Interaction of CNAME and A records with regard to TTL

bert hubert ahu at
Fri Jan 17 10:33:30 UTC 2003

On Fri, Jan 17, 2003 at 11:00:51AM +0100, Kross Joachim ICM N PG U ID A 1 wrote:

> and some of the related A records are still in the cache of the name server.
> What is the behavior of the name server: does it return the CNAME record and
> the A records it still has, i.e. an incomplete set with regard to all the
> applicable records that exist, or does it forward the CNAME request to make
> sure it has all the related A records?

This situation is not supposed to happen as all members of the A set are
supposed to have the same TTL. That the dns protocol allows differing TTLs
is sad enough as it it.

So, when we step outside the bounds of how DNS is supposed to work, we may
get some weird things. The PowerDNS recursing nameserver takes the TTL of
the 'first' A record as the TTL of the whole set to determine if any records
are present.  However, subsequent records in that set may have timedout
already, so the actual answer may not contain all TTLs.

The moral of this story is that it is definitely possible for multiple
entries not to all appear at a client nameserver, which holds even with
identical TTLs.



--      Open source, database driven DNS Software           Linux Advanced Routing & Traffic Control HOWTO                         Consulting

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