On March 24, 2016 at 2:16:27 , Mark Risher ([email protected]) wrote:
Hi, Chris:
Thanks for the 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.
In addition to the possible difficulty in migrating a domain off of a server
(particularly in a multi-tenant config), we also felt that introducing a new
SMTP verb might be dramatically more complicated than deploying in parallel,
because at least the DNS/webpki reporting can be added offline without changes
to the core MTA.
The SMTP-policy in DNS model allows advertising receiver policy without
changing the MTA, but it still requires a core MTA upgrade to the sender before
it actually improves security for the domain. Most MTA software includes both
receiver and sender in the same package, so sites do need to upgrade their core
MTA package to get better security. I don’t think it’s a good idea for a site
to have receiver and sender software of different product versions deployed
(except during a transition period). So I see no actual deployment benefit for
the SMTP policy negotiation by putting it in DNS.
Was that not a concern in DEEP? Or simply unavoidable?
Both, plus I’m not a fan of adding new unnecessary protocol traffic. Having the
negotiation added to the existing SMTP EHLO negotiation doesn’t add new packets
to the core mail transfer operation as the DNS-policy approach does.
I may be biased as a email server developer, but it seems much easier to add
protocol extensions at the application layer than to change
performance-critical lower level connection logic and DNS lookups (often deeply
buried in badly designed APIs). In addition, the application protocols tend to
have good error reporting mechanisms (human readable strings) which are not
present in DNS or other layers so it’s going to be easier to get an
application-layer solution working in practice.
Now for the SMTP relay scenario, decoupling the reporting mechanism from the
core MTA makes a lot of sense to me. It may also make sense for
IMAP/POP/Submission, but I need to think about that some more as there are
subtle deployment tradeoffs.
- Chris
/m
--
Mark E. Risher | Group Product Manager | [email protected] |
650-253-3123
On Thu, Mar 24, 2016 at 1:45 AM, Daniel Margolis <[email protected]> wrote:
Hey,
Of course we reviewed DEEP during the drafting process, but as you say, the
targets are slightly different. I've responded to some individual points
inline; in summary, though, I think you raise some actionable points about
using the DEEP policy framework (which I will read up on), changing the
reporting syntax, and recording cipher usage. I have questions for you below on
some of the other points.
Thanks.
On Thu, Mar 24, 2016 at 4:44 AM, Chris Newman <[email protected]> wrote:
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).
Agreed that there's some overlap. One rather obvious observation, to frame the
discussion, though, is that DEEP addresses a case where clients connect to
relatively few *novel* servers (you rarely configure a new IMAP or SMTP MSA
server) and have user-facing UIs, so some of the challenges around discovering
and remembering policies and handling failures are a bit different. But we
should certainly try to align the proposals nonetheless.
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.
If I read correctly, you're making two different comments here:
a. We should communicate the STS policies within SMTP, not using DNS.
b. We should authenticate the STS policies directly using the SMTP-TLS server
certificate, not some HTTPS side-channel. (Of course this follows from (a), for
the most part.)
Those are spot-on comments. We had initially proposed solely extending SMTP for
policy exchange, similarly to Jim's proposal in "REQUIRETLS" but in reverse
(https://tools.ietf.org/html/draft-fenton-smtp-require-tls-01). However, for
our proposed semantics (i.e. "always require TLS for my domain in the future,
unless I give you a signed change of policy"), this is tricky in a very
specific case: if changing the policy requires serving a new signed policy, and
serving policies is done via SMTP, then users of hosted mail services who turn
on "require TLS" will be unable to revoke their published policy if they move
to a hosted provider who doesn't support the protocol to begin with.
(It also means these domains' SMTP servers will have to support SNI and serve a
server certificate that actually matches the recipient domain, whereas the
current mechanism allows one to run a separate HTTPS server with client.com's
cert but to "bless" SMTP connections to mailhost.com's cert.)
So as far as I can see, to satisfy this scenario, the policy exchange *must* be
some out-of-band protocol. HTTPS is purely a choice of convenience; I had
originally proposed signed blobs stuck in DNS RR, but big resource records seem
problematic in practice (even if the RFC supports them) and the whole thing
gets ugly fast.
Can you think of a way to handle the hosted scenario with SMTP, or some other
more elegant solution?
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.
I think this is mostly pending the discussion for #1: if we need DNS for the
reason outlined above, it's convenient to use it for this, too. If we don't, of
course, it may not be.
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).
I will have to reread this part of your proposal more closely, but unifying the
policy framework certainly sounds reasonable on its face.
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.
Is there a particular mail thread I should read to get the context here?
Sorry...
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.
No strong feeling here on my end. XML is in theory convenient because it
supports XSD; in practice I think the XSD I wrote in the draft has some bugs.
;) Using JSON is also quite reasonable, and *feels* like a natural choice if we
want to support reporting via HTTP/S POST as well, which we may.
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.
Thanks, makes sense.
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?
DEEP is a pretty cool acronym, but yeah, STS has this dis/advantage of
referencing a mostly known item (HSTS). Insofar as I was trying to hew as
closely to the HSTS trust model as possible (within the confines of the
protocol) I think that's a helpful analogy. But probably not the biggest point
right now. ;)
- 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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