On Mon, 9 Feb 1998, Dave C. wrote:
> Now, the twist. Some of our customers for which we either do primary
> and backup MX, or just backup MX, have someone else running the primary
> DNS for their domains. To facilitate my moving these around without
> having to explain to some (usually clueless) DNS operator (usually
> running NT) what an MX record is and why I need it changed, I have them
> install something like this as their MX records:
>
>
> theirdomain.com. MX 10 theirdomain.com10.mx.mydomain.com.
> theirdomain.com. MX 20 theirdomain.com20.mx.mydomain.com.
>
> and then
>
> theirdomain.com10.mx.mydomain.com. CNAME mail.mydomain.com.
> theirdomain.com20.mx.mydomain.com. CNAME smtp.mydomain.com.
>
> so that I can change the CNAMES quickly if/when I need to.
This is of course not the way you are supposed to do things. The right-
hand side of an MX record is supposed to be the name of an A record.
Somewhere in the RFCs it says this. However, Exim is supposed to cope
with CNAMEs because so many people insist on using them. Nevertheless,
there is always the possibility of a bug in the code.
> However, when I do this, the two servers that are "smtp.mydomain.com"
> play hot-potato with the message every retry cycle, until eventually
> there are too many Received headers and it bounces.
Can you try running an address test on one of the servers, with
debugging turned on? Something like
exim -d9 -bt xxx@???
or possibly even with -d10 to see what the DNS resolver calls are? I've
just reviewed the code, and I can't see any problems, but that doesn't
mean to say there aren't any. :-) Send me the output from the debugging
run and I'll see if it tells me anything.
> How can I tell the two servers that are "smtp.mydomin.com" not to pass
> mail to each other, under any circumstance?
There is one sure way, independent of anything else, but it is tedious
if you have a lot of such domains. It is to set up a domainlist router
for the domain, so that it doesn't go near the DNS at all. However, I'd
rather find out what it is actually doing in the DNS lookup.
--
Philip Hazel University Computing Service,
ph10@??? New Museums Site, Cambridge CB2 3QG,
P.Hazel@??? England. Phone: +44 1223 334714
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