knock knock, Neo...

Port Knocking is a wicked clever idea to greatly increase the safety and crack resistance of a publicly accessible server.

The basic idea is to have the server operating in stealth-mode (no port query is every answered, just silently ignored) until a specific sequence of ports are touched, causing the server to open a port and enable a specific service. Trivial example: I touch ports 2022, 2122, 2222 and then SSH on port 22 is enabled for, say 60 seconds or until an active SSH session ends.

Neat idea? Wait, there's more. Now add dead zones of ignored ports, source IP address encoding, time stamp, and other data contained in which ports are touched. Now sprinkle on some military-grade encryption and secure hashing and you have something which basically can't be re-used; even if you send the same exact sequence again the port won't open. Also note that the act of opening the port is just the first step; the normal security of whatever you are connecting to still applies.

It is always important to remember that security through obscurity is not something you can depend upon; but this would make your server awfully obscure...