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 the specific port(s). A variant called single packet authorization exists, where only a single "knock" is needed, consisting of an encrypted packet.
The primary purpose of port knocking is to prevent an attacker from scanning a system for potentially exploitable services by doing a port scan because unless the attacker sends the correct knock sequence, the protected ports will appear closed.
During a discussion, I came up with the following question: Considering an admin (as in server admin) or even an attacker (as in the bad guy trying to get access to the machine) a port knocking method can be considered a form of authentication towards that server?