Torrent File Strip

You can skip my explanation and download the tool here,

git clone git://

My main computer recently stopped working for me, and, until I have a replacement, I've been using my wife's old 2004-era desktop, with only 500MB of memory. That has required me to make use of a lighter-weight BitTorrent client than I have in the past: rTorrent.

Because rTorrent's interface is built on ncurses, when combined with GNU Screen it behaves very much like a daemon. I have configured it to watch a certain directory for new torrent files. When I want to start downloading a new torrent, I just put the torrent file there and rTorrent gets to work on it. Share this watched directory on a network, and rTorrent becomes a network service.

# rTorrent configuration
directory = /torrents/
session = /torrents/.session/
schedule = watch_directory,5,5,load_start=/torrents/watch/*.torrent
schedule = untied_directory,5,5,stop_untied=
schedule = tied_directory,5,5,start_tied=

Unfortunately, the rTorrent documentation has not been kept up to date, and appears to be inaccurate. I prefer to rely completely on the distributed hash table (DHT) rather than use normal trackers (so long as the torrent is free of DRM). My last BitTorrent client allowed me to do this, but the documented rTorrent option for doing this, enable_trackers, doesn't seem to work at all.

enable_trackers = no

My current workaround is to strip away all of the tracker information from the torrent file before giving it to rTorrent. It's a Perl script, with all the hard work being done by the Bencode module. Stripping out the trackers is trivial,

my $decoded = bdecode do { local $/; <STDIN> };
$$decoded{'announce'} = "";
delete $$decoded{'announce-list'};
print( bencode $decoded );

The encoding of a torrent file is of critical importance, due to the info_hash: the hash of all of the torrent data and metadata, which uniquely identifies that torrent. If any aspect of the info field in the torrent file changes, you get a different hash and will not be able to participate in the original torrent. The reason the above code guaranteed to work properly is because, for any possible bencode data structure, there is exactly one possible way to bencode it.

There are four data types in the bencode format: integers, byte strings, lists, and associative arrays. Integers are encoded in ASCII as base-10 (making it unaffected by endianess). Byte strings are stored as an integer indicating its length, followed by the literal string itself. Lists are stored as a list of objects in order, with sentinel. And associative arrays are stored as a list of key pairs. Most importantly, those key pairs are stored in alphabetical order, enforcing a single encoding for any given associative array.

The simple code above only works on stdin to stdout, but it would be more convenient to edit torrent files in place. So I buffed it up a bit in my working version. It has two command line switches: one for operating on the file in-place (-i), and one for setting the tracker URL manually, rather than inserting a dummy value (-t). It also strips out some of the extra, optional fields, cutting the torrent file down to its minimal size.

Once you've installed the Bencode module, drop that script in your PATH somewhere and you're ready to go.

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Chris Wellons (PGP)
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