According to a story by the Washington Post, last Monday, the DNS root servers that provide fundamental DNS support for global Internet operations suffered a Distributed Denial of Service (DDoS) attack. The report said that the DDoS attack struck all 13 of the root servers about 5:00 P.M. Monday.
The root servers provide crucial tables that point to the next-level DNS servers for all .com, .net, .org, .edu, .gov, and several other domain suffixes. The attack lasted about 1 hour, and most users probably didn't notice any trouble; however if the attack had lasted longer, it would have affected an exponentially growing number of users because ISP-level DNS servers would have began to fail during DNS lookups. According to the Post story, the FBI didn't speculate on who might have caused the attack, but the bureau is looking into the matter.