Security of Patched DNS

In spite of the availability of DNSSEC, which protects against cache poisoning even by MitM attackers, many caching DNS resolvers still rely for their security against poisoning on merely validating that DNS responses contain some ‘unpredictable’ values, copied from the request. These values include the 16 bit identifier field, and other fields, randomised and validated by different ‘patches’ to DNS. We investigate the prominent patches, and show how attackers can circumvent all of them, namely: – We show how attackers can circumvent source port randomisation, in the (common) case where the resolver connects to the Internet via different NAT devices. – We show how attackers can circumvent IP address randomisation, using some (standard-conforming) resolvers. – We show how attackers can circumvent query randomisation, including both randomisation by prepending a random nonce and case randomisation (0x20 encoding). We present countermeasures preventing our attacks; however, we believe that our attacks provide additional motivation for adoption of DNSSEC (or other MitM-secure defenses).