Massive distributed denial-of-service attacks on Spamhaus this week focused widespread attention on the huge security threats posed by millions of poorly configured Internet Domain Name System (DNS) servers.

The attacks on Spamhaus that began March 19 were apparently launched by a group opposed to the Geneva, Switzerland-based volunteer organization's antispam work.

Several security firms described the attacks on the organization as the largest -- by far -- ever publicly known DDoS attacks to date.

In DDoS attacks, hackers typically try to take down a network by directing huge volumes of useless traffic to it. The traffic is usually generated using large botnets of compromised computers.

Large DDoS attacks have typically tended to involve between 4 gigabits per second to 10Gbps of traffic.

The Spamhaus attacks involved traffic volumes that reached a staggering 300Gbps -- said to be three times larger than the largest DDoS traffic seen to date and magnitudes greater than the traffic involved in a majority of past denial-of-service attacks.

The perpetrators behind the attack employed the well-known but infrequently used method DNS reflection to generate the huge stream of DDoS traffic directed against Spamhaus.

DNS servers are used primarily to look up and resolve domain names such as www.computerworld.com and www.idg.com to their corresponding IP addresses. If a DNS server does not have the domain information in its database or cache, it queries other nearby DNS servers for the information.

Ideally, DNS servers should be configured only to handle look-up requests coming from within a specific domain or IP address range. So a DNS server belonging to an ISP should handle only requests coming from within its IP address range.

In reality, however, millions of DNS servers are configured by default to be open DNS resolvers that accept and respond to queries from outside their own domain, making them vulnerable to exploitation by attackers because virtually anyone on the Internet can use an open DNS server to handle genuine or malicious queries.

For instance, to generate DDoS traffic, the attackers behind the Spamhaus attack sent queries with a spoofed source address to tens of thousands of open DNS resolvers, said Matthew Prince, CEO of CloudFlare, which has been helping Spamhaus deal with the recent attacks.

The lookup requests were made to appear as if they came from Spamhaus. So the responses to the requests from the tens of thousands of open DNS resolvers were sent to Spamhaus, generating a huge volume of traffic.

To magnify the volume of traffic, the attackers crafted the look-up queries in such a manner as to get each open DNS server 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 300Gbps 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 situation 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.

DNS server operators also need to have egress filtering controls in place to ensure that the DNS traffic leaving their networks originated from inside their network, he said.

The Open DNS Resolver Project also calls on DNS server operators to consider implementing rate-limiting software to prevent the sort of traffic amplification that was used in the Spamhaus attacks.

"There are things that need to get cleaned up. That is why we need some awareness of the problem," Smith said.

Jaikumar Vijayan covers data security and privacy issues, financial services security and e-voting for Computerworld. Follow Jaikumar on Twitter at @jaivijayan, or subscribe to Jaikumar's RSS feed . His e-mail address is jvijayan@computerworld.com.