DNS puzzle

Ceri Davies setantae at submonkey.net
Wed Aug 28 19:21:55 BST 2002

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On Wed, Aug 28, 2002 at 12:11:22PM -0500, Jeff LaCoursiere wrote:
> As I write this I am envisioning how I could setup a quick test case to
> find the answer, but I am lazy at heart.  Imagine the following:
> * Zone a.com with TTL of 5 days
> * Zone b.com with TTL of 1 hour
> * 'A' record in b.com called 'www' with a value
> * 'CNAME' record in a.com called 'www' pointing to www.b.com
> Now the issue:
> A client asks his name server (not authoritative for either a.com or
> b.com - lets say caching only) to resolve www.a.com and gets an answer.
> Five minutes later the admin of b.com changes the address of the A
> record.  Two hours later the client asks his name server again to resolve
> www.a.com.
> What answer does he get?  The old value or the new value?

The new value is still the same as the old one, i.e.:

$ORIGIN a.com
www	IN	CNAME	www.b.com.

The resulting query for the A record for www.b.com will of course, yield the
new answer.

So to answer what you thought you were asking:
	The new one.

To answer what you were actually asking:
	Trick question, there is no old/new value as it hasn't changed.


you can't see when light's so strong
you can't see when light is gone

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