Routing mail to a sub-domain

[Mark Damrose]

"Iain Firkins" <iain.firkins at btinternet.com> wrote in message
news:aqde3k$tld$1 at isrv4.isc.org...
>
> "Kevin Darcy" <kcd at daimlerchrysler.com> wrote in message
> news:aqcap3$futd$1 at isrv4.isc.org...
> >
> >
> > If you *must* anonymize the mail server names, please at least use
> server1,
> > server2 or whatever so that we know which server is which...
> >
>
> That's a fair point, let me make those changes and see if it becomes
> clearer.
>
> Lets say that mailserver1 is the main email server, and mailserver2 is the
> email server in the subdomain. The main domain will be called 'domain' and
> the sub-domain 'sub.domain'.
>
> In the external DNS for the domain, there is an entry which says:
>
> sub.domain    IN    MX    12 mailserver1 (FQDN)
>
> In the internal DNS servers, there are the following entries:
> sub.domain    86400    IN    NS    ns0.sub.domain (FQDN)
>                      86400    IN    NS    ns1.sub.domain (FQDN)
>                      86400    IN    NS    ns2.sub.domain (FQDN)
>                      86400    IN    NS    ns3.sub.domain (FQDN)
>
> If I do an nslookup and use ns0.sub.domain through ns3.sub.domain (with
> type=MX), I get the following:
>
> sub.domain    preference = 11, mail exchanger = mailserver2
> ....
>
> If I do an nslookup with the internal DNS (with type=MX), I get the
> following:
>
> Non-authoritative answer:
> sub.domain   preference = 12, mail exchanger = mailserver1
>
> Authoritative answers can be found from:
> sub.domain   nameserver = ns0.sub.domain (FQDN)
> sub.domain   nameserver = ns1.sub.domain (FQDN)
> sub.domain   nameserver = ns2.sub.domain (FQDN)
> sub.domain   nameserver = ns3.sub.domain (FQDN)
> mailserver1       internet address = xxx.xxx.xxx.xxx
>
> I think this is where the problem lies. With this setup, my understanding
is
> that mailserver1 would receive an email for sub.domain because of the
> external DNS entry. mailserver1 would try to resolve the sub.domain name
and
> get the NS records in the internal DNS and then check with ns0.sub.domain.
> It would then get mailserver2 and pass the mail on. But I wonder if the
> non-authoritative answer is causing the problem. Nowhere is there an MX
> record for sub.domain pointing to mailserver1 except for the external DNS
so
> I don't know why mailserver1 would be looking there for an answer.
>
> To re-iterate, I get a "554 MX list for sub.domain points back to
> mailserver1" error whenever I send an email to user at sub.domain.
>
> Sorry for any confusion in my last posting. I hope this is a little
clearer
> now.
>
> Regards,
>
> Iain

If it's a recent enough version of sendmail, on mailserver1 insert in the
mailertable
sub.domain  smtp:[mailserver2]
or
sub.domain esmtp:[mailserver2]
depending on if you want to forward on DSN, etc.

This will force mailserver1 to deliver all mail addressed to sub.domain to
mailserver2.  The brackets tell sendmail to look up only the address
record(s) and not an MX.