The various measures "seem to have helped, since the later attacks did not affect our systems as much as the first one," CSL and EIS said. They added that to provide further protection, they will "significantly increase and spread" the number of Joker.com name servers and take other undisclosed steps.

CSL and EIS said that about 15 percent of the Joker.com domains were affected by the attacks. According to Netcraft Ltd., an Internet performance monitoring company in Bath, England, more than 550,000 domains are registered with Joker.com.

Attacks against DNS servers have been rare until now but are viewed as being dangerous because they can bring down large numbers of Web sites.

Earlier last month, VeriSign Inc. said that about 1,500 organizations worldwide had been hit this year by unknown hackers who used botnets and DNS servers to launch denial-of-service attacks. In those cases, though, the DNS servers were used to amplify the effects of the attacks and weren't really targets themselves.