What exactly is a trigger port and how do I figure out which one it is?

Question

I have this router now since a few weeks and I need port forwarding for certain games and/or gaming services such as Steam.

Now normally configuring this is a non-issue for me, but what boggles my mind is that my router is asking for a trigger port.

Now what exactly is this trigger port and how do I figure out which of the ports, e.g. provided by Steam, is the one to enter there?

Answer 1

When using classic port forwarding you've got one specific issue: You've got a 1:1 relation between the incoming port and the LAN member this is forwarded to.

For example, you can specify that incoming traffic on port 12345 (bad idea; never use that port!) to the machine 192.168.2.2. But if you, for example, play that game on more than one computer, you'd have to change the forward every time you switch machines.

Port triggering is an interesting approach to counter these disadvantages:

• The port is not necessarily open all the time (depends on whether your LAN IP changes).
• The port will always forward to the proper machine.

You'll still run into issues, if you want to use multiple machines at the same time, or you want to passively listen on ports (e.g. a web server), but that's something different.

So how does it work? Pretty easy.

Let's assume you'd like to play some game where you'd like to host single matches. If there's some authentication server your game will be connected to all the time, you can use the port that server is using as the trigger port. Let's assume it's port 5000.

To host games, other players have to be able to reach your PC on port 5001. So you create a new dynamic port activation with the following parameters:

• Triggering port: 5000
• Triggering protocol: (depends on the game, most likely TCP for non time sensitive stuff like authentication)
• Ports to be opened: 5001 to 5001 (typically UDP for most games)

And that's it.

Now, whenever you start the game and try to authenticate/login, the game will create an outgoing connection to its authentication server on port 5000.

The router will notice this, since it's the "trigger port", and enable/reroute port forwarding to the computer that made that connection.

The port forwarding is now active, you can play and host your matches, etc.

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Also note that this feature isn't provided by all routers. Some use a different strategy to reroute ports in a changing environment. For example, AVM routers typically got an option to always assign port forwards to the same machine, even if its IP changes due to DHCP. While this is very convenient for things that work without outgoing connections (dedicated servers in general), it won't allow you to dynamically switch machines and use the same ports on them without reconfiguring port forwarding.

Answer 2

The Wikipedia article on port triggering (what your router seems to call "dynamic port activation") has a good overview.

Essentially, with the settings you have specified, any outgoing connection from your computer to a server on port 4380 will automatically forward the specified incoming port ranges ("Ports to be opened") to your computer. This is particularly helpful if you don't have a fixed IP address (either a statically-configured IP or "static DHCP", which most reasonably new routers support).

This is likely not what you actually want, though. It's not clear from that Steam page which port the initial outgoing connection will use. You probably want to look under "Port diversions and port forwarding" and forward the port ranges you need there.