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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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migrated from Apr 13 '11 at 15:20

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Some clients use HTTP also. – Moab Apr 13 '11 at 16:28
@Moab, but not for uploading... – Tarnay Kálmán Apr 14 '11 at 21:02

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.

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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.

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Yeah, but some torrent clients(for example utorrent) don't initate upload-only connections, and this is what causes the confusion. – Tarnay Kálmán Apr 13 '11 at 15:36
And that would be µTorrent's problem. – Ignacio Vazquez-Abrams Apr 13 '11 at 15:38

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