About nc
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
nc 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]
Options
| -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 will not be 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 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 | Specifies the IP of the interface which is 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 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 | 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 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. |
nc 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.