path: root/doc/rfc/rfc9076.txt

Internet Engineering Task Force (IETF)                  T. Wicinski, Ed.
Request for Comments: 9076                                     July 2021
Obsoletes: 7626                                                         
Category: Informational                                                 
ISSN: 2070-1721


                       DNS Privacy Considerations

Abstract

   This document describes the privacy issues associated with the use of
   the DNS by Internet users.  It provides general observations about
   typical current privacy practices.  It is intended to be an analysis
   of the present situation and does not prescribe solutions.  This
   document obsoletes RFC 7626.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9076.

Copyright Notice

   Copyright (c) 2021 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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Scope
   3.  Risks
   4.  Risks in the DNS Data
     4.1.  The Public Nature of DNS Data
     4.2.  Data in the DNS Request
       4.2.1.  Data in the DNS Payload
     4.3.  Cache Snooping
   5.  Risks on the Wire
     5.1.  Unencrypted Transports
     5.2.  Encrypted Transports
   6.  Risks in the Servers
     6.1.  In the Recursive Resolvers
       6.1.1.  Resolver Selection
       6.1.2.  Active Attacks on Resolver Configuration
       6.1.3.  Blocking of DNS Resolution Services
       6.1.4.  Encrypted Transports and Recursive Resolvers
     6.2.  In the Authoritative Name Servers
   7.  Other Risks
     7.1.  Re-identification and Other Inferences
     7.2.  More Information
   8.  Actual "Attacks"
   9.  Legalities
   10. Security Considerations
   11. IANA Considerations
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Appendix A.  Updates since RFC 7626
   Acknowledgments
   Contributions
   Author's Address

1.  Introduction

   This document is an analysis of the DNS privacy issues, in the spirit
   of Section 8 of [RFC6973].

   The Domain Name System (DNS) is specified in [RFC1034], [RFC1035],
   and many later RFCs, which have never been consolidated.  It is one
   of the most important infrastructure components of the Internet and
   is often ignored or misunderstood by Internet users (and even by many
   professionals).  Almost every activity on the Internet starts with a
   DNS query (and often several).  Its use has many privacy
   implications, and this document is an attempt at a comprehensive and
   accurate list.

   Let us begin with a simplified reminder of how the DNS works (see
   also [RFC8499]).  A client, the stub resolver, issues a DNS query to
   a server called the recursive resolver (also called caching resolver,
   full resolver, or recursive name server).  Let's use the query "What
   are the AAAA records for www.example.com?" as an example.  AAAA is
   the QTYPE (Query Type), and www.example.com is the QNAME (Query
   Name).  (The description that follows assumes a cold cache, for
   instance, because the server just started.)  The recursive resolver
   will first query the root name servers.  In most cases, the root name
   servers will send a referral.  In this example, the referral will be
   to the .com name servers.  The resolver repeats the query to one of
   the .com name servers.  The .com name servers, in turn, will refer to
   the example.com name servers.  The example.com name servers will then
   return the answers.  The root name servers, the name servers of .com,
   and the name servers of example.com are called authoritative name
   servers.  It is important, when analyzing the privacy issues, to
   remember that the question asked to all these name servers is always
   the original question, not a derived question.  The question sent to
   the root name servers is "What are the AAAA records for
   www.example.com?", not "What are the name servers of .com?".  By
   repeating the full question, instead of just the relevant part of the
   question to the next in line, the DNS provides more information than
   necessary to the name server.  In this simplified description,
   recursive resolvers do not implement QNAME minimization as described
   in [RFC7816], which will only send the relevant part of the question
   to the upstream name server.

   DNS relies heavily on caching, so the algorithm described above is
   actually a bit more complicated, and not all questions are sent to
   the authoritative name servers.  If the stub resolver asks the
   recursive resolver a few seconds later, "What are the SRV records of
   _xmpp-server._tcp.example.com?", the recursive resolver will remember
   that it knows the name servers of example.com and will just query
   them, bypassing the root and .com.  Because there is typically no
   caching in the stub resolver, the recursive resolver, unlike the
   authoritative servers, sees all the DNS traffic.  (Applications, like
   web browsers, may have some form of caching that does not follow DNS
   rules, for instance, because it may ignore the TTL.  So, the
   recursive resolver does not see all the name resolution activity.)

   It should be noted that DNS recursive resolvers sometimes forward
   requests to other recursive resolvers, typically bigger machines,
   with a larger and more shared cache (and the query hierarchy can be
   even deeper, with more than two levels of recursive resolvers).  From
   the point of view of privacy, these forwarders are like resolvers
   except that they do not see all of the requests being made (due to
   caching in the first resolver).

   At the time of writing, almost all this DNS traffic is currently sent
   unencrypted.  However, there is increasing deployment of DNS over TLS
   (DoT) [RFC7858] and DNS over HTTPS (DoH) [RFC8484], particularly in
   mobile devices, browsers, and by providers of anycast recursive DNS
   resolution services.  There are a few cases where there is some
   alternative channel encryption, for instance, in an IPsec VPN tunnel,
   at least between the stub resolver and the resolver.  Some recent
   analysis on the service quality of encrypted DNS traffic can be found
   in [dns-over-encryption].

   Today, almost all DNS queries are sent over UDP [thomas-ditl-tcp].
   This has practical consequences when considering encryption of the
   traffic as a possible privacy technique.  Some encryption solutions
   are only designed for TCP, not UDP, although new solutions are still
   emerging [RFC9000] [DPRIVE-DNSOQUIC].

   Another important point to keep in mind when analyzing the privacy
   issues of DNS is the fact that DNS requests received by a server are
   triggered for different reasons.  Let's assume an eavesdropper wants
   to know which web page is viewed by a user.  For a typical web page,
   there are three sorts of DNS requests being issued:

   Primary request:
      This is the domain name in the URL that the user typed, selected
      from a bookmark, or chose by clicking on a hyperlink.  Presumably,
      this is what is of interest for the eavesdropper.

   Secondary requests:
      These are the additional requests performed by the user agent
      (here, the web browser) without any direct involvement or
      knowledge of the user.  For the Web, they are triggered by
      embedded content, Cascading Style Sheets (CSS), JavaScript code,
      embedded images, etc.  In some cases, there can be dozens of
      domain names in different contexts on a single web page.

   Tertiary requests:
      These are the additional requests performed by the DNS service
      itself.  For instance, if the answer to a query is a referral to a
      set of name servers and the glue records are not returned, the
      resolver will have to send additional requests to turn the name
      servers' names into IP addresses.  Similarly, even if glue records
      are returned, a careful recursive server will send tertiary
      requests to verify the IP addresses of those records.

   It can also be noted that, in the case of a typical web browser, more
   DNS requests than strictly necessary are sent, for instance, to
   prefetch resources that the user may query later or when
   autocompleting the URL in the address bar.  Both are a significant
   privacy concern since they may leak information even about non-
   explicit actions.  For instance, just reading a local HTML page, even
   without selecting the hyperlinks, may trigger DNS requests.

   Privacy-related terminology is from [RFC6973].  This document
   obsoletes [RFC7626].

2.  Scope

   This document focuses mostly on the study of privacy risks for the
   end user (the one performing DNS requests).  The risks of pervasive
   surveillance [RFC7258] are considered as well as risks coming from a
   more focused surveillance.  In this document, the term "end user" is
   used as defined in [RFC8890].

   This document does not attempt a comparison of specific privacy
   protections provided by individual networks or organizations; it
   makes only general observations about typical current practices.

   Privacy risks for the holder of a zone (the risk that someone gets
   the data) are discussed in [RFC5155] and [RFC5936].

   Privacy risks for recursive operators (including access providers and
   operators in enterprise networks) such as leakage of private
   namespaces or blocklists are out of scope for this document.

   Non-privacy risks (e.g., security-related considerations such as
   cache poisoning) are also out of scope.

   The privacy risks associated with the use of other protocols that
   make use of DNS information are not considered here.

3.  Risks

   The following four sections outline the privacy considerations
   associated with different aspects of the DNS for the end user.  When
   reading these sections, it needs to be kept in mind that many of the
   considerations (for example, recursive resolver and transport
   protocol) can be specific to the network context that a device is
   using at a given point in time.  A user may have many devices, and
   each device might utilize many different networks (e.g., home, work,
   public, or cellular) over a period of time or even concurrently.  An
   exhaustive analysis of the privacy considerations for an individual
   user would need to take into account the set of devices used and the
   multiple dynamic contexts of each device.  This document does not
   attempt such a complex analysis; instead, it presents an overview of
   the various considerations that could form the basis of such an
   analysis.

4.  Risks in the DNS Data

4.1.  The Public Nature of DNS Data

   It has been stated that "the data in the DNS is public".  This
   sentence makes sense for an Internet-wide lookup system, and there
   are multiple facets to the data and metadata involved that deserve a
   more detailed look.  First, access control lists (ACLs) and private
   namespaces notwithstanding, the DNS operates under the assumption
   that public-facing authoritative name servers will respond to "usual"
   DNS queries for any zone they are authoritative for, without further
   authentication or authorization of the client (resolver).  Due to the
   lack of search capabilities, only a given QNAME will reveal the
   resource records associated with that name (or that name's
   nonexistence).  In other words: one needs to know what to ask for in
   order to receive a response.  There are many ways in which supposedly
   "private" resources currently leak.  A few examples are DNSSEC NSEC
   zone walking [RFC4470], passive DNS services [passive-dns], etc.  The
   zone transfer QTYPE [RFC5936] is often blocked or restricted to
   authenticated/authorized access to enforce this difference (and maybe
   for other reasons).

   Another difference between the DNS data and a particular DNS
   transaction (i.e., a DNS name lookup): DNS data and the results of a
   DNS query are public, within the boundaries described above, and may
   not have any confidentiality requirements.  However, the same is not
   true of a single transaction or a sequence of transactions; those
   transactions are not / should not be public.  A single transaction
   reveals both the originator of the query and the query contents; this
   potentially leaks sensitive information about a specific user.  A
   typical example from outside the DNS world is that the website of
   Alcoholics Anonymous is public but the fact that you visit it should
   not be.  Furthermore, the ability to link queries reveals information
   about individual use patterns.

4.2.  Data in the DNS Request

   The DNS request includes many fields, but two of them seem
   particularly relevant for the privacy issues: the QNAME and the
   source IP address.  "Source IP address" is used in a loose sense of
   "source IP address + maybe source port number", because the port
   number is also in the request and can be used to differentiate
   between several users sharing an IP address (behind a Carrier-Grade
   NAT (CGN), for instance [RFC6269]).

   The QNAME is the full name sent by the user.  It gives information
   about what the user does ("What are the MX records of example.net?"
   means they probably want to send email to someone at example.net,
   which may be a domain used by only a few persons and is therefore
   very revealing about communication relationships).  Some QNAMEs are
   more sensitive than others.  For instance, querying the A record of a
   well-known web statistics domain reveals very little (everybody
   visits websites that use this analytics service), but querying the A
   record of www.verybad.example where verybad.example is the domain of
   an organization that some people find offensive or objectionable may
   create more problems for the user.  Also, sometimes, the QNAME embeds
   the software one uses, which could be a privacy issue (for instance,
   _ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org.
   There are also some BitTorrent clients that query an SRV record for
   _bittorrent-tracker._tcp.domain.example.

   Another important thing about the privacy of the QNAME is future
   usages.  Today, the lack of privacy is an obstacle to putting
   potentially sensitive or personally identifiable data in the DNS.  At
   the moment, your DNS traffic might reveal that you are exchanging
   emails but not with whom.  If your Mail User Agent (MUA) starts
   looking up Pretty Good Privacy (PGP) keys in the DNS [RFC7929], then
   privacy becomes a lot more important.  And email is just an example;
   there would be other really interesting uses for a more privacy-
   friendly DNS.

   For the communication between the stub resolver and the recursive
   resolver, the source IP address is the address of the user's machine.
   Therefore, all the issues and warnings about collection of IP
   addresses apply here.  For the communication between the recursive
   resolver and the authoritative name servers, the source IP address
   has a different meaning; it does not have the same status as the
   source address in an HTTP connection.  It is typically the IP address
   of the recursive resolver that, in a way, "hides" the real user.
   However, hiding does not always work.  The edns-client-subnet (ECS)
   EDNS0 option [RFC7871] is sometimes used (see one privacy analysis in
   [denis-edns-client-subnet]).  Sometimes the end user has a personal
   recursive resolver on their machine.  In both cases, the IP address
   originating queries to the authoritative server is as sensitive as it
   is for HTTP [sidn-entrada].

   A note about IP addresses: there is currently no IETF document that
   describes in detail all the privacy issues around IP addressing in
   general, although [RFC7721] does discuss privacy considerations for
   IPv6 address generation mechanisms.  In the meantime, the discussion
   here is intended to include both IPv4 and IPv6 source addresses.  For
   a number of reasons, their assignment and utilization characteristics
   are different, which may have implications for details of information
   leakage associated with the collection of source addresses.  (For
   example, a specific IPv6 source address seen on the public Internet
   is less likely than an IPv4 address to originate behind an address-
   sharing scheme.)  However, for both IPv4 and IPv6 addresses, it is
   important to note that source addresses are propagated with queries
   via the ECS option and comprise metadata about the host, user, or
   application that originated them.

4.2.1.  Data in the DNS Payload

   At the time of writing, there are no standardized client identifiers
   contained in the DNS payload itself (ECS, as described in [RFC7871],
   is widely used; however, [RFC7871] is only an Informational RFC).

   DNS Cookies [RFC7873] are a lightweight DNS transaction security
   mechanism that provides limited protection against a variety of
   increasingly common denial-of-service and amplification/forgery or
   cache poisoning attacks by off-path attackers.  It is noted, however,
   that they are designed to just verify IP addresses (and should change
   once a client's IP address changes), but they are not designed to
   actively track users (like HTTP cookies).

   There are anecdotal accounts of Media Access Control (MAC) addresses
   (https://lists.dns-oarc.net/pipermail/dns-
   operations/2016-January/014143.html) and even user names being
   inserted in nonstandard EDNS(0) options [RFC6891] for stub-to-
   resolver communications to support proprietary functionality
   implemented at the resolver (e.g., parental filtering).

4.3.  Cache Snooping

   The content of recursive resolvers' caches can reveal data about the
   clients using it (the privacy risks depend on the number of clients).
   This information can sometimes be examined by sending DNS queries
   with RD=0 to inspect cache content, particularly looking at the DNS
   TTLs [grangeia.snooping].  Since this also is a reconnaissance
   technique for subsequent cache poisoning attacks, some
   countermeasures have already been developed and deployed
   [cache-snooping-defence].

5.  Risks on the Wire

5.1.  Unencrypted Transports

   For unencrypted transports, DNS traffic can be seen by an
   eavesdropper like any other traffic.  (DNSSEC, specified in
   [RFC4033], explicitly excludes confidentiality from its goals.)  So,
   if an initiator starts an HTTPS communication with a recipient, the
   HTTP traffic will be encrypted, but the DNS exchange prior to it will
   not be.  When other protocols become more and more privacy aware and
   secured against surveillance (e.g., [RFC8446], [RFC9000]), the use of
   unencrypted transports for DNS may become "the weakest link" in
   privacy.  It is noted that, at the time of writing, there is ongoing
   work attempting to encrypt the Server Name Identification (SNI) in
   the TLS handshake [RFC8744], which is one of the last remaining non-
   DNS cleartext identifiers of a connection target.

   An important specificity of the DNS traffic is that it may take a
   different path than the communication between the initiator and the
   recipient.  For instance, an eavesdropper may be unable to tap the
   wire between the initiator and the recipient but may have access to
   the wire going to the recursive resolver or to the authoritative name
   servers.

   The best place to tap, from an eavesdropper's point of view, is
   clearly between the stub resolvers and the recursive resolvers,
   because traffic is not limited by DNS caching.

   The attack surface between the stub resolver and the rest of the
   world can vary widely depending upon how the end user's device is
   configured.  By order of increasing attack surface:

   *  The recursive resolver can be on the end user's device.  In
      (currently) a small number of cases, individuals may choose to
      operate their own DNS resolver on their local machine.  In this
      case, the attack surface for the connection between the stub
      resolver and the caching resolver is limited to that single
      machine.  The recursive resolver will expose data to authoritative
      resolvers as discussed in Section 6.2.

   *  The recursive resolver may be at the local network edge.  For
      many/most enterprise networks and for some residential networks,
      the caching resolver may exist on a server at the edge of the
      local network.  In this case, the attack surface is the local
      network.  Note that in large enterprise networks, the DNS resolver
      may not be located at the edge of the local network but rather at
      the edge of the overall enterprise network.  In this case, the
      enterprise network could be thought of as similar to the Internet
      Access Provider (IAP) network referenced below.

   *  The recursive resolver can be in the IAP network.  For most
      residential networks and potentially other networks, the typical
      case is for the user's device to be configured (typically
      automatically through DHCP or relay agent options) with the
      addresses of the DNS proxy in the Customer Premises Equipment
      (CPE), which in turn points to the DNS recursive resolvers at the
      IAP.  The attack surface for on-the-wire attacks is therefore from
      the end user system across the local network and across the IAP
      network to the IAP's recursive resolvers.

   *  The recursive resolver can be a public DNS service (or a privately
      run DNS resolver hosted on the public Internet).  Some machines
      may be configured to use public DNS resolvers such as those
      operated by Google Public DNS or OpenDNS.  The user may have
      configured their machine to use these DNS recursive resolvers
      themselves -- or their IAP may have chosen to use the public DNS
      resolvers rather than operating their own resolvers.  In this
      case, the attack surface is the entire public Internet between the
      user's connection and the public DNS service.  It can be noted
      that if the user selects a single resolver with a small client
      population (even when using an encrypted transport), it can
      actually serve to aid tracking of that user as they move across
      network environments.

   It is also noted that, typically, a device connected _only_ to a
   modern cellular network is

   *  directly configured with only the recursive resolvers of the IAP
      and

   *  afforded some level of protection against some types of
      eavesdropping for all traffic (including DNS traffic) due to the
      cellular network link-layer encryption.

   The attack surface for this specific scenario is not considered here.

5.2.  Encrypted Transports

   The use of encrypted transports directly mitigates passive
   surveillance of the DNS payload; however, some privacy attacks are
   still possible.  This section enumerates the residual privacy risks
   to an end user when an attacker can passively monitor encrypted DNS
   traffic flows on the wire.

   These are cases where user identification, fingerprinting, or
   correlations may be possible due to the use of certain transport
   layers or cleartext/observable features.  These issues are not
   specific to DNS, but DNS traffic is susceptible to these attacks when
   using specific transports.

   Some general examples exist; for example, certain studies highlight
   that the OS fingerprint values (http://netres.ec/?b=11B99BD) of IPv4
   TTL, IPv6 Hop Limit, or TCP Window size can be used to fingerprint
   client OSes or that various techniques can be used to de-NAT DNS
   queries [dns-de-nat].

   Note that even when using encrypted transports, the use of cleartext
   transport options to decrease latency can provide correlation of a
   user's connections, e.g., using TCP Fast Open [RFC7413].

   Implementations that support encrypted transports also commonly reuse
   connections for multiple DNS queries to optimize performance (e.g.,
   via DNS pipelining or HTTPS multiplexing).  Default configuration
   options for encrypted transports could, in principle, fingerprint a
   specific client application.  For example:

   *  TLS version or cipher suite selection

   *  session resumption

   *  the maximum number of messages to send and

   *  a maximum connection time before closing a connections and
      reopening.

   If libraries or applications offer user configuration of such options
   (e.g., [getdns]), then they could, in principle, help to identify a
   specific user.  Users may want to use only the defaults to avoid this
   issue.

   While there are known attacks on older versions of TLS, the most
   recent recommendations [RFC7525] and the development of TLS 1.3
   [RFC8446] largely mitigate those.

   Traffic analysis of unpadded encrypted traffic is also possible
   [pitfalls-of-dns-encryption] because the sizes and timing of
   encrypted DNS requests and responses can be correlated to unencrypted
   DNS requests upstream of a recursive resolver.

6.  Risks in the Servers

   Using the terminology of [RFC6973], the DNS servers (recursive
   resolvers and authoritative servers) are enablers: "they facilitate
   communication between an initiator and a recipient without being
   directly in the communications path".  As a result, they are often
   forgotten in risk analysis.  But, to quote [RFC6973] again, "Although
   [...] enablers may not generally be considered as attackers, they may
   all pose privacy threats (depending on the context) because they are
   able to observe, collect, process, and transfer privacy-relevant
   data".  In [RFC6973] parlance, enablers become observers when they
   start collecting data.

   Many programs exist to collect and analyze DNS data at the servers --
   from the "query log" of some programs like BIND to tcpdump and more
   sophisticated programs like PacketQ [packetq] and DNSmezzo
   [dnsmezzo].  The organization managing the DNS server can use this
   data itself, or it can be part of a surveillance program like PRISM
   [prism] and pass data to an outside observer.

   Sometimes this data is kept for a long time and/or distributed to
   third parties for research purposes [ditl] [day-at-root], security
   analysis, or surveillance tasks.  These uses are sometimes under some
   sort of contract, with various limitations, for instance, on
   redistribution, given the sensitive nature of the data.  Also, there
   are observation points in the network that gather DNS data and then
   make it accessible to third parties for research or security purposes
   ("passive DNS" [passive-dns]).

6.1.  In the Recursive Resolvers

   Recursive resolvers see all the traffic since there is typically no
   caching before them.  To summarize: your recursive resolver knows a
   lot about you.  The resolver of a large IAP, or a large public
   resolver, can collect data from many users.

6.1.1.  Resolver Selection

   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.  The choice by a user to join a particular network (e.g.,
   by physically plugging in a cable or selecting a network in an OS
   dialogue) typically updates a number of system resources -- these can
   include IP addresses, the 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 their default system DNS
   settings and so implicitly accept the network settings for the DNS.
   The network resolvers have therefore historically been the sole
   destination for all of the DNS queries from a device.  These
   resolvers may have varied 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 the
   network.

   All major OSes expose the system DNS settings and allow users to
   manually override them if desired.

   More recently, some networks and users have actively chosen to use a
   large public resolver, e.g., Google Public DNS
   (https://developers.google.com/speed/public-dns), Cloudflare
   (https://developers.cloudflare.com/1.1.1.1/setting-up-1.1.1.1/), or
   Quad9 (https://www.quad9.net).  There can be many reasons: cost
   considerations for network operators, better reliability, or anti-
   censorship considerations are just a few.  Such services typically do
   provide a privacy policy, and the user can get an idea of the data
   collected by such operators by reading one, e.g., Google Public DNS -
   Your Privacy (https://developers.google.com/speed/public-dns/
   privacy).

   In general, as with many other protocols, issues around
   centralization also arise with DNS.  The picture is fluid with
   several competing factors contributing, where these factors can also
   vary by geographic region.  These include:

   *  ISP outsourcing, including to third-party and public resolvers

   *  regional market domination by one or only a few ISPs

   *  applications directing DNS traffic by default to a limited subset
      of resolvers (see Section 6.1.1.2)

   An increased proportion of the global DNS resolution traffic being
   served by only a few entities means that the privacy considerations
   for users are highly dependent on the privacy policies and practices
   of those entities.  Many of the issues around centralization are
   discussed in [centralisation-and-data-sovereignty].

6.1.1.1.  Dynamic Discovery of DoH and Strict DoT

   While support for opportunistic DoT can be determined by probing a
   resolver on port 853, there is currently no standardized discovery
   mechanism for DoH and Strict DoT servers.

   This means that clients that might want to dynamically discover such
   encrypted services, and where users are willing to trust such
   services, are not able to do so.  At the time of writing, efforts to
   provide standardized signaling mechanisms to discover the services
   offered by local resolvers are in progress [DNSOP-RESOLVER].  Note
   that an increasing number of ISPs are deploying encrypted DNS; for
   example, see the Encrypted DNS Deployment Initiative [EDDI].

6.1.1.2.  Application-Specific Resolver Selection

   An increasing number of applications are offering application-
   specific encrypted DNS resolution settings, rather than defaulting to
   using only the system resolver.  A variety of heuristics and
   resolvers are available in different applications, including hard-
   coded lists of recognized DoH/DoT servers.

   Generally, users are not aware of application-specific DNS settings
   and may not have control over those settings.  To address these
   limitations, users will only be aware of and have the ability to
   control such settings if applications provide the following
   functions:

   *  communicate the change clearly to users when the default
      application resolver changes away from the system resolver

   *  provide configuration options to change the default application
      resolver, including a choice to always use the system resolver

   *  provide mechanisms for users to locally inspect, selectively
      forward, and filter queries (either via the application itself or
      use of the system resolver)

   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.

6.1.2.  Active Attacks on Resolver Configuration

   The previous section discussed DNS privacy, assuming that all the
   traffic was directed to the intended servers (i.e., those that would
   be used in the absence of an active attack) and that the potential
   attacker was purely passive.  But, in reality, there can be active
   attackers in the network.

   The Internet Threat model, as described in [RFC3552], assumes that
   the attacker controls the network.  Such an attacker can completely
   control any insecure DNS resolution, both passively monitoring the
   queries and responses and substituting their own responses.  Even if
   encrypted DNS such as DoH or DoT is used, unless the client has been
   configured in a secure way with the server identity, an active
   attacker can impersonate the server.  This implies that opportunistic
   modes of DoH/DoT as well as modes where the client learns of the DoH/
   DoT server via in-network mechanisms such as DHCP are vulnerable to
   attack.  In addition, if the client is compromised, the attacker can
   replace the DNS configuration with one of its own choosing.

6.1.3.  Blocking of DNS Resolution Services

   User privacy can also be at risk if there is blocking of access to
   remote recursive servers that offer encrypted transports, e.g., when
   the local resolver does not offer encryption and/or has very poor
   privacy policies.  For example, active blocking of port 853 for DoT
   or blocking of specific IP addresses could restrict the resolvers
   available to the user.  The extent of the risk to user privacy is
   highly dependent on the specific network and user context; a user on
   a network that is known to perform surveillance would be compromised
   if they could not access such services, whereas a user on a trusted
   network might have no privacy motivation to do so.

   As a matter of policy, some recursive resolvers use their position in
   the query path to selectively block access to certain DNS records.
   This is a form of rendezvous-based blocking as described in
   Section 4.3 of [RFC7754].  Such blocklists often include servers
   known to be used for malware, bots, or other security risks.  In
   order to prevent circumvention of their blocking policies, some
   networks also block access to resolvers with incompatible policies.

   It is also noted that attacks on remote resolver services, e.g.,
   DDoS, could force users to switch to other services that do not offer
   encrypted transports for DNS.

6.1.4.  Encrypted Transports and Recursive Resolvers

6.1.4.1.  DoT and DoH

   Use of encrypted transports does not reduce the data available in the
   recursive resolver and ironically can actually expose more
   information about users to operators.  As described in Section 5.2,
   use of session-based encrypted transports (TCP/TLS) can expose
   correlation data about users.

6.1.4.2.  DoH-Specific Considerations

   DoH inherits the full privacy properties of the HTTPS stack and as a
   consequence introduces new privacy considerations when compared with
   DNS over UDP, TCP, or TLS [RFC7858].  Section 8.2 of [RFC8484]
   describes the privacy considerations in the server of the DoH
   protocol.

   A brief summary of some of the issues includes the following:

   *  HTTPS presents new considerations for correlation, such as
      explicit HTTP cookies and implicit fingerprinting of the unique
      set and ordering of HTTP request header fields.

   *  The User-Agent and Accept-Language request header fields often
      convey specific information about the client version or locale.

   *  Utilizing the full set of HTTP features enables DoH to be more
      than an HTTP tunnel, but it is at the cost of opening up
      implementations to the full set of privacy considerations of HTTP.

   *  Implementations are advised to expose the minimal set of data
      needed to achieve the desired feature set.

   [RFC8484] specifically makes selection of HTTPS functionality vs.
   privacy an implementation choice.  At the extremes, there may be
   implementations that attempt to achieve parity with DoT from a
   privacy perspective at the cost of using no identifiable HTTP
   headers, and there might be others that provide feature-rich data
   flows where the low-level origin of the DNS query is easily
   identifiable.  Some implementations have, in fact, chosen to restrict
   the use of the User-Agent header so that resolver operators cannot
   identify the specific application that is originating the DNS
   queries.

   Privacy-focused users should be aware of the potential for additional
   client identifiers in DoH compared to DoT and may want to only use
   DoH client implementations that provide clear guidance on what
   identifiers they add.

6.2.  In the Authoritative Name Servers

   Unlike what happens for recursive resolvers, the observation
   capabilities of authoritative name servers are limited by caching;
   they see only the requests for which the answer was not in the cache.
   For aggregated statistics ("What is the percentage of LOC queries?"),
   this is sufficient, but it prevents an observer from seeing
   everything.  Similarly, the increasing deployment of QNAME
   minimization [ripe-qname-measurements] reduces the data visible at
   the authoritative name server.  Still, the authoritative name servers
   see a part of the traffic, and this subset may be sufficient to
   violate some privacy expectations.

   Also, the user often has some legal/contractual link with the
   recursive resolver (they have chosen the IAP, or they have chosen to
   use a given public resolver) while having no control and perhaps no
   awareness of the role of the authoritative name servers and their
   observation abilities.

   As noted before, using a local resolver or a resolver close to the
   machine decreases the attack surface for an on-the-wire eavesdropper.
   But it may decrease privacy against an observer located on an
   authoritative name server.  This authoritative name server will see
   the IP address of the end client instead of the address of a big
   recursive resolver shared by many users.

   This "protection", when using a large resolver with many clients, is
   no longer present if ECS [RFC7871] is used because, in this case, the
   authoritative name server sees the original IP address (or prefix,
   depending on the setup).

   As of today, all the instances of one root name server, L-root,
   receive together around 50,000 queries per second.  While most of it
   is "junk" (errors on the Top-Level Domain (TLD) name), it gives an
   idea of the amount of big data that pours into name servers.  (And
   even "junk" can leak information; for instance, if there is a typing
   error in the TLD, the user will send data to a TLD that is not the
   usual one.)

   Many domains, including TLDs, are partially hosted by third-party
   servers, sometimes in a different country.  The contracts between the
   domain manager and these servers may or may not take privacy into
   account.  Whatever the contract, the third-party hoster may or may
   not be honest; in any case, it will have to follow its local laws.
   For example, requests to a given ccTLD may go to servers managed by
   organizations outside of the ccTLD's country.  Users may not
   anticipate that when doing a security analysis.

   Also, it seems (see the survey described in [aeris-dns]) that there
   is a strong concentration of authoritative name servers among
   "popular" domains (such as the Alexa Top N list).  For instance,
   among the Alexa Top 100K (https://www.alexa.com/topsites), one DNS
   provider hosts 10% of the domains today.  The ten most important DNS
   providers together host one-third of all domains.  With the control
   (or the ability to sniff the traffic) of a few name servers, you can
   gather a lot of information.

7.  Other Risks

7.1.  Re-identification and Other Inferences

   An observer has access not only to the data they directly collect but
   also to the results of various inferences about this data.  The term
   "observer" here is used very generally; for example, the observer
   might passively observe cleartext DNS traffic or be in the network
   that is actively attacking the user by redirecting DNS resolution, or
   it might be a local or remote resolver operator.

   For instance, a user can be re-identified via DNS queries.  If the
   adversary knows a user's identity and can watch their DNS queries for
   a period, then that same adversary may be able to re-identify the
   user solely based on their pattern of DNS queries later on regardless
   of the location from which the user makes those queries.  For
   example, one study [herrmann-reidentification] found that such re-
   identification is possible so that "73.1% of all day-to-day links
   were correctly established, i.e. user u was either re-identified
   unambiguously (1) or the classifier correctly reported that u was not
   present on day t + 1 any more (2)".  While that study related to web
   browsing behavior, equally characteristic patterns may be produced
   even in machine-to-machine communications or without a user taking
   specific actions, e.g., at reboot time if a characteristic set of
   services are accessed by the device.

   For instance, one could imagine that an intelligence agency
   identifies people going to a site by putting in a very long DNS name
   and looking for queries of a specific length.  Such traffic analysis
   could weaken some privacy solutions.

   The IAB Privacy and Security Program also has a document [RFC7624]
   that considers such inference-based attacks in a more general
   framework.

7.2.  More Information

   Useful background information can also be found in [tor-leak]
   (regarding the risk of privacy leaks through DNS) and in a few
   academic papers: [yanbin-tsudik], [castillo-garcia],
   [fangming-hori-sakurai], and [federrath-fuchs-herrmann-piosecny].

8.  Actual "Attacks"

   A very quick examination of DNS traffic may lead to the false
   conclusion that extracting the needle from the haystack is difficult.
   "Interesting" primary DNS requests are mixed with useless (for the
   eavesdropper) secondary and tertiary requests (see the terminology in
   Section 1).  But, in this time of "big data" processing, powerful
   techniques now exist to get from the raw data to what the
   eavesdropper is actually interested in.

   Many research papers about malware detection use DNS traffic to
   detect "abnormal" behavior that can be traced back to the activity of
   malware on infected machines.  Yes, this research was done for the
   greater good, but technically it is a privacy attack and it
   demonstrates the power of the observation of DNS traffic.  See
   [dns-footprint], [dagon-malware], and [darkreading-dns].

   Passive DNS services [passive-dns] allow reconstruction of the data
   of sometimes an entire zone.  Well-known passive DNS services keep
   only the DNS responses and not the source IP address of the client,
   precisely for privacy reasons.  Other passive DNS services may not be
   so careful.  And there are still potential problems with revealing
   QNAMEs.

   The revelations from the Edward Snowden documents, which were leaked
   from the National Security Agency (NSA), provide evidence of the use
   of the DNS in mass surveillance operations [morecowbell].  For
   example, the MORECOWBELL surveillance program uses a dedicated covert
   monitoring infrastructure to actively query DNS servers and perform
   HTTP requests to obtain meta-information about services and to check
   their availability.  Also, the QUANTUMTHEORY
   (https://theintercept.com/document/2014/03/12/nsa-gchqs-
   quantumtheory-hacking-tactics/) project, which includes detecting
   lookups for certain addresses and injecting bogus replies, is another
   good example showing that the lack of privacy protections in the DNS
   is actively exploited.

9.  Legalities

   To our knowledge, there are no specific privacy laws for DNS data in
   any country.  Interpreting general privacy laws, like the European
   Union's [data-protection-directive] or GDPR (https://gdpr.eu/tag/
   gdpr/), in the context of DNS traffic data is not an easy task, and
   there is no known court precedent.  See an interesting analysis in
   [sidn-entrada].

10.  Security Considerations

   This document is entirely about security -- more precisely, privacy.
   It just lays out the problem; it does not try to set requirements
   (with the choices and compromises they imply), much less define
   solutions.  Possible solutions to the issues described here are
   discussed in other documents (currently too many to all be
   mentioned); see, for instance, "Recommendations for DNS Privacy
   Operators" [RFC8932].

11.  IANA Considerations

   This document has no IANA actions.

12.  References

12.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

12.2.  Informative References

   [aeris-dns]
              Vinot, N., "Vie privée: et le DNS alors? [Privacy: what
              about DNS?]", February 2015,
              <https://blog.imirhil.fr/vie-privee-et-le-dns-alors.html>.

   [cache-snooping-defence]
              ISC, "DNS Cache snooping - should I be concerned?",
              October 2018, <https://kb.isc.org/docs/aa-00482>.

   [castillo-garcia]
              Castillo-Perez, S. and J. Garcia-Alfaro, "Anonymous
              Resolution of DNS Queries", Lecture Notes in Computer
              Science, Vol. 5332, DOI 10.1007/978-3-540-88873-4_5, 2008,
              <https://dl.acm.org/doi/10.1007/978-3-540-88873-4_5>.

   [centralisation-and-data-sovereignty]
              De Filippi, P. and S. McCarthy, "Cloud Computing:
              Centralization and Data Sovereignty", European Journal of
              Law and Technology, Vol. 3, No. 2, October 2012,
              <https://papers.ssrn.com/sol3/
              papers.cfm?abstract_id=2167372>.

   [dagon-malware]
              Dagon, D., "Corrupted DNS Resolution Paths: The Rise of a
              Malicious Resolution Authority", ISC/OARC Workshop, 2007,
              <https://www.dns-oarc.net/files/workshop-2007/Dagon-
              Resolution-corruption.pdf>.

   [darkreading-dns]
              Lemos, R., "Got Malware? Three Signs Revealed In DNS
              Traffic", May 2013,
              <https://www.darkreading.com/analytics/security-
              monitoring/got-malware-three-signs-revealed-in-dns-
              traffic/d/d-id/1139680>.

   [data-protection-directive]
              European Parliament, "Directive 95/46/EC of the European
              Parliament and of the Council of 24 October 1995 on the
              protection of individuals with regard to the processing of
              personal data and on the free movement of such data",
              Official Journal L 281, pp. 31-50, November 1995,
              <https://eur-lex.europa.eu/LexUriServ/
              LexUriServ.do?uri=CELEX:31995L0046:EN:HTML>.

   [day-at-root]
              Castro, S., Wessels, D., Fomenkov, M., and K. Claffy, "A
              Day at the Root of the Internet", ACM SIGCOMM Computer
              Communication Review, Vol. 38, No. 5,
              DOI 10.1145/1452335.1452341, October 2008,
              <https://www.sigcomm.org/sites/default/files/ccr/
              papers/2008/October/1452335-1452341.pdf>.

   [denis-edns-client-subnet]
              Denis, F., "Security and privacy issues of edns-client-
              subnet", August 2013,
              <https://00f.net/2013/08/07/edns-client-subnet/>.

   [ditl]     CAIDA, "A Day in the Life of the Internet (DITL)",
              <https://www.caida.org/projects/ditl/>.

   [dns-de-nat]
              Orevi, L., Herzberg, A., Zlatokrilov, H., and D. Sigron,
              "DNS-DNS: DNS-based De-NAT Scheme", January 2017,
              <https://www.researchgate.net/publication/320322146_DNS-
              DNS_DNS-based_De-NAT_Scheme>.

   [dns-footprint]
              Stoner, E., "DNS Footprint of Malware", OARC Workshop,
              October 2010, <https://www.dns-oarc.net/files/workshop-
              201010/OARC-ers-20101012.pdf>.

   [dns-over-encryption]
              Lu, C., Liu, B., Li, Z., Hao, S., Duan, H., Zhang, M.,
              Leng, C., Liu, Y., Zhang, Z., and J. Wu, "An End-to-End,
              Large-Scale Measurement of DNS-over-Encryption: How Far
              Have We Come?", IMC '19: Proceedings of the Internet
              Measurement Conference, pp. 22-35,
              DOI 10.1145/3355369.3355580, October 2019,
              <https://dl.acm.org/citation.cfm?id=3355369.3355580>.

   [dnsmezzo] Bortzmeyer, S., "DNSmezzo", <http://www.dnsmezzo.net/>.

   [DNSOP-RESOLVER]
              Sood, P., Arends, R., and P. Hoffman, "DNS Resolver
              Information Self-publication", Work in Progress, Internet-
              Draft, draft-ietf-dnsop-resolver-information-01, 11
              February 2020, <https://datatracker.ietf.org/doc/html/
              draft-ietf-dnsop-resolver-information-01>.

   [DPRIVE-DNSOQUIC]
              Huitema, C., Dickinson, S., and A. Mankin, "Specification
              of DNS over Dedicated QUIC Connections", Work in Progress,
              Internet-Draft, draft-ietf-dprive-dnsoquic-03, 12 July
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              dprive-dnsoquic-03>.

   [EDDI]     EDDI, "Encrypted DNS Deployment Initiative",
              <https://www.encrypted-dns.org>.

   [fangming-hori-sakurai]
              Zhao, F., Hori, Y., and K. Sakurai, "Analysis of Privacy
              Disclosure in DNS Query", MUE '07: Proceedings of the 2007
              International Conference on Multimedia and Ubiquitous
              Engineering, pp. 952-957, DOI 10.1109/MUE.2007.84,
              ISBN 0-7695-2777-9, April 2007,
              <https://dl.acm.org/citation.cfm?id=1262690.1262986>.

   [federrath-fuchs-herrmann-piosecny]
              Federrath, H., Fuchs, K.-P., Herrmann, D., and C.
              Piosecny, "Privacy-Preserving DNS: Analysis of Broadcast,
              Range Queries and Mix-based Protection Methods", ESORICS
              2011, pp. 665-683, DOI 10.1007/978-3-642-23822-2_36,
              ISBN 978-3-642-23822-2, 2011, <https://svs.informatik.uni-
              hamburg.de/publications/2011/2011-09-14_FFHP_PrivacyPreser
              vingDNS_ESORICS2011.pdf>.

   [getdns]   "getdns", <https://getdnsapi.net>.

   [grangeia.snooping]
              Grangeia, L., "Cache Snooping or Snooping the Cache for
              Fun and Profit", 2005,
              <https://www.semanticscholar.org/paper/Cache-Snooping-or-
              Snooping-the-Cache-for-Fun-and-
              1-Grangeia/9b22f606e10b3609eafbdcbfc9090b63be8778c3>.

   [herrmann-reidentification]
              Herrmann, D., Gerber, C., Banse, C., and H. Federrath,
              "Analyzing Characteristic Host Access Patterns for Re-
              Identification of Web User Sessions", Lecture Notes in
              Computer Science, Vol. 7127,
              DOI 10.1007/978-3-642-27937-9_10, 2012, <https://epub.uni-
              regensburg.de/21103/1/Paper_PUL_nordsec_published.pdf>.

   [morecowbell]
              Grothoff, C., Wachs, M., Ermert, M., and J. Appelbaum,
              "NSA's MORECOWBELL: Knell for DNS", January 2015, <https:/
              /pdfs.semanticscholar.org/2610/2b99bdd6a258a98740af8217ba8
              da8a1e4fa.pdf>.

   [packetq]  DNS-OARC, "A tool that provides a basic SQL-frontend to
              PCAP-files", Release 1.4.3, commit 29a8288, October 2020,
              <https://github.com/DNS-OARC/PacketQ>.

   [passive-dns]
              Weimer, F., "Passive DNS Replication", 17th Annual FIRST
              Conference, April 2005,
              <https://www.first.org/conference/2005/papers/florian-
              weimer-slides-1.pdf>.

   [pitfalls-of-dns-encryption]
              Shulman, H., "Pretty Bad Privacy: Pitfalls of DNS
              Encryption", WPES '14: Proceedings of the 13th Workshop on
              Privacy in the Electronic Society, pp. 191-200,
              DOI 10.1145/2665943.2665959, November 2014,
              <https://dl.acm.org/citation.cfm?id=2665959>.

   [prism]    Wikipedia, "PRISM (surveillance program)", July 2015,
              <https://en.wikipedia.org/w/index.php?title=PRISM_(surveil
              lance_program)&oldid=673789455>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4470]  Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
              and DNSSEC On-line Signing", RFC 4470,
              DOI 10.17487/RFC4470, April 2006,
              <https://www.rfc-editor.org/info/rfc4470>.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
              <https://www.rfc-editor.org/info/rfc5155>.

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
              <https://www.rfc-editor.org/info/rfc5936>.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,
              <https://www.rfc-editor.org/info/rfc6269>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <https://www.rfc-editor.org/info/rfc7413>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/info/rfc7624>.

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,
              <https://www.rfc-editor.org/info/rfc7626>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.

   [RFC7754]  Barnes, R., Cooper, A., Kolkman, O., Thaler, D., and E.
              Nordmark, "Technical Considerations for Internet Service
              Blocking and Filtering", RFC 7754, DOI 10.17487/RFC7754,
              March 2016, <https://www.rfc-editor.org/info/rfc7754>.

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
              <https://www.rfc-editor.org/info/rfc7816>.

   [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>.

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
              <https://www.rfc-editor.org/info/rfc7873>.

   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,
              <https://www.rfc-editor.org/info/rfc7929>.

   [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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

   [RFC8744]  Huitema, C., "Issues and Requirements for Server Name
              Identification (SNI) Encryption in TLS", RFC 8744,
              DOI 10.17487/RFC8744, July 2020,
              <https://www.rfc-editor.org/info/rfc8744>.

   [RFC8890]  Nottingham, M., "The Internet is for End Users", RFC 8890,
              DOI 10.17487/RFC8890, August 2020,
              <https://www.rfc-editor.org/info/rfc8890>.

   [RFC8932]  Dickinson, S., Overeinder, B., van Rijswijk-Deij, R., and
              A. Mankin, "Recommendations for DNS Privacy Service
              Operators", BCP 232, RFC 8932, DOI 10.17487/RFC8932,
              October 2020, <https://www.rfc-editor.org/info/rfc8932>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [ripe-qname-measurements]
              de Vries, W., "Making the DNS More Private with QNAME
              Minimisation", April 2019,
              <https://labs.ripe.net/Members/wouter_de_vries/make-dns-a-
              bit-more-private-with-qname-minimisation>.

   [sidn-entrada]
              Hesselman, C., Jansen, J., Wullink, M., Vink, K., and M.
              Simon, "A privacy framework for 'DNS big data'
              applications", November 2014,
              <https://www.sidnlabs.nl/downloads/
              yBW6hBoaSZe4m6GJc_0b7w/2211058ab6330c7f3788141ea19d3db7/
              SIDN_Labs_Privacyraamwerk_Position_Paper_V1.4_ENG.pdf>.

   [thomas-ditl-tcp]
              Thomas, M. and D. Wessels, "An Analysis of TCP Traffic in
              Root Server DITL Data", DNS-OARC 2014 Fall Workshop,
              October 2014, <https://indico.dns-
              oarc.net/event/20/session/2/contribution/15/material/
              slides/1.pdf>.

   [tor-leak] Tor, "Tor FAQs: I keep seeing these warnings about SOCKS
              and DNS information leaks. Should I worry?",
              <https://www.torproject.org/docs/
              faq.html.en#WarningsAboutSOCKSandDNSInformationLeaks>.

   [yanbin-tsudik]
              Yanbin, L. and G. Tsudik, "Towards Plugging Privacy Leaks
              in Domain Name System", June 2010,
              <https://arxiv.org/abs/0910.2472>.

Appendix A.  Updates since RFC 7626

   Many references were updated.  Discussions of encrypted transports,
   including DoT and DoH, and sections on DNS payload, authentication of
   servers, and blocking of services were added.  With the publishing of
   [RFC7816] on QNAME minimization, text, references, and initial
   attempts to measure deployment were added to reflect this.  The text
   and references on the Snowden revelations were updated.

   The "Risks Overview" section was changed to "Scope" to help clarify
   the risks being considered.  Text on cellular network DNS, blocking,
   and security was added.  Considerations for recursive resolvers were
   collected and placed together.  A discussion on resolver selection
   was added.

Acknowledgments

   Thanks to Nathalie Boulvard and to the CENTR members for the original
   work that led to this document.  Thanks to Ondrej Sury for the
   interesting discussions.  Thanks to Mohsen Souissi and John Heidemann
   for proofreading and to Paul Hoffman, Matthijs Mekking, Marcos Sanz,
   Francis Dupont, Allison Mankin, and Warren Kumari for proofreading,
   providing technical remarks, and making many readability
   improvements.  Thanks to Dan York, Suzanne Woolf, Tony Finch, Stephen
   Farrell, Peter Koch, Simon Josefsson, and Frank Denis for good
   written contributions.  Thanks to Vittorio Bertola and Mohamed
   Boucadair for a detailed review of the -bis.  And thanks to the IESG
   members for the last remarks.

Contributions

   Sara Dickinson and Stephane Bortzmeyer were the original authors of
   the document, and their contribution to the initial draft of this
   document is greatly appreciated.

Author's Address

   Tim Wicinski (editor)
   Elkins, WV 26241
   United States of America

   Email: tjw.ietf@gmail.com