client behavior - cname vs A record

[Mark_Andrews at isc.org]

> Assuming that a client follows RFC standards (I know I assume a bit too much 
> ;) ), how would a client behave in this situation?
> 
> A client makes a request for a hostname which is actually cnamed to another 
> hostname.
> 
> Now, the answer he will receive will have the cname record with a ttl, as 
> well as the A record (for the cname target hostname) with a ttl value as 
> well.
> 
> What ttl value will be honored? In other words, will the client re-request 
> name resolution when the cname or A record ttl is reached?

	99.9% of clients don't see the TTL.  It is hidden by the
	hostname lookup API's gethostbyname(), getaddrinfo().
	The client either uses the address returned forever,
	uses the address immediately then forgets it re-querying
	as required or has a fixed TTL.  The first two are most
	common.  The last is rare.

	DNS caches will expire each of the records independently.
 
> Thanks
> Wang
> 
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--
Mark Andrews, Internet Software Consortium
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark.Andrews at isc.org