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Internet Engineering Task Force (IETF) S. BortzmeyerRequest for Comments: 7626 AFNICCategory: Informational August 2015ISSN: 2070-1721DNS Privacy ConsiderationsAbstract 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 document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; seeSection 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7626.Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject toBCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) 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 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Bortzmeyer Informational [Page 1]

RFC 7626                       DNS Privacy                   August 2015Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.1.  The Alleged Public Nature of DNS Data . . . . . . . . . .42.2.  Data in the DNS Request . . . . . . . . . . . . . . . . .52.3.  Cache Snooping  . . . . . . . . . . . . . . . . . . . . .62.4.  On the Wire . . . . . . . . . . . . . . . . . . . . . . .72.5.  In the Servers  . . . . . . . . . . . . . . . . . . . . .82.5.1.  In the Recursive Resolvers  . . . . . . . . . . . . .82.5.2.  In the Authoritative Name Servers . . . . . . . . . .92.5.3.  Rogue Servers . . . . . . . . . . . . . . . . . . . .102.6.  Re-identification and Other Inferences  . . . . . . . . .112.7.  More Information  . . . . . . . . . . . . . . . . . . . .113.  Actual "Attacks"  . . . . . . . . . . . . . . . . . . . . . .114.  Legalities  . . . . . . . . . . . . . . . . . . . . . . . . .125.  Security Considerations . . . . . . . . . . . . . . . . . . .126.  References  . . . . . . . . . . . . . . . . . . . . . . . . .126.1.  Normative References  . . . . . . . . . . . . . . . . . .126.2.  Informative References  . . . . . . . . . . . . . . . . .13   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .17   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .171.  Introduction   This document is an analysis of the DNS privacy issues, in the spirit   ofSection 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 [DNS-TERMS].)  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?" 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, inBortzmeyer                    Informational                     [Page 2]

RFC 7626 DNS Privacy August 2015 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). There are a few cases where there is some channel encryption, for instance, in an IPsec VPN, at least between the stub resolver and the resolver. 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:Bortzmeyer Informational [Page 3]

RFC 7626 DNS Privacy August 2015 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 eavesdropper. Secondary requests: these are the additional requests performed by the user agent (here, the web browser) without any direct involvement or knowledge of the user. For the Web, they are triggered by embedded content, Cascading Style Sheets (CSS), JavaScript code, embedded images, etc. In some cases, there can be dozens of domain names in different contexts on a single web page. Tertiary requests: these are the additional requests performed by the DNS 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 [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] as well as 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] and [RFC5155]. 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 privateBortzmeyer Informational [Page 4]

RFC 7626 DNS Privacy August 2015 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 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 differentiate between several users sharing an IP address (behind a Carrier-Grade NAT (CGN), for instance [RFC6269]). The QNAME is the full name sent by the user. It gives information about what the user does ("What are the MX records of example.net?" means 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 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, _ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org. There are also some BitTorrent clients that query an SRV record for _bittorrent-tracker._tcp.domain.example.Bortzmeyer Informational [Page 5]

RFC 7626 DNS Privacy August 2015 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 [DANE-OPENPGPKEY], then privacy becomes a lot more important. And email is just an example; there would be other really interesting uses for a more privacy- friendly DNS. For the communication between the stub resolver and the recursive resolver, the source IP address is the address of the user's machine. Therefore, all the issues and warnings about collection of IP addresses apply here. For the communication between the recursive resolver and the authoritative name servers, the source IP address has a different meaning; it does not have the same status as the source address in an HTTP connection. It is now the IP address of the recursive resolver that, in a way, "hides" the real user. However, hiding does not always work. Sometimes [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 [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.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.Bortzmeyer Informational [Page 6]

RFC 7626                       DNS Privacy                   August 20152.4.  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 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, the DNS may 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   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 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-Bortzmeyer                    Informational                     [Page 7]

RFC 7626                       DNS Privacy                   August 2015      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 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.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 ofBortzmeyer                    Informational                     [Page 8]

RFC 7626                       DNS Privacy                   August 2015   the data collected by reading the privacy policy of a big public   resolver, e.g., <https://developers.google.com/speed/public-dns/privacy>.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 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 [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 50,000 queries per second.  While most of it   is "junk" (errors on the Top-Level Domain (TLD) name), it gives an   idea of the amount of big data that pours into name servers.  (And   even "junk" can leak information; for instance, if there is a typing   error in the TLD, the user will send data to a TLD that is not the   usual one.)   Many domains, including TLDs, are partially hosted by third-party   servers, sometimes in a different country.  The contracts between the   domain manager and these servers may or may not take privacy into   account.  Whatever the contract, the third-party hoster may 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 organizationsBortzmeyer                    Informational                     [Page 9]

RFC 7626 DNS Privacy August 2015 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, [ripe-atlas-turkey]). A practical consequence of this section is that solutions for DNS privacy may have to address authentication of the server, not just passive sniffing.Bortzmeyer Informational [Page 10]

RFC 7626 DNS Privacy August 20152.6. 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.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 [federrath-fuchs-herrmann-piosecny].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 inSection 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 ofBortzmeyer Informational [Page 11]

RFC 7626                       DNS Privacy                   August 2015   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] (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 [sidn-entrada].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, [QNAME-MINIMIZATION] for the minimization of data or   [TLS-FOR-DNS] about encryption.6.  References6.1.  Normative References   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",              STD 13,RFC 1034, DOI 10.17487/RFC1034, November 1987,              <http://www.rfc-editor.org/info/rfc1034>.   [RFC1035]  Mockapetris, P., "Domain names - implementation and              specification", STD 13,RFC 1035, DOI 10.17487/RFC1035,              November 1987, <http://www.rfc-editor.org/info/rfc1035>.Bortzmeyer                    Informational                    [Page 12]

RFC 7626                       DNS Privacy                   August 2015   [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,              <http://www.rfc-editor.org/info/rfc6973>.   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an              Attack",BCP 188,RFC 7258, DOI 10.17487/RFC7258, May              2014, <http://www.rfc-editor.org/info/rfc7258>.6.2.  Informative References   [aeris-dns]              Vinot, N., "Vie privee: et le DNS alors?", (In French),              2015,              <https://blog.imirhil.fr/vie-privee-et-le-dns-alors.html>.   [castillo-garcia]              Castillo-Perez, S. and J. Garcia-Alfaro, "Anonymous              Resolution of DNS Queries", 2008,              <http://deic.uab.es/~joaquin/papers/is08.pdf>.   [CLIENT-SUBNET]              Contavalli, C., Gaast, W., Lawrence, D., and W. Kumari,              "Client Subnet in DNS Queries", Work in Progress,draft-ietf-dnsop-edns-client-subnet-02, July 2015.   [dagon-malware]              Dagon, D., "Corrupted DNS Resolution Paths: The Rise of a              Malicious Resolution Authority", ISC/OARC Workshop, 2007,              <https://www.dns-oarc.net/files/workshop-2007/Dagon-Resolution-corruption.pdf>.   [DANE-OPENPGPKEY]              Wouters, P., "Using DANE to Associate OpenPGP public keys              with email addresses", Work in Progress,draft-ietf-dane-openpgpkey-03, April 2015.   [darkreading-dns]              Lemos, R., "Got Malware? Three Signs Revealed In DNS              Traffic", InformationWeek Dark Reading, May 2013,              <http://www.darkreading.com/analytics/security-monitoring/got-malware-three-signs-revealed-in-dns-traffic/d/d-id/1139680>.Bortzmeyer                    Informational                    [Page 13]

RFC 7626                       DNS Privacy                   August 2015   [data-protection-directive]              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-lex.europa.eu/LexUriServ/              LexUriServ.do?uri=CELEX:31995L0046:EN:HTML>.   [day-at-root]              Castro, S., Wessels, D., Fomenkov, M., and K. Claffy, "A              Day at the Root of the Internet", ACM SIGCOMM Computer              Communication Review, Vol. 38, Number 5, DOI              10.1145/1452335.1452341, October 2008,              <http://www.sigcomm.org/sites/default/files/ccr/papers/2008/October/1452335-1452341.pdf>.   [denis-edns-client-subnet]              Denis, F., "Security and privacy issues of edns-client-              subnet", August 2013, <https://00f.net/2013/08/07/edns-client-subnet/>.   [ditl]     CAIDA, "A Day in the Life of the Internet (DITL)", 2002,              <http://www.caida.org/projects/ditl/>.   [dns-footprint]              Stoner, E., "DNS Footprint of Malware", OARC Workshop,              October 2010, <https://www.dns-oarc.net/files/workshop-201010/OARC-ers-20101012.pdf>.   [DNS-TERMS]              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS              Terminology", Work in Progress,draft-ietf-dnsop-dns-terminology-03, June 2015.   [dnschanger]              Wikipedia, "DNSChanger", October 2013,              <https://en.wikipedia.org/w/index.php?title=DNSChanger&oldid=578749672>.   [dnsmezzo] Bortzmeyer, S., "DNSmezzo", 2009,              <http://www.dnsmezzo.net/>.Bortzmeyer                    Informational                    [Page 14]

RFC 7626                       DNS Privacy                   August 2015   [fangming-hori-sakurai]              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,              <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", Computer              Security ESORICS 2011, Springer, page(s) 665-683, ISBN              978-3-642-23821-5, 2011,              <https://svs.informatik.uni-hamburg.de/publications/2011/2011-09-14_FFHP_PrivacyPreservingDNS_ESORICS2011.pdf>.   [grangeia.snooping]              Grangeia, L., "DNS Cache Snooping or Snooping the Cache              for Fun and Profit", February 2004,              <http://www.msit2005.mut.ac.th/msit_media/1_2551/nete4630/materials/20080718130017Hc.pdf>.   [herrmann-reidentification]              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-regensburg.de/21103/1/Paper_PUL_nordsec_published.pdf>.   [morecowbell]              Grothoff, C., Wachs, M., Ermert, M., and J. Appelbaum,              "NSA's MORECOWBELL: Knell for DNS", GNUnet e.V., January              2015, <https://gnunet.org/morecowbell>.   [packetq]  Dot SE, "PacketQ, a simple tool to make SQL-queries              against PCAP-files", 2011,              <https://github.com/dotse/packetq/wiki>.   [packetq-list]              PacketQ, "PacketQ Mailing List",              <http://lists.iis.se/mailman/listinfo/packetq>.   [passive-dns]              Weimer, F., "Passive DNS Replication", April 2005,              <http://www.enyo.de/fw/software/dnslogger/#2>.Bortzmeyer                    Informational                    [Page 15]

RFC 7626                       DNS Privacy                   August 2015   [prism]    Wikipedia, "PRISM (surveillance program)", July 2015,              <https://en.wikipedia.org/w/index.php?title=PRISM_              (surveillance_program)&oldid=673789455>.   [QNAME-MINIMIZATION]              Bortzmeyer, S., "DNS query name minimisation to improve              privacy", Work in Progress,draft-ietf-dnsop-qname-minimisation-04, June 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,              <http://www.rfc-editor.org/info/rfc4033>.   [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,              <http://www.rfc-editor.org/info/rfc5155>.   [RFC5936]  Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol              (AXFR)",RFC 5936, DOI 10.17487/RFC5936, June 2010,              <http://www.rfc-editor.org/info/rfc5936>.   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and              P. Roberts, "Issues with IP Address Sharing",RFC 6269,              DOI 10.17487/RFC6269, June 2011,              <http://www.rfc-editor.org/info/rfc6269>.   [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,              <http://www.rfc-editor.org/info/rfc7624>.   [ripe-atlas-turkey]              Aben, E., "A RIPE Atlas View of Internet Meddling in              Turkey", March 2014,              <https://labs.ripe.net/Members/emileaben/a-ripe-atlas-view-of-internet-meddling-in-turkey>.   [sidn-entrada]              Hesselman, C., Jansen, J., Wullink, M., Vink, K., and M.              Simon, "A privacy framework for 'DNS big data'              applications", November 2014,              <https://www.sidnlabs.nl/uploads/tx_sidnpublications/SIDN_Labs_Privacyraamwerk_Position_Paper_V1.4_ENG.pdf>.Bortzmeyer                    Informational                    [Page 16]

RFC 7626 DNS Privacy August 2015 [thomas-ditl-tcp] Thomas, M. and D. Wessels, "An Analysis of TCP Traffic in Root Server DITL Data", DNS-OARC 2014 Fall Workshop, October 2014, <https://indico.dns-oarc.net/event/20/session/2/contribution/15/material/slides/1.pdf>. [TLS-FOR-DNS] Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., and P. Hoffman, "TLS for DNS: Initiation and Performance Considerations", Work in Progress,draft-ietf-dprive-start-tls-for-dns-01, July 2015. [tor-leak] Tor, "DNS leaks in Tor", 2013, <https://www.torproject.org/docs/faq.html.en#WarningsAboutSOCKSandDNSInformationLeaks>. [yanbin-tsudik] Yanbin, L. and G. Tsudik, "Towards Plugging Privacy Leaks in the Domain Name System", October 2009, <http://arxiv.org/abs/0910.2472>.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.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/Bortzmeyer Informational [Page 17]

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