[zaarn]

And the router has a clue about those UDP "sessions"? They're not sessions either, it's just an application declaring that incoming UDP packets with a certain destination port (and optionally destination IP, source IP or source Port) be delivered to it. Nothing about sessions.

[foxfluff]

If you send from local ip/port X to remote ip/port Y, your router will see both pairs. The router has no problem sending responses back your way after it has stored the tuple, assuming you're receiving responses on the same port you sent from. UDP connection tracking is nothing new at all.

If you haven't sent anything at all, then you're not a normal client, you're a server and need port forwarding anyway (or you're ftp and should be shot).

[zaarn]

UDP Connection Tracking is not well implemented on all routers, more than once I've found that forwarding a UDP port makes that UDP port unavailable for other devices to use.

[grosswait]

If connection tracking wasn’t a thing, every UDP reply would be sent to every device on the network.

[zaarn]

Yes but your router might not interact well with Connection Tracking and UDP port forwards. Especially with such wide range ones. I know more than one case where port forwarding disables connection tracking for UDP on those ports.