DOH                                                         J. Livingood
Internet-Draft                                                   Comcast
Intended status: Informational                            M. Antonakakis
Expires: September 25, 2019              Georgia Institute of Technology
                                                               B. Sleigh
                                                                  BT Plc
                                                             A. Winfield
                                                          March 24, 2019

    Centralized DNS over HTTPS (DoH) Implementation Issues and Risks


   The DNS over HTTPS (DoH) protocol is specified in RFC8484.  This
   document considers Centralized DoH deployment, which seems one likely
   way that DoH may be implemented, based on recent industry discussions
   and testing.  This describes that implementation model, as well the
   potential associated risks and issues.  The document also makes
   recommendations pertaining to the implementation of DoH, as well as
   recommendations for further study prior to widespread adoption.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 25, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Separating the Protocol from Implementation Issues  . . . . .   2
   3.  Network Operators Are Interested in Deploying DoH . . . . . .   3
   4.  Centralized DoH Defined . . . . . . . . . . . . . . . . . . .   3
   5.  Centralization vs. De-Centralization of Services  . . . . . .   4
   6.  Centralized DoH Assumption: Enabled/Centralized by Default  .   5
   7.  Potential for Rapid Centralized DoH Adoption  . . . . . . . .   5
   8.  Potential Technical Risks . . . . . . . . . . . . . . . . . .   6
   9.  Potential Business Risks  . . . . . . . . . . . . . . . . . .  14
   10. Recommendations . . . . . . . . . . . . . . . . . . . . . . .  15
   11. Document Reviewer Acknowlegedments  . . . . . . . . . . . . .  18
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   14. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  19
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     15.1.  Informative References . . . . . . . . . . . . . . . . .  19
     15.2.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  23
   Appendix B.  Open Issues  . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   The DNS over HTTPS (DoH) protocol is specified in [RFC8484].  This
   document considers Centralized DoH deployment, which seems one likely
   way that DoH may be implemented, based on recent industry discussions
   and testing.  This describes that implementation model, as well the
   potential associated risks and issues.  The document also makes
   recommendations pertaining to the implementation of DoH, as well as
   recommendations for further study prior to widespread adoption.

2.  Separating the Protocol from Implementation Issues

   This document is not intended as a critique of the DoH protocol
   itself, which can be a valued addition to the Internet and appears to
   have many helpful uses.  Rather, this document focuses solely on how
   DoH is now being implemented and/or might be implemented.  Thus, in

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   no way should this document be read as critical of the DoH protocol
   itself, which can bring positive benefits to end user privacy.

   In addition, conventional DNS generally uses UDP port 53, though in
   some cases TCP is used.  DoH is still DNS but it uses an entirely
   different protocol to send and receive queries.  This document does
   not delve into the particulars of the DoH protocol.  For those
   details, please refer to [RFC8484].

3.  Network Operators Are Interested in Deploying DoH

   Network operators, ranging from ISPs to enterprises, schools, and
   others work hard to provide outstanding DNS and network performance,
   as well as to protect the security and privacy of users.  In
   addition, most also provide DNS-based services such as opt-in
   parental controls for consumers or malware/security protection in
   enterprises, content filtering in schools, etc.  These operators are
   also interested in adding support for DoH (as well as DNS over TLS,
   DoT).  However, the current Centralized DoH implementation model does
   not appear to make it possible for these operators to continue to
   play a value added role in the delivery of network services, or to
   continue to provide DNS-related services, and may even cause problems
   beyond that.

   In addition, the DoH resolvers that network operators might provide
   would likely not be open recursive resolvers but would instead mirror
   the current model whereby the resolver has an access control list
   (ACL) so that the servers only respond to clients on that network,
   which helps to reduce abuse (see the Open Resolver Project) [1].
   This does not appear to be compatible with the current Centralized
   DoH implementation model that appears to assume that DoH resolvers
   are openly accessible from any network.

   Finally, network operators also typically have a direct, trusted
   relationship with users, often bound by legal agreements including
   Terms of Service and a Privacy Policy.  Depending upon the particular
   country/region there are laws and regulations, such as the General
   Data Protection Regulation (GDPR), that may also govern user privacy,
   data collection, and data handling.  All of those things would apply
   to network operator DoH services as they do to conventional DNS

4.  Centralized DoH Defined

   DoH clients have been implemented in a number of platforms, including
   in the Mozilla Firefox web browser (see Mozilla blog) [2], and Google
   Chrome (Chromium) web browser (see Google Public DNS Developer web
   site) [3].  In deployments thus far, Mozilla has defaulted to

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   Cloudflare (see Mozilla blog) [4], and Google's Chrome browser have
   used Google Public DNS (see Google Public DNS Developer web site)

   Since directing DoH-based DNS traffic to a small number of commercial
   DNS providers represents a high degree of concentration and
   centralization of operation and control, this document describes this
   potential implementation model as "Centralized DoH".

5.  Centralization vs. De-Centralization of Services

   DNS is the most highly distributed global database on the Internet,
   with millions of people and organizations independently changing and
   adding to this database in a loosely coupled system.  The web (HTTP/
   HTTPS) is also highly distributed, with countless parties creating
   and publishing content on the web.  Both the DNS and HTTP protocols
   demonstrate the key architectural tenets of the Internet, such as
   loose coupling between systems and layers, loose coordination between
   different entities that use a particular protocol, and broadly de-
   centralized distribution of the protocol and associated systems.

   But over the past decade, there has been a trend of most web traffic
   shifting towards a small number of very large web platforms.  For
   example in 2009, Craig Labovitz described the rise of the so-called
   hyper-giants (see Arbor Networks report) [6], with 30 companies being
   responsible for 30% of global traffic.  A subsequent paper from
   Harvard's Berkman Center [7] highlighted the security, independent
   media, and human rights implications of this development as a result
   of attacks against that small number of key platforms, among other
   issues.  Many others have measured, studied, and debated this trend
   since 2009.  A recent Sandvine paper [8] noted that Google's YouTube
   comprised 35% of mobile Internet traffic, and Netfix comprised 13.75%
   of global Internet traffic (see Sandvine blog) [9].

   This trend towards centralization onto a small number of large
   platforms has given rise to efforts to "re-decentralize the web".
   Proponents of the effort to resist and reverse the further
   centralization of the web and the Internet more generally include Sir
   Tim Berners-Lee (see Ars Technica article [10], Gizmodo article [11],
   New York Times article [12]), Vint Cerf (see blog [13]),
   Brewster Kahle (see IEEE Spectrum article [14]), MIT's Digital
   Currency Initiative [15], participants in the Decentralized Web
   summit [16], and others.

   In addition, IETF contributors are also considering the challenges
   posed by consolidation, which in the DoH context is analogous to
   Centralized DoH.  For example, the Internet Architecture Board (IAB)
   posted a draft entitled "Considerations on Internet Consolidation and

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   the Internet Architecture" [17] that delves into these issues, as
   also noted on the IETF blog [18].  The Internet Society also
   published their 2019 Global Internet Report on the Future of the
   Internet [19] that is primarily focused on consolidation.

   While traffic to certain destinations is increasingly concentrated,
   at the same time the physical destinations (e.g. servers) are often
   highly distributed by these platforms in order to deliver good
   performance to users around the world.  But while edge servers may
   remain relatively physically distributed, their control,
   administration, and operation is still centralized.  But even if a
   Centralized DoH provider's physical servers are highly distributed it
   still would not come close to wide distribution of conventional DNS
   today, which is typically distributed down to the network level,
   ranging from a local region of an ISP's network (e.g. metropolitan
   London area) to a small business' local area network (LAN),
   enterprise network, school LAN, etc.

   There are many potential implications to increased centralization and
   consolidation of the DNS.  These can include technical, business, and
   other implications and risks that are explored later in this document
   in Section 8 and Section 9.

6.  Centralized DoH Assumption: Enabled/Centralized by Default

   This document assumes a potential Centralized DoH environment where a
   few large scale implementers have enabled DoH by default.  This means
   that there are assumed to be a small number of Centralized DoH
   providers, rather than a large number of distributed DoH resolver
   providers and in stark contrast to the highly distributed nature of
   the DNS today.  This is explored further in Section 5.  While each
   implementer has so far configured DoH off by default, and users can
   opt-in, the apparent design target of some or all key implementations
   is to enable Centralized DoH by default at some point in the future
   (see Internet Society blog [20].  Thus, the current opt-in model is
   assumed to be temporary.

7.  Potential for Rapid Centralized DoH Adoption

   Implementation of some new protocols, such as IPv6 (see World IPv6
   Launch site [21], Wikipedia article [22], LinkedIn blog [23]) or
   DNSSEC (see DNSSEC deployment history [24]), depended upon broad
   community technical coordination, extensive open measurement,
   extensive technical discussions over several years, and gradual
   adoption.  But adoption of IPv6 and DNSSEC grew organically over time
   in part due to the wide variety and great number of parties that
   needed to independently take action to adopt those protocols.  In
   contrast, there are far fewer major web browsers and operating

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   systems than network operators, websites, authoritative domain name
   server operators, and so on.

   The result of this comparatively greater concentration is that if
   just two organizations implement DoH, then the adoption of
   Centralized DoH could increase quite rapidly, and quickly overtake
   and displace conventional non-DoH DNS query traffic.  It appears to
   be unprecedented that a new protocol could be so rapidly deployed and
   thus displace an existing, long-standing, highly distributed protocol
   based on implementation by just two implementers.

   This extraordinary potential for rapid Centralized DoH deployment
   alone suggests the need for a high degree of testing, discussion, and
   consensus in the global Internet community that is broader than the
   much more limited consensus necessary for adoption of proposed IETF

   To illustrate the potential for rapid Centralized DoH, if just two
   organizations, Google and Mozilla, were to implement Centralized DoH
   in Android, Chrome, and Firefox, then global adoption of DoH could
   occur rapidly and represent the majority of DNS queries on the

   All the above being said, it is important to note that this is not
   necessarily a criticism of the motivations of the potential
   Centralized DoH providers and that scale and market share are neither
   objectively good or bad attributes.  Indeed, as IETF chair Alissa
   Cooper noted in an interview about consolidation with the Internet
   Society [25], "In some cases larger entities can have faster,
   broader, positive impacts on end users.  Today, if one or a small
   handful of the largest web properties, content delivery networks, or
   email service providers chooses to deploy a new security technology
   or implement a performance-enhancing feature, those improvements can
   benefit millions or billions of users on short order."

   Thus, on the one hand rapid adoption of new security protocols can be
   good and adoption can be hastened by the actions of a few key
   players.  But it is important to also acknowledge that this may
   simultaneously be in tension with other goals for the design and
   operation of the Internet, requiring thoughtful consideration of the
   pros and cons and extensive discussion in the Internet community and

8.  Potential Technical Risks

   There are a variety of potentially significant risks to the security,
   stability, and performance of the Internet as a result of Centralized

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   DoH implementation, and resulting consolidation of DNS operations.
   These include the following potential and speculative risks:

   1.   Significant Operational Shift of Global Internet Infrastructure:
        Shifting from a large quantity of highly distributed DNS
        resolvers to a few centralized ones will likely have significant
        impacts on how the Internet operates, is administered, and how
        routine troubleshooting is performed.  The full implications of
        such a significant and potentially sudden change require deep
        study by a range of actors across the Internet ecosystem.

   2.   Decreased Stability:
        Significant centralization can increase the fragility of a
        technical system, because there are fewer points of failure and
        thus the impact of any individual failure can be quite high.
        The net effect suggests that if DNS operations become
        significantly centralized as a result of DoH, then the stability
        of the DNS is likely to be negatively impacted.  While not
        directly related to DoH, there are examples of widespread
        Internet outages when large DNS-related platforms experience
        technical faults or attacks, such as Cloudflare (see Cloudflare
        blog [26]), DynDNS (see Dyn blog [27]), and many others.

        Even without the advent of Centralized DoH, which appears to be
        a potentially significant concentrator of DNS traffic, a recent
        paper from Harvard University ( see Zittrain et. al., "Evidence
        of Decreasing Internet Entropy, The Lack of Redundancy in DNS
        Resolution by Major Websites and Services" [28]) raises concerns
        that may only worsen with DoH.  They write, "We find an
        increasing concentration of DNS services in a small number of
        dominant cloud services companies.  Coupled with domains
        apparent tendency not to employ DNS services from multiple DNS
        providers, this concentration could pose a fundamental threat to
        the distributed resilience of the Internet.  Our results also
        suggest ways to mitigate these issues... The Dyn attack provides
        a vivid illustration of how DNS infrastructure vulnerabilities -
        and DNS space concentration - can wreak havoc on the stability
        of the Internet."

   3.   Increased Security Threats:
        Centralization due to DoH could mean a dramatic reduction in the
        number of recursive DNS operators.  This seems likely to lead to
        fewer points of failure on which attackers can focus,
        potentially altering the return on investment (ROI) necessary
        for a large scale attack, compromise, or disruption to succeed.
        Such threats may include the outsized effect of Border Gateway

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        Protocol (BGP) hijacks (see Internet Society blog [29], Freedom
        to Tinker blog [30], CSO Online article [31], ZDNet article
        [32], Ars Technica article [33], Google whitepaper [34],
        Distributed Denial of Service (DDoS) attacks (see Dyn blog
        [35]), or regular Denial of Service (DoS) attacks made against a
        small number of systems.

   4.   Loss of Security Threat Visibility:
        Some users will lose or have a degraded ability to use DNS
        blocklists, which are one of the primary and most effective ways
        to protect a network and its users against malware, phishing,
        spam, DDoS attacks, etc.

        This is because there has long been a notion that as a network
        connects to the Internet, that the network can exercise some
        degree of local policy control, which remains local to that
        network and does not propagate beyond the boundary of their
        administrative domain.  This includes monitoring and protecting
        the security of the network and devices on that network.  Over
        time, one of the practices that has evolved and become
        widespread is the use of the DNS to monitor for, remediate, and/
        or prevent malware infection or other security problems.  This
        functionality is deployed in many types of networks, from ISPs
        to networks used by enterprises, small businesses, government
        offices, schools, churches, libraries, and others.  In some
        cases the DNS server may reside inside the network, while in
        other cases it may be external (aka cloud-based, such as
        OpenDNS).  To illustrate this, many networks monitor the FQDNs
        of DNS queries for matches against lists of well-known malware
        command and control hosts.  In some cases when matches occur,
        the device owner or local network administrator may be notified
        of a potential malware infection.  In other cases, the DNS is
        configured to re-write the response for a query of a malware-
        associated FQDN, providing an address that points of a server
        alerting the end user of a malware risk, providing a NXDOMAIN
        response to cause the DNS lookup to terminate in failure, or
        other response.  This functionality will fail if DNS queries
        bypass the servers that perform this function to Centralized DoH
        resolvers.  Thus, Centralized DoH can create blind spots in this
        critical area of security threat visibility.

   5.   Loss of Parental Controls or other Content Controls:
        Similar to using the DNS on local networks to monitor for and
        prevent security issues, the DNS is often used to implement
        local content controls such as parental controls.  With these
        controls a parent can configure a service on their home network

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        to prevent children from accessing inappropriate or other
        disallowed content.  For example, a parent may configure
        policies to bar their two children, aged 5 and 7 years, from
        accessing any sites associated with social media, gambling,
        illegal drug use, pornography, and so on.  Such opt-in services
        are highly popular, especially because they can work across
        device types (e.g.  PC and mobile) and device ecosystems (e.g.
        Android and Apple), software (e.g.  Mac and Windows), and
        platform ecosystems (e.g.  Google, Apple, and Amazon).

        Similar to the malware example, this is usually implemented via
        DNS response matching and re-writing, with the end user
        presented with either a redirection to a content block page or
        receiving an NXDOMAIN response.  This functionality will fail if
        DNS queries bypass the servers that perform this function to
        Centralized DoH resolvers.  And while one suggestion may be that
        the Centralized DoH provider offer such services, this is not a
        choice users are being permitted to make.  They may be perfectly
        satisfied with their current solution, not want to take the time
        to setup a new service, or not want to use the Centralized DoH
        provider for this function.  In addition, mature and highly
        customizable parental control and content control systems that
        meet the needs of enterprises, schools, parents, and others do
        not appear to be offered by Centralized DoH providers.  To the
        extent that similar solutions seem to exist, they appear to be
        very basic, and lacking in the customization and functionality
        that has developed in this marketplace over the last 20 years.

        In addition, if users wish to configure their own independent
        DNS resolver that provides features such as parental controls,
        as many do today, this may become more complicated and varied
        with Centralized DoH to the extent that some software like
        browsers or other applications are over-riding configurations
        set by users in their operating system.

   6.   Split DNS Problems:
        Split DNS [RFC8499] is an implementation in which separate DNS
        servers are provided for internal and external networks as a
        means of security and privacy management.  This is most often
        used in enterprise, education, and government networks.  In
        practice this means that there are names that only resolve on an
        internal network, or that resolve to special internal hosts for
        internal network users and publicly accessible hosts for users
        outside of the network.  For example, an enterprise may have an
        internal service named "Accounting-System" reachable on the web
        via https://accounting-system or https://accounting-, and connected to via internal, non-routable

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        RFC1918 IPv4 addresses such as  These domains or
        fully qualified domain names (FQDNs) are maintained only on a
        network's local DNS resolvers, and are not resolvable using the
        Internet's authoritative DNS infrastructure.  These names will
        no longer be resolvable based on the expected implementation of
        DoH because the local resolvers that can provide a valid
        response for these names is no longer in the resolution path for
        the end user on that network - they are skipping past the local
        resolver to a centralized resolver on the Internet.  It is
        certainly possible to criticize the use of split DNS, like
        Network Address Translation (NAT), but whether it is a
        supposedly good practice or not, it seems a pervasive practice
        nonetheless and should be considered by new DNS protocol

   7.   Enterprise Data Leaks:
        When split DNS names are used, as noted in the above example for
        a user on an enterprise network attempting to connect to a host
        at, the lookup with a
        centralized DoH resolver will typically fail (NXDOMAIN).  But
        because the internal name was sent to the centralized DoH
        resolver, that private name has "leaked" outside of the local /
        enterprise network.  Similarly, lookups of reverse DNS names
        ( will leak private IP addresses as well.  The leak
        of IP address data could occur regardless of whether or not
        split DNS is used.

   8.   Potentially Reduced Software Diversity:
        Consolidation of recursive DNS functions to a few Centralized
        DoH providers suggests that there will be fewer types of DNS
        server software over time, or at least that a very small number
        of DNS server software packages will account for the
        overwhelming volume of DNS traffic.  This leads to less software
        diversity over time, which is in some cases considered a
        negative in this realm (see IEEE Explore paper [36], Freedom to
        Tinker blog [37]).  This may also shift most DNS traffic away
        from platforms using open source software to proprietary
        software.  In addition, the impact of any DNS software exploits
        (such as the BIND "packet of death" [38]) against the software
        used by the few key Centralized DoH operators seems likely to
        have an outsized impact on the global Internet.

   9.   Potential for Increased Commercial Use of DNS Data:
        As a result of the highly distributed nature of the DNS today,
        and of recursive DNS operations specifically, there are

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        essentially no - or at least few - global data sets of end user
        DNS queries.  The possible exception to that is for large public
        DNS operators that receive hundreds of billions of queries per
        day.  But as Centralized DoH potentially leads to consolidation
        onto a few large platforms, very large DNS query datasets will
        emerge, which carries the risk that organizations will be
        tempted to or find it otherwise necessary or advantageous to
        make commercial use of that data.  This might especially be the
        case of those operators that offer DNS services for "free".
        Furthermore, even if data sets are in some manner "anonymized",
        it seems likely that some organizations will possess enough
        other datasets that the combination of the two may trivially
        enable de-anonymization.  See [Narayanan-Shmatikov-1],
        [Narayanan-Shmatikov-2], and [Narayanan-Shmatikov-3].  In
        addition, a user may be uncomfortable with or unhappy with
        having their DNS traffic sent to a pre-configured Centralized
        DoH with whom they have no relationship.  As noted earlier in
        the document, this can be particularly problematic in light of
        the GDPR and other laws and regulations around the world.

   10.  Potentially Negative Impact on Content Delivery Network (CDN)
        It seems there is some risk that some CDNs may be less able to
        provide good content localization with Centralized DoH,
        equivalent to the localization that they provide today.  This is
        because CDN localization today depends upon accurately
        estimating the rough location from which client queries
        originate, whether derived from EDNS-Client-Subnet (EDNS0)
        [RFC6891] [RFC7871] or some other method employed by a CDN.
        This location information is then used to dynamically generate
        authoritative DNS responses that provide different responses
        based on that client location.  The goal of the CDN is to
        provide highly localized responses, such as directing a client
        to content cached in their local city or region rather than that
        which is further away, such as across an ocean or across many

        The technical impacts of reduced CDN localization might include
        slower access to Internet content for end users and more traffic
        traversing backbone and sub-optimal peering points as opposed to
        localized points of direct interconnection between networks.  It
        is difficult to estimate what, if any, the impact would be.  But
        large-scale measurement platforms such as the SamKnows system
        that is used by many regulators around the world such as OFCOM
        and the FCC may be useful for exploring this further, as noted
        in Section 10.

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   11.  Use of DoH for Malware Command and Control:
        Related to the loss of security threat visibility, it seems
        clear that DoH is also now being used as a new and undetectable
        malware command and control channel.  One example is DoHC DoHC2
        [39].  As a result, from the perspective of a variety of
        networks, DoH is sometimes considered a security threat given
        its adoption as a covert malware command and control
        communications channel and thus may be considered a new avenue
        for abuse.

   12.  Disruption of Legally-Mandated National-Level DNS Blocks:
        In an increasing number of countries, network operators are
        required by law to implement DNS-based blocking of names.  Some
        democratic countries have developed laws and regulations in this
        area, including the UK, Sweden, Switzerland, France, Italy,
        Brazil, and others [CITATIONS NEEDED].  As a result, Centralized
        DoH resolvers appear unlikely to comply with these local laws
        and so legally-mandated national DNS blocks will become
        ineffective.  National regulators may take a dim view of this
        and require that Centralized DoH resolvers comply with
        applicable national laws mandating DNS blocking.  Whether or not
        national-level DNS blocking is either good, effective, or easily
        circumvented matters little; organizations operating within a
        given country are typically expected to comply with such
        applicable laws and so Centralized DoH resolvers will need to
        determine how to comply.  (This point is not to be confused with
        the sorts of blocks that have historically been imposed on major
        content destinations by repressive political regimes and/or
        those with extensive censorship in place to limit or control

   13.  Potentially Negative Impact on End User Broadband Performance:
        As noted above, it seems possible that the extent of
        localization of CDN-based content may decline somewhat.  Should
        that be the case, this means that the performance or speed of
        access of CDN-based content will decline.  Since most web
        content is CDN-based, this suggests the possibility that the
        general end user performance of the Internet will decline.  An
        initial study by Mozilla [40] has suggested that the protocol
        itself (DoH vs. UDP/53 DNS) is roughly 6ms slower.  But that
        limited study only considers how fast it takes to get *an*
        answer, not how *local* or good that answer is from an end user
        perspective.  More measurement is certainly necessary here,
        which can consider how local the answers are and what the end-
        to-end performance of web-based content is for users of
        centralized DoH vs. non-users.

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   14.  Unknown Server-Side Performance and Scaling:
        Many new protocols are implemented organically, which means the
        growth or adoption of the protocol is relatively gradual.  As a
        result, software developers and system administrators have a
        longer period of time to learn about performance tuning and
        scaling, and to methodically test and deploy improvements,
        compared to a "flash-cut" sort of migration where a significant
        shift to the protocol or system occurs in a short period.  In
        the case of the likely DoH implementation on which this document
        speculates, it seems a rapid shift is more likely.  This raises
        the risk of instability and reduces the ability of technical
        personnel involved in development and deployment to learn and
        make technical changes gradually and with relatively minimal
        impact on systems and users due to relatively high levels of
        usage inherent in a flash cut or rapid migration scenario.

   15.  Increase in Exploits Targeting Individual DNS Engineers and
        Given the likely consolidation of most recursive DNS traffic to
        a very small number of operators, it seems logical to conclude
        that attackers will realize that a fairly small number of DNS
        engineering and operations/administration personnel (less than
        two dozen?) will control a key function of the Internet.  As a
        result, it seems likely that a savvy attacker would target
        exploits such as spear-phishing [41] or advanced persistent
        threats (APT) [42] against this small number of people in order
        to gain access to systems that administer or control recursive
        DNS functions.

   16.  Increased Complexity and Cost of End User Troubleshooting:
        Today, ISPs and other network operators can guide users through
        troubleshooting to determine, often via a simple command line
        interface, what DNS servers they have been assigned and what
        responses they are receiving from those resolvers.  In the
        Centralized DoH model, determining what DoH servers are being
        used and testing responses from those servers seems likely to be
        relatively more complicated and varied.  This could increase the
        complexity and cost of routine end user troubleshooting.

   17.  Disruption of ISP Walled Garden Functions and Other Captive
        Many ISP networks utilize a DNS-based walled garden for
        customers to provision new service, to activate a new device, to
        re-establish an existing service after non-payment, and so on.
        It appears that Centralized DoH disrupts these widely used

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        functions, because the browser is over-riding the specially
        assigned walled-garden DNS server addresses and is instead
        attempting to use a Centralized DoH resolver.  Similarly, other
        captive portals that may be affected include those to access
        WiFi networks on campuses, in airplanes and coffee shops, and so
        on.  While new standards in the IETF's CAPPORT Working Group
        [43] may avoid this in the future, CAPPORT standards are still
        developing and/or are not widely deployed.

9.  Potential Business Risks

   New IETF standards are not introduced in a vacuum.  Rather, IETF
   standards have real-world impacts on technologies, markets, and
   societies.  As a result, potentially rapid shifts in adoption of
   these standards means that relevant IETF working groups cannot ignore
   potential real-world, technical and non-technical impacts.

   If Centralized DoH is implemented quickly based on the business
   decisions of one or two organizations with significant operating
   system and/or web browser market share, with the resulting effect of
   greater centralization, then the following potential and speculative
   business risks are worth considering:

   1.  Smaller DNS Software Marketplace:
       The market for DNS server software may be disrupted, as default
       DoH resolver choices override typical DNS settings that direct
       DNS traffic today.  As a result, the number of DNS server
       software developers may dwindle.  This is because if ~70% of the
       world's DNS queries rapidly moves to two Centralized DoH resolver
       operators, then there is a diminished need for conventional DNS
       operators to continue to maintain their DNS infrastructure.  This
       could impact DNS server software developers in both commercial
       and open source markets, such as Akamai, Cisco, CZ.NIC, Infoblox,
       PowerDNS, NLnet Labs, etc.

   2.  Fewer Public DNS Operator Choices:
       The market for public DNS resolution will likely be disrupted, as
       default Centralized DoH resolver choices override end-user-
       configured DNS settings.  For example, an end user may configure
       their operating system to use a "public" DNS service that
       implements parental control functions.  But when using their web
       browser, the browser sends its DNS queries - which are likely the
       great majority of queries from the user based on the prevalence
       of the web as an application - to a Centralized DoH resolver
       instead.  After the adoption of these public DNS services
       declines dramatically, these organizations may struggle to

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       continue to justify the resources necessary to maintain the
       service.  This could impact all those public DNS resolvers that
       are not the default partner of a Centralized DoH implementer
       (e.g.  Cloudflare) or are not themselves DoH implementers (e.g.
       Google), such as Cisco's OpenDNS and Quad9.

   3.  Reduced CDN Localization and Competition:
       The CDN market may be disrupted, as noted above, should some or
       most existing CDNs be less able to provide good content
       localization.  This is because content localization is one of the
       key reasons an organization would purchase a CDN's services.  So
       if this key reason goes away or is negatively impacted, there may
       be less demand for CDN services or the price of those services
       may decline as a result of reduced benefits to customers.  This
       may also affect competition if some providers exit the market or
       alter their market behavior as a result.

   4.  Smaller DNS Labor Market:
       The labor market for DNS engineering and operations expertise may
       also be disrupted.  This is likely due to there being fewer
       independent developers of DNS software, as well as fewer
       recursive DNS operators, which can be expected to reduce the need
       for DNS technical resources over time.

10.  Recommendations

   This document makes the following recommendations:

   1.   Develop a Standardized DoH Resolver Discovery and Selection
        [RFC8484] does not specify a mechanism to discover whether a DoH
        server is available as part of the local network configuration
        and configuration of the URI template used the construct the URL
        for resolution is explicitly stated as being out of band from
        the DoH protocol.  Without such a discovery mechanism, there is
        little choice for DoH clients to use any other mechanism than
        pre-configured DoH servers, which by implication would be almost
        certainly be outside of the network of the ISP or other network
        operator, even if they offered a DoH service.

        There are efforts underway to discover and automatically
        associate a DoH server with a resolver, for example draft-ietf-
        doh-resolver-associated-doh [44].  Such configuration
        mechanisms, if adopted by DoH clients, would potentially
        ameliorate many of the issues with DoH deployment expressed in

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        this document, since it would provide a way for end-users to use
        the DoH server provided by the local network operator.

   2.   Conventional DNS Providers Should Begin Testing DoH:
        ISPs and other network operators, as well as any other
        conventional DNS providers should begin to test DoH as a new
        protocol that will be added to their existing DNS services.  In
        particular, it seems critical to develop appropriate, scalable,
        reliable, and cost effective deployment design that can deliver
        DNS resolutions at least at the level of performance that users
        expect of conventional DNS.

   3.   More Measurement is Needed:
        Limited measurements collected thus far have been in some cases
        shared publicly, but the underlying datasets remain confidential
        and private.  In the future, significantly more measurement data
        needs to collected, shared publicly, and debated in the Internet
        technical community.  Past deployments of new protocols can be a
        guide here, such as measurements undertaken in support of the
        deployment of IPv6, such as World IPv6 Day and Launch [45].

   4.   Defaults Matter - Consider Them Carefully:
        Make opt-in by default during initial deployment.  Off by
        default is less risky for initial deployment.  That should
        likely remain so until there has been significantly more
        technical testing, global measurement, and Internet community

   5.   Internet Corporation for Assigned Names and Numbers (ICANN)
        ICANN coordinates and/or administers key Internet functions,
        such as the Internet Assigned Numbers Authority (IANA) and the
        global Domain Name System (DNS).  ICANN maintains a group of
        technical experts in the SSAC [46] that advises the ICANN Board
        and community on the security and integrity of the Internet's
        naming and address allocation systems, including domain name
        operations, administration, and registration.  Given that SSAC
        focused on threat assessments and risks related to the stability
        and security of the DNS, they seem like one appropriate party to
        assess some of the risks that have been briefly explored in this
        document or other DoH implementation-related risks and issues.

   6.   Additional Expert Reviews:

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        In addition to the ICANN SSAC, several other organization to be
        appropriate parties that are well situated to assess risks and
        issues as they pertain to those organizations or their members/
        participants.  They include, but are not limited to:
        - DNS Operations Analysis and Research Center (DNS-OARC)
        - Messaging Malware Mobile Anti-Abuse Working Group (M3AAWG)
        - Network Operator Groups (NOGs), such as the African Network
        Operators Group (AfNOG), Australian Network Operators Group
        (AUSNOG), Caribbean Network Operators Group (CaribNOG), Latin
        American and Caribbean Region Network Operators Group (LACNOG),
        North American Network Operators Group (NANOG), Pacific Network
        Operators Group (PACNOG), Reseaux IP Europeens Network
        Coordination Centre (RIPE NCC), Slovenian Network Operators
        Group (SiNOG), etc.
        - DNS registries outside of ICANN, such as Council of European
        National Top-Level Domain Registries (CENTR).

   7.   Detailed Community Assessment of Risks and Issues:
        All of the risks in Section 8 and Section 9 should be assessed.
        In addition, when a change needs to happen to a protocol or a
        system, it seems important to debate a few other key things such
        - What is the threat model that makes this change important
        enough to justify?
        - What are the security and privacy implications of this change?
        - What are the implications for stability, operations, network
        and systems administration, software development and diversity,
        and other key issues?
        - Do the benefits outweigh the drawbacks?
        - What alternatives should be considered or developed?

   8.   Continue to Focus on DNSSEC Adoption:
        To the extent that one of the underlying concerns motivating DoH
        adoption pertains to modification of DNS responses via man in
        the middle attacks, it seems that DNSSEC signing of domain names
        and DNSSEC validation (including in clients) may be able to
        mitigate that issue.  DNSSEC remains important because DoH,
        whether centralized or distributed, only provides security for
        the transmission of DNS queries/responses over the wire (in
        transit, or channel security), and does not provide assurance
        that the response itself is secure and unmodified (content
        security).  This issue is explored in more detail in an APNIC
        blog [47] and an ICANN presentation [48].

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   9.   Conduct Enterprise, Education, and Government Network Testing:
        This documents several issues related to these non-ISP types of
        networks.  Additional testing should be conducted by these
        entities in order to document actual and/or potential issues,
        including the extent or severity of those issues, and provide
        that feedback to appropriate standards and industry groups.

   10.  DoH Client Software Developers Should Investigate Region-
        Specific Differences:
        DoH can improve user privacy, especially in certain countries/
        regions with known surveillance and/or manipulation of DNS
        queries and other data, which can pose human rights risks in
        these areas.  But many other countries/regions have more
        privacy-protective expectations, rules, regulations, and laws.
        It may be worthwhile for DoH client software developers to
        consider developing application logic that enables Centralized
        DoH in the high risk areas, while not leveraging DoH or
        leveraging a distributed approach to DoH in the low risk areas.

   11.  Develop Centralized DoH Data Privacy Guidelines/Frameworks:
        Assuming Centralized DoH is a viable model for implementation of
        DoH, what sort of measures are needed to limit the potential for
        problematic behavior by Centralized DoH providers?  Should there
        be a code of conduct (or equivalent) and, if so, who will
        develop/maintain that?  Likely topics for some guidelines or
        framework might include how DoH client data may be collected,
        retained, processed, shared, monetized, and/or combined with
        other data sets, etc., whether and what limits there may be on
        generating unique-per-used FQDNs, whether and what limits there
        may be on web-related cookies/tracking mechanisms, detection of
        DoH servers that return bogus or bad/false data, policy
        statements from DoH providers on how client data is used,
        measures to ensure DoH client data is suitably anonymized to
        minimize the risk of re-identification of individuals by
        combining DoH data with other data sources, etc.

11.  Document Reviewer Acknowlegedments

   The authors thank the several individuals for performing a detailed
   review of this document, noting that this acknowledgement is not
   intended to imply that they endorse the document.  We specifically
   wish to thank: Greg Aaron, Rob Alderfer, Bill Check, Neil Cook, Joe

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   Crowe, Glenn Deen, Andy Fidler, Peter Hagopian, Paul Hoffman, Yiu
   Lee, Adam Roach, Jim Reid, and Ralf Weber.

12.  IANA Considerations

   RFC Editor: Please remove this section before publication.

   This memo includes no requests to or actions for IANA.

13.  Security Considerations

   This document does not introduce any new security considerations.
   However, it does highlight a number of potential security
   considerations related to how [RFC8484] might be implemented in the
   future, especially Centralized DoH.  For example, an attacker could
   substantially disrupt the global Internet by targeting one or two
   major platform providers.  See Section 8 for more information.

14.  Privacy Considerations

   This document does not introduce any new security considerations.
   However, it does highlight a number of potential privacy
   considerations related to how [RFC8484] might be implemented in the
   future, especially Centralized DoH.  For example, with Centralized
   DoH there will be a small number of large commercial platforms that
   have an extensive business collecting and leveraging user-related
   data that could extend and augment these data sets as a result of the
   data they can collect by handling the majority of global DNS traffic.
   See Section 9 for more information.

15.  References

15.1.  Informative References

              Narayanan, A. and V. Shmatikov, "Robust de-anonymization
              of large sparse datasets", IEEE Security and Privacy 2008,

              Narayanan, A. and V. Shmatikov, "De-anonymizing social
              networks", 2009 30th IEEE Symposium on Security and
              Privacy 2009, 2009.

              Narayanan, A. and V. Shmatikov, "Myths and fallacies of
              personally identifiable information", Communications of
              the ACM 53.6, 2010.

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   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <>.

15.2.  URIs










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Appendix A.  Change Log

   RFC Editor: Please remove this appendix before publication.

   o  -00: First version published

   o  -01: Fixed formatting issue with title at top of each page

   o  -02: Removal of 2 ICANN-related recommendations that don't appear

   o  -03: Corrected cited stat from Mozilla study from 5% slower to 6ms

Appendix B.  Open Issues

   Section will be removed before final publication

   o  Citations are needed in the legally-mandated DNS blocking section.

   o  Improve the formatting of extended quotations.

Authors' Addresses

   Jason Livingood


   Manos Antonakakis
   Georgia Institute of Technology


   Bob Sleigh
   BT Plc


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   Alister Winfield


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