The packet always rings twice

Linux Journal has an innovative way of remotely accessing your otherwise firewall’ed-up server – port knocking. This trick is accomplished by using a pre-arranged sequence of connection attempts to a unique set of ports within a unique time interval as a method of authentication. In other words, like knocking in a certain pattern on a locked door of a speakeasy. A daemon can be set to monitor the firewall logs for this secret “knock”, and open a port (say, port 22 for secure shell login) when the sequence is played. The beauty of this system is that the host firewall can still be silently blocking all ports.