knockd is a port-knock server. It 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.
