DNS TTL adherence

Simon Waters simonw at zynet.net
Wed Mar 15 08:56:36 UTC 2006

On Wednesday 15 Mar 2006 02:32, Joe Maimon wrote:
> And the dnscache resolver cache service in win2k and up.
> http://support.microsoft.com/kb/318803/en-us
> http://support.microsoft.com/kb/245437/EN-US/

Both these article say the DNS TTL is honoured by the cache. Microsoft may 
have done some horrid things with DNS over the years, but returning stale 
data just breaks things.

> If you are expecting hot cutovers to anything by utilizing DNS, sure
> seems that you need to expect to support traffic to the values of the
> old records for some time.

Nope. You can bin traffic as soon as the TTL for it expires. Everything else 
is broken, and experience here is that it is either spam/zombie generated or 
googlebots <sigh - there is always one>.

> And if you are expecting very long TTL's to give you extra insurance for
> outages and what-nots, expect spotty effectiveness.

Agreed, there is no requirement to cache records for the whole of the 
advertised TTL (or -ve TTL).

In answer to the original question, I'm not aware of any DNS servers that 
don't expire data at the end of the TTL period correctly. Failing to expire 
such data would be a good way of breaking things, and people would just not 
use such broken software.

I'm not sure why the OP thinks someone would research such a bug in detail, my 
experience is they would just fix it.

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