[dns-operations] Surprising behaviour by certain authoritative name servers

[Greg Choules]

Hi Niall.
Yes, we (Three UK) have seen this before, from AWS DNS and Cloudflare.
Cloudflare say this is intentional because of the way that their CNAME
flattening works.
I don't think it's a protocol violation, but (if you're an ISP and run
large cacheing servers) it can really muck up your cache!

Another thing to watch out for is TTL=0. If the original authoritative TTL
is (say) 60s, but repeated queries show an AA answer with decrementing TTL,
we have noticed that this can decrement to 0.
An AA answer with TTL=0 is legal, but it cannot be cached and it also
messed with a piece of equipment that had issues with 0 TTLs (but that's
just our problem).

I hope that helps.
cheers, Greg

On Tue, 7 Jan 2020 at 12:24, Niall O'Reilly <niall.oreilly at ucd.ie> wrote:

> Hi.
>
> I've had my attention drawn to some surprising behaviour by
> certain authoritative name servers. I'm not sure how best
> to categorize this behaviour, and wonder how some of you
> might view it.
>
> What's surprising is that an authoritative name server
> shows both a decremented TTL value (as if it were answering
> from cache) and the AA flag.
>
> I'm not sure which of the following labels is the best fit
> for this behaviour:
>
>    - normal and expected (but so far outside my experience),
>    - strange but harmless,
>    - downright wrong.
>
> Thanks in advance to whomever is minded to reply.
>
> Thanks especially to Mats Dufberg who, diligently
> investigating what I had mistakenly guessed was a problem
> in zonemaster, took time to identify, and make me aware of,
> what was causing occasional trouble reports.
>
> Niall
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>
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