Port knocking is an authentication method that can be used by a
network administrator to control access to computers or other network
devices behind a firewall. Port knocking takes advantage of firewall
rules to allow a client who knows the "secret knock" to enter the
network through a closed port by performing a sequence of connection
attempts (called a knock sequence). The correct knock sequence for
any given port is created for specific IP addresses by the network
administrator. A small program called a daemon monitors the firewall
log files for connection requests and determines whether or not a
client seeking the network is on the list of approved IP addresses
and has performed the correct knock sequence. If the answer is yes,
it opens the associated port and allows access. Of course, if
unauthorized personnel discover the knock sequence, then they, too,
can gain access.
That was interesting... never thought about something like that... it'd certainly protect from intruders while keeping the system secure. And if you made the knock long enough, it'd work (possibly) quite well. And on multiple ports, even. And have the server open a ssh tunnel to the client... although that might be going too far.