Will BitTorrent harm my computer?

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Summary: BitTorrent, the peer-to-peer file sharing technology, is often suspect in computer problems. It's not BitTorrent at fault, but the files being shared.

BitTorrent itself is highly unlikely to harm your computer.

However what you download using BitTorrent - well, that's a different story. Some caution is called for.

And I know some of you are asking ... "what's a BitTorrent?"

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Traditionally, when you download a file from the internet, it's really just copying a file from one computer to another. Typically that means you're copying a file from some centralized server to your computer:

The "problems" with that approach include the fact that there's a central server - if it goes down, for example, you can't download the file. Another problem is that everyone is picking up the file from one place - that means that for a popular download, that server better have a very fast or "high bandwidth" connection to the internet that all people downloading at the same time can share:

BitTorrent is one implementation of something called "peer to peer" file sharing. Rather than downloading from a single central server, the download is divided into pieces, and those pieces are downloaded from different computers:

Note that we no longer think of those computers serving up the files as "servers". Rather, they're your peers - other computers pretty much just like yours. Hence the term "peer-to-peer".

By copying peer-to-peer the bandwidth used is spread out across many different paths through the internet. Peer #1 might be down the street, peer #2 might be across the country, and peer #3 might be across the planet. Each will serve up parts of the file as fast as they can, and your BitTorrent client patches those pieces together as they arrive.

Now, how did those peers all get the file in the first place? The same way you're downloading it. And how did they start serving up pieces of the file? By running the BitTorrent client. One important aspect of peer-to-peer file sharing is that as soon as you've downloaded even a piece of a file, your file sharing client can then make that piece available for someone else to download from you. Your computer becomes one of the peers that can serve the file to others.

A more accurate diagram of peer-to-peer file sharing is this:

In this diagram every computer interested in sharing or downloading a particular file is, effectively, connected to every other interested in that same file, sometimes called a "swarm". If a computer doesn't have all the parts of the file, it keeps asking other peers for the missing pieces until it has the complete copy. It can then remain in the swarm, making all the pieces available to any other peers that ask.

So that, in a nutshell, is BitTorrent, and peer-to-peer file sharing. In and of itself, it's just a different technology to download files, and there's absolutely nothing inherently wrong with that. BitTorrent itself will not harm your computer in any way.

But there is a problem.

The problem is not with the technology, but with how it's frequently used.

As you can see from that last diagram, there is no single authoritative site for a particular file being shared on a peer-to-peer network. If you take out any one of the computers in the network, the rest can continue to share and copy the files quite happily.

That means it's extremely difficult to stop a file from being shared.

It also means that it's very difficult to track down all the sites sharing a file.

In turn, that means that sharing illegal or pirated copies of files is much, much easier, because it's much less likely that any single sharer will be tracked down and prosecuted. Possible? Yes. But definitely more difficult.

So, many peer-to-peer networks have a lot of illegal content.

Even so, downloading illegal copies of legitimate software, music or videos won't harm your computer. It's wrong, but it won't harm you.

Spyware and viruses, on the other hand, will.

What many hackers and malware creators have realized is that there are a lot of people downloading illegal software from peer-to-peer networks. Since there's almost no accountability for what gets placed on a peer-to-peer network, it's trivial for them to put up lucrative files that have been infested with malware. For example a file sharing network might offer "Microsoft Office", and it might even be a copy of the latest and greatest copy of Office. But it's quite possible, perhaps even likely, that the person that first shared that copy added to it spyware or viruses in the hopes that people would be tempted by a free copy of an expensive product only to install much more than they bargained for.

And that's what will harm your computer - the malware that often accompanies "free" software available on peer-to-peer networks.

Because, let's face it, who would you complain to if you find that your latest free download infected your machine with spyware? As I said, there's no accountability, and nowhere to turn.

Besides the moral and ethical reasons for not downloading illegal software, the risk of infection is a very practical reason to stay legit.