MX record ordering

[Chip Old]

On Fri, 30 Mar 2001, lawrence.a.kravets at us.arthurandersen.com wrote:

> We have recently upgrade to Bind 9.1.0 from Bind 4.9.3. Given the
> following, we have reason to believe that the order of response of the
> following has changed. We expect that it would split equally between
> server1 and server2. Unfortunately we see more traffic on server1 than
> server2.  Has something changed in Bind 9?
>
> mailhosts      IN   MX   10   server1.zone.com.
> mailhosts      IN   MX   10   server2.zone.com.
> mailhosts      IN   MX   20   server3.zone.com.
> mailhosts      IN   MX   30   server4.zone.com.

Unless BIND 9 does something I don't know about to MX records (like change
their preference values), that's an MTA issue, not a BIND issue.
Unfortunately not all MTAs deal with MX records the same way.

When Sendmail 8 encounters multiple MX records with identical preference
values, it selects randomly among them.  If the first selection doesn't
respond, Sendmail drops that MX record and again randomlt selects from the
remaining same-valued MX records.  If it tries all the same-valued MX
records unsuccessfully, it moves on to the next higher-valued one.

I have no idea how other MTAs do it, but am sure others here will know.

-- 
Chip Old (Francis E. Old)               E-Mail:  fold at bcpl.net
Manager, BCPL Network Services          Voice:   410-887-6180
Manager, BCPL.NET Internet Services     FAX:     410-887-2091
320 York Road
Towson, Maryland 21204 U.S.A.