How DNS Cache Poisoning Works

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Advantage: Attacker

But the dynamics of the race have been dramatically altered in favor of the attacker with this new vulnerability because a security researcher figured out a way to eliminate the narrow time window. This is accomplished by rapidly firing questions at the caching server that an attacker knows the server will not be able to answer. For instance, an attacker can ask where 1q2w3e.google.com is, knowing a caching server is unlikely to have such an entry. That provokes subsequent questions from the caching server and creates millions of opportunities to send fake answers.

Instead of only one race the attacker can have millions, creating more chances to guess the right values for query parameters and making the attack dangerous. In fact, it was demonstrated that open source DNS servers could be compromised in 10 seconds.

Poisoning an entry for 1q2w3e.google.com is not useful since no one will ever request that domain, but this is where the last part of the attack comes into play: In the fake answers the attacker also points the caching server to a fake name-server for the domain the attacker wants to compromise. The caching server stores both of these pieces of information.

Since the attacker now controls the name server for the domain, every subsequent query for the domain will be directed to the attacker's server. This means the attacker now controls addressing for all the subdomains for the domain: www.bigbank.com, mail.bigbank.com, etc. This is extraordinarily powerful; any request for any subdomain can be directed to a server of the attacker's choosing.

To address these problems it was decided that the UDP port used for a query should no longer be the default port 53, but rather a port randomly chosen from the entire range of UDP ports (less the reserved ports). UDP source port randomization, or UDP SPR, as it is called, makes it harder for an attacker to guess query parameters since now both the 16-bit query ID and as many as 16 additional bits for the UDP port must be correct, for a total of up to 4 billion combinations.

But enterprises discovered their DNS servers were situated behind various devices that provide network address translation (NAT). Most NATs de-randomized the UDP ports used by the DNS server, rendering the new fix less effective. IT managers were also not enthusiastic about opening up the full range of UDP ports in their firewalls. To make matters worse, another security researcher demonstrated that it was still possible to poison a DNS server even with the protection afforded by randomization across 64,000 UDP ports.

Now what?

It's time to consider alternative means of securing the DNS. UDP source port randomization is a useful defense, but a balance needs to be struck between the protection afforded the DNS by UDP SPR and the exposure created by opening a range of ports or degrading firewall performance. Secure modes of operation for DNS servers, such as switching to a TCP connection when potential attacks are detected, are another useful defense.

Additional defenses are needed when an attacker gets lucky and guesses the necessary parameters to spoof a query response. This means DNS servers need to get smarter and analyze query responses so potentially harmful information can be discarded from fake answers sent by an attacker.

Bob Halley is a principal architect at Nominum.

This story, "How DNS Cache Poisoning Works" was originally published by Network World.

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