[JohnStrange]

No it's not the tracker, it also works for magnet links and people get letters for downloading.

There are companies who join the download swarm and register all other downloading parties. That's very easy with bittorrent, since the protocol is (originally) designed for fast download sharing without any regard to anonymity or pseudonymity.[1] The process is not reliable for providing evidence of copyright infringement, though, and the German system mostly works by scare tactics of lawyers - many people don't want to risk a lawsuit even if they could win it.

[1] https://torrentfreak.com/thousands-of-spies-are-watching-tra...

[zaarn]

These companies are the scum of the scum, tbh, I recall I once got a letter claiming I must pay about 6000€ for illegally downloading "Debian 5 Linux Netboot ISO" and "Ubuntu 12.04 x86 Full ISO" or something along those lines.

They sent some awfully scary letters for what amounts to legally obtaining an ISO file.

[notzorbo3]

I used to run an abandoned warez site when I was young. I received a lot of cease and desist letters from "lawyers". They usually failed to identify the infringing material, failed to show they had the right to act on the copywriters behalf and a staggering amount of them confused trademark infringement with copyright infringement. Also, every last one I received via email. Yeah, right, like that's going to hold up. I ignored all of them and never got even so much as a follow up.

In other words, such things are considered low-hanging fruit by these companies. Just throw it out there and see what sticks.

[zaarn]

Luckily the german system is less strict than the DMCA, you can fact-check any letters you get, you only need to act if you know (for certain) it's illegal

[floatboth]

How do these companies know who to contact for a given IP address? Do ISPs just cooperate with them?

[zaarn]

WHOIS records.

Though that only tells you the RIPE member for it.

They might have still leased the IP elsewhere (which is totally informal, if you operate a IP range all you need is a letter saying you can do it with the owner's signature)