[dnstap] Caching status of Client Response messages

[Paul Ebersman]

matt> The request was for a field to indicate when a CR is filled from
matt> cached information or results in iterative queries.
[...]
matt> The second approach is to use a bit field. Since protobufs don't
matt> have a bit or flag type this would most likely be implemented as a
matt> uint that name servers and consumers would need to do bit math
matt> on.  The same three items would be defined in the bit field, with
matt> some other bits reserved for future expansion, and anything
matt> remaining available for implementation-specific values.

matt> Since there were no large recursive operators in the conversation
matt> we weren't sure if there was a use case for consuming the more
matt> complex data.

As a former large recursive operator who did DNSSEC validation, the 2nd
option would be very handy for giving an idea of what mix of cached and
iterative data goes into answering queries. Would also give me some idea
of what damange folks doing lots of short TTLs but DNSSEC signing were
doing to me in terms of cache misses vs hits.