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I have been testing this "bittorent sync" software (a peer to peer application) as an experiment. I decided to block the application using the Windows firewall on the computer that is running the software. I then tried to sync from external computer outside the LAN and it still worked fine.

I then went to my router and disable UPNP and reboot and I am still able to connect to my machine; I made sure that I block all TCP and UDP from external access and this thing is still transmitting UDP packets.

How does this technology work? Is it firewall proof? Do i need a firewall that read each packet to block it? I know that some firewalls allow you to explicitly block peer to peer applications, is that what is needed in this case?

I am a little confused please help...

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migrated from serverfault.com Jul 3 '13 at 20:36

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1 Answer

It sounds like either the firewalls you tested with are broken or you do not know how to use them. Are you sure you didn't just block externally initiated traffic? If you really blocked all TCP and UDP, then web access shouldn't work. If web access works, then you didn't block all TCP.

UDP traffic can appear to both sides as if the other initiated it. So blocking only externally initiated traffic won't always block UDP. (TCP can too, but this almost never happens whereas it's common with UDP.)

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I only have tcp 3389 open on the firewall, let's forget about the windows firewall for a minute. This thing still transferring data. Somehow it is bypassing the netgear router firewall or it is using some other technology. It is possible to block internal udp traffic to the outside. –  winteck Jul 3 '13 at 21:16
    
What exactly did you enable in the router firewall that you are expecting will block this traffic? –  David Schwartz Jul 3 '13 at 21:27
    
Actually nothing. I am thinking that the router would block everything by default unless i explicitly open the ports. There is obviously something that i am not getting... –  winteck Jul 3 '13 at 21:33
    
@winteck: Most routers, by default, try to make things work. Generally, firewalling takes explicit configuration. There's a myth that routers provide firewalling just because they can't make something work. But firewalling is, pretty much by definition, intentional filtering to stop traffic. The router is making this work just as it makes other things work, as is its job. Why should it block this if it can make it work? What reason does it have to think this is something it should prohibit rather than make work? –  David Schwartz Jul 3 '13 at 21:37
    
If that is the case why arent't all my TCP ports open by default? a nnmap scan only reveals my 3389 as the only port open, hence my assumption... FYI i am using the netgear R6100. I am thinking on using my own firewall... –  winteck Jul 3 '13 at 21:46
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