Internet advertising company DoubleClick was shut down yesterday (Tuesday) by a denial of service (DOS) attack.

The massive attack crippled the company's website and its advertising servers, which distribute Web advertisements to other websites on the Internet, including Techworld's. 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.

Keynote measurements for the period covered by the attacks show that the "base page" - or basic Hypertext Markup Language (HTML) 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, said Lloyd Taylor, vice president of operations at Keynote.

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, said 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 (IP) addresses used by machines on the Internet to route traffic.

The company does not know what the source of the slowdowns was, but the behaviour of the pages is consistent with a DOS attack on DoubleClick, Taylor said. The performance of DoubleClick's servers had returned to normal by late afternoon, East Coast time, Taylor said.

Tthe situation at DoubleClick has improved over the last few hours and staff are taking steps to "resolve the situation permanently," Blum said.