RE: Overriding DNS lookups for a specific domain

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Overriding DNS lookups for a specific domain
\ andy k (14 Mar 2008)
. \ Jay Chandler (14 Mar 2008)
. . \ andy k (14 Mar 2008)

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Subject:	RE: Overriding DNS lookups for a specific domain
Group:	Postfix-users
From:	andy k
Date:	14 Mar 2008

Okay I see how a transport can be used to define alternate delivery for an entire domain; to make this less abstract, domain def.org is Postini. Joe's real inbox is joe. Currently, inbound email to joe is forwarded directly to joe. Domain xyz.org is set up for spam filtering with Postini. I'd like inbound email to abc.org to get routed to xyz.org's externally-defined MX host, which is Postini; Postini will then forward it to xyz.org.

dig -t mx xyz.org (from outside of our network) gives something like

xyz.org. 1800 IN MX 20 xyz.org.s2a3.psmtp.com.
xyz.org. 1800 IN MX 30 xyz.org.s4b5.psmtp.com.
xyz.org. 1800 IN MX 10 xyz.org.s6a7.psmtp.com.

dig -t mx xyz.org (from *inside* our network, on abc.org mail server) gives

xyz.org. 1200 IN MX 100 smtp.xyz.org.

On the server handling abc.org email, I can't simply add

xyz.org smtp:postini.com
or

xyz.org smtp:psmtp.com

to the transport file, because inbound mail must use one of the 3 MX hosts above to be filtered through Postini.

I suppose I could put

xyz.org smtp:xyz.org.s6a7.psmtp.com

in transport and force it to go to one of the MX servers, but this seems like bad practice and kind of brittle since any external DNS changes to xyz.org MX would kill it.

Can I force postfix to use a different (external) nameserver *just* for the xyz.org domain? We can't simply change the internal DNS for xyz.org MX, it affects other systems.

Thanks again,

Andy

> andy k wrote:
> > Thanks for the reply Wietse -- however, I don't think this will work
> > for me (I did not give all the details, my bad -- def.org will not
> > just accept all forwarded emails for xyz.org -- it will only accept
> > them if they go through the specific MX hosts configured for xyz.org
> > in external DNS -- for example, xyz.org.server2.def.org).
> >
> > But now I have a different question -- how do I override DNS for the
> > whole xyz.org domain on my server? To clarify:
> >
> > abc.org has multiple mail accounts which are simple alias forwards to
> > xyz.org accounts. abc.org currently uses internal nameservers in
> > resolv.conf, which resolve xyz.org to another internal server.
> > However, xyz.org resolves to def.org's MX records for external DNS
> > lookups. def.org is an external/third-party server. I want to force
> > my internal server abc.org to resolve xyz.org to def.org's mail
> > servers, but still use the nameservers in resolv.conf for resolving
> > domains other than xyz.org.
> >
> > Can I do this with postfix?
> >
> > Thanks again,
> >
>
> man 5 transport
>
> --
> Jay Chandler / KB1JWQ
> Living Legend / Systems Exorcist
> Today's Excuse: Someone is standing on the ethernet cable, causing a kink in the cable
>

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