How to go about forwarding UPnP/SSDP etc. between guest network and "lan"?


I am wondering if there is a sensible method to forward UPnP/SSDP requests between networks.

I have a DAB radio which also has an Ethernet socket and I want it to have limited outside connectivity. However, what’s more is that I’d like the device to be discoverable from the “lan” zone. Now accessing from the “lan” zone appears to be no problem with the defaults. Alas, because the (local) broadcasts used by UPnP are probably not being routed (and rightly so), the device isn’t visible “on the other side”.

Now I was thinking of giving the Omnia another IP and running rinetd on that as I have used this sort of setup - among others - to pretend to be another remote machine in order to fetch LetsEncrypt certificates centrally for several servers I maintain.

However, I reckon this would not work alone because of the subnet issues. After all even with rinetd the radio system won’t have an idea it’s also present in that other subnet (which constitutes the “lan” zone).

Any ideas how to go about this? I found this thread, but it doesn’t quite apply to my issue.

Should I be successful on my own, I’ll post the solution here. Thanks for reading.