Routing mail to a sub-domain

[Kevin Darcy]

Iain Firkins wrote:

> Hello people,
>
> Bit of a conundrum here that after hours of looking at various sites and
> talking with various people seems to point to a DNS config problem.
>
> The situation is this. I want to route mail from the main email server to
> another email server on a subdomain. The main email server is the only one
> with port 25 access to the outside world, so obviously mail is delivered
> there in the first instance. There is an entry in the external DNS server:
>
> subdomain    IN    MX    12 mail server (FQDN)
>
> I'm pretty confident this is working as it should. In the internal DNS
> servers, I point that same subdomain to its specific DNS servers:
>
> subdomain    86400    IN    NS    ns0.subdomain (FQDN)
>                     86400    IN    NS    ns1.subdomain (FQDN)
>                     86400    IN    NS    ns2.subdomain (FQDN)
>                     86400    IN    NS    ns3.subdomain (FQDN)
>
> If I do an nslookup, set type to MX, and query those DNS servers, I get the
> correct name for the mail server for that subdomain. However, if I do an
> nslookup up on the internal DNS server, I get the following:
>
> Non-authoritative answer:
> subdomain   preference = 12, mail exchanger = mail server
>
> Authoritative answers can be found from:
> subdomain   nameserver = ns0.subdomain (FQDN)
> subdomain   nameserver = ns1.subdomain (FQDN)
> subdomain   nameserver = ns2.subdomain (FQDN)
> subdomain   nameserver = ns3.subdomain (FQDN)
> mail server       internet address = xxx.xxx.xxx.xxx
>
> I think this is where the problems are. Whenever I send an email to the
> subdomain, I get "554 MX list for subdomain points back to mail server".
> Mostly every page I've found that talks about this particular error message
> says update Cw in sendmail.cf. I've checked with the person responsible for
> the subdomain and asked about the update of Cw in sendmail.cf, but his reply
> was that the mail server is not the endpoint for this message, as it should
> be getting passed to the mail server in the subdomain.
>
> I've found a couple of postings on Google Groups with situations like this,
> but never a real solution. Is this situation so unusual ??

Well, it's pretty hard to answer this question when there are at least 2 mail
servers involved, yet you insist on "anonymizing" the MX target to just "mail
server".

If the MX target is the name of the subdomain mail server, and the message is
bouncing *after* being delivered to that mail server, with a bounce message of
"554 MX list for subdomain points back to {name of subdomain mail server}",
then it's quite clearly a Cw problem. In that case, the admin's answer of "the
mail server is not the endpoint for this message" makes no sense whatsoever
because the subdomain mail server *is* the endpoint of the message, right? But
the admin's reponses, as you've expressed it, seems to be making a distinction
between, on the one hand, "the mail server", and, on the other hand, "the mail
server in the subdomain". So this just adds to the confusion.

On the other hand, if you're pointing the MX target to your own mail server,
then basically you're looping to yourself and it's not surprising that mail is
failing.

If you *must* anonymize the mail server names, please at least use server1,
server2 or whatever so that we know which server is which...


- Kevin