Beginner's question

[Kevin Darcy]

The purpose of an MX record is to publish the name of a server which will
accept mail for a given domain name. If you want a server to forward mail
somewhere *other* than where the MX record(s) tell(s) it to, then this is no
longer a DNS issue; you have to somehow configure/program that logic into the
mail server. In the case of sendmail,  you could probably use a mailertable to
override the normal MX-record processing, or just add the domain to Cw and then
use MAIL_HUB or LUSER_RELAY to selectively forward only those addresses which
don't exist on MX2.yyy.org as aliases and/or local users ("local users" doesn't
just mean flesh-and-blood users, of course, it also includes mail accounts like
"root" or other system accounts).

In your particular situation, the "TryNullMXList" option, which I've seen in
the documentation but never used, would probably be the _simplest_ way to make
things work, but it is "disrecommended", and I believe would affect *all* mail
routing by MX2.yyy.org's sendmail, not just mail for the yyy.org domain.
Something more selective would probably serve you better.

As with any non-MX-based mail routing architecture, beware of mail loops.


- Kevin

Alain G. Fabry wrote:

> Reading DNS and BIND 3rd Edition, I have a question concerning MX exchanger.
> I have 2 systems running sendmail, say MX1.yyy.org and MX2.yyy.org
> All my users' mail is received at MX1.yyy.org (yyy.org) using sendmail.
> MX2.yyy.org does not have any users on the system but is running sendmail.
> Is it possible to configure the DNS to have MX2 forward mail to MX1 without
> adding MX1 as a mail exchanger in the DNS.
>
> Example, can I
>
> yyy.org.    MX mx2.yyy.org.
> yyy.org.    a 1.2.3.4
>
> If yyy.org is running sendmail, should it receive the mail forwarded from
> mx2.yyy.org?
>
> I've tried a similar setup but the mail gets returned to the sender with
> ****MX list for yyy.org points back to mx2.yyy.org
> ****.......Local configuration error
>
> If I add Cw yyy.org in the sendmail.cf file on mx2.yyy.com, will mx2 forward
> the mail to mx1 (yyy.com)?
>
> I guess it would be a similar setup for some ISP to forward mail to a domain
> from one of his clients, right?
>
> I hope this makes sense and thanks for any help.
>
> FYI, right now I have:
> yyy.org.    MX mx1.yyy.org
> yyy.org    A 1.2.3.4
> And this works since mx1.yyy.org is yyy.org
>
>  _Alain
>
> ______________________
> Alain G. Fabry
> Sr. LAN Administrator
> The University of Texas - Pan American
> External Affairs - CoSERVE
> Phone : (956) 381-3361