nullprogram.com/blog/2011/02/19/
You can skip my explanation and download the tool here,
git clone git://github.com/skeeto/btstrip.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> };
$$decoded{'announce'} = "http://127.0.0.1/";
delete $$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.