The Nmap Scripting Engine (NSE) is one of Nmap's most
powerful and flexible features. It allows users to write (and
share) simple scripts to automate a wide variety of networking
tasks. Those scripts are then executed in parallel with the speed
and efficiency you expect from Nmap. Users can rely on the
growing and diverse set of scripts distributed with Nmap, or write
their own to meet custom needs.
We designed NSE to to versatile, with the following tasks in mind
- Network discovery
This is Nmap's bread and butter. Examples include
looking up whois data based on the target domain,
querying ARIN, RIPE, or APNIC for the target IP to determine ownership,
performing identd lookups on open ports, SNMP queries, and
listing available NFS/SMB/RPC shares and services.
-
More sophisticated version detection
The Nmap version detection system (Chapter 7, Service and Application Version Detection)
is able to recognize thousands of different services through
its probe and regular expression 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
the section called “Version Detection Using NSE”.
-
Vulnerability detection
When a new vulnerability is discovered, you often want
to scan your networks quickly to identify vulnerable systems
before the bad guys do. While Nmap isn't a
comprehensive vulnerability scanner,
NSE is powerful enough to handle even demanding vulnerability
checks. Many vulnerability detection scripts have already been
written and we plan to distribute more as they are written.
- Backdoor detection
Many attackers and some automated worms leave
backdoors to enable later reentry. Some of these can be
detected by Nmap's regular expression based version detection.
For example, within hours of the MyDoom worm hitting the
Internet,
Jay Moran
posted an Nmap version detection probe and
signature so that others could quickly scan their networks.
For more complex worms and backdoors, NSE is needed
for reliable detection.
- Vulnerability exploitation
As a general scripting language, NSE can even
be used to exploit vulnerabilities rather than just find them.
The capability to add custom exploit scripts may be valuable
for some people (particularly
penetration testers),
though we aren't
planning to turn Nmap into an exploitation framework like
Metasploit.
The listed items were our initial goals, but we expect that Nmap
users will come up with inventive uses for NSE.
Scripts are written in the
embedded
Lua programming language.
The language itself is well documented in the books
Programming
in Lua, Second Edition and
Lua
5.1 Reference Manual.
The reference manual is also
freely available
online, as is the
first edition of Programming in
Lua. Given the availability of these excellent general
Lua programming references, this document only covers aspects and
extensions specific to Nmap's scripting engine.
NSE is activated with the -sC option (or
--script if you wish to specify a custom set of
scripts) and results are integrated into Nmap
normal
and XML output.
Two types of scripts are supported: service and host
scripts. Service scripts relate to a certain open port
(service) on the target host, and any results they produce are included
next to that port in the Nmap output port table. Host scripts,
on the other hand, run no more than once against each target IP
and produce results below the port table. Example 9.1 shows a typical script scan. Examples of
service scripts producing output are: Stealth SSH
version, which tricks some SSH servers into divulging
version information without logging the attempt as they normally
would; Service Owner, which connects to open
ports, then performs a reverse-identd query to determine what
username each is running under; and HTML Title,
which simply grabs the title of the root path of any web servers
found. A sample host script is RIPE Query,
which looks up and reports target IP ownership
information.
Example 9.1. Typical NSE output
$ nmap -sC localhost -p 22,23,80,113
Starting Nmap ( http://nmap.org )
Interesting ports on localhost (127.0.0.1):
PORT STATE SERVICE
22/tcp open ssh
|_ Stealth SSH version: SSH-1.99-OpenSSH_4.2
|_ SSH protocol version 1: Server supports SSHv1
23/tcp closed telnet
80/tcp open http
|_ HTML title:Test Page for Apache Installation
113/tcp closed auth
Host script results:
|_ RIPE Query: IP belongs to: Internet Assigned Numbers Authority
Nmap finished: 1 IP address (1 host up) scanned in 0.907 seconds
Intro
