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]]
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:
- simple TCP proxies
- shell-script based HTTP clients and servers
- network daemon testing
- a SOCKS or HTTP ProxyCommand for ssh(1)
|UDP otherwise default TCP.
|Do not attempt to read from stdin.
| delay time interval between lines of text sent and received. Also causes a delay time between connections to multiple ports.
| listen for an incoming connection rather than initiate a connection to a remote host.|
-p, -s, or
-z , -w
| keep listening for another connection after its current connection is completed. used with |
| scan for listening daemons, without sending any data to them. ncompatable with |
| IP of the interface which is used to send the packets. incompatible with |
|source port incompatable with |
|randomly choose source and/or destination ports instead of sequentially within a range or in the order
that the system assigns them.
|interface to bind the socket to.
| send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. to script telnet sessions.
| no DNS or service lookups
| use Unix Domain Sockets.
| 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.
| protocol when talking to the proxy server. SOCKS v.4, connect(HTTPS proxy) SOCKS v.5(default.
| 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).
|IPv4 addresses only.
|IPv6 addresses only.
|Enable debugging on the socket.
|Forces nc not to use cellular data context.
| 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).
|can be single integers or ranges. Ranges are in the form |
In general, a destination port must be specified, unless the -U option is given (in which case a socket must be specified).
On one console, start nc listening on a specific port for a connection.
$ 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').
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
Body of email.
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
220 host.example.com IMS SMTP Receiver Version 0.84 Ready
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
Original implementation by *Hobbit* .
Rewritten with IPv6 support by Eric Jackson .
UDP port scans will always succeed (i.e. report the port as open), rendering the -uz combination of flags relatively useless.