On Fri, 7 Sep 2001, Jeff Green wrote:
> The MTA for the domain is a (dialup) Linux host we'll call m1.mydomain.com
> using Exim 3.12. The following MX records are for the dialup and the ISP's
> queue, respectively:
>
> mydomain.com preference = 10, mail exchanger = m1.mydomain.com
> mydomain.com preference = 100, mail exchanger = queue.isp.net
You shouldn't really point MX records at hosts that are not always
accessible. It is less of a nasty if a caller gets an instant "no route
to host". It is most anti-social if the caller has to suffer a timeout
each time. The reason it's bad is that it wastes resources for the
caller, even possibly costing Real Money if the caller is on a
connection that has traffic-based or time-based charges.
> Port 25 is open only to queue.isp.net and internal machines.
That's getting pretty bad. As a result of the previous thread on this
list, I have now submitted a draft BCP RFC for discussion by the
relevant IETF working group. It says you must not point MX records at
hosts that cannot be reached (typical examples: hosts using private IP
addresses and hosts behind firewalls). Of course, this has yet to be
accepted, but several networking experts I've asked support it.
This sounds like another variation: a host that can be reached some of
the time, but when it is reached, it rejects calls. (But if by "open"
you mean it's otherwise firewall-blocked, then it falls into the
category I've cited in the draft.)
> I'd like the
> ISP to reconfigure to eliminate the delays that result from seeing the
> local computer in either a disconnected state or unwilling to accept
> connections.
Hear, hear! That's why I've submitted the draft RFC.
> The ISP mail/DNS admin seems to believe
> that without the first mx record, or reversed priority (above) all mail would
> bounce as there is no local delivery option.
That's rubbish if the MTA is Exim. You just need to set up a domainlist
router to specify where to send mail for the domain. I am sure other
MTAs have equally straightforward methods of manual routing.
Philip
--
Philip Hazel University of Cambridge Computing Service,
ph10@??? Cambridge, England. Phone: +44 1223 334714.
