Does a torrent tracker pass file data or just the list of addresses?

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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?

Saturnix

Posted 2013-05-22T21:56:54.317

Reputation: 155

2^ see updated question. Thanks for your note. – Saturnix – 2013-05-22T22:09:25.313

1Thank 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 – 2013-05-22T22:10:33.370

Answers

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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).

Breakthrough

Posted 2013-05-22T21:56:54.317

Reputation: 32 927

then how come that I can seed, having no public IP? – Saturnix – 2013-05-22T22:00:34.407

"newer torrents can use distributed hash tables" - Don't you mean torrent clients? – Karan – 2013-05-22T22:00:35.403

2@Saturnix: What do you mean you have no public IP? How are you connected to the internet? – Karan – 2013-05-22T22:00:59.280

@Karan indeed I did, thanks for the correction. Also note that the actual torrents themselves also require explicit DHT support. – Breakthrough – 2013-05-22T22:03:01.723

"Also note that the actual torrents themselves also require explicit DHT support." - I suppose you're talking about the Private flag? If so, yes, if that flag's set then the torrent can only be used with private trackers and DHT is not allowed. – Karan – 2013-05-22T22:05:36.210

Take the "shared" folder on my computer - if I install Apache, enable web sharing and put an "index.html" file in it, I will still be unable to publish that folder as a website. It has been explained to me that this is due to the fact that my IP is shared in the neighborhood and not specifically redirecting to my computer. Is this wrong? If not, I don't see how someone can connect to my torrent client and take data packets. – Saturnix – 2013-05-22T22:07:08.930

Oh, I see you've edited the reply - now it makes sense. Thanks! I'll accept it in 3 minutes. – Saturnix – 2013-05-22T22:07:51.743

@Karan no, when creating a DHT-based torrent, the actual torrent is missing an announce key-value pair, and instead contains one or more node key-value pairs. See this article for details.

– Breakthrough – 2013-05-22T22:09:06.707

Thanks for the link. Yeah, that would be for a DHT-only trackerless torrent, whereas the Private flag would be for a tracked torrent with DHT blocked. – Karan – 2013-05-22T22:11:48.897

1@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 – 2013-05-22T22:18:53.413

I've also seen mention of bad/hacked clients that don't respect the Private flag. – Karan – 2013-05-22T22:21:13.243

@Saturnix To answer your "How can they connect to me if I don't have a public IP" question: How are you getting data back from the SuperUser.com servers? You connected to them and asked for the data and they sent it back to you. BitTorrent with no open ports (google Port Forwarding) work the same way, You connect to another client who does have ports open and ask him "Do you have any data I need and do I have any data you neeed?". You could NOT connect to another person who also has no open ports, one side must have open ports, but 90% of most users have ports open now. – Scott Chamberlain – 2013-05-22T22:45:39.313

^ yeah, that makes sense - thanks for your explanation. – Saturnix – 2013-05-23T00:30:46.027

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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.

Jasper Lingers

Posted 2013-05-22T21:56:54.317

Reputation: 1

Asked: 2013-05-22T21:56:54.317

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Active: 2013-08-20T11:51:45.033