VeriSign has warned against a new type of denial-of-service attack that is much more powerful than previous methods.

Traditional attacks work by sending thousands of queries to overwhelm victim PCs. The new attack methodology works by sending requests to domain name servers naming the target of the attack as the place to send details for a DNS query.

This makes the attacks almost impossible to block, as they originate from thousands of machines and there is no single program or bot which can be blocked. 

"These attacks have been significantly larger than anything we've seen," said Ken Silva, VeriSign's chief technology officer.

The attacks first started in December 2005, reaching a peak in January before dying off, Silva said. He added that VeriSign has recorded 1,500 such attacks that have brought down commercial websites.