MX record Question

[Kevin Darcy]

Waltner, Steve wrote:

> > The question is how does this work?
> >
> > MX 10 mail01.domain.com
> > MX 10 mail02.domain.com
> >
> >
> > If for example this in theory will round robin, what would happen if
> > mail01
> > were to go down, would mail02 be the next on the list even after being
> > resolved?  My email admin wanted to know how this would work.  We think
> > that
> > it should go to the next one in line, this of course depending on the MTA.
> > Could someone shed some light.
> >
> > Thanks,
> >
> > Mark
> >
> >
> When BIND returns a response to an MX query for this domain name, it will
> return the two entries ordered in round-robin fashion. So if mail01 was
> listed before mail02 in the response, the remote mailserver would first
> attempt to contact mail01, and then after a minute the TCP connection to
> port 25 (SMTP server) would fail, so the remote server would then try mail02
> and the mail would go through.

Um, no. Read RFC 2821, Section 5. The remote mailserver must pick one of the MX
targets at *random*. This is so load-balancing can occur even when the
DNS servers don't round-robin (round-robin address-record ordering is not
mandated by any RFC, and in fact can be turned off in BIND).


- Kevin