Re: [Gen-art] [dns-privacy] Genart last call review of draft-ietf-dprive-bcp-op-07

[Eric Rescorla <[e-mail]>]

On Mon, Feb 10, 2020 at 8:14 AM Mohit Sethi M <mohit.m.sethi=
[e-mail]> wrote:

> Hi Sara,
>
> I understand the desire to get this done with. However, I have some
> further comments in-line:
> On 1/24/20 5:29 PM, Sara Dickinson wrote:
>
> Mohit,
>
> I’m out of the office next week so in order to try to move the draft along
> I have published an -08 version which I think addresses most of your
> comments (there were a few questions in my response below). Please let me
> know if any are still outstanding.
>
> Best regards
>
> Sara.
>
> On 17 Jan 2020, at 15:33, Sara Dickinson <[e-mail]> wrote:
>
>
> On 29 Dec 2019, at 13:50, Mohit Sethi via Datatracker <[e-mail]>
> wrote:
>
> Reviewer: Mohit Sethi
> Review result: On the Right Track
>
> I am the assigned Gen-ART reviewer for this draft. The General Area
> Review Team (Gen-ART) reviews all IETF documents being processed
> by the IESG for the IETF Chair.  Please treat these comments just
> like any other last call comments.
>
> For more information, please see the FAQ at
>
> <https://trac.ietf.org/trac/gen/wiki/GenArtfaq>.
>
> Document: draft-ietf-dprive-bcp-op-07
> Reviewer: Mohit Sethi
> Review Date: 2019-12-29
> IETF LC End Date: 2020-01-02
> IESG Telechat date: Not scheduled for a telechat
>
> Summary:
> This draft discusses privacy challenges for recursive DNS resolvers. It
> then
> describes policy and security considerations that DNS service providers
> can use
> for enhanced user privacy. The draft is 'On the Right Track' but not yet
> ready.
>
>
> Many thanks for the detailed review! Ben, Rob I hope theses fixes also
> address your comments.
>
>
> Major issues:
>
> I wonder if section 5.1.2.1/5.1.3.1 should also talk about recommending
> OCSP
> stapling (RFC 6066)? I looked at RFC 8310 and it mentions RFC 7525. Do you
> want
> to mention it here in section 5.1.2.1/5.1.3.1?
>
>
> What exactly are you thinking of here - something that just says “Server
> operators should also follow the best practices with regard to OCSP as
> described in RFC7525”? If something more could you please suggest text?
>
> I was hoping that the text would be more precise rather than a cursory
> reference to another RFC. You could say something along the lines: 'The TLS
> client and server MUST use Certificate Status Requests [RFC6066] for the
> server's certificate chain and the client MUST treat a CertificateEntry
> (except the trust anchor) without a valid CertificateStatus extension as
> invalid and abort the handshake with an appropriate alert.'. In the same
> vein, I would expect that you would strongly mandate the use of TLS 1.3
> with DoH and DNS over TLS?
>
Mohit: I actually don't think we should require OCSP at all. For instance,
most browsers are moving away from it. We should require or at least
strongly encourage revocation checking.

-Ekr



>
> In section 5.1.2.1, what is meant by 'authentication domain names'? Later,
> the
> text says 'authentication name for the service'. I guess you are implying
> the
> authentic domain name of the DNS resolver service that the client software
> should verify through the common name (CN) in the certificate? Some more
> explanation here would be beneficial.
>
>
> It is defined in the terminology section of RFC8310:
>
> "Authentication domain name: A domain name that can be used to
> authenticate a privacy-enabling DNS server.  Sources of authentication
> domain names are discussed in Section 7."
>
> I have added a reference for RFC8310 after the first use of
> ‘authentication domain name’ and made sure every instance of
> 'authentication name' is updated to 'authentication domain name' for
> clarity.
>
> Personally, I find the phrase 'Authentication domain name' very unclear.
> From the phrase, it doesn't look like it has anything to do with DNS? On
> first reading, I interpreted it as the domain name where I should
> authenticate. Since the IESG let this through for RFC 8310, I guess we will
> have to live with this (poor) choice.
>
>
>
> In section 5.1.4, should 'DNS Roadblock avoidance' be 'DNSSEC Roadblock
> avoidance'? And please add a reference to RFC 8027 here if that is the
> case.
>
>
> Yes, good catch - will do.
>
>
> Section 5.1.7 says "discussion on the use of Bloom Filters in Appendix A".
> It
> is pointing to the wrong appendix.
>
>
> Fixed - thanks.
>
> Also, this section talks about implementing
> traffic monitoring by the DNS service provider. I would argue that in most
> deployments, the traffic monitoring is required (and implemented) by a
> different entity. Think of a home network router that has a parental
> control.
> Or an enterprise restricting employees from visiting certain sites (to
> prevent
> insider attacks)? The impact of encryption is more serious for them and
> less so
> for a DNS service provider. What is the BCP advice for them?
>
>
> You are correct that the there are differing concerns but I don’t believe
> this document should tackle that - the audience of the document is
> specifically operators of DNS privacy services, typically monitoring to
> prevent DDoS or similar, not network operators in general (although they
> may be both in some cases). For the more general case I think the impact is
> covered in RFC8404 'Effects of Pervasive Encryption on Operators’?
>
> I do notice a couple of places where just ‘operators’ is used in the text
> so could add ‘DNS Privacy Service’ before that to clarify?
>
> I leave it to Alissa and IESG to decide what they want here.
>
>
> Also, is it fair
> to say that this is a best current practice? It feels that we need more
> experience before we start recommending it as an optimization.
>
>
> Given the specific scope discussed above I think it is fair. The privacy
> policies of most of the public resolver operators and ISPs that offer
> encrypted DNS are pretty good today in terms of how they try to minimise
> the user data retained and I’m sure they all still have monitoring in
> place…
>
> My question was specifically about using Bloom filters? Which DNS
> operators are currently using it? Do we have sufficient evidence other than
> the research publication that this is 'Best Current Practice'?
>
>
>
> This comment applies to all 5.1.1-5.1.8. Each subsection starts rather
> abruptly
> by discussing threats. It would be nice if you add a sentence at the
> beginning
> of each sub-section telling the reader what are they heading into. This is
> probably most obvious from section 5.1.8. Without even telling the reader
> what
> is a pure TLS proxy, you start listing the DNS privacy threats. Only later
> on
> you mention option that operators may implement DNS-over-TLS using a TLS
> proxy.
>
>
> If we go down this road then I think to be consistent we would need to add
> text for all 16 sections 5.1.1 to 5.3.3. I could do this but I have a
> feeling it would be come quite repetitive with respect to the section title
> text and I think if we could get the titles correct (and possibly add more
> text to section 5) this might be unnecessary. I suggest:
>
> Section 5: Add a first paragraph:
> “In the following sections we first outline the threats relevant to the
> specific topic and then discuss the potential actions that can be taken to
> mitigate them.”thorita
>
> And for section 5.1.8 change the title to “Limitations of fronting a DNS
> Privacy Service with a pure TLS proxy”.
> Happy to update any other headers you thought too vague.
>
> Would that address the issue or do you still think additional text is
> required?
>
>
> In section 5.3.2 'way and OUGHT obfuscate'. OUGHT is not part of RFC
> 2119/8174.
> Why is it capitalized? And, 'ought' ought to be followed by a 'to’.
>
>
> I was confused by this and then realised it is a hangover from a much
> earlier version of the draft that used EXPECTED/OUGHT/MIGHT keywords
> defined in the draft to described the various levels of actions…. so as you
> suggest:
>
> OLD: “For example, a resolver with a very small community of users risks
> exposing data in this way and OUGHT obfuscate this...”
>
> NEW: “For example, a resolver with a very small community of users risks
> exposing data in this way and ought to obfuscate this..."
>
>
> At the beginning of section 5, you describe three classes of actions.
> However,
> none of the subsections contain clear "Additional options" that operators
> need
> to follow for "maximal compliance”?
>
>
> Sections 5.3.1 and 5.3.2 do have them. In earlier versions several more
> sections but they have been removed. thorita
>
> Okay. Thanks. I hadn't seen this. I have a question related to the
> following text:
>
>    o  Aggressive Use of DNSSEC-Validated Cache [RFC8198 <https://tools.ietf.org/html/rfc8198>] and [RFC8020 <https://tools.ietf.org/html/rfc8020>]
>       (NXDOMAIN: There Really Is Nothing Underneath) to reduce the
>       number of queries to authoritative servers to increase privacy.
>
> Suppose I own the domain mydomain.com? My application requires clients to
> lookup for non-existent sub domains. So if a client sends a query to the
> local resolver for non-existent1.mydomain.com and
> non-existent2.mydomain.com, will both be sent to the authoritative server
> for mydomain.com?
>
>
>
> The document seems focused on TLS 1.2 (and does not talk about TLS 1.3). In
> fact, RFC 8446 is not even in the list of references even though section
> 5.1.3.1 mentions it. Similarly, Appendix A.2 mentions TLS session
> resumption
> without server-side state. How about servers using TLS 1.3 and PSK
> resumption?
> RFC 8446 has text on client tracking in appendix C.4:
> https://tools.ietf.org/html/rfc8446#appendix-C.4.
>
>
> A slight hangover from the fact the DNS-over-TLS spec was published in
> 2015 before TLS 1.3 was standardised (so it just says TLS 1.2 or later) and
> so most of the early DoT services used just TLS 1.2.
>
> Section 5.1.3.1 had the text ‘RFC8446’ but it wasn’t actually a reference
> so I’ve fixed that - thanks.
>
> I’ve added a bullet point to Appendix A.2
> “RFC8446 Appendix C.4 describes Client Tracking Prevention in TLS 1.3"
>
>
> There is something wrong about the last sentence of Appendix A.2 'Note that
> depending on the specifics of the implementation [RFC8484] may also provide
> increased tracking'. You already mention RFC 8484 in Appendix A.1 as a
> means to
> increase privacy. Perhaps you wanted to cite a different RFC here?
>
>
> The point is that the use of HTTP headers in DoH can add additional
> privacy concerns over the other DNS transports, but that RFC8484 leaves
> that as an implementation decision. I suggest replacing the text with
>
> “Note that Section 8 of RFC8484 outlines the privacy considerations of
> DoH. Depending on the specifics of a DoH implementation there may be
> increased identification and tracking compared to other DNS transports."
>
> Think of the reader of this document. Appendix A.1 says that DoH can
> increase privacy. The Appendix A.2 says that with DoH 'identification and
> tracking may be increased'  Should I use DoH or not? I recommend to
> re-phrase the text in A.2 along the lines 'While DoH can increase privacy,
> section 8 of RFC 8484 outlines potential mechanisms that can nonetheless be
> used by on-path adversaries for correlation and tracking. As recommended in
> RFC 8484, DNS resolvers that offer DoH need to consider the benefit and
> privacy impact of these features, and their deployment context when
> deciding what features are enabled. Resolver implementations are advised to
> expose the minimal set of data needed to achieve the desired feature set.'
>
>
>
> Minor issues:
>
> Nits/editorial comments:
>
> There is mixed usage of Anonymisation (in Table 1) vs Anonymization. The
> same
> with Pseudoanonymisation (in Table 1) vs pseudonymization in text. Please
> check
> with the RFC editor on what is expected and use that consistently. Also
> noticed
> optimisation.
>
>
> Thanks - have fixed. I have now used the American English forms (z not s)
> throughout.
>
>
> In Table 1, Crytpographic should be Cryptographic.
>
>
> Ack.
>
>
> Maybe you could use an Oxford comma when using lists of items.
>
>
> Had it some places, but not all - should be fixed now.
>
>
> In section 5.1.2.1, there is stray space character at the end of the
> bullet on
> "Follow the guidance in Section 6.5 of [RFC7525] with regards to
> certificate
> revocation .”
>
>
> Perhaps expand DNSSEC on first usage: Domain Name System Security
> Extensions
> (DNSSEC).
>
> In section 5.1.6 'in terms of such options as filtering' should instead be
> 'in
> terms of options such as filtering'.
>
> In section 5.1.8 'a DNS aware proxy such as [dnsdist] which offer custom
> (similar to that proposed in'. Consider using 'offers' instead of 'offer'
> and
> 'similar to those proposed in' instead of 'similar to that proposed in'.
>
> In section 5.2.2 'presents and overview' should be 'presents an overview'.
> Consider rephrasing 'the better to resist brute force'. Also, in 'agreed
> solution or any Standards to inform', why is standards capitalized?
>
> In section 5.2.4 'queries on multiple TCP session' should be 'queries on
> multiple TCP sessions'. Please expand CPE on first usage.
>
> In section 6.3 'This is by analogy with e.g. several TLS or website' could
> instead be 'This is analogous to several TLS or website'.
>
> In Appendix A.1 'documents apply to recursive to authoritative DNS'
> shouldn't
> there be an 'and’?
>
>
> All fixed - thanks.
>
>
> In Appendix C.1, consider changing the format for the sub bullets of '2.
> Data
> collection and sharing.'. Instead of numbering them with 1/2/3, perhaps use
> a/b/c.
>
>
> I’m currently using markdown which won’t let me do that…. :-( I do think
> that would be better so I suggest adding a note that this is done at RFC
> editor time….?
>
> Yes, please do. It will help with readability.
>
> Thanks for the rest of the changes.
>
> --Mohit
>
>
>
> In Appendix C.1 'of use of system' could be 'of system use'. Also why is
> there
> a line break between 'items that are' and 'included'? There is an
> extraneous
> 'the' in 'available with the our threat intelligence'. Consider re-wording
> parts of paragraph '3. Sharing of data. '. At one place you say 'with our
> threat intelligence partners' and a few words later you say 'with its
> threat
> intelligence partners’.
>
>
> Fixed.
>
>
> In Appendix C.1 'In the event of events or observed behaviors' is a bit
> hard to
> parse. Consider rephrasing the 'event of events' part.
>
>
> Replaced events with actions.
>
> Best regards
>
> Sara.
>
>
>
>
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