Online attackers have briefly disrupted service on at least two of the 13 "root" servers that are used to direct traffic on the Internet.

The attack, which began Tuesday at about 5:30 a.m. Eastern time, was the most significant attack against the root servers since an October 2002 DDoS attack, said Ben Petro, senior vice president of services with Internet service provider Neustar. Root servers manage the Internet's DNS, used to translate Web addresses such as Amazon.com into the numerical IP addresses used by machines.

The attack appeared to have been launched by a group of compromised PCs, called a botnet, Petro said.

"Two of the root servers suffered badly, although they did not completely crash; some of the others also saw heavy traffic," said John Crain, chief technical officer with the Internet Corporation for Assigned Names and Numbers (ICANN).

The two hardest-hit servers are maintained by the U.S. Department of Defense and ICANN, he added.

The botnet briefly overwhelmed these servers with useless requests, causing them to occasionally hang, but did not disrupt Internet service, Petro said. By 10:30 a.m., Internet service providers were able to filter enough of the traffic from the botnet machines that traffic to and from the root servers was essentially back to normal.

Compared to other DDoS attacks, this one did not have a lot of firepower. Petro said the bandwidth of the attack could be measured in megabytes, as opposed to the gigabyte-level attacks that are now frequently seen. "It was a small attack, but it was focused and targeted at the roots," he said.

It is unclear where the attack originated, Crain said. "The effect of this traffic on the end users was negligible," he added.

Petro said engineers are still "scratching their heads" about the reason for the attack. Although some DDoS attacks are launched for political reasons or to extort money from Web site owners, this one appeared to have no such purpose, he said.

Crain ventured a guess as to the attack's effect, however. "I suspect that the largest effect of the DNS traffic was to deprive some engineers of sleep and generate press," he wrote.