According to this it's possible to have incoming connections to Bitcoin client by forwarding port 8333 on the router. I was wondering if there is some specific port range Bitcoin Core uses for making outgoing connections which could be used as a triggering rule? This way when the client is not running, port would be closed, plus it could be used by different machines behind the router without configuration changes (I know I could forward different ports to different machines and then change the incoming port in client configs)
@NateEldredge is right.
My current setup is as follow:
Rule name: Bitcoin
Triggered range: 8333-8333
Forwarded range: 8333-8333
If Bitcoin Core is running the port is open (according to http://www.canyouseeme.org/). If it's closed the port is closed too.
The only catch is it won't work if all 8 outgoing connections will use non default port but it shouldn't happen too often if ever.