Slate’s Explainer On BitTorrent Investigations

How’d they know I downloaded Meet the Fockers?

[…] How do investigators find their targets?

They join the networks. In general, the movie and recording industries search out illegal file-sharers by hiring security firms to monitor popular file-sharing communities and report back on any activity that appears illegal. A company like MediaSentry, for example, will hop on to a file-sharing network and start searching for specific files. (The client provides a list of copyrighted material to check up on.) Investigators use customized versions of standard torrent trackers to sniff out the IP addresses of anyone who makes a given file available. Then they’ll take snapshots of all the other files those users offer. They may also try to connect to one of these targets to see if they can make an illegal download.