Netcat

netcat

Developer(s)	*Hobbit*
Stable release	1.10 / March 20, 1996
Operating system	UNIX
Type	Network utility
License	Permissive free software
Website	http://nc110.sourceforge.net/

Netcat is a computer networking service for reading from and writing network connections using TCP or UDP. Netcat is designed to be a dependable “back-end” device that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of correlation you would need and has a number of built-in capabilities.

In 2000, according to www.insecure.org, Netcat was voted the second most functional network security tool. Also, in 2003 and 2006 it gained fourth place in the same category. Netcat is often referred to as a "Swiss-army knife for TCP/IP." Its list of features includes port scanning, transferring files, and port listening, and it can be used as a backdoor.

Features

Some of netcat's major features are:

• Outbound or inbound connections, TCP or UDP, to or from any ports
• Full DNS forward/reverse checking, with appropriate warnings
• Ability to use any local source port
• Ability to use any locally-configured network source address
• Built-in port-scanning capabilities, with randomization
• Built-in loose source-routing capability
• Can read command line arguments from standard input
• Slow-send mode, one line every N seconds
• Hex dump of transmitted and received data
• Optional ability to let another program service established connections
• Optional telnet-options responder
• Featured tunneling mode which allows also special tunneling such as UDP to TCP, with the possibility of specifying all network parameters (source port/interface, listening port/interface, and the remote host allowed to connect to the tunnel.

Examples

Opening a raw connection to port 25 (like telnet)

nc mail.server.net 25

Setting up a one-shot webserver on port 8080 to present a file

{ echo -ne "HTTP/1.0 200 OK\r\n\r\n"; cat some.file; } | nc -l 8080

The file can then be accessed via a webbrowser under http://servername:8080/. Netcat only serves the file once to the first client that connects and then exits.

Checking if UDP ports (-u) 80-90 are open on 192.168.0.1 using zero mode I/O (-z)

nc -vzu 192.168.0.1 80-90

PS: UDP tests will always show as “open”. The -uz argument is useless.

Pipe via UDP (-u) with a wait time (-w) of 1 second to 'loggerhost' on port 514

echo '<0>message' | nc -w 1 -u loggerhost 514

Port scanning

An uncommon use of netcat is port scanning. Netcat is not considered the best tool for this job, but it can be sufficient (a more advanced tool is Nmap)

nc -v -n -z -w 1 192.168.1.2 1-1000

The “-n” parameter here prevents DNS lookup, “-z” makes nc not receive any data from the server, and “-w 1″ makes the connection timeout after 1 second of inactivity.

Proxying

Another useful behavior is using netcat as a proxy. Both ports and hosts can be redirected. Look at this example:

nc -l 12345 | nc www.google.com 80

Port 12345 represents the request

This starts a nc server on port 12345 and all the connections get redirected to google.com:80. If a web browser makes a request to nc, the request will be sent to google but the response will not be sent to the web browser. That is because pipes are unidirectional. This can be worked around with a named pipe to redirect the input and output.

mkfifo backpipe
nc -l 12345  0<backpipe | nc www.google.com 80 1>backpipe

The "-c" option may also be used:

nc -l 12345 -c 'nc www.google.com 80'

Making any process a server

On a computer A with IP 192.168.1.2:

nc -l -p 1234 -e /bin/bash

The “-e” option spawns the executable with its input and output redirected via network socket. It connects to computer A from any other computer on the same network:

nc 192.168.1.2 1234

ls -las

total 4288
4 drwxr-xr-x 15 imsovain users 4096 2009-02-17 07:47 .
4 drwxr-xr-x 4 imsovain users 4096 2009-01-18 21:22 ..
8 -rw------- 1 imsovain users 8192 2009-02-16 19:30 .bash_history
4 -rw-r--r-- 1 imsovain users 220 2009-01-18 21:04 .bash_logout
...

The consequences are that nc is a popular cracker tool as it is so easy to create a backdoor on any computer. On a Linux computer you may spawn /bin/bash and on a Windows computer cmd.exe to have total control over it.

Port Forwarding or Port Mapping

On Linux, NetCat can be used for port forwarding. Below are nine different ways to do port forwarding in NetCat (-c switch not supported though):

nc -l -p port1 -c ' nc -l -p port2'
nc -l -p port1 -c ' nc host2 port2'
nc -l -p port1 -c ' nc -u -l -p port2'
nc -l -p port1 -c ' nc -u host2 port2'
nc host1 port1 -c ' nc host2 port2'
nc host1 port1 -c ' nc -u -l -p port2'
nc host1 port1 -c ' nc -u host2 port2'
nc -u -l -p port1 -c ' nc -u -l -p port2'
nc -u -l -p port1 -c ' nc -u host2 port2'

Example, see Proxying Netcat#Proxying

Variants

The original version of netcat was a Unix program. The last version (1.1) was released in March 1996 and is currently mirrored at http://nc110.sf.net/.

There are several implementations on POSIX systems, including rewrites from scratch like GNU netcat or OpenBSD netcat (this last has also new features like IPv6 support). Mac OS X users can use the Netcat Darwin Port. There is also a Microsoft Windows version of netcat created by Chris Wysopal, and a Cygwin version is available.

Known ports for embedded systems includes versions for the Windows CE (named Netcat 4 wince) or for the iPhone.

BusyBox includes by default a lightweight version of netcat.

Socat is a more complex variant of netcat. It is larger and more flexible and has more options that must be configured for a given task.

Cryptcat is a version of netcat with integrated transport encryption capabilities.

Middle 2005 the Nmap announced another netcat incarnation called Ncat. It features new possibilities such as "Connection Brokering", TCP/UDP Redirection, SOCKS4 client and server support, ability to "Chain" Ncat processes, HTTP CONNECT proxying (and proxy chaining), SSL connect/listen support and IP address/connection filtering. Like Nmap, Ncat is cross-platform.

On some systems, modified versions or similar netcat utilities go by the command name(s) nc, ncat, pnetcat, socat, sock, socket, sbd.

See also

• List of Unix programs
• Metasploit Project
• Nmap
• Telnet

External links

• OpenBSD nc(1) man page via OpenBSD
• GNU netcat
• Socat
• Netcat for Windows CE (PocketPC, Windows Mobile)
• Advanced Netcat Tutorial
• Netcat - a couple of useful examples
• Netcat useful examples
• major features
• Netcat for Windows Mirror