Re: [dns-privacy] Datatracker State Update Notice: <draft-ietf-dprive-rfc7626-bis-04.txt>

Sara Dickinson <sara@sinodun.com> Mon, 02 March 2020 15:16 UTC

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From: Sara Dickinson <sara@sinodun.com>
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Date: Mon, 2 Mar 2020 15:16:25 +0000
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Cc: Brian Dickson <brian.peter.dickson@gmail.com>, "Eric Vyncke (evyncke)" <evyncke@cisco.com>, "dprive-chairs@ietf.org" <dprive-chairs@ietf.org>, "dns-privacy@ietf.org" <dns-privacy@ietf.org>, "draft-ietf-dprive-rfc7626-bis@ietf.org" <draft-ietf-dprive-rfc7626-bis@ietf.org>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [dns-privacy] Datatracker State Update Notice: <draft-ietf-dprive-rfc7626-bis-04.txt>
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> On 29 Feb 2020, at 02:03, Eric Rescorla <ekr@rtfm.com> wrote:
> 
> 
> 
> On Fri, Feb 28, 2020 at 4:50 PM Brian Dickson <brian.peter.dickson@gmail.com <mailto:brian.peter.dickson@gmail.com>> wrote:
> 
> 
> On Fri, Feb 28, 2020 at 3:15 PM Eric Rescorla <ekr@rtfm.com <mailto:ekr@rtfm.com>> wrote:
> 
> 
> On Wed, Feb 26, 2020 at 6:19 AM Sara Dickinson <sara@sinodun.com <mailto:sara@sinodun.com>> wrote:
> 
> 
>> On 23 Jan 2020, at 13:53, Eric Rescorla <ekr@rtfm.com <mailto:ekr@rtfm.com>> wrote:
>> 
>> On Thu, Jan 23, 2020 at 5:08 AM Sara Dickinson <sara@sinodun.com <mailto:sara@sinodun.com>> wrote:
>> 
>>> 
>>> 
>>> On 20 Jan 2020, at 22:37, Eric Rescorla <ekr@rtfm.com <mailto:ekr@rtfm.com>> wrote:
>>> 
>>> Review comments attached:
>>> 
>>> 
>>> COMMENTS
>>> S 3.1.
>>> >      described above, and may not have any confidentiality requirements.
>>> >      However, the same is not true of a single transaction or a sequence
>>> >      of transactions; that transaction is not / should not be public.  A
>>> >      typical example from outside the DNS world is: the web site of
>>> >      Alcoholics Anonymous is public; the fact that you visit it should not
>>> >      be.
>>> 
>>> Well, technically what you want to conceal is the origin of the
>>> transaction or its linkage to other transactions. The existence of the
>>> query for that A record isn't secret.
>> 
>> Suggest:
>> 
>> “that transaction (and specifically the origin of that transaction) is not / should not be public."
>> 
>> The parenthetical swallows the main sentence. The accurate thing to say is:
>> 
>> In general, the existence of a single query is not sensitive -- though there may be exceptions
>> for some very low use domains. However, the origin of a given query may leak information
>> about specific users and the ability to link queries reveals information about individual use
>> patterns.
> 
> To fit with existing text suggest:
> 
>  However, the same is not true of a single transaction or a sequence
>  of transactions; those transaction are not / should not be public. A single
>  transactions reveals both the originator of the query and the query 
>  contents which potentially leaks sensitive information about a specific user. A
>  typical example from outside the DNS world is: the web site of
>  Alcoholics Anonymous is public; the fact that you visit it should not
>  be.  
> 
> I find this text a bit clumsy, but OK.
> 
> 
> Furthermore, the ability to link queries reveals information about individual use patterns. 
> 
> The key thing is "link queries as being from the same user”

OK - will include that. 


> 
> 
> <snip>
> 
>> 
>> 
>>> 
>>> 
>>> 
>>> 
>>> S 3.4.2.
>>> >      services available.  Given this, the choice of a user to configure a
>>> >      single resolver (or a fixed set of resolvers) and an encrypted
>>> >      transport to use in all network environments can actually serve to
>>> >      identify the user as one that desires privacy and can provide an
>>> >      added mechanism to track them as they move across network
>>> >      environments.
>>> 
>>> I don't understand this paragraph. It's not the choice of specific
>>> resolvers that leaks data, but the choice to turn on encryption, In
>>> fact, from an identification purpose, the more resolvers that support
>>> encrypted transport, the better signal this is.
>> 
>> This was driven by an observation that many early adopters set up their own DoT server and used that from all their devices, which could act as a way to identify that user. 
>> 
>> 1. This seems like an edge case.
>> 2. We already have plenty of people running their own unencrypted resolvers for other reasons.
> 
> I can live with removing this text, but it does seem there is something to say about the fact configuring a single resolver for a device could differentiate a user….
> 
> Sure, but this has nothing to do with DoH or DoT.
> 
> 
> I think the text might be clearer if the bit "as one that desires privacy and" were removed.
> The issue isn't whether a specific user desires privacy, it is whether a specific user can be identified and tracked.
> 
> This RFC update, is about privacy. 
> This particular section is on encrypted transport. 
> This paragraph is highlighting that configuring a static list of resolvers, even if those use encrypted transport, may expose the user to privacy risks due to that constant set of resolvers, as the user moves between networks.
> And this risk is inversely proportional to the number of users of the resolver.
> Maybe this last bit needs to be added to emphasis why this is a risk?
> 
> It's about the fact that DoH/DoT don't protect against this or mitigate it, even if the payload is encrypted. I.e. it doesn't not have anything to do with DoH/DoT; it's bigger than DoH/DoT, and DoH/DoT don't fix the problem.
> 
> Yes, I agree with that, but this risk is generic to DNS even if you use your own resolver (e.g., pihole) which we know people do. Therefore it does not belong in this section as a risk of DoH/DoT..

I think the more general case is alluded to in the forth bullet point of section 3.4.1 without being called out specifically….  

One option would be update the first sentence of that bullet point to say “The recursive resolver can be a public DNS service (or a privately run DNS resolver hosted on the public internet).” and to add the following sentence to the end of the bullet point

“It can be noted that the choice of a user to configure a single resolver (even when using an encrypted transport) can actually serve to aid tracking of that user as they move across network  environments."

Then remove the fifth paragraph from section 3.4.2?


> 
>  
> 
> 
>> 
>>> 
>>> 
>>> S 3.5.1..1.2.
>>> >   
>>> >      o  communicate clearly the change in default to users
>>> >   
>>> >      o  provide configuration options to change the default
>>> >   
>>> >      o  provide configuration options to always use the system resolver
>>> 
>>> I get that you think this is neutral, but all of this is equally true
>>> about dynamic discovery via DHCP, it's just that that's the status
>>> quo, so you don't notice it. In this context, this text is tendentious.
>> 
>> The first paragraph of section 3.5.1.1 talks about the variability of DNS resolution privacy with network (assuming DHCP). I can add some text there to better explain the status quo if you think it is needed. Suggest:
>> 
>> “Typically joining a new network requires some form of user action (e.g physically plugging in a cable or selecting a network in a OS dialogue) and so a user is also implicitly choosing to use the DHCP-provided resolver via this action too."
>> 
>> The user has no idea that such a DHCP resolver even exists. And you could say the same thing about the user's choice to install an application with its own resolver selection logic.
>> 
>> 
>> I can’t think of a mobile or desktop OS that doesn’t allow users to override the DHCP provided DNS settings…. but I could text about that in section 5.1.1. if you really think it is needed?
>> 
>> This isn't about that. AFAICT most major applications also allow you to use the system resolver choices. Rather, the issue here is the repeated arguments in this discussion (which you implicitly accept above) that the current status quo somehow represents user choice whereas an application choosing its own resolver does not. And both this text and your proposed text implicitly takes a position on that.
> 
> I think that is reading quite a bit into the text however, to reduce any implicit misinterpretation I propose trying to further align the two sections… 
> 
> 1) Replace the first paragraph in section 3.5.1.1 with the following:
> 
> "Given all the above considerations, the choice of recursive resolver has direct privacy considerations for end users.  Historically, end user devices have used the DHCP-provided local network recursive resolver. Because of this, the choice by a user to join a particular network  (e.g. by physically plugging in a cable or selecting a network in a OS dialogue) also determines the default system DNS resolver selection; the two are directly coupled in this model.
> 
> The vast majority of users do not change their default system DNS settings and so implicitly accept the network settings for DNS. The network resolvers have therefore historically been the sole destination for all of the DNS queries from a device. These resolvers may have strong, medium, or weak privacy policies depending on the network.  Privacy policies for these servers may or may not be available and users need to be aware that privacy guarantees will vary with network.
> 
> All major OS’s expose the system DNS settings and allow users to manually override them if desired.”
> 
> 
> 2) And replace the second paragraph of 3.5.1.1.2  with this:
> 
> "Such application-specific settings introduce new control points on end user devices for DNS resolution settings in addition to the historically used system settings discussed in Section 3.5.1.1. They therefore alter the DNS privacy considerations for the device by introducing additional destinations for DNS queries. 
> 
> Users will only be aware that a new control point (with new defaults) exists if an application clearly communicates this to the user on install or upgrade. 
> 
> This reflects status quo bias. If you want to say this, then you need to point out that essentially no OS clearly communicates that it is selecting the DHCP resolver.
> 
> IMHO, this is fair, and probably adds a bit of clarity; perhaps some wordsmithing on the start of 3.5.1.1 would benefit both this and other subsections of 3.5.1.1.
> Suggestion:
> 
> Old: "Historically, end user devices have used the DHCP-provided local network recursive resolver, which may have strong, medium, or weak privacy policies depending on the network."
> 
> New: "Historically, end user devices have used the OS-selected recursive resolver, whose identity is not exposed or discoverable. The OS may have been configured with a static set of resolvers, or may use a DHCP-provided local network resolver. This OS-selected resolver may have strong, medium, or weak privacy policies, which for DHCP provided resolvers may depend on the network."
> 
> The gist of this is, over-riding the default has effects that the application cannot know, because the OS resolver choice is generally not known, and by extension, the privacy policies cannot be known.
> 
> I don't believe this is accurate. In general there are (not that portable) mechanisms for determining the DNS configuration, as is done in Chromium:
> https://cs.chromium.org/chromium/src/net/dns/?sq=package:chromium&dr=CSs&g=0 <https://cs.chromium.org/chromium/src/net/dns/?sq=package:chromium&dr=CSs&g=0>
> 

I’m fine with Ekr’s original suggestion by replacing the last sentence in the first paragraph with the following:

"The choice by a user to join a particular network  (e.g. by physically plugging in a cable or selecting a network in a OS dialogue) typically updates a number of system resources - these can include IP addresses, availability of IPv4/IPv6,  DHCP server, and DNS resolver. These individual changes, including the change in DNS resolver, are not normally communicated directly to the user by the OS when the network is joined. The choice of network has historically determined the default system DNS resolver selection; the two are directly coupled in this model.


> 
> 
> 
> 
>  
> 
> 
> The vast majority of users do not change default application-specific settings and so implicitly accept the application-specific settings for DNS. As with network resolvers, these resolvers may have strong, medium, or weak privacy policies depending on the application.  Privacy policies for these servers may or may not be available and users need to be aware that privacy guarantees will vary with each application. 
> 
> For users to have the ability to inspect and control the application-specific DNS settings in a similar fashion to the OS DNS settings, each application must also:
> 
>    o  expose the default settings to the user
> 
>    o  provide configuration options to manually override the default settings
> 
>    o  provide configuration options to always use the system resolver"
> 
> This last point is wrong. The parallel here would be to use the *network provided* resolver, not the system resolver. I would suggest that you be less prescriptive about this and just say "applications will need to provide user-visible options to configure the resolver." I would avoid "must" (even in lower case) because this is a statement of fact, not a normative one.

> 
> No, system resolver is accurate. In addition to not knowing what possible resolver is offered by DHCP, the application can't know if DHCP (i.e. network provided) is even being used -- the system could be using a static choice of resolver, or even other non-DNS resolution services (e.g. NIS+).
> 
> It turns out that there are at least three options here:
> 
> - Select your own resolver
> - Select the resolver *configured into the system* (AIUI this is what Chrome does).
> - Use the system resolver via the conventional API calls.

Suggest: 

"For users to have the ability to inspect and control the application-specific DNS settings in a similar fashion to the OS DNS settings, each application also needs to:

   o  expose the default settings to the user

   o  provide configuration options to manually override the default settings so that resolution is performed via
          * user specified resolvers
          * the resolvers configured into the system settings
          * the system resolver via conventional API calls

If all such applications are updated to use the system resolver, as described in the last bullet point, the device reverts to a single point of control for all DNS queries."

Sara. 

> 
> -Ekr
> 
>  
> Brian 
> 
> 
>> 
>> 
>> The point in section 3.5..1.1.2  terms of privacy considerations is that any application that uses an application specific DNS setting introduces another control point on the device for the DNS resolution setting (which with the current offerings is independent of the network used), and if or how that is exposed is entirely up to the application. I suggest adding this text at the start of the second paragraph:
>> 
>> "Such application-specific settings introduce new control points on end user devices for DNS resolution settings in addition to the historically used system settings."
>> 
>> Well, I suppose this is true, but it's also true of (for instance) allowing the user to set a proxy, which we've done forever.
>> 
>>> 
>>> 
>>> S 3.5.1.1.2.
>>> >   
>>> >      Application-specific changes to default destinations for users' DNS
>>> >      queries might increase or decrease user privacy - it is highly
>>> >      dependent on the network context and the application-specific
>>> >      default.  This is an area of active debate and the IETF is working on
>>> >      a number of issues related to application-specific DNS settings.
>>> 
>>> This, too, could be said about the current situation.
>> 
>> See above. 
>> 
>> And my response above..
>> 
>> -Ekr
>> 
>> 
>> Sara. 
>> 
>>> 
>>> On Mon, Jan 20, 2020 at 1:47 PM Eric Vyncke (evyncke) <evyncke@cisco.com <mailto:evyncke@cisco.com>> wrote:
>>> Thanks to Sara and Stéphane for the -04 revised I-D. 
>>> 
>>> After reading the -04, I think that most of the IETF Last Call comments are addressed (and consensus needs to be balanced -- even for informational document) and that the document sticks to facts.
>>> 
>>> But, as section 3.5.1 ("in the recursive resolvers") raised a lot of discussions during the first IETF Last Call, and as the authors reacted to those comments by deep changes in the text, let's have a new IETF Last Call before proceeding with IESG evaluation.
>>> 
>>> Again, thank you to the reviewers and the authors
>>> 
>>> Regards,
>>> 
>>> -éric
>>> 
>>> 
>>> On 20/01/2020, 22:34, "IETF Secretariat" <ietf-secretariat-reply@ietf.org <mailto:ietf-secretariat-reply@ietf.org>> wrote:
>>> 
>>>     IESG state changed:
>>> 
>>>     New State: Last Call Requested
>>> 
>>>     (The previous state was Waiting for AD Go-Ahead::AD Followup)
>>> 
>>>     The previous last call raised several points. The authors have worked on those points and this new informational IETF draft has substantive changes; enough to go trigger a new IETF Last Call.
>>> 
>>>     -éric
>>> 
>>>     Datatracker URL: https://datatracker.ietf.org/doc/draft-ietf-dprive-rfc7626-bis/ <https://datatracker.ietf.org/doc/draft-ietf-dprive-rfc7626-bis/>
>>> 
>>> 
>>> 
>>> _______________________________________________
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>>> https://www.ietf.org/mailman/listinfo/dns-privacy <https://www.ietf.org/mailman/listinfo/dns-privacy>
>>> 
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