I have built a program to filter out excess DNS requests so that SimpleDNS does not have to process them. Recently we encountered a problem with excessive type 99 (SPF) requests, which our server does not support. The bulk of these requests are coming from Google and Amazon (3100 from Amazon alone on one morning from dozens of servers). Why these biggies are asking for a deprecated DNS type and not accepting the answer is beyond me, and to make matters worse, they started asking for another deprecated type A6. So I added them both to the filter. It worked like a charm when I tested it with an SPF request (timed out), but there are still queries that are making it through the filter. SimpleDNS responds to SPF requests with:
Request from 22.214.171.124 for ?99?-record for xxxxxxx.com.
-> Header: Not implemented.