To magnify the volume of traffic, the attackers crafted the look up queries in such a manner as to get each open DNS servers to respond with much larger volumes of data than normal, Prince said.
Denial-of-service attacks that take advantage of open DNS resolvers are not new.
As far back as in 2006, more than 1,500 organizations around the world were hit by a series of similar attacks, prompting wide concern from security experts.
Then, as now, many security experts warned that ISPs and others operating DNS servers must ensure that their systems are properly configured to prevent attacks such as the one launched against Spamhaus. The problem remains as pervasive as ever despite the warnings, experts note today.
The Open DNS Resolver Project , an effort by a group of security experts to draw attention to the issue, estimates that there are currently about 27 million DNS servers that are open resolvers. About 25 million of those pose a significant threat, according to the project's website.
According to Prince, barely 100,000 of the open resolvers were used to direct 300 Gbps of traffic against the organization. "What's spooky here is that only a tiny fraction of the open resolvers were used," he said. The attackers could easily have co-opted more DNS servers, Prince noted.
"This is a situations where some configuration changes on the DNS server side can help prevent the attacks," said Alex Cox, a principal security researcher with RSA Security's FirstWatch team.
But the required changes are difficult to get without a broad collaboration among ISPs. "The problem with a DNS attack is you can't really turn your DNS servers off," without causing widespread disruption, Cox said. "Once this thing blows over it will be interesting to see how some of the folks whose infrastructure was used, will respond."
The perpetrators of this week's attacks knew that Spamhaus had a good infrastructure in place to deal with denial of service attacks and therefore had to do something really big, said Dan Holden, director of the security and engineering response team at Arbor Networks.
Such attacks are not fully defendable but can be mitigated by ensuring that DNS servers are configured properly, he said. "The good news is that these open DNS resolvers will get a lot more visibility," following the attacks he said. "So hopefully the issue will get fixed."
Several standards are readily available to help ISPs and others operating DNS servers to configure systems to ensure they respond only to requests from their own users, said Mike Smith director of the customer security incident response team at Akamai.