nc

nc is the command which runs netcat, a simple Unix utility that reads and writes data across network connections, using the TCP or UDP protocol.
It is designed to be a reliable “back-end” tool that can be used directly or driven by other programs and scripts.
At the same time, it is a feature-rich network debugging and exploration tool, since it can create almost any kind of connection
you would need and has several interesting built-in capabilities. Common uses include:

• Simple TCP proxies
• Shell-script based HTTP clients and servers
• Network daemon testing
• A Socks or HTTP ProxyCommand for ssh

Common Options

-l Used to specify that nc should listen for an incoming connection rather than initiate a connection to a remote host.

-p source_port Specifies the source port nc should use, subject to privilege restrictions and availability.

-s source Specifies the IP of the interface which is used to send the packets. For UNIX-domain datagram sockets, specifies the local temporary socket file to create and use so that datagrams can be received. It is an error to use this option in conjunction with the -l option.

-U Specifies to use UNIX-domain sockets.
-u Use UDP instead of the default option of TCP. For UNIX-domain sockets, use a datagram socket instead of a stream socket. If a UNIX-domain socket is used, a temporary receiving socket is created in /tmp unless the -s flag is given.

-X proxy_protocol Requests that nc should use the specified protocol when talking to the proxy server. Supported protocols are “4” (SOCKS v.4), “5” (SOCKS v.5) and “connect” (HTTPS proxy). If the protocol is not specified, SOCKS version 5 is used.

-x proxy_address[:port] Requests that nc should connect to destination using a proxy at proxy_address and port. If port is not specified, the well-known port for the proxy protocol is used (1080 for SOCKS, 3128 for HTTPS).

-z Specifies that nc should just scan for listening daemons, without sending any data to them. It is an error to use this option in conjunction with the -l option.

-v Have nc give more verbose output.

Examples

(1) Client/Server Model

It is quite simple to build a very basic client/server model using nc. On one console, start nc listening on a specific port for a connection.

For example:

$ nc -l 8000

nc is now listening on port 1234 for a connection. Open another console (or a second machine), connect to the machine and port being listened on:

$ nc localhost 8000

Now, a connection should have been set up between the ports. Anything typed at the second console will be sent to the first, and vice-versa.

Execute command from client side

Server side open a port and allow client connected execute arbitrary command on it could be dangerous! If we really want to do this, we can do this:

On ‘server’ side:

$ mkfifo /tmp/f
$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

On ‘client’ side:

$ nc localhost 1234

By doing this, you create a fifo at /tmp/f and make nc listen at port 1234 of address 127.0.0.1 on ‘server’ side, when a ‘client’ establishes a connection successfully to that port, /bin/sh gets executed on ‘server’ side and the shell prompt is given to ‘client’ side.

When connection is terminated, nc quits as well. Use -k if you want it keep listening, but if the command quits this option won’t restart it or keep nc running. Also don’t forget to remove the file descriptor once you don’t need it anymore:

$ rm -f /tmp/f

(2) Data Transfer

The example in the previous section can be expanded to build a basic data transfer model. Any information input into one end of the connection will be output to the other end, and input and output can be easily captured in order to emulate file transfer.

Start by using nc to listen on a specific port, with output captured into a file:

$ nc -l 1234 > filename.out

Using a second machine, connect to the listening nc process, feeding it the file which is to be transferred:

$ nc host.example.com 1234 < filename.in

(3) Port Scanning

It may be useful to know which ports are open and running services on a target machine. The -z flag can be used to tell nc to report open ports, rather than initiate a connection. Usually it’s useful to turn on verbose output to stderr by use this option in conjunction with -v option.

For example:

$ nc -zv host.example.com 20-30
Connection to host.example.com 22 port [tcp/ssh] succeeded!
Connection to host.example.com 25 port [tcp/smtp] succeeded!

The port range was specified to limit the search to ports 20 - 30, and is scanned by increasing order.
You can also specify a list of ports to scan, for example:

$ nc -zv host.example.com 80 20 22

The ports are scanned by the order you given.
Alternatively, it might be useful to know which server software is running, and which versions. This information is often contained within the greeting banners. To retrieve these, it is necessary to first make a connection, and then break the connection when the banner has been retrieved. This can be accomplished by specifying a small timeout with the -w flag, or perhaps by issuing a “QUIT” command to the server:

$ echo "QUIT" | nc host.example.com 20-30
SSH-1.99-OpenSSH_3.6.1p2 Protocol mismatch. 
220 host.example.com IMS SMTP Receiver Version 0.84 Ready

More examples

$ nc -p 31337 -w 5 host.example.com 42

Opens a TCP connection to port 42 of host.example.com, using port 31337 as the source port, with a timeout of 5 seconds.

$ nc -u host.example.com 53

Opens a UDP connection to port 53 of host.example.com.

$ nc -s 10.1.2.3 host.example.com 42

Opens a TCP connection to port 42 of host.example.com using 10.1.2.3 as the IP for the local end of the connection.

$ nc -lU /var/tmp/dsocket

Creates and listens on a UNIX-domain stream socket.

$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42

Connects to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, port 8080. This example could also be used by ssh.

$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42

The same as the above example, but this time enabling proxy authentication with username “ruser” if the proxy requires it.

Reference
http://www.computerhope.com/unix/nc.htm