How can Sync connect from outside when no Firewall ports are open?

[mcmbit120]

I have set up BittorrentSync on my laptop computer (windows 10), on my server (Windows 2012 server) and on my iPhone. All devices are located at home. On the Windows 10 computer I have installed the Desktop Version of BittorrentSync, on the Windows Server 2012 I have installed BittorrentSync as a Windows Service.

My strategy is to sync several folders when I am located at home, so "LAN only". I am not interested to sync stuff when I am on the road.

But I was very surprise to find out that when I am outside my LAN (office, public Wifi etc), BIttorrentSync on my iPhone shows my home peers as connected an also data is synced. I wonder how this can happen, as I never opend a firewall Port on my home's LAN router. As I understood, no BittorrentSync central servers are used.

Somebody can help my understand the technology? 

Thanks!

[GreatMarko]

If you have the "Use UPnP Port Mapping" setting enabled in Sync (which by default it is), Sync will automatically attempt to open the necessary port(s) on your router.

For further information, please see: https://help.getsync.com/hc/en-us/articles/204762669-Sync-Preferences

[mcmbit120]

Thanks for this hint.

Hmm, I do not have and never had enabled UPnP on my router, as I want to be aware about what is happening on my network. I just double checked the settings on my (Fritzbox) router.

Are there any other reasons that could be the reason for finding my peers over the internet? Maybe I am just too blind ...

[Helen]

for UDP connections to tracker/relay UPnP is not obligatory, Sync can do without. 

Unless you have explicitly blocked port 3000 on your router and/or firewall, your peers will be able to connect over relay or tracker.