Linux nc command

Updated: 11/06/2021 by Computer Hope
nc command

On Unix-like operating systems, the nc command runs Netcat, a utility for sending raw data over a network connection.

This page covers the Linux version of nc.


Netcat is a 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, 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:


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]


-4 Forces nc to use IPv4 addresses only.
-6 Forces nc to use IPv6 addresses only.
-b Allow broadcast.
-C Send CRLF as line-ending.
-D Enable debugging on the socket.
-d Do not attempt to read from stdin.
-h Prints out nc help.
-I length Specifies the size of the TCP receive buffer.
-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.
-O length Specifies 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 is not attempted. Proxy authentication is only supported for HTTP CONNECT proxies at present.
-p source_port Specifies 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.
-r Specifies that source or destination ports should be chosen randomly instead of sequentially in a range or in the order the system assigns them.
-S Enables the RFC 2385 TCP MD5 signature option.
-s source Specifies the IP of the interface used to send the packets. For UNIX-domain datagram sockets, specifies the local temporary socket file to create and use so that datagrams can be received. It is an error to use this option in conjunction with the -l option.
-T toskeyword Change IPv4 TOS value. toskeyword 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.
-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. For UNIX-domain sockets, use a datagram socket instead of a stream socket. If a UNIX-domain 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. The default is 0.
-v Have nc give more verbose output.
-w timeout Connections that cannot be established or are idle timeout after timeout seconds. The -w flag has no effect on the -l option, i.e., nc listens forever for a connection, with or without the -w flag. The default is no timeout.
-X proxy_protocol 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 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).
-Z DCCP mode.
-z Specifies that nc should only scan for listening daemons, without sending any data to them. It is an error to use this option in conjunction with the -l option.

destination is 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 (where the local host is used). For UNIX-domain sockets, a destination is required and is the socket path to connect to (or listen on if the -l option is given).

port is a single integer or a range of ports. Ranges are in the form nn-mm. In general, a destination port must be specified, unless the -U option is given.

Client/server model

It is quite simple to build a 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 1234

There should now be a connection between the ports. Anything typed at the second console is concatenated to the first, and vice versa. After the connection was set up, nc does not really care which side is used as a ‘server’ and which side is 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 is established by redirecting file descriptors. Be cautious here because opening a port and letting anyone connected execute arbitrary commands on your site is DANGEROUS. If you 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 also quits. 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 to one end of the connection is output to the other end, and input and output can be captured to emulate file transfer.

To start, use 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 that is to be transferred:

nc 1234 <

After the file is transferred, the connection closes 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 homepage of a website:

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 like 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 e-mail may be submitted to an SMTP server using:

nc [-C] localhost 25 << EOF
MAIL FROM:<[email protected]>
RCPT TO:<[email protected]>
Body of e-mail.

Port scanning

It may be useful to know which ports are open and running services on a target machine. The -z flag can tell nc to report open ports, rather than initiate a connection. Usually, it's useful to turn on verbose output to stderr using this option with the -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.

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

nc -zv 80 20 22
nc: connect to 80 (tcp) failed: Connection refused 
nc: connect to 20 (tcp) failed: Connection refused 
Connection to 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 in the greeting banners. To retrieve these, it is necessary to first make a connection, and then break the connection when the banner was 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 20-30
SSH-1.99-OpenSSH_3.6.1p2 Protocol mismatch. 
220 IMS SMTP Receiver Version 0.84 Ready


nc -p 31337 -w 5 42

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

nc -u 53

Opens a UDP connection to port 53 of

nc -s 42

Opens a TCP connection to port 42 of using 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 42

Connects to port 42 of via an HTTP proxy at, port 8080. This example could also be used by ssh.

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

The same as the example above, but this time enabling proxy authentication with username "ruser" if the proxy requires it.

ifconfig — View or modify the configuration of network interfaces.