Re: problem with AOL addresses
On Fri, 2003-05-23 at 04:37, Paul Johnson wrote:
> Because it's looking at a list of dynamically assigned networks. Lack
> of MX is not proof that there isn't supposed to be a mail server
> there, just means that the mail exchanger for that host isn't that
> host.
>
> Having something like
>
> ursine.dyndns.org. IN A 12.225.10.14
> ursine.dyndns.org. IN MX ursine.dyndns.org
>
> is being uselessly redundant and costs sites sending to you an extra
> DNS lookup that they shouldn't have to do. Why bother adding an MX if
> you don't have to?
Even if you don't have an MX record, the system sending you mail doesn't
know that, and is still going to do an MX query for your domain. That
query will fail and then the sender will fall back to an A query.
Conversely, if you do have an MX record that points to itself, then the
response to the MX query will contain the MX record, plus other
interesting records, including the A record. All of these response
records will be used to populate the DNS server cache of the sender's
DNS server.
So, the sender has to do two queries either way, but if you have a MX
record, the answer to the second query will usually come from the cache
of the sender's local DNS server, which is pretty cheap. It's generally
considered the best practice to have a MX record for all systems that
accept mail for your domain.
Regards,
--
Dave Carrigan
Seattle, WA, USA
dave@rudedog.org | http://www.rudedog.org/ | ICQ:161669680
UNIX-Apache-Perl-Linux-Firewalls-LDAP-C-C++-DNS-PalmOS-PostgreSQL-MySQL
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