Secret Port Knocking
via BoingBoing, an interesting article from 2003 about a firewall access technique called Port Knocking. It works by listening for a pre-defined sequence of connection attempts to certain ports. If the sequence matches the pattern within a certain time limit, then a specific port is open to the requesting IP address to access. Here is an implementation of the technique on SourceForge. I'm a little surprised we haven't seen wider support of this technique. It seems it would be fairly trivial to integrate the client-side functionality into an SSH client like PuTTY to start some interest and encourage server-side support in software and hardware firewalls. Quite intuitive as well. Who didn't have a secret knock when they were a kid?