IP assignment for virtual webhosting

> > RFC 1912: "Common DNS Operational and Configuration Errors"
>
> I believe the intention here is that primary MX for the domain is
> mail.provider.com (A record, easily changed at the DNS), and the
> SMTP server setting for customers' MUAs is mail.customerdomain.com
> (valid use of CNAME, easily changed at the DNS instead of manual
> reconfiguration).

That's a valid use of CNAME records in conjunction with mail
handling, but about the only one.  You still shouldn't combine MX
and CNAME, which is what RFC1912 talks about in the previously
quoted section.

However, the text from RFC1912 is still valid (the example lacks
the MX priority field, though ;-), but doesn't point out what the
reason for the RFC 974 restriction is.  So if you'll excuse me
for flying off on a DNS tangent...

The mail server presents itself with one name in its greeting
message.  Hopefully this name is a fully qualified domain name
(probably mail.provider.com in the above example).  This is the
name you should be using as the target in an MX record when you
want to use the mail service of that host.

If you adhere to this rule, your mail server will correctly
handle being a "secondary" mail handler for a domain (modulo any
configured relaying restrictions), and recognize "itself" in a
list of MX targets, and prune off itself and any lower-
prioritized entries.

If you don't, well..., you're doing something which is similar to
walking around pointing a loaded and unsecured gun in the
direction of your own foot.

- Håvard