How a Denial-of-Service Attack Works

Investigators are piecing together details about one of the most aggressive computer attacks in recent memory -- a powerful "denial-of-service" assault that overwhelmed computers at U.S. and South Korean government agencies, companies and institutions, in some cases for days.

How does this type of cyber attack work? And how can people make sure their computers are safe?

Here are some questions and answers about the attack.

Q: What is a "denial-of-service" attack?

A: Think about what would happen if you and all your friends called the same restaurant over and over and ordered things you didn't even really want. You'd jam the phone lines and overwhelm the kitchen to the point that it couldn't take any more new orders.

That's what happens to Web sites when criminals hit them with denial-of-service attacks. They're knocked offline by too many junk requests from computers controlled by the attackers.

The bad guys' main weapon in such an attack is "botnets," or networks of "zombie" personal computers they've infected with a virus. The virus lets the criminals remotely control innocent people's machines, which are programmed to contact certain Web sites over and over until that overwhelms the servers that host the sites. The servers become too busy to respond to anything, and the Web site slows or stops working altogether.

It's different from what usually happens when you try to access a Web site. Normally, you just make one request to see the site, and unless there's a crush of traffic from something like a big news event, the servers respond well. Hijacked PCs, on the other hand, are programmed to send way more traffic than a normal user could generate on his or her own.