Yes, I have been able to use Syncthing on two firewalled devices behind AirVPN. For this test, I also disabled "Local Discovery" to rule out false positives. 1. Do not change the "Global Discovery Servers" and do not open/forward their ports! Keep it to the default:
udp4://announce.syncthing.net:22026, udp6://announce-v6.syncthing.net:22026
2. On both/all of your devices, check Syncthing's web interface and look for the "Global Discovery" field. It has to say 1/2 or 2/2, meaning that it's been able to connect to at least one of the discovery servers. If it says 0/2, check your internet connectivity, VPN connectivity and the default discovery servers (step 1). 3. For each device you want to sync, create forwarded ports in Air's client area. As an example, let's pretend you have 2 devices and get ports 12345 and 12346. Set the "Sync Protocol Listen Address" on device A to
0.0.0.0:12345
and on device B to
0.0.0.0:12346
and don't forget to restart each Syncthing instance after changing ports! 4. (only for firewalled devices) If your devices' firewalls block all incoming traffic by default, allow traffic to port 12345 or 12346. Example for ufw:
ufw allow in on tun0 to 0.0.0.0/0 port 12345
Example for iptables:
iptables -A INPUT -i tun0 -p tcp --dport 12345 -j ACCEPT 5. In Air's client area, do the TCP port check for each device. If Syncthing is running on the given device/port and your firewall allows incoming traffic (see step 4), you will get a green light. If you don't, check all the previous steps. 6. If you do get a green light for both devices, you can go ahead and pair your devices. On device A, click "Add Device" and type in device B's Device ID. You will now see a message on device A:
Device B ([AirVPN IP]:some_port) wants to connect. Add new device?
Choose "Add" and you're good to go