Re: [DNSOP] ANAME in answer or additional section [issue #62]

[Brian Dickson <[e-mail]>]

On Thu, Jun 13, 2019 at 1:51 PM Bob Harold <[e-mail]> wrote:

>
> On Thu, Jun 13, 2019 at 1:50 PM Brian Dickson <
> [e-mail]> wrote:
>
>>
>>
>> On Wed, Jun 12, 2019 at 1:11 AM Matthijs Mekking <[e-mail]>
>> wrote:
>>
>>> Brian,
>>>
>>> Thanks for the detailed background on why DNAME worked. There are a few
>>> things that caught my attention:
>>>
>>> > When a recursive queried an authority server, if it got back a DNAME
>>> but did not understand it, it ignored the DNAME but processed the CNAME
>>> (as if only the CNAME existed) (plus any other data like chained CNAMEs
>>> or A/AAAA records)
>>>
>>> > All of this is unfortunate, because of the fact that there is no
>>> genuinely backward compatible record similar to ANAME that can be used,
>>> without a very strong likelihood of breaking things. From authority to
>>> recursive: You can't return an ANAME and a CNAME (as a
>>> backward-compatible rewrite signal that corresponds to the ANAME), since
>>> the CNAME will effectively obscure other RRTYPEs that might coexist
>>> (e.g. at the zone apex).
>>>
>>> This is fine, because that is not what we want: We would like to add the
>>> ANAME in the answer section with the A/AAAA records (not a CNAME).
>>>
>>> > The real problem here, is the "other" record for backward
>>> compatibility isn't a rewrite-type (such as CNAME or DNAME), but is a
>>> "promoted" A/AAAA record of potentially limited utility and questionable
>>> provenance (due to geo-ip stuff, TTL stuff, and RRSIG problems).
>>>
>>> I actually see the A/AAAA record as the backward compatibility records:
>>> An ANAME-aware resolver would understand the ANAME and can act upon it,
>>> an ANAME-unaware resolver will use the A/AAAA records that the
>>> authoritative returned.
>>>
>>
>> So, this is where the analogy to DNAME diverges from reality of ANAME,
>> and IMHO is the the crux of one of the main problems with ANAME.
>>
>> In the DNAME/CNAME example, the A/AAAA records are returned ONLY IF the
>> server that is authoritative for the DNAME is also authoritative for the
>> DNAME "target" (right-hand-side/RDATA).
>> If the DNAME auth server is not, it will only return DNAME+CNAME records.
>>
>> The only "legitimate" (in my opinion) reason that the ANAME authoritative
>> server should also return A/AAAA records, is if it is also authoritative
>> for the ANAME "target" (right-hand-side/RDATA).
>>
>> (And the reason that having the ANAME authoritative server obtain and
>> return A/AAAA records itself leads to what I called:
>>
>>> potentially limited utility and questionable provenance (due to geo-ip
>>> stuff, TTL stuff, and RRSIG problems).
>>>
>>
>> I have elaborated on this problem previously, but will do so again for
>> completeness/context:
>>
>>    - There can be differences (possibly significant differences) in the
>>    results returned for resolution of the "target" between the ANAME
>>    authoritative server, and the querying resolver.
>>       - E.g. Any sort of "stupid DNS tricks" that return different
>>       values based on either physical topology (anycast instance) or geo-ip
>>       (client-subnet)
>>       - That discrepancy can direct clients to a suboptimal server,
>>       where suboptimal can even be, from a user perspective, badly broken (e..g.
>>       wrong language, illegal content, etc.)
>>    - The interactions on TTLs and the need for repeated lookups can have
>>    adverse impacts on both clients, resolvers, and auth servers
>>       - An auth server might want to use longer TTLs to reduce query
>>       volume, for ANAME values that do not change frequently (A/AAAA TTL set to
>>       same as ANAME TTL)
>>       - The original A/AAAA TTL (for the "target" owner name's A/AAAA
>>       RRDATA) might be short because it changes frequently (e.g. CDNs)
>>    - If the "sibling" data is only a hint, non-upgraded resolvers will
>>    serve A/AAAA records that are either poor (longer latency, higher loss),
>>    wrong (incorrect language due to wrong CDN node), broken (long TTL -> wrong
>>    server), or slow (requery required)
>>
>> I don't have a better suggestion on how to fix this within the context of
>> ANAME; IMNSHO it is an intractable issue, a fundamental problem with ANAME
>> if sibling records are required.
>>
>> Brian
>>
>
> I see two main cases:
>
>    - ANAME replaces a CNAME record, so that other records can be attached
>    to the same name. I don't think this is likely to be a big use case. In
>    this case, all your concerns apply.
>
>
>    - ANAME replaces A/AAAA records, most likely at a zone apex, where
>    CNAME was desired but not allowed. I think this is the main case for ANAME.
>    And in this case, the old A/AAAA records are returned as previously, but
>    with the added ANAME record. Your concerns only apply if they already
>    applied to the A/AAAA records - nothing has gotten any worse.
>
> Is there another major case I am missing?
>

Yes, ANAME placed at apex, where no A/AAAA exists or existed. Typically
when there is a "www" CNAME, and the zone owner wants to make a "magic apex
CNAME" that points the same place as "www" currently does.

I think that (addition of ANAME) is going to be 95% of the deployments,
versus a 5% migration of already-existing proprietary RRTYPEs or
pseudo-types (vertical integration) to ANAME. (If those weren't the
expected levels of deployment, I don't the the efforts on ANAME would be
worthwhile.)

Brian