. Unfortunately NSE only gives us information about the currently scanned port.
To find out if port 113 is open, we use the nmap.get_port_state function. If the auth port was not
scanned, the get_port_state function returns nil. So we check that the table is not nil. We also
check that both ports are in the open state. If this is the case, the action is executed, otherwise we skip the
action.
portrule = function(host, port)
local auth_port = { number=113, protocol="tcp" }
local identd = nmap.get_port_state(host, auth_port)
if
identd ~= nil
and identd.state == "open"
and port.protocol == "tcp"
and port.state == "open"
then
return true
else
return false
end
end
8.3. The Mechanism
At last we implement the actual functionality! The script first connects to the port on which we expect to
find the identification server, then it will connect to the port we want information about. Doing so involves
first creating two socket options by calling nmap.new_socket. Next we define an error-handling catch
function which closes those sockets if failure is detected. At this point we can safely use object methods such
as open, close, send and receive to operate on the network socket. In this case we call connect to
make the connections. NSE's exception handling mechanism is used to avoid excessive error-handling code.
8. Script Writing Tutorial
25
We simply wrap the networking calls in a try call which will in turn call our catch function if anything
goes wrong.
If the two connections succeed, we construct a query string and parse the response. If we received a satisfactory
response, we return the retrieved information.
action = function(host, port)
local owner = ""
local client_ident = nmap.new_socket()
local client_service = nmap.new_socket()
local catch = function()
client_ident:close()
client_service:close()
end
local try = nmap.new_try(catch)
try(client_ident:connect(host.ip, 113))
try(client_service:connect(host.ip, port.number))
local localip, localport, remoteip, remoteport =
try(client_service:get_info())
local request = port.number .. ", " .. localport .. "\n"
try(client_ident:send(request))
owner = try(client_ident:receive_lines(1))
if string.match(owner, "ERROR") then
owner = nil
else
owner = string.match(owner, "USERID : .+ : (.+)\n", 1)
end
try(client_ident:close())
try(client_service:close())
return owner
end
Note that because we know that the remote port is stored in port.number, we could have ignored the last
two return values of client_service:get_info() like this:
local localip, localport = try(client_service:get_info())
In this example we exit quietly if the service responds with an error. This is done by assigning nil to the
owner variable which will be returned. NSE scripts generally only return messages when they succeed, so
they don't flood the user with pointless alerts.
26
8. Script Writing Tutorial
9. Writing Script Documentation (NSEDoc)
Scripts are used by more than just their authors, so they require good documentation. NSE modules need
documentation so developers can use them in their scripts. NSE's documentation system, described in this
section, aims to meet both these needs. While reading this section, you may want to browse NSE's online
documentation, which is generated using this system. It is at http://nmap.org/nsedoc/.
NSE uses a customized version of the LuaDoc10 documentation system called NSEDoc. The documentation
for scripts and modules is contained in their source code, as comments with a special form. Example 4 is an
NSEDoc comment taken from the stdnse.print_debug() function.
Example 4. An NSEDoc comment for a function
--- Prints a formatted debug message if the current verbosity level is greater
-- than or equal to a given level.
--- This is a convenience wrapper around
-- <code>nmap.print_debug_unformatted()</code>. The first optional numeric
-- argument, <code>verbosity</code>, is used as the verbosity level necessary
-- to print the message (it defaults to 1 if omitted). All remaining arguments
-- are processed with Lua's <code>string.format()</code> function.
-- @param level Optional verbosity level.
-- @param fmt Format string.
-- @param ... Arguments to format.
Documentation comments start with three dashes: ---. The body of the comment is the description of the
following code. The first paragraph of the description should be a brief summary, with the following paragraphs
providing more detail. Special tags starting with @ mark off other parts of the documentation. In the above
example you see @param, which is used to describe each parameter of a function. A complete list of the
documentation tags is found in Section 9.1, “NSE Documentation Tags” [29].
Text enclosed in the HTML-like <code> and </code> tags will be rendered in a monospace font. This
should be used for variable and function names, as well as multi-line code examples. When a sequence of
lines start with the characters “* ”, they will be rendered as a bulleted list.
It is good practice to document every public function and table in a script or module. Additionally every
script and module should have its own file-level documentation. A documentation comment at the beginning
of a file (one that is not followed by a function or table definition) applies to the entire file. File-level
documentation can and should be several paragraphs long, with all the high-level information useful to a
developer using a module or a user running a script. Example 5 shows documentation for the comm module
(with a few paragraphs removed to save space).
10
http://luadoc.luaforge.net/
9. Writing Script Documentation (NSEDoc)
27
Example 5. An NSEDoc comment for a module
--- Common communication functions for network discovery tasks like
-- banner grabbing and data exchange.
--- These functions may be passed a table of options, but it's not required. The
-- keys for the options table are <code>"bytes"</code>, <code>"lines"</code>,
-- <code>"proto"</code>, and <code>"timeout"</code>. <code>"bytes"</code> sets
-- a minimum number of bytes to read. <code>"lines"</code> does the same for
-- lines. <code>"proto"</code> sets the protocol to communicate with,
-- defaulting to <code>"tcp"</code> if not provided. <code>"timeout"</code>
-- sets the socket timeout (see the socket function <code>set_timeout()</code>
-- for details).
-- @author Kris Katterjohn 04/2008
-- @copyright Same as Nmap--See http://nmap.org/book/man-legal.html
There are some special considerations for documenting scripts rather than functions and modules. In particular,
scripts have special variables for some information which would otherwise belongs in @-tag comments
(script variables are described in Section 3, “Script Format” [10]). In particular, a script's description belongs
in the description variable rather than in a documentation comment, and the information that would go
in @author and @copyright belong in the variables author and license instead. NSEDoc knows
about these variables and will use them in preference to fields in the comments. Scripts should also have an
@output tag showing sample output, as well as @args and @usage where appropriate. Example 6 shows
proper form for script-level documentation, using a combination of documentation comments and NSE
variables.
28
9. Writing Script Documentation (NSEDoc)
Example 6. An NSEDoc comment for a script
description = [[
Maps IP addresses to autonomous system (AS) numbers.
The script works by sending DNS TXT queries to a DNS server which in
turn queries a third-party service provided by Team Cymru
(team-cymru.org) using an in-addr.arpa style zone set up especially for
use by Nmap.
]]
---- @usage
-- nmap --script asn-query.nse [--script-args dns=<DNS server>] <target>
-- @args dns The address of a recursive nameserver to use (optional).
-- @output
-- Host script results:
-- | AS Numbers:
-- | BGP: 64.13.128.0/21 | Country: US
-- |
Origin AS: 10565 SVCOLO-AS - Silicon Valley Colocation, Inc.
-- |
Peer AS: 3561 6461
-- | BGP: 64.13.128.0/18 | Country: US
-- |
Origin AS: 10565 SVCOLO-AS - Silicon Valley Colocation, Inc.
-- |_
Peer AS: 174 2914 6461
author = "jah, Michael"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
categories = {"discovery", "external"}
Compiled NSE modules are also documented with NSEDoc, even though they have no Lua source code.
Each compiled module has a file <modulename>.luadoc that is kept in the nselib directory alongside
the Lua modules. This file lists and documents the functions and tables in the compiled module as though
they were written in Lua. Only the name of each function is required, not its definition (not even end). You
must use the @name and @class tags when documenting a table to assist the documentation parser in
identifying it. There are several examples of this method of documentation in the Nmap source distribution
(including nmap.luadoc, bit.luadoc, and pcre.luadoc).
9.1. NSE Documentation Tags
The following tags are understood by NSEDoc:
@param
Describes a function parameter. The first word following @param is the name of the parameter being
described. The tag should appear once for each parameter of a function.
@see
Adds a cross-reference to another function or table.
@return
Describes a return value of a function. @return may be used multiple times for multiple return values.
9. Writing Script Documentation (NSEDoc)
29
@usage
Provides a usage example of a function, script, or module. In the case of a function, the example is Lua
code; for a script it is an Nmap command line; and for a module it is usually a code sample. @usage
may be given more than once. If it is omitted in a script, NSEDoc generates a default standardized usage
example.
@name
Defines a name for the function or table being documented. This tag is normally not necessary because
NSEDoc infers names through code analysis.
@class
Defines the “class” of the object being documented: function, table, or module. Like @name,
this is normally inferred automatically.
@field
In the documentation of a table, @field describes the value of a named field.
@args
Describes a script argument, as used with the --script-args option (see Section 2.4, “Arguments
to Scripts” [9]). The first word after @args is the name of the argument, and everything following
that is the description. This tag is special to script-level comments.
@output
This tag, which is exclusive to script-level comments, shows sample output from a script.
@author
This tag, which may be given multiple times, lists the authors of an NSE module. For scripts, use the
author variable instead.
@copyright
This tag describes the copyright status of a module. For scripts, use the license variable instead.
10. Script Parallelism in NSE
Before now, we have only lightly touched on the steps NSE takes to allow multiple scripts to execute in
parallel. Usually, the author need not concern himself with how any of this is implemented; however, there
are a couple cases that warrant discussion that we will cover in this section. As a script writer, you may need
to control how multiple scripts interact in a library; you may require multiple threads to work in parallel; or
perhaps you need to serialize access to a remote resource.
The standard mechanism for parallel execution is a thread. A thread encapsulates execution flow and data
of a script using the Lua thread or coroutine. A Lua thread allows us to yield the current script at
arbitrary points to continue work on another script. Typically, these yield points are blocking calls to the
NSE Socket library. The yield back to NSE is also transparent; the script is unaware of the transition and
views each socket method as a blocking call.
Let's go over some common terminology. A script is analogous to a binary executable; it holds the information
necessary to execute our script. A thread (a Lua coroutine) is analogous to a process; it runs a script against
a host and possibly port. We sometimes abuse our terminology throughout the book by referring to a thread
30
10. Script Parallelism in NSE
as a running script. We are really saying the "instantiation of the script", in the same sense that a process is
the instantiation of an executable.
NSE provides the bare-bone essentials you need to expand your degree of parallelism beyond the basic script
thread: new independent threads, Mutexes, and Condition Variables. We will go into depth on each of these
mechanisms in the following sections.
10.1. Worker Threads
There are several instances where a script needs finer control with respect to parallel execution beyond what
is offered by default with a generic script. The common reason for this need is the inability for a script to
read from multiple sockets concurrently. For example, an HTTP spidering script may want to have multiple
Lua threads querying web server resources in parallel. To solve this problem, NSE offers the function
stdnse.new_thread to create worker threads. These worker threads have all the power of independent
scripts with the only restriction that they may not report Script Output.
Each worker thread launched by a script is given a main function and a variable number of arguments to be
passed to the main function by NSE:
worker_thread, status_function = stdnse.new_thread(main, ...)
You are given back the Lua thread (coroutine) that uniquely identifies your worker thread and a status query
function that queries the status of your new worker.
The status query function returns two values:
status, error_object = status_function()
The first return value, status, is simply the return value of coroutine.status on the worker thread
coroutine (more precisely, the base coroutine, read more about base coroutine in the section called “The
Base Thread” [36]). The second return value contains the error object thrown that ended the worker thread
or nil if no error was thrown. This object is typically a string, like most Lua errors. However, recall that
any Lua type can be an error object, even nil! You should inspect the error object, the second return value,
only if the status of your worker is "dead".
NSE discards all return values from the main function when the worker thread finishes execution. You should
communicate with your worker through the use of main function parameters, upvalues, or function
environments. You will see how to do this in Example 7.
Finally, when using worker threads you should always use condition variables and Mutexes to coordinate
with your worker threads. Keep in mind that Nmap is single threaded so there are no (memory) issues in
synchronization to worry about; however, there is resource contention. Your resources are usually network
bandwidth, network sockets, etc. Condition variables are also useful if the work for any single thread is
dynamic. For example, a web server spider script with a pool of workers will initially have a single root html
document. Following the retrieval of the root document, the set of resources to be retrieved (the worker's
work) will become very large (an html document adds many new hyperlinks (resources) to fetch).
10. Script Parallelism in NSE
31
Example 7. Worker Thread Example
local requests = {"/", "/index.html", --[[ long list of objects ]]}
function thread_main (host, port, responses, ...)
local condvar = nmap.condvar(responses);
local what = {n = select("#", ...), ...};
local allReqs = nil;
for i = 1, what.n do
allReqs = http.pGet(host, port, what[i], nil, nil, allReqs);
end
local p = assert(http.pipeline(host, port, allReqs));
for i, response in ipairs(p) do responses[#responses+1] = response end
condvar "signal";
end
function many_requests (host, port)
local threads = {};
local responses = {};
local condvar = nmap.condvar(responses);
local i = 1;
repeat
local j = math.min(i+10, #requests);
local co = stdnse.new_thread(thread_main, host, port, responses,
unpack(requests, i, j));
threads[co] = true;
i = j+1;
until i > #requests;
repeat
condvar "wait";
for thread in pairs(threads) do
if coroutine.status(thread) == "dead" then threads[thread] = nil end
end
until next(threads) == nil;
return responses;
end
For brevity, this example omits typical behavior of a traditional web spider. The requests table is assumed
to contain a number of objects (hundreds or thousands) to warrant the use of worker threads. Our example
will dispatch a new thread with 11 relative Uniform Resource Identifiers (URI) to request, up to the length
of the requests table. Worker threads are very cheap so we are not afraid to create a lot of them. After
we dispatch this large number of threads, we wait on our Condition Variable until every thread has finished
then finally return the responses table.
You may have noticed that we did not use the status function returned by stdnse.new_thread. You
will typically use this for debugging or if your program must stop based on the error thrown by one of your
worker threads. Our simple example did not require this but a fault tolerant library may.
10.2. Thread Mutexes
Recall from the beginning of this section that each script execution thread (e.g. ftp-anon running against
an FTP server on a target host) yields to other scripts whenever it makes a call on network objects (sending
32
10. Script Parallelism in NSE
or receiving data). Some scripts require finer concurrency control over thread execution. An example is the
whois script which queries whois servers for each target IP address. Because many concurrent queries often
result in getting one's IP banned for abuse, and because a single query may return additional information for
targets other threads are running against, it is useful to have other threads pause while one thread performs
a query.
To solve this problem, NSE includes a mutex function which provides a mutex11 (mutual exclusion object)
usable by scripts. The Mutex allows for only one thread to be working on an object. Competing threads
waiting to work on this object are put in the waiting queue until they can get a "lock" on the Mutex. A solution
for the whois problem above is to have each thread block on a Mutex using a common string, thus ensuring
that only one thread is querying whois servers at once. When finished querying the remote servers, the thread
can store results in the NSE registry and unlock the Mutex. Other scripts waiting to query the remote server
can then obtain a lock, check for usable results retrieved from previous queries, make their own queries, and
unlock the Mutex. This is a good example of serializing access to a remote resource.
The first step in using a Mutex is to create one via a call to the nmap library:
mutexfn = nmap.mutex(object)
The mutexfn returned is a function which works as a Mutex for the object passed in. This object can
be any Lua data type except nil, booleans, and numbers. The returned function allows you to lock,
try to lock, and release the Mutex. Its first and only parameter must be one of the following:
"lock"
Make a blocking lock on the Mutex. If the Mutex is busy (another thread has a lock on it), then the
thread will yield and wait. The function returns with the Mutex locked.
"trylock"
Makes a non-blocking lock on the Mutex. If the Mutex is busy then it immediately returns with a return
value of false. Otherwise the Mutex locks the Mutex and returns true.
"done"
Releases the Mutex and allows another thread to lock it. If the thread does not have a lock on the Mutex,
an error will be raised.
"running"
Returns the thread locked on the Mutex or nil if the Mutex is not locked. This should only be used for
debugging as it interferes with garbage collection of finished threads.
NSE maintains a weak reference to the Mutex so other calls to nmap.mutex with the same object will
return the same function (Mutex); however, if you discard your reference to the Mutex then it may be
collected; and, subsequent calls to nmap.mutex with the object will return a different Mutex function!
Thus you should save your Mutex to a (local) variable that persists for the entire time you require.
A simple example of using the API is provided in Example 8. For real-life examples, read the
asn-query.nse and whois.nse scripts in the Nmap distribution.
11
http://en.wikipedia.org/wiki/Mutual_exclusion
10. Script Parallelism in NSE
33
Example 8. Mutex manipulation
local mutex = nmap.mutex("My Script's Unique ID");
function action(host, port)
mutex "lock";
-- Do critical section work - only one thread at a time executes this.
mutex "done";
return script_output;
end
10.3. Condition Variables
Condition Variables arose out of a need to coordinate with worker threads created using the
stdnse.new_thread function. A Condition Variable allows one or more threads to wait on an object
and one or more threads to awaken one or all threads waiting on the object. Said differently, multiple threads
may unconditionally block on the Condition Variable by waiting. Other threads may wake up one or all
of the waiting threads via signalling the Condition Variable.
As an example, we may dispatch multiple worker threads that will produce results for us to use, like our
earlier Example 7 [32]. Until all the workers finish, our master thread must sleep. Note that we cannot poll
for results like in a traditional Operating System thread because NSE does not preempt Lua threads. Instead,
we use a Condition Variable that the master thread waits on until awakened by a worker. The master will
continually wait until all workers have terminated.
The first step in using a Condition Variable is to create one via a call to the nmap library:
condvarfn = nmap.condvar(object)
The semantics for Condition Variables are similar to Mutexes. The condvarfn returned is a function which
works as a Condition Variable for the object passed in. This object can be any Lua data type except nil,
booleans, and numbers. The returned function allows you to wait, signal, and broadcast on the Condition
Variable. Its first and only parameter must be one of the following:
"wait"
Wait on the Condition Variable. This adds your thread to the waiting queue for the Condition Variable.
You will resume execution when another thread signals or broadcasts on the Condition Variable.
"signal"
Signal the Condition Variable. A thread in the Condition Variable's waiting queue will be resumed.
"broadcast"
Signal all threads in the Condition Variable's waiting queue.
Like with Mutexes, NSE maintains a weak reference to the Condition Variable so other calls to
nmap.condvar with the same object will return the same function (Condition Variable); however, if you
discard your reference to the Condition Variable then it may be collected; and, subsequent calls to
nmap.condvar with the object will return a different Condition Variable function! Thus you should save
your Condition Variable to a (local) variable that persists for the entire time you require.
34
10. Script Parallelism in NSE
When using Condition Variables, it is important to check the predicate before and after waiting. A predicate
is a test on whether to continue doing work within your worker or master thread. For your worker threads,
this will at the very least include a test to see if the master thread is still alive. You do not want to continue
doing work when no thread will use your results. A typical test before waiting may be: check whether the
master is still running, if not then quit; check that there is work to be done; if not then wait.
NSE does not guarantee spurious wakeups will not occur; that is, there is no guarantee your thread will not
be awakened when no thread called "signal" or "broadcast" on the Condition Variable. The typical,
but not only, reason for a spurious wakeup is the termination of a thread using a Condition Variable. This is
an important guarantee NSE makes that allows you to avoid deadlock where a worker or master waits for a
thread to wake them up that ended without signaling the Condition Variable.
10.4. Collaborative Multithreading
One of Lua's least known features is collaborative multithreading through coroutines. A coroutine provides
an independent execution stack that is resumable. The standard coroutine provides access to the creation
and manipulation of coroutines. Lua's online first edition of Programming in Lua12 contains an excellent
introduction to coroutines. We will provide an overview of the use of coroutines here for completeness but
this is no replacement for reviewing PiL.
We have mentioned coroutines throughout this section as threads. This is the type (thread) of a coroutine
in Lua. Users of NSE that have any parallel programming experience with Operating System threads may
be confused by this. As a reminder, Nmap is single threaded. Lua threads provide the basis for parallel
scripting but only one thread is ever running at a time.
A Lua function executes on top of a Lua thread. The thread maintains a stack of active functions, local
variables, and the current instruction. We can switch between coroutines by explicitly yielding the running
thread. The coroutine which resumed the yielded thread resumes operation. Example 9 shows a brief use of
coroutines to print numbers.
Example 9. Basic Coroutine Use
local function main ()
coroutine.yield(1)
coroutine.yield(2)
coroutine.yield(3)
end
local co = coroutine.create(main)
for i = 1, 3 do
print(coroutine.resume(co))
end
--> true
1
--> true
2
--> true
3
What you should take from this example is the ability to transfer between flows of control extremely easily
through the use of coroutine.yield. This is an extremely powerful concept that enables NSE to run
scripts in parallel. All scripts are run as coroutines that yield whenever they make a blocking socket function
12
http://www.lua.org/pil/
10. Script Parallelism in NSE
35
call. This enables NSE to run other scripts and later resume the blocked script when its I/O operation has
completed.
As a script writer, there are times when coroutines are the best tool for a job. One common use in socket
programming is to filter data. You may produce a function that generates all the links from an HTML
document. An iterator using string.gmatch only catchs a single pattern. Because some complex matches
may take many different Lua patterns, it is more appropriate to use a coroutine. Example 10 shows how to
do this.
Example 10. Link Generator
function links (html_document)
local function generate ()
for m in string.gmatch(html_document, "url%((.-)%)") do
coroutine.yield(m) -- css url
end
for m in string.gmatch(html_document, "href%s*=%s*\"(.-)\"") do
coroutine.yield(m) -- anchor link
end
for m in string.gmatch(html_document, "src%s*=%s*\"(.-)\"") do
coroutine.yield(m) -- img source
end
end
return coroutine.wrap(generate)
end
function action (host, port)
-- ... get HTML document and store in html_document local
for link in links(html_document) do
links[#links+1] = link; -- store it
end
-- ...
end
There are many other instances where coroutines may provide an easier solution to a problem. It takes
experience from use to help identify those cases.
The Base Thread
Because scripts may use coroutines for their own multithreading, it is important to be able to identify an
owner of a resource or to establish whether the script is still alive. NSE provides the function stdnse.base
for this purpose.
Particularly when writing a library that attributes ownership of a cache or socket to a script, you may use the
base thread to establish whether the script is still running. coroutine.status on the base thread will
give the current state of the script. In cases where the script is "dead", you will want to release the resource.
Be careful with keeping references to these threads; NSE may discard a script even though it has not finished
executing. The thread will still report a status of "suspended". You should keep a weak reference to the
thread in these cases so that it may be collected.
36
10. Script Parallelism in NSE
11. Version Detection Using NSE
The version detection system built into Nmap was designed to efficiently recognize the vast majority of
protocols with a simple probe and pattern matching syntax. Some protocols require more complex
communication than version detection can handle. A generalized scripting language as provided by NSE is
perfect for these tough cases.
NSE's version category contains scripts that enhance standard version detection. Scripts in this category
are run whenever you request version detection with -sV; you don't need to use -sC to run these. This cuts
the other way too: if you use -sC, you won't get version scripts unless you also use -sV.
One protocol which we were unable to detect with normal version detection is Skype version 2. The protocol
was likely designed to frustrate detection out of a fear that telecom-affiliated Internet service providers might
consider Skype competition and interfere with the traffic. Yet we did find one way to detect it. If Skype
receives an HTTP GET request, it pretends to be a web server and returns a 404 error. But for other requests,
it sends back a chunk of random-looking data. Proper identification requires sending two probes and comparing
the two responses—an ideal task for NSE. The simple NSE script which accomplishes this is shown in
Example 11.
11. Version Detection Using NSE
37
Example 11. A typical version detection script (Skype version 2 detection)
description = [[
Detects the Skype version 2 service.
]]
author = "Brandon Enright"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
categories = {"version"}
require "comm"
portrule = function(host, port)
return (port.number == 80 or port.number == 443 or
port.service == nil or port.service == "" or
port.service == "unknown")
and port.protocol == "tcp" and port.state == "open"
and port.service ~= "http" and port.service ~= "ssl/http"
end
action = function(host, port)
local status, result = comm.exchange(host, port,
"GET / HTTP/1.0\r\n\r\n", {bytes=26, proto=port.protocol})
if (not status) then
return
end
if (result ~= "HTTP/1.0 404 Not Found\r\n\r\n") then
return
end
-- So far so good, now see if we get random data for another request
status, result = comm.exchange(host, port,
"random data\r\n\r\n", {bytes=15, proto=port.protocol})
if (not status) then
return
end
if string.match(result, "[^%s!-~].*[^%s!-~].*[^%s!-~]") then
-- Detected
port.version.name = "skype2"
port.version.product = "Skype"
nmap.set_port_version(host, port, "hardmatched")
return
end
return
end
If the script detects Skype, it augments its port table with now-known name and product fields. It then
sends this new information to Nmap by calling nmap.set_port_version. Several other version fields
are available to be set if they are known, but in this case we only have the name and product. For the full list
of version fields, refer to the nmap.set_port_version documentation.
Notice that this script does nothing unless it detects the protocol. A script shouldn't produce output (other
than debug output) just to say it didn't learn anything.
38
11. Version Detection Using NSE
12. Example Script: finger.nse
The finger script (finger.nse) is a perfect example of a short and simple NSE script.
First the information fields are assigned. A detailed description of what the script actually does goes in the
description field.
description = [[
Attempts to get a list of usernames via the finger service.
]]
author = "Eddie Bell"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
The categories field is a table containing all the categories the script belongs to—These are used for
script selection with the --script option:
categories = {"default", "discovery"}
You can use the facilities provided by the nselib (Section 6, “NSE Libraries” [13]) with require. Here
we want to use common communication functions and shorter port rules:
require "comm"
require "shortport"
We want to run the script against the finger service. So we test whether it is using the well-known finger
port (79/tcp), or whether the service is named “finger” based on version detection results or in the port
number's listing in nmap-services:
portrule = shortport.port_or_service(79, "finger")
First, the script uses nmap.new_try to create an exception handler that will quit the script in case of an
error. Next, it passes control to comm.exchange, which handles the network transaction. Here we have
asked to wait in the communication exchange until we receive at least 100 lines, wait at least 5 seconds, or
until the remote side closes the connection. Any errors are handled by the try exception handler. The script
returns a string if the call to comm.exchange() was successful.
action = function(host, port)
local try = nmap.new_try()
return try(comm.exchange(host, port, "\r\n",
{lines=100, proto=port.protocol, timeout=5000}))
end
13. Implementation Details
Now it is time to explore the NSE implementation details in depth. Understanding how NSE works is useful
for designing efficient scripts and libraries. The canonical reference to the NSE implementation is the source
code, but this section provides an overview of key details. It should be valuable to folks trying to understand
and extend the NSE source code, as well as to script authors who want to better-understand how their scripts
are executed.
12. Example Script: finger.nse
39
13.1. Initialization Phase
During its initialization stage, Nmap loads the Lua interpreter and its provided libraries. These libraries are
fully documented in the Lua Reference Manual13. Here is a summary of the libraries, listed alphabetically
by their namespace name:
debug
The debug library provides a low-level API to the Lua interpreter, allowing you to access functions
along the execution stack, retrieve function closures and object metatables, and more.
io
The Input/Output library offers functions such as reading from files or from the output from programs
you execute.
math
Numbers in Lua usually correspond to the double C type, so the math library provides access to
rounding functions, trigonometric functions, random number generation, and more.
os
The Operating System library provides system facilities such as filesystem operations (including file
renaming or removal and temporary file creation) and system environment access.
package
Among the functions provided by Lua's package-lib is require, which is used to load nselib modules.
string
The string library provides functions for manipulating Lua strings, including printf-style string formatting,
pattern matching using Lua-style patterns, substring extraction, and more.
table
The table manipulation library is essential for operating on Lua's central data structure (tables).
In addition to loading the libraries provided by Lua, the nmap namespace functions are loaded. The search
paths are the same directories that Nmap searches for its data files, except that the nselib directory is
appended to each. At this stage any provided script arguments are stored inside the registry.
The next phase of NSE initialization is loading the selected scripts, based on the defaults or arguments
provided to the --script option. The version category scripts are loaded as well if version detection
was enabled. NSE first tries to interpret each --script argument as a category. This is done with a Lua
C function in nse_init.cc named entry based on data from the script.db script categorization
database. If the category is found, those scripts are loaded. Otherwise Nmap tries to interpret --script
arguments as files or directories. If no files or directories with a given name are found in Nmap's search path,
an error is raised and the Script Engine aborts.
If a directory is specified, all of the .nse files inside it are loaded. Each loaded file is executed by Lua. If
a portrule is present, it is saved in the porttests table with a portrule key and file closure value. Otherwise,
if the script has a hostrule, it is saved in the hosttests table in the same manner.
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40
13. Implementation Details
13.2. Matching Scripts with Targets
After initialization is finished, the hostrules and portrules are evaluated for each host in the current
target group. The rules of every chosen script is tested against every host and (in the case of service scripts)
each open and open|filtered port on the hosts. The combination can grow quite large, so portrules
should be kept as simple as possible. Save any heavy computation for the script's action.
Next, a Lua thread is created for each of the matching script-target combinations. Each thread is stored with
pertinent information such as its dependencies, target, target port (if applicable), host and port tables (passed
to the action), and the script type (service or host script). The mainloop function then processes each
runlevel grouping of threads in order.
13.3. Script Execution
Nmap performs NSE script scanning in parallel by taking advantage of Nmap's Nsock parallel I/O library
and the Lua coroutines 14 language feature. Coroutines offer collaborative multi-threading so that scripts can
suspend themselves at defined points and allow other coroutines to execute. Network I/O, particularly waiting
for responses from remote hosts, often involves long wait times, so this is when scripts yield to others. Key
functions of the Nsock wrapper cause scripts to yield (pause). When Nsock finishes processing such a request,
it makes a callback which causes the script to be pushed from the waiting queue back into the running queue
so it can resume operations when its turn comes up again.
The mainloop function moves threads between the waiting and running queues as needed. A thread which
yields is moved from the running queue into the waiting list. Running threads execute until they either yield,
complete, or fail with an error. Threads are made ready to run (placed in the running queue) by calling
process_waiting2running. This process of scheduling running threads and moving threads between
queues continues until no threads exist in either queue.
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13. Implementation Details
41
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