What is port knocking?
Port knocking is a technique to manage access to a range of ports using information gleaned from the firewall logs. Basically, protects a tcp port from getting connected to without "authenticating" first, much like the common system of telnetting to a particular port on a router to change it's firewall rules. The difference is that this doesn't require that service be running to receive anything on those ports -> it gleans the information from the paket logs on those ports.

The net result is that you can operate a box without any "open" ports, and still have it be remotely accessible from any IP, offering a viable replacement for access control lists when managing sensitive ports.

For more information about port knocking in general, and the perl/ipchains implementation that I based this off of, go to Martin Krzywinski's excellent site. In particular, you should read this description of how port knocking works.

For more information about my simplified, BASH implementation of portknocking, read on!
To learn how to install it, click here.