Detecting DNS cache poisoning

I spoke with a good friend of mine last week about his recent trip to NANOG. While he was there, he listened to a talk about detecting DNS cache poisoning. However, this was detection at the authoritative server, not at the cache itself. This is a bit different than detection at a cache because most cache poisoning will happen outside of your domain.

I initially wrote about the Kaminsky DNS bug a while back, and this builds somewhat on that discussion. When a cache poisoning attack is underway, the attacker must spoof the source IP of the DNS response. From what I can tell, this is because the resolver is told by the root servers who the authoritative server is for the domain. Thus, if a response comes back from a non-authoritative IP, it won’t be accepted.

So let’s look at the attack briefly. The attacker starts requesting a large number of addresses, something to the tune of,, etc. While those packets are being sent, the attacker sends out the responses with the spoofed headers. Since we are now guessing both the QID *and* the port, we miss a lot because the port is incorrect.

When the server receives a packet on a port that is not expecting data, it responds with an ICMP message, “Destination Port Unreachable.” That ICMP message is sent to the source IP of the packet, which is the spoofed authoritative IP. This is known as ICMP backscatter.

Administrators of authoritative name servers can monitor for ICMP backscatter and identify possible cache poisoning attacks. In most cases, there is nothing that can be done directly to mitigate these attacks, but it is possible to identify the cache being attacked and notify the admin. Cooperation between administrators can lead to a complete mitigation of the attack and protection of clients who may be harmed.

This is an excellent example of the type of data you can identify simple through passive monitoring on your local network.

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One Response to “Detecting DNS cache poisoning”

  1. Michael Costello says:

    The request for the non-existent host can be sourced from any IP. The caching server then goes through the recursive lookup, eventually asking the authoritative server for the answer. Here, destination IP is the authoritative server, destination port 53, and source IP is the caching server and source port 53 if you’re really insecure or something (highly) random (if you’re patched). This is where the attacker needs to start spoofing the authoritative answer. The ICMP backscatter will be caused by the attacker guessing the source port incorrectly and will be sent to the authoritative server because that is the address that has been spoofed.

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