1. Disable use of tracker and relay in share preferences and in power user settings. Sync will then no longer connect to any remote devices (outside your local network).

  2. to ensure peer discovery, enable multicasts in LAN and ensure that these go through Wi-Fi <-> Ethernet if peers are in different subnets.

  3. Put sync.conf with the following content into Sync's storage folder.

{
  "service_folders.use_relay" : false, 
  "service_folders.use_tracker" : false
}
  1. Please note: If your peer established connection at least once over the Internet, Sync will cache the global IP address and will continue syncing over the Internet as before. To make sure syncing is done over LAN, you'll need to clear the cache on each desktop:

    1. Set the "Peer expiration time (days)" in Advanced preferences - More Options to "0".
    2. Restart the client.
    3. Set the preference back to "7".