Spamhaus DDoS Spotlights DNS Server Security Challenge

When the Spamhaus Project was recently hit with a tsunami of distributed denial-of-service attack (DDoS) traffic, the impact of the incident caused a stir in the world of network security.

The attack took advantage of open DNS resolvers, allowing the attackers to control the flow of massive amounts of traffic. At its height, the attack peaked at some 300 gigabits per second, sparking a renewed focus for some on developing plans for mitigating the risk of similar situations.

"Attackers can amplify their DDoS attacks by spoofing requests to open DNS servers from their victims' IP addresses," explains Tom Cross, research director at Lancope. "The DNS responses that get sent back to the victim can be much larger than the requests that the attacker is generating, so this makes it easier for the attacker to consume the victims' bandwidth. These attacks can generate so much traffic that they will easily overwhelm the connectivity between the victims' data center and the Internet."

The right way to mitigate them is by routing traffic to a scrubbing center on the Internet that can remove unsolicited DNS responses while allowing the rest of the traffic to flow through to the destination, he adds.

In a FAQ about the situation, Spamhaus -- which tracks the world's spam organizations -- advised organizations to police traffic to ensure that traffic with spoofed sending addresses does not leave their networks. Referring to the attacks as "a call to action for the Internet community as a whole," the group also urged organizations to lock down any open DNS resolvers.

It is fair to say that despite a number of recommendations having been made during the past decade, several million recursive name servers remain configured in a generally promiscuous mode, observes Ken Silva, senior vice president of cyberstrategy within ManTech International's Mission Cyber & Intelligence Solutions Group.

"Best Current Practice 38 [BCP38], a set of recommendations which would limit the effectiveness of some DDoS attacks, was written 13 years ago, but still is not as widely implemented as it should be," he says. "Also, several best practice configuration guides outline good DNS server hygiene, but many remain configured poorly. This goes to the heart of the problem in dealing with this issue in general...in order to properly combat the threat, it involves participants from ISPs to enterprises to small network operators. And that's a lot of people and a lot of coordination."

The challenge of addressing DDoS, however, is not going away. On March 28, American Express experienced this firsthand when a DDoS attack struck its site for about two hours. The attack has been attributed to the Izz ad-Din al-Qassam group, which is believed to be behind a wave of attacks against financial institutions in the U.S. that began last year. During the past several months, the group has been tied to attacks against Bank of America, Citibank, and JPMorgan Chase.

There are several different kinds of denial-of-service attacks, each requiring a different kind of defense, notes Cross.

"The kind of attack that people usually think of when they think of DDoS is a huge traffic flood that overwhelms the bandwidth of the victim's Internet connectivity," he tells Dark Reading. "This kind of attack needs to be stopped at the service provider or out in the Internet, before the traffic reaches the victim's final hop.

"Many companies offer network-based scrubbing services that can block large traffic floods while letting legitimate traffic through," he adds. "However, these network-based solutions are not appropriate for every kind of attack. Some denial-of-service attacks can impact the availability of services without creating the sort of traffic flood that would trigger a network-based scrubbing solution."

Equipment such as firewall and intrusion detection systems can block certain classes of traditional DDOS, such as SYN Floods, but they usually don't have the features needed to deal with application-layer DDoS and other attacks hitting networks today, Cross says.

"Enterprises should not attempt to outgun the threat," advises Silva. "It's too costly, and the threat has more resources available to them. First and foremost, an enterprise needs to engage services that protect them even before the threat reaches them.

"Second, have a plan for what to do when you are under attack. Even a service provider could wind up severely degraded enough that a significant amount of traffic makes it through. Or the advisory can adapt to your countermeasures. Expect that could happen and have a plan. Lastly, make sure that you are not unwittingly participating in these attacks."

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