SAN FRANCISCO — On a quiet Sunday in mid-February, something curious attracted the attention of the behind-the-scenes engineers who scour the Internet for signs of trouble. There, among the ubiquitous boasts posted by the hacking collective Anonymous, was a call to attack some of the network’s most crucial parts.

The message called it Operation Global Blackout, and rallied Anonymous supporters worldwide to attack the Domain Name System, which converts human-friendly domain names like google.com into numeric addresses that are more useful for computers.

It declared when the attack would be carried out: March 31. And it detailed exactly how: by bombarding the Domain Name System with junk traffic in an effort to overwhelm it altogether.

There was no way to know for sure whether this was a pre-April Fool’s Day hoax or a credible threat. After all, this was Anonymous, a decentralized movement with no leaders and no coherent ideology, but a track record of considerable damage. The call to arms would have to be treated as one would treat a bomb threat called in to a high school football game. The engineers would have to prepare.