NAME¶

nc — arbitrary TCP and UDP connections and listens

SYNOPSIS¶

DESCRIPTION¶

The nc (or netcat) utility is used for just about anything under the sun involving TCP or UDP. It can open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6. Unlike telnet(1), nc scripts nicely, and separates error messages onto standard error instead of sending them to standard output, as telnet(1) does with some.

Common uses include:

• simple TCP proxies
• shell-script based HTTP clients and servers
• network daemon testing
• a SOCKS or HTTP ProxyCommand for ssh(1)
• and much, much more

The options are as follows:

-4

Forces nc to use IPv4 addresses only.

-6

Forces nc to use IPv6 addresses only.

-D

Enable debugging on the socket.

-d

Do not attempt to read from stdin.

-h

Prints out nc help.

-i interval

Specifies a delay time interval between lines of text sent and received. Also causes a delay time between connections to multiple ports.

-k

Forces nc to stay listening for another connection after its current connection is completed. It is an error to use this option without the -l option.

-l

Used to specify that nc should listen for an incoming connection rather than initiate a connection to a remote host. It is an error to use this option in conjunction with the -p, -s, or -z options. Additionally, any timeouts specified with the -w option are ignored.

-n

Do not do any DNS or service lookups on any specified addresses, hostnames or ports.

-p source_port

Specifies the source port nc should use, subject to privilege restrictions and availability. It is an error to use this option in conjunction with the -l option.

-r

Specifies that source and/or destination ports should be chosen randomly instead of sequentially within a range or in the order that the system assigns them.

-S

Enables the RFC 2385 TCP MD5 signature option.

-s source_ip_address

Specifies the IP of the interface which is used to send the packets. It is an error to use this option in conjunction with the -l option.

-T ToS

Specifies IP Type of Service (ToS) for the connection. Valid values are the tokens “lowdelay”, “throughput”, “reliability”, or an 8-bit hexadecimal value preceded by “0x”.

-C

Send CRLF as line-ending

-t

Causes nc to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. This makes it possible to use nc to script telnet sessions.

-U

Specifies to use Unix Domain Sockets.

-u

Use UDP instead of the default option of TCP.

-v

Have nc give more verbose output.

-w timeout

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 proxy_version

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 hostname 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.

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¶

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 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:

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:

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

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 [-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. 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:

Open a UDP connection to port 53 of host.example.com:

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:

Create and listen on a Unix Domain Socket:

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.

SEE ALSO¶

cat(1), ssh(1)

AUTHORS¶

Original implementation by *Hobbit* ⟨hobbit@avian.org⟩.
Rewritten with IPv6 support by Eric Jackson ⟨ericj@monkey.org⟩.

CAVEATS¶

UDP port scans will always succeed (i.e. report the port as open), rendering the -uz combination of flags relatively useless.