NetCat is a tool for connecting stdin(3)/stdout(3) to a TCP/UDP connection, giving Shell scripts the power to excercise or even provide network services.

It was initially written for Unix by someone known only as Hobbit and released as public domain. Development of this tool continued up to version 1.10. nc(1) is the ManPage for this tool, and it can be installed on Debian using apt-get install netcat.

It is freely given away to the Internet community in the hope that it will be useful, with no restrictions except giving credit where it is due. No GPLs, Berkeley copyrights or any of that nonsense.

There is now a fully POSIX-compatible rewrite known as GNU Netcat. It is maintained by Giovanni Giacobbi and has been referred to as "netcat ran through indent".

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.

To complicate matters, there is also another rewrite called netcat6 to adds support for IPv6 networks to the original utility.

CLUG's wiki has a page on NetCat with some examples.