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Recommendations for DNS Privacy Service Operators

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Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8932.
Authors Sara Dickinson , Benno Overeinder , Roland van Rijswijk-Deij , Allison Mankin
Last updated 2019-03-11
Replaces draft-dickinson-dprive-bcp-op
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dprive                                                      S. Dickinson
Internet-Draft                                                Sinodun IT
Intended status: Best Current Practice                     B. Overeinder
Expires: September 12, 2019                         R. van Rijswijk-Deij
                                                              NLnet Labs
                                                               A. Mankin
                                                          March 11, 2019

           Recommendations for DNS Privacy Service Operators


   This document presents operational, policy and security
   considerations for DNS operators who choose to offer DNS Privacy
   services.  With these recommendations, the operator can make
   deliberate decisions regarding which services to provide, and how the
   decisions and alternatives impact the privacy of users.

   This document also presents a framework to assist writers of DNS
   Privacy Policy and Practices Statements (analogous to DNS Security
   Extensions (DNSSEC) Policies and DNSSEC Practice Statements described
   in [RFC6841]).

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 September 12, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Privacy related documents . . . . . . . . . . . . . . . . . .   5
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Recommendations for DNS privacy services  . . . . . . . . . .   6
     5.1.  On the wire between client and server . . . . . . . . . .   7
       5.1.1.  Transport recommendations . . . . . . . . . . . . . .   7
       5.1.2.  Authentication of DNS privacy services  . . . . . . .   7
       5.1.3.  Protocol recommendations  . . . . . . . . . . . . . .   9
       5.1.4.  Availability  . . . . . . . . . . . . . . . . . . . .  10
       5.1.5.  Service options . . . . . . . . . . . . . . . . . . .  11
       5.1.6.  Impact on Operators . . . . . . . . . . . . . . . . .  11
       5.1.7.  Limitations of using a pure TLS proxy . . . . . . . .  12
     5.2.  Data at rest on the server  . . . . . . . . . . . . . . .  12
       5.2.1.  Data handling . . . . . . . . . . . . . . . . . . . .  12
       5.2.2.  Data minimization of network traffic  . . . . . . . .  13
       5.2.3.  IP address pseudonymization and anonymization methods  14
       5.2.4.  Pseudonymization, anonymization or discarding of
               other correlation data  . . . . . . . . . . . . . . .  15
       5.2.5.  Cache snooping  . . . . . . . . . . . . . . . . . . .  16
     5.3.  Data sent onwards from the server . . . . . . . . . . . .  16
       5.3.1.  Protocol recommendations  . . . . . . . . . . . . . .  16
       5.3.2.  Client query obfuscation  . . . . . . . . . . . . . .  17
       5.3.3.  Data sharing  . . . . . . . . . . . . . . . . . . . .  18
   6.  DNS privacy policy and practice statement . . . . . . . . . .  19
     6.1.  Recommended contents of a DPPPS . . . . . . . . . . . . .  19
       6.1.1.  Policy  . . . . . . . . . . . . . . . . . . . . . . .  19
       6.1.2.  Practice  . . . . . . . . . . . . . . . . . . . . . .  20
     6.2.  Current policy and privacy statements . . . . . . . . . .  21
     6.3.  Enforcement/accountability  . . . . . . . . . . . . . . .  21
   7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  22
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  22
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22
   11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24

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     12.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     12.2.  Informative References . . . . . . . . . . . . . . . . .  26
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  27
   Appendix A.  Documents  . . . . . . . . . . . . . . . . . . . . .  29
     A.1.  Potential increases in DNS privacy  . . . . . . . . . . .  29
     A.2.  Potential decreases in DNS privacy  . . . . . . . . . . .  29
     A.3.  Related operational documents . . . . . . . . . . . . . .  30
   Appendix B.  Encryption and DNSSEC  . . . . . . . . . . . . . . .  30
   Appendix C.  IP address techniques  . . . . . . . . . . . . . . .  30
     C.1.  Google Analytics non-prefix filtering . . . . . . . . . .  31
     C.2.  dnswasher . . . . . . . . . . . . . . . . . . . . . . . .  32
     C.3.  Prefix-preserving map . . . . . . . . . . . . . . . . . .  32
     C.4.  Cryptographic Prefix-Preserving Pseudonymisation  . . . .  32
     C.5.  Top-hash Subtree-replicated Anonymisation . . . . . . . .  33
     C.6.  ipcipher  . . . . . . . . . . . . . . . . . . . . . . . .  33
     C.7.  Bloom filters . . . . . . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   The Domain Name System (DNS) is at the core of the Internet; almost
   every activity on the Internet starts with a DNS query (and often
   several).  However the DNS was not originally designed with strong
   security or privacy mechanisms.  A number of developments have taken
   place in recent years which aim to increase the privacy of the DNS
   system and these are now seeing some deployment.  This latest
   evolution of the DNS presents new challenges to operators and this
   document attempts to provide an overview of considerations for
   privacy focused DNS services.

   In recent years there has also been an increase in the availability
   of "public resolvers" [I-D.ietf-dnsop-terminology-bis] which users
   may prefer to use instead of the default network resolver because
   they offer a specific feature (e.g. good reachability, encrypted
   transport, strong privacy policy, filtering (or lack of), etc.).
   These open resolvers have tended to be at the forefront of adoption
   of privacy related enhancements but it is anticipated that operators
   of other resolver services will follow.

   Whilst protocols that encrypt DNS messages on the wire provide
   protection against certain attacks, the resolver operator still has
   (in principle) full visibility of the query data and transport
   identifiers for each user.  Therefore, a trust relationship exists.
   The ability of the operator to provide a transparent, well
   documented, and secure privacy service will likely serve as a major
   differentiating factor for privacy conscious users if they make an
   active selection of which resolver to use.

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   It should also be noted that the choice of a user to configure a
   single resolver (or a fixed set of resolvers) and an encrypted
   transport to use in all network environments has both advantages and
   disadvantages.  For example the user has a clear expectation of which
   resolvers have visibility of their query data however this resolver/
   transport selection may provide an added mechanism to track them as
   they move across network environments.  Commitments from operators to
   minimize such tracking are also likely to play a role in user
   selection of resolvers.

   More recently the global legislative landscape with regard to
   personal data collection, retention, and pseudonymization has seen
   significant activity.  It is an untested area that simply using a DNS
   resolution service constitutes consent from the user for the operator
   to process their query data.  The impact of recent legislative
   changes on data pertaining to the users of both Internet Service
   Providers and public DNS resolvers is not fully understood at the
   time of writing.

   This document has two main goals:

   o  To provide operational and policy guidance related to DNS over
      encrypted transports and to outline recommendations for data
      handling for operators of DNS privacy services.

   o  To introduce the DNS Privacy Policy and Practice Statement (DPPPS)
      and present a framework to assist writers of this document.  A
      DPPPS is a document that an operator can publish outlining their
      operational practices and commitments with regard to privacy
      thereby providing a means for clients to evaluate the privacy
      properties of a given DNS privacy service.  In particular, the
      framework identifies the elements that should be considered in
      formulating a DPPPS.  This document does not, however, define a
      particular Policy or Practice Statement, nor does it seek to
      provide legal advice or recommendations as to the contents.

   Community insight [or judgment?] about operational practices can
   change quickly, and experience shows that a Best Current Practice
   (BCP) document about privacy and security is a point-in-time
   statement.  Readers are advised to seek out any errata or updates
   that apply to this document.

2.  Scope

   "DNS Privacy Considerations" [I-D.bortzmeyer-dprive-rfc7626-bis]
   describes the general privacy issues and threats associated with the
   use of the DNS by Internet users and much of the threat analysis here
   is lifted from that document and from [RFC6973].  However this

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   document is limited in scope to best practice considerations for the
   provision of DNS privacy services by servers (recursive resolvers) to
   clients (stub resolvers or forwarders).  Privacy considerations
   specifically from the perspective of an end user, or those for
   operators of authoritative nameservers are out of scope.

   This document includes (but is not limited to) considerations in the
   following areas (taken from [I-D.bortzmeyer-dprive-rfc7626-bis]):

   1.  Data "on the wire" between a client and a server

   2.  Data "at rest" on a server (e.g. in logs)

   3.  Data "sent onwards" from the server (either on the wire or shared
       with a third party)

   Whilst the issues raised here are targeted at those operators who
   choose to offer a DNS privacy service, considerations for areas 2 and
   3 could equally apply to operators who only offer DNS over
   unencrypted transports but who would like to align with privacy best

3.  Privacy related documents

   There are various documents that describe protocol changes that have
   the potential to either increase or decrease the privacy of the DNS.
   Note this does not imply that some documents are good or bad, better
   or worse, just that (for example) some features may bring functional
   benefits at the price of a reduction in privacy and conversely some
   features increase privacy with an accompanying increase in
   complexity.  A selection of the most relevant documents are listed in
   Appendix A for reference.

4.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] and [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   DNS terminology is as described in [I-D.ietf-dnsop-terminology-bis]
   with one modification: we restate the clause in the original
   definition of Privacy-enabling DNS server in [RFC8310] to include the
   requirement that a DNS over (D)TLS server should also offer at least
   one of the credentials described in Section 8 and implement the
   (D)TLS profile described in Section 9 of [RFC8310].

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   Other Terms:

   o  DPPPS: DNS Privacy Policy and Practice Statement, see Section 6.

   o  DNS privacy service: The service that is offered via a privacy-
      enabling DNS server and is documented either in an informal
      statement of policy and practice with regard to users privacy or a
      formal DPPPS.

5.  Recommendations for DNS privacy services

   We describe two classes of threats:

   o  'Privacy Considerations for Internet Protocols' [RFC6973] Threats

      *  Privacy terminology, threats to privacy and mitigations as
         described in Sections 3, 5 and 6 of [RFC6973].

   o  DNS Privacy Threats

      *  These are threats to the users and operators of DNS privacy
         services that are not directly covered by [RFC6973].  These may
         be more operational in nature such as certificate management or
         service availability issues.

   We describe three classes of actions that operators of DNS privacy
   services can take:

   o  Threat mitigation for well understood and documented privacy
      threats to the users of the service and in some cases to the
      operators of the service.

   o  Optimization of privacy services from an operational or management

   o  Additional options that could further enhance the privacy and
      usability of the service

   This document does not specify policy only best practice, however for
   DNS Privacy services to be considered compliant with these best
   practice guidelines they SHOULD implement (where appropriate) all:

   o  Threat mitigations to be minimally compliant

   o  Optimizations to be moderately compliant

   o  Additional options to be maximally compliant

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5.1.  On the wire between client and server

   In this section we consider both data on the wire and the service
   provided to the client.

5.1.1.  Transport recommendations

   [RFC6973] Threats:

   o  Surveillance:

      *  Passive surveillance of traffic on the wire
         [I-D.bortzmeyer-dprive-rfc7626-bis] Section 2.4.2.

   DNS Privacy Threats:

   o  Active injection of spurious data or traffic


   A DNS privacy service can mitigate these threats by providing service
   over one or more of the following transports

   o  DNS-over-TLS [RFC7858] and [RFC8310]

   o  DoH [RFC8484]

   It is noted that a DNS privacy service can also be provided over DNS-
   over-DTLS [RFC8094], however this is an Experimental specification
   and there are no known implementations at the time of writing.

   It is also noted that DNS privacy service might be provided over
   IPSec, DNSCrypt or VPNs.  However, use of these transports for DNS
   are not standardized and any discussion of best practice for
   providing such a service is out of scope for this document.

   Whilst encryption of DNS traffic can protect against active injection
   this does not diminish the need for DNSSEC, see Appendix B.

5.1.2.  Authentication of DNS privacy services

   [RFC6973] Threats:

   o  Surveillance:

      *  Active attacks that can redirect traffic to rogue servers
         [I-D.bortzmeyer-dprive-rfc7626-bis] Section 2.5.3.

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   DNS privacy services should ensure clients can authenticate the
   server.  Note that this, in effect, commits the DNS privacy service
   to a public identity users will trust.

   When using DNS-over-TLS clients that select a 'Strict Privacy' usage
   profile [RFC8310] (to mitigate the threat of active attack on the
   client) require the ability to authenticate the DNS server.  To
   enable this, DNS privacy services that offer DNS-over-TLS should
   provide credentials in the form of either X.509 certificates or SPKI

   When offering DoH [RFC8484], HTTPS requires authentication of the
   server as part of the protocol.

   NOTE: At this time the reference to the TLS DNSSEC chain extension
   draft has been removed as it is no longer considered an active TLS WG


   DNS privacy services can also consider the following capabilities/

   o  As recommended in [RFC8310] providing DANE TLSA records for the

      *  In particular, the service could provide TLSA records such that
         authenticating solely via the PKIX infrastructure can be
         avoided.  Certificate management

   Anecdotal evidence to date highlights the management of certificates
   as one of the more challenging aspects for operators of traditional
   DNS resolvers that choose to additionally provide a DNS privacy
   service as management of such credentials is new to those DNS

   It is noted that SPKI pinset management is described in [RFC7858] but
   that key pinning mechanisms in general have fallen out of favor
   operationally for various reasons such as the logistical overhead of
   rolling keys.

   DNS Privacy Threats:

   o  Invalid certificates, resulting in an unavailable service.

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   o  Mis-identification of a server by a client e.g. typos in URLs or
      authentication domain names


   It is recommended that operators:

   o  Follow the guidance in Section 6.5 of [RFC7525] with regards to
      certificate revocation

   o  Choose a short, memorable authentication name for the service

   o  Automate the generation and publication of certificates

   o  Monitor certificates to prevent accidental expiration of

5.1.3.  Protocol recommendations  DNS-over-TLS

   DNS Privacy Threats:

   o  Known attacks on TLS such as those described in [RFC7457]

   o  Traffic analysis, for example: Pitfalls of DNS Encryption [1]

   o  Potential for client tracking via transport identifiers

   o  Blocking of well known ports (e.g. 853 for DNS-over-TLS)


   In the case of DNS-over-TLS, TLS profiles from Section 9 and the
   Countermeasures to DNS Traffic Analysis from section 11.1 of
   [RFC8310] provide strong mitigations.  This includes but is not
   limited to:

   o  Adhering to [RFC7525]

   o  Implementing only (D)TLS 1.2 or later as specified in [RFC8310]

   o  Implementing EDNS(0) Padding [RFC7830] using the guidelines in

   o  Clients should not be required to use TLS session resumption
      [RFC5077] or Domain Name System (DNS) Cookies [RFC7873].

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   o  A DNS-over-TLS privacy service on both port 853 and 443.  This
      practice may not be possible if e.g. the operator deploys DoH on
      the same IP address.


   o  Concurrent processing of pipelined queries, returning responses as
      soon as available, potentially out of order as specified in
      [RFC7766].  This is often called 'OOOR' - out-of-order responses.
      (Providing processing performance similar to HTTP multiplexing)

   o  Management of TLS connections to optimize performance for clients
      using either

      *  [RFC7766] and EDNS(0) Keepalive [RFC7828] and/or

      *  DNS Stateful Operations [I-D.ietf-dnsop-session-signal]

   Additional options that providers may consider:

   o  Offer a .onion [RFC7686] service endpoint  DoH

   DNS Privacy Threats:

   o  Known attacks on TLS such as those described in [RFC7457]

   o  Traffic analysis, for example: DNS Privacy not so private: the
      traffic analysis perspective [2]

   o  Potential for client tracking via transport identifiers


   o  Clients must be able to forego the use of HTTP Cookies [RFC6265]
      and still use the service

   o  Clients should not be required to include any headers beyond the
      absolute minimum to obtain service from a DoH server.  (See
      Section 6.1 of [I-D.ietf-httpbis-bcp56bis].)

5.1.4.  Availability

   DNS Privacy Threats:

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   o  A failed DNS privacy service could force the user to switch
      providers, fallback to cleartext or accept no DNS service for the


   A DNS privacy service must be engineered for high availability.
   Particular care should to be taken to protect DNS privacy services
   against denial-of-service attacks, as experience has shown that
   unavailability of DNS resolving because of attacks is a significant
   motivation for users to switch services.  See, for example
   Section IV-C of Passive Observations of a Large DNS Service: 2.5
   Years in the Life of Google [3].

5.1.5.  Service options

   DNS Privacy Threats:

   o  Unfairly disadvantaging users of the privacy service with respect
      to the services available.  This could force the user to switch
      providers, fallback to cleartext or accept no DNS service for the


   A DNS privacy service should deliver the same level of service as
   offered on un-encrypted channels in terms of such options as
   filtering (or lack thereof), DNSSEC validation, etc.

5.1.6.  Impact on Operators

   DNS Privacy Threats:

   o  Increased use of encryption impacts operator ability to manage
      their network [RFC8404]

   Many monitoring solutions for DNS traffic rely on the plain text
   nature of this traffic and work by intercepting traffic on the wire,
   either using a separate view on the connection between clients and
   the resolver, or as a separate process on the resolver system that
   inspects network traffic.  Such solutions will no longer function
   when traffic between clients and resolvers is encrypted.  There are,
   however, legitimate reasons for operators to inspect DNS traffic,
   e.g. to monitor for network security threats.  Operators may
   therefore need to invest in alternative means of monitoring that
   relies on either the resolver software directly, or exporting DNS
   traffic from the resolver using e.g. dnstap [4].

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   When implementing alternative means for traffic monitoring, operators
   of a DNS privacy service should consider using privacy conscious
   means to do so (see, for example, the discussion on the use of Bloom
   Filters in the #documents appendix in this document).

5.1.7.  Limitations of using a pure TLS proxy

   DNS Privacy Threats:

   o  Limited ability to manage or monitor incoming connections using
      DNS specific techniques

   o  Misconfiguration of the target server could lead to data leakage
      if the proxy to target server path is not encrypted.


   Some operators may choose to implement DNS-over-TLS using a TLS proxy
   (e.g.  nginx [5], haproxy [6] or stunnel [7]) in front of a DNS
   nameserver because of proven robustness and capacity when handling
   large numbers of client connections, load balancing capabilities and
   good tooling.  Currently, however, because such proxies typically
   have no specific handling of DNS as a protocol over TLS or DTLS using
   them can restrict traffic management at the proxy layer and at the
   DNS server.  For example, all traffic received by a nameserver behind
   such a proxy will appear to originate from the proxy and DNS
   techniques such as ACLs, RRL or DNS64 will be hard or impossible to
   implement in the nameserver.

   Operators may choose to use a DNS aware proxy such as dnsdist [8]
   which offer custom options (similar to that proposed in
   [I-D.bellis-dnsop-xpf]) to add source information to packets to
   address this shortcoming.  It should be noted that such options
   potentially significantly increase the leaked information in the
   event of a misconfiguration.

5.2.  Data at rest on the server

5.2.1.  Data handling

   [RFC6973] Threats:

   o  Surveillance

   o  Stored data compromise

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   o  Correlation

   o  Identification

   o  Secondary use

   o  Disclosure

   Other Threats

   o  Contravention of legal requirements not to process user data?


   The following are common activities for DNS service operators and in
   all cases should be minimized or completely avoided if possible for
   DNS privacy services.  If data is retained it should be encrypted and
   either aggregated, pseudonymized or anonymized whenever possible.  In
   general the principle of data minimization described in [RFC6973]
   should be applied.

   o  Transient data (e.g. that is used for real time monitoring and
      threat analysis which might be held only memory) should be
      retained for the shortest possible period deemed operationally

   o  The retention period of DNS traffic logs should be only those
      required to sustain operation of the service and, to the extent
      that such exists, meet regulatory requirements.

   o  DNS privacy services should not track users except for the
      particular purpose of detecting and remedying technically
      malicious (e.g.  DoS) or anomalous use of the service.

   o  Data access should be minimized to only those personnel who
      require access to perform operational duties.


   o  Consider use of full disk encryption for logs and data capture

5.2.2.  Data minimization of network traffic

   Data minimization refers to collecting, using, disclosing, and
   storing the minimal data necessary to perform a task, and this can be
   achieved by removing or obfuscating privacy-sensitive information in
   network traffic logs.  This is typically personal data, or data that

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   can be used to link a record to an individual, but may also include
   revealing other confidential information, for example on the
   structure of an internal corporate network.

   The problem of effectively ensuring that DNS traffic logs contain no
   or minimal privacy-sensitive information is not one that currently
   has a generally agreed solution or any Standards to inform this
   discussion.  This section presents and overview of current techniques
   to simply provide reference on the current status of this work.

   Research into data minimization techniques (and particularly IP
   address pseudonymization/anonymization) was sparked in the late
   1990s/early 2000s, partly driven by the desire to share significant
   corpuses of traffic captures for research purposes.  Several
   techniques reflecting different requirements in this area and
   different performance/resource tradeoffs emerged over the course of
   the decade.  Developments over the last decade have been both a
   blessing and a curse; the large increase in size between an IPv4 and
   an IPv6 address, for example, renders some techniques impractical,
   but also makes available a much larger amount of input entropy, the
   better to resist brute force re-identification attacks that have
   grown in practicality over the period.

   Techniques employed may be broadly categorized as either
   anonymization or pseudonymization.  The following discussion uses the
   definitions from [RFC6973] Section 3, with additional observations
   from van Dijkhuizen et al. [9]

   o  Anonymization.  To enable anonymity of an individual, there must
      exist a set of individuals that appear to have the same
      attribute(s) as the individual.  To the attacker or the observer,
      these individuals must appear indistinguishable from each other.

   o  Pseudonymization.  The true identity is deterministically replaced
      with an alternate identity (a pseudonym).  When the
      pseudonymization schema is known, the process can be reversed, so
      the original identity becomes known again.

   In practice there is a fine line between the two; for example, how to
   categorize a deterministic algorithm for data minimization of IP
   addresses that produces a group of pseudonyms for a single given

5.2.3.  IP address pseudonymization and anonymization methods

   As [I-D.bortzmeyer-dprive-rfc7626-bis] makes clear, the big privacy
   risk in DNS is connecting DNS queries to an individual and the major
   vector for this in DNS traffic is the client IP address.

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   There is active discussion in the space of effective pseudonymization
   of IP addresses in DNS traffic logs, however there seems to be no
   single solution that is widely recognized as suitable for all or most
   use cases.  There are also as yet no standards for this that are
   unencumbered by patents.  The following table presents a high level
   comparison of various techniques employed or under development today
   and classifies them according to categorization of technique and
   other properties.  The list of techniques includes the main
   techniques in current use, but does not claim to be comprehensive.
   Appendix C provides a more detailed survey of these techniques and
   definitions for the categories and properties listed below.

   Figure showing comparison of IP address techniques (SVG) [10]

   The choice of which method to use for a particular application will
   depend on the requirements of that application and consideration of
   the threat analysis of the particular situation.

   For example, a common goal is that distributed packet captures must
   be in an existing data format such as PCAP [pcap] or C-DNS
   [I-D.ietf-dnsop-dns-capture-format] that can be used as input to
   existing analysis tools.  In that case, use of a format-preserving
   technique is essential.  This, though, is not cost-free - several
   authors (e.g.  Brenker & Arnes [11]) have observed that, as the
   entropy in an IPv4 address is limited, given a de-identified log from
   a target, if an attacker is capable of ensuring packets are captured
   by the target and the attacker can send forged traffic with arbitrary
   source and destination addresses to that target, any format-
   preserving pseudonymization is vulnerable to an attack along the
   lines of a cryptographic chosen plaintext attack.

5.2.4.  Pseudonymization, anonymization or discarding of other
        correlation data

   DNS Privacy Threats:

   o  IP TTL/Hoplimit can be used to fingerprint client OS

   o  Tracking of TCP sessions

   o  Tracking of TLS sessions and session resumption mechanisms

   o  Resolvers _might_ receive client identifiers e.g.  MAC addresses
      in EDNS(0) options - some CPE devices are known to add them.

   o  HTTP headers


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   o  Data minimization or discarding of such correlation data

   TODO: More analysis here.

5.2.5.  Cache snooping

   [RFC6973] Threats:

   o  Surveillance:

      *  Profiling of client queries by malicious third parties


   o  See ISC Knowledge database on cache snooping [12] for an example
      discussion on defending against cache snooping

   TODO: Describe other techniques to defend against cache snooping

5.3.  Data sent onwards from the server

   In this section we consider both data sent on the wire in upstream
   queries and data shared with third parties.

5.3.1.  Protocol recommendations

   [RFC6973] Threats:

   o  Surveillance:

      *  Transmission of identifying data upstream.


   As specified in [RFC8310] for DNS-over-TLS but applicable to any DNS
   Privacy services the server should:

   o  Implement QNAME minimization [RFC7816]

   o  Honor a SOURCE PREFIX-LENGTH set to 0 in a query containing the
      EDNS(0) Client Subnet (ECS) option and not send an ECS option in
      upstream queries.


   o  The server should either

      *  not use the ECS option in upstream queries at all, or

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      *  offer alternative services, one that sends ECS and one that
         does not.

   If operators do offer a service that sends the ECS options upstream
   they should use the shortest prefix that is operationally feasible
   (NOTE: the authors believe they will be able to add a reference for
   advice here soon) and ideally use a policy of whitelisting upstream
   servers to send ECS to in order to minimize data leakage.  Operators
   should make clear in any policy statement what prefix length they
   actually send and the specific policy used.

   Whitelisting has the benefit that not only does the operator know
   which upstream servers can use ECS but also allows the operator to
   decide which upstream servers apply privacy policies that the
   operator is happy with.  However some operators consider whitelisting
   to incur significant operational overhead compared to dynamic
   detection of ECS on authoritative servers.

   Additional options:

   o  Aggressive Use of DNSSEC-Validated Cache [RFC8198] to reduce the
      number of queries to authoritative servers to increase privacy.

   o  Run a copy of the root zone on loopback [RFC7706] to avoid making
      queries to the root servers that might leak information.

5.3.2.  Client query obfuscation

   Additional options:

   Since queries from recursive resolvers to authoritative servers are
   performed using cleartext (at the time of writing), resolver services
   need to consider the extent to which they may be directly leaking
   information about their client community via these upstream queries
   and what they can do to mitigate this further.  Note, that even when
   all the relevant techniques described above are employed there may
   still be attacks possible, e.g.  [Pitfalls-of-DNS-Encryption].  For
   example, a resolver with a very small community of users risks
   exposing data in this way and OUGHT obfuscate this traffic by mixing
   it with 'generated' traffic to make client characterization harder.
   The resolver could also employ aggressive pre-fetch techniques as a
   further measure to counter traffic analysis.

   At the time of writing there are no standardized or widely recognized
   techniques to perform such obfuscation or bulk pre-fetches.

   Another technique that particularly small operators may consider is
   forwarding local traffic to a larger resolver (with a privacy policy

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   that aligns with their own practices) over an encrypted protocol so
   that the upstream queries are obfuscated among those of the large

5.3.3.  Data sharing

   [RFC6973] Threats:

   o  Surveillance

   o  Stored data compromise

   o  Correlation

   o  Identification

   o  Secondary use

   o  Disclosure

   DNS Privacy Threats:

   o  Contravention of legal requirements not to process user data?


   Operators should not provide identifiable data to third-parties
   without explicit consent from clients (we take the stance here that
   simply using the resolution service itself does not constitute

   Even when consent is granted operators should employ data
   minimization techniques such as those described in Section 5.2.1 if
   data is shared with third-parties.

   Operators should consider including specific guidelines for the
   collection of aggregated and/or anonymized data for research
   purposes, within or outside of their own organization.  See SURFnet's
   policy [13] on data sharing for research as an example.

   TODO: More on data for research vs operations... how to still
   motivate operators to share anonymized data?

   TODO: Guidelines for when consent is granted?

   TODO: Applies to server data handling too.. could operators offer
   alternatives services one that implies consent for data processing,
   one that doesn't?

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6.  DNS privacy policy and practice statement

6.1.  Recommended contents of a DPPPS

6.1.1.  Policy

   1.  Make an explicit statement that IP addressses are treated as PII

   2.  State if IP addresses are being logged

   3.  Specify clearly what data (including whether it is aggregated,
       pseudonymized or anonymized and the conditions of data transfer)

       *  Collected and retained by the operator, and for what period it
          is retained

       *  Shared with partners

       *  Shared, sold or rented to third-parties

   4.  Specify any exceptions to the above, for example technically
       malicious or anomalous behavior

   5.  Declare any partners, third-party affiliations or sources of

   6.  Whether user DNS data is correlated or combined with any other
       personal information held by the operator

   7.  Result filtering.  This section should explain whether the
       operator filters, edits or alters in any way the replies that it
       receives from the authoritative servers for each DNS zone, before
       forwarding them to the clients.  For each category listed below,
       the operator should also specify how the filtering lists are
       created and managed, whether it employs any third-party sources
       for such lists, and which ones.

       *  Specify if any replies are being filtered out or altered for
          network and computer security reasons (e.g. preventing
          connections to malware-spreading websites or botnet control

       *  Specify if any replies are being filtered out or altered for
          mandatory legal reasons, due to applicable legislation or
          binding orders by courts and other public authorities

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       *  Specify if any replies are being filtered out or altered for
          voluntary legal reasons, due to an internal policy by the
          operator aiming at reducing potential legal risks

       *  Specify if any replies are being filtered out or altered for
          any other reason, including commercial ones

6.1.2.  Practice

   This section should explain the current operational practices of the

   1.  Specify any temporary or permanent deviations from the policy for
       operational reasons

   2.  With reference to section Section 5 provide specific details of
       which capabilities are provided on which client facing addresses
       and ports

   3.  Specify the authentication name to be used (if any) and if TLSA
       records are published (including options used in the TLSA

   4.  Specify the SPKI pinsets to be used (if any) and policy for
       rolling keys

   5.  Provide contact/support information for the service

   6.  Jurisdiction.  This section should communicate the applicable
       jurisdictions and law enforcement regimes under which the service
       is being provided.

       *  Specify the entity or entities that will control the data and
          be responsible for their treatment, and their legal place of

       *  Specify, either directly or by pointing to the applicable
          privacy policy, the relevant privacy laws that apply to the
          treatment of the data, the rights that users enjoy in regard
          to their own personal information that is treated by the
          service, and how they can contact the operator to enforce them

       *  Specify the countries in which the servers handling the DNS
          requests and the data are located (if the operator applies a
          geolocation policy so that requests from certain countries are
          only served by certain servers, this should be specified as

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       *  Specify whether the operator has any agreement in place with
          law enforcement agencies, or other public and private parties
          dealing with security and intelligence, to give them access to
          the servers and/or to the data

   7.  Describe how consent is obtained from the user of the DNS privacy
       service differentiating

       *  Uninformed users for whom this trust relationship is implicit

       *  Privacy-conscious users, that make an explicit trust choice

       (this may prove relevant in the context of e.g. the GDPR as it
       relates to consent)

6.2.  Current policy and privacy statements

   A tabular comparison of existing policy and privacy statements from
   various DNS Privacy service operators based on the proposed DPPPS
   structure can be found on [14].

   We note that the existing set of policies vary widely in style,
   content and detail and it is not uncommon for the full text for a
   given operator to equate to more than 10 pages of moderate font sized
   A4 text.  It is a non-trivial task today for a user to extract a
   meaningful overview of the different services on offer.

6.3.  Enforcement/accountability

   Transparency reports may help with building user trust that operators
   adhere to their policies and practices.

   Independent monitoring or analysis could be performed where possible

   o  ECS, QNAME minimization, EDNS(0) padding, etc.

   o  Filtering

   o  Uptime

   This is by analogy with e.g. several TLS or website analysis tools
   that are currently available e.g.  SSL Labs [15] or [16].

   Additionally operators could choose to engage the services of a third
   party auditor to verify their compliance with their published DPPPS.

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7.  IANA considerations


8.  Security considerations

   Security considerations for DNS-over-TCP are given in [RFC7766], many
   of which are generally applicable to session based DNS.

   TODO: e.g.  New issues for DoS defence, server admin policies

9.  Acknowledgements

   Many thanks to Amelia Andersdotter for a very thorough review of the
   first draft of this document.  Thanks to John Todd for discussions on
   this topic, and to Stephane Bortzmeyer, Puneet Sood and Vittorio
   Bertola for review.  Thanks to Daniel Kahn Gillmor, Barry Green, Paul
   Hoffman, Dan York, John Reed, Lorenzo Colitti for comments at the
   mic.  Thanks to Loganaden Velvindron for useful updates to the text.

   Sara Dickinson thanks the Open Technology Fund for a grant to support
   the work on this document.

10.  Contributors

   The below individuals contributed significantly to the document:

   John Dickinson
   Sinodun Internet Technologies
   Magdalen Centre
   Oxford Science Park
   Oxford OX4 4GA
   United Kingdom

   Jim Hague
   Sinodun Internet Technologies
   Magdalen Centre
   Oxford Science Park
   Oxford OX4 4GA
   United Kingdom

11.  Changelog


   o  Change 'open resolver' for 'public resolver'

   o  Minor editorial changes

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   o  Remove recommendation to run a separate TLS 1.3 service

   o  Move TLSA to purely a optimisation in Section 5.2.1

   o  Update reference on minimal DoH headers.

   o  Add reference on user switching provider after service issues in
      Section 5.1.4

   o  Add text in Section 5.1.6 on impact on operators.

   o  Add text on additional threat to TLS proxy use (Section 5.1.7)

   o  Add reference in Section 5.3.1 on example policies.


   o  Many minor editorial fixes

   o  Update DoH reference to RFC8484 and add more text on DoH

   o  Split threat descriptions into ones directly referencing RFC6973
      and other DNS Privacy threats

   o  Improve threat descriptions throughout

   o  Remove reference to the DNSSEC TLS Chain Extension draft until new
      version submitted.

   o  Clarify use of whitelisting for ECS

   o  Re-structure the DPPPS, add Result filtering section.

   o  Remove the direct inclusion of privacy policy comparison, now just
      reference and an example of such work.

   o  Add an appendix briefly discussing DNSSEC

   o  Update affiliation of 1 author


   o  Initial commit of re-named document after adoption to replace

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12.  References

12.1.  Normative References

              Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
              Lemon, T., and T. Pusateri, "DNS Stateful Operations",
              draft-ietf-dnsop-session-signal-20 (work in progress),
              December 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011, <https://www.rfc-

   [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, <https://www.rfc-

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <>.

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,

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   [RFC7828]  Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", RFC 7828,
              DOI 10.17487/RFC7828, April 2016, <https://www.rfc-

   [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
              DOI 10.17487/RFC7830, May 2016, <https://www.rfc-

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <>.

   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016, <https://www.rfc-

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018, <https://www.rfc-

   [RFC8404]  Moriarty, K., Ed. and A. Morton, Ed., "Effects of
              Pervasive Encryption on Operators", RFC 8404,
              DOI 10.17487/RFC8404, July 2018, <https://www.rfc-

   [RFC8467]  Mayrhofer, A., "Padding Policies for Extension Mechanisms
              for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467,
              October 2018, <>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

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12.2.  Informative References

              Bellis, R., Dijk, P., and R. Gacogne, "DNS X-Proxied-For",
              draft-bellis-dnsop-xpf-04 (work in progress), March 2018.

              Bortzmeyer, S. and S. Dickinson, "DNS Privacy
              Considerations", draft-bortzmeyer-dprive-rfc7626-bis-02
              (work in progress), January 2019.

              Dickinson, J., Hague, J., Dickinson, S., Manderson, T.,
              and J. Bond, "C-DNS: A DNS Packet Capture Format", draft-
              ietf-dnsop-dns-capture-format-10 (work in progress),
              December 2018.

              Kristoff, J. and D. Wessels, "DNS Transport over TCP -
              Operational Requirements", draft-ietf-dnsop-dns-tcp-
              requirements-03 (work in progress), January 2019.

              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", draft-ietf-dnsop-terminology-bis-14 (work in
              progress), September 2018.

              Nottingham, M., "Building Protocols with HTTP", draft-
              ietf-httpbis-bcp56bis-08 (work in progress), November

   [pcap], "PCAP", 2016, <>.

              Shulman, H., "Pretty Bad Privacy: Pitfalls of DNS
              Encryption", 2014, <

   [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,

   [RFC6235]  Boschi, E. and B. Trammell, "IP Flow Anonymization
              Support", RFC 6235, DOI 10.17487/RFC6235, May 2011,

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   [RFC6841]  Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
              Framework for DNSSEC Policies and DNSSEC Practice
              Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,

   [RFC7457]  Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
              Known Attacks on Transport Layer Security (TLS) and
              Datagram TLS (DTLS)", RFC 7457, DOI 10.17487/RFC7457,
              February 2015, <>.

   [RFC7686]  Appelbaum, J. and A. Muffett, "The ".onion" Special-Use
              Domain Name", RFC 7686, DOI 10.17487/RFC7686, October
              2015, <>.

   [RFC7706]  Kumari, W. and P. Hoffman, "Decreasing Access Time to Root
              Servers by Running One on Loopback", RFC 7706,
              DOI 10.17487/RFC7706, November 2015, <https://www.rfc-

   [RFC8094]  Reddy, T., Wing, D., and P. Patil, "DNS over Datagram
              Transport Layer Security (DTLS)", RFC 8094,
              DOI 10.17487/RFC8094, February 2017, <https://www.rfc-

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <>.

12.3.  URIs









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Appendix A.  Documents

   This section provides an overview of some DNS privacy related
   documents, however, this is neither an exhaustive list nor a
   definitive statement on the characteristic of the document.

A.1.  Potential increases in DNS privacy

   These documents are limited in scope to communications between stub
   clients and recursive resolvers:

   o  'Specification for DNS over Transport Layer Security (TLS)'
      [RFC7858], referred to here as 'DNS-over-TLS'.

   o  'DNS over Datagram Transport Layer Security (DTLS)' [RFC8094],
      referred to here as 'DNS-over-DTLS'.  Note that this document has
      the Category of Experimental.

   o  'DNS Queries over HTTPS (DoH)' [RFC8484] referred to here as DoH.

   o  'Usage Profiles for DNS over TLS and DNS over DTLS' [RFC8310]

   o  'The EDNS(0) Padding Option' [RFC7830] and 'Padding Policy for
      EDNS(0)' [RFC8467]

   These documents apply to recursive to authoritative DNS but are
   relevant when considering the operation of a recursive server:

   o  'DNS Query Name minimization to Improve Privacy' [RFC7816]
      referred to here as 'QNAME minimization'

A.2.  Potential decreases in DNS privacy

   These documents relate to functionality that could provide increased
   tracking of user activity as a side effect:

   o  'Client Subnet in DNS Queries' [RFC7871]

   o  'Domain Name System (DNS) Cookies' [RFC7873])

   o  'Transport Layer Security (TLS) Session Resumption without Server-
      Side State' [RFC5077] referred to here as simply TLS session

   o  'A DNS Packet Capture Format' [I-D.ietf-dnsop-dns-capture-format]

   o  Passive DNS [I-D.ietf-dnsop-terminology-bis]

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   Note that depending on the specifics of the implementation [RFC8484]
   may also provide increased tracking.

A.3.  Related operational documents

   o  'DNS Transport over TCP - Implementation Requirements' [RFC7766]

   o  'Operational requirements for DNS-over-TCP'

   o  'The edns-tcp-keepalive EDNS0 Option' [RFC7828]

   o  'DNS Stateful Operations' [I-D.ietf-dnsop-session-signal]

Appendix B.  Encryption and DNSSEC

   The addition of encryption to DNS does not remove the need for DNSSEC
   [RFC4033] - they are independent and fully compatible protocols, each
   solving different problems.  The use of one does not diminish the
   need nor the usefulness of the other.

   All DNS privacy services SHOULD offer a DNS privacy service that
   performs DNSSEC validation.  In addition they SHOULD be able to
   provide the DNSSEC RRs to the client so that it can perform its own

   While the use of an authenticated and encrypted transport protects
   origin authentication and data integrity between a client and a DNS
   privacy service it provides no proof (for a non-validating client)
   that the data provided by the DNS privacy service was actually DNSSEC

Appendix C.  IP address techniques

   Data minimization methods may be categorized by the processing used
   and the properties of their outputs.  The following builds on the
   categorization employed in [RFC6235]:

   o  Format-preserving.  Normally when encrypting, the original data
      length and patterns in the data should be hidden from an attacker.
      Some applications of de-identification, such as network capture
      de-identification, require that the de-identified data is of the
      same form as the original data, to allow the data to be parsed in
      the same way as the original.

   o  Prefix preservation.  Values such as IP addresses and MAC
      addresses contain prefix information that can be valuable in
      analysis, e.g. manufacturer ID in MAC addresses, subnet in IP

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      addresses.  Prefix preservation ensures that prefixes are de-
      identified consistently; e.g. if two IP addresses are from the
      same subnet, a prefix preserving de-identification will ensure
      that their de-identified counterparts will also share a subnet.
      Prefix preservation may be fixed (i.e. based on a user selected
      prefix length identified in advance to be preserved ) or general.

   o  Replacement.  A one-to-one replacement of a field to a new value
      of the same type, for example using a regular expression.

   o  Filtering.  Removing (and thus truncating) or replacing data in a
      field.  Field data can be overwritten, often with zeros, either
      partially (grey marking) or completely (black marking).

   o  Generalization.  Data is replaced by more general data with
      reduced specificity.  One example would be to replace all TCP/UDP
      port numbers with one of two fixed values indicating whether the
      original port was ephemeral (>=1024) or non-ephemeral (>1024).
      Another example, precision degradation, reduces the accuracy of
      e.g. a numeric value or a timestamp.

   o  Enumeration.  With data from a well-ordered set, replace the first
      data item data using a random initial value and then allocate
      ordered values for subsequent data items.  When used with
      timestamp data, this preserves ordering but loses precision and

   o  Reordering/shuffling.  Preserving the original data, but
      rearranging its order, often in a random manner.

   o  Random substitution.  As replacement, but using randomly generated
      replacement values.

   o  Cryptographic permutation.  Using a permutation function, such as
      a hash function or cryptographic block cipher, to generate a
      replacement de-identified value.

C.1.  Google Analytics non-prefix filtering

   Since May 2010, Google Analytics has provided a facility [17] that
   allows website owners to request that all their users IP addresses
   are anonymized within Google Analytics processing.  This very basic
   anonymization simply sets to zero the least significant 8 bits of
   IPv4 addresses, and the least significant 80 bits of IPv6 addresses.
   The level of anonymization this produces is perhaps questionable.
   There are some analysis results [18] which suggest that the impact of
   this on reducing the accuracy of determining the user's location from

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   their IP address is less than might be hoped; the average discrepancy
   in identification of the user city for UK users is no more than 17%.

   Anonymization: Format-preserving, Filtering (grey marking).

C.2.  dnswasher

   Since 2006, PowerDNS have included a de-identification tool dnswasher
   [19] with their PowerDNS product.  This is a PCAP filter that
   performs a one-to-one mapping of end user IP addresses with an
   anonymized address.  A table of user IP addresses and their de-
   identified counterparts is kept; the first IPv4 user addresses is
   translated to, the second to and so on.  The de-
   identified address therefore depends on the order that addresses
   arrive in the input, and running over a large amount of data the
   address translation tables can grow to a significant size.

   Anonymization: Format-preserving, Enumeration.

C.3.  Prefix-preserving map

   Used in TCPdpriv [20], this algorithm stores a set of original and
   anonymised IP address pairs.  When a new IP address arrives, it is
   compared with previous addresses to determine the longest prefix
   match.  The new address is anonymized by using the same prefix, with
   the remainder of the address anonymized with a random value.  The use
   of a random value means that TCPdrpiv is not deterministic; different
   anonymized values will be generated on each run.  The need to store
   previous addresses means that TCPdpriv has significant and unbounded
   memory requirements, and because of the need to allocated anonymized
   addresses sequentially cannot be used in parallel processing.

   Anonymization: Format-preserving, prefix preservation (general).

C.4.  Cryptographic Prefix-Preserving Pseudonymisation

   Cryptographic prefix-preserving pseudonymisation was originally
   proposed as an improvement to the prefix-preserving map implemented
   in TCPdpriv, described in Xu et al. [21] and implemented in the
   Crypto-PAn tool [22].  Crypto-PAn is now frequently used as an
   acronym for the algorithm.  Initially it was described for IPv4
   addresses only; extension for IPv6 addresses was proposed in Harvan &
   Schoenwaelder [23] and implemented in snmpdump.  This uses a
   cryptographic algorithm rather than a random value, and thus
   pseudonymity is determined uniquely by the encryption key, and is
   deterministic.  It requires a separate AES encryption for each output
   bit, so has a non-trivial calculation overhead.  This can be

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   mitigated to some extent (for IPv4, at least) by pre-calculating
   results for some number of prefix bits.

   Pseudonymization: Format-preserving, prefix preservation (general).

C.5.  Top-hash Subtree-replicated Anonymisation

   Proposed in Ramaswamy & Wolf [24], Top-hash Subtree-replicated
   Anonymisation (TSA) originated in response to the requirement for
   faster processing than Crypto-PAn.  It used hashing for the most
   significant byte of an IPv4 address, and a pre-calculated binary tree
   structure for the remainder of the address.  To save memory space,
   replication is used within the tree structure, reducing the size of
   the pre-calculated structures to a few Mb for IPv4 addresses.
   Address pseudonymization is done via hash and table lookup, and so
   requires minimal computation.  However, due to the much increased
   address space for IPv6, TSA is not memory efficient for IPv6.

   Pseudonymization: Format-preserving, prefix preservation (general).

C.6.  ipcipher

   A recently-released proposal from PowerDNS [25], ipcipher [26] is a
   simple pseudonymization technique for IPv4 and IPv6 addresses.  IPv6
   addresses are encrypted directly with AES-128 using a key (which may
   be derived from a passphrase).  IPv4 addresses are similarly
   encrypted, but using a recently proposed encryption ipcrypt [27]
   suitable for 32bit block lengths.  However, the author of ipcrypt has
   since indicated [28] that it has low security, and further analysis
   has revealed it is vulnerable to attack.

   Pseudonymization: Format-preserving, cryptographic permutation.

C.7.  Bloom filters

   van Rijswijk-Deij et al. [29] have recently described work using
   Bloom filters to categorize query traffic and record the traffic as
   the state of multiple filters.  The goal of this work is to allow
   operators to identify so-called Indicators of Compromise (IOCs)
   originating from specific subnets without storing information about,
   or be able to monitor the DNS queries of an individual user.  By
   using a Bloom filter, it is possible to determine with a high
   probability if, for example, a particular query was made, but the set
   of queries made cannot be recovered from the filter.  Similarly, by
   mixing queries from a sufficient number of users in a single filter,
   it becomes practically impossible to determine if a particular user
   performed a particular query.  Large numbers of queries can be
   tracked in a memory-efficient way.  As filter status is stored, this

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   approach cannot be used to regenerate traffic, and so cannot be used
   with tools used to process live traffic.

   Anonymized: Generalization.

Authors' Addresses

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


   Benno J. Overeinder
   NLnet Labs
   Science Park 400
   Amsterdam  1098 XH
   The Netherlands


   Roland M. van Rijswijk-Deij
   NLnet Labs
   Science Park 400
   Amsterdam  1098 XH
   The Netherlands


   Allison Mankin


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