knock sequence as a one-time password defeat the aim of simplified administration. In practice, port knocking must be combined with other forms of authentication that are not vulnerable to replay or man-in-the-middle attacks for the whole system to be effective. References External links SilentKnock: Practical, Provably Undetectable Authentication "An Analysis of Port Knocking and Single Packet Authorization" MSc Thesis by Sebastien Jeanquier "Implementing a Port Knocking system in C" Honors Thesis by Matt Doyle. List of resources about PK and SPA (papers, implementations, presentations...) PortKnocking - A system for stealthy authentication across closed ports. (archived) Linux Journal: Port Knocking (2003) A Critique of Port Knocking (2004) fwknop: An implementation that combines port knocking and passive OS fingerprinting WebKnock: An online Port Knocking and Single Packet Authorization (SPA) client based on fwknop webknocking: Using webpages instead of ports. Port knocking to hide a backdoor Port knocking to hide communication channel for malware, example from the wild Techniques for Lightweight Concealment and Authentication in IP Networks (2002) Internet security