This document is providing STS functionality for SMTP relay, while DEEP is
providing STS functionality for SMTP submission (plus IMAP & POP). I believe
it’s important to align these two proposals and am open to changing DEEP to do
so (https://tools.ietf.org/html/draft-ietf-uta-email-deep-04).
Note that early versions of DEEP also provided STS for SMTP relay but solving
the STS problem for MUAs is sufficiently difficult and different that I think
it’s good to have a separate document for SMTP relay STS. If you are open to
editing this as a WG document with rough consensus changes, I would welcome the
collaboration.
I agree with most of Viktor’s comments so I won’t repeat them. Here are some
new comments:
1. I personally dislike using DNS records for any of this proposal. I believe
SMTP security policy is best communicated within SMTP as this minimizes attack
surface, eliminates the need for TOFU in some scenarios, and puts the policy
configuration closer to the server operator. We can use the PKIX trust model
with SMTP/STARTTLS just as we use it with HTTPS (with Viktor’s notable caveats)
so HTTPS is not needed and shouldn’t be used to validate SMTP security policy.
2. I think using HTTPS to validate MX records in the non-DNSSEC scenario is a
very interesting idea. But MX records are routing rather than policy and thus
don’t belong in a policy record. How about just defining a well-known HTTPS URL
that contains a copy of the actual binary MX record for the domain (or a
semantically-identical syntax transformation of that binary record)? Then we
just need the SMTP server to advertise the binary policy flag Viktor mentioned
to get a non-DNSSEC model to reasonably trustworthy MX records.
3. DEEP goes through a lot of trouble to create an extensible framework for
policy by creating a registry (while still keeping the model simple). Your
proposal notes the need for future policy work but lacks a model to add new
policies. I think the SMTP STS document is incomplete without an extensibility
model and you can get it by referencing DEEP (and if we need to change the
syntax or model in DEEP to make that more palatable, I’m open to that
discussion).
4. UTA already discussed the idea of timeouts for security policy in DEEP and
reached rough consensus not to include timeouts. I doubt the SMTP relay use
case is sufficiently different to alter that rough consensus. So I suspect we
can drop timeouts from SMTP STS after a discussion, but if there is rough
consensus to keep timeouts in SMTP STS then I’d like to reconsider that
decision for DEEP on a proposal-alignment basis.
5. DEEP’s reporting mechanism for SMTP submission (the CLIENT command) is an
as-it-happens rather than after-the-fact mechanism. I’d like to investigate the
idea of an after-the-fact mechanism for MUAs although I’m not sure it’s a good
fit. Regardless, we need to use the same reporting syntax for submission SMTP
and relay SMTP, so I’m open to changing DEEP’s current submission reporting
syntax as needed to align them.
6. I think JSON would be a better format than XML for reporting. The reports
are coming from an untrusted source and will be parsed. JSON has a much smaller
attack surface than XML and as this is part of a security mechanism, I think
that’s an important consideration. The one potential advantage I see to XML is
the wide availability of SAX-style parsers that can handle large volumes of
data, but I think the attack surface argument should win in this case. I know a
number of XML parser libraries are insecure-by-default due to schema URI
resolution. Regardless, this is an interesting design trade-off for a rough
consensus decision.
7. I think your document should reference DEEP section 6 so SMTP relay cipher
usage is recorded for trace purposes; that’s another aspect of your
transparency goal.
7. I note in passing that DEEP allows use of the PKIX trust model and the DANE
trust model at the same time. I think that’s good.
I’m wondering if I should rename DEEP to MUA STS?
- Chris
On March 21, 2016 at 20:11:06 , Daniel Margolis ([email protected]) wrote:
Thanks for the feedback to both of you. I don't disagree; I think Viktor makes
a very solid point in favor of simplicity. In addition, a report-only protocol
could be extended to support arbitrary error reporting; an out-of-band (e.g.
HTTP) channel to share delivery failures between domains strikes me as useful
in the general case.
Separately, because we're already assuming providers (both sending and
receiving) make a choice on implementing DANE and/or webPKI, I don't think
actually splitting the two makes it any more or less complex to implement, or
should discourage adoption of one or the other mechanism.
So I would say I'm feeling a bit in favor of Viktor's suggestion, but I'd like
to chat a bit more with my co-authors and think about it first. ;)
Viktor, as an aside regarding the hosted mail scenario, we already had the
suggestion to move the HTTPS endpoint to something like
"_smtp_sts.example.com/current". If we do that, the customer (example.com) can
make this a CNAME for "_smtp_sts.hostingdomain.com", who can use SNI to serve
the policy with the customer's cert (assuming the customer trusts the hosting
provider with this; for domains that don't operate their own HTTPS endpoint
this seems to me to be likely). For the more complex case, the cron setup you
describe doesn't seem too onerous, I agree.
Thanks again for the feedback.
On Tue, Mar 22, 2016 at 10:21 AM, Neil Cook <[email protected]> wrote:
> On 22 Mar 2016, at 08:49, Viktor Dukhovni <[email protected]> wrote:
>
> On Tue, Mar 22, 2016 at 08:58:25AM +0100, Daniel Margolis wrote:
>
> My (strong) suggestion: use DNS for just cache invalidation, and
> perhaps also publication (via a separate record) of the "rua"
> reporting URI. Do not duplicate data which one must in any case
> obtain and cache via HTTPS in DNS.
>
> Do not attempt to hedge your bets and support DANE/DNSSEC via STS,
> I don't think that makes much sense either.
>
I agree with the “don’t hedge your bets” part. I was quite surprised to see all
the justification for STS in the first part of the document, including “the
mechanism described here presents a variant for systems not yet supporting
DNSSEC”, and yet then goes on to include DNSSEC as one of the policy
authentication mechanisms.
> * Allow (DANE or other) domains to publish just the RUA,
> the feature is not STS-specific.
>
+1
Neil
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