ICMP Knock Server

This program uses a concept similar to port knocking, which basically involves waiting for a range of packets that match given criteria before executing some action (typically exposing a port, e.g. for SSH connections, that is kept closed or blocked otherwise). But instead of listening for TCP or UDP packets to arrive on specific ports in a specific order, it accepts ICMP echo requests and checks if their payload lengths and order of arrival match the given pattern. The advantage is that this approach works with the standard PING tool. Programs with custom code or (although common) networking tools like netcat plus the ability to execute them are not required.