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Let's suppose a file is listed on a torrent site. Obviously, the site doesn't have the file but the tracker does have a list of address to which I can connect to download such file.

My question is: does the tracker also pass packets of the file or do I just connect directly to the computer(s) hosting it?

I can seed at 3/4mbs speed but I have no public IP: how is this even possible without a third part effectively taking the packet and redirect it to another user?

From a legal standpoint this makes a huge difference.

No one in the world can possibly connect to my computer without passing through a third part server as my IP is shared across the neighborhood.

So who is doing this thing? The tracker? If yes, then why most newspaper and blogs reports that "a tracker doesn't host the file but just a list of addresses" when a torrent site is shut down?

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^ see updated question. Thanks for your note. –  Saturnix May 22 '13 at 22:09
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Thank you for updating the question and making this question valid for Super User. I've edited it a slight bit more, and believe it is now on-topic and can stay open. –  Breakthrough May 22 '13 at 22:10
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My question is: does the tracker also pass packets of the [redacted] file or do I just connect directly to the computer(s) hosting it?

No, a tracker does not have any actual data packets pass through it. If that were the case, the tracker would incur very high bandwidth usage, and the throughput of all clients connected to the tracker would be very limited. Finally, newer BitTorrent clients can use distributed hash tables for peer lists (given the torrent was created as a DHT torrent), completely negating the need for a centralized tracker.

I can seed at 3/4mbs speed but I have no public IP: how is this even possible without a third part effectively taking the packet and redirect it to another user?

Your computer makes a direct TCP/UDP connection to any peers. No data traffic ever passes through the tracker whatsoever. One can analyze the actual data packets being sent to and forth from a connected peer using a network traffic analyzer like Wireshark, which already contains high-level support for BitTorrent packets (including display filters).

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then how come that I can seed, having no public IP? –  Saturnix May 22 '13 at 22:00
    
"newer torrents can use distributed hash tables" - Don't you mean torrent clients? –  Karan May 22 '13 at 22:00
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@Saturnix: What do you mean you have no public IP? How are you connected to the internet? –  Karan May 22 '13 at 22:00
    
@Karan indeed I did, thanks for the correction. Also note that the actual torrents themselves also require explicit DHT support. –  Breakthrough May 22 '13 at 22:03
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@Karan yes, this is correct. By setting the private flag, you force clients to route their request traffic through the tracker (where the administrators of the tracker can then limit access to the torrent for unauthorized clients). Without this flag, it would be possible for an unauthorized client to begin downloading from another client completely bypassing the tracker through the DHT. –  Breakthrough May 22 '13 at 22:18
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A tracker backend only exchanges a IP list. Nothing else, except for possible "stats" that your client does (how much has it send, how much has it received, and if a full download succeeded), but overal, nothing else. Basically, if you download a IP list on the net, and paste it in your client, is already enough to exchange the content. A backend tracker does this for you.

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