Comment by Dylan16807

Doesn't it need to go through the CNAME chain no matter what? If it's doing that, isn't filtering at most tracking all the records that matched? That requires a trivial data structure.

Parsing the answer section in a single pass requires more finesse, but does it need fancier data structures than a string to string map? And failing that you can loop upon CNAME. I wouldn't call a depth limit like 20 "a rather low limit on the number of CNAMEs in a response", and max 20 passes through a max 64KB answer section is plenty fast.