Netcat

Features

The original netcat's features include:

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.

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

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.). In some versions of netcat like netcat-traditional in Debian 8.6, you need to specify -p before the port number.

Checking whether 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 -z argument is useless.

Test whether 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 backpipenc -l 12345 0<backpipe | nc www.google.com 80 1>backpipe

The -c option may also be used with the ncat implementation:

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.

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 address 192.168.1.2, run this command:

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

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.

Examples by netcat Debian 8.6 (2017) man pages

Connect to example.org on TCP port 8080. : ncat example.org 8080

Listen for connections on TCP port 8080. : ncat -l 8080

Redirect TCP port 8080 on the local machine to host on port 80. : ncat --sh-exec "ncat example.org 80" -l 8080 --keep-open

Bind to TCP port 8081 and attach /bin/bash for the world to access freely. : ncat --exec "/bin/bash" -l 8081 --keep-open

Bind a shell to TCP port 8081, limit access to hosts on a local network, and limit the maximum number of simultaneous connections to 3.

ncat --exec "/bin/bash" --max-conns 3 --allow 192.168.0.0/24 -l 8081 --keep-open

Connect to smtphost:25 through a SOCKS4 server on port 1080: ncat --proxy socks4host --proxy-type socks4 --proxy-auth user smtphost 25

Create an HTTP proxy server on localhost port 8888. : ncat -l --proxy-type http localhost 8888

Send a file over TCP port 9899 from host2 (client) to host1 (server).

Transfer in the other direction, turning Ncat into a “one file” server.

Encrypted file transfer

Suppose you have an ssh tunnel, and you want to copy a file to the remote machine. You could just scp it directly, but that opens up another connection. The goal is to re-use the existing connection. You can use netcat to do this:

When you SSH in, add -L 31000:127.0.0.1:31000

On the remote: nc -lvnp 31000 127.0.0.1 > file

On the local: nc -v -w 2 127.0.0.1 31000 < file

No extra overhead. TCP takes care of error correction. SSH has already encrypted the pipe.

Ports and reimplementations

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

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

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

BusyBox includes by default a lightweight version of netcat.

Solaris 11 includes netcat implementation based on OpenBSD 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. 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. [1] 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. [2]

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

In the middle of 2005, 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.