Port Knocking

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port triggering

Port triggering is a feature included in some routers (e.g. LinkSys BEFSR81, see review at extremetech.com and screenshot of port triggering on the LinkSys BEFSR81) which mitigates the fact that the internal IP address is fixed during port forwarding.

Suppose you have two hosts in an internal network and you would like to host some service from one or the other machine (e.g. Quake or Roger Wilco server). Depending on which internal machine you use to host the service, you will need to either reprogram the port forwarding rules or change the internal IPs to match the rules. That's annoying and this is exactly where port triggering comes in.

Triggering forwards incoming connections to a particular internal host when this internal host initiates a connection. The act of the internal host initiating a connection is the trigger. For example, if an internal host connects to port snmp/25 on a remote server, triggering could be configured to then allow incoming connections from the remote server to port auth/113 of the internal host. The nice thing about triggering is that the internal host's IP/MAC address does not need to be configured anywhere - the router will perform the forwarding smartly. See details on Networking FAQ at broadbandreports.com. Thus, port triggering is similar to port fowarding, except that instead of configuring the forwarding rules yourself, they are automatically configured based on outgoing connections.