Hackers have struck DNS servers at Network Solutions with a denial-of-service attack, resulting in a brief performance degradation for customers. The attack, which took place on Monday, is the second DDoS attack on a domain-name registrar within a couple of days, following an attack on German company Joker.com.

The attacks were targeted at the Network Solution’s WorldNIC name servers and resulted in a service degradation for about 25 minutes before the server was restored to normal, a spokeswoman for the company said.

According to internet performance-monitoring company Netcraft, more than 550,000 domains are registered with Joker. "Any of those domains that use Joker's DNS servers are likely to be affected," Netcraft said.

Attacks against DNS servers, though relatively rare until now, are considered especially critical because of their potential to bring down large numbers of webstes.

In October 2002, all 13 of the Internet's root DNS servers were victims of a massive DDoS attack that raised concerns about the Internet’s infrastructure but did little damage otherwise.

Just a week ago, VeriSign said that about 1,500 organisations worldwide had been attacked earlier this year by unknown hackers who employed botnets and DNS servers to launch a particularly devastating form of denial of service.

In the attacks described by VeriSign, DNS servers were used to amplify the affects of denial-of-service attacks and were not really targets themselves. But security experts said they believe that DNS servers could just as easily become targets.