allowedNetworks is an advanced device setting that affects connection handling. The default is for this setting to be empty, indicating that there is no restriction on the allowed networks for a device.

By setting this to a comma separated list of networks, connections to the given device will be limited to those networks. The networks refer to the address of the remote device, not the network that the local device is presently on.

Given a value of,, 2001:db8::/32 Syncthing will:

  • Allow connections from the device from addresses in the specified networks.

  • Reject connections from the device from addresses outside the specified networks.

  • Attempt connections to addresses in the specified networks (manually configured or discovered).

  • Not attempt connections to addresses outside the specified networks, regardless of whether manually configured or automatically discovered.

Allowed values are numeric IPv4 and IPv6 prefixes in CIDR format, as in the example. Hostnames, netmasks in octet format, etc., are not supported.

If the value is not empty it will be enforced for all connections. Mentioning only an IPv4 prefix will thus deny all IPv6 connections and vice versa. Use (IPv4) or ::/0 (IPv6) to allow all connections on that address family.