AT FIRST, IT LOOKED LIKE typical network congestion. So the system administrators weren't too concerned when TypePad blogs and LiveJournal social networks flickered like a light bulb in a faulty socket. But 15 minutes later, at 4 pm on May 2, 2006, the sites went dark, and so did the mood at Six Apart, the company that owns them. In the blink of an eye, 10 million blogs and online communities disappeared. "It looked like the servers had freaked out," CEO Barak Berkowitz recalls. Flash floods of data thundered into one network port, stopped inexplicably, then reappeared to overwhelm another. The engineers pored over logs, desperately looking for a cause. After an agonizing hunt, they found it: a distributed denial-of-service attack, or DDoS. Six Apart's servers had been inundated with so ma...