[dnsdist] not cached long TXT queries.

[Nico]

Hi,
small question, to check what we may be missing.

We have a big traffic increment from dnsdist to clients and
backend-servers (5 times to clients, an 20 times from backends).

The reason we found is a lot of TXT queries for c9-11.com which results in
almost 4k size answer.

Besides this being something abnormal, the question is why dnsdist is not
caching this answers.

Our cachehit dropped from usual 98% to 84%, and we can see that the queries
are forwarded to the backends, (unbound and pdns) which answer from theirs
cache. The ttl is large.
And the backend queries went from 1kqps to almost 5kqps.

So, the traffic form customers to dnsdist is understandable, but not the
traffic from dnsdist to
backend servers.

If it helps, at the same time the metrics for "cache deferred lookups per
poool" and "cache deferred
inserts per pool"  incremented proportionally.

Thanks!!
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