DNS over HTTPS (DoH) Considerations for Operator Networks
draft-doh-reid-operator-00

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DoH                                                            A. Fidler
Internet-Draft                                                    BT plc
Intended status: Informational                                 B. Hubert
Expires: March 12, 2020                                      OpenXchange
                                                            J. Livingood
                                                                 Comcast
                                                                 J. Reid
                                                                RTFM llp
                                                              N. Leymann
                                                     Deutsche Telekom AG
                                                       September 9, 2019

       DNS over HTTPS (DoH) Considerations for Operator Networks
                       draft-doh-reid-operator-01

Abstract

   The introduction of DNS over HTTPS (DoH), defined in RFC8484,
   presents a number of challenges to network operators.  These are
   described in this document.  The objective is to document the problem
   space and make suggestions that could help inform network operators
   on how to take account of DoH deployment.  This document also
   identifies topics that may require further analysis.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on March 12, 2020.

Copyright Notice

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

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Contrasting DoH and Conventional DNS  . . . . . . . . . . . .   3
   4.  Regulatory and Policy Considerations  . . . . . . . . . . . .   4
     4.1.  Local Policy Constraints  . . . . . . . . . . . . . . . .   4
     4.2.  Regulatory and Legal Impacts  . . . . . . . . . . . . . .   5
     4.3.  Regulatory Constraints  . . . . . . . . . . . . . . . . .   5
   5.  Network Operations  . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Impact on DNS query logging . . . . . . . . . . . . . . .   6
     5.2.  CDN endpoint selection  . . . . . . . . . . . . . . . . .   6
     5.3.  Redirection for captive portals . . . . . . . . . . . . .   7
     5.4.  Managed network services  . . . . . . . . . . . . . . . .   7
     5.5.  Resolver capacity management  . . . . . . . . . . . . . .   7
     5.6.  Failure recovery  . . . . . . . . . . . . . . . . . . . .   8
     5.7.  Impact on Network Address Translation . . . . . . . . . .   8
     5.8.  Load balancing and failover . . . . . . . . . . . . . . .   8
   6.  User Support  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Provisioning  . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Privacy Concerns  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. Human Rights Considerations . . . . . . . . . . . . . . . . .  13
   11. Open Issues for Further Study . . . . . . . . . . . . . . . .  14
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     14.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   Traditional DNS traffic between stub resolvers, recursive servers and
   authoritative servers is not encrypted.  This can pose a privacy
   challenge for Internet users, because their access to named network
   resources can potentially be tracked through their DNS queries.  In
   principle, any network element along the path between the user and

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   resolving or authoritative servers could observe this unencrypted
   traffic.  DoT (DNS over TLS) [RFC7858] is one proposal for providing
   privacy of DNS queries and DNS over HTTPS (DoH) [RFC8484] is another.
   Both DoH and DoT encrypt the communications between the end client
   (user) and recursive resolver.  Plaintext DNS traffic between
   recursive and authoritative servers is generally less of a privacy
   concern because it usually does not contain information such as the
   source address of the initial query that could identify the end
   client.

2.  Terminology

   DoH Server: A server supporting the DNS over HTTPS is called a "DoH
   server" to differentiate it from a "DNS server" (one that only
   provides DNS service over one or more of the other transport
   protocols standardised for DNS).  Similarly, a client that supports
   the DNS over HTTPS is called a "DoH client".

   Operator: A large Internet service provider, typically a cable
   company or fixed/mobile telco with a national or international
   network.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Contrasting DoH and Conventional DNS

   With conventional DNS today, using UDP or TCP port 53, most users are
   assigned the IP addresses of several recursive resolvers via DHCP or
   similar network bootstrapping mechanism.  These are usually the IP
   addresses of recursive resolvers that are administered by the network
   operator.  There is currently no equivalent to this for DoH.  Users
   sometimes also change to third-party recursive resolvers.  In some
   cases, they may even operate their own local resolver.  It is not yet
   clear how or if DoH will offer these possibilities.

   RFC 8484 defines the protocol for DNS over HTTPS (DoH).  When DoH is
   used, client and server DNS traffic is encrypted using a TLS RFC 8446
   [RFC8446] channel, typically to port 443.  DoH clients will have
   little need for conventional DNS apart from an initial bootstrap
   query to find the IP addresses of a suitable DoH server.  This will
   mean the bulk of their DNS resolver traffic would bypass an
   operator's DNS resolver infrastructure because that traffic will make
   use of the resolver service provided by the DoH server.

   When DoH is used, the traditional DNS client-server model of clients
   making queries and waiting for a reply from a server might well

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   change.  It can be expected that DoH servers will sometimes use DoH
   opportunistically.  For instance a web server could include
   application/dns-message elements in the returned HTML data,
   anticipating the domain names that the web browser might need to
   resolve before rendering some web page.  In this case, the client
   would not need to lookup those names with DoH or conventional DNS
   because the relevant DNS data have already been supplied.

   It is likely that DoH will be widely implemented and deployed by
   browser vendors and DNS providers.  Some deployment has already taken
   place.

   Adoption of DoH is expected to be driven by browser vendors adding
   DoH support in future releases of their software.  They may even make
   DoH the default for browser DNS lookups rather than conventional port
   53 queries.

   DoH changes the current, well established business model where an end
   user (customer) pays for Internet connectivity and recursive DNS
   service is part of that offering from the ISP.  When DoH is used, the
   customer will probably be dependent on DoH servers operated by third
   parties and have no contractual or business relationship with those
   providers.  It also cannot be assumed that these DoH servers will be
   operating under the same policy and regulatory conditions that are
   applied by the end user's ISP.

4.  Regulatory and Policy Considerations

4.1.  Local Policy Constraints

   Operator networks often have local policy constraints which require
   some form of DNS blocking or rewriting - for example to offer
   customers parental controls, to restrict access to illegal content or
   to mimimise end user exposure to malware, phishing attacks and so on.
   These tend to be implemented by using data from threat intelligence
   providers, usually some sort of RPZ feed that is incorporated into
   the configuration of the operator's DNS resolver infrastructure.

   It is not yet clear how or if this functionality can be made
   available by DoH servers.  These protective measures will be less
   effective once DoH is used because end user DNS traffic will largely
   bypass the operator's DNS infrastructure, rendering such content and
   security protections useless.  Some of these measures may be offered
   by some DoH servers, but as yet there is no defined mechanism to
   ensure that all local policy is implemented.

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4.2.  Regulatory and Legal Impacts

   Operators can also be required to perform DNS blocking and filtering
   or rewriting for legal reasons: handling takedown notices or
   complying with court orders.  This may also be necessary for
   operational and/or security reasons such as dealing with botnets and
   DDoS attacks.  [CSRIC]

   As before, it is not yet clear how or if DoH servers will provide
   this functionality.  Some of these measures may be offered by some
   DoH servers, but there is no defined mechanism to ensure that all
   local policy is implemented, particularly those required in certain
   jurisdictions today.  Current protective measures will be less
   effective once DoH is used because customer DNS traffic will largely
   bypass the operator's DNS infrastructure.

   Conventional recursive DNS services are generally located in the
   country where an operator is based.  Since third-party DoH service
   providers are likely to be based and/or operated from outside those
   local countries, different protections and regulatory considerations
   may apply to the protection, storage and processing of user data
   processed on those servers.  Typical regulations that could apply
   include General Data Protection Regulation (EU) 2016/679 [GDPR] and
   the EU-US Privacy Shield Framework [USPS].  These can sometimes have
   global scope - GDPR for instance.  Overseas regulations may have
   lower, higher or even no commitments governing such services compared
   to those that would apply to a local operator.  The potential impact
   of these regulatory obligations with respect to DoH services is
   unclear, including whether or not they apply or even could be applied
   at all.

4.3.  Regulatory Constraints

   Logs containing individual DNS queries and the IP addresses or other
   data correlating those queries to specific users or homes may in some
   legal jurisdictions be considered as personally identifying
   information (PII).  In such jurisdictions detailed DNS query logs may
   be subject to data protection and retention regulations, or other
   legal and/or compliance requirements.

   Operators can also be subject to regulation or other legal
   instruments that require DNS query logs to be retained for a certain
   period of time and made available for law enforcement purposes as
   needed, such as under a court order or other legal process.

   Since DoH potentially bypasses conventional DNS resolvers on which
   these privacy, regulatory, and legal requirements are imposed, it
   will reduce or eliminate the potential social value of these rules,

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   and may even be viewed by some countries as a potential breach of
   regulatory compliance (whether by ISPs, DoH server operators, or
   others).

5.  Network Operations

5.1.  Impact on DNS query logging

   Analysis of resolver query data is an important task in most operator
   networks.  This can help with traffic management, load balancing and
   capacity planning as well as network and user security.  Widespread
   uptake of DoH will mean an operator has reduced visibility of the DNS
   traffic in their network.  Query traffic logged by traditional
   resolving servers will be less representative (or even completely
   unrepresentative) of the overall DNS activity in an operator's
   network.

5.2.  CDN endpoint selection

   End user queries made with DoH could mean that lookups return answers
   that are sub-optimal. i.e. directing clients to a distant CDN node
   that is outside the operator's network instead of to the localised
   CDN node(s) installed inside that network or directly interconnected
   with that network.  Those DNS responses would be keyed on the source
   IP address of a resolving DoH server, possibly operated by a third
   party, rather than an address of one of the operator's resolving DNS
   servers or end client IP address information that those resolving
   servers might choose to provide through the Client Subnet EDNS0
   option RFC 7871 [RFC7871].

   The impact to an operator of directing clients to a distant CDN node
   that is outside the operator's network is not only slower access to
   resources provided by the CDN.  It also incurs higher costs for the
   operator because traffic is routed over the operator's backbone and
   peering links rather than remaining within a part of the network that
   is geographically or topologically close to the end-user.

   Additionally, operators have powerful technical, operational and
   business incentives to provide optimal user experience for their
   customers, particularly in terms of latency and speed of Internet
   services.  This involves working with multiple CDN and content
   providers to ensure best performance when delivering those services,
   for example by providing Client Subnet EDNS0 option information.  One
   risk is that DoH services could be provided by operators or
   distributors of web content who have different motivations.  For
   instance a provider of DoH service may choose to offer fast access to
   the content that they host or distribute, but may decide not to offer

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   the geographic information of the end-user (for privacy, policy or
   business reasons) to competing content providers/distributors.

5.3.  Redirection for captive portals

   Network operators also often use captive portal DNS to provide
   customer self-service activation and related customer account
   provisioning, billing and support activities.  For example, captive
   portal DNS is used extensively to support functions such as self-
   service provisioning of customer owned and managed Customer Premises
   Equipment (CPE), service support, mobile pay as you go top up and
   access to national/regional WiFi hot spots.  DoH traffic may bypass
   these operator-supplied functions that are essential for managing the
   network.  This would significantly disrupt the manner in which
   networks are operated and managed.

5.4.  Managed network services

   The provision of managed network services, for instance to corporate
   or other enterprise clients will be affected by DoH.  It could
   negatively affect bring-your-own-device policies which might
   introduce devices into these networks that are configured to use
   third party DoH servers.  For instance there is a risk that internal
   domain names used extensively in such networks could leak to external
   DoH servers, presenting obvious privacy and security issues.

5.5.  Resolver capacity management

   Large operator networks are likely to operate their own DoH servers
   because of local policy or business considerations.  This could mean
   an increase in TCP-based DNS traffic to port 443 as DoH displaces
   conventional UDP-based queries to port 53.  Transitioning from a
   primarily UDP-based service to TCP-based DoH would likely require
   substantial network capacity enhancements to an operator's DNS
   infrastructure.  This might also require changes to existing load
   balancing and failover architectures.  Establishing a DoH service in
   these environments would absolutely impact operational management and
   support.

   It is unclear how much end-user DNS traffic will migrate to DoH and
   how quickly that happens since this will depend on the uptake of DoH-
   capable applications.  There is also uncertainty about the default
   behaviour of these applications, for instance try DoH first then fall
   back to conventional DNS, use DoH only, try DoH and DNS in parallel
   and accept whichever answers first, etc.  These unknowns have a
   further obvious impact on capacity planning and network operations.

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5.6.  Failure recovery

   It is not clear how DoH services will affect customers' approach to
   disaster recovery and fault reporting or influence their business
   continuity planning.  For instance, if a client loses connectivity or
   access to their chosen DoH provider(s), they may lose Internet
   service even though they remain connected to the operator's network
   and could otherwise use conventional DNS resolution services.  It is
   assumed, but cannot be guaranteed, that DoH-capable applications will
   fall back to conventional DNS whenever DoH service fails.
   Applications might however be configured to only use DoH apart from
   an initial bootstrapping query that uses DNS.

5.7.  Impact on Network Address Translation

   Techniques such as DNS64 [RFC6147] and NAT64 [RFC6146] are widely
   used for devices with IPv6-only transport, particularly in mobile
   networks to ensure continued access to parts of the Internet that are
   IPv4-only.  These generally require the operator's DNS resolver
   server to carry out some form of IP address mapping.  It is not known
   what impact DoH will have in these environments.  It is unlikely that
   this will work with third party DoH providers because they will not
   have information about the operator's network that would allow them
   to map these IPv6 addresses.

   In networks where the translator prefix is not the well-known prefix
   defined by RFC6146, the client's use of a DoH resolver outside the
   operator's network will prevent access to IPv4-only content, because
   the resolver will not know the correct prefix to use in its response.
   Even when the well-known prefix is used, the DoH resolver may not be
   configured to correctly use it in its response.

5.8.  Load balancing and failover

   Operator networks make extensive use of DNS-based solutions for load
   balancing and service failover.  These might not work as expected
   with DoH clients which bypass the operator's DNS resolver
   infrastructure.  Further operational problems may arise if stale DNS
   data are held in a DoH client's cache.

6.  User Support

   o  Adoption of DoH is likely to decouple DNS from the provision of
      Internet connectivity.  For most users, DNS resolution is
      currently part of the service provided by their ISP.  With DoH,
      users can be expected to rely on DoH service providers and are
      likely to have no business or contractual relationship with those
      providers.

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   o  Getting meaningful consent from users - how?

   o  How will users be able to opt in/out of DoH services?

   o  Users may want to give meaningful consent to use DNS filters.
      Therefore, there should be an option for users to enable and
      disable DoH with neither behaviour assumed.  Such permissions
      should also apply to DoH queries made by web-based apps using an
      API, not just the queries directly entered by the user.

   o  How do users select their "trusted" DoH Provider?  NB This is
      BEFORE draft-hoffman-resolver-associated-doh or whatever is used
      by a DoH client to connect to its chosen DoH server/service.  i.e.
      How is a user or application supplied with a list of DoH
      providers?  How does it choose between them and what are the
      selection criteria?

   o  Clarification is needed on trusted certificate approach, e.g. is
      it enforced at application rather than the kernel/operating system
      layer?

   o  Can/should discrete apps be able to choose their own DoH server?
      Suppose a banking app is configured to use the bank's DoH
      provider.  Can that default be over-ridden?  Should it?

   o  How does a user get told about (and approve) a change of DoH
      service for a phone/tablet when they're roaming between mobile
      telcos or using whatever DoH service is offered in $coffeeshop?

   o  How is an operator expected to support the customer or
      troubleshoot problems caused by accidental or intentional change
      of DoH server?  If the DoH provider deletes all their historic DoH
      traffic, how do they support the ISP customer regarding
      troubleshooting?

   o  How will DoH provisioning take account of existing customer
      parental control/malware protection settings and flag the
      consequences of selecting a new provider on these?

   o  How will browsers/applications explain DoH/DNS options to
      customers so that they can make an informed decision, as many will
      not appreciate what DNS is.  If they select a third party DoH
      provider, that may bypass their existing network operator's
      content and malware protection controls.  The end user will
      presumably need to set these up again with their new DoH provider.

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   o  How to explain to customers that they may need to check/contact
      both their DoH provider(s) and network provider to resolver
      performance and outage issues.

7.  Provisioning

   o  Some list or registry of "trusted" DoH servers is probably
      necessary.  Who/what is going to maintain this and manage it?
      What criteria and procedures are needed for adding or removing
      entries from that list?  How does a DoH provider get trusted and
      become untrusted?

   o  What are the requirements to become a DoH trusted recursive
      resolver?  Will browsers or applications only show global DoH
      provider options?  How can regional network operators offering DoH
      just to their customer base be supported?  How will browsers and
      applications know which regional or local options exist and which
      of these should and should not be honoured?

   o  Need an industry approach for DoH discovery, trust and selection
      that operates in an open and transparent manner, giving the
      customer meaningful consent options.

   o  How to configure CPE and other edge devices (e.g. smartphone) to
      use the operator's chosen DoH provider.

   o  Can/should the operator's choice be overridden by the customer?

   o  Clients need to know the IP address of the DoH server they want to
      use.  How is this discovered, draft-hoffman-resolver-associated-
      doh?

   o  How do web applications get to specify the DoH server they want?
      If web apps get to choose the DoH server, they could be pointing
      to a malicious server (security issue) or allowing a DNS provider
      other than that defined by the user to see the DNS queries
      (privacy issue).

   o  How will DoH provisioning take account of existing customer
      parental control/malware protection settings and flag the
      consequences of selecting a new provider on these?

   o  If a browser or other edge device can do DoH, what determines if
      the DoH is the preferred the choice?, e.g. if CPE or set top box
      devices also supports DNS over TLS, should DoH be an option?  If
      multiple options for DNS resolution are available, what decision
      process is used to make the customer recommendation and how is
      this trusted?

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8.  Privacy Concerns

   Compared to traditional DNS, DoH offers more privacy protection
   against passive surveillance because requests and replies are carried
   over an encrypted channel.  DoH offers an equivalent amount of
   privacy protection against passive surveillance as DoT does because
   both rely on TLS for their security properties.

   Content Delivery Networks use techniques like EDNS-Client-Subnet
   (ECS) to return DNS answers that direct a client to an optimal
   location, for instance the CDN's node in the operator's network which
   serves the end user.  DoH has the potential to be more privacy
   intrusive than ECS, largely because DoH is based on HTTP and can
   leverage the rich per-user and per-device tracking that pervades the
   web today.  The implications of that are not yet well understood.

   A DoH server will have a direct HTTPS connection to the client,
   assuming there are no middleboxes in the path between them.  That
   could for example enable DoH servers operated by CDNs to carry out
   much more fine-grained redirection and content delivery, perhaps even
   on a per-user or per-user-session basis.  They would be able to serve
   content and advertisements based on the end user's choice of
   operating system, their browser and that browser's configuration in
   addition to the client's source IP address, web cookie data, or other
   factors as is prevalent on the web today.

   Global DoH providers will have access to significantly more DNS query
   data, and therefore be able to perform richer big data analytics,
   combining these insights with those obtained from other global
   platforms (search engines, operating systems, browsers, ad trackers,
   analytics services, web sites, mobile apps, payment systems,
   e-commerce stores, social networks, in store Bluetooth beacons,
   etc.), potentially leading to a poor privacy outcome for consumers.

   The DoH provider may adhere to different privacy policies than the
   operator's DNS service, particularly where they are located in
   different jurisdictions.  This may lead to better or worse privacy
   outcomes for users.

   Operators in some jurisdictions are required to perform DNS filtering
   functions on traditional DNS queries and responses.  If this
   functionality has to be provided using DoH, the only available option
   may be to fully proxy the HTTPS traffic.  That represents more of a
   privacy intrusion than filtering alone.

   It is feasible that individual applications might have the ability to
   select their own DoH server, bypassing the system- or operator-
   defined DoH settings.  That could lead to privacy violations because

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   DoH queries get sent to an arbitrary DoH server with unknown privacy
   policies.

9.  Security Considerations

   DoH will give new opportunities for bad actors to propagate malware,
   spam and botnets.  Once they use DoH, as some botnets have already
   started doing for command-and-control traffic, their DNS traffic will
   be encrypted and anonymised, making it hard to deploy countermeasures
   to protect against and mitigate these serious security threats.  This
   is likely to have an adverse impact on cybersecurity both at a
   network/country level as well as for end users.  Use of DoH could
   make it slower to identify DNS-based DDoS attacks, more difficult to
   attribute patient-zero for malware infections and harder to block
   access to botnet command-and-control nodes.  A proof of concept
   exfiltration channel tool based on DoH [GODOH] already exists and it
   is reasonable to expect others which are much less benign will emerge
   in the future.

   DoH queries and responses will be intermingled with other HTTPS port
   443 traffic.  This provides good traffic flow security for the
   client, because it's not readily clear when a DoH request or reply is
   taking place (unlike DoT).  However network analytics may fail to
   detect when a malware implant on the client is making DoH requests,
   which would present a security risk.

   Security of DoH relies on the TLS session for the HTTPS connection.
   Therefore it inherits the security guarantees that TLS provides.
   There may be interactions between DoH and TLS, for example issues
   arising from DoH servers handling large numbers of TLS connections to
   DoH clients simultaneously, that have not yet been explored.

   DNS query traffic is often made available to providers of threat
   intelligence and reputation services.  These providers typically
   aggregate such data from many operators, process these datasets and
   then generate whitelists and blocklists which operators can then
   apply in their networks.  DoH is likely to mean there will be less
   query data readily available for this sort of analysis.  Overall DNS
   query traffic would be spread across a combination of operator-run
   DNS resolver servers and a number of DoH servers who might (or might
   not) make their query traffic available to providers of threat
   intelligence and reputation services.

   This will have two unwelcome results.  First, threat intelligence and
   reputation services will have fewer data to analyse and therefore
   have a significantly less complete perspective of end users' DNS
   behaviour.  Second, the quality and effectiveness of the data
   provided by threat intelligence and reputation services will be

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   materially diminished.  This seems likely to reduce the security of
   networks and users as a result.

   Although DoH uses TLS to provides authentication and data integrity
   of the channel between client and resolver, this does not guarantee
   that the resolver is returning correct DNS data to the client.  DoH
   clients may need to perform DNSSEC validation to verify data received
   from DoH servers.

   There is a risk that internal domain names used extensively in
   managed enterprise networks could leak to external DoH servers,
   presenting obvious privacy and security issues.

   DoH can be implemented within the browser, rather than the kernel or
   an operating system library.  It is not yet clear if that will make
   endpoint-based malware detection more or less effective.

   Browser APIs will allow web applications to make DoH queries.  If
   individual applications have the ability to select their own DoH
   server, it is not clear if that change would only apply to DoH
   lookups by that application or if they had broader scope.  When these
   changes over-ride system- or operator-defined DoH settings, they will
   affect other processes running on the DoH client and effectively
   hijack their DNS traffic by rerouting it to the application's DoH
   provider.

   The interactions between infrastructure using Network Address
   Translation (NAT) [RFC3022] and DoH is unclear.  In situations where
   a third party DoH server can return security threat data back to the
   operator of the originating network, its value is likely to be
   diminished due to the IP address sharing inherent in using NAT.

10.  Human Rights Considerations

   Parental control systems relying on DNS filtering can be bypassed
   using DoH.  This may lead to increased ability of minors to access
   restricted or otherwise inappropriate content on the Internet.

   Using DoH to bypass local DNS filtering and provide anonymity for end
   users is a mixed blessing.  Using DoH to bypass country-based DNS
   filtering may provide end users a way of bypassing censorship
   mechanisms put in place by restrictive regimes.  On the other hand,
   DoH could also help criminals to evade detection by obscuring the
   source of their attacks or botnet control nodes, while increasing the
   commercial tracking of user activity and trade in that data.

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11.  Open Issues for Further Study

   o  DoH's reliance on TLS raises a number of concerns and unknowns.
      These include scalability in managing session tickets, handling
      session resumption and the duration of TLS sessions.  These will
      need careful analysis, particularly on DoH servers which get
      queries from large numbers of DoH clients.

   o  The impact of DNS traffic migrating from UDP and port 53 to TCP
      and port 443 needs to be modelled because of the extra packets and
      round-trip times needed for TCP connection setup and the TLS
      handshake: performance, capacity planning, network engineering and
      so on.

   o  DoH can leverage the rich per-user and per-device tracking that
      pervades the web today.  Since the implications of that are not
      yet well understood, further work in this area is needed.

   o  How DoH services will develop new functionality to overcome any
      inherent performance impact from moving the service out of the
      operator network.  For instance, optimisations to reduce latency
      in 3/4/5G mobile networks.

   o  Clarification is needed around ECS blocking and options to avoid
      impacting existing network operator on-net caching strategy.

   o  What DoH service metrics will be available for users to compare
      DoH providers?

12.  IANA Considerations

   This memo includes no request to IANA.

13.  Acknowledgements

   Fill this in later

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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14.2.  Informative References

   [CSRIC]    FCC, "Cybersecurity Risk Management and Best Practices",
              <https://transition.fcc.gov/to-be-confirmed>.

   [GDPR]     European Union, "General Data Protection Regulation (EU)
              2016/679",
              <https://eur-lex.europa.eu/eli/reg/2016/679/oj>.

   [GODOH]    Sensepost, "DNS exfiltration using DoH",
              <https://sensepost.com/blog/2018/waiting-for-godoh/>.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,
              <https://www.rfc-editor.org/info/rfc3022>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,
              <https://www.rfc-editor.org/info/rfc6147>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [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,
              <https://www.rfc-editor.org/info/rfc7871>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

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   [USPS]     US Secretary of Commerce, "EU-U.S. Privacy Shield
              Framework",
              <https://www.privacyshield.gov/EU-US-Framework>.

Authors' Addresses

   Andy Fidler
   BT plc
   BT Adastral Park
   Martlesham Heath, Ipswich  IP5 3RE
   UK

   Email: andrew.fidler@bt.com

   Bert Hubert
   OpenXchange
   Rollnerstrasse 14
   Nuremberg  90408
   Germany

   Email: bert.hubert@open-xchange.com

   Jason Livingood
   Comcast
   1800 Arch Street
   Philadelphia  PA 19118
   USA

   Email: jason_livingood@comcast.com

   Jim Reid
   RTFM llp
   St Andrews House
   382 Hillington Road, Glasgow  G51 4BL
   Scotland

   Email: jim@rfc1035.com

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   Nic Leymann
   Deutsche Telekom AG
   Friedrich-Ebert-Allee 140
   Bonn  53113
   Germany

   Email: N.Leymann@telekom.de

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