We describe a novel, practical and simple technique to make DNS queries more resistant to poisoning attacks: mix the upper and lower case spelling of the domain name in the query. Fortuitously, almost all DNS authority servers preserve the mixed case encoding of the query in answer messages. Attackers hoping to poison a DNS cache must therefore guess the mixed-case encoding of the query, in addition to all other fields required in a DNS poisoning attack. This increases the difficulty of the attack.

We describe and measure the additional protections realized by this technique. Our analysis includes a basic model of DNS poisoning, measurement of the benefits that come from case-sensitive query encoding, implementation of the system for recursive DNS servers, and large-scale real-world experimental evaluation. Since the benefits of our technique can be significant, we have simultaneously made this DNS encoding system a proposed IETF standard. Our approach is practical enough that, just weeks after its disclosure, it is being implemented by numerous DNS vendors.