Netcat is a featured networking utility which reads and writes data across network connections, using the TCP/IP protocol. It is designed to be a reliable “back-end” tool that can be used directly or easily 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.

It provides access to the following main features:

• Outbound and inbound connections, TCP or UDP, to or from any ports.
• Featured tunneling mode which allows also special tunneling such as UDP to TCP, with the possibility of specifying all network parameters (source port/interface, listening port/interface, and the remote host allowed to connect to the tunnel.
• Built-in port-scanning capabilities, with randomizer.
• Advanced usage options, such as buffered send-mode (one line every N seconds), and hexdump (to stderr or to a specified file) of trasmitted and received data.
• Optional RFC854 telnet codes parser and responder.

The GNU Netcat is distributed freely under the GNU General Public License (GPL).

echo  -e "GET / HTTP/1.0\r\n" | nc 192.168.1.1 80

Desde el servidor

cat backup.iso | nc -l 3333

Desde el cliente

nc 192.168.0.1 3333 > backup.iso

Si queremos poner un contador

nc 192.168.0.1 3333 | pv -b > backup.iso

Otro ejemplo

Desde el servidor

dd if=/dev/hdb5 | gzip -9 | nc -l 3333

Desde el cliente

nc 192.168.0.1 3333 | pv -b > myhdb5partition.img.gz

for port in {1..65535}; do
  nc -w 1 localhost $port > /dev/null && echo "Port $port open" || echo "Port $port closed";
done

Fuente : http://adamish.com/blog/archives/567

http://netcat.sourceforge.net/

Tutorial con ejemplos prácticos : http://crysol.org/node/28

Mas ejemplos : http://www.g-loaded.eu/2006/11/06/netcat-a-couple-of-useful-examples/