Efficacy of using short timeout values for an A record

Mark Andrews marka at isc.org
Tue Feb 14 22:16:59 UTC 2012


In message <0B215138-0162-4FE0-835A-9FC611A6E648 at mac.com>, Chuck Swiger writes:
> On Feb 14, 2012, at 2:59 AM, goran kent wrote:
> > I need to setup an A record for a machine who's IP might change
> > unexpectedly, and I need to ensure PCs out there cache it for as short
> > a time as possible:
> > 
> >    host1    300  IN A 10.10.10.10
> > 
> > Does anyone know whether MS windows PCs will in fact honour that 300s,
> > then force a re-lookup?  Can I use even shorter values?  eg, 60?
> 
> ISC's BIND has (or had) a MINTTL value of 5 minutes / 300 seconds.
> It's probably unreasonable to expect other platforms to refetch DNS
> records faster than that.

To the best of my knowlege this is just plain wrong.  If a answer
had a TTL it was honoured.  If a negative answer didn't have a TTL
(no SOA record in the authority section) then one was choosen.

> Aside from DNS, you're going to run into layer-2 problems with MAC-to-IP
> mappings in your switches if you try to move an IP around at sub-minute
> intervals.
> 
> What problem are you actually trying to solve?  It's likely that a tool
> or mechanism like load-balancing onto a pool of boxes would provide a
> much better solution than expecting to move a box around so rapidly....
> 
> Regards,
> -- 
> -Chuck
> 
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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org



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