If this port is rocking, don't come a knocking
Port knocking is a method of establishing extra security between a networked computer and the outside world. It seems like a great idea. For hosts that are running private services that need to be connected to the net, require clients to make a specific series of connection attempts to various ports. After the correct sequence is made, the firewall opens up the port for the client. This would be very difficult to uncover via a port scan.