Configuring different TTLs in multiple RRs for the same domain name, TYPE, and CLASS

Tony Finch dot at
Thu Mar 24 22:20:11 UTC 2016

Dave Warren <davew at> wrote:
> On 2016-03-24 09:46, Ray Bellis wrote:
> > On 24/03/2016 16:41, Tony Finch wrote:
> >
> > > >When I changed our TTLs from 24h to 1h last year, it didn't have a visible
> > > >effect on authoritative server query load, much to my surprise.
> >
> > I'm not that surprised - there's definitely not a linear correlation
> > between the TTL of an RRset and how frequently it's queried.
> >
> > Unless your TTL is very short, forced expulsion from cache (due to
> > cache-size limits) would cause many clients to re-query for a record far
> > more frequently than once-per-TTL.
> Has anyone ever done any evaluation on this? For average resolvers, what
> is the longest TTL that has any utility?

There was a great paper published 15 years ago describing a study of DNS
cache effectiveness at MIT.

It concluded (amongst other things) that NS records (and associated
address records) are really important, but leaf records that users ask for
don't matter so much. (Based on cache hits before TTL expiry, IIRC.)

I don't know of a similar study performed more recently. is also interesting.

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