US-CERT: NAT/PAT Affects DNS Cache Poisoning Mitigation
US-CERT released a Current Activity entry and a Vulnerability Note on July 8, 2008 regarding deficiencies in DNS implementations. These deficiencies could leave an affected system vulnerable to cache poisoning. Technical details regarding this vulnerability have been posted to public websites. Attackers could use these details to construct exploit code. Users are encouraged to patch systems or apply workarounds immediately.
A number of patches implement source port randomization in the name server as a way to reduce the practicality of cache poisoning attacks. Administrators should be aware that in infrastructures where nameservers exist behind Network Address Translation (NAT) and Port Address Translation (PAT) devices, port randomization in the nameserver may be overwritten by the NAT/PAT device and a sequential port address could be allocated. This may weaken the protection offered by source port randomization in the nameserver.
US-CERT encourages users to consider one of the following workarounds:
- Place the nameserver outside of the NAT/PAT device in the network infrastructure.
- Configure the NAT/PAT device to perform source port randomization.
- Configure the NAT/PAT device to preserve the source port assigned by the nameserver.
Additional information can be found in US-CERT Vulnerability Note VU#800113.
More information will be provided as it becomes available.