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I just downloaded a linux image using bittorrent. Although the port seems to be closed I noticed that my client was also uploading. As far as I know UPNP and NAT-PMP are disabled in my current network. Does my client (Transmission) utilize other techniques like TCP-hole-punching? How is uploading with closed ports possible?
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Your computer is connecting to other computers through the tracker. Because your computer is initiating those connections, they go through even though the port is closed. You would get more connections to upload if the ports were not blocked.
Most people would say you should open the ports so you can help others download what you did.
Joshua Nurczyk
Posted 2011-04-13T11:48:37.700
Reputation: 2 316
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The ports being closed doesn't inhibit upload or download; all it does it prevent external torrent clients from initiating connections with yours.
Ignacio Vazquez-Abrams
Posted 2011-04-13T11:48:37.700
Reputation: 100 516
Yeah, but some torrent clients(for example utorrent) don't initate upload-only connections, and this is what causes the confusion. – Tarnay Kálmán – 2011-04-13T15:36:25.153
And that would be µTorrent's problem. – Ignacio Vazquez-Abrams – 2011-04-13T15:38:02.750
Some clients use HTTP also. – Moab – 2011-04-13T16:28:11.913
@Moab, but not for uploading... – Tarnay Kálmán – 2011-04-14T21:02:38.680