KnockOnD is a simple port knocking client compatible with knockd or any other port knocking server. It supports both UDP and TCP protocols, interpacket delays. Easy to configure and use.
Port knocking is a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports. Once a correct sequence of connection attempts is received, the firewall rules are dynamically modified to allow the host which sent the connection attempts to connect over specific port(s). For detailed explanation see http://www.portknocking.org/
Using KnockOnD one can define destination host, delay between packets (in milliseconds) and port numbers to which packets are to be sent as a "sequence". Once saved these sequences are easily accessible for "knocking".