Forgive me for adding to this for the less technical people. I had this problem and struggled for a while, and while I am a software developer, I found the networking instructions confusing. Here's what it took to solve it for me... I was using a D-LINK DIR-645 home router - which is one of the more sophisticated and programmable home routers. The problem was indeed that the Router internal firewall was blocking the communications. But clearing the block did not require manually opening firewall ports, rather I had to adjust the default firewall security level. To do this: This is ADVANCED settings on the router, and FIREWALL settings. The default setting for NAT ENDPOINT FILTERING - UDP ENDPOINT FILTERING is "Port and Address Restricted", which the help describes as 'The NAT does not forward any incoming connection requests with the same port address as an already establish connection.' But, as described at the beginning of this thread, this forwarding is required for the Sync to make the connection. So changing to just "Address Restricted", which is described as 'The NAT forwards incoming connection requests to a LAN-side host only when they come from the same IP address with which a connection was established. This allows the remote application to send data back through a port different from the one used when the outgoing session was created.' seemed to be appropriate, and indeed it worked. Summary - changing ADVANCED - FIREWALL SETTINGS from UDP - Endpoint Filtering of Port and Address Restricted to just Address Restricted allowed BitTorrent Sync to work for connections from outside my home network into my home network.