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Re: [DNSOP] RFC 8482 (the ANY -> HINFO hack) and DNAME

Viktor Dukhovni <ietf-dane@dukhovni.org> Sun, 17 November 2019 06:20 UTC

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From: Viktor Dukhovni <ietf-dane@dukhovni.org>
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Date: Sun, 17 Nov 2019 01:20:43 -0500
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Subject: Re: [DNSOP] RFC 8482 (the ANY -> HINFO hack) and DNAME
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> On Nov 16, 2019, at 9:41 AM, John Levine <johnl@taugh.com> wrote:
> 
> Remember that it's invalid for an NS or MX to point to a CNAME so I assume
> it's equally invalid for them to point to a DNAME.

There's no need for NS RRs pointing the non-canonical names, the DNAMEs
are there for continuity (or to support alternative representations) of
actual application services.  That's not a real barrier to DNAME use.

And speaking of application services, the prohibition of MX pointing to
CNAME is not enforced by any MTA I'm aware, and a small, but non-negligible
fraction of domains do have MX RRs that point to CNAMEs.

In my DANE survey, out of of 8503 MX hosts, 130 (1.5%) are CNAME aliases.
None presently via DNAMEs, but Tony Finch has a DNAME redirecting TLSA
lookups:

  _tcp.hummus.csx.cam.ac.uk. IN DNAME _hummus_tcp.exim.org.

and three other domains also have DNAME-redirected _tcp subtrees.

-- 
	Viktor.

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