X-Ray: How BitTorrent works

We’re going to need a tracker
Upon opening a '.torrent' -- a file that contains information about the file you want to download -- a user connects to what’s known as a tracker.

Essentially, a tracker is a server that allows the users’ clients to communicate and tell each other which blocks of the files they have (and in turn, how much of the file they’ve downloaded in total), how many peers and seeds there are in the swarm and so on. If a tracker fails for whatever reason, users won’t even be able to start their download.

Recently, there have been two major developments that have attempted to make the BitTorrent protocol more resistant to tracker failure. The first of these is the ability to assign multiple trackers to a torrent, meaning the death of one tracker is no big deal and users can continue their download. The second of these is the ability to have 'tracker-less' torrents. Despite what the name suggests, these torrents do have a tracker, just not a centralised one. Instead, it considers every peer in a swarm a tracker.

Such torrents are not widely supported however, with only three clients (BitComet, Azureus and the original BitTorrent software) allowing for them. Whether a client supports this feature or not, trackers aren’t aware of what files are being distributed -- that’s what an index is for. Unlike the ill-fated KaZaA, the BitTorrent protocol doesn’t feature an in-built search function.

If someone is after a certain file, they have to look around on the internet until they find a torrent for it (assuming one exists). In general, the best place to find a torrent is on a site like TorrentSpy (www.torrentspy.com) or Linuxtracker (linuxtracker.org).

Such sites have torrents available for download and list all relevant details about files, as well as providing users with information about the status of the torrent’s tracker and the amount of seeds and peers currently in the swarm. Some index sites run their own trackers, but most simply host torrents for download.

Due to the nature of the protocol, it’s often difficult to find older files as index sites usually remove torrents as their popularity decreases.

Like most file sharing tools, Bittorrent can be used to download illegal files, and despite what you may have heard, the BitTorrent protocol isn’t impenetrable to prying eyes. While trackers might not reveal the specifics of what’s been downloaded by a particular user, they certainly keep a log of IP addresses. Not only that, most clients allow you to easily see who you’re connected to. For this reason, some BitTorrent users employ programs such as PeerGuardian (phoenixlabs.org) to ensure their they retain their privacy.