DNS privacy considerations
draft-ietf-dprive-problem-statement-01

DNS PRIVate Exchange (dprive) Working Group                S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Informational                           January 7, 2015
Expires: July 11, 2015

                       DNS privacy considerations
                 draft-ietf-dprive-problem-statement-01

Abstract

   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 DPRIVE working
   group mailing list [dprive].

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 11, 2015.

Copyright Notice

   Copyright (c) 2015 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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   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 . . . . . . . . . . . . . . . . .   5
     2.3.  Cache snooping  . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  On the wire . . . . . . . . . . . . . . . . . . . . . . .   6
     2.5.  In the servers  . . . . . . . . . . . . . . . . . . . . .   7
       2.5.1.  In the recursive resolvers  . . . . . . . . . . . . .   8
       2.5.2.  In the authoritative name servers . . . . . . . . . .   8
       2.5.3.  Rogue servers . . . . . . . . . . . . . . . . . . . .   9
   3.  Actual "attacks"  . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Legalities  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

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 begin with a simplified reminder of how the DNS works.
   (REMOVE BEFORE PUBLICATION: We hope that the document
   [I-D.hoffman-dns-terminology] will be published as a RFC so most of
   this section could be replaced by a reference to it.)  A client, the
   stub resolver, issues a DNS query to a server, the recursive resolver
   (also called caching resolver or full resolver or simply resolver
   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
   recursive resolver will first query the root nameservers.  In most
   cases, the root nameservers will send a referral.  In this example,
   the referral will be to .com nameservers.  The resolver repeats the
   query to one of the .com nameservers.  The .com nameserver, in turn,
   will refer to the example.com nameservers.  The example.com

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   nameserver will then return the answer.  The root name servers, the
   name servers of .com and those 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.
   Unlike what many "DNS for dummies" articles say, 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 nameserver.

   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 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 everything.

   It should be noted that DNS recursive resolvers sometimes forward
   requests to bigger machines, with a larger and more shared cache, the
   forwarders (and the query hierarchy can be even deeper, with more
   than two levels of recursive resolvers).  From the point of view of
   privacy, forwarders are like resolvers, except that the caching in
   the recursive resolvers before them decreases the amount of data they
   can see.

   All this DNS traffic is today sent in clear (unencryted), except a
   few cases when the IP traffic is protected, for instance in an IPsec
   VPN.

   Today, almost all DNS queries are sent over UDP.  This has practical
   consequences, when considering a possible privacy technique,
   encryption of the traffic: 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 mix of many sort of DNS requests received by a
   server.  Let's assume the eavesdropper wants to know which Web page
   is viewed by an user.  For a typical Web page displayed by the user,
   there are three sorts of DNS requests being issued:

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

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      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, CSS sheets, 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 is not returned,
      the resolver will have to do tertiary requests to turn 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 are sent, for instance to prefetch resources that the
   user may query later, or when autocompleting the URL in the address
   bar (which obviously is a big privacy concern).

   For privacy-related terms, we will use here the terminology of
   [RFC6973].

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]) and also risks coming from a more
   focused surveillance.  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 scope.

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 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, 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, more dubious reasons).

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   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 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
   be.

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 sort out several users sharing an IP
   address (behind a 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 therefore very
   revealing about communication relationships).  Some qnames are more
   sensitive than others.  For instance, querying the A record of
   google-analytics.com 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.
   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 a SRV record for _bittorrent-
   tracker._tcp.domain.example.

   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 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 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

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   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
   recursive resolver which, in a way "hides" the real user.  However,
   hiding does not always work.  Sometimes
   [I-D.vandergaast-edns-client-subnet] 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.

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

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.  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]
   explicitly 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 other protocols will become more and more privacy-aware and
   secured against surveillance, the DNS risks to become "the weakest
   link" in privacy.

   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

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   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 machine.

   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 IAP network referenced below.

   The recursive 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
   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.  Some machines
   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 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.

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

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   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] and DNSmezzo
   [dnsmezzo].  The organization managing the DNS server can use these
   data itself or it can be part of a surveillance program like PRISM
   [prism] and pass data to an outside observer.

   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
   [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 [1].

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
   expectations.

   Also, the end user has typically 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.

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   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 TLDs), but a
   large TLDs 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.  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 [I-D.vandergaast-edns-client-subnet] 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 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.

   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 be honest
   or not but, in any case, it will have to follow its local laws.  It
   may be surprising for an end-user that requests to a given ccTLD may
   go to servers managed by organisations outside of the country.

   Also, it seems (TODO: actual numbers requested) that there is a
   strong concentration of authoritative name servers among "popular"
   domains (such as the Alexa Top N list).  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

   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 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 malware like DNSchanger[dnschanger] which changes the
   recursive resolver in the machine's configuration, or with
   transparent DNS proxies in the network that will divert the traffic
   intended for a legitimate DNS server (for instance
   [turkey-googledns]).

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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 a 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

   This document is entirely about security, more precisely privacy.  It
   just lays down the problem, it does not try to set requirments (with
   the choices and compromises they imply), much less to define
   solutions.  A document on requirments for DNS privacy is
   [I-D.hallambaker-dnse].  Possible solutions to the issues described
   here are discussed in other documents (currently too many to be
   listed here).

6.  Acknowledgments

   Thanks to Nathalie Boulvard and to the CENTR members for the original
   work which leaded to this document.  Thanks to Ondrej Sury for the
   interesting discussions.  Thanks to Mohsen Souissi and John Heidemann
   for proofreading, to Paul Hoffman, Marcos Sanz and Warren Kumari for
   proofreading, technical remarks, and many readability improvements.
   Thanks to Dan York, Suzanne Woolf, Tony Finch, Peter Koch and Frank
   Denis for good written contributions.

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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
              2013.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, May 2014.

7.2.  Informative References

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

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

   [I-D.vandergaast-edns-client-subnet]
              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.

   [I-D.hallambaker-dnse]
              Hallam-Baker, P., "DNS Privacy and Censorship: Use Cases
              and Requirements.", draft-hallambaker-dnse-01 (work in
              progress), May 2014.

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

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   [I-D.hoffman-dns-terminology]
              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", draft-hoffman-dns-terminology-00 (work in
              progress), November 2014.

   [dprive]   IETF, DPRIVE., "The DPRIVE working group", March 2014,
              <http://www.ietf.org/mail-archive/web/dns-privacy/>.

   [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/>.

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

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

   [darkreading-dns]
              Lemos, R., "Got Malware? Three Signs Revealed In DNS
              Traffic", May 2013,
              <http://www.darkreading.com/monitoring/
              got-malware-three-signs-revealed-in-dns/240154181>.

   [dnschanger]
              Wikipedia, , "DNSchanger", November 2011,
              <http://en.wikipedia.org/wiki/DNSChanger>.

   [packetq]  Dot SE, , "PacketQ, a simple tool to make SQL-queries
              against PCAP-files", 2011,
              <https://github.com/dotse/packetq/wiki>.

   [dnsmezzo]
              Bortzmeyer, S., "DNSmezzo", 2009,
              <http://www.dnsmezzo.net/>.

   [prism]    NSA, , "PRISM", 2007, <http://en.wikipedia.org/wiki/
              PRISM_%28surveillance_program%29>.

   [ditl]     CAIDA, , "A Day in the Life of the Internet (DITL)", 2002,
              <http://www.caida.org/projects/ditl/>.

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   [turkey-googledns]
              Bortzmeyer, S., "Hijacking of public DNS servers in
              Turkey, through routing", 2014,
              <http://www.bortzmeyer.org/
              dns-routing-hijack-turkey.html>.

   [data-protection-directive]
              Europe, , "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, <http://eur-lex.europa.eu/LexUriServ/
              LexUriServ.do?uri=CELEX:31995L0046:EN:HTML>.

   [passive-dns]
              Weimer, F., "Passive DNS Replication", April 2005,
              <http://www.enyo.de/fw/software/dnslogger/#2>.

   [tor-leak]
              Tor, , "DNS leaks in Tor", 2013,
              <https://trac.torproject.org/projects/tor/wiki/doc/TorFAQ#
              IkeepseeingthesewarningsaboutSOCKSandDNSandinformationleak
              s.ShouldIworry>.

   [yanbin-tsudik]
              Yanbin, L. and G. Tsudik, "Towards Plugging Privacy Leaks
              in the Domain Name System", 2009,
              <http://arxiv.org/abs/0910.2472>.

   [castillo-garcia]
              Castillo-Perez, S. and J. Garcia-Alfaro, "Anonymous
              Resolution of DNS Queries", 2008,
              <http://deic.uab.es/~joaquin/papers/is08.pdf>.

   [fangming-hori-sakurai]
              Fangming, , Hori, Y., and K. Sakurai, "Analysis of Privacy
              Disclosure in DNS Query", 2007,
              <http://dl.acm.org/citation.cfm?id=1262690.1262986>.

   [federrath-fuchs-herrmann-piosecny]
              Federrath, H., Fuchs, K., Herrmann, D., and C. Piosecny,
              "Privacy-Preserving DNS: Analysis of Broadcast, Range
              Queries and Mix-Based Protection Methods", 2011,
              <https://svs.informatik.uni-hamburg.de/publications/2011/2
              011-09-14_FFHP_PrivacyPreservingDNS_ESORICS2011.pdf>.

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7.3.  URIs

   [1] https://developers.google.com/speed/public-dns/privacy

Author's Address

   Stephane Bortzmeyer
   AFNIC
   1, rue Stephenson
   Montigny-le-Bretonneux  78180
   France

   Phone: +33 1 39 30 83 46
   Email: bortzmeyer+ietf@nic.fr
   URI:   http://www.afnic.fr/

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