[dns-operations] TTL=0

Greg Choules gregchoules at googlemail.com
Thu Jan 17 22:23:01 UTC 2019


Hello.
My question concerns TTL on answers given to clients by recursive servers
that have obtained those answers from downstream authoritative servers.
If the authoritative operator sets a static TTL of zero on a record, which
the recursive server is then required to fetch, the behaviour of the
recursive server (as I understand and observe it) is to NOT cache the
answer and to send the answer with TTL still equal to zero to the client.

However, if the authoritative operator sets a static TTL of >zero on a
record, which the recursive server is then required to fetch, the behaviour
of the recursive server will be to cache the answer with that TTL and send
the answer to the client, again with that TTL. The recursive server now
starts to count backwards, decrementing the TTL of the answer in cache
until it reaches zero.

Is there ever a case, for cached answers, that the recursive server would
answer the client with TTL=0? Or would that be illegal? RFC1034 states that
records with TTL=0 "should not be cached". Note "should" and not "must".

Any help would be appreciated.
thanks, Greg
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