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

Dave Warren davew at
Fri Mar 25 07:09:49 UTC 2016

On 2016-03-24 18:28, Barry Margolin wrote:
> In article <mailman.454.1458858570.73610.bind-users at>,
>   Dave Warren <davew at> wrote:
>> On 2016-03-24 15:20, Tony Finch wrote:
>>> 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.
>> The internet was a very different place 15 years ago, in particular,
>> this was before every Windows client machine had it's own DNS cache
>> service and largely before today's connected mobile devices were a thing.
> But it was also before the widespread use of CDNs (Akamai was founded
> only 3 years earlier). These days, the most heavily used web sites use
> CDNs, which make heavy use of short TTLs for the leaf CNAME and A
> records.

Yeah, that's a factor too.

I'm more interested in the impact from the perspective of an 
authoritative server operator and in some respects sites that use short 
TTLs will increase the odds of my longer-TTL's records staying in the 
cache longer before it gets hit by a cache-size limit, but none of my 
zones are really large enough to do A/B testing.

Dave Warren

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