Resurrecting a dead torrent tracker and finding 3M peers

> Is this legal?

Why wouldn't it be? You're not actually hosting a tracker in this case, only looking at incoming connections. And even if you do run a tracker, hard to make the case that the tracker itself is illega. Hosting something like opentrackr is like hosting a search engine, how they respond to legal takedown requests is where the crux is at, and whatever infra sits around the tracker, so police and courts can see/assume the intent. But trackers are pretty stupid coordination server software, would be crazy if they became illegal.

Because knowingly helping people commit crimes generally counts the same as committing the crime yourself. I.e. federally in the U.S. under 18 USC 2a https://www.law.cornell.edu/uscode/text/18/2 The software you're running being "simple" isn't a defence for doing illegal things with it - like aiding others commit crimes.

There are a few internet/copyright safe harbor provisions (in the US) that might maybe (probably not) make it not a crime, I don't know, I'm not a lawyer. But your general thought when you hear "helping someone else commit a crime" ought to be "that's probably a crime itself".