High Quality Content by WIKIPEDIA articles! In computer networking, 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). A variant called Single Packet Authorization exists, where only a single `knock` is needed, consisting of an encrypted packet.