Network Working Group                                      S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Informational                            April 27, 2014
Expires: October 29, 2014

                       DNS privacy considerations


   This document describes the privacy issues associated with the use of
   the DNS by Internet users.  It is intended to be mostly an analysis
   of the present situation, in the spirit of section 8 of [RFC6973] and
   it does not prescribe solutions.

   Discussions of the document should take place on the dns-privacy
   mailing list [dns-privacy].

Status of This Memo

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   This Internet-Draft will expire on October 29, 2014.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The alleged public nature of DNS data . . . . . . . . . .   4
     2.2.  Data in the DNS request . . . . . . . . . . . . . . . . .   4
     2.3.  Cache snooping  . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  On the wire . . . . . . . . . . . . . . . . . . . . . . .   6
     2.5.  In the servers  . . . . . . . . . . . . . . . . . . . . .   7
       2.5.1.  In the resolvers  . . . . . . . . . . . . . . . . . .   8
       2.5.2.  In the authoritative name servers . . . . . . . . . .   8
       2.5.3.  Rogue servers . . . . . . . . . . . . . . . . . . . .   9
   3.  Actual "attacks"  . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Legalities  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The Domain Name System is specified in [RFC1034] and [RFC1035].  It
   is one of the most important infrastructure components of the
   Internet and one of the most often ignored or misunderstood.  Almost
   every activity on the Internet starts with a DNS query (and often
   several).  Its use has many privacy implications and we try to give
   here a comprehensive and accurate list.

   Let us start with a small reminder of the way the DNS works (with
   some simplifications).  A client, the stub resolver, issues a DNS
   query to a server, the resolver (also called caching resolver or full
   resolver or recursive name server).  For instance, the query is "What
   are the AAAA records for".  AAAA is the qtype (Query
   Type) and the qname (Query Name).  To get the answer,
   the resolver will query first the root nameservers, which will, most
   of the times, send a referral.  Here, the referral will be to .com
   nameservers.  In turn, they will send a referral to the
   nameservers, which will provide the answer.  The root name servers,
   the name servers of .com and those of 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.
   Unlike what many "DNS for dummies" articles say, the question sent to

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   the root name servers is "What are the AAAA records for", not "What are the name servers of .com?".  So, the
   DNS leaks more information than it should.

   Because the DNS uses caching heavily, not all questions are sent to
   the authoritative name servers.  If the stub resolver, a few seconds
   later, asks to the resolver "What are the SRV records of _xmpp-", the resolver will remember that it knows
   the name servers of and will just query them, bypassing
   the root and .com.  Because there is typically no caching in the stub
   resolver, the resolver, unlike the authoritative servers, sees

   Almost all the DNS queries are today sent over UDP, and this has
   practical consequences if someone thinks of encrypting this traffic
   (some encryption solutions are typically done for TCP, not UDP).

   I should be noted to that DNS resolvers sometimes forward requests to
   bigger machines, with a larger and more shared cache, the forwarders.
   From the point of view of privacy, forwarders are like resolvers,
   except that the caching in the resolver before them decreases the
   amount of data they can see.

   Another important point to keep in mind when analyzing the privacy
   issues of DNS is the mix of many sort of DNS requests received by a
   server.  Let's assume the eavesdropper want to know which Web page is
   visited by an user.  For a typical Web page displayed by the user,
   there are three sorts of DNS requests:

      Primary request: this is the domain name that the user typed or
      selected from a bookmark or choosed by clicking on an hyperklink.
      Presumably, this is what is of interest for the eavesdropper.

      Secondary requests: these are the requests performed by the user
      agent (here, the Web browser) without any direct involvment or
      knowledge of the user.  For the Web, they are triggered by
      included content, CSS sheets, JavaScript code, embedded images,
      etc.  In some cases, there can be dozens of domain names in a
      single page.

      Tertiary requests: these are the 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 is not returned,
      the resolver will have to do tertiary requests to turn name
      servers' named into IP addresses.

   For privacy-related terms, we will use here the terminology of

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2.  Risks

   This draft focuses mostly on the study of privacy risks for the end-
   user (the one performing DNS requests).  Privacy risks for the holder
   of a zone (the risk that someone gets the data) are discussed in
   [RFC5936].  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 data and meta data that deserve a more
   detailed look.  First, access control lists and private name spaces
   nonwithstanding, 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 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, more dubious reasons).

   Another differentiation to be applied is between the DNS data as
   mentioned above and a particular transaction, most prominently but
   not limited to a DNS name lookup.  The fact that the results of a DNS
   query are public within the boundaries described in the previous
   paragraph and therefore might have no confidentiality requirements
   does not imply the same for a single or a sequence of transactions.
   A typical example from outside the DNS world: 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 specially
   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 +
   may be source port", because the port is also in the request and can
   be used to sort out several users sharing an IP address (CGN for

   The qname is the full name sent by the original user.  It gives
   information about what the user does ("What are the MX records of"  means he probably wants to send email to someone at, which may be a domain used by only a few persons and

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   therefore very revealing).  Some qnames are more sensitive than
   others.  For instance, querying the A record of
   reveals very little (everybody visits Web sites which use Google
   Analytics) but querying the A record of www.verybad.example where
   verybad.example is the domain of an illegal or very offensive
   organization may create more problems for the user.  Another example
   is when the qname embeds the software one uses.  For instance,  Or
   some BitTorrent clients that query a SRV record for _bittorrent-

   Another important thing about the privacy of the qname is the future
   usages.  Today, the lack of privacy is an obstacle to putting
   interesting data in the DNS.  At the moment your DNS traffic might
   reveal that you are doing email but not who with.  If your MUA starts
   looking up PGP keys in the DNS [I-D.wouters-dane-openpgp] then
   privacy becomes a lot more important.  And email is just an example,
   there will be other really interesting uses for a more secure (in the
   sense of privacy) DNS.

   For the communication between the stub resolver and the resolver, the
   source IP address is the one of the user's machine.  Therefore, all
   the issues and warnings about collection of IP addresses apply here.
   For the communication between the 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 a HTTP connection.  It
   is now the IP address of the resolver which, in a way "hides" the
   real user.  However, it does not always work.  Sometimes
   [I-D.vandergaast-edns-client-subnet] is used.  Sometimes the end user
   has a personal resolver on her machine.  In that case, the IP address
   is as sensitive as it is for HTTP.

   A note about IP addresses: there is currently no IETF document which
   describes in detail the privacy issues of IP addressing.  In the mean
   time, 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.

2.3.  Cache snooping

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   The content of resolvers can reveal data about the clients using it.
   This information can sometimes be examined by sending DNS queries
   with RD=0 to inspect cache content, particularly looking at the DNS
   TTLs.  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

   DNS traffic can be seen by an eavesdropper like any other traffic.
   It is typically not encrypted.  (DNSSEC, specified in [RFC4033]
   explicitely excludes confidentiality from its goals.)  So, if an
   initiator starts a HTTPS communication with a recipient, while the
   HTTP traffic will be encrypted, the DNS exchange prior to it will not
   be.  When the other protocols will become more or more privacy-aware
   and secured against surveillance, the DNS risks to become "the
   weakest link" in privacy.

   What also makes the DNS traffic different 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 resolver, or to the authoritative name servers.

   The best place, from an eavesdropper's point of view, is clearly
   between the stub resolvers and the resolvers, because he 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 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 stub resolver to caching resolver connection is limited to
   that single machine.

   The resolver can be in the IAP (Internet Access Provider) 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 resolver 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 resolvers.

   The resolver may also be at the local network edge.  For many/most
   enterprise networks and for some residential users the caching

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   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 IAP network referenced above.

   The resolver can be a public DNS service.  Some end users may be
   configured to use public DNS resolvers such as those operated by
   Google Public DNS or OpenDNS.  The end user may have configured their
   machine to use these DNS resolvers themselves - or their IAP may
   choose 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

2.5.  In the servers

   Using the terminology of [RFC6973], the DNS servers (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] reference 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 attacker.

   Sometimes, these data are kept for a long time and/or distributed to
   third parties, for research purposes [ditl], for security analysis,
   or for surveillance tasks.  Also, there are observation points in the
   network which gather DNS data and then make it accessible to third-
   parties for research or security purposes ("passive DNS

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2.5.1.  In the resolvers

   The resolvers see the entire traffic since there is typically no
   caching before them.  They are therefore well situated to observe the
   traffic.  To summarize: your 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 [1].

2.5.2.  In the authoritative name servers

   Unlike the resolvers, they are limited by caching.  They see only a
   part of the requests.  For aggregated statistics ("what is the
   percentage of LOC queries?"), it is sufficient but it may prevent an
   observer to observe everything.  Nevertheless, the authoritative name
   servers sees a part of the traffic and this sample may be sufficient
   to defeat some privacy expectations.

   Also, the end user has typically some legal/contractual link with the
   resolver (he has chosen the IAP, or he has chosen to use a given
   public resolver) while he is often not even aware of the role of the
   authoritative name servers and their observation abilities.

   It is an interesting question whether the privacy issues are bigger
   in the root or in a large TLD.  The root sees the traffic for all the
   TLDs (and the huge amount of traffic for non-existing TLD) but a
   large TLD has less caching before it.

   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 since the authoritative name server will
   see the IP address of the end client, and not the address of a big
   resolver shared by many users.  This is no longer true if
   [I-D.vandergaast-edns-client-subnet] is used because, in this case,
   the authoritative name server sees the original IP prefix or address
   (depending on the setup).

   As of today, all the instances of one root name server, L-root,
   receive together around 20 000 queries per second.  While most of it
   is junk (errors on the TLD name), it gives an idea of the amount of
   big data which pours into name servers.

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   Many domains, including TLD, 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.  But it may be surprising for an end-user that requests to a
   given ccTLD may go to servers managed by organisations outside of the

2.5.3.  Rogue servers

   A rogue DHCP server can direct you to a rogue resolver.  Most of the
   times, 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.  Same thing for malwares like
   DNSchanger[dnschanger] which changes the resolver in the machine's

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) second 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 you're actually
   interested in.

   Many research papers about malware detection use DNS traffic to
   detect "abnormal" behaviour 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.  It is used for many reasons, some good,
   some bad.  It is an example of privacy issue even when no source IP
   address is kept.

4.  Legalities

   To our knowledge, there are no specific privacy laws for DNS data.
   Interpreting general privacy laws like [data-protection-directive]
   (European Union) in the context of DNS traffic data is not an easy
   task and it seems there is no court precedent here.

5.  Security considerations

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   This document is entirely about security, more precisely privacy.
   Possible solutions to the issues described here are discussed in
   [I-D.bortzmeyer-dnsop-privacy-sol] (qname minimization, local caching
   resolvers), [I-D.hzhwm-start-tls-for-dns] (encryption of traffic) or
   in [I-D.wijngaards-dnsop-confidentialdns] (encryption also).
   Attempts have been made to encrypt the resource record data

6.  Acknowledgments

   Thanks to Nathalie Boulvard and to the CENTR members for the original
   work which leaded to this draft.  Thanks to Ondrej Sury for the
   interesting discussions.  Thanks to Mohsen Souissi for proofreading.
   Thanks to Dan York, Suzanne Woolf, Tony Finch, Peter Koch and Frank
   Denis for good written contributions.

7.  References

7.1.  Normative References

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

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973, July

7.2.  Informative References

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

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

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              Contavalli, C., Gaast, W., Leach, S., and E. Lewis,
              "Client Subnet in DNS Requests", draft-vandergaast-edns-
              client-subnet-02 (work in progress), July 2013.

              Bortzmeyer, S., "Possible solutions to DNS privacy
              issues", draft-bortzmeyer-dnsop-privacy-sol-00 (work in
              progress), December 2013.

              Wijngaards, W., "Confidential DNS", draft-wijngaards-
              dnsop-confidentialdns-00 (work in progress), November

              Timms, B., Reid, J., and J. Schlyter, "IANA Registration
              for Encrypted ENUM", draft-timms-encrypt-naptr-01 (work in
              progress), July 2008.

              Zi, Z., Zhu, L., Heidemann, J., Mankin, A., and D.
              Wessels, "Starting TLS over DNS", draft-hzhwm-start-tls-
              for-dns-00 (work in progress), February 2014.

              Wouters, P., "Using DANE to Associate OpenPGP public keys
              with email addresses", draft-wouters-dane-openpgp-02 (work
              in progress), February 2014.

              IETF, , "The dns-privacy mailing list", March 2014.

   [dnsop]    IETF, , "The dnsop mailing list", October 2013.

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

              Stoner, E., "DNS footprint of malware", October 2010.

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

              Wikipedia, , "DNSchanger", November 2011.

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              Denis, F., "DNSCrypt", .

              Bernstein, D., "DNScurve", .

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

              Bortzmeyer, S., "DNSmezzo", 2009.

   [prism]    NSA, , "PRISM", 2007.

   [crime]    Rizzo, J. and T. Dong, "The CRIME attack against TLS",

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

              , "European directive 95/46/EC on the protection of
              individuals with regard to the processing of personal data
              and on the free movement of such data", November 1995.

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

              , "DNS leaks in Tor", 2013.

Author's Address

   Stephane Bortzmeyer
   Immeuble International
   Saint-Quentin-en-Yvelines  78181

   Phone: +33 1 39 30 83 46

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