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Internet advertising company DoubleClick was shut down this week by a denial of service attack launched from computers on the Internet, a company spokesperson confirms.

The massive attack began at around 10:30 a.m. Eastern Time Tuesday in the United States and crippled the company's Web site and its advertising servers, which distribute Web advertisements to other Web sites on the Internet. Ripple effects from the attacks were felt across the Internet, as Web pages that display DoubleClick ads struggled to retrieve them from the company's servers, causing "severe disruption" for DoubleClick customers, according to a company statement.

Leading Web sites all experienced significant slow downs during the period covered by the attack, including Web pages for the Washington Post, New York Times, CNet Networks, Nortel Networks, and InfoWorld magazine, according to Keynote Systems, a Web performance measurement company in San Mateo, California.

Keynote measurements for the period covered by the attacks show that the "base page"--or basic Hypertext Markup Language documents--served by those Web sites loaded quickly, but that the "full page," which includes any content the Web page points to, suddenly began to load very slowly, says Lloyd Taylor, vice president of operations at Keynote.

Unidentified Source

DoubleClick's domain name system (DNS) servers were the target of the attack, which came from "outside sources" that it has not identified and lasted for approximately four hours, says Jennifer Blum, vice president of corporate communications at DoubleClick.

DNS is the system of servers that matches up reader-friendly names such as DoubleClick.net with the numeric Internet Protocol addresses used by machines on the Internet to route traffic.

Keynote recorded a threefold slowdown in response time for Web pages beginning at around 7:00 a.m. Eastern Time and ending at 1:30 p.m. Eastern Time. The company does not know what the source of the slowdowns was, but the behavior of the pages is consistent with a DOS attack on DoubleClick, Taylor says.

The performance of DoubleClick's servers had returned to normal by late Tuesday afternoon, East Coast time, Taylor says.

The situation at DoubleClick has improved and staff are taking steps to "resolve the situation permanently," Blum says.