December 7, 2010

DNS Spoofing

Filed under: Geekery — Tags: , , , — csgillespie @ 3:34 pm

Midway through 2008 a new (serious) internet vulnerability was discovered by Dan Kaminsky. Dan realised that there was a serious flaw in the way the Internet’s domain name system works. This flaw was critical, because it allowed the bad guys to redirect users to malicious sites without detection.

What is DNS

The Domain name system (DNS) converts easy to remember web addresses into their numerical IP counter parts. For example, rather than trying to remember, we just need to remember www.ncl.ac.uk. So when you type an URL into your address bar, you first ask your DNS resolver if it knows the IP address. If the resolver doesn’t know it will ask a DNS server for directions.

In the good old days, no-one worried about security and consequently the DNS protocol was lax. First, all queries to the DNS server were typically through the same port. Second, each request had a unique ID (from 1- 65,536), however, the ids were generated in a completely predicable manner, i.e. 1, 2, 3, ….65536. This meant that an attacker could respond to the resolver (with a fake IP address) before the DNS server got a chance.

The obvious solution to this is for each request to use a (pseudo) random port and id.

Checking your DNS address

All this is old news, and most DNS resolvers should have been updated – however a few haven’t. To test your whether your DNS resolver is doing something sensible, run the DNS spoofing test provided by grc. Essentially, the test makes a few thousand consecutive calls to your DNS resolver and stores the id and port number of the request.

The test takes a few minutes and is completely painless. At the end you get a few graphs showing your port and id numbers.  My ID distribution graph seems to indicate that the resolver that I’m using is issuing random query transaction IDs.

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