Closed port authentication with port knocking

Abstract

Port knocking is a promising new technology to further secure remote services. This technology can be used to keep all TCP ports closed until a user has authenticated with a port knock sequence. During the port knock sequence all ports remain closed, thus rendering the server invisible to any malicious port scans. After a valid knock sequence has been verified by the system, a predetermined TCP or UDP port is opened allowing for a standard connection for a predefined service. This allows an extra layer of authentication at the transport layer without requiring changes to the application. A review of the current implementations is given.