CNAME for MX records

[David Botham]

> -----Original Message-----
> From: bind-users-bounce at isc.org [mailto:bind-users-bounce at isc.org] On
> Behalf Of Terry Poperszky
> Sent: Wednesday, August 07, 2002 3:46 PM
> To: comp-protocols-dns-bind at isc.org
> Subject: CNAME for MX records
> 
> Is there any reaon not to use CNAMEs for your MX records? I have been

Yes, it has to do with the Mail Exchanger Algorithm.  Basically, mta's
will obtain a complete list of mx records for a given domain.  They will
sort the list by preference (lowest to highest).  Then, they will look
for themselves in the list and delete themselves and any mx records with
a higher preference (why send the mail further away from its final
destination than it already is?).  However, it is important to note that
when looking for itself in the list, it will only find its canonical
name.  Therefore, if an mx record has a cname record on the RHS, the mta
in question would not delete itself from the list.  This could obviously
cause a mailing loop.

I have heard many arguments for using cname records in the rhs of an mx
record, like "my mail servers don't act like that...".  However, it all
goes back to making a list of all the things you could do to make your
life, or the life of the next person to come along, more difficult.
Using a cname on the rhs of an mx is close to the top of that list.

BTW:  all of what I have discussed above comes right out of DNS and
BIND, 4th Edition, page 98.  Get It, read It, love It, It is your friend
:)

Dave...

> doing
> it internally with no problem for several years, but my ISP objected
when
> I
> changed my external records to a similar format.
> 
> Terry