Problem with Multiple MX records

[Kevin Darcy]

kevinmadsen at my-deja.com wrote:

> I maintain many domains on my DNS server.  These domains have all been
> configured to have 2 MX records, the primary with a preference of 10
> and the secondary with a preference of 50.
>
> For Example:
>
>         IN      mx      10 server1.mydomain.com.
>         IN      mx      50 server2.mydomain.com.
>
> According to documentation I have read, the mail clients should attempt
> delivery to the lowest number preference server first.  However, I have
> several E-Mail clients that seem to ignore the preference and deliver
> to the MX record that is given first via a DNS query.

Those clients are in direct violation of RFC 974. Fix them.

> The problem is, my DNS server first replies as follows (using DIG):
>
> ;; ANSWER SECTION:
> mydomain.com.           1D IN MX        10 server1.mydomain.com.
> mydomain.com.           1D IN MX        50 server2.mydomain.com.
>
> Then, the second time I query it, my DNS server replies as follows
> (using DIG):
>
> ;; ANSWER SECTION:
> mydomain.com.           1D IN MX        50 server2.mydomain.com.
> mydomain.com.           1D IN MX        10 server1.mydomain.com.
>
> It alternates each time.  I have read about BIND allowing multiple A
> records and alternating the answers for each query (or some variant of
> this concept), but cannot find this article anymore.
>
> Are there any settings to control this or stop this behavior?

You can define a "fixed" rrset-order for that particular name, however it
will be of limited usefulness unless you define this on all of the slaves
as well as the master, and even then, other nameservers over which you
have no control may cache the answer and then "round-robin" the answers
they give out from their cache.

To truly solve this problem, you need to fix the broken clients.


- Kevin