HOWTO: Outgoing port translation

[drivera]

Hi!

I'm trying to do something that on the surface appears simple enough, but I'm having a hell of a time configuring. I have a VPN through which I want to route all traffic that the firewall detects going to a specific port number (say...8888).  Simple enough, and done easily enough.

The trick is that I also want that outgoing traffic's destination port number to change from "8888" to "7777", without touching the destination address... and this is where I'm having a hell of a time.

The Outbound rules UI allows me to change the source port, the source address, the destination address... but not the destination port.

I also can't use a port forward since I don't know the request's possible destination beforehand and the port forwarding UI (at least) requires a destination IP to forward the traffic to...

I refuse to believe this "simple" (right?) functionality is impossible in OPNSense, and instead I prefer to think that the issue is that I'm uneducated/incompetent in OPNSense management and configuration, since that seems to me the more likely scenario.

So.... any ideas?

Thanks!