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. |
iptables 
libpcap