Knockd and knock are a port-knock server and client, respectively. Knockd 
listens to all traffic on an ethernet (or PPP) interface, looking for 
special "knock" sequences of port-hits. A client makes these port-hits 
by sending a TCP (or UDP) packet to a port on the server. This port 
need not be open -- since knockd listens at the link-layer level, it 
sees all traffic even if it's destined for a closed port. When the 
server detects a specific sequence of port-hits, it runs a command 
defined in its configuration file. This can be used to open up holes in 
a firewall for quick access.