Re: [dns-privacy] Benjamin Kaduk's No Objection on draft-ietf-dprive-rfc7626-bis-06: (with COMMENT)

Benjamin Kaduk <kaduk@mit.edu> Mon, 12 October 2020 03:04 UTC

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Date: Sun, 11 Oct 2020 20:04:32 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
To: Tim Wicinski <tjw.ietf@gmail.com>
Cc: The IESG <iesg@ietf.org>, draft-ietf-dprive-rfc7626-bis@ietf.org, dprive-chairs@ietf.org, DNS Privacy Working Group <dns-privacy@ietf.org>, Brian Haberman <brian@innovationslab.net>
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Subject: Re: [dns-privacy] Benjamin Kaduk's No Objection on draft-ietf-dprive-rfc7626-bis-06: (with COMMENT)
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Hi Tim,

On Fri, Oct 09, 2020 at 12:42:08PM -0400, Tim Wicinski wrote:
> On Wed, Oct 7, 2020 at 12:16 AM Benjamin Kaduk via Datatracker <
> noreply@ietf.org> wrote:
> 
> > Benjamin Kaduk has entered the following ballot position for
> > draft-ietf-dprive-rfc7626-bis-06: No Objection
> >
> > When responding, please keep the subject line intact and reply to all
> > email addresses included in the To and CC lines. (Feel free to cut this
> > introductory paragraph, however.)
> >
> >
> > Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> > for more information about IESG DISCUSS and COMMENT positions.
> >
> >
> > The document, along with other ballot positions, can be found here:
> > https://datatracker.ietf.org/doc/draft-ietf-dprive-rfc7626-bis/
> >
> >
> >
> > ----------------------------------------------------------------------
> > COMMENT:
> > ----------------------------------------------------------------------
> >
> > Section 1
> >
> >    At the time of writing, almost all this DNS traffic is currently sent
> >    in clear (i.e., unencrypted).  However there is increasing deployment
> >
> > nit: I think that "in the clear" is the term of art (add "the").
> >
> 
> Another comment suggested changing "in clear (i.e., unencrypted)"
> to just be "unencrypted."
> 
> Would this work for you?

Yes, that sounds fine.

> 
> >    Today, almost all DNS queries are sent over UDP [thomas-ditl-tcp].
> >
> > It looks like
> > (
> > https://mailarchive.ietf.org/arch/msg/dns-privacy/1pZL1FA57hzE1e09mQ2HMg2aWYY/
> > )
> > Sara was going to follow up with the DITL authors to try and ascertain
> > whether "almost all queries" is still accurate for the "UDP" aspect,
> > though the IETF mailarchive search doesn't seem to find any more recent
> > traffic on that topic.  Do we know if anyone actually heard back about
> > this (or the "sent in [the] clear" a few lines previously)?
> > I do not pretend to have the expertise needed to judge how the changes
> > deployed by major browser affect the statistics for "all DNS traffic"
> > (which presumably includes both stub-to-resolver and
> > resolver-to-authoritative).
> >
> 
> 
> I  talked with Sara, who did reach out to the authors, and they informed us
> that there  was no updates.

It's good that we heard back, though I guess this still doesn't match with
my intuition (and I guess Alissa made this into a discuss point).  But, I
don't trust my intuition here without data to back it up :)

> >    This has practical consequences when considering encryption of the
> >    traffic as a possible privacy technique.  Some encryption solutions
> >    are only designed for TCP, not UDP and new solutions are still
> >    emerging [I-D.ietf-quic-transport] [I-D.huitema-quic-dnsoquic].
> >
> > [It looks like dnsoquic became draft-huitema-dprive-dnsoquic.]
> >
> >
> Oh good catch - and in *our* WG!
> 
> 
> 
> > Section 3
> >
> >    multiple dynamic contexts of each device.  This document does not
> >    attempt such a complex analysis, instead it presents an overview of
> >    the various considerations that could form the basis of such an
> >    analysis.
> >
> > nit: looks like a comma splice.
> >
> 
> I changed "such a complex analysis, instead it" to
> "such a complex analysis, but instead it"

+1

> > Section 4.1
> >
> >    authentication or authorization of the client (resolver).  Due to the
> >    lack of search capabilities, only a given QNAME will reveal the
> >    resource records associated with that name (or that name's non-
> >    existence).  In other words: one needs to know what to ask for, in
> >
> > I agree with Warren that this statement ("only [...] will reveal [...]
> > or that name's non-existence") is overly strong.
> 
> 
> > Section 4.2
> >
> >    The DNS request includes many fields, but two of them seem
> >    particularly relevant for the privacy issues: the QNAME and the
> >    source IP address. "source IP address" is used in a loose sense of
> >    "source IP address + maybe source port number", because the port
> >
> > In other contexts I've seen this combination referred to as the
> > "transport address".
> >
> >    The QNAME is the full name sent by the user.  It gives information
> >    about what the user does ("What are the MX records of example.net?"
> >    means he probably wants to send email to someone at example.net,
> >    which may be a domain used by only a few persons and is therefore
> >    very revealing about communication relationships).  [...]
> >
> > (editorial) something like not-a-secret-cabal.example might make the
> > example more visceral than example.net does.
> >
> >    create more problems for the user.  Also, sometimes, the QNAME embeds
> >    the software one uses, which could be a privacy issue.  For instance,
> >    _ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org.
> >
> > (nit) I trust that this can be made into a complete sentence while
> > addressing Warren's more-substantive comment.
> >
> >    There are also some BitTorrent clients that query an SRV record for
> >    _bittorrent-tracker._tcp.domain.example.
> >
> > In a similar vein, I'm not sure what domain.example is supposed to
> > represent here -- the domain of the author of the BitTorrent client?
> >
> >    Therefore, all the issues and warnings about collection of IP
> >    addresses apply here.  For the communication between the recursive
> >
> > I mostly assume that this is intended to be a reference to the generic
> > concerns about "IP addresses are PII", etc., that one is ambiently
> > exposed to by reading enough about the Internet.  (There does not seem
> > to be previous discussion of "collection of IP addresses" in this
> > document, which would seem to indicate that it is not trying to refer
> > back to previous text.)  If so, perhaps an extra word or two would help
> > ("all the standard issues and warnings", "all the generic issues and
> > warnings", etc.) clarify the intent of the reference.
> >
> >
> I want to get back to this.

Okay.

> >    However, hiding does not always work.  Sometimes EDNS(0) Client
> >    subnet [RFC7871] is used (see its privacy analysis in
> >    [denis-edns-client-subnet]).  [...]
> >
> > (nit) The wording here ("its privacy analysis") suggests that the
> > referenced document is an authoritative/official IETF position, but it
> > seems to be a blog post by a single individual.  Using "one" or "a"
> > rather than "its" would convey a less-authoritative connotation.
> >
> >
> I replaced this with "one"

Thanks.

> >                                        In both cases, the IP address
> >    originating queries to the authoritative server is as sensitive as it
> >    is for HTTP [sidn-entrada].
> >
> > I don't see how [sidn-entrada] supports the claim that end-user-adjacent
> > DNS client IP addresses are equally sensitive as HTTP client IP
> > Addresses; it mentions "sensitive" only twice (as "privacy-sensitive",
> > admittedly, applying to such IP addresses, but as an assertion without
> > justification) and "http" only in URLs (mostly in the references) and as
> > an example request.  It would feel more natural to use an IETF reference
> > here, as well -- e.g., RFC 7624 discusses correlating client IP
> > addresses with end users, RFC 7239 clearly covers privacy considerations
> > for sending client IP addresses in the "forwarded" header field, and
> > there are no doubt others -- though I do note the contents of the
> > paragraph after this one.
> >
> >
> Skipping this for now.

Okay.

> >                      However, for both IPv4 and IPv6 addresses, it is
> >    important to note that source addresses are propagated with queries
> >    and comprise metadata about the host, user, or application that
> >    originated them.
> >
> > (This "propagated with queries" is still contingent on EDNS(0) Client
> > Subnet from the previous paragraph, right?)
> >
> >
> Yes it is - do you think it needs to be re-mentioned?

I'd prefer if it was, perhaps "are propagated with queries via EDNS(0)
Client subnet"

> > Section 4.2.1
> >
> >    cache poisoning attacks by off-path attackers.  It is noted, however,
> >    that they are designed to just verify IP addresses (and should change
> >    once a client's IP address changes), they are not designed to
> >    actively track users (like HTTP cookies).
> >
> > nit: comma splice.
> >
> 
> Would this change work for you:
> 
> update "IP address changes), they are not designed to"
> 
> with  "IP address changes), but they are not designed to"

Sure.

> > Section 5.1
> >
> >    not be.  When other protocols will become more and more privacy-aware
> >    and secured against surveillance (e.g., [RFC8446],
> >    [I-D.ietf-quic-transport]), the use of unencrypted transports for DNS
> >    may become "the weakest link" in privacy.  It is noted that at the
> >    time of writing there is on-going work attempting to encrypt the SNI
> >    in the TLS handshake [I-D.ietf-tls-sni-encryption].
> >
> > This mention of encrypted "SNI" (now encrypted ClientHello) comes as a
> > bit of a non sequitur.  I suggest a bit of transition such as an
> > additional clause at the end of the sentences ", which is one of the
> > last remaning non-DNS cleartext identifiers of a connection target".
> > (While the actual work itself has progressed to encrypting the entire
> > ClientHello, I think it's okay to focus the exposition here on the SNI,
> > as the relevant attribute.)
> >
> >
> You're suggesting adding your clause at the end of that sentence? Like so:
> 
>     It is noted that at the time of writing
>    there is on-going work attempting to encrypt the SNI in the TLS handshake
>    [@RFC8744], which is one of the
>    last remaining non-DNS cleartext identifiers of a connection target.

Right, that's the suggestion.  (You don't have to use it, of course.)

>                                                          It can be noted
> >       that if the user selects a single resolver with a small client
> >       population (even when using an encrypted transport) it can
> >       actually serve to aid tracking of that user as they move across
> >       network environment.
> >
> > I wonder if it is worth adding another clause at the end: ", and that an
> > attacker in a position to observe the moving user is likely also able to
> > observe the likely-unencrypted DNS queries from the resolver to the
> > authoritative servers"
> > Also, nit: "environments" plural.
> >
> 
> Thanks fixed.
> 
> >
> > Section 5.2
> >
> >    Traffic analysis of unpadded encrypted traffic is also possible
> >    [pitfalls-of-dns-encryption] because the sizes and timing of
> >    encrypted DNS requests and responses can be correlated to unencrypted
> >    DNS requests upstream of a recursive resolver.
> >
> > We could (but don't have to) note that effective padding policies remain
> > an open area of research.
> >
> > Section 6.1.1.2
> >
> >    o communicate clearly the change in default to users
> >
> > I think this is intending to say "when the default application resolver
> > changes away from the system resolver", but the present text is perhaps
> > a little unclear about what "the change" is referring to.
> >
> >
> I see.  Is this better?
> 
> "communicate clearly the change to users when the default application
>      resolver changes away from the system resolver"

Better, yes.  Though maybe "communicate clearly to users the change when
[...]" is better still?

> > Section 6.1.2
> >
> >                                                                 Even if
> >    encrypted DNS such as DoH or DoT is used, unless the client has been
> >    configured in a secure way with the server identity, an active
> >    attacker can impersonate the server.  [...]
> >
> > More than the server identity is needed -- the credentials or trust
> > anchor needed to authenticate a peer as that identity are also needed.
> >
> > Section 6.1.3
> >
> >    User privacy can also be at risk if there is blocking (by local
> >    network operators or more general mechanisms) of access to remote
> >    recursive servers that offer encrypted transports when the local
> >    resolver does not offer encryption and/or has very poor privacy
> >    policies.  [...]
> >
> > I suggest adding "e.g." before "when the local resolver" to avoid giving
> > the impression that this is an exhaustive list.
> >
> 
> Good idea.  Added
> 
> >
> >    This is a form of Rendezvous-Based Blocking as described in
> >    Section 4.3 of [RFC7754].  Such blocklists often include servers know
> >    to be used for malware, bots or other security risks.  In order to
> >    prevent circumvention of their blocking policies, some networks also
> >    block access to resolvers with incompatible policies.
> >
> > Perhaps this is touching too much on the controversial topic, but it
> > seems to me that the networks in question "attempt to block access";
> > whether or not they fully and reliably succeed at doing so is not clear.
> > (See also the near-impossibility of closing covert channels in
> > protocols.)
> >
> >    It is also noted that attacks on remote resolver services, e.g., DDoS
> >    could force users to switch to other services that do not offer
> >    encrypted transports for DNS.
> >
> > nit: comma after DDoS.
> >
> 
> Fixed
> 
> >
> > Section 6.1.4.2
> >
> >    Some implementations have, in fact, chosen to restrict the use of the
> >    'User-Agent' header so that resolver operators cannot identify the
> >    specific application that is originating the DNS queries.
> >
> > With similar disclaimer as previously, perhaps "trivially identify"?
> > There are other fingerprinting techniques possible even at, e.g., the TLS
> > layer (that we discussed previously in this document!), which still
> > apply to DoH.
> >
> > Section 6.2
> >
> >    This "protection", when using a large resolver with many clients, is
> >    no longer present if ECS [RFC7871] is used because, in this case, the
> >    authoritative name server sees the original IP address (or prefix,
> >    depending on the setup).
> >
> > (side note) this has always been a bit confusing to me -- ECS is "client
> > subnet", not "client address", and I don't really understand why someone
> > would set the prefix length to the full 128 (or 32) bits of the address.
> > Is there really a lot of non-truncated client addresses being sent
> > around like this?  How did that happen?
> >
> 
> I'm not sure how there are non-truncated client addresses, but I would
> say if something can be done in the DNS, it is being done.
> 
> >
> >                                                                     So,
> >    requests to a given ccTLD may go to servers managed by organizations
> >    outside of the ccTLD's country.  End users may not anticipate that,
> >    when doing a security analysis.
> >
> > (Is this a "for example"?  It seems plausibly relevant for non-cc TLDs
> > as well.)
> 
> 
> yes, I changed "So" to "For example"
> 
> 
> > Section 7.1
> >
> >    The IAB privacy and security program also have a work in progress
> >    [RFC7624] that considers such inference-based attacks in a more
> >    general framework.
> >
> > I do not really think the final RFC constitutes a "work in progress"
> > anymore.
> >
> 
> Correct - this is a good catch.
> 
> >
> > Section 8
> >
> >    Passive DNS systems [passive-dns] allow reconstruction of the data of
> >    sometimes an entire zone.  They are used for many reasons -- some
> >    good, some bad.  Well-known passive DNS systems keep only the DNS
> >    responses, and not the source IP address of the client, precisely for
> >    privacy reasons.  Other passive DNS systems may not be so careful.
> >
> > Perhaps not so well-intentioned, either...
> >
> >    The revelations from the Edward Snowden documents, which were leaked
> >    from the National Security Agency (NSA) provide evidence of the use
> >
> > nit: comma after "(NSA)".
> >
> >
> Fixed
> 
> > Section 9
> >
> >    To our knowledge, there are no specific privacy laws for DNS data, in
> >    any country.  Interpreting general privacy laws like
> >    [data-protection-directive] or GDPR [10] applicable in the European
> >    Union in the context of DNS traffic data is not an easy task, and we
> >    do not know a court precedent here.  See an interesting analysis in
> >    [sidn-entrada].
> >
> > This text is essentially unchanged since RFC 7626; did we do much of a
> > search for whether the past five years have brought about changes in the
> > legal landscape?
> >
> 
> I can speak from personal experience in my previous employer and I
> worked with corporate legal to track anything useful.
> 
> 
> I know I missed some of your comments, I am not blowing you off. I wanted
> to capture the editorial fixes first while I think through a few of the
> others.

Thanks for noting that, and of course for the updates already made.

-Ben