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DNS privacy considerations

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7626.
Author Stéphane Bortzmeyer
Last updated 2014-10-26
Replaces draft-bortzmeyer-dnsop-dns-privacy
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
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IESG IESG state Became RFC 7626 (Informational)
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Network Working Group                                      S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Informational                          October 26, 2014
Expires: April 29, 2015

                       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 DPRIVE working
   group mailing list [dprive].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

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

   This Internet-Draft will expire on April 29, 2015.

Copyright Notice

   Copyright (c) 2014 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
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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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  . . . . . . . . . . . . . . . . . . . . .   6
     2.4.  On the wire . . . . . . . . . . . . . . . . . . . . . . .   6
     2.5.  In the servers  . . . . . . . . . . . . . . . . . . . . .   7
       2.5.1.  In the resolvers  . . . . . . . . . . . . . . . . . .   7
       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  . . . . . . . . . . . . . . . . . . . . . . . .  13

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.  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).  Let's use the query "What are the AAAA records for" as an example.  AAAA is the qtype (Query type), and is the qname (Query Name).  The 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 .com nameserver, in turn, will refer to the nameservers.  The nameserver will then return
   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"

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   articles say, the question sent to the root name servers is "What are
   the AAAA records for", 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 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

   Today, almost all DNS queries are sent over UDP.  This has practical
   consequences, when considering the encryption of this traffic: some
   encryption solutions are only designed for TCP, not UDP.

   It should be noted 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 a 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 involvement 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.

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

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, 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).

   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 data 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 + 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 instance).

   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

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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
   potentially sensitive or personally identifiable 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 privacy-friendly DNS.

   For the communication between the stub resolver and the 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 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 (see its
   privacy analysis in [denis-edns-client-subnet]).  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.

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2.3.  Cache snooping

   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.

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   The resolver may also 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 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] 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

2.5.1.  In the resolvers

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   Resolvers see all the 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 resolvers, authoritative name servers are limited by caching;
   they see only a part of the requests.  For aggregated statistics
   ("What is the percentage of LOC queries?"), this is sufficient; but
   it may prevent 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
   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.  This authoritative name server will see
   the IP address of the end client, instead of the address of a big
   resolver shared by many users.  A possible solution is to have a
   local resolver and to forward the cache misses to a big resolver.

   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 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.  Whatever the contract, the third-party hoster may be honest

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   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.

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

   This document is entirely about security, more precisely privacy.  A
   document on requirments for DNS privacy is [I-D.hallambaker-dnse].
   Possible solutions to the issues described here are discussed in
   [I-D.ietf-dnsop-qname-minimisation] (qname minimization), in

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   [I-D.bortzmeyer-dnsop-privacy-sol] (local caching resolvers,
   gratuitous queries), [I-D.hzhwm-start-tls-for-dns] (encryption of
   traffic), in [I-D.wijngaards-dnsop-confidentialdns] (encryption also)
   or in many other documents (there are many proposals to encrypt the
   DNS).  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
   and to Warren Kumari for proofreading and many readability
   improvements.  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.

              Bortzmeyer, S., "DNS query name minimisation to improve
              privacy", draft-ietf-dnsop-qname-minimisation-00 (work in
              progress), October 2014.

              Wijngaards, W. and G. Wiley, "Confidential DNS", draft-
              wijngaards-dnsop-confidentialdns-01 (work in progress),
              March 2014.

              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.

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

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

   [dprive]   IETF, ., "The DPRIVE working group", March 2014,

   [dnsop]    IETF, ., "The DNSOP working group", October 2013,


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

              Dagon, D., "Corrupted DNS Resolution Paths: The Rise of a
              Malicious Resolution Authority", 2007, <https://www.dns-

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

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

              Wikipedia, ., "DNSchanger", November 2011,

              Denis, F., "DNSCrypt", , <>.

              Bernstein, D., "DNScurve", , <>.

   [packetq]  Dot SE, ., "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]     CAIDA, ., "A Day in the Life of the Internet (DITL)",
              2002, <>.

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              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, <

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

              Tor, ., "DNS leaks in Tor", 2013, <https://

Author's Address

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

   Phone: +33 1 39 30 83 46

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