nc - netcat

establish arbitrary TCP and UDP connections and listens

nc [-46DCdhklnrtUuvz] [-b eaboundif] [-s source_ip_address][-p source_port]
[-i interval] [-w timeout]
[-X proxy_protocol] [-x proxy_address[:port]]
          [hostname] [port[s]]

open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6.
scripts nicely, and separates error messages to standard error

    Common uses include:
  1. simple TCP proxies
  2. shell-script based HTTP clients and servers
  3. network daemon testing
  4. a SOCKS or HTTP ProxyCommand for ssh(1)
-u UDP otherwise default TCP.
-v verbose
-d Do not attempt to read from stdin.
-i interval delay time interval between lines of text sent and received. Also causes a delay time between connections to multiple ports.
-l listen for an incoming connection rather than initiate a connection to a remote host.

Incompatible with -p, -s, or -z , -w

-k keep listening for another connection after its current connection is completed. used with -l
-z scan for listening daemons, without sending any data to them. ncompatable with -l
-s source_ip_address IP of the interface which is used to send the packets. incompatible with -l
-p source_port source port incompatable with -l .
-r randomly choose source and/or destination ports instead of sequentially within a range or in the order that the system assigns them.
-b if interface to bind the socket to.
-t send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. to script telnet sessions.
-n no DNS or service lookups
-U use Unix Domain Sockets.
-w econds If a connection and stdin are idle for more than timeout seconds, then the connection is silently closed.
The -w flag has no effect on the -l option, i.e. nc will listen forever for a connection, with or without the -w flag. The default is no timeout.
-X 4|5|connect protocol when talking to the proxy server. SOCKS v.4, connect(HTTPS proxy) SOCKS v.5(default.
-x proxy_address[:port] connect 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).
-4 IPv4 addresses only.
-6 IPv6 addresses only.
-D Enable debugging on the socket.
-C Forces nc not to use cellular data context.
-h help.
hostname can be a numerical IP address or a symbolic hostname (unless the -n option is given).
In general, a hostname must be specified, unless the -l option is given (in which case the local host is used).
port[s] can be single integers or ranges. Ranges are in the form nn-mm.
In general, a destination port must be specified, unless the -U option is given (in which case a socket must be specified).

Client/Server Model

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 127.0.0.1 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').

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
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 web site: $ echo -n "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80 Note that this also displays the headers sent by the web server. They can be filtered, using a tool such as sed(1), 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 localhost 25 << EOF HELO host.example.com MAIL FROM: RCPT TO: DATA Body of email. . QUIT EOF 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. For example: $ nc -z 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. 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 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 EXAMPLES Open a TCP connection to port 42 of host.example.com, using port 31337 as the source port, with a timeout of 5 seconds: $ nc -p 31337 -w 5 host.example.com 42 Open a UDP connection to port 53 of host.example.com: $ nc -u host.example.com 53 Open 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 -s 10.1.2.3 host.example.com 42 Create and listen on a Unix Domain Socket: $ nc -lU /var/tmp/dsocket Connect 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(1); see the ProxyCommand directive in ssh_config(5) for more information. $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42 SEE ALSO cat(1), ssh(1) AUTHORS Original implementation by *Hobbit* . Rewritten with IPv6 support by Eric Jackson . CAVEATS UDP port scans will always succeed (i.e. report the port as open), rendering the -uz combination of flags relatively useless.