Linux 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:
- Simple TCP proxies
- Shell-script based HTTP clients and servers
- Network daemon testing
- A Socks or HTTP ProxyCommand for ssh
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.|
|-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||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.
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 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 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 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
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 host.example.com 1234 < filename.in
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 host.example.com 80
Note that this also displays the headers sent by the web server. They can be filtered, using a tool like sed, if necessary.
nc [-C] localhost 25 << EOF HELO host.example.com MAIL FROM:<[email protected]> RCPT TO:<[email protected]> DATA Body of e-mail. . QUIT EOF
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.
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 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 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
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 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.