DNS / Mail problem

Mark_Andrews at isc.org Mark_Andrews at isc.org
Sat Dec 14 11:51:17 UTC 2002

> glederrey at swissonline.ch wrote:
> > Here is the DNS configuration :
> > 
> > In the zone "genam.ch" :
> > 
> > cobalt IN A
> > 
> > In the zone "oppersdorff.net"
> > 
> > von IN CNAME cobalt.genam.ch.
> so far so good.
> > Now there are two problems (maybe unrelated). First : when this client s
> > ends mail (Outlook, with her ISP's smtp server) the "From:" and
> > "Reply-to:" headers get rewritten from "user at von.oppersdorff.net" to
> > "user at cobalt.genam.ch".
> You say she's sending email from her ISP's smtp server.  Okay.  So why
> would the server rewrite anything?  To me it sounds like a setting in
> her ISP's smtp server, though why, I can't tell you.

	Well this is actually a RFC requirement.  Aliases are supposed to 
	be re-written.
> > I dont have this problem with any other domain
> > that I host, but those domains have an MX record, and not a CNAME ; you
> > cant have both an MX an d a CNAME for the same name, that is the
> > following is wrong :
> > 
> > von IN CNAME cobalt.genam.ch.
> > von IN MX 0 cobalt.genam.ch.
> You can.  What you can't have is:
>   von in cname cobalt.genam.ch.
>   von in mx 0 von.oppersdorff.net.

	Neither is legal.  CNAME and other data is illegal.  The other
	data in this case is the MX record.
> because an mx record can't point to a host defined by a cname record.
> In your example, the mx is pointing to cobalt.genam.ch, which is defined
> by an A record, which is fine.
> > The question being : how can I avoid this header rewrite ?

	Don't use a CNAME.

> Try putting in the cname, but I still don't see how it's got anything to
> do with the header rewrite.  mx defines where mail is delivered to; it's
> got nothing to do with sending mail.
> > The next problem is that with this configuration, I need to have the
> > same host for the mail and for the web (Cant have both MX and CNAME). Is
> > there a better solution ?
> Just put in the cname record.  You can.

	No you can't.  Please go read the section on aliases in RFC 1034
	to understand why you can't.
> > I can declare von.oppersdorff.net as an A record, but that doesnt seems
> > "clean", if the web host moves, then I have to edit all DNS zones, and
> > not only the "main" one.

	That is the way you do it today.  Eventually you should be able to
	use SRV records once the browser vendors add support.
> > I am probably missing something, or I have some of the concepts wrong.
> I think you've got some of the concept wrong.

	No.  You have the concept wrong.

> Jeff
> -- 
> Jeff Lasman, nobaloney.net, P. O. Box 52672, Riverside, CA  92517 US
> Internet & Unix/Linux/Sun/Cobalt Consulting +1 909 778-9980
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Mark Andrews, Internet Software Consortium
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