Fred Stluka from Bristle.com just introduced me to port knocking -- a method of opening ports on a firewall by trying to connect with a series of ports in a pre-determined order. The sequence of "knocks" acts like a secret access code -- like tapping out the first couple of bars from your favorite song to tell someone inside that it's you. Similar secret taps were used during Prohibition to ensure that only those who knew the secret knock would be allowed into the speakeasies. These days, port knocking provides a way to forge access through a firewall by using the code.