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February 6th, 2009

New DNS Amplification Method Allows Distributed Denial Of Service Attack

A new DDoS attack, known as DNS Amplification, has been used sporadically since December. It started getting attention last month when ISPrime, a small New York Internet service provider, started getting hit hard with what’s known as a distributed denial of service (DDOS) attack. The attack was launched by the operator of a pornographic Web site who was trying to shut down a competitor, hosted on ISPrime’s network, according to Phil Rosenthal, the company’s chief technology officer. The fight between two pornographic Web sites turned nasty when one figured out how to take down the other by exploiting a previously unknown vulnerability in the Internet’s Domain Name System (DNS).

The attack on ISPrime started on the morning of Sunday, Jan. 18. It lasted about a day, but what was remarkable was that a relatively small number of PCs were able to generate a very large amount of traffic on the network. One day later, a similar attack followed, lasting three days. Before ISPrime was able to filter the unwanted traffic, attackers were able to use up about 5GB/second of the company’s bandwidth.

With a bit of work, Rosenthal’s staff was able to filter out the hostile traffic, but in an e-mail interview he said that the attack “represents a disturbing trend in the sophistication of denial of service attacks.”

According to Don Jackson, director of threat intelligence at security vendor SecureWorks, we may soon see a lot more of these DNS Amplification attacks. Late last week, the botnet operators, who rent out their networks of hacked computers to the highest bidder, started adding custom DNS Amplification tools to their networks.

One of the things that makes a DNS amplification attack particularly nasty is the fact that by sending a very small packet to a legitimate DNS server, say 17 bytes, the attacker can then trick the server into sending a much larger packet, about 500 bytes, to the victim of the attack. By spoofing the source of the packet, the attacker can direct it at specific parts of his victim’s network.

Jackson estimates that the 5GB/second attack against ISPrime was achieved with just 2,000 computers, which sent out spoofed packets to thousands of legitimate nameservers, all of which started flooding the ISPrime network. ISPrime’s Rosenthal says that about 750,000 legitimate DNS servers were used in the attack on his network.

One of the things that makes the attack particularly nasty is that it’s very hard to protect against. The only real defense is to ask upstream provider to filter the malicious traffic.  It’s not something the victim can do by themselves.

DNS-OARC has published some information on how to prevent BIND DNS servers from being used in one of these attacks. Microsoft was unable to immediately provide information on how to mitigate this particular attack on its own products.
Here can be found technical analysis of the DNS Amplification attack by SecureWorks.

Credit: Robert McMillan, IDG News Service, PC World

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