Netcat is a computer networking service for reading from and writing network connections using TCP or UDP. Netcat is designed to be a dependable “back-end” device that can be used candidly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of correlation you would need and has a number of built-in capabilities.
In 2000 according to www.insecure.org Netcat was voted the second most functional network security tool. Also, in 2003 and 2006 it gained fourth place in the same category. Netcat is often referred to as a "Swiss-army knife for TCP/IP." Its list of features includes port scanning, transferring files, and port listening, and it can be used as a backdoor.
According to http://nc110.sourceforge.net/, some of netcat's major features are:
* Outbound or inbound connections, TCP or UDP, to or from any ports
* Full DNS forward/reverse checking, with appropriate warnings
* Ability to use any local source port
* Ability to use any locally-configured network source address
* Built-in port-scanning capabilities, with randomization
* Built-in loose source-routing capability
* Can read command line arguments from standard input
* Slow-send mode, one line every N seconds
* Hex dump of transmitted and received data
* Optional ability to let another program service established connections
* Optional telnet-options responder
* 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.