http://docs.syncthing.net/users/firewall.html
Local Firewall
If your PC has a local firewall, you will need to open the following ports for **incoming traffic**:
- Port 22000/TCP (or the actual listening port if you have changed the
Sync Protocol Listen Address setting.)
- Port 21025/UDP (for discovery broadcasts on IPv4)
- Port 21026/UDP (for discovery multicasts on IPv6)
I had some drops for outgoing traffic for the last 2 ports. The doc just talks of incoming ports. I would correct it by myself but I am not sure if the last 2 ports are only for outgoing or both.