Netcat

netcat

Developer(s)	*Hobbit*
Stable release	1.10 / 2 January 2007 (2007-01-02)
Operating system	Unix
Type	Network utility
License	Original version: custom, permissive license GNU Version: GPL OpenBSD Version: BSD
Website	nc110.sourceforge.net

Netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP. Netcat is designed to be a dependable back-end 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 connection its user could need and has a number of built-in capabilities.

Its list of features includes port scanning, transferring files, and port listening, and it can be used as a backdoor.

Features

The original netcat's features include:^[1]

• 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 establish connections
• Optional telnet-options responder
• Featured tunneling mode which permits user-defined tunneling, e.g., UDP or 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).

Rewrites like GNU's and OpenBSD's support additional features. For example, OpenBSD's nc supports TLS.

Examples

Opening a raw connection to port 25 (like SMTP)

nc mail.server.net 25

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

 { echo -ne "HTTP/1.0 200 OK\r\nContent-Length: $(wc -c <some.file)\r\n\r\n"; cat some.file; } | nc -l 8080

The file can then be accessed via a web browser under http://servername:8080/. Netcat only serves the file once to the first client that connects and then exits, it also provides the content length for browsers that expect it. (This should work fine in a LAN, but may potentially fail with any kind of firewall between.).

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

Note that UDP tests will always show as "open". The -uz argument is useless.

Test if UDP port is open: simple UDP server and client

This test is useful, if you have shell access to the server that should be tested, but you do not know whether there is a firewall blocking a specific UDP port on the server.

On the listening host, i.e. on the server whose port needs to be checked, do the following:

nc -ul 7000

On the sending host, do the following – note that servname is the hostname of the listening host:

nc -u servname 7000

If text typed on the sending host (type something and hit enter) is displayed also on the listening host, then the UDP port 7000 is open. If it is not open, you will get an error such as "Connection refused".

There is a caveat. On some machines, IPv6 may be the default IP version to use by netcat. Thus, the host specified by the hostname is contacted using IPv6, and the user might not know about this. Ports may appear closed in the test, even though they would be open when using IPv4. This can be difficult to notice and may cause the false impression that the port is blocked, while it is actually open. You can force the use of IPv4 by using adding -4 to the options of the nc commands.

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 behaviour 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 with the 'ncat' implementation:^[2]

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

Using a named pipe is a more reliable method because using "-c" option provides only a one-shot proxy.

Another useful feature is to proxy SSL connections. This way, the traffic can not be viewed in wire sniffing applications such as wireshark. This can be accomplished on UNIXes by utilizing mkfifo, netcat, and openssl.

 mkfifo tmp
 mkfifo tmp2
 nc -l 8080 -k > tmp < tmp2 &
 while [ 1 ]
 do
  openssl s_client -connect www.google.com:443 -quiet < tmp > tmp2
 done

Making any process a server

netcat can be used to make any process a network server. It can listen on a port and pipe the input it receives to that process.

The -e option spawns the executable with its input and output redirected via network socket.

For example, it is possible to expose a bourne shell process to remote computers.

To do so, on a computer A with IP 192.168.1.2, run this command:

$ nc -l -p 1234 -e /bin/sh

Then, from any other computer on the same network, one could run this nc command:

$ 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
...

In this way, the -e option can be used to create a rudimentary backdoor. Some administrators perceive this as a risk, and thus do not allow netcat on a computer.

Ports and reimplementations

The original version of netcat was a Unix program. The last version (1.10) was released in March 1996.^[1]

There are several implementations on POSIX systems, including rewrites from scratch like GNU netcat^[3] or OpenBSD netcat,^[4] the latter of which supports IPv6 and TLS. The OpenBSD version has been ported to the FreeBSD base^[5] and Windows/Cygwin.^[6] Mac OS X users can use MacPorts to install a netcat variant.^[7] There is also a Microsoft Windows version of netcat available.^[8]

Known ports for embedded systems includes versions for Windows CE (named "Netcat 4 wince"^[9]) or for the iPhone.^[10]

BusyBox includes by default a lightweight version of netcat.

Solaris 11 includes netcat implementation based on OpenBSD netcat.

Socat^[11] 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. On February 1, 2016, Santiago Zanella-Beguelin and Microsoft Vulnerability Research issued a security advisory regarding a composite Diffie-Hellman parameter which had been hard-coded into the OpenSSL implementation of socat. The implausibility that a composite might have been unintentionally introduced where a prime number is required has led to the suspicion of sabotage to introduce a backdoor software vulnerability.

Cryptcat^[12] is a version of netcat with integrated transport encryption capabilities.

In the middle of 2005, Nmap announced another netcat incarnation called Ncat.^[13] 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
• Nmap
• OpenSSL
• Telnet
• Plink
• Packet Sender

References

External links

• Official website
• nc(1) – Linux User Commands Manual