Portknocking

Port knocking is a method of remotely administering the opening of ports on a firewall. It is a form of network access control. The system listens for connection attempts on a predefined sequence of closed ports. When the correct sequence of connection attempts is detected, the firewall is configured to open a specific port to the originating IP address, allowing a connection to be made. For example, a user might send SYN packets to ports 1000, 2000, and 3000 in that specific order. Upon receiving these packets, the server would then open port 22 (SSH) to the user's IP address.

The primary purpose of port knocking is to enhance security by making services appear unavailable to unauthorized users. By keeping the required port closed by default, network scanners will not see the service as open, thus reducing the attack surface. It adds a layer of obscurity, as the sequence of "knocks" is not inherently discoverable without prior knowledge. However, port knocking is not a substitute for strong authentication methods. If the knocking sequence is discovered or intercepted, the security is compromised.

Implementations of port knocking typically involve a daemon running on the server that monitors network traffic for the specific sequence. Once the sequence is recognized, the daemon dynamically modifies the firewall rules to allow access. While it can be an effective security measure when implemented correctly, it can also introduce complexity in network management and troubleshooting. Alternative security measures, such as VPNs or robust access control lists, are often considered more standard and maintainable.