- Nmap Network Scanning
- Chapter9.Nmap Scripting Engine
Table of Contents
- Introduction
- Usage and Examples
- Script Categories
- Script Types and Phases
- Command-line Arguments
- Script Selection
- Arguments to Scripts
- Complete Examples
- Script Format
- description Field
- categories Field
- author Field
- license Field
- dependencies Field
- Rules
- Action
- Environment Variables
- Script Language
- Lua Base Language
- NSE Scripts
- NSE Libraries
- List of All Libraries
- Hacking NSE Libraries
- Adding C Modules to Nselib
- Nmap API
- Information Passed to a Script
- Network I/O API
- Connect-style network I/O
- Raw packet network I/O
- Structured and Unstructured Output
- Exception Handling
- The Registry
- Script Writing Tutorial
- The Head
- The Rule
- The Action
- Writing Script Documentation (NSEDoc)
- NSE Documentation Tags
- Script Parallelism in NSE
- Worker Threads
- Mutexes
- Condition Variables
- Collaborative Multithreading
- The base thread
- Version Detection Using NSE
- Example Script: finger
- Implementation Details
- Initialization Phase
- Script Scanning
Introduction
The Nmap Scripting Engine (NSE) is one of Nmap's most powerful and flexible features. It allows users to write (and share) simple scripts to automate a wide variety of networking tasks. Those scripts are then executed in parallel with the speed and efficiency you expect from Nmap. Users can rely on the growing and diverse set of scripts distributed with Nmap, or write their own to meet custom needs.
We designed NSE to be versatile, with the following tasks in mind:
- Network discovery
This is Nmap's bread and butter. Examples include looking up whois data based on the target domain, querying ARIN, RIPE, or APNIC for the target IP to determine ownership, performing identd lookups on open ports, SNMP queries, and listing available NFS/SMB/RPC shares and services.
- More sophisticated version detection
The Nmap version detection system (Chapter7, Service and Application Version Detection) 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 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. When the Heartbleed bug affected hundreds of thousands of systems worldwide, Nmap's developers responded with the
ssl-heartbleeddetection script within 2 days. Many vulnerability detection scripts are already available 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, 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.
- Vulnerability exploitation
As a general scripting language, NSE can even be used to exploit vulnerabilities rather than just find them. The capability to add custom exploit scripts may be valuable for some people (particularly penetration testers), though we aren't planning to turn Nmap into an exploitation framework such as Metasploit.
These listed items were our initial goals, and we expect Nmap users to come up with even more inventive uses for NSE.
Scripts are written in the embedded Lua programming language, version 5.3. The language itself is well documented in the books Programmingin Lua, Fourth Edition and Lua5.2 Reference Manual.The reference manual, updated for Lua 5.3, 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.
A typical script scan is shown in the Example9.1. Service scripts producing output in this example are ssh-hostkey, which provides the system's RSA and DSA SSH keys, and rpcinfo, which queries portmapper to enumerate available services. The only host script producing output in this example is smb-os-discovery, which collects a variety of information from SMB servers. Nmap discovered all of this information in a third of a second.
Example9.1.Typical NSE output
# nmap -sC -p22,111,139 -T4 localhostStarting Nmap ( https://nmap.org )Nmap scan report for flog (127.0.0.1)PORT STATE SERVICE22/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-ssnHost script results:| smb-os-discovery: Unix| LAN Manager: Samba 3.0.31-0.fc8|_Name: WORKGROUPNmap done: 1 IP address (1 host up) scanned in 0.33 secondsA 38-minute video introduction to NSE is available athttps://nmap.org/presentations/BHDC10/. Thispresentation was given by Fyodor and David Fifield at Defcon and theBlack Hat Briefings in 2010.
