Netcat

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). 

Check Ports

Check to see if port is open and get a response:

   nc -zv IPADDRESS 443

Network Speed Test

Set one server to listen on port 5000 (after opening the port):

   nc -vvklnp 5000 >/dev/null

Run this on the other server to test the network speed between the two and change out the XX for the proper IP:

   dd if=/dev/zero bs=1M count=1K | nc -vvn 192.168.1.XX 5000 -q 1