Facebook's vanishing act explained

On Monday, Facebook vanished from the Internet, along with the company's other platforms, Instagram and WhatsApp. For the more than five hours during which they were down, Facebook potentially lost tens of millions of dollars in revenue. 

According to Facebook, the problems started during a routine bit of maintenance on the company's internal backbone. Facebook uses larger data centers to hold all the content on its websites and apps, and smaller servers that handle Domain Name System (DNS) queries. During some maintenance on Monday, a particular tool, instead of taking one line down for maintenance, sent out a command to take down every line.

Shiv Panwar, professor of electrical and computer engineering at NYU Tandon and a researcher at NYU Wireless, explains that DNS is hierarchical—if a DNS query runs into a problem, it will check a wider range of servers to see if it can locate the information it needs. It's the equivalent of switching from a local phonebook to a regional one.

Panwar suggests that the tool may have been designed to take down a particular route to test how the rest of the backbone picked up the slack. In other words, a tool designed to test the network's redundancy.