On 21 Nov 1996, Neal Becker wrote:
> If I want to split the load, can I just put 2 MX with equal priority?
> It doesn't appear to work, it looks like exim always chooses the
> first.
Exim chooses the first, but modern nameservers do round robinning so
should return them in different orders. It would therefore be a bad idea
if Exim started messing around as well. We certainly see mail coming in
to both of the machines that service cus.cam.ac.uk with equal MX values.
> Is this covered in the RFCs?
RFC 1123 (October 1989) says
The following information is to be used to rank the host
addresses:
(1) Multiple MX Records -- these contain a preference
indication that should be used in sorting. If there are
multiple destinations with the same preference and there
is no clear reason to favor one (e.g., by address
preference), then the sender-SMTP SHOULD pick one at
random to spread the load across multiple mail exchanges
for a specific organization; note that this is a
refinement of the procedure in [DNS:3].
but I don't believe that is in fact a good idea now that the DNS does
round-robinning. (And in any case it is only a SHOULD, not a MUST.)
--
Philip Hazel University Computing Service,
ph10@??? New Museums Site, Cambridge CB2 3QG,
P.Hazel@??? England. Phone: +44 1223 334714
