Section: User Commands (1)
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nc - arbitrary TCP and UDP connections and listens  


nc [-46bCDdFhklNnrStUuvZz [-I length ] ] [-i interval ] [-M ttl ] [-m minttl ] [-O length ] [-P proxy_username ] [-p source_port ] [-q seconds ] [-s source ] [-T keyword ] [-V rtable ] [-W recvlimit ] [-w timeout ] [-X proxy_protocol ] [-x proxy_address [: port ] ] [destination ] [port ]  


The nc (or netcat utility is used for just about anything under the sun involving TCP, UDP, or UNIX sockets. 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:

The options are as follows:

Use IPv4 addresses only.
Use IPv6 addresses only.
Allow broadcast.
Send CRLF as line-ending. Each line feed (LF) character from the input data is translated into CR+LF before being written to the socket. Line feed characters that are already preceded with a carriage return (CR) are not translated. Received data is not affected.
Enable debugging on the socket.
Do not attempt to read from stdin.
Pass the first connected socket using sendmsg(2) to stdout and exit. This is useful in conjunction with -X to have nc perform connection setup with a proxy but then leave the rest of the connection to another program (e.g. ssh(1) using the ssh_config5 ProxyUseFdpass option). Cannot be used with -U
Print out the nc help text and exit.
-I length
Specify the size of the TCP receive buffer.
-i interval
Sleep for interval seconds between lines of text sent and received. Also causes a delay time between connections to multiple ports.
When a connection is completed, listen for another one. Requires -l When used together with the -u option, the server socket is not connected and it can receive UDP datagrams from multiple hosts.
Listen for an incoming connection rather than initiating a connection to a remote host. The destination and port to listen on can be specified either as non-optional arguments, or with options -s and -p respectively. Cannot be used together with -x or -z Additionally, any timeouts specified with the -w option are ignored.
-M ttl
Set the TTL / hop limit of outgoing packets.
-m minttl
Ask the kernel to drop incoming packets whose TTL / hop limit is under minttl
shutdown(2) the network socket after EOF on the input. Some servers require this to finish their work.
Do not do any DNS or service lookups on any specified addresses, hostnames or ports.
-O length
Specify the size of the TCP send buffer.
-P proxy_username
Specifies a username to present to a proxy server that requires authentication. If no username is specified then authentication will not be attempted. Proxy authentication is only supported for HTTP CONNECT proxies at present.
-p source_port
Specify the source port nc should use, subject to privilege restrictions and availability.
-q seconds
after EOF on stdin, wait the specified number of seconds and then quit. If seconds is negative, wait forever (default). Specifying a non-negative seconds implies -N
Choose source and/or destination ports randomly instead of sequentially within a range or in the order that the system assigns them.
Enable the RFC 2385 TCP MD5 signature option.
-s source
Send packets from the interface with the source IP address. For UNIX datagram sockets, specifies the local temporary socket file to create and use so that datagrams can be received. Cannot be used together with -x
-T keyword
Change the IPv4 TOS/IPv6 traffic class value. keyword may be one of critical inetcontrol lowcost lowdelay netcontrol throughput reliability or one of the DiffServ Code Points: ef af11 ... af43 cs0 ... cs7 or a number in either hex or decimal.
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.
Use UNIX sockets. Cannot be used together with -F or -x
Use UDP instead of TCP. Cannot be used together with -x For UNIX sockets, use a datagram socket instead of a stream socket. If a UNIX socket is used, a temporary receiving socket is created in /tmp unless the -s flag is given.
-V rtable
Set the routing table to be used.
Produce more verbose output.
-W recvlimit
Terminate after receiving recvlimit packets from the network.
-w timeout
Connections which cannot be established or are idle timeout after timeout seconds. 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_protocol
Use proxy_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 ]
Connect to destination 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). An IPv6 address can be specified unambiguously by enclosing proxy_address in square brackets. A proxy cannot be used with any of the options -lsuU
DCCP mode.
Only scan for listening daemons, without sending any data to them. Cannot be used together with -l

destination can be a numerical IP address or a symbolic hostname (unless the -n option is given). In general, a destination must be specified, unless the -l option is given (in which case the local host is used). For UNIX sockets, a destination is required and is the socket path to connect to (or listen on if the -l option is given).

port can be a specified as a numeric port number, or as a service name. Ports may be specified in a range of the form nn - mm In general, a destination port must be specified, unless the -U option is given.  


It is quite simple to build a very basic client/server model using . 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 this 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



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 -N 1234 <

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


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:
$ 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(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
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. 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 (unless the -r flag is set).

You can also specify a list of ports to scan, for example:

$ nc -zv http 20 22-23
nc: connect to 80 (tcp) failed: Connection refused
nc: connect to 20 (tcp) failed: Connection refused
Connection to port [tcp/ssh] succeeded!
nc: connect to 23 (tcp) failed: Connection refused

The ports are scanned by the order you given (unless the -r flag is set).

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 Qq Dv QUIT command to the server:

$ echo "QUIT" | nc 20-30
Protocol mismatch.
220 IMS SMTP Receiver Version 0.84 Ready


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

$ nc -p 31337 -w 5 42

Open a UDP connection to port 53 of

$ nc -u 53

Open a TCP connection to port 42 of using as the IP for the local end of the connection:

$ nc -s 42

Create and listen on a UNIX stream socket:

$ nc -lU /var/tmp/dsocket

Connect to port 42 of via an HTTP proxy at, port 8080. This example could also be used by ssh(1); see the ProxyCommand directive in ssh_config5 for more information.

$ nc -x10.2.3.4:8080 -Xconnect 42

The same example again, this time enabling proxy authentication with username ``ruser'' if the proxy requires it:

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


cat(1), ssh(1)  


Original implementation by An *Hobbit* Aq Mt .
Rewritten with IPv6 support by An Eric Jackson Aq Mt .
Modified for Debian port by Aron Xu Aq .  


UDP port scans using the -uz combination of flags will always report success irrespective of the target machine's state. However, in conjunction with a traffic sniffer either on the target machine or an intermediary device, the -uz combination could be useful for communications diagnostics. Note that the amount of UDP traffic generated may be limited either due to hardware resources and/or configuration settings.