Re: [DNSOP] I-D Action: draft-ietf-dnsop-svcb-https-05.txt

Petr Špaček <pspacek@isc.org> Wed, 21 April 2021 17:12 UTC

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Date: Wed, 21 Apr 2021 19:12:38 +0200
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Subject: Re: [DNSOP] I-D Action: draft-ietf-dnsop-svcb-https-05.txt
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On 21. 04. 21 19:00, Ben Schwartz wrote:
> On Wed, Apr 21, 2021 at 12:44 PM Wellington, Brian 
> <bwelling=40akamai.com@dmarc.ietf.org 
> <mailto:40akamai.com@dmarc.ietf.org>> wrote:
> 
>     I’m not a fan of the new text in section 4.3:
> 
>         Recursive resolvers MUST be able to convey SVCB records
>         with unrecognized SvcParamKeys or malformed SvcParamValues.
> 
> 
>     It is perfectly reasonable for a recursive resolver to reject any
>     malformed DNS record.  There’s a significant difference between
>     “malformed” and “unknown”.  A recursive resolver should definitely
>     allow records with unknown SvcParamKeys, but if the format of a
>     record is known, a resolver should be allowed (encouraged, in fact)
>     to check that it conforms to that format.
> 
> 
> The concern here was about differing interpretations of future 
> standards.  For example, if some SvcParamValue is defined as a URL (for 
> some future key), implementations are likely to disagree about whether 
> certain bytestrings are valid URLs.  (There are several definitions and 
> a zillion edge cases.)  The resolver has no use for these values, so the 
> most compatible behavior is to treat them as opaque bytestrings.
> 
> We very deliberately chose the phrase "MUST be able to convey" to 
> recognize that resolvers might be configured not to convey a record with 
> a malformed value, but it should be possible.
> 
> Do you think we should change this sentence?

I can see the same ambiguity as Brian.

What about this text as replacement?
---
Recursive resolvers MUST be able to convey SVCB records with 
unrecognized SvcParamKeys if they conform to RDATA wire format (section 
2.2).
---

What I mean: Resolvers must be able to handle _unknown_ SvcParams as 
opaque byte strings.

Various resolvers will do some sort of filtering on individual params, 
but such filtering is "local policy" and nothing we write into the RFC 
can change local policy anyway. For that reason I would not go to great 
lengths to explain what to do with unrecognized URLs etc.

-- 
Petr Špaček