NETCAT some useful commands(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


nc [-46bCDdhklnrStUuvZz] [-I length] [-i interval] [-O length] 
   [-P proxy_username] [-p source_port] [-q seconds] [-s source] 
   [-T toskeyword] [-V rtable] [-w timeout] [-X proxy_protocol] 
   [-x proxy_address[:port]] [destination] [port]

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 1234

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

nc 1234
There should now be a connection between the ports. Anything typed at the second console will be concatenated to the first, and vice-versa. After the connection has been set up, nc does not really care which side is being used as a ‘server’ and which side is being used as a ‘client’. The connection may be terminated using an EOF (‘^D’).
There is no -c or -e option in modern netcat, but you still can execute a command after connection being established by redirecting file descriptors. Be cautious here because opening a port and let anyone connected execute arbitrary command on your site is DANGEROUS. If you really need to do this, here is an example:
On ‘server’ side:

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

On ‘client’ side:
nc 1234
(shell prompt from
By doing this, you create a fifo at /tmp/f and make nc listen at port 1234 of address 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

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 1234 <

After the file has been transferred, the connection will close automatically.

Talking To Servers

It is sometimes useful to talk to servers “by hand” rather than through a user interface. It can aid in troubleshooting, when it might be necessary to verify what data a server is sending in response to commands issued by the client. For example, to retrieve the home page of a website:

printf "GET / HTTP/1.0\r\n\r\n" | nc 80

Note that this also displays the headers sent by the web server. They can be filtered, using a tool such as sed, if necessary.
More complicated examples can be built up when the user knows the format of requests required by the server. As another example, an email may be submitted to an SMTP server using:

nc [-C] localhost 25 << EOF

Body of email.

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 20-30
Connection to 22 port [tcp/ssh] succeeded!
Connection to 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 80 20 22
nc: connect to 80 (tcp) failed: Connection refused 
nc: connect to 20 (tcp) failed: Connection refused 
Connection to port [tcp/ssh] succeeded! 

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. In order 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 20-30

SSH-1.99-OpenSSH_3.6.1p2 Protocol mismatch. 
220 IMS SMTP Receiver Version 0.84 Ready


nc -p 31337 -w 5 42

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

nc -u 53

Opens a UDP connection to port 53 of

nc -s 42

Opens a TCP connection to port 42 of using 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 42

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

nc -x10.2.3.4:8080 -Xconnect -Pruser 42

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