Port Knocking

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port forwarding

What's the difference between port forwarding and port knocking?

In a port forwarding scenario, a firewall passes all incoming connections for a particular port, or group of ports, to a host within the internal network . During this process the port value could be remapped. For example, you can configure your firewall so that all connections from external hosts made to port ssh/22 are forwarded to port ssh/22 of an internal host, thereby setting up a firewalled, internal SSH server. You could, for the sake of example, forward port telnet/23 to another internal host's port ssh/22 so that you can now select which host to connect to by the port number.

Before you rush off and reconfigure your firewal, understand the implications. By forwarding ports you are creating a hole in your firewall and you are are allowing packets to enter your internal network. Nevertheless, port forwarding is often useful if you want to

• provide an external service from an internal firewalled host
• provide multiple instances of a service from internal firewalled hosts for the purpose of load balancing.

Most routers provide port forwarding feature (screenshot of port forwarding on the LinkSys BEFSR81). Port triggering is another method to remap traffic.