CODEWITHSUNDEEP
c#bittorrent
Kevin Montrose
Kevin Montrose

Reputation: 22601

BitTorrent tracker announce problem

I've been throwing a little bit of spare time at writing a BitTorrent client, mostly out of curiosity but partly out of a desire to improve my c# skills.

I've been using the theory wiki as my guide. I've built up a library of classes for handling BEncoding, which I'm quite confident in; basically because the sanity check is to regenerate the original .torrent file from my internal representation immediately after parsing, then hash and compare.

The next stage is to get tracker announces working. Here I hit a stumbling block, because trackers reject my requests without terribly useful error messages.

Take, for instance, the latest stack overflow database dump. My code generates the following announce URI:

http://208.106.250.207:8192/announce?info_hash=-%CA8%C1%C9rDb%ADL%ED%B4%2A%15i%80Z%B8%F%C&peer_id=01234567890123456789&port=6881&uploaded=0&downloaded=0&left=0&compact=0&no_peer_id=0&event=started

The tracker's response to my code:

d14:failure reason32:invalid info hash and/or peer ide

The tracker's response to that string dropped into Chrome's address bar:

d8:completei2e11:external ip13:168.7.249.11110:incompletei0e8:intervali600e5:peerslee

The peer_id is (valid) garbage, but changing it to something sensible (impersonating a widely used client) doesn't change anything.

Like I said, I'm pretty sure I'm pulling the info dictionary out properly and hashing (SHA1) like I should, and the peer id is well formed.

My guess is I'm doing some minor thing stupidly wrong, and would appreciate any help in spotting what it is exactly.

Its kind of hard to guess what code would be pertinent (and there's far to much to just post). However, I'll try and post anything asked for.

EDIT
I wasn't hex encoding the info_hash, which sort of helps.

This is the code that takes the generates URI and try's to fetch a response:

//uri is the above
WebRequest req = WebRequest.Create(uri);
WebResponse resp = req.GetResponse();
Stream stream = resp.GetResponseStream();

Upvotes: 3

Views: 5044

Answers (4)

Encombe
Encombe

Reputation: 2089

There is a error in the URL %-encoding of the info_hash. The leading zeros in the two last bytes of the info_hash has been removed. 

It is:     info_hash=-%CA8%C1%C9rDb%ADL%ED%B4%2A%15i%80Z%B8%F%C
Should be: info_hash=-%CA8%C1%C9rDb%ADL%ED%B4%2A%15i%80Z%B8%0F%0C

When the announce string is dropped into Chrome's address bar it's probably auto-corrected by the browser.

Upvotes: 1

Matthew Scharley
Matthew Scharley

Reputation: 132454

What exactly are you hashing? You should only hash the info section, not the whole torrent file... So basically, decode the file, reencode the info section, hash that.

ie. For the torrent posted, all you should be hashing is:

d6:lengthi241671490e4:name20:so-export-2009-07.7z12:piece lengthi262144e6:pieces18440:<lots of binary data>e

Upvotes: 1

Chris Thompson
Chris Thompson

Reputation: 16871

This isn't an answer to your problem, but it may help for testing.

There are open-source PHP-based torrent trackers out there. They are incredibly inefficient (I know, I wrote a caching mechanism for one back in the day), but you could set up your own local tracker and modify the PHP code to help debug your client as it communicates with the tracker. Having a local client-server setup would make troubleshooting a lot easier.

Upvotes: 1

dtb
dtb

Reputation: 217391

MonoTorrent is a BitTorrent implementation that comes with Mono. In the HTTPTracker class there is a CreateAnnounceString method. Maybe you can compare your implementation with how that method is doing it? (You probably need to hunt down where the AnnounceParameters instance is created.)

Upvotes: 3

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