Syncthing / UPnP / Multi VLAN / no connection

[anyonebutnoone]

Hello together,

please move if I'm in the wrong category.

I'm running an up to date 2.4.4-p3 with several internal networks, some virtual and some hard on links, no packages installed.
The firewall has a public IP on the WAN interface.

In two (maybe three) of my networks I'd like to run Syncthing clients/users.

I have setup UPnP so the clients can punch holes in the firewall to be reachable - I am aware of the given security implications and use ACLs to lock it down as much as possible.

Both devices, one on each network successfully signup in the UPnP service.

Now while both devices are behind the same public IP they do not connect to each other. Both receive the target public IP and port from the syncthing network but the connection times out.

I know the firewall needs no additional rules as that is how UPnP works as far as I understand AND because when I VPN out/put one device behind another public IP I can connect to the device behind the pfsense just fine, this goes with one and the other.

What am I missing?
Thanks in advance!

[jimp]

You can't hairpin NAT like that with FreeBSD/pf. They won't be able to reach each other through the WAN address.

With manual port forward rules, you can enable NAT reflection which adds more rules behind the scenes to cover that scenario. That is not possible with UPnP.