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
Syntax
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 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’).
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 127.0.0.1 1234 > /tmp/f
On ‘client’ side:
nc host.example.com 1234
(shell prompt from host.example.com)
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:
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 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 website:
printf "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, 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
HELO host.example.com
MAIL FROM:<user@host.example.com>
RCPT TO:<user2@host.example.com>
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. 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
nc: connect to host.example.com 80 (tcp) failed: Connection refused
nc: connect to host.example.com 20 (tcp) failed: Connection refused
Connection to host.example.com 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 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
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.