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<chapter id="nse"><title>Nmap Scripting Engine</title>
<indexterm class="startofrange" id="nse-indexterm"><primary>Nmap Scripting Engine (NSE)</primary></indexterm>
<indexterm><primary>scripting</primary><see>Nmap Scripting Engine</see></indexterm>
<indexterm><primary>NSE</primary><see>Nmap Scripting Engine</see></indexterm>
<sect1 id="nse-intro">
<title>Introduction</title>
<para>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.</para>
<para>We designed NSE to be versatile, with the following tasks in mind:</para>
<variablelist>
<varlistentry>
<term>Network discovery</term>
<listitem>
<para>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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary>version detection</primary><secondary>using NSE</secondary></indexterm>
More sophisticated version detection</term>
<listitem>
<para>The Nmap version detection system (<xref linkend="vscan"/>)
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
<xref linkend="nse-vscan"/>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary>vulnerability detection</primary></indexterm>
Vulnerability detection</term>
<listitem>
<para>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 <ulink role="hidepdf" url="https://sectools.org/vuln-scanners.html">vulnerability scanner</ulink>,
NSE is powerful enough to handle even demanding vulnerability
checks. When the Heartbleed bug affected hundreds of thousands of
systems worldwide, Nmap's developers responded with the
<literal>ssl-heartbleed</literal> detection script within 2 days.
Many vulnerability detection scripts are already available and we plan to distribute more as they are written.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Backdoor detection</term>
<listitem>
<para>
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, but more complex worms
and backdoors require NSE's advanced capabilities to reliably detect.
NSE has been used to detect the Double Pulsar NSA backdoor in SMB and
backdoored versions of UnrealIRCd, vsftpd, and ProFTPd.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Vulnerability exploitation</term>
<listitem>
<para>
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),<indexterm><primary>penetration testing</primary></indexterm>
though we aren't
planning to turn Nmap into an exploitation framework such as
<ulink url="http://www.metasploit.com">Metasploit</ulink>.<indexterm><primary><application>Metasploit</application></primary></indexterm>
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
These listed items were our initial goals, and we expect Nmap
users to come up with even more inventive uses for NSE.
</para>
<para>
Scripts are written in the
embedded
<ulink url="http://www.lua.org/">Lua programming language</ulink>, version 5.3.<indexterm><primary>Lua programming language</primary><seealso>Nmap Scripting Engine</seealso></indexterm>
The language itself is well documented in the books
<web>
<citetitle><ulink url="http://www.amazon.com/dp/8590379868?tag=secbks-20">Programming
in Lua, Fourth Edition</ulink></citetitle> and
<citetitle><ulink url="http://www.amazon.com/dp/9888381229?tag=secbks-20">Lua
5.2 Reference Manual</ulink></citetitle>.
</web>
<print>
<citetitle>Programming in Lua, Fourth Edition</citetitle> and
<citetitle>Lua 5.2 Reference Manual</citetitle>.
</print>
The reference manual, updated for Lua 5.3, is also
<ulink url="http://www.lua.org/manual/5.3/">freely available
online</ulink>, as is the
<ulink url="http://www.lua.org/pil/">first edition of <citetitle>Programming in
Lua</citetitle></ulink>. Given the availability of these excellent general
Lua programming references, this document only covers aspects and
extensions specific to Nmap's scripting engine.
</para>
<para>
NSE is activated with the <option>-sC</option> option (or
<option>--script</option> if you wish to specify a custom set of
scripts) and results are integrated into Nmap
normal<indexterm><primary>normal output</primary></indexterm>
and XML output.<indexterm><primary>XML output</primary></indexterm>
</para>
<para>
A typical script scan is shown in the
<xref linkend="nse-ex1" xrefstyle="select: label nopage"/>.
Service scripts producing output in this example are
<literal>ssh-hostkey</literal>, which provides the system's RSA and DSA SSH keys, and <literal>rpcinfo</literal>, which queries
portmapper to enumerate available services. The only host
script producing output in this example
is <literal>smb-os-discovery</literal>, which collects a variety of
information from SMB servers.<indexterm><primary>script names, examples
of</primary></indexterm> Nmap discovered all of this information in a third of a second.</para>
<example id="nse-ex1"><title>Typical NSE output</title><indexterm><primary><option>-sC</option></primary><secondary>example of</secondary></indexterm>
<screen>
# <userinput>nmap -sC -p22,111,139 -T4 localhost</userinput>
Starting Nmap ( https://nmap.org )
Nmap scan report for 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
</screen>
</example>
<para>A 38-minute video introduction to NSE is available at
<ulink url="https://nmap.org/presentations/BHDC10/"/>. This
presentation was given by Fyodor and David Fifield at Defcon and the
Black Hat Briefings in 2010.</para>
</sect1>
<sect1 id="nse-usage">
<title>Usage and Examples</title>
<para>
While NSE has a complex implementation for efficiency, it is
strikingly easy to use. Simply specify
<option>-sC</option><indexterm><primary><option>-sC</option></primary></indexterm>
to enable the most common scripts. Or specify the
<option>--script</option><indexterm><primary><option>--script</option></primary></indexterm>
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
<option>--script-args</option><indexterm><primary><option>--script-args</option></primary></indexterm> and <option>--script-args-file</option><indexterm><primary><option>--script-args-file</option></primary></indexterm>
options.
The <option>--script-help</option><indexterm><primary><option>--script-help</option></primary></indexterm>
shows a description of what each selected script does.
The two remaining options,
<option>--script-trace</option><indexterm><primary><option>--script-trace</option></primary></indexterm>
and <option>--script-updatedb</option>,<indexterm><primary><option>--script-updatedb</option></primary></indexterm>
are generally only used for script debugging and development. Script scanning is also included as part of the <option>-A</option> (aggressive scan) option.
</para>
<para>
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 <option>-sn</option> 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 <option>-Pn -sn</option> options together with
<option>-sC</option> or <option>--script</option>. Every host will
be assumed up and still only host scripts will be run. This
technique is useful for scripts like
<filename>whois-ip</filename><indexterm><primary><filename>whois-ip</filename> script</primary></indexterm>
that only use the remote system's address and don't require it to be
up.
</para>
<para>
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.
</para>
<sect2 id="nse-categories"><title>Script Categories</title>
<indexterm><primary>script categories</primary></indexterm>
<para>NSE scripts define a list of categories they belong to.
Currently defined categories are
<literal>auth</literal>,
<literal>broadcast</literal>,
<literal>brute</literal>,
<literal>default</literal>.
<literal>discovery</literal>,
<literal>dos</literal>,
<literal>exploit</literal>,
<literal>external</literal>,
<literal>fuzzer</literal>,
<literal>intrusive</literal>,
<literal>malware</literal>,
<literal>safe</literal>,
<literal>version</literal>, and
<literal>vuln</literal>.
Category names are not case sensitive. The following list describes each category.</para>
<variablelist>
<varlistentry>
<term id="nse-category-auth">
<indexterm><primary sortas="auth script category">&ldquo;<literal>auth</literal>&rdquo; script category</primary></indexterm>
<option>auth</option>
</term>
<listitem>
<para>These scripts deal with authentication credentials (or bypassing them) on the target system. Examples include <literal>x11-access</literal>, <literal>ftp-anon</literal>, and <literal>oracle-enum-users</literal>. Scripts which use brute force attacks to determine credentials are placed in the <literal>brute</literal> category instead.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-broadcast">
<indexterm><primary sortas="broadcast script category">&ldquo;<literal>broadcast</literal>&rdquo; script category</primary></indexterm>
<option>broadcast</option>
</term>
<listitem>
<para>Scripts in this category typically do discovery of hosts
not listed on the command line by broadcasting on the local network.
Use the
<varname>newtargets</varname><indexterm><primary><varname>newtargets</varname> script argument</primary></indexterm>
script argument to allow these scripts to automatically add the
hosts they discover to the Nmap scanning queue.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-brute">
<indexterm><primary sortas="brute script category">&ldquo;<literal>brute</literal>&rdquo; script category</primary></indexterm>
<option>brute</option>
</term>
<listitem>
<para>These scripts use brute force attacks to guess authentication credentials of a remote server. Nmap contains scripts for brute forcing dozens of protocols, including <literal>http-brute</literal>, <literal>oracle-brute</literal>, <literal>snmp-brute</literal>, etc.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-default">
<indexterm><primary sortas="default script category">&ldquo;<literal>default</literal>&rdquo; script category</primary></indexterm>
<option>default</option>
</term>
<listitem>
<para>These scripts are the default set and are run when
using the <option>-sC</option> or <option>-A</option>
options rather than listing scripts
with <option>--script</option>. This category can also be
specified explicitly like any other
using <option>--script=default</option>. Many factors are
considered in deciding whether a script should be run by
default:</para>
<variablelist>
<varlistentry>
<term>Speed</term>
<listitem><para>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.</para></listitem>
</varlistentry>
<varlistentry>
<term>Usefulness</term>
<listitem><para>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.</para></listitem>
</varlistentry>
<varlistentry>
<term>Verbosity</term>
<listitem><para>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 <literal>default</literal> 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 is discovered.</para></listitem>
</varlistentry>
<varlistentry>
<term>Reliability</term>
<listitem><para>Many scripts use heuristics and fuzzy signature matching to reach conclusions about the target host or service. Examples include <literal>sniffer-detect</literal> and <literal>sql-injection</literal>. If the script is often wrong, it doesn't belong in the <literal>default</literal> 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.</para></listitem>
</varlistentry>
<varlistentry>
<term>Intrusiveness</term>
<listitem><para>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 <literal>default</literal> category. Default scripts are almost always in the <literal>safe</literal> category too, though we occasionally allow <literal>intrusive</literal> scripts by default when they are only mildly intrusive and score well in the other factors.</para></listitem>
</varlistentry>
<varlistentry>
<term>Privacy</term>
<listitem><para>Some scripts, particularly those in the <literal>external</literal> category described later, divulge information to third parties by their very nature. For example, the <literal>whois</literal> 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 <literal>default</literal> category inclusion.</para></listitem>
</varlistentry>
</variablelist>
<para>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 <literal>default</literal> category. A few default scripts are <literal>identd-owners</literal> (determines the username running remote services using identd), <literal>http-auth</literal> (obtains authentication scheme and realm of web sites requiring authentication), and <literal>ftp-anon</literal> (tests whether an FTP server allows anonymous access).</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-discovery">
<indexterm><primary sortas="discovery script category">&ldquo;<literal>discovery</literal>&rdquo; script category</primary></indexterm>
<option>discovery</option>
</term>
<listitem>
<para>These scripts try to actively discover more about the
network by querying public registries, SNMP-enabled
devices, directory services, and the like. Examples include <literal>html-title</literal> (obtains the title of the root path of web sites), <literal>smb-enum-shares</literal> (enumerates Windows shares), and <literal>snmp-sysdescr</literal> (extracts system details via SNMP).</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-dos">
<indexterm><primary sortas="dos script category">&ldquo;<literal>dos</literal>&rdquo; script category</primary></indexterm>
<option>dos</option>
</term>
<listitem>
<para>Scripts in this category may cause a denial of
service. Sometimes this is done to test vulnerability to
a denial of service method, but more commonly it is
an undesired by necessary side effect of testing for
a traditional vulnerability. These tests sometimes crash
vulnerable services.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-exploit">
<indexterm><primary sortas="exploit script category">&ldquo;<literal>exploit</literal>&rdquo; script category</primary></indexterm>
<option>exploit</option>
</term>
<listitem>
<para>These scripts aim to actively exploit some vulnerability. Examples include <literal>jdwp-exec</literal> and <literal>http-shellshock</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-external">
<indexterm><primary sortas="external script category">&ldquo;<literal>external</literal>&rdquo; script category</primary></indexterm>
<option>external</option>
</term>
<listitem>
<para>Scripts in this category may send data to a
third-party database or other network resource. An example
of this is <literal>whois-ip</literal>, which makes a
connection to
whois<indexterm><primary>whois</primary></indexterm> 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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-fuzzer">
<indexterm><primary sortas="fuzzer script category">&ldquo;<literal>fuzzer</literal>&rdquo; script category</primary></indexterm>
<option>fuzzer</option>
</term>
<listitem>
<para>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 <literal>dns-fuzz</literal>, which bombards a DNS server with slightly flawed domain requests until either the server crashes or a user specified time limit elapses.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-intrusive">
<indexterm><primary sortas="intrusive script category">&ldquo;<literal>intrusive</literal>&rdquo; script category</primary></indexterm>
<option>intrusive</option>
</term>
<listitem>
<para>These are scripts that cannot be classified in the
<literal>safe</literal> 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 <literal>http-open-proxy</literal> (which attempts to
use the target server as an HTTP proxy)
and <literal>snmp-brute</literal> (which tries to guess a
device's SNMP community string by sending common values
such
as <literal>public</literal>, <literal>private</literal>,
and <literal>cisco</literal>). Unless a script is in the special <literal>version</literal> category, it should be categorized as either <literal>safe</literal> or <literal>intrusive</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-malware">
<indexterm><primary sortas="malware script category">&ldquo;<literal>malware</literal>&rdquo; script category</primary></indexterm>
<option>malware</option>
</term>
<listitem>
<para>These scripts test whether the target platform is
infected by malware or backdoors. Examples include <literal>smtp-strangeport</literal>, which watches for SMTP servers running on unusual port numbers, and <literal>auth-spoof</literal>, 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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-safe">
<indexterm><primary sortas="safe script category">&ldquo;<literal>safe</literal>&rdquo; script category</primary></indexterm>
<option>safe</option>
</term>
<listitem>
<para>Scripts
which weren't designed to crash services, use large
amounts of network bandwidth or other resources, or
exploit security holes are categorized as <literal>safe</literal>. 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
<literal>ssh-hostkey</literal> (retrieves an SSH host key) and
<literal>html-title</literal> (grabs the title from a
web page). Scripts in the <literal>version</literal> category are not categorized by safety, but any other scripts which aren't in <literal>safe</literal> should be placed in <literal>intrusive</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-version">
<indexterm><primary sortas="version script category">&ldquo;<literal>version</literal>&rdquo; script category</primary></indexterm>
<indexterm><primary>version detection</primary><seealso>&ldquo;<literal>version</literal>&rdquo; script category</seealso></indexterm>
<option>version</option>
</term>
<listitem>
<para>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 (<option>-sV</option>) was requested.
Their output cannot be distinguished from version
detection output and they do not produce service or host
script results. Examples
are <literal>skypev2-version</literal>, <literal>pptp-version</literal>,
and <literal>iax2-version</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term id="nse-category-vuln">
<indexterm><primary sortas="vuln script category">&ldquo;<literal>vuln</literal>&rdquo; script category</primary></indexterm>
<option>vuln</option>
</term>
<listitem>
<para>These scripts check for specific known vulnerabilities and
generally only report results if they are found. Examples include <literal>realvnc-auth-bypass</literal> and <literal>afp-path-vuln</literal>.</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="nse-script-types">
<title>Script Types and Phases</title>
<para>
NSE supports four types of scripts, which are distinguished by the kind of targets they take and the scanning phase in which they are run. Individual scripts may support multiple types of operation.
</para>
<variablelist>
<varlistentry>
<term>Prerule scripts</term>
<listitem>
<para>These scripts run before any of Nmap's scan phases, so
Nmap has not collected any information about its targets
yet. They can be useful for tasks which don't depend on
specific scan targets, such as performing network broadcast
requests to query DHCP and DNS SD servers. Some of these
scripts can generate new targets for Nmap to scan (only if
you specify
the <ulink url="https://nmap.org/nsedoc/lib/target.html">newtargets</ulink>
NSE argument). For example, <ulink role="hidepdf"
url="https://nmap.org/nsedoc/scripts/dns-zone-transfer.html">dns-zone-transfer</ulink>
can obtain a list of IPs in a domain using a zone transfer
request and then automatically add them to Nmap's scan
target list. Prerule scripts can be identified by containing a <literal>prerule</literal> function (see <xref linkend="nse-format-rules"/>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Host scripts</term>
<listitem>
<para>Scripts in this phase run during Nmap's normal
scanning process after Nmap has performed host discovery,
port scanning, version detection, and OS detection against
the target host. This type of script is invoked once
against each target host which matches
its <literal>hostrule</literal> function. Examples
are <ulink role="hidepdf"
url="https://nmap.org/nsedoc/scripts/whois-ip.html">whois-ip</ulink>,
which looks up ownership information for a target IP,
and <ulink role="hidepdf"
url="https://nmap.org/nsedoc/scripts/path-mtu.html">path-mtu</ulink>
which tries to determine the maximum IP packet size which
can reach the target without requiring fragmentation.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Service scripts</term>
<listitem>
<para>These scripts run against specific services listening
on a target host. For example, Nmap includes more than 15
http service scripts to run against web servers. If a host
has web servers running on multiple ports, those scripts may
run multiple times (one for each port). These are the most
commong Nmap script type, and they are distinguished by
containing a <literal>portrule</literal> function for
deciding which detected services a script should run
against.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Postrule scripts</term>
<listitem>
<para>These scripts run after Nmap has scanned all of its
targets. They can be useful for formatting and presenting
Nmap output. For example, <ulink role="hidepdf"
url="https://nmap.org/nsedoc/scripts/ssh-hostkey.html">ssh-hostkey</ulink>
is best known for its service (portrule) script which
connects to SSH servers, discovers their public keys, and
prints them. But it also includes a postrule which checks
for duplicate keys amongst all of the hosts scanned, then
prints any that are found. Another potential use for a
postrule script is printing a reverse-index of the Nmap
output&mdash;showing which hosts run a particular service
rather than just listing the services on each host.
Postrule scripts are identified by containing a
<literal>postrule</literal> function.
</para>
<para>Many scripts could potentially run as either a prerule
or postrule script. In those cases, we recommend using a
prerule for consistency.</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="nse-cmd-line-args">
<title>Command-line Arguments</title>
<para>
These are the five command-line arguments specific to script scanning:
</para>
<variablelist>
<varlistentry>
<term>
<indexterm><primary><option>-sC</option></primary></indexterm>
<option>-sC</option>
</term>
<listitem>
<para>Performs a script scan using the default set of scripts. It is
equivalent to <option>--script=default</option>. Some of the
scripts in this <literal>default</literal> category are considered intrusive and should
not be run against a target network without permission. </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary><option>--script</option></primary></indexterm>
<option>--script <replaceable>filename</replaceable>|<replaceable>category</replaceable>|<replaceable>directory</replaceable>/|<replaceable>expression</replaceable><optional>,...</optional></option></term>
<listitem>
<para>
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
<literal>all</literal> makes every script in Nmap's script database
eligible to run. The <literal>all</literal> argument should be used with caution as NSE may contain dangerous scripts including exploits, brute force authentication crackers, and denial of service attacks.
</para>
<para>
Each element in the script expression list may be prefixed with a
<literal>+</literal> character to force the given script(s) to run
regardless of the conditions in their <literal>portrule</literal> or
<literal>hostrule</literal> functions. This is generally only done by
advanced users in special cases. For example, you might want to do a
configuration review on a bunch of MS SQL servers, some of which are
running on nonstandard ports. Rather than slow the Nmap scan by
running extensive version detection (<option>-sV
--version-all</option>) so that Nmap will recognize the <literal>ms-sql</literal>
service, you can force the <literal>ms-sql-config</literal> script to run against all the
targeted hosts and ports by specifying <option>--script
+ms-sql-config</option>.</para>
<para>
File and directory names may be relative or absolute. Absolute names are
used directly. Relative paths are searched for in the
<filename>scripts</filename> subdirectory of each of the following places until
found:
<indexterm><primary>data files</primary><secondary>directory search order</secondary></indexterm><indexterm><primary>scripts, location of</primary></indexterm>
<simplelist>
<member><option>--datadir</option></member>
<member><envar>$NMAPDIR</envar><indexterm><primary><envar>NMAPDIR</envar> environment variable</primary></indexterm></member>
<member><filename>~/.nmap</filename> (not searched on Windows)<indexterm><primary sortas="nmap directory"><filename>.nmap</filename> directory</primary></indexterm></member>
<member><filename><replaceable>APPDATA</replaceable>\nmap</filename> (only on Windows)<indexterm><primary sortas="nmap directory"><filename>.nmap</filename> directory</primary></indexterm></member>
<member>the directory containing the <filename>nmap</filename>
executable</member>
<member>the directory containing the <filename>nmap</filename>
executable, followed by <filename>../share/nmap</filename> (not searched on Windows)</member>
<member><varname>NMAPDATADIR</varname><indexterm><primary><varname>NMAPDATADIR</varname></primary></indexterm> (not searched on Windows)</member>
<member>the current directory.</member>
</simplelist>
</para>
<para>
When a directory name ending in <literal>/</literal> is given, Nmap loads every file in the directory
whose name ends with <filename>.nse</filename>. All other files are
ignored and directories are not searched recursively. When a filename is
given, it does not have to have the <filename>.nse</filename> extension;
it will be added automatically if necessary.
</para>
<para>
See <xref linkend="nse-script-selection"/> for examples and a full
explanation of the <option>--script</option> option.
</para>
<indexterm><primary>script database</primary><see><filename>script.db</filename></see></indexterm>
<para>Nmap scripts are stored in a <filename>scripts</filename>
subdirectory of the Nmap data directory by default (see
<xref linkend="data-files"/>). For efficiency, scripts are indexed in
a database stored
in <filename>scripts/script.db</filename>,<indexterm><primary><filename>script.db</filename></primary></indexterm>
which lists the category or categories in which each script belongs.
The argument <literal>all</literal> 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.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary><option>--script-args</option></primary></indexterm>
<option>--script-args <replaceable>args</replaceable></option>
</term>
<listitem>
<para>Provides arguments to the scripts. See
<xref linkend="nse-args"/> for a detailed explanation.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary><option>--script-args-file</option></primary></indexterm>
<option>--script-args-file <replaceable>filename</replaceable></option>
</term>
<listitem>
<para>This option is the same as
<option>--script-args</option> except that you pass the
arguments in a file rather than on the command-line. See
<xref linkend="nse-args"/> for a detailed
explanation.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary><option>--script-help</option></primary></indexterm>
<option>--script-help <replaceable>filename</replaceable>|<replaceable>category</replaceable>|<replaceable>directory</replaceable>|<replaceable>expression</replaceable>|all<optional>,...</optional></option>
</term>
<listitem>
<para>
Shows help about scripts. For each script matching the given
specification, Nmap prints the script name, its categories, and its
description. The specifications are the same as those accepted by
<option>--script</option>; so for example if you want help about
the <literal>ssl-enum-ciphers</literal> script, you would run
<command>nmap --script-help ssl-enum-ciphers</command>. A sample of script
help is shown in <xref linkend="nse-script-help"/>.
</para>
<example id="nse-script-help">
<indexterm><primary><option>--script-help</option></primary><secondary>example of</secondary></indexterm>
<title>Script help</title>
<screen>
$ nmap --script-help "afp-* and discovery"
Starting Nmap 7.40 ( https://nmap.org ) at 2017-04-21 14:15 UTC
afp-ls
Categories: discovery safe
https://nmap.org/nsedoc/scripts/afp-ls.html
Attempts to get useful information about files from AFP volumes.
The output is intended to resemble the output of <code>ls</code>.
afp-serverinfo
Categories: default discovery safe
https://nmap.org/nsedoc/scripts/afp-serverinfo.html
Shows AFP server information. This information includes the server's
hostname, IPv4 and IPv6 addresses, and hardware type (for example
<code>Macmini</code> or <code>MacBookPro</code>).
afp-showmount
Categories: discovery safe
https://nmap.org/nsedoc/scripts/afp-showmount.html
Shows AFP shares and ACLs.
</screen>
</example>
<para>
If the
<option>-oX</option><indexterm><primary><option>-oX</option></primary></indexterm>
option is used, an XML representation of the script help will be
written to the given file.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary><option>--script-trace</option></primary></indexterm>
<option>--script-trace</option>
</term>
<listitem>
<para>
This option is similar to
<option>--packet-trace</option>, 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 <option>--packet-trace</option> enables script
tracing too.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary><option>--script-updatedb</option></primary></indexterm>
<option>--script-updatedb</option>
</term>
<listitem>
<para>This option updates the script database found
in <filename>scripts/script.db</filename> 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 <filename>scripts</filename> directory or if you
have changed the categories of any script. This option is
used by
itself without arguments: <command>nmap --script-updatedb</command>.</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Some other Nmap options have effects on script scans. The most
prominent of these is
<option>-sV</option>.<indexterm><primary><option>-sV</option></primary></indexterm>
A version scan automatically executes
the scripts in the
<literal>version</literal> category.<indexterm><primary sortas="version script category">&ldquo;<literal>version</literal>&rdquo; script category</primary></indexterm>
The scripts
in this category are slightly different from other scripts because their
output blends in with the version scan results and they do not produce any
script scan output to the screen. If the
<option>-oX</option><indexterm><primary><option>-oX</option></primary></indexterm>
option is used, typical script output will still be available in the
XML output file.
</para>
<para>
Another option which affects the scripting engine is
<option>-A</option>.<indexterm><primary><option>-A</option></primary><secondary>features enabled by</secondary></indexterm>
The aggressive Nmap mode implies
the <option>-sC</option> option.
</para>
<para>
</para>
</sect2>
<sect2 id="nse-script-selection">
<title>Script Selection</title>
<indexterm><primary><option>--script</option></primary></indexterm>
<indexterm><primary>script selection</primary></indexterm>
<para>
The <option>--script</option> option takes a comma-separated list
of categories, filenames, and directory names. Some simple
examples of its use:
</para>
<variablelist>
<varlistentry>
<term><command>nmap --script default,safe</command></term>
<listitem>
<para>Loads all scripts in the <literal>default</literal> and
<literal>safe</literal> categories.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script smb-os-discovery</command></term>
<listitem>
<para>Loads only the <filename>smb-os-discovery</filename>
script. Note that the <filename>.nse</filename> extension is
optional.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script default,banner,/home/user/customscripts</command></term>
<listitem>
<para>Loads the script in the <literal>default</literal>
category, the <filename>banner</filename> script, and all
<filename>.nse</filename> files in the directory
<filename>/home/user/customscripts</filename>.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm><primary>wildcards</primary><secondary>in script selection</secondary></indexterm>
<para>
When referring to scripts from <filename>script.db</filename> by
name, you can use a shell-style &lsquo;<literal>*</literal>&rsquo;
wildcard.
</para>
<variablelist>
<varlistentry>
<term><command>nmap --script "http-*"</command></term>
<listitem>
<para>Loads all scripts whose name starts with
<filename>http-</filename>, such as
<filename>http-auth</filename> and
<filename>http-open-proxy</filename>. The argument to
<option>--script</option> had to be in quotes to protect the
wildcard from the shell.</para>
</listitem>
</varlistentry>
</variablelist>
<indexterm><primary>Boolean expressions in script selection</primary></indexterm>
<para>
More complicated script selection can be done using the
<literal>and</literal>, <literal>or</literal>, and
<literal>not</literal> operators to build Boolean expressions. The
operators have the same
<ulink role="hidepdf" url="http://www.lua.org/manual/5.3/manual.html#3.4.8">precedence</ulink>
as in Lua: <literal>not</literal> is the highest, followed by
<literal>and</literal> and then <literal>or</literal>. You can
alter precedence by using parentheses. Because expressions contain
space characters it is necessary to quote
them.
</para>
<variablelist>
<varlistentry>
<term><command>nmap --script "not intrusive"</command></term>
<listitem>
<para>Loads every script except for those in the
<literal>intrusive</literal> category.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script "default or safe"</command></term>
<listitem>
<para>This is functionally equivalent to
<command>nmap --script "default,safe"</command>. It loads all
scripts that are in the <literal>default</literal> category or
the <literal>safe</literal> category or both.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script "default and safe"</command></term>
<listitem>
<para>Loads those scripts that are in
<emphasis>both</emphasis> the <literal>default</literal> and
<literal>safe</literal> categories.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script "(default or safe or intrusive) and not http-*"</command></term>
<listitem>
<para>Loads scripts in the <literal>default</literal>,
<literal>safe</literal>, or <literal>intrusive</literal>
categories, except for those whose names start with
<filename>http-</filename>.</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Names in a Boolean expression may be a category, a filename from
<filename>script.db</filename>, or <literal>all</literal>. A name
is any sequence of characters not containing
&lsquo;<literal>&nbsp;</literal>&rsquo;,
&lsquo;<literal>,</literal>&rsquo;,
&lsquo;<literal>(</literal>&rsquo;,
&lsquo;<literal>)</literal>&rsquo;, or
&lsquo;<literal>;</literal>&rsquo;, except for the sequences
<literal>and</literal>, <literal>or</literal>, and
<literal>not</literal>, which are operators.
</para>
</sect2>
<sect2 id="nse-args">
<title>Arguments to Scripts</title>
<indexterm><primary>script arguments</primary></indexterm>
<para>
Arguments may be passed to NSE scripts using the
<option>--script-args</option> 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
<varname>nmap.registry.args</varname>, though they are normally accessed through the <literal>stdnse.get_script_args</literal> function.
</para>
<para>
The syntax for script arguments is similar to Lua's table constructor
syntax. Arguments are a comma-separated list of
<literal>name=value</literal> pairs. Names and values may be strings not
containing whitespace or the characters
&lsquo;<literal>{</literal>&rsquo;,
&lsquo;<literal>}</literal>&rsquo;,
&lsquo;<literal>=</literal>&rsquo;, or
&lsquo;<literal>,</literal>&rsquo;.
To include one of these characters in a string, enclose the string in
single or double quotes. Within a quoted string,
&lsquo;<literal>\</literal>&rsquo; 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.
</para>
<para>
Values may also be tables enclosed in <literal>{}</literal>, 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.
</para>
<para>Script arguments are often qualified with the relevant
script name so that a user doesn't unintentionally affect multiple
scripts with a single generic name. For example, you can set
the timeout for responses to the
<literal>broadcast-ping</literal> script (and only that script)
by setting <literal>broadcast-ping.timeout</literal> to the
amount of time you're willing to wait. Sometimes,
however, you want a script argument applied more widely. If you
remove the qualification and specify just
<literal>timeout=250ms</literal>, you will be setting the value
for more than a dozen scripts in addition to
<literal>broadcast-ping</literal>. You can even combine
qualified and unqualified arguments, and the most specific match
takes precedence. For example, you could specify
<literal>rlogin-brute.timeout=20s,timeout=250ms</literal>. In
that case, the timeout will be 20 seconds for the
<literal>rlogin-brute</literal> script, and 250 milliseconds for all other
scripts which support this variable
(<literal>broadcast-ping</literal>,
<literal>lltd-discovery</literal>, etc.)</para>
<para>Rather than pass the arguments on the command line with
<option>--script-args</option>, you may store them in a file
(separated by commas or newlines) and specify just the file name
with <option>--script-args-file</option>. Options specified
with <option>--script-args</option> on the command-line take
precedence over those given in a file. The filename may be
given as an absolute path or relative to Nmap's usual
search path (NMAPDIR, etc.)
</para>
<para>Here is a typical Nmap invocation with script arguments:
<informalexample>
<indexterm><primary><option>--script-args</option></primary><secondary>example of</secondary></indexterm>
<literallayout>
<command>nmap -sC --script-args 'user=foo,pass=",{}=bar",paths={/admin,/cgi-bin},xmpp-info.server_name=localhost'</command>
</literallayout>
</informalexample>
Notice that the script arguments are surrounded in single quotes. For the
Bash shell, this prevents the shell from interpreting the double quotes
and doing automatic string concatenation. Naturally, different shells may
require you to escape quotes or to use different quotes. See your
relevant manual. The command results in this Lua table:
<programlisting>
nmap.registry.args = {
user = "foo",
pass = ",{}=bar",
paths = {
"/admin",
"/cgi-bin"
},
xmpp-info.server_name="localhost"
}
</programlisting>
While you could access the values directly from <literal>nmap.registry.args</literal>, it is normally better to use the <literal>stdnse.get_script_args</literal> function like this:
<programlisting>
local server_name = stdnse.get_script_args("xmpp-info.server_name")
</programlisting>
</para>
<para>
All script arguments share a global namespace, the
<literal>nmap.registry.args</literal> table. For this reason, short or
ambiguous names like <literal>user</literal> are not recommended. Some
scripts prefix their arguments with their script name, like
<literal>smtp-open-relay.domain</literal>.
Arguments used by libraries, which can
affect many scripts, usually have names beginning with the name of the
library, like <literal>smbuser</literal> and
<literal>creds.snmp</literal>.
</para>
<para>
The online NSE Documentation Portal at <ulink
url="https://nmap.org/nsedoc/"/> lists the arguments that each script
accepts, including any library arguments that may influence the script.
</para>
</sect2>
<sect2 id="nse-usage-examples">
<title>Complete Examples</title>
<variablelist>
<varlistentry>
<term><command>nmap -sC example.com</command></term>
<listitem>
<para>A simple script scan using the default set of
scripts.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap -sn -sC example.com</command></term>
<listitem>
<para>A script scan without a port scan; only host scripts are
eligible to run.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap -Pn -sn -sC example.com</command></term>
<listitem>
<para>A script scan without host discovery or a port scan. All
hosts are assumed up and only host scripts are eligible to
run.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary><option>--script-trace</option></primary><secondary>example of</secondary></indexterm>
<command>nmap --script smb-os-discovery --script-trace example.com</command></term>
<listitem>
<para>Execute a specific script with script tracing.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<indexterm><primary><option>--script-args</option></primary><secondary>example of</secondary></indexterm>
<command>nmap --script snmp-sysdescr --script-args creds.snmp=admin example.com</command></term>
<listitem>
<para>Run an individual script that takes a script
argument.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>nmap --script mycustomscripts,safe example.com</command></term>
<listitem>
<para>Execute all scripts in the
<filename>mycustomscripts</filename> directory as well as all
scripts in the <literal>safe</literal> category.</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="nse-script-format">
<title>Script Format</title>
<para>NSE scripts consist of a handful of descriptive fields, a rule defining when the script should be executed, and an <literal>action</literal> function 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.</para>
<sect2 id="nse-format-description">
<title><literal>description</literal> Field</title>
<indexterm><primary sortas="description script variable">&ldquo;<varname>description</varname>&rdquo; script variable</primary></indexterm>
<para>The <literal>description</literal> field describes what a script is testing
for and any important notes the user should be aware of. Depending on script complexity, descriptions may vary in length 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.
</para>
</sect2>
<sect2 id="nse-format-categories">
<title><literal>categories</literal> Field</title>
<indexterm><primary sortas="categories script variable">&ldquo;<varname>categories</varname>&rdquo; script variable</primary></indexterm>
<para>The <literal>categories</literal> field defines one or
more categories to which a script belongs (see
<xref linkend="nse-categories"/>). 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:</para>
<programlisting>
categories = {"default", "discovery", "safe"}
</programlisting>
</sect2>
<sect2 id="nse-format-author">
<title><literal>author</literal> Field </title>
<indexterm><primary sortas="author script variable">&ldquo;<varname>author</varname>&rdquo; script variable</primary></indexterm>
<para>
The <literal>author</literal> 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 from the NSEDoc web site. This optional field is not used by NSE, but gives script authors their due credit or blame.
</para>
</sect2>
<sect2 id="nse-format-license">
<title><literal>license</literal> Field </title>
<indexterm><primary sortas="license script variable">&ldquo;<varname>license</varname>&rdquo; script variable</primary></indexterm>
<indexterm><primary>copyright</primary><secondary>of scripts</secondary></indexterm>
<para>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 <literal>license</literal> 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 <xref linkend="nmap-copyright"/>). They include
the following line:</para>
<programlisting>
license = "Same as Nmap--See https://nmap.org/book/man-legal.html"
</programlisting>
<para>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. For a BSD-style license, please include this line:
<programlisting>
license = "Simplified (2-clause) BSD license--See https://nmap.org/svn/docs/licenses/BSD-simplified"
</programlisting>
</para>
</sect2>
<sect2 id="nse-format-dependencies">
<title><literal>dependencies</literal> Field</title>
<indexterm><primary sortas="dependencies script variable">&ldquo;<varname>dependencies</varname>&rdquo; script variable</primary></indexterm>
<indexterm><primary>script dependencies</primary></indexterm>
<para>
The <literal>dependencies</literal> field is an array containing the
names of scripts that should run before this script, if they are also selected. This is used when
one script can make use of the results of another. For example, most of
the <filename>smb-*</filename> scripts depend on
<filename>smb-brute</filename>,<indexterm><primary><filename>smb-brute</filename> script</primary></indexterm>
because the accounts found by <filename>smb-brute</filename> may allow
the other scripts to get more information. Listing a script in
<literal>dependencies</literal> doesn't cause that script to be run; it
still has to be selected through the <option>--script</option> option
or otherwise. <literal>dependencies</literal> merely forces an ordering
among the scripts that <emphasis>are</emphasis> selected. This is an
example of a <literal>dependencies</literal> table, from
<filename>smb-os-discovery</filename>:<indexterm><primary><filename>smb-os-discovery</filename> script</primary></indexterm>
<programlisting>
dependencies = {"smb-brute"}
</programlisting>
The dependencies table is optional. NSE will assume
the script has no dependencies if the field is omitted.
</para>
<para>
Dependencies establish an internal ordering of scripts, assigning each
one a number called a <quote>runlevel</quote><footnote><para>Up through
Nmap version 5.10BETA2, dependencies didn't exist and script authors
had to set a <varname>runlevel</varname> field manually.</para></footnote>.<indexterm><primary>runlevel</primary></indexterm>
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:
<screen>
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.
</screen>
</para>
</sect2>
<sect2 id="nse-format-rules">
<title>Rules</title>
<indexterm><primary sortas="prerule script variable">&ldquo;<varname>prerule</varname>&rdquo; script variable</primary></indexterm>
<indexterm><primary sortas="portrule script variable">&ldquo;<varname>portrule</varname>&rdquo; script variable</primary></indexterm>
<indexterm><primary sortas="hostrule script variable">&ldquo;<varname>hostrule</varname>&rdquo; script variable</primary></indexterm>
<indexterm><primary sortas="postrule script variable">&ldquo;<varname>postrule</varname>&rdquo; script variable</primary></indexterm>
<indexterm><primary>rules in NSE</primary><see>&ldquo;<varname>prerule</varname>&rdquo;, &ldquo;<varname>portrule</varname>&rdquo;, &ldquo;<varname>hostrule</varname>&rdquo; and &ldquo;<varname>postrule</varname>&rdquo;</see></indexterm>
<para>
Nmap uses the script rules to determine whether a script should be
run against a target. A rule is a Lua function that returns either
<literal>true</literal> or <literal>false</literal>. The script
<literal>action</literal> function is only performed if the rule
evaluates to <literal>true</literal>.
</para>
<para>
A script must contain one or more of the following functions that
determine when the script will be run:
<simplelist>
<member><literal>prerule()</literal></member>
<member><literal>hostrule(host)</literal></member>
<member><literal>portrule(host, port)</literal></member>
<member><literal>postrule()</literal></member>
</simplelist>
<literal>prerule</literal> scripts run once, before any hosts are
scanned, during the script pre-scanning
phase.<indexterm><primary>script
pre-scanning</primary></indexterm> <literal>hostrule</literal> and
<literal>portrule</literal> scripts run after each batch of hosts
is scanned. <literal>postrule</literal> scripts run once after all
hosts have been scanned, in the script post-scanning
phase.<indexterm><primary>script post-scanning
phase</primary></indexterm> A script may run in more than one
phase if it has several rules.
</para>
<para>
<literal>prerule</literal> and <literal>postrule</literal> do not
accept arguments. <literal>hostrule</literal> accepts a host table
and may test, for example, the IP address or hostname of the
target. <literal>portrule</literal> accepts both a host table and
a port table for any port in the
<literal>open</literal><indexterm><primary><literal>open</literal> port state</primary></indexterm>,
<literal>open|filtered</literal><indexterm><primary><literal>open|filtered</literal> port state</primary></indexterm>,
or <literal>unfiltered</literal><indexterm><primary><literal>unfiltered</literal> port state</primary></indexterm>
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 <xref
linkend="nse-tutorial-rule"/>.
</para>
<para>Advanced users may force a script to run regardless of the
results of these rule functions by prefixing the script name (or
category or other expression) with a <literal>+</literal> in the
<option>--script</option> argument.</para>
<para>
The current standard to choose between a
<literal>prerule</literal> or a <literal>postrule</literal> is
this: if the script is doing host discovery or any other network
operation then the <literal>prerule</literal> should be used.
<literal>postrule</literal> is reserved for reporting of data and
statistics that were gathered during the scan.
</para>
</sect2>
<sect2 id="nse-format-action"><title>Action</title>
<indexterm><primary sortas="action script variable">&ldquo;<varname>action</varname>&rdquo; script variable</primary></indexterm>
<para>
The action is the heart of an NSE script. It contains all of the
instructions to be executed when the script's prerule, portrule, hostrule or postrule
triggers. It is a Lua function which accepts the same arguments as the
rule. The return value of the action value may be a table of
name&ndash;value pairs, a string, or <code>nil</code>. For an example of
an NSE action refer to <xref linkend="nse-tutorial-action"/>.
</para>
<para>
If the output of the action is a table, it is automatically formatted in
a structured fashion for inclusion in the normal (<option>-oN</option>)
and XML (<option>-oX</option>) output formats. If a string, the text is
displayed directly in normal output, and written as an XML attribute in
XML output, No output is produced if the script returns
<literal>nil</literal>. See <xref linkend="nse-structured-output"/> for
details of how different return values are handled.
</para>
</sect2>
<sect2 id="nse-format-environment"><title>Environment Variables</title>
<indexterm><primary sortas="environment script variable">&ldquo;<varname>environment</varname>&rdquo; script variable</primary></indexterm>
<para>Each script has its own set of environment variables:</para>
<variablelist>
<varlistentry>
<term><literal>SCRIPT_PATH</literal></term>
<listitem>
<para>
The script path.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SCRIPT_NAME</literal></term>
<listitem>
<para>
The script name. This variable can be used in debug output.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>SCRIPT_TYPE</literal></term>
<listitem>
<para>
Since a script can have multiple rule functions, this
environment variable will show which rule has activated
the script, this would be useful if the script wants to
share some code between different Script Scan phases.
It will take one of these four string values:
<literal>"prerule"</literal>, <literal>"hostrule"</literal>,
<literal>"portrule"</literal> or
<literal>"postrule"</literal>.
This variable is only available during and after the evaluation
of the rule functions.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
This is an example of a debug code that uses the previous
environment variables, followed by the output message, from dns-zone-transfer:
<programlisting>
stdnse.print_debug(3, "Skipping '%s' %s, 'dnszonetransfer.server' argument is missing.", SCRIPT_NAME, SCRIPT_TYPE)
</programlisting>
<screen>
Initiating NSE at 15:31
NSE: Skipping 'dns-zone-transfer' prerule, 'dnszonetransfer.server' argument is missing.
</screen>
</para>
</sect2>
</sect1>
<sect1 id="nse-language">
<title>Script Language</title>
<indexterm><primary>Nmap Scripting Engine (NSE)</primary><secondary>parts of</secondary></indexterm>
<para>
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.
</para>
<indexterm><primary>Nmap Scripting Engine (NSE)</primary><secondary>library</secondary></indexterm>
<para>
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 <xref linkend="nse-library"/>.</para>
<sect2 id="nse-lua">
<title>Lua Base Language</title>
<indexterm><primary>Lua programming language</primary></indexterm>
<para>
The Nmap scripting language is an embedded <ulink
url="http://www.lua.org/">Lua</ulink> interpreter which is
extended with libraries for interfacing with Nmap. The Nmap
API is in the Lua namespace <literal>nmap</literal>. This
means that all calls to resources provided by Nmap have an
<literal>nmap</literal> prefix.<indexterm><primary><varname>nmap</varname> NSE library</primary></indexterm>
<literal>nmap.new_socket()</literal>, 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.
</para>
<para>
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
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 <application>Wireshark</application> sniffer and <application>Snort</application> IDS.
</para>
</sect2>
</sect1>
<sect1 id="nse-scripts">
<indexterm><primary>Nmap Scripting Engine (NSE)</primary><secondary>list of scripts</secondary></indexterm>
<title>NSE Scripts</title>
<print>
<para>This section lists (alphabetically) all NSE scripts packaged
with Nmap at the time of this writing. It comes straight from the
script source code thanks to the NSEDoc documentation system
described in <xref linkend="nsedoc"/>. Because of space limitations,
only script names, categories, and brief summaries of operation are
shown. Of course no paper documentation can stay current with
software developed as actively as NSE is. For complete and
up-to-date documentation, including script arguments and output
samples, see the online NSE Documentation Portal at
<ulink url="https://nmap.org/nsedoc/"/>.
</para>
&nse-scripts;
</print>
<web>
<para>This section (a long list of NSE scripts with brief
summaries) is only provided in the printed edition of this book
because we already provide a better online interface to the
information at the <ulink url="https://nmap.org/nsedoc/">NSE
Documentation Portal</ulink>.</para>
</web>
</sect1>
<sect1 id="nse-library">
<indexterm><primary>Nmap Scripting Engine (NSE)</primary><secondary>list of modules</secondary></indexterm>
<title>NSE Libraries</title>
<para>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, <filename>nselib</filename>, which
is installed in the configured Nmap data directory. Scripts need only
<ulink url="http://www.lua.org/manual/5.2/manual.html#pdf-require"><literal>require</literal></ulink> the default libraries in order to use them.
</para>
<sect2 id="nse-library-list">
<title>List of All Libraries</title>
<para>
This list is just an overview to give an idea of what libraries
are available. Developers will want to consult the complete
documentation at <ulink url="https://nmap.org/nsedoc/"/>.
</para>
&nse-modules;
</sect2>
<sect2 id="hacking-nse-libraries">
<title>Hacking NSE Libraries</title>
<para>
A common mistake when editing libraries is to accidentally use a
global variable instead of a local one. Different libraries using the
same global variable can be the cause of mysterious bugs. Lua's scope
assignment is global by default, so this mistake is easy to make.
</para>
<para>
To help correct this problem, NSE uses a library adapted from
the standard Lua distribution called
<filename>strict.lua</filename>.<indexterm><primary><filename>strict</filename> NSE library</primary></indexterm>
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 <literal>nil</literal>) in the file scope.
</para>
</sect2>
<sect2 id="nse-library-c-modules">
<title>Adding C Modules to Nselib</title>
<indexterm><primary>Nmap Scripting Engine (NSE)</primary><secondary>C modules</secondary></indexterm>
<para>
A few of the modules included in nselib are written in C or C++
rather than Lua. Two examples are <literal>bit</literal>
and <literal>pcre</literal>. 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 <literal>pcre</literal> and <literal>openssl</literal>
modules) you are linking to an existing C library. This section
describes how to write your own compiled extensions to nselib.
</para>
<para>
The Lua C API is described at length in
<web><ulink url="http://www.amazon.com/dp/8590379825?tag=secbks-20"><citetitle>Programming in Lua, Second Edition</citetitle></ulink>,</web>
<print><citetitle>Programming in Lua, Second Edition</citetitle>,</print>
so this is a short summary. C modules consist of functions that
follow the protocol of the
<ulink url="http://www.lua.org/manual/5.2/manual.html#lua_CFunction"><type>lua_CFunction</type></ulink>
type. The functions are registered with Lua and assembled into a
library by calling the
<function>luaL_newlib</function><indexterm><primary><function>luaL_newlib</function></primary></indexterm>
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
<function>luaopen_<replaceable>module</replaceable></function>.
</para>
<para>
The most straightforward compiled module that comes with NSE is
<literal>openssl</literal>.<indexterm><primary><varname>openssl</varname> NSE library</primary></indexterm>
This module serves as a good example for a beginning module
writer. The
source code for
<literal>openssl</literal> source is in <filename>nse_openssl.cc</filename> and
<filename>nse_openssl.h</filename>. Most of the other compiled modules
follow this <literal>nse_<replaceable>module name</replaceable>.cc</literal> naming convention.
</para>
<para>
Reviewing the <literal>openssl</literal> module shows that one of the
functions in <filename>nse_openssl.cc</filename> is
<function>l_md5</function>, which calculates an MD5 digest. Its
function prototype is:</para>
<programlisting>
static int l_md5(lua_State *L);
</programlisting>
<para>The prototype shows that <function>l_md5</function> matches the
<type>lua_CFunction</type> 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,
<function>l_md5</function> is entered into an array of type
<type>luaL_Reg</type> and associated with the name
<function>md5</function>:</para>
<programlisting>
static const struct luaL_Reg openssllib[] = {
{ "md5", l_md5 },
{ NULL, NULL }
};
</programlisting>
<para>This function will now be known as <function>md5</function> to NSE. Next the library is registered with a call to
<function>luaL_newlib</function> inside the initialization
function <function>luaopen_openssl</function>, as shown
next. Some lines relating to the registration of
OpenSSL <type>BIGNUM</type> types have been omitted:</para>
<programlisting>
LUALIB_API int luaopen_openssl(lua_State *L) {
luaL_newlib(L, openssllib);
return 1;
}
</programlisting>
<para>The function <function>luaopen_openssl</function>
is the only function in the file that is exposed in
<filename>nse_openssl.h</filename>. <varname>OPENSSLLIBNAME</varname> is simply the string
<literal>"openssl"</literal>.
</para>
<para>
After a compiled module is written, it must be added to NSE by including
it in the list of standard libraries in
<filename>nse_main.cc</filename>. Then the module's
source file names must be added to
<filename>Makefile.in</filename> 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
<filename>mswin32/nmap.vcproj</filename> project file using MS Visual Studio (see <xref linkend="inst-win-source"/>).
</para>
</sect2>
</sect1>
<sect1 id="nse-api">
<title>Nmap API</title>
<indexterm class="startofrange" id="nse-nmap-indexterm"><primary><varname>nmap</varname> NSE library</primary></indexterm>
<indexterm><primary>Nmap Scripting Engine (NSE)</primary><secondary>API</secondary></indexterm>
<para>
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<indexterm><primary>Nsock</primary><secondary>in NSE</secondary></indexterm>
library
for efficient network I/O.
</para>
<sect2 id="nse-api-arguments">
<title>Information Passed to a Script</title>
<para>
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
<literal>action</literal> method.<indexterm><primary sortas="action script variable">&ldquo;<varname>action</varname>&rdquo; script variable</primary></indexterm>
The arguments, <literal>host</literal> and
<literal>port</literal>, 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
<literal>host</literal> table, and if it matched a portrule it
gets both <literal>host</literal> and <literal>port</literal>.
The following list describes each variable in these two tables.
</para>
<para>
<variablelist>
<varlistentry>
<term><option>host</option>
</term>
<listitem>
<para>
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 <option>-O</option> switch was supplied), the
IP address and the host name of the scanned target.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.os</option>
</term>
<listitem>
<para>
An array of OS match tables. An OS match consists of a
human-readable name and an array of OS classes. Each OS
class consists of a vendor, OS family, OS generation,
device type, and an array of
CPE<indexterm><primary>CPE</primary></indexterm> entries
for the class. (See <xref linkend="osdetect-ref-format"/>
for a description of OS match fields.) Fields may be
<varname>nil</varname> if they are not defined. The
<varname>host.os</varname> table has this overall
structure:
</para>
<programlisting>
host.os = {
{
name = <replaceable>string</replaceable>,
classes = {
{
vendor = <replaceable>string</replaceable>,
osfamily = <replaceable>string</replaceable>,
osgen = <replaceable>string</replaceable>,
type = <replaceable>string</replaceable>,
cpe = {
"cpe:/<replaceable>...</replaceable>",
<optional>More CPE</optional>
}
},
<optional>More classes</optional>
},
},
<optional>More OS matches</optional>
}
</programlisting>
<para>
For example, an OS match on this
<filename>nmap-os-db</filename><indexterm><primary><filename>nmap-os-db</filename></primary></indexterm>
entry:
</para>
<programlisting>
Fingerprint Linux 2.6.32 - 3.2
Class Linux | Linux | 2.6.X | general purpose
CPE cpe:/o:linux:linux_kernel:2.6
Class Linux | Linux | 3.X | general purpose
CPE cpe:/o:linux:linux_kernel:3
</programlisting>
<para>
will result in this <varname>host.os</varname> table:
</para>
<programlisting>
host.os = {
{
name = "Linux 2.6.32 - 3.2",
classes = {
{
vendor = "Linux",
osfamily = "Linux",
osgen = "2.6.X",
type = "general purpose",
cpe = { "cpe:/o:linux:linux_kernel:2.6" }
},
{
vendor = "Linux",
osfamily = "Linux",
osgen = "3.X",
type = "general purpose",
cpe = { "cpe:/o:linux:linux_kernel:3" }
}
},
}
}
</programlisting>
<para>
Only entries corresponding to perfect OS matches are put
in the <varname>host.os</varname> table. If Nmap was run
without the <option>-O</option> option, then
<literal>host.os</literal> is <literal>nil</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.ip</option>
</term>
<listitem>
<para>Contains a string representation of the IP address of the
target host. If the scan was run against a host name and its
DNS lookup returned more than one IP addresses, then the
same IP address is used as the one chosen for the scan.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.name</option>
</term>
<listitem>
<para>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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.targetname</option>
</term>
<listitem>
<para>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 <literal>nil</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.reason</option>
</term>
<listitem>
<para>
Contains a string representation of the reason why the target host is in
its current state. The reason is given by the type of the packet that
determined the state. For example, an <literal>echo-reply</literal> from
an alive host.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.reason_ttl</option>
</term>
<listitem>
<para>
Contains the TTL value of the response packet, that was used to determine
the status of the target host, when it arrived. This response packet is the
packet that is also used to set <literal>host.reason</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.directly_connected</option>
</term>
<listitem>
<para> 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.mac_addr</option>
</term>
<listitem>
<para>MAC address<indexterm><primary>MAC address</primary></indexterm>
of the destination host (six-byte-long binary
string) if available, otherwise <literal>nil</literal>. The MAC address is generally only available for hosts directly connected on a LAN and only if Nmap is doing a raw packet scan such as SYN scan.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.mac_addr_next_hop</option>
</term>
<listitem>
<para>MAC address
of the first hop in the route to the host, or
<literal>nil</literal> if not available.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.mac_addr_src</option>
</term>
<listitem>
<para>Our own MAC address, which was used to connect to the
host (either our network card's, or (with
<option>--spoof-mac</option>)<indexterm><primary><option>--spoof-mac</option></primary></indexterm>
the spoofed address).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.interface</option>
</term>
<listitem>
<para>A string containing the interface name
(dnet-style)<indexterm><primary>libdnet</primary></indexterm>
through
which packets to the host are sent.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.interface_mtu</option>
</term>
<listitem>
<para>The MTU (maximum transmission unit) for <literal>host.interface</literal>,
or 0 if not known.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.bin_ip</option>
</term>
<listitem>
<para>The target host's IP address as a 4-byte (IPv4) or 16-byte (IPv6) string.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.bin_ip_src</option>
</term>
<listitem>
<para>Our host's (running Nmap) source IP address as a 4-byte (IPv4) or 16-byte (IPv6) string.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.times</option>
</term>
<listitem>
<para>This table contains Nmap's timing data for the host (see
<xref linkend="scan-methods-rtt"/>). Its keys are <literal>srtt</literal> (smoothed
round trip time), <literal>rttvar</literal> (round trip time variance), and <literal>timeout</literal>
(the probe timeout), all given in floating-point seconds.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.traceroute</option>
</term>
<listitem>
<para>
This is an array of traceroute hops, present when the
<option>--traceroute</option> option was used. Each entry is a
host table with fields <literal>name</literal>,
<literal>ip</literal> and <literal>srtt</literal> (round
trip time). The TTL for an entry is implicit given its position
in the table. An empty table represents a timed-out hop.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>host.os_fp</option>
</term>
<listitem>
<para>If OS detection was performed, this is a string containing the OS
fingerprint for the host. The format is described in
<xref linkend="osdetect-fingerprint-format"/>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>port</option>
</term>
<listitem>
<para>
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 <literal>nmap.get_port_state()</literal> and <literal>nmap.get_ports()</literal> calls.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>port.number</option>
</term>
<listitem>
<para>
Contains the port number of the target port.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>port.protocol</option>
</term>
<listitem>
<para>
Defines the protocol of the target port. Valid values are
<literal>"tcp"</literal> and <literal>"udp"</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>port.service</option>
</term>
<listitem>
<para>
Contains a string representation of the service running on
<literal>port.number</literal> as detected by the Nmap service
detection. If the <literal>port.version.service_dtype</literal> field is
<literal>"table"</literal>, 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
<literal>port.version.name</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>port.reason</option>
</term>
<listitem>
<para>
Contains a string representation of the reason why the target port is in
its current state (given by <literal>port.state</literal>). The reason is
given by the type of the packet that determined the state. For example, a
<literal>RST</literal> packet from a closed port or
<literal>SYN-ACK</literal> from an open port.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>port.reason_ttl</option>
</term>
<listitem>
<para>
Contains the TTL value of the response packet, that was used to determine
the status of the target port, when it arrived. This response packet is the
packet that is also used to set <literal>port.reason</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>port.version</option>
</term>
<listitem>
<para>
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
<literal>nil</literal>. The meaning of each value is given in the following table:</para>
<table id="scripting-tbl-port-version-values">
<title><literal>port.version</literal> values</title>
<tgroup cols="2">
<colspec colwidth="2*" />
<colspec colwidth="5*" />
<thead><row>
<entry>Name</entry>
<entry>Description</entry>
</row></thead>
<tbody>
<row>
<entry align="left"><literal>name</literal></entry>
<entry>Contains the service name Nmap decided on for the port.</entry>
</row>
<row>
<entry align="left"><literal>name_confidence</literal></entry>
<entry>Evaluates how confident Nmap is about the accuracy of
<literal>name</literal>, from 1 (least confident) to 10. If
<literal>port.version.service_dtype</literal> is
<literal>"table"</literal>, this is 3.</entry>
</row>
<row>
<entry align="left"><literal>product</literal>, <literal>version</literal>, <literal>extrainfo</literal>, <literal>hostname</literal>, <literal>ostype</literal>, <literal>devicetype</literal></entry>
<entry>These five variables are the same as those described under <replaceable>versioninfo</replaceable> in <xref linkend="vscan-db-match"/>.
</entry>
</row>
<row>
<entry align="left"><literal>service_tunnel</literal></entry>
<entry>Contains the string <literal>"none"</literal> or <literal>"ssl"</literal> based on whether or not Nmap used SSL tunneling to detect the service.</entry>
</row>
<row>
<entry align="left"><literal>service_fp</literal></entry>
<entry>The service fingerprint, if any, is provided in this value. This is described in
<xref linkend="vscan-community"/>.
</entry>
</row>
<row>
<entry align="left"><literal>service_dtype</literal></entry>
<entry>Contains the string <literal>"table"</literal> or
<literal>"probed"</literal> based on whether or not Nmap deduced
<literal>port.version.name</literal> from the
<filename>nmap-services</filename> file or from a service probe match.
</entry>
</row>
<row>
<entry align="left"><literal>cpe</literal></entry>
<entry>List of CPE codes for the detected service. As described in the
<ulink url="http://cpe.mitre.org">official CPE specification</ulink> these strings
all start with the <literal>cpe:/</literal> prefix.</entry>
</row>
</tbody></tgroup></table>
</listitem>
</varlistentry>
<varlistentry>
<term><option>port.state</option>
</term>
<listitem>
<para>
Contains information on the state of the port.
Service scripts are only run against ports in the
<literal>open</literal> or
<literal>open|filtered</literal> states, so
<literal>port.state</literal> generally contains one
of those values. Other values might appear if the port
table is a result of the
<literal>get_port_state</literal> or <literal>get_ports</literal>
functions. You can adjust the port state using the
<literal>nmap.set_port_state()</literal> call. This is
normally done when an <literal>open|filtered</literal>
port is determined to be <literal>open</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</sect2>
<sect2 id="nse-api-networkio">
<title>Network I/O API</title>
<para>
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 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.
</para>
<sect3 id="nse-api-networkio-connect">
<title>Connect-style network I/O</title>
<indexterm><primary>sockets in NSE</primary></indexterm>
<para>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.
</para>
<para>
An NSE socket is created by calling
<function>nmap.new_socket</function>, which returns a socket object.
The socket object supports the usual <function>connect</function>,
<function>send</function>, <function>receive</function>, and
<function>close</function> methods. Additionally the functions
<function>receive_bytes</function>,
<function>receive_lines</function>, and
<function>receive_buf</function> allow greater control
over data reception.
<xref linkend="nse-api-networkio-connect-example" xrefstyle="select: label nopage"/>
shows the use of connect-style network operations. The
<function>try</function> function is used for error handling, as described in
<xref linkend="nse-exceptions"/>.
</para>
<example id="nse-api-networkio-connect-example">
<title>Connect-style I/O</title>
<programlisting>
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()
</programlisting>
</example>
</sect3>
<sect3 id="nse-api-networkio-raw">
<title>Raw packet network I/O</title>
<indexterm><primary>raw packets</primary><secondary>in NSE</secondary></indexterm>
<para>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.</para>
<para>Raw packet reception is handled through a
Libpcap<indexterm><primary>libpcap</primary></indexterm>
wrapper inside the Nsock
library.<indexterm><primary>Nsock</primary></indexterm>
The steps are to open a capture device, register listeners
with the device, and then process packets as they are
received.</para>
<para>The <function>pcap_open</function> 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 <function>pcap_register</function>
function. The packet hash callback will normally extract some
portion of the packet, such as its source address.</para>
<para>The pcap reader is instructed to listen for certain
packets using the <function>pcap_register</function> 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
<function>pcap_receive</function> method. Register the empty
string to receive all packets.</para>
<para>A script receives all packets for which a listener has
been registered by calling the
<function>pcap_receive</function> method. The method blocks
until a packet is received or a timeout occurs.</para>
<para>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
<function>nmap.new_socket</function> and later close the socket
with <function>socket_object:close</function>&mdash;just like
with the connection-based network I/O.</para>
<para>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 <function>nmap.new_dnet</function>
function is called to create the required object for sending. After
this, a raw socket or Ethernet interface handle can be opened for use.</para>
<para>Once the dnet object is created, the function <function>ip_open</function>
can be called to initialize the object for IP sending. <function>ip_send</function>
sends the actual raw packet, which must start with the IP 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 <function>ip_close</function>.</para>
<para>For sending at a lower level than IP, NSE provides functions for
writing Ethernet frames. <function>ethernet_open</function> initializes
the dnet object for sending by opening an Ethernet interface. The raw
frame is sent with <function>ethernet_send</function>. To close the
handle, call <function>ethernet_close</function>.</para>
<para>Sometimes the easiest ways to understand complex APIs is by
example. The
<filename>ipidseq</filename><indexterm><primary><filename>ipidseq</filename> script</primary></indexterm>
script included with
Nmap uses raw IP packets to test hosts for suitability for Nmap's
Idle Scan (<option>-sI</option>). The
<filename>sniffer-detect</filename><indexterm><primary><filename>sniffer-detect</filename> script</primary></indexterm>
script also included with Nmap uses raw Ethernet frames in an attempt
to detect promiscuous-mode machines on the network (those running
sniffers).</para>
</sect3>
</sect2>
<sect2 id="nse-structured-output">
<title>Structured and Unstructured Output</title>
<indexterm>structured script output</indexterm>
<para>
NSE scripts should usually return a table representing their
output, one that is nicely organized and has thoughtfully chosen
keys. Such a table will be automatically formatted for screen
output and will be stored as nested elements in XML output.
Having XML output broken down logically into keys and values
makes it easier for other tools to make use of script output.
It is possible for a script to return only a string, but doing
so is deprecated. In the past, scripts could only return a
string, and their output was simply copied to the XML as a blob
of text&ndash;this is now known as <quote>unstructured
output</quote>.
</para>
<para>
Suppose a script called <filename>user-list</filename> returns a
table as shown in this code sample. The following paragraphs
show how it appears in normal and XML output.
</para>
<programlisting>
local output = stdnse.output_table()
output.hostname = "slimer"
output.users = {}
output.users[#output.users + 1] = "root"
output.users[#output.users + 1] = "foo"
output.users[#output.users + 1] = "bar"
return output
</programlisting>
<para>
A Lua table is converted to a string for normal output. The way
this works is: each nested table gets a new level of
indentation. Table entries with string keys are preceded by the
key and a colon; entries with integer keys simply appear in
order.
Unlike normal Lua tables, which are unordered, a table that
comes from <code>stdnse.output_table</code> will keep its keys in
the order they were inserted.
<xref linkend="nse-normal-structured-output"/> shows how the
example table appears in normal output.
</para>
<example id="nse-normal-structured-output">
<title>Automatic formatting of NSE structured output</title>
<screen>
PORT STATE SERVICE
1123/tcp open unknown
| user-list:
| hostname: slimer
| users:
| root
| foo
|_ bar
</screen>
</example>
<para>
The XML representation of a Lua table is constructed as follows.
Nested table become <code>table</code> elements. Entries of
tables that are not themselves tables become <code>elem</code>
elements. Entries (whether <code>table</code> or
<code>elem</code>) with string keys get a <code>key</code>
attribute (e.g.
<code>&lt;elem key="username"&gt;foo&lt;/elem&gt;</code>);
entries with integer keys have no <code>key</code> element and
their key is implicit in the order in which they appear.
</para>
<para>
In addition to the above, whatever normal output the script
produces (even if automatically generated) is copied to the
<code>output</code> attribute of the <code>script</code>
element. Newlines and other special characters will be encoded
as XML character entities, for example <code>&amp;#xa;</code>.
<xref linkend="nse-xml-structured-output"/> shows how the example
table appears in XML.
</para>
<example id="nse-xml-structured-output">
<title>NSE structured output in XML</title>
<screen><![CDATA[<script id="t" output="&#xa;hostname: slimer&#xa;users: &#xa; root&#xa; foo&#xa; bar">
<elem key="hostname">slimer</elem>
<table key="users">
<elem>root</elem>
<elem>foo</elem>
<elem>bar</elem>
</table>
</script>
]]></screen>
</example>
<para>
Some scripts need more control their normal output. This is the
case, for example, with scripts that need to display complex
tables. For complete control over the output, these scripts may
do either of these things:
<simplelist>
<member>return a string as second return value, or</member>
<member>set the <code>__tostring</code> metamethod on the
returned table.</member>
</simplelist>
The resulting string will be used in normal output, and the
table will be used in XML as usual. The formatted string may
contain newline characters to appear as multiple lines.
</para>
<para>
If the above code example were modified in this way to return a
formatted string,
<programlisting>
local output = stdnse.output_table()
output.hostname = "slimer"
output.users = {}
output.users[#output.users + 1] = "root"
output.users[#output.users + 1] = "foo"
output.users[#output.users + 1] = "bar"
local output_str = string.format("hostname: %s\n", output.hostname)
output_str = output_str .. "\n" .. stringaux.strjoin(", ", output.users)
return output, output_str
</programlisting>
then the normal output would appear as follows:
<screen>
PORT STATE SERVICE
1123/tcp open unknown
| user-list:
| hostname: slimer
|_ users: root, foo, bar
</screen>
</para>
<sect3 id="nse-structured-output-conventions">
<para>
There are conventions regarding the formatting of certain kinds
of data in structured output. Users of NSE output benefit by
being able to assume that some kinds of data, for instance dates
and times, are formatted the same way, even in different
scripts.
</para>
<para>
Network addresses, for example IPv4, IPv6, and MAC, are
represented as strings.
</para>
<para>
Long hexadecimal strings such as public key fingerprints should
be written using lower-case alphabetical characters and without
separators such as colons.
</para>
<para>
Dates and times are formatted according to
<ulink role="hidepdf" url="http://www.rfc-editor.org/rfc/rfc3339.txt">RFC
3339</ulink><indexterm><primary>RFC 3339</primary></indexterm>.
If the time zone offset is known, they should appear like these
examples:
<screen>
2012-09-07T23:37:42+00:00
2012-09-07T23:37:42+02:00
</screen>
If the time zone offset is not known (representing some
unspecified local time), leave off the offset part:
<screen>
2012-09-07T23:37:42
</screen>
The library function
<code>datetime.format_timestamp</code> code exists to format times
for structured output. It takes an optional time zone offset in
seconds and automatically shifts the date to be correct within
that offset.
<screen>
datetime.format_timestamp(os.time(), 0) --> "2012-09-07T23:37:42+00:00"
</screen>
</para>
</sect3>
</sect2>
<sect2 id="nse-exceptions">
<title>Exception Handling</title>
<indexterm><primary>exceptions in NSE</primary></indexterm>
<para>
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 <function>nmap.new_try</function> 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
<function>new_try</function> which will be called
if an exception is caught. The function would generally perform any required cleanup operations.
</para>
<para>
<xref linkend="nse-exception-handling" xrefstyle="select: label nopage"/> shows cleanup
exception handling at work. A new function named
<function>catch</function> 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&mdash;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.
</para>
<example id="nse-exception-handling">
<title>Exception handling example</title>
<programlisting>
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))
</programlisting>
</example>
<para>
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 <literal>true</literal> upon successful completion of the function and
<literal>false</literal> (or <literal>nil</literal>) 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
<literal>nil</literal> or <literal>false</literal> it is
treated as <literal>true</literal> so you can return your
value in the normal case and return <literal>nil, <replaceable>error description</replaceable></literal>
if an error occurs.
</para>
</sect2>
<sect2 id="nse-api-registry">
<title>The Registry</title>
<indexterm><primary>registry (NSE)</primary></indexterm>
<para>Scripts can share information by storing values in a
<firstterm>register</firstterm>, which is a special table that can be
accessed by all scripts. There is a global registry with the name
<varname>nmap.registry</varname>, shared by all scripts. Each host
additionally has its own registry called
<varname>host.registry</varname>, where <varname>host</varname> is the
<link linkend="nse-api-arguments">host table</link> passed to a script.
Information in the registries is not stored between Nmap
executions.</para>
<para>The global registry persists throughout an entire scan session.
Scripts can use it, for example, to store values that will later be
displayed by a postrule script. The per-host registries, on the other
hand, only exist while a host is being scanned. They can be used to send
information from one script to another one that runs against the same
host. When possible, use the per-host registry; this not only saves you
from having to make key names unique across hosts, but also allows the
memory used by the registry to be reclaimed when it is no longer
needed.</para>
<para>
Here are examples of using both registries:
<simplelist>
<member>The portrule of the <filename>ssh-hostkey</filename> script collects SSH key fingerprints
and stores them in the global <varname>nmap.registry</varname> so they
can be printed later by the postrule.</member>
<member>The <filename>ssl-cert</filename> script collects SSL certificates and
stores them in the per-host registry so that the
<filename>ssl-google-cert-catalog</filename> script can use them without
having to make another connection to the server.</member>
</simplelist>
</para>
<para>Because every script can write to the global registry table, it is
important to make the keys you use unique, to avoid overwriting the keys
of other scripts (or the same script running in parallel).</para>
<para>Scripts that use the results of another script must declare it using
the <literal>dependencies</literal> variable to make sure that the earlier
script runs first.</para>
</sect2>
<indexterm class="endofrange" startref="nse-nmap-indexterm"/>
</sect1>
<sect1 id="nse-tutorial">
<title>Script Writing Tutorial</title>
<indexterm class="startofrange" id="nse-tutorial-indexterm"><primary>Nmap Scripting Engine (NSE)</primary><secondary>tutorial</secondary></indexterm>
<para>
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<indexterm><primary>auth service</primary></indexterm> 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 <literal><replaceable>port-on-server</replaceable>,
<replaceable>port-on-client</replaceable></literal> 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 <ulink role="hidepdf"
url="http://www.rfc-editor.org/rfc/rfc1413.txt">RFC
1413</ulink>, 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&mdash;one to the identification server
and one to the listening port you wish to query. Both obstacles
are easily overcome with NSE.</para>
<para>
The anatomy of a script is described in <xref linkend="nse-script-format"/>.
In this section we will show how the described structure is utilized.
</para>
<sect2 id="nse-tutorial-head">
<title>The Head</title>
<para>
The head of the script is essentially its meta information. This
includes the
fields: <literal>description</literal>, <literal>categories</literal>, <literal>dependencies</literal>, <literal>author</literal>, and <literal>license</literal> as well as
initial NSEDoc information such as usage, args, and output
tags (see <xref linkend="nsedoc"/>).
</para>
<para>
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 <literal>description</literal>. 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:
</para>
<para>
<indexterm><primary><literal>auth-owners</literal> script</primary></indexterm>
<indexterm><primary sortas="description script variable">&ldquo;<varname>description</varname>&rdquo; script variable</primary></indexterm>
<programlisting>
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.
]]
</programlisting>
</para>
<para>Next comes NSEDoc information. This script is missing the
common <literal>@usage</literal> and <literal>@args</literal> tags
since it is so simple, but it does have an
NSEDoc <literal>@output</literal> tag:</para>
<programlisting>
---
--@output
-- 21/tcp open ftp ProFTPD 1.3.1
-- |_ auth-owners: nobody
-- 22/tcp open ssh OpenSSH 4.3p2 Debian 9etch2 (protocol 2.0)
-- |_ auth-owners: root
-- 25/tcp open smtp Postfix smtpd
-- |_ auth-owners: postfix
-- 80/tcp open http Apache httpd 2.0.61 ((Unix) PHP/4.4.7 ...)
-- |_ auth-owners: dhapache
-- 113/tcp open auth?
-- |_ auth-owners: nobody
-- 587/tcp open submission Postfix smtpd
-- |_ auth-owners: postfix
-- 5666/tcp open unknown
-- |_ auth-owners: root
</programlisting>
<para>
Next come the <literal>author</literal>, <literal>license</literal>, and <literal>categories</literal> tags.
This script belongs to the
<literal>safe</literal><indexterm><primary><literal>safe</literal>
script category</primary></indexterm> 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
<literal>default</literal><indexterm><primary><literal>default</literal>
script category</primary></indexterm>
category. Here are the variables in context:</para>
<indexterm><primary sortas="categories script variable">&ldquo;<varname>categories</varname>&rdquo; script variable</primary></indexterm>
<programlisting>
author = "Diman Todorov"
license = "Same as Nmap--See https://nmap.org/book/man-legal.html"
categories = {"default", "safe"}
</programlisting>
</sect2>
<sect2 id="nse-tutorial-rule">
<title>The Rule</title>
<para>
The rule section is a Lua method which decides whether to skip
or execute the script's action. This decision is usually based on
the type of the rule and the host and port information passed to
it. A <literal>prerule</literal> or a
<literal>postrule</literal> will always evaluate to true. 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.</para>
<para>To find out if port 113 is open, we use the
<function>nmap.get_port_state</function> function. If the auth
port was not scanned, the <literal>get_port_state</literal>
function returns <literal>nil</literal>. So we check that
the table is not <literal>nil</literal>. We also
check that both ports are in the <literal>open</literal> state.
If this is the case, the action is executed, otherwise we skip
the action.
</para>
<para>
<indexterm><primary sortas="portrule script variable">&ldquo;<varname>portrule</varname>&rdquo; script variable</primary></indexterm>
<programlisting>
portrule = function(host, port)
local auth_port = { number=113, protocol="tcp" }
local identd = nmap.get_port_state(host, auth_port)
return identd ~= nil
and identd.state == "open"
and port.protocol == "tcp"
and port.state == "open"
end
</programlisting>
</para>
</sect2>
<sect2 id="nse-tutorial-action">
<title>The Action</title>
<para>
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 <function>nmap.new_socket</function>. Next we define an error-handling <function>catch</function> function which closes those sockets if failure is detected. At this point we can safely use object methods such as <function>open</function>,
<function>close</function>,
<function>send</function> and
<function>receive</function> to operate on the network socket. In this case we call <function>connect</function> to make the connections. NSE's exception handling mechanism<indexterm><primary>exceptions in NSE</primary></indexterm>
is used to avoid excessive error-handling code. We simply wrap the networking calls in a <function>try</function> call which will in turn call our <function>catch</function> function if anything goes wrong.</para>
<para>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.
</para>
<para>
<indexterm><primary sortas="action script variable">&ldquo;<varname>action</varname>&rdquo; script variable</primary></indexterm>
<programlisting>
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 .. "\r\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,
"%d+%s*,%s*%d+%s*:%s*USERID%s*:%s*.+%s*:%s*(.+)\r?\n")
end
try(client_ident:close())
try(client_service:close())
return owner
end
</programlisting>
</para>
<para>Note that because we know that the remote port is stored
in <literal>port.number</literal>, we could have ignored the last two
return values of <literal>client_service:get_info()</literal> like
this:</para>
<programlisting>
local localip, localport = try(client_service:get_info())
</programlisting>
<para>In this example we exit quietly if the service responds with an error. This is done by assigning <literal>nil</literal> to the <varname>owner</varname> variable which will be returned. NSE scripts generally only return messages when they succeed, so they don't flood the user with pointless alerts.</para>
</sect2>
<indexterm class="endofrange" startref="nse-tutorial-indexterm"/>
</sect1>
<sect1 id="nsedoc">
<title>Writing Script Documentation (NSEDoc)</title>
<indexterm class="startofrange" id="nsedoc-indexterm"><primary>Nmap Scripting Engine (NSE)</primary><secondary>documentation in</secondary></indexterm>
<indexterm class="startofrange" id="nse-nsedoc-indexterm"><primary>NSEDoc</primary></indexterm>
<para>
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
<ulink url="https://nmap.org/nsedoc/"/>.
</para>
<para>
NSE uses a customized version of the
<ulink url="http://luadoc.luaforge.net/">LuaDoc</ulink><indexterm><primary>LuaDoc</primary></indexterm>
documentation system called NSEDoc.
The documentation for scripts
and modules is contained in their source code, as
comments with a special form.
<xref linkend="nsedoc-comment" xrefstyle="select: label nopage"/>
is an NSEDoc comment taken from the
<function>stdnse.print_debug()</function> function.
</para>
<!-- From stdnse.lua. -->
<!-- Be careful to change <code> to &lt;code&gt; when you copy code.
<code> is a DocBook tag so it will disappear within a programlisting! -->
<example id="nsedoc-comment">
<title>An NSEDoc comment for a function</title>
<programlisting>
---
-- 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
-- &lt;code&gt;nmap.log_write&lt;/code&gt;. The first optional numeric
-- argument, &lt;code&gt;level&lt;/code&gt;, is used as the debugging level necessary
-- to print the message (it defaults to 1 if omitted). All remaining arguments
-- are processed with Lua's &lt;code&gt;string.format&lt;/code&gt; function.
-- @param level Optional debugging level.
-- @param fmt Format string.
-- @param ... Arguments to format.
</programlisting>
</example>
<para>
Documentation comments start with three dashes:
<literal>---</literal>. 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 <literal>@</literal> mark off
other parts of the documentation. In the above example you see
<literal>@param</literal>, which is used to describe each parameter
of a function. A complete list of the documentation tags is found
in <xref linkend="nsedoc-tags"/>.
</para>
<para>
Text enclosed in the HTML-like <literal>&lt;code&gt;</literal> and
<literal>&lt;/code&gt;</literal> 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 <quote><literal>* </literal></quote>, they will
be rendered as a bulleted list. Each list item must be entirely on
one physical line.
</para>
<para>
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.
<xref linkend="nsedoc-module" xrefstyle="select: label nopage"/>
shows documentation for the <literal>comm</literal> module (with a
few paragraphs removed to save space).
</para>
<example id="nsedoc-module">
<title>An NSEDoc comment for a module</title>
<programlisting>
---
-- 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 &lt;code&gt;"bytes"&lt;/code&gt;, &lt;code&gt;"lines"&lt;/code&gt;,
-- &lt;code&gt;"proto"&lt;/code&gt;, and &lt;code&gt;"timeout"&lt;/code&gt;. &lt;code&gt;"bytes"&lt;/code&gt; sets
-- a minimum number of bytes to read. &lt;code&gt;"lines"&lt;/code&gt; does the same for
-- lines. &lt;code&gt;"proto"&lt;/code&gt; sets the protocol to communicate with,
-- defaulting to &lt;code&gt;"tcp"&lt;/code&gt; if not provided. &lt;code&gt;"timeout"&lt;/code&gt;
-- sets the socket timeout (see the socket function &lt;code&gt;set_timeout&lt;/code&gt;
-- for details).
--
-- @author Kris Katterjohn 04/2008
-- @copyright Same as Nmap--See https://nmap.org/book/man-legal.html
</programlisting>
</example>
<para>
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
<xref linkend="nse-script-format"/>). In particular, a script's
description belongs in the <varname>description</varname> variable
rather than in a documentation comment, and the information that
would go in <literal>@author</literal> and
<literal>@copyright</literal> belong in the variables
<varname>author</varname> and <varname>license</varname> instead.
NSEDoc knows about these variables and will use them in preference
to fields in the comments. Scripts should also have
<varname>@output</varname> and <varname>@xmloutput</varname> tags showing sample output, as well as <varname>@args</varname> and <varname>@usage</varname> where appropriate.
<xref linkend="nsedoc-script" xrefstyle="select: label nopage"/>
shows proper form for script-level documentation, using a
combination of documentation comments and NSE variables.
</para>
<!-- From asn-query.nse. -->
<example id="nsedoc-script">
<title>An NSEDoc comment for a script</title>
<programlisting>
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. The responses to these queries contain both Origin and Peer
ASNs and their descriptions, displayed along with the BGP Prefix and
Country Code. The script caches results to reduce the number of queries
and should perform a single query for all scanned targets in a BGP
Prefix present in Team Cymru's database.
Be aware that any targets against which this script is run will be sent
to and potentially recorded by one or more DNS servers and Team Cymru.
In addition your IP address will be sent along with the ASN to a DNS
server (your default DNS server, or whichever one you specified with the
&lt;code&gt;dns&lt;/code&gt; script argument).
]]
---
-- @usage
-- nmap --script asn-query [--script-args dns=&lt;DNS server&gt;] &lt;target&gt;
-- @args dns The address of a recursive nameserver to use (optional).
-- @output
-- Host script results:
-- | asn-query:
-- | 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 https://nmap.org/book/man-legal.html"
categories = {"discovery", "external", "safe"}
</programlisting>
</example>
<indexterm><primary>NSEDoc</primary><secondary>for C modules</secondary></indexterm>
<para>
Compiled NSE modules are also documented with NSEDoc, even though
they have no Lua source code. Each compiled module has a file
<filename><replaceable>modulename</replaceable>.luadoc</filename><indexterm><primary sortas="luadoc filename extension"><filename>.luadoc</filename> filename extension</primary></indexterm>
that is kept in the <filename>nselib</filename> 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 <literal>end</literal>). You must use the
<literal>@name</literal> and <literal>@class</literal> 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 <literal>nmap.luadoc</literal>, <literal>lfs.luadoc</literal>, and <literal>pcre.luadoc</literal>).
</para>
<sect2 id="nsedoc-tags">
<title>NSE Documentation Tags</title>
<para>
The following tags are understood by NSEDoc:
</para>
<variablelist>
<varlistentry>
<term><option>@param</option></term>
<listitem>
<para>
Describes a function parameter. The first word following
<literal>@param</literal> is the name of the parameter
being described. The tag should appear once for each
parameter of a function.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@see</option></term>
<listitem>
<para>
Adds a cross-reference to another function or table.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@return</option></term>
<listitem>
<para>
Describes a return value of a function.
<literal>@return</literal> may be used multiple times for
multiple return values.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@usage</option></term>
<listitem>
<para>
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.
<literal>@usage</literal> may be given more than once. If it is
omitted in a script, NSEDoc generates a default standardized
usage example.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@name</option></term>
<listitem>
<para>
Defines a name for the function or table being documented.
This tag is normally not necessary because NSEDoc infers
names through code analysis.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@class</option></term>
<listitem>
<para>
Defines the <quote>class</quote> of the object being
documented: <literal>function</literal>,
<literal>table</literal>, or <literal>module</literal>.
Like <literal>@name</literal>, this is normally inferred
automatically.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@field</option></term>
<listitem>
<para>
In the documentation of a table, <varname>@field</varname> describes the value of a
named field.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@args</option></term>
<listitem>
<para>
Describes a script argument, as used with the
<option>--script-args</option> option (see
<xref linkend="nse-args"/>). The first word after
<literal>@args</literal> is the name of the argument, and
everything following that is the description. This tag is
special to script-level comments.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@output</option></term>
<listitem>
<para>
This tag, which is exclusive to
script-level comments, shows sample output from a script.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@xmloutput</option></term>
<listitem>
<para>
Shows what the script's
<xref linkend="nse-structured-output">structured output</xref>
looks like when written to XML. The XML sample should not include
the enclosing <literal>&lt;script&gt;</literal> and
<literal>&lt;/script&gt;</literal> tags and should be indented to
show hierarchy.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@author</option></term>
<listitem>
<para>
This tag, which may be given multiple times, lists the authors of an NSE module. For scripts, use the
<varname>author</varname> variable instead.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>@copyright</option></term>
<listitem>
<para>
This tag describes the copyright status of a module. For scripts,
use the <varname>license</varname>
variable instead.
</para>
</listitem>
</varlistentry>
</variablelist>
<!-- These tags are undocumented here: @description, @summary, and
@release. @documentation and @summary are automatically extracted
from the contents of a comment. @release has not been used with
NSEDoc. -->
</sect2>
<indexterm class="endofrange" startref="nsedoc-indexterm"/>
<indexterm class="endofrange" startref="nse-nsedoc-indexterm"/>
</sect1>
<sect1 id="nse-parallelism">
<title>Script Parallelism in NSE</title>
<para>
In <xref linkend="nse-api-networkio"/>, it was mentioned that NSE
automatically parallelizes network operations. Usually this process is
transparent to a script author, but there are some advanced techniques
that require knowledge of how it works. The techniques covered in this
section are controlling how multiple scripts interact in a library, using
multiple threads in parallel, and disabling parallelism for special
cases.
</para>
<para>
The standard mechanism for parallel execution is a thread. A thread
encapsulates the execution flow and data of a script.
Lua thread may be yielded at arbitrary locations to continue
work on another script. Typically, these yield locations are blocking
socket operations in the
<literal>nmap</literal><indexterm><primary><literal>nmap</literal> NSE library</primary></indexterm>
library. The yield back to the script is also transparent, a side effect
of the socket operation.
</para>
<para>
Let's go over some common terminology. A <emphasis>script</emphasis> is
analogous to a binary executable; it holds the information necessary to
execute a script. A <emphasis>thread</emphasis> (a Lua coroutine) is
analogous to a process; it runs a script against a host and possibly
port. Sometimes we abuse terminology and refer to a running thread
as a running <quote>script</quote>, but what this really means is an
instantiation of a script, in the same way that a process is the
instantiation of an executable.
</para>
<para>
NSE provides the bare-bone essentials needed to expand parallelism
basic model of one thread per script: new independent threads,
mutexes, and condition variables.
</para>
<sect2 id="nse-parallelism-threads">
<title>Worker Threads</title>
<para>
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. A common need is 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 answer this need, NSE offers the
function <literal>stdnse.new_thread</literal> 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.
</para>
<para>
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:
</para>
<para>
<literal>worker_thread, status_function = stdnse.new_thread(main, ...)</literal>
</para>
<para>
<literal>stdnse.new_thread</literal> returns two values: 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:
</para>
<para>
<literal>status, error_object = status_function()</literal>
</para>
<para>
The first return value is simply the return
value of <literal>coroutine.status</literal> run on the worker thread
coroutine. (More precisely, the <literal>base</literal> coroutine. Read
more about <literal>base</literal> coroutine in <xref
linkend="nse-parallelism-base"/>.) The second return value contains
an error object that caused the termination of the worker thread, or
<literal>nil</literal> if no error was thrown. This object is typically
a string, like most Lua errors. However, any Lua type can
be an error object, even <literal>nil</literal>. Therefore
inspect the error object, the second return value, only if the status
of the worker is <literal>"dead"</literal>.
</para>
<para>
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 <literal>main</literal> function parameters,
upvalues, or function environments. See
<xref linkend="nse-worker-example" xrefstyle="select: label nopage"/>
for an example.
</para>
<para>
Finally, when using worker threads you should always use condition
variables or mutexes to coordinate them. Nmap is single-threaded so
there are no memory synchronization issues to worry about; but there
<emphasis>is</emphasis> contention for resources.
These resources include usually network bandwidth and sockets.
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) may become very large as each new
document adds new URLs to fetch.
</para>
<example id="nse-worker-example">
<title>Worker threads</title>
<programlisting>
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
for thread in pairs(threads) do
if coroutine.status(thread) == "dead" then threads[thread] = nil end
end
if ( next(threads) ) then
condvar "wait"
end
until next(threads) == nil;
return responses;
end
</programlisting>
</example>
<para>
For brevity, this example omits typical behavior of a traditional web
spider. The requests table is assumed to contain enough objects
to warrant the use of worker threads. The code in this example
dispatches a new thread with as many as 11 relative URLs.
Worker threads are cheap, so don't be afraid to create a lot of them.
After dispatching all these threads, the code waits on a
condition variable until every thread
has finished, then finally return the responses table.
</para>
<para>
You may have noticed that we did not use the status function returned
by <literal>stdnse.new_thread</literal>. 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 more fault-tolerant library may.
</para>
</sect2>
<sect2 id="nse-parallelism-mutex">
<title>Mutexes</title>
<indexterm><primary>threads in NSE</primary></indexterm>
<indexterm><primary>mutexes in NSE</primary></indexterm>
<para>
Recall from the beginning of this section that each script execution
thread (e.g. <literal>ftp-anon</literal> running against an FTP server
on a target host) yields to other scripts whenever it makes a call
on network objects (sending or receiving data). Some scripts require
finer concurrency control over thread execution. An example is the
<literal>whois</literal><indexterm><literal>whois</literal> script</indexterm>
script which queries
whois<indexterm><primary>whois</primary></indexterm> servers for each
target IP address. Because many concurrent queries can get your
IP banned for abuse, and because a single query may
return the same information another instance of the script is about to
request, it is useful to have other threads pause while one thread
performs a query.
</para>
<para>
To solve this problem, NSE includes a <literal>mutex</literal> function
which provides a <ulink
url="http://en.wikipedia.org/wiki/Mutual_exclusion">mutex</ulink>
(mutual exclusion object) usable by scripts. The mutex allows for only
one thread to be working on an object at a time. Competing threads
waiting to
work on this object are put in the waiting queue until they can get a
<quote>lock</quote> on the mutex. A solution for the <literal>whois</literal>
problem above is to have each thread block on a mutex using a common
string, ensuring that only one thread at a time is querying a server.
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 the cache
for a usable result from a previous query, make their own queries, and
unlock the mutex. This is a good example of serializing access to a
remote resource.
</para>
<para>
The first step in using a mutex is to create one with a call to
<literal>nmap.mutex</literal>.
</para>
<programlisting>
mutexfn = nmap.mutex(object)
</programlisting>
<para>
The <literal>mutexfn</literal> returned is a function which works as a
mutex for the <literal>object</literal> passed in. This object can be
any <ulink role="hidepdf"
url="http://www.lua.org/manual/5.2/manual.html#2.1">Lua data
type</ulink> except <literal>nil</literal>,
Boolean, and number. The
returned function allows you to lock, try to lock, and release the
mutex. Its sole argument must be one of the
following:
</para>
<variablelist>
<varlistentry>
<term><literal>"lock"</literal></term>
<listitem>
<para>
Makes 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>"trylock"</literal></term>
<listitem>
<para>
Makes a non-blocking lock on the mutex. If the mutex is busy then
it immediately returns with a return value of
<literal>false</literal>. Otherwise, locks the mutex and
returns <literal>true</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>"done"</literal></term>
<listitem>
<para>
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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>"running"</literal></term>
<listitem>
<para>
Returns the thread locked on the mutex or <literal>nil</literal>
if the mutex is not locked. This should only be used for
debugging as it interferes with garbage collection of finished
threads.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
NSE maintains a weak reference to the mutex so other calls to
<literal>nmap.mutex</literal> with the same object will return the same
mutex function. However, if you discard your reference to the mutex
then it may be collected and subsequent calls to
<literal>nmap.mutex</literal> with the object will return a different
function. Therefore save your mutex to a (local) variable
that persists as long as you need it.
</para>
<para>
A simple example of using the API is provided in <xref
linkend="nse-mutex-handling" xrefstyle="select: label nopage"/>. For
real-life examples, read the
<filename>asn-query</filename><indexterm><primary><filename>asn-query</filename> script</primary></indexterm>
and
<filename>whois</filename><indexterm><primary><filename>whois</filename> script</primary></indexterm>
scripts in the Nmap
distribution.
</para>
<example id="nse-mutex-handling">
<title>Mutex manipulation</title>
<programlisting>
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
</programlisting>
</example>
</sect2>
<sect2 id="nse-parallelism-condvar">
<title>Condition Variables</title>
<para>
Condition variables arose out of a need to coordinate with worker
threads created by the <literal>stdnse.new_thread</literal>
function. A condition variable allows many threads to wait on
one object, and one or all of them to be awakened when some condition
is met. Said differently, multiple threads may unconditionally
<literal>block</literal> on the condition variable by
<emphasis>waiting</emphasis>. Other threads may use the condition
variable to wake up the waiting threads.
</para>
<para>
For example, consider the earlier <xref
linkend="nse-worker-example"/>. Until all
the workers finish, the controller thread must sleep. Note that we cannot
<literal>poll</literal> for results like in a traditional operating
system thread because NSE does not preempt Lua threads. Instead,
we use a condition variable that the controller thread
<emphasis>waits</emphasis> on until awakened by a worker. The controller
will continually wait until all workers have terminated.
</para>
<para>
The first step in using a condition variable is to create one
with a call to <literal>nmap.condvar</literal>.
</para>
<para><literal>condvarfn = nmap.condvar(object)</literal></para>
<para>
The semantics for condition variables are similar to those of mutexes. The
<literal>condvarfn</literal> returned is a function which works as a
condition variable for the <literal>object</literal> passed in. This
object can be any <ulink role="hidepdf"
url="http://www.lua.org/manual/5.2/manual.html#2.1">Lua data
type</ulink> except <literal>nil</literal>,
Boolean, and number. The
returned function allows you to wait, signal, and broadcast on the
condition variable. Its sole argument must be one of the
following:
</para>
<variablelist>
<varlistentry>
<term><literal>"wait"</literal></term>
<listitem>
<para>
Wait on the condition variable. This adds the current thread to the
waiting queue for the condition variable. It will resume
execution when another thread signals or broadcasts on the
condition variable.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>"signal"</literal></term>
<listitem>
<para>
Signal the condition variable. One of the threads in the condition
variable's waiting queue will be resumed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>"broadcast"</literal></term>
<listitem>
<para>
Resume all the threads in the condition variable's waiting queue.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
Like with mutexes, NSE maintains a weak reference to the condition
variable so other calls to <literal>nmap.condvar</literal> with the
same object will return the same function. However, if you discard your reference to the condition variable then
it may be collected and subsequent calls to
<literal>nmap.condvar</literal> with the object will return a different
function. Therefore save your condition
variable to a (local) variable that persists as long as you need it.
</para>
<para>
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 a worker or controller thread. For worker
threads, this will at the very least include a test to see if the
controller is still alive. You do not want to continue doing work
when there's no thread to use your results. A typical test before waiting
may be: Check whether the controller is still running; if not, then quit.
Check if is work to be done; if not, then wait.
</para>
<para>
A thread waiting on a condition variable may be resumed without any
other thread having called <literal>"signal"</literal> or
<literal>"broadcast"</literal> on the condition variable (a spurious
wakeup).
The usual, but not only, reason that this may happen
is the termination of one of the threads using the condition variable. This
is an important guarantee NSE makes that allows you to avoid deadlock
where a worker or controller waits for a thread to wake them up that ended
without signaling the condition variable.
</para>
</sect2>
<sect2 id="nse-parallelism-cm">
<title>Collaborative Multithreading</title>
<para>
One of Lua's least-known features is collaborative multithreading
through <emphasis>coroutines</emphasis>. A coroutine provides an
independent execution stack that can be yielded and resumed.
The standard <literal>coroutine</literal> table provides access to the
creation and manipulation of coroutines. Lua's online first
edition of <ulink url="http://www.lua.org/pil/"><citetitle>Programming in
Lua</citetitle></ulink> contains an excellent introduction to
coroutines. What follows is an overview of the
use of coroutines here for completeness, but this is no replacement for
the definitive reference.
</para>
<para>
We have mentioned coroutines throughout this section as
<emphasis>threads</emphasis>. This is the <emphasis>type</emphasis>
(<literal>"thread"</literal>) of a coroutine in Lua. They are not the
preemptive threads that programmers may be expecting. Lua threads
provide the basis for parallel scripting but only one thread is ever
running at a time.
</para>
<para>
A Lua function executes on top of a Lua
thread. The thread maintains a stack of active
functions, local variables, and the current instruction pointer. We can switch
between coroutines by explicitly yielding the
running thread. The coroutine which resumed the
yielded thread resumes operation.
<xref linkend="nse-cm-coroutines" xrefstyle="select: label nopage"/>
shows a brief use of coroutines to print numbers.
</para>
<example id="nse-cm-coroutines">
<title>Basic Coroutine Use</title>
<programlisting>
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
</programlisting>
</example>
<para>
Coroutines are the facility
that enables NSE to run scripts in parallel. All scripts are
run as coroutines that yield whenever they make a blocking socket
function call. This enables NSE to run other scripts and later resume
the blocked script when its I/O operation has completed.
</para>
<para>
Sometimes coroutines are the best
tool for a job within a single script. One common use in socket programming is filtering
data. You may write a function that generates all the links from an
HTML document. An iterator using <literal>string.gmatch</literal>
can catches only a single pattern. Because some complex matches may take
many different Lua patterns, it is more appropriate to use a
coroutine.
<xref linkend="nse-cm-links" xrefstyle="select: label nopage"/>
shows how to do this.
</para>
<example id="nse-cm-links">
<title>Link Generator</title>
<programlisting>
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
</programlisting>
</example>
<sect3 id="nse-parallelism-base">
<title>The base thread</title>
<para>
Because scripts may use coroutines for their own multithreading,
it is important to be able to identify the <emphasis>owner</emphasis>
of a resource or to establish whether the script is still alive.
NSE provides the function <literal>stdnse.base</literal> for this
purpose.
</para>
<para>
Particularly when writing a library that attributes
ownership of a cache or socket to a script, you can use the
base thread to establish whether the script is still running.
<literal>coroutine.status</literal> on the base thread will give
the current state of the script. In cases where the script is
<literal>"dead"</literal>, 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 <literal>"suspended"</literal>.
You should keep a weak reference to the thread in these cases
so that it may be collected.
</para>
</sect3>
</sect2>
</sect1>
<sect1 id="nse-vscan">
<title>Version Detection Using NSE</title>
<indexterm class="startofrange" id="nse-sample-indexterm"><primary>Nmap Scripting Engine (NSE)</primary><secondary>sample scripts</secondary></indexterm>
<indexterm><primary>version detection</primary><secondary>using NSE</secondary></indexterm>
<para>
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.
</para>
<para>
NSE's <literal>version</literal><indexterm><primary><varname>version</varname> script category</primary></indexterm>
category contains scripts that enhance standard version
detection. Scripts in this category are run whenever you request
version detection with <option>-sV</option>; you don't need to use
<option>-sC</option> to run these. This cuts
the other way too: if you use <option>-sC</option>, you won't get
<literal>version</literal> scripts unless you also use
<option>-sV</option>.
</para>
<para>
One protocol which we were unable to detect with normal version
detection is Skype<indexterm><primary>Skype</primary></indexterm>
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&mdash;an ideal
task for NSE. The simple NSE script which accomplishes this is
shown in
<xref linkend="nse-skypev2-script" xrefstyle="select: label nopage"/>.
</para>
<example id="nse-skypev2-script">
<title>A typical version detection script (Skype version 2 detection)</title>
<indexterm><primary><filename>skypev2-version</filename> script</primary></indexterm>
<programlisting>
description = [[
Detects the Skype version 2 service.
]]<indexterm><primary sortas="description script variable">&ldquo;<varname>description</varname>&rdquo; script variable</primary></indexterm>
---
-- @output
-- PORT STATE SERVICE VERSION
-- 80/tcp open skype2 Skype
author = "Brandon Enright"<indexterm><primary>Enright, Brandon</primary></indexterm><indexterm><primary sortas="author script variable">&ldquo;<varname>author</varname>&rdquo; script variable</primary></indexterm>
license = "Same as Nmap--See https://nmap.org/book/man-legal.html"<indexterm><primary sortas="license script variable">&ldquo;<varname>license</varname>&rdquo; script variable</primary></indexterm>
categories = {"version"}
require "comm"
require "shortport"
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"
and not(shortport.port_is_excluded(port.number,port.protocol))
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)
return
end
return
end
</programlisting>
</example>
<para>
If the script detects Skype, it augments its <varname>port</varname>
table with now-known <varname>name</varname> and
<varname>product</varname> fields. It then sends this new
information to Nmap by calling
<function>nmap.set_port_version</function>. 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 <ulink role="hidepdf" url="https://nmap.org/nsedoc/modules/nmap.html"><function>nmap.set_port_version</function> documentation</ulink>.
</para>
<para>
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.
</para>
</sect1>
<sect1 id="nse-example-scripts">
<title>Example Script: <filename>finger</filename></title>
<indexterm><primary><literal>finger</literal> script</primary></indexterm>
<para>The <filename>finger</filename> script is a perfect
example of a short and simple NSE script.
</para>
<para>First the information fields are assigned.
A detailed description of what the script
actually does goes in the <literal>description</literal> field.</para>
<programlisting>
description = [[
Attempts to get a list of usernames via the finger service.
]]<indexterm><primary sortas="description script variable">&ldquo;<varname>description</varname>&rdquo; script variable</primary></indexterm>
author = "Eddie Bell"<indexterm><primary>Bell, Eddie</primary></indexterm><indexterm><primary sortas="author script variable">&ldquo;<varname>author</varname>&rdquo; script variable</primary></indexterm>
license = "Same as Nmap--See https://nmap.org/book/man-legal.html"<indexterm><primary sortas="license script variable">&ldquo;<varname>license</varname>&rdquo; script variable</primary></indexterm>
</programlisting>
<para>The <literal>categories</literal> field is a table
containing all the categories the script belongs to. These are used for
script selection with the <option>--script</option> option:</para>
<programlisting>
categories = {"default", "discovery", "safe"}<indexterm><primary sortas="categories script variable">&ldquo;<varname>categories</varname>&rdquo; script variable</primary></indexterm>
</programlisting>
<para>
Every good script comes with a sample of its output in an NSEDoc comment.
</para>
<programlisting>
---
-- @output
-- PORT STATE SERVICE
-- 79/tcp open finger
-- | finger:
-- | Welcome to Linux version 2.6.31.12-0.2-default at linux-pb94.site !
-- | 01:14am up 18:54, 4 users, load average: 0.14, 0.08, 0.01
-- |
-- | Login Name Tty Idle Login Time Where
-- | Gutek Ange Gutek *:0 - Wed 06:19 console
-- | Gutek Ange Gutek pts/1 18:54 Wed 06:20
-- | Gutek Ange Gutek *pts/0 - Thu 00:41
-- |_Gutek Ange Gutek *pts/4 3 Thu 01:06
</programlisting>
<para>You can use the facilities provided by the nselib (<xref
linkend="nse-library"/>) with <literal>require</literal>. Here
we want to use common communication functions and shorter port rules:</para>
<programlisting>
require "comm"
require "shortport"
</programlisting>
<para>We want to run the script against the finger service. So we
test whether it is using the well-known finger port (<literal>79/tcp</literal>), or
whether the service is named <quote>finger</quote> based on version
detection results or in the port number's listing
in <filename>nmap-services</filename>:</para>
<programlisting>
portrule = shortport.port_or_service(79, "finger")<indexterm><primary sortas="portrule script variable">&ldquo;<varname>portrule</varname>&rdquo; script variable</primary></indexterm>
</programlisting>
<para>First, the script uses <function>nmap.new_try</function> to
create an exception handler that will quit the script in case of an
error. Next, it passes control to <function>comm.exchange</function>,
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
<function>try</function> exception handler. The script returns a string
if the call to <literal>comm.exchange()</literal> was successful.</para>
<programlisting>
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
</programlisting>
<indexterm class="endofrange" startref="nse-sample-indexterm"/>
</sect1>
<sect1 id="nse-implementation">
<title>Implementation Details</title>
<indexterm><primary>Nmap Scripting Engine (NSE)</primary><secondary>implementation</secondary></indexterm>
<para>
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.
</para>
<sect2 id="nse-implementation-init">
<title>Initialization Phase</title>
<para>
NSE is initialized before any scanning when Nmap first starts, by the
<literal>open_nse</literal> function. <literal>open_nse</literal>
creates a fresh Lua state that will persist across
host groups,<indexterm><primary>host groups</primary><secondary>persistence of NSE through</secondary></indexterm>
until the program exits.
It then loads the standard Lua libraries and compiled NSE libraries.
The standard Lua libraries are
documented in the <ulink
url="http://www.lua.org/manual/5.2/manual.html">Lua Reference
Manual</ulink>. The standard Lua libraries available to NSE are
<literal>debug</literal>,
<literal>io</literal>,
<literal>math</literal>,
<literal>os</literal>,
<literal>package</literal>,
<literal>string</literal>, and
<literal>table</literal>.
Compiled NSE libraries are those that are defined in a C++ file instead
of a Lua file. They include
<literal>nmap</literal>,
<literal>pcre</literal>,
<literal>bin</literal>,
<literal>bit</literal>, and
<literal>openssl</literal> (if available).
</para>
<para>
After loading the basic libraries, <literal>open_nse</literal> loads
the file
<literal>nse_main.lua</literal>. The NSE core is in this
file&mdash;Lua code manages scripts and sets up the appropriate
environment. In this situation Lua really shines as a glue language.
C++ is used to provide the network framework and low-level libraries.
Lua is used to structure data, determine which scripts to load,
and schedule and execute scripts.
</para>
<para>
<literal>nse_main.lua</literal> sets up the Lua
environment to be ready for script scanning later on. It
loads all the scripts the user has chosen and returns a function
that does the actual script scanning to
<literal>open_nse</literal>.
</para>
<para>
The <literal>nselib</literal> directory is added to the Lua path to
give scripts access to the standard NSE library. NSE loads replacements
for the standard coroutine functions so that yields initiated by NSE are
caught and propagated back to the NSE scheduler.
</para>
<para>
<literal>nse_main.lua</literal> next defines classes and functions
to be used during setup. The script arguments
(<literal>--script-args</literal>) are loaded into
<literal>nmap.registry.args</literal>.
A script database is created if one doesn't already exist or if this
was requested with <option>--script-updatedb</option>.
</para>
<para>
Finally, the scripts listed on the command line are loaded.
The <literal>get_chosen_scripts</literal> function works to find
chosen scripts by comparing categories, filenames, and directory names.
The scripts are loaded into memory for later use.
<literal>get_chosen_scripts</literal> works by transforming the
argument to <literal>--script</literal> into a block of Lua code and
then executing it. (This is how the <literal>and</literal>,
<literal>or</literal>, and <literal>not</literal> operators are
supported.) Any specifications that don't directly match a category or
a filename from
<filename>script.db</filename><indexterm><primary><filename>script.db</filename></primary></indexterm>
are checked against file and directory names. If the specification is a
regular file, it's loaded. If a directory, all the
<literal>*.nse</literal> files within it are loaded. Otherwise, the
engine raises an error.
</para>
<para>
<literal>get_chosen_scripts</literal> finishes by arranging the
selected scripts according to their dependencies (see
<xref linkend="nse-format-dependencies"/>).
Scripts that have no dependencies are in runlevel 1. Scripts that
directly depend on these are in runlevel 2, and so on.
When a script scan is run, each runlevel is run separately and in
order.
</para>
<para>
<literal>nse_main.lua</literal> defines two classes:
<literal>Script</literal> and <literal>Thread</literal>. These classes
are the objects that represent NSE scripts and their script threads.
When a script is loaded, <literal>Script.new</literal>
creates a new Script object. The script file is loaded into Lua
and saved for later use. These classes and their methods are intended
to encapsulate the data needed for each script and its threads.
<literal>Script.new</literal> also contains sanity checks to ensure that the
script has required fields such as the <literal>action</literal>
function.
</para>
</sect2>
<sect2 id="nse-implementation-scan">
<title>Script Scanning</title>
<para>
When NSE runs a script scan, <literal>script_scan</literal> is called
in <literal>nse_main.cc</literal>. Since there are three script scan
phases, <literal>script_scan</literal> accepts two arguments, a
script scan type which can be one of these values:
<literal>SCRIPT_PRE_SCAN</literal> (Script Pre-scanning phase) or
<literal>SCRIPT_SCAN</literal> (Script scanning phase) or
<literal>SCRIPT_POST_SCAN</literal> (Script Post-scanning phase),
and a second argument which is a list of targets to scan if
the script scan phase is <literal>SCRIPT_SCAN</literal>.
These targets will be passed to the <literal>nse_main.lua</literal>
main function for scanning.
</para>
<para>
The main function for a script scan generates a number of script
threads based on whether the <literal>rule</literal> function
returns true. The generated threads are stored in a list of runlevel
lists. Each runlevel list of threads is passed separately to the
<literal>run</literal> function. The <literal>run</literal> function
is the main worker function for NSE where all the magic happens.
</para>
<para>
The <literal>run</literal> function's purpose is run all the threads
in a runlevel until they all finish. Before doing this however,
<literal>run</literal> redefines some Lua registry values that
help C code function. One such function,
<literal>_R[WAITING_TO_RUNNING]</literal>, allows the network library
binding written in C to move a thread from the waiting queue to the
running queue. Scripts are run until the
running and waiting queues are both empty. Threads that yield are
moved to the waiting queue; threads that are ready to continue
are moved back to the running queue. The cycle continues until
the thread quits or ends in error. Along with the waiting and running
queues, there is a pending queue. It serves as a temporary location
for threads moving from the waiting queue to the running queue before
a new iteration of the running queue begins.
</para>
</sect2>
</sect1>
<indexterm class="endofrange" startref="nse-indexterm"/>
</chapter>
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