How does Resilio Sync connect from the internet when there's a firewall and no port is listening?

Resilio Sync, formally BitTorrent Sync is able to somehow find a way around 2 NAT firewalls I have all while no port is setup to listen externally nor port forwarding on the routers. When I say I have 2 NAT firewalls I mean that I have an ISP router and then inside of that a personal router. The WAN port of the personal router is connected to the LAN port of the ISP router. The WAN port of the ISP router is connected to the modem.

Resilio Sync syncs files. I really didn't expect it to be able to connect from my mobile phone 50 miles away from my house. Ofcourse there's no port forwarding setup on either router. So no direct external traffic would have been allowed.

I did notice in resilio Sync it offers options for Universal Plug and Play (UPnP) and thought maybe this had something to do with how this worked. I do have this enabled on my routers.

Any ideas how this software magically was able to connect to what I thought was a pretty insular network?

3 Sorted by: Reset to default

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