Sync Via Openvpn Uses Relay Server Instead Of Direct Connection


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today I noticed a (for me) very dangerous problem with sync.

If you use BitTorrent Sync through a fresh OpenVPN connection, the peer will be discovered through a relay server instead of the broadcast through TUN!


Let me explain my situation:

I am a student and my student housing is behind a very restrictive firewall that blocks my IP in seconds when I start using peer-2-peer protocols. As I love the idea of BitTorrent Sync I use one of my servers in the Google Compute Engine to host a btsync daemon besides an OpenVPN server that lets me bypass this firewall. The setup is actually working pretty well.


As I don't want to route all my traffic through the VPN I am blocking the outgoing bittorrent tracker and sync packets at my local NAT-Router and switch on the VPN from time to time to sync with the server.

And here comes the problem:


Initially the OpenVPN is switched off and the server peer is offline.

When I establish the VPN connection it takes a few seconds and the server peer is detected and sync works fine.

Unfortunately if the server checked "Use relay server when required", this new connection is going through a relay server instead of the direct connection! And it remains that way!!

If you instead uncheck "Use relay server when required" the connection is also established but NOT through the relay server.


I realised that something was wrong when I observed the outgoing traffic of my server.

Of course that's not really a widespread setup but I think it points to a little (not so obvious) problem with the peer discovery.


Of course this is hardly enough information to check if that is really a bug, but if you want to investigate this I could provide you with additional setup info.


-- Andre

Edited by ankoh
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