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

Mark Andrews <marka@isc.org> Mon, 18 November 2019 01:35 UTC

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Date: Mon, 18 Nov 2019 12:35:48 +1100
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Subject: Re: [DNSOP] RFC 8482 (the ANY -> HINFO hack) and DNAME
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> On 17 Nov 2019, at 17:20, Viktor Dukhovni <ietf-dane@dukhovni.org> wrote:
> 
>> 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.

Just because broken configuration don’t always cause problems doesn’t mean
that they don’t sometimes.  MTA’s need to know what names they are known
by to properly remove MX records from consideration when performing store and
forward. Email forwarding loops still occur.

> 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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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka@isc.org