syncthing-networking - Man Page

If you have a NAT router which supports UPnP, the easiest way to get a working port forward is to make sure UPnP setting is enabled on both Syncthing and the router – Syncthing will try to handle the rest. If it succeeds you will see a message in the console saying:

Created UPnP port mapping for external port XXXXX on UPnP device YYYYY.

If this is not possible or desirable, you should set up a port forwarding for ports 22000/TCP and 22000/UDP (or whichever port is set in the Sync Protocol Listen Address setting). The external forwarded ports and the internal destination ports have to be the same (e.g. 22000/TCP).

Communication in Syncthing works both ways. Therefore if you set up port forwards for one device, other devices will be able to connect to it even when they are behind a NAT network or firewall.

In the absence of port forwarding, Relaying may work well enough to get devices connected and synced, but will perform poorly in comparison to a direct connection.

The router needs to allow/forward broad-/multicasts for local discovery to work. Usually these are allowed by default in a single local subnet, but may be blocked between different subnets or even between a bridged Wi-Fi and LAN.

If you are unable to set up your router thus or your firewall as shown below, and your devices have static IP addresses, you can specify them directly by changing the default dynamic setting for Addresses to something like: tcp://192.168.1.xxx:22000, dynamic.

If you’re using ufw on Linux and have installed the Syncthing package <https://apt.syncthing.net/>, you can allow the necessary ports by running:

sudo ufw allow syncthing

If you also want to allow external access to the Syncthing web GUI, run:

sudo ufw allow syncthing-gui

Allowing external access is not  necessary for a typical installation.

You can then verify that the ports mentioned above are allowed:

sudo ufw status verbose

In case you installed Syncthing manually you can follow the instructions to manually add the syncthing preset <https://github.com/syncthing/syncthing/tree/main/etc/firewall-ufw> to ufw.

If you are using Firewalld <https://firewalld.org/> it has included support for syncthing (since version 0.5.0, January 2018), and you can enable it with:

sudo firewall-cmd --zone=public --add-service=syncthing --permanent
sudo firewall-cmd --reload

Similarly there is also a syncthing-gui service.

If you have SSH access to the machine running Syncthing but would rather not open the web GUI port to the outside world, you can access it through a SSH tunnel instead. You can start a tunnel with a command like the following:

ssh -L 9999:localhost:8384 machine

This will bind to your local port 9999 and forward all connections from there to port 8384 on the target machine. This still works even if Syncthing is bound to listen on localhost only.