DAve wrote:
Good morning.

I have been asked by my co-workers and sales why I always create a A record for new domains we host instead of a CNAME.

The issue I run into lately with some domains is that a client has a website with a industry host such as frank.relator.com and he wants to have DNS point www.frank.com to frank.relator.com with a CNAME. The client does not want an A record for frank.com.

Somewhere, in a class far far away, I was taught a DNS zone had to have a A record to function properly. I can't seem to locate anything in the RFCs.

Am I wrong?

Yes, you're wrong.

In terms of web service, you can use either an A record or a CNAME record
to provide the address part of a site's URL[*].  As far as the web server is
concerned, it looks for the 'Host=' line in the HTTP packet to decide what
name-based VHOST to dispatch the query to internally, and doesn't necessarily
do any DNS lookups at all.  Web clients just do a gethostbyname(3) or 
getaddrinfo(3) call to resolve the  site name into an IP, and anything 
supported by those (/etc/hosts, NIS, LDAP, DNS) will do the trick.

In terms of the DNS a 'Zone' is a delegated block of the name space under
a single administrative control.  Typically with BIND this maps onto a single
'Zone file' containing all of the DNS resource records for the zone.  The only 
records a zone *has* to have are:

  * 1 SOA record, with the zone serial number

  * Some number of NS records giving the nameservers for the zone.

It's perfectly permissible to have a zone that doesn't contain any A
records (or AAAA records) and in fact, reasonably common: reverse domains
generally contain mostly PTR records.


[*] Possibly others, but A and CNAME are the vast majority.  Being able to
use SRV for webservers would be cool.

Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                 Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey     Ramsgate
                                                 Kent, CT11 9PW

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