dprive                                                     S. Bortzmeyer
Internet-Draft                                                     AFNIC
Obsoletes: 7626 (if approved)                               S. Dickinson
Intended status: Informational                                Sinodun IT
Expires: January 3, 2019                                    July 2, 2018

                       DNS Privacy Considerations


   This document describes the privacy issues associated with the use of
   the DNS by Internet users.  It is intended to be an analysis of the
   present situation and does not prescribe solutions.

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
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   This Internet-Draft will expire on January 3, 2019.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The Alleged Public Nature of DNS Data . . . . . . . . . .   5
     2.2.  Data in the DNS Request . . . . . . . . . . . . . . . . .   5
       2.2.1.  Data in the DNS payload . . . . . . . . . . . . . . .   7
     2.3.  Cache Snooping  . . . . . . . . . . . . . . . . . . . . .   7
     2.4.  On the Wire . . . . . . . . . . . . . . . . . . . . . . .   7
       2.4.1.  Unencrypted Transports  . . . . . . . . . . . . . . .   7
       2.4.2.  Encrypted Transports  . . . . . . . . . . . . . . . .   9
     2.5.  In the Servers  . . . . . . . . . . . . . . . . . . . . .  10
       2.5.1.  In the Recursive Resolvers  . . . . . . . . . . . . .  10
       2.5.2.  In the Authoritative Name Servers . . . . . . . . . .  12
       2.5.3.  Rogue Servers . . . . . . . . . . . . . . . . . . . .  13
       2.5.4.  Authentication of servers . . . . . . . . . . . . . .  13
       2.5.5.  Blocking of services  . . . . . . . . . . . . . . . .  14
     2.6.  Re-identification and Other Inferences  . . . . . . . . .  14
     2.7.  More Information  . . . . . . . . . . . . . . . . . . . .  15
   3.  Actual "Attacks"  . . . . . . . . . . . . . . . . . . . . . .  15
   4.  Legalities  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  17
     8.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

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

   The Domain Name System 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 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 is an attempt at a comprehensive and accurate list.

   Let us begin with a simplified reminder of how the DNS works.  (See
   also [I-D.ietf-dnsop-terminology-bis].)  A client, the stub resolver,
   issues a DNS query to a server, called the recursive resolver (also
   called caching resolver or full resolver or recursive name server).
   Let's use the query "What are the AAAA records for www.example.com?"

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   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 server will then return the answer.  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.

   Because DNS relies on caching heavily, the algorithm described just
   above is actually a bit more complicated, and not all questions are
   sent to the authoritative name servers.  If a few seconds later the
   stub resolver asks the recursive resolver, "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).

   Almost all this DNS traffic is currently sent in clear (unencrypted).
   At the time of writing there is increasing deployment of DNS-over-TLS
   [RFC7858] and work underway on DoH [I-D.ietf-doh-dns-over-https].
   There are a few cases where there is some alternative channel
   encryption, for instance, in an IPsec VPN, at least between the stub
   resolver and the resolver.

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   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.

   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 by 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 an
   hyperlink.  Presumably, this is what is of interest for the

   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 system 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 do additional requests to turn
   the name servers' names into IP addresses.  Similarly, even if glue
   records are returned, a careful recursive server will do tertiary
   requests to verify the IP addresses of those records.

   It can be noted also 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 big 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.

   For privacy-related terms, we will use the terminology from

2.  Risks

   This document focuses mostly on the study of privacy risks for the
   end user (the one performing DNS requests).  We consider the risks of
   pervasive surveillance [RFC7258] as well as risks coming from a more
   focused surveillance.  Privacy risks for the holder of a zone (the

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   risk that someone gets the data) are discussed in [RFC5936] and
   [RFC5155].  Non-privacy risks (such as cache poisoning) are out of

2.1.  The Alleged Public Nature of DNS Data

   It has long been claimed that "the data in the DNS is public".  While
   this sentence makes sense for an Internet-wide lookup system, there
   are multiple facets to the data and metadata involved that deserve a
   more detailed look.  First, access control lists 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 non-
   existence).  In other words: one needs to know what to ask for, in
   order to receive a response.  The zone transfer QTYPE [RFC5936] is
   often blocked or restricted to authenticated/authorized access to
   enforce this difference (and maybe for other reasons).

   Another differentiation to be considered is between the DNS data
   itself and a particular 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; that transaction is not / should not be public.  A
   typical example from outside the DNS world is: the web site of
   Alcoholics Anonymous is public; the fact that you visit it should not

2.2.  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", because the port 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

   The QNAME is the full name sent by the user.  It gives information
   about what the user does ("What are the MX records of example.net?"
   means he probably wants to send email to someone at example.net,
   which may be a domain used by only a few persons and is therefore
   very revealing about communication relationships).  Some QNAMEs are
   more sensitive than others.  For instance, querying the A record of a
   well-known web statistics domain reveals very little (everybody

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   visits web sites 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,
   There are also some BitTorrent clients that query an SRV record for

   Another important thing about the privacy of the QNAME is the 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 doing email
   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

   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 now the IP address of
   the recursive resolver that, in a way, "hides" the real user.
   However, hiding does not always work.  Sometimes EDNS(0) Client
   subnet [RFC7871] is used (see its privacy analysis in
   [denis-edns-client-subnet]).  Sometimes the end user has a personal
   recursive resolver on her machine.  In both cases, the IP address 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
   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 a CGN or other NAT.)  However, for both
   IPv4 and IPv6 addresses, it's important to note that source addresses
   are propagated with queries and comprise metadata about the host,
   user, or application that originated them.

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2.2.1.  Data in the DNS payload

   At the time of writing there are no standardized client identifiers
   contained in the DNS payload itself (ECS [RFC7871] while widely used
   is only of Category Informational).

   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), they are not designed to
   actively track users (like HTTP cookies).

   There are anecdotal accounts of MAC addresses [1] and even user names
   being inserted in non-standard EDNS(0) options for stub to resolver
   communications to support proprietary functionality implemented at
   the resolver (e.g. parental filtering).

2.3.  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 counter
   measures have already been developed and deployed.

2.4.  On the Wire

2.4.1.  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, while
   the HTTP traffic will be encrypted, the DNS exchange prior to it will
   not be.  When other protocols will become more and more privacy-aware
   and secured against surveillance (e.g.  [I-D.draft-ietf-tls-tls130,
   [I-D.draft-ietf-quic-transport]), the use of unencrypted transports
   for DNS may become "the weakest link" in privacy.  It is noted that
   there is on-going work attempting to encrypt the SNI in the TLS
   handshake but that this is a non-trivial problem [I-D.ietf-tls-sni-

   An important specificity of the DNS traffic is that it may take a
   different path than the communication between the initiator and the

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   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 computer is
   configured.  By order of increasing attack surface:

     The recursive resolver can be on the end user's computer.  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

     The recursive resolver may be at the local network edge.  For
     many/most enterprise networks and for some residential users, 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 premises.  For most
     residential users and potentially other networks, the typical case
     is for the end user's computer to be configured (typically
     automatically through DHCP) with the addresses of 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

     The recursive resolver can be a public DNS service.  Some machines
     may be configured to use public DNS resolvers such as those
     operated today by Google Public DNS or OpenDNS.  The end 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
     end user's connection and the public DNS service.

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2.4.2.  Encrypted Transports

   The use of encrypted transports directly mitigates passive
   surveillance of the DNS payload, however there are still some privacy
   attacks possible.

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

   There are some general examples, for example, certain studies have
   highlighted that IP TTL or TCP Window sizes os-fingerprint [2] values
   can be used to fingerprint client OS's or that various techniques can
   be used to de-NAT DNS queries dns-de-nat [3].

   The use of clear text transport options to decrease latency may also
   identify a user e.g. using TCP Fast Open [RFC7413].

   More specifically, (since the deployment of encrypted transports is
   not widespread at the time of writing) users wishing to use encrypted
   transports for DNS may in practice be limited in the resolver
   services available.  Given this, the choice of a user to configure a
   single resolver (or a fixed set of resolvers) and an encrypted
   transport to use in all network environments can actually serve to
   identify the user as one that desires privacy and can provide an
   added mechanism to track them as they move across network

   Users of encrypted transports are also highly likely to re-use
   sessions for multiple DNS queries to optimize performance (e.g. via
   DNS pipelining or HTTPS multiplexing).  Certain configuration options
   for encrypted transports could also in principle fingerprint a user,
   for example session resumption, the maximum number of messages to
   send or a maximum connection time before closing a connections and

   Whilst there are known attacks on older versions of TLS the most
   recent recommendations [RFC7525] and developments [I-D.draft-ietf-
   tls-tls13] in this area largely mitigate those.

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

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2.5.  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 again [RFC6973], "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] [packetq-list] 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]).

2.5.1.  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.  You may get an idea of
   the data collected by reading the privacy policy of a big public
   resolver, e.g., <https://developers.google.com/speed/public-dns/
   privacy>.  Encrypted transports

   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 mentioned in Section 2.4
   use of session based encrypted transports (TCP/TLS) can expose
   correlation data about users.  Such concerns in the TCP/TLS layers
   apply equally to DNS-over-TLS and DoH which both use TLS as the
   underlying transport.

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   The proposed specification for DoH [I-D.ietf-doh-dns-over-https]
   includes a Privacy Considerations section which highlights some of
   the differences between HTTP and DNS.  As a deliberate design choice
   DoH inherits the privacy properties of the HTTPS stack and as a
   consequence introduces new privacy concerns when compared with DNS
   over UDP, TCP or TLS [RFC7858].  The rationale for this decision is
   that retaining the ability to leverage the full functionality of the
   HTTP ecosystem is more important than placing specific constraints on
   this new protocol based on privacy considerations (modulo limiting
   the use of HTTP cookies).

   In analyzing the new issues introduced by DoH it is helpful to
   recognize that there exists a natural tension between

   o  the wide practice in HTTP to use various headers to optimize HTTP
      connections, functionality and behaviour (which can facilitate
      user identification and tracking)

   o  and the fact that the DNS payload is currently very tightly
      encoded and contains no standardized user identifiers.

   DNS-over-TLS, for example, would normally contain no client
   identifiers above the TLS layer and a resolver would see only a
   stream of DNS query payloads originating within one or more
   connections from a client IP address.  Whereas if DoH clients
   commonly include several headers in a DNS message (e.g. user-agent
   and accept-language) this could lead to the DoH server being able to
   identify the source of individual DNS requests not only to a specific
   end user device but to a specific application.

   Additionally, depending on the client architecture, isolation of DoH
   queries from other HTTP traffic may or may not be feasible or
   desirable.  Depending on the use case, isolation of DoH queries from
   other HTTP traffic may or may not increase privacy.

   The picture for privacy considerations and user expectations for DoH
   with respect to what additional data may be available to the DoH
   server compared to DNS over UDP,TCP or TLS is complex and requires a
   detailed analysis for each use case.  In particular the choice of
   HTTPS functionality vs privacy is specifically made an implementation
   choice in DoH and users may well have differing privacy expectations
   depending on the DoH use case and implementation.

   At the extremes, there may be implementations that attempt to achieve
   parity with DNS-over-TLS from a privacy perspective at the cost of
   using no identifiable headers, there might be others that provide

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   feature rich data flows where the low-level origin of the DNS query
   is easily identifiable.

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

2.5.2.  In the Authoritative Name Servers

   Unlike what happens for recursive resolvers, 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.  Still, the authoritative name servers see a part of the
   traffic, and this subset may be sufficient to violate some privacy

   Also, the end user typically has some legal/contractual link with the
   recursive resolver (he has chosen the IAP, or he has 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

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   account.  Whatever the contract, the third-party hoster may be honest
   or not but, in any case, it will have to follow its local laws.  So,
   requests to a given ccTLD may go to servers managed by organizations
   outside of the ccTLD's country.  End users may not anticipate that,
   when doing a security analysis.

   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, one DNS provider hosts today 10% of the
   domains.  The ten most important DNS providers host together one
   third of the domains.  With the control (or the ability to sniff the
   traffic) of a few name servers, you can gather a lot of information.

2.5.3.  Rogue Servers

   The previous paragraphs discussed DNS privacy, assuming that all the
   traffic was directed to the intended servers and that the potential
   attacker was purely passive.  But, in reality, we can have active
   attackers redirecting the traffic, not to change it but just to
   observe it.

   For instance, a rogue DHCP server, or a trusted DHCP server that has
   had its configuration altered by malicious parties, can direct you to
   a rogue recursive resolver.  Most of the time, it seems to be done to
   divert traffic by providing lies for some domain names.  But it could
   be used just to capture the traffic and gather information about you.
   Other attacks, besides using DHCP, are possible.  The traffic from a
   DNS client to a DNS server can be intercepted along its way from
   originator to intended source, for instance, by transparent DNS
   proxies in the network that will divert the traffic intended for a
   legitimate DNS server.  This rogue server can masquerade as the
   intended server and respond with data to the client.  (Rogue servers
   that inject malicious data are possible, but it is a separate problem
   not relevant to privacy.)  A rogue server may respond correctly for a
   long period of time, thereby foregoing detection.  This may be done
   for what could be claimed to be good reasons, such as optimization or
   caching, but it leads to a reduction of privacy compared to if there
   was no attacker present.  Also, malware like DNSchanger [dnschanger]
   can change the recursive resolver in the machine's configuration, or
   the routing itself can be subverted (for instance,

2.5.4.  Authentication of servers

   Both Strict mode for DNS-over-TLS and DoH require authentication of
   the server and therefore as long as the authentication credentials
   are obtained over a secure channel then using either of these

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   transports defeats the attack of re-directing traffic to rogue
   servers.  Of course attacks on these secure channels are also
   possible, but out of the scope of this document.

2.5.5.  Blocking of services

   User privacy can also be at risk if there is blocking (by local
   network operators or more genearl mechanisms) of access to recursive
   servers that offer encrypted transports.  For example active blocking
   of port 853 for DNS-over-TLS or of specific IP addresses (e.g. could restrict the resolvers available to the client.
   Similarly attacks on such services e.g.  DDoS could force users to
   switch to other services that do not offer encrypted transports for

2.6.  Re-identification and Other Inferences

   An observer has access not only to the data he/she directly collects
   but also to the results of various inferences about this data.

   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 have a work in progress
   [RFC7624] that considers such inference-based attacks in a more
   general framework.

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2.7.  More Information

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

3.  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
   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 systems [passive-dns] allow reconstruction of the data of
   sometimes an entire zone.  They are used for many reasons -- some
   good, some bad.  Well-known passive DNS systems keep only the DNS
   responses, and not the source IP address of the client, precisely for
   privacy reasons.  Other passive DNS systems may not be so careful.
   And there is still the potential problems with revealing QNAMEs.

   The revelations (from the Edward Snowden documents, which were leaked
   from the National Security Agency (NSA)) of the MORECOWBELL
   surveillance program [morecowbell], which uses the DNS, both
   passively and actively, to surreptitiously gather information about
   the users, is another good example showing that the lack of privacy
   protections in the DNS is actively exploited.

4.  Legalities

   To our knowledge, there are no specific privacy laws for DNS data, in
   any country.  Interpreting general privacy laws like
   [data-protection-directive] or GDPR [4] applicable in the European
   Union in the context of DNS traffic data is not an easy task, and we
   do not know a court precedent here.  See an interesting analysis in

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5.  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'

6.  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,
   Tim Wicinski, 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.  And thanks to the IESG members for
   the last remarks.

7.  Changelog


   o  Initial commit.  Differences to RFC7626:

      *  Update many references

      *  Add discussions of encrypted transports including DNS-over-TLS
         and DoH

      *  Add section on DNS payload

      *  Add section on authentication of servers

      *  Add section on blocking of services

8.  References

8.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

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   [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,

   [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>.

8.2.  Informative References

              Vinot, N., "Vie privee: et le DNS alors?", (In French),

              Castillo-Perez, S. and J. Garcia-Alfaro, "Anonymous
              Resolution of DNS Queries", 2008,

              Dagon, D., "Corrupted DNS Resolution Paths: The Rise of a
              Malicious Resolution Authority", ISC/OARC Workshop, 2007,

              Lemos, R., "Got Malware? Three Signs Revealed In DNS
              Traffic", InformationWeek Dark Reading, May 2013,

              European Parliament, "Directive 95/46/EC of the European
              Pariament and of the council 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. 0031 - 0050, November 1995, <http://eur-

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              Castro, S., Wessels, D., Fomenkov, M., and K. Claffy, "A
              Day at the Root of the Internet", ACM SIGCOMM Computer
              Communication Review, Vol. 38, Number 5,
              DOI 10.1145/1452335.1452341, October 2008,

              Denis, F., "Security and privacy issues of edns-client-
              subnet", August 2013,

   [ditl]     CAIDA, "A Day in the Life of the Internet (DITL)", 2002,

              Stoner, E., "DNS Footprint of Malware", OARC Workshop,
              October 2010, <https://www.dns-oarc.net/files/workshop-

              Wikipedia, "DNSChanger", October 2013,

              Bortzmeyer, S., "DNSmezzo", 2009,

              Fangming, Z., Hori, Y., and K. Sakurai, "Analysis of
              Privacy Disclosure in DNS Query", 2007 International
              Conference on Multimedia and Ubiquitous Engineering (MUE
              2007), Seoul, Korea, ISBN: 0-7695-2777-9, pp. 952-957,
              DOI 10.1109/MUE.2007.84, April 2007,

              Federrath, H., Fuchs, K., Herrmann, D., and C. Piosecny,
              "Privacy-Preserving DNS: Analysis of Broadcast, Range
              Queries and Mix-based Protection Methods", Computer
              Security ESORICS 2011, Springer, page(s) 665-683,
              ISBN 978-3-642-23821-5, 2011, <https://svs.informatik.uni-

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              Grangeia, L., "DNS Cache Snooping or Snooping the Cache
              for Fun and Profit", February 2004,

              Herrmann, D., Gerber, C., Banse, C., and H. Federrath,
              "Analyzing Characteristic Host Access Patterns for Re-
              Identification of Web User Sessions",
              DOI 10.1007/978-3-642-27937-9_10, 2012, <http://epub.uni-

              Dickinson, S., Rijswijk-Deij, R., and A. Mankin,
              "Recommendations for DNS Privacy Service Operators",
              draft-dickinson-bcp-op-00 (work in progress), March 2018.

              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", draft-ietf-dnsop-terminology-bis-10 (work in
              progress), April 2018.

              Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", draft-ietf-doh-dns-over-https-12 (work in
              progress), June 2018.

              Grothoff, C., Wachs, M., Ermert, M., and J. Appelbaum,
              "NSA's MORECOWBELL: Knell for DNS", GNUnet e.V., January
              2015, <https://gnunet.org/morecowbell>.

   [packetq]  Dot SE, "PacketQ, a simple tool to make SQL-queries
              against PCAP-files", 2011,

              PacketQ, "PacketQ Mailing List",

              Weimer, F., "Passive DNS Replication", April 2005,

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              Shulman, H., "Pretty Bad Privacy:Pitfalls of DNS
              Encryption", <https://www.ietf.org/mail-archive/web/dns-

   [prism]    Wikipedia, "PRISM (surveillance program)", July 2015,

   [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,

   [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,

   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
              (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,

   [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,

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [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,

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   [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,

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,

   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,

              Aben, E., "A RIPE Atlas View of Internet Meddling in
              Turkey", March 2014,

              Hesselman, C., Jansen, J., Wullink, M., Vink, K., and M.
              Simon, "A privacy framework for 'DNS big data'
              applications", November 2014,

              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-

              Tor, "DNS leaks in Tor", 2013,

              Yanbin, L. and G. Tsudik, "Towards Plugging Privacy Leaks
              in the Domain Name System", October 2009,

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8.3.  URIs

   [1] https://lists.dns-oarc.net/pipermail/dns-

   [2] http://netres.ec/?b=11B99BD

   [3] https://www.researchgate.net/publication/320322146_DNS-DNS_DNS-

   [4] https://www.eugdpr.org/the-regulation.html

Authors' Addresses

   Stephane Bortzmeyer
   1, rue Stephenson
   France  78180

   Email: bortzmeyer+ietf@nic.fr

   Sara Dickinson
   Sinodun IT
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA
   United Kingdom

   Email: sara@sinodun.com

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