Bittorrent is a swarm file transfer system in which downloaders help share pieces of a file with other downloaders in a tit-for-tat exchange. It helps alleviate network load on servers that have popular files. The LNX-BBC project offers its ISO images and snapshot tarballs as bittorrent files, and encourages others to help share and mirror them with bittorrent.
This document will explain how to set up bittorrent for various situations, and help the reader understand the way in which bittorrent functions.
Bittorrent isn't your typical "put file up on server, wait for people to download" sort of protocol. It's more of a peer to peer protocol, but one that's designed for reliability and load balancing.
The .torrent files contain the location of a tracker, which is a brokering server that connects users with other users who have pieces of files that can be swapped. It deals entirely in sha1 hashes, and thus has no concept of "filenames". The contents of the files never pass through the tracker.
The key is that users grab pieces from each other. This means that to put a file up available for download, you actually start a downloader process, but with the file already completely in place! This can be confusing when you are used to HTTP and FTP servers, but realize that once you have completed a download, the tracker will know that you have all pieces of the file available for others to grab.
This is why often you'll see torrent sites admonishing users to keep their clients open after downloads are complete. The more people do this, the more widely available a file is. BitTorrent rewards popularity, unlike HTTP and FTP servers, which punish popularity by their susceptibility to bandwidth bottlenecks.
The clients decide whether or not to provide pieces of a file to another client by using a tit-for-tat strategy. That means that the more you upload to people, the faster your download gets. It also means that your client will tend toward the closer (in the network sense, i.e. faster) peer.
We here at the LNX-BBC project have already set up and installed a tracker, and all our .torrent files point to it. You can see a brief statistics page by visiting http://www.lnx-bbc.com:3623/. All you need to do to help us mirror is to run a downloader! How you choose to do that depends on your preference.
The stock BitTorrent package comes with a few different python scripts for downloading: ptdownloadgui, which uses wxgtk (but requires a ridiculously recent version, so beware); btdownloadcurses, which uses a nice curses readout; and btdownloadheadless, which spits to stdout, but falls silent if started in the background. There are also "launchmany" versions of these programs available, which we will discuss shortly.
The most user-friendly way to use bittorrent is to follow the instructions in the docs for associating the downloaders with the application/x-bittorrent MIME type. This way, you can just select a .torrent file in your Web browser and let the MIME handler take care of the rest. You'll see a torrent window pop up, and start connecting to peers. Couldn't be simpler, right?
You may also wish to run your downloader in a screen session or something, in which case you could run the downloader at the command line:
$ btdownloadcurses.py --url http://www.lnx-bbc.com/images/bbc-2.0.iso.torrent
But if you want to really help out, you'll mirror more than just one file. For this, there is the launchmany program. The launchmany versions of the programs accept as their parameters the path to a directory full of .torrent files. They regularly poll the directory for new or removed .torrents and update their actions appropriately. As you may guess, this makes them useful as daemons.
For starters, let's look at the curses version, which I typically use when I'm keeping my downloads in a screen session and want to be able to check up on their status.
[zork(~/torrent)] wget http://www.lnx-bbc.com/images/bbc-2.0.iso.torrent [zork(~/torrent)] wget http://www.lnx-bbc.com/images/bbc-2.0-src.iso.torrent [zork(~)] btlaunchmanycurses.py ~/torrent/
Now I end up with a curses readout showing me the download progress, as well as the upload speed and running totals of my bandwidth use. I can throw in .torrent files or take them out of my ~/torrent directory, and torrent downloads will begin and end as needed.
But for real long-term use, it's best to keep things headless. To this end, we have btlaunchmany.py. This script will fall silent if run in the background, and is ideal for keeping a mirror going long-term.
For one of our Debian GNU/Linux systems, I have an init script that I use. It ensures that bittorrent starts and stops at the appropriate runlevels, and keeps these files available in the client space.
For yet another of our systems, we use a simple crontab entry to make sure that bittorrent is run on startup:
@reboot nohup btlaunchmany.py /var/www/lnx-bbc.com/images/
This method may work best if you lack root access to the machine you wish to use.
You may wish to set up regular cron jobs to wget all files of the mime type application/x-bittorrent, or to grab all files ending in .torrent from http://www.lnx-bbc.com/images/ or http://www.lnx-bbc.com/builds/. Whatever you do, it is helpful to make sure that the .torrent files in your launchmany directory are up-to-date, so that you are serving recent files.
Absolutely not. You only transmit the files for which you have chosen .torrent files. We here at the LNX-BBC project only put up images that have passed at least a cursory test of license integrity, similar to that of Debian's. All of the torrents we provide are freely modifiable and redistributable.
The tracker attempts to compensate for this by notifying your machine to connect to the downloader. This means that you can't upload anything to anyone who is also behind NAT, but you can still do your part.
No. The .torrent files we provide have the lnx-bbc tracker hard-coded into them. You just need to download in a "file and forget" fashion, and you'll be helping Join Us Now And Share The Software!
The default python for Debian is 2.1 in Testing, but the latest packages seem to have been built against 2.2. Fortunately there's no real compatability issue. You can safely solve this one by doing:
$ cd /usr/local/lib/python2.1/site-packages/ $ ln -sf /usr/lib/python2.2/site-packages/BitTorrent/
This is the most robust solution, since it won't interfere with any future upgrades (unless you muck about in /usr/local/lib/python2.1 a lot).