Re: [Uta] Eric Rescorla's Yes on draft-ietf-uta-email-deep-09: (with COMMENT)

["Chris Newman" <[e-mail]>]

On 26 Oct 2017, at 19:56, Keith Moore wrote:
> On 10/26/2017 07:18 PM, Chris Newman wrote:
>
>>>> Line 304
>>>>        preference to services supporting STARTTLS (if 
>>>> offered).  (See
>>>>        also Section 4.5.)
>>>> I note that 6186 is kind of unclear on what should go in SNI. It 
>>>> obviously
>>>> needs to be what you are checking against (which 6186 gets right) 
>>>> but maybe
>>>> it's worth clarifying in this document somewhere.
>>> Hmm.    I might need to come back to that one.   Lots of layers 
>>> to sift through.  Feel free to suggest text.
>>
>> I believe RFC 7817 handles this issue sufficiently.
>
> Not sure whether EKR was referring specifically to the use of SNI with 
> SRV records or not, but that's what I had assumed he meant.  So far I 
> haven't found any specific advice for (a) what name the MUA should 
> specify in SNI, or (b) what names the server should recognize and have 
> certificates for.   It's pretty clear that the server needs to have 
> a certificate for the name on the right hand side of the SRV record, 
> but should it also have a certificate for the name on the left hand 
> side?  (e.g. _pop3s._tcp.example.com?)  That would potentially make 
> SRV discovery more secure.
>
> But I think that's well beyond what the WG (and IESG) approved.   So 
> I guess I'm inclined to leave the -10 text as it is now without 
> specifically addressing this issue.

I just re-read the related text a bit more carefully. Although RFC 7817 
adequately covers what subjectAltName identities mail servers need to 
support in certificates, it doesn't cover what goes in SNI (although it 
mentions SNI). Ditto for RFC 6186. This is annoying; the issue really 
should have been addressed in 7817 or 6125 :-(

I suspect in practice that most MUAs use the post-SRV-lookup hostname in 
TLS SNI. And server support for that name form in SNI will be necessary 
to interoperate with MUAs that don't support RFC 6186 (or use an 
alternative autodiscovery mechanism in preference to RFC 6186). So the 
remaining question is would it be useful/helpful for a server to also 
support an SNI based on the SRV record or SRV-ID name? I don't think the 
answer to that question is important enough to justify a normative 
change to this document.

To address this, I suggest we add a sentence to the end of section 4.4 
(just before the sentence referencing 7817):

--
Mail servers supporting SNI need to support the post-SRV hostname to 
interoperate with MUAs that have not implemented RFC 6186.
--

I view that as a statement of fact rather than a normative change so I 
think it's kosher. Saying more on the topic would probably cross the 
normative change line so I'd prefer not to go there.

		- Chris