Re: [dbound] [art] [DNSOP] not DNAME, was Related Domains By DNS (RDBD) Draft

Tony Finch <> Thu, 28 February 2019 13:32 UTC

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Date: Thu, 28 Feb 2019 13:31:51 +0000
From: Tony Finch <>
To: John C Klensin <>
cc: John R Levine <>,,
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Subject: Re: [dbound] [art] [DNSOP] not DNAME, was Related Domains By DNS (RDBD) Draft
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John C Klensin <> wrote:
> John R Levine <> wrote:
> > There's the additional issue that an MX with a target of a
> > CNAME or DNAME doesn't work reliably,
> "doesn't work reliably" may or may not be a synonym for "is
> explicitly prohibited by SMTP" but the latter is true is any
> event.

Actually the issue here is not the target of the MX but its owner,
i.e. the mail domain itself.

As you know, professor (to coin a phrase) if I have:

foo.example MX
bar.example CNAME foo.example

This is allowed per SMTP. However there are disagreements between various
versions of the specification and various implementations about what this

Some treat it as equivalent to

bar.example MX

Others treat it as equivalent to a directive that says addresses
@bar.example should have the domain part rewritten to foo.example.

So if you have CNAME (or DNAME) pointing at MX and you want the alias to
be usable as a mail domain, you have to be super careful with the setup of
the target mail servers to work around the interoperability gotchas.

On the other hand, although MX pointing at CNAME is formally frowned on,
AIUI it works fine in practice.

f.anthony.n.finch  <>
East Forties: Northerly 4 or 5. Slight or moderate. Fair. Good.