CNAME vs. A record

Mark_Andrews at isc.org Mark_Andrews at isc.org
Sun Jul 7 21:37:31 UTC 2002


> Quick question:  I just submitted some dns changes to my ISP similar to the
> following:
> 
> host          IN A    1.2.3.4
> 
> www        IN CNAME    host
> ftp            IN CNAME    host
> 
> My ISP said that the recommended way to do this is with A records, as
> follows:
> 
> host        IN A     1.2.3.4
> www      IN A     1.2.3.4
> ftp          IN A     1.2.3.4
> 
> Any insights?  Could this be to facilitate reverse lookups?
> Any downside to using all A records?
> 
> Thanks,
> TD

	For most things it does not matter which way you do it.

	However if you want to support email to user at www and user at ftp
	(especially if they need to be delivered to different people)
	they need to be adddress records.  CNAMEs cause address
	re-writting in the sending MTA.  There are also some cases
	where CNAMEs can't be used (i.e. nameservers can't be CNAMEs).

	As for PTR records you only need one PTR record for the
	address.  No application looks at multiple PTR records.  It
	causes problems for access control based on name as the PTR
	records can be returned in any order.  Also continually
	adding PTR records for ever virtual machine will eventually
	cause the answer to the PTR query to exceed what can be
	supported by the DNS protocol.

	Mark
--
Mark Andrews, Internet Software Consortium
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark.Andrews at isc.org


More information about the bind-users mailing list