draft-ietf-dprive-bcp-op.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE rfc SYSTEM 'rfc2629.dtd' []>
<rfc ipr="trust200902" category="bcp" docName="draft-ietf-dprive-bcp-op-14">
<?rfc toc="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<?rfc private=""?>
<?rfc topblock="yes"?>
<?rfc comments="no"?>
<front>
<title abbrev="DNS Privacy Service Recommendations">Recommendations for DNS Privacy Service Operators</title>

<author initials="S." surname="Dickinson" fullname="Sara Dickinson">
<organization>Sinodun IT</organization>
<address>
<postal>
<street></street>
<street>Magdalen Centre</street>
<street>Oxford Science Park</street>
<city>Oxford</city>
<code>OX4 4GA</code>
<country>United Kingdom</country>
<region></region>
</postal>
<phone></phone>
<email>sara@sinodun.com</email>
<uri></uri>
</address>
</author>
<author initials="B." surname="Overeinder" fullname="Benno J. Overeinder">
<organization>NLnet Labs</organization>
<address>
<postal>
<street></street>
<street>Science Park 400</street>
<city>Amsterdam</city>
<code>1098 XH</code>
<country>The Netherlands</country>
<region></region>
</postal>
<phone></phone>
<email>benno@nlnetLabs.nl</email>
<uri></uri>
</address>
</author>
<author initials="R." surname="van Rijswijk-Deij" fullname="Roland M. van Rijswijk-Deij">
<organization>NLnet Labs</organization>
<address>
<postal>
<street></street>
<street>Science Park 400</street>
<city>Amsterdam</city>
<code>1098 XH</code>
<country>The Netherlands</country>
<region></region>
</postal>
<phone></phone>
<email>roland@nlnetLabs.nl</email>
<uri></uri>
</address>
</author>
<author initials="A." surname="Mankin" fullname="Allison Mankin">
<organization>Salesforce</organization>
<address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
<email>allison.mankin@gmail.com</email>
<uri></uri>
</address>
</author>
<date year="2020" month="July" day="13"/>

<area>Internet</area>
<workgroup>dprive</workgroup>
<keyword>DNS</keyword>


<abstract>
<t>This document presents operational, policy, and security considerations for DNS
recursive resolver 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.
</t>
<t>This document also presents a non-normative framework to assist writers of a Recursive
operator Privacy Statement (analogous to DNS Security Extensions (DNSSEC)
Policies and DNSSEC Practice Statements described in RFC6841).
</t>
</abstract>


</front>

<middle>

<section anchor="introduction" title="Introduction">
<t>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.
</t>
<t>In recent years there has also been an increase in the availability of &quot;public
resolvers&quot; <xref target="RFC8499"/> which users may prefer to use instead of the default
network resolver either because they offer a specific feature (e.g., good
reachability or encrypted transport) or because the network resolver lacks a
specific feature (e.g., strong privacy policy or unfiltered responses). These
public 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.
</t>
<t>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 (whether explicit or implicit) is assumed to exist between
each user and the operator of the resolver(s) used by that user. 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.
</t>
<t>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 resolver
operators to minimize such tracking as users move between networks are also
likely to play a role in user selection of resolvers.
</t>
<t>More recently the global legislative landscape with regard to personal data
collection, retention, and pseudonymization has seen significant activity.
Providing detailed practice advice about these areas to the operator is out of
scope, but <xref target="data-sharing"/> describes some mitigations of data sharing risk.
</t>
<t>This document has two main goals:
</t>
<t>
<list style="symbols">
<t>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.</t>
<t>To introduce the Recursive operator Privacy Statement (RPS) and present a
framework to assist writers of an RPS. An RPS is a
document that an operator should publish which outlines their operational
practices and commitments with regard to privacy, thereby providing a means
for clients to evaluate both the measurable and claimed privacy properties of
a given DNS privacy service. The framework identifies a set of elements and
specifies an outline order for them. This document does not, however, define a
particular privacy statement, nor does it seek to provide legal advice as to
the contents.</t>
</list>
</t>
<t>A desired operational impact is that all operators (both those providing
resolvers within networks and those operating large public services) can
demonstrate their commitment to user privacy thereby driving all DNS resolution
services to a more equitable footing. Choices for users would (in this ideal
world) be driven by other factors, e.g., differing security policies or minor
difference in operator policy, rather than gross disparities in privacy concerns.
</t>
<t>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 updates that apply to this document.
</t>
</section>

<section anchor="scope" title="Scope">
<t>&quot;DNS Privacy Considerations&quot; <xref target="RFC7626"/> 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 <xref target="RFC6973"/>. However
this 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). Choices that are made exclusively by
the end user, or those for operators of authoritative nameservers are out
of scope.
</t>
<t>This document includes (but is not limited to) considerations in the following
areas:
</t>
<t>
<list style="numbers">
<t>Data &quot;on the wire&quot; between a client and a server.</t>
<t>Data &quot;at rest&quot; on a server (e.g., in logs).</t>
<t>Data &quot;sent onwards&quot; from the server (either on the wire or shared with a
third party).</t>
</list>
</t>
<t>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
otherwise like to align with privacy best practice.
</t>
</section>

<section anchor="privacyrelated-documents" title="Privacy-related documents">
<t>There are various documents that describe protocol changes that have the
potential to either increase or decrease the privacy properties of the DNS in
various ways. 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 <xref target="documents"/> for reference.
</t>
</section>

<section anchor="terminology" title="Terminology">
<t>The key words &quot;MUST&quot;, &quot;MUST NOT&quot;, &quot;REQUIRED&quot;, &quot;SHALL&quot;, &quot;SHALL NOT&quot;, &quot;SHOULD&quot;,
&quot;SHOULD NOT&quot;, &quot;RECOMMENDED&quot;, &quot;NOT RECOMMENDED&quot;, &quot;MAY&quot;, and &quot;OPTIONAL&quot; in this
document are to be interpreted as described in BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.
</t>
<t>DNS terminology is as described in <xref target="RFC8499"/> with one modification: we restate
the clause in the original definition of Privacy-enabling DNS server in
<xref target="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 of <xref target="RFC8310"/> and
implement the (D)TLS profile described in Section 9 of <xref target="RFC8310"/>.
</t>
<t>Other Terms:
</t>
<t>
<list style="symbols">
<t>RPS: Recursive operator Privacy Statement, see
<xref target="recursive-operator-privacy-statement-rps"/>.</t>
<t>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 RPS.</t>
</list>
</t>
</section>

<section anchor="recommendations-for-dns-privacy-services" title="Recommendations for DNS privacy services">
<t>In the following sections we first outline the threats relevant to the specific topic and then discuss the potential actions that can be taken to mitigate them.
</t>
<t>We describe two classes of threats:
</t>
<t>
<list style="symbols">
<t>Threats described in <xref target="RFC6973"/> 'Privacy Considerations for Internet Protocols'
<list style="symbols">
<t>Privacy terminology, threats to privacy, and mitigations as described in
Sections 3, 5, and 6 of <xref target="RFC6973"/>.</t>
</list></t>
<t>DNS Privacy Threats
<list style="symbols">
<t>These are threats to the users and operators of DNS privacy services that
are not directly covered by <xref target="RFC6973"/>. These may be more operational in
nature such as certificate management or service availability issues.</t>
</list></t>
</list>
</t>
<t>We describe three classes of actions that operators of DNS privacy
services can take:
</t>
<t>
<list style="symbols">
<t>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.</t>
<t>Optimization of privacy services from an operational or management perspective.</t>
<t>Additional options that could further enhance the privacy and usability of the
service.</t>
</list>
</t>
<t>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:
</t>
<t>
<list style="symbols">
<t>Threat mitigations to be minimally compliant.</t>
<t>Optimizations to be moderately compliant.</t>
<t>Additional options to be maximally compliant.</t>
</list>
</t>
<t>The rest of this document does not use normative language but instead refers
only to the three differing classes of action which correspond to the three
named levels of compliance stated above. However, compliance (to the indicated
level) remains a normative requirement.
</t>

<section anchor="on-the-wire-between-client-and-server" title="On the wire between client and server">
<t>In this section we consider both data on the wire and the service provided to
the client.
</t>

<section anchor="transport-recommendations" title="Transport recommendations">
<t><xref target="RFC6973"/> Threats:
</t>
<t>
<list style="symbols">
<t>Surveillance:
<list style="symbols">
<t>Passive surveillance of traffic on the wire</t>
</list></t>
</list>
</t>
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Active injection of spurious data or traffic.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>A DNS privacy service can mitigate these threats by providing service over one
or more of the following transports
</t>
<t>
<list style="symbols">
<t>DNS over TLS (DoT) <xref target="RFC7858"/> and <xref target="RFC8310"/>.</t>
<t>DNS over HTTPS (DoH) <xref target="RFC8484"/>.</t>
</list>
</t>
<t>It is noted that a DNS privacy service can also be provided over DNS over DTLS
<xref target="RFC8094"/>, however this is an Experimental specification and there are no known
implementations at the time of writing.
</t>
<t>It is also noted that DNS privacy service might be provided over IPSec,
DNSCrypt, or VPNs. However, there are no specific RFCs that cover the use of
these transports for DNS and any discussion of best practice for providing such
a service is out of scope for this document.
</t>
<t>Whilst encryption of DNS traffic can protect against active injection on the
paths traversed by the encrypted connection this does not diminish the need for
DNSSEC, see <xref target="dnssec"/>.
</t>
</section>

<section anchor="authentication-of-dns-privacy-services" title="Authentication of DNS privacy services">
<t><xref target="RFC6973"/> Threats:
</t>
<t>
<list style="symbols">
<t>Surveillance:
<list style="symbols">
<t>Active attacks on client resolver configuration</t>
</list></t>
</list>
</t>
<t>Mitigations:
</t>
<t>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.
</t>
<t>When using DoT, clients that select a 'Strict Privacy' usage profile <xref target="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 need to provide credentials that will be accepted by the client's
trust model, in the form of either X.509 certificates <xref target="RFC5280"/> or Subject
Public Key Info (SPKI) pin sets <xref target="RFC8310"/>.
</t>
<t>When offering DoH <xref target="RFC8484"/>, HTTPS requires authentication of the server as
part of the protocol.
</t>
<t>Server operators should also follow the best practices with regard to
certificate revocation as described in <xref target="RFC7525"/>.
</t>

<section anchor="certificate-management" title="Certificate management">
<t>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 operators.
</t>
<t>It is noted that SPKI pin set management is described in <xref target="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.
</t>
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Invalid certificates, resulting in an unavailable service which might force a
user to fallback to cleartext.</t>
<t>Mis-identification of a server by a client e.g., typos in DoH URL templates
<xref target="RFC8484"/> or authentication domain names <xref target="RFC8310"/> which accidentally direct
clients to attacker controlled servers.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>It is recommended that operators:
</t>
<t>
<list style="symbols">
<t>Follow the guidance in Section 6.5 of <xref target="RFC7525"/> with regards to certificate
revocation.</t>
<t>Automate the generation, publication, and renewal of certificates. For example,
ACME <xref target="RFC8555"/> provides a mechanism to actively manage certificates through
automation and has been implemented by a number of certificate authorities.</t>
<t>Monitor certificates to prevent accidental expiration of certificates.</t>
<t>Choose a short, memorable authentication domain name for the service.</t>
</list>
</t>
</section>
</section>

<section anchor="protocol-recommendations" title="Protocol recommendations">

<section anchor="dot" title="DoT">
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Known attacks on TLS such as those described in <xref target="RFC7457"/>.</t>
<t>Traffic analysis, for example: <xref target="Pitfalls-of-DNS-Encryption"/>.</t>
<t>Potential for client tracking via transport identifiers.</t>
<t>Blocking of well known ports (e.g., 853 for DoT).</t>
</list>
</t>
<t>Mitigations:
</t>
<t>In the case of DoT, TLS profiles from Section 9 of <xref target="RFC8310"/> and the
Countermeasures to DNS Traffic Analysis from section 11.1 of <xref target="RFC8310"/>
provide strong mitigations. This includes but is not limited to:
</t>
<t>
<list style="symbols">
<t>Adhering to <xref target="RFC7525"/>.</t>
<t>Implementing only (D)TLS 1.2 or later as specified in <xref target="RFC8310"/>.</t>
<t>Implementing EDNS(0) Padding <xref target="RFC7830"/> using the guidelines in
<xref target="RFC8467"/> or a successor specification.</t>
<t>Servers should not degrade in any way the query service level provided to
clients that do not use any form of session resumption mechanism, such as TLS
session resumption <xref target="RFC5077"/> with TLS 1.2, section 2.2 of <xref target="RFC8446"/>, or Domain
Name System (DNS) Cookies <xref target="RFC7873"/>.</t>
<t>A DoT privacy service on both port 853 and 443. If the operator deploys DoH on the same IP address this requires the use of the 'dot' ALPN value <xref target="dot-ALPN"/>.</t>
</list>
</t>
<t>Optimizations:
</t>
<t>
<list style="symbols">
<t>Concurrent processing of pipelined queries, returning responses as soon as
available, potentially out of order as specified in <xref target="RFC7766"/>. This is often
called 'OOOR' - out-of-order responses (providing processing performance
similar to HTTP multiplexing).</t>
<t>Management of TLS connections to optimize performance for clients using
<xref target="RFC7766"/> and EDNS(0) Keepalive <xref target="RFC7828"/></t>
</list>
</t>
<t>Additional Options:
</t>
<t>Management of TLS connections to optimize performance for clients using DNS Stateful Operations <xref target="RFC8490"/>.
</t>
</section>

<section anchor="doh" title="DoH">
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Known attacks on TLS such as those described in <xref target="RFC7457"/>.</t>
<t>Traffic analysis, for example: <xref target="DNS-Privacy-not-so-private"/>.</t>
<t>Potential for client tracking via transport identifiers.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>
<list style="symbols">
<t>Clients must be able to forgo the use of HTTP Cookies <xref target="RFC6265"/> and still
use the service.</t>
<t>Use of HTTP/2 padding and/or EDNS(0) padding as described in Section 9 of
<xref target="RFC8484"/></t>
<t>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
<xref target="I-D.ietf-httpbis-bcp56bis"/>.)</t>
</list>
</t>
</section>
</section>

<section anchor="dnssec" title="DNSSEC">
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Users may be directed to bogus IP addresses which, depending on the
application, protocol and authentication method, might lead users to reveal
personal information to attackers. One example is a website that doesn't use
TLS or its TLS authentication can somehow be subverted.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>
<list style="symbols">
<t>All DNS privacy services must offer a DNS privacy service that performs Domain
Name System Security Extensions (DNSSEC) validation. In addition they must be
able to provide the DNSSEC RRs to the client so that it can perform its own
validation.</t>
</list>
</t>
<t>The addition of encryption to DNS does not remove the need for DNSSEC
<xref target="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.
</t>
<t>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 authenticated. As with cleartext DNS the
user is still solely trusting the AD bit (if present) set by the resolver.
</t>
<t>It should also be noted that the use of an encrypted transport for DNS actually
solves many of the practical issues encountered by DNS validating clients e.g.
interference by middleboxes with cleartext DNS payloads is completely avoided.
In this sense a validating client that uses a DNS privacy service which supports
DNSSEC has a far simpler task in terms of DNSSEC Roadblock avoidance <xref target="RFC8027"/>.
</t>
</section>

<section anchor="availability" title="Availability">
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>A failed DNS privacy service could force the user to switch providers,
fallback to cleartext or accept no DNS service for the outage.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>A DNS privacy service should strive to engineer encrypted services to the same
availability level as any unencrypted services they provide. 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 <xref target="Passive-Observations-of-a-Large-DNS"/>.
</t>
<t>Techniques such as those described in Section 10 of <xref target="RFC7766"/> can be of use to operators to defend against such attacks.
</t>
</section>

<section anchor="service-options" title="Service options">
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>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 outage.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>A DNS privacy service should deliver the same level of service as offered on
un-encrypted channels in terms of options such as filtering (or lack thereof),
DNSSEC validation, etc.
</t>
</section>

<section anchor="impact-of-encryption-on-monitoring-by-dns-privacy-service-operators" title="Impact of Encryption on Monitoring by DNS Privacy Service Operators">
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Increased use of encryption can impact DNS privacy service operator ability to
monitor traffic and therefore manage their DNS servers <xref target="RFC8404"/>.</t>
</list>
</t>
<t>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.
Many DNS privacy service operators still have need 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., <xref target="dnstap"/>.
</t>
<t>Optimization:
</t>
<t>When implementing alternative means for traffic monitoring, operators of a DNS
privacy service should consider using privacy conscious means to do so (see
section <xref target="data-at-rest-on-the-server"/> for more details on data handling and also
the discussion on the use of Bloom Filters in <xref target="ip-address-techniques"/>.
</t>
</section>

<section anchor="limitations-of-fronting-a-dns-privacy-service-with-a-pure-tls-proxy" title="Limitations of fronting a DNS privacy service with a pure TLS proxy">
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Limited ability to manage or monitor incoming connections using DNS specific
techniques.</t>
<t>Misconfiguration (e.g., of the target server address in the proxy
configuration) could lead to data leakage if the proxy to target server path
is not encrypted.</t>
</list>
</t>
<t>Optimization:
</t>
<t>Some operators may choose to implement DoT using a TLS proxy (e.g.
<xref target="nginx"/>, <xref target="haproxy"/>, or
<xref target="stunnel"/>) 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.
</t>
<t>Operators may choose to use a DNS aware proxy such as
<xref target="dnsdist"/> which offers custom options (similar to that
proposed in <xref target="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.
</t>
</section>
</section>

<section anchor="data-at-rest-on-the-server" title="Data at rest on the server">

<section anchor="data-handling" title="Data handling">
<t><xref target="RFC6973"/> Threats:
</t>
<t>
<list style="symbols">
<t>Surveillance.</t>
<t>Stored data compromise.</t>
<t>Correlation.</t>
<t>Identification.</t>
<t>Secondary use.</t>
<t>Disclosure.</t>
</list>
</t>
<t>Other Threats
</t>
<t>
<list style="symbols">
<t>Contravention of legal requirements not to process user data.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>The following are recommendations relating to common activities for DNS service
operators and in all cases data retention 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 <xref target="RFC6973"/> should be
applied.
</t>
<t>
<list style="symbols">
<t>Transient data (e.g., that is used for real time monitoring and threat analysis
which might be held only in memory) should be retained for the shortest
possible period deemed operationally feasible.</t>
<t>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.</t>
<t>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.</t>
<t>Data access should be minimized to only those personnel who require access to
perform operational duties. It should also be limited to anonymized or
pseudonymized data where operationally feasible, with access to full logs (if
any are held) only permitted when necessary.</t>
</list>
</t>
<t>Optimizations:
</t>
<t>
<list style="symbols">
<t>Consider use of full disk encryption for logs and data capture storage.</t>
</list>
</t>
</section>

<section anchor="data-minimization-of-network-traffic" title="Data minimization of network traffic">
<t>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 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.
</t>
<t>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 an
overview of current techniques to simply provide reference on the current
status of this work.
</t>
<t>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.
</t>
<t>Techniques employed may be broadly categorized as either anonymization or
pseudonymization. The following discussion uses the definitions from <xref target="RFC6973"/>
Section 3, with additional observations from <xref target="van-Dijkhuizen-et-al."/>
</t>
<t>
<list style="symbols">
<t>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.</t>
<t>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.</t>
</list>
</t>
<t>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 address.
</t>
</section>

<section anchor="ip-address-pseudonymization-and-anonymization-methods" title="IP address pseudonymization and anonymization methods">
<t>A major 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.
</t>
<t>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.
</t>
<t><xref target="ip-address-techniques"/> provides a more detailed survey of various techniques
employed or under development in 2019.
</t>
</section>

<section anchor="pseudonymization-anonymization-or-discarding-of-other-correlation-data" title="Pseudonymization, anonymization, or discarding of other correlation data">
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Fingerprinting of the client OS via various means including: IP TTL/Hoplimit,
TCP parameters (e.g., window size, ECN support, SACK), OS specific DNS query
patterns (e.g., for network connectivity, captive portal detection, or OS
specific updates).</t>
<t>Fingerprinting of the client application or TLS library by, e.g., HTTP headers
(e.g., User-Agent, Accept, Accept-Encoding), TLS version/Cipher suite
combinations, or other connection parameters.</t>
<t>Correlation of queries on multiple TCP sessions originating from the same IP
address.</t>
<t>Correlating of queries on multiple TLS sessions originating from the same
client, including via session resumption mechanisms.</t>
<t>Resolvers <spanx style="emph">might</spanx> receive client identifiers, e.g., MAC addresses in EDNS(0)
options - some Customer-premises equipment (CPE) devices are known to add them <xref target="MAC-address-EDNS"/>.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>
<list style="symbols">
<t>Data minimization or discarding of such correlation data.</t>
</list>
</t>
</section>

<section anchor="cache-snooping" title="Cache snooping">
<t><xref target="RFC6973"/> Threats:
</t>
<t>
<list style="symbols">
<t>Surveillance:
<list style="symbols">
<t>Profiling of client queries by malicious third parties.</t>
</list></t>
</list>
</t>
<t>Mitigations:
</t>
<t>
<list style="symbols">
<t>See <xref target="ISC-Knowledge-database-on-cache-snooping"/> for an example discussion on
defending against cache snooping. Options proposed include limiting access to
a server and limiting non-recursive queries.</t>
</list>
</t>
</section>
</section>

<section anchor="data-sent-onwards-from-the-server" title="Data sent onwards from the server">
<t>In this section we consider both data sent on the wire in upstream queries and
data shared with third parties.
</t>

<section anchor="protocol-recommendations-1" title="Protocol recommendations">
<t><xref target="RFC6973"/> Threats:
</t>
<t>
<list style="symbols">
<t>Surveillance:
<list style="symbols">
<t>Transmission of identifying data upstream.</t>
</list></t>
</list>
</t>
<t>Mitigations:
</t>
<t>As specified in <xref target="RFC8310"/> for DoT but applicable to any DNS Privacy
services the server should:
</t>
<t>
<list style="symbols">
<t>Implement QNAME minimization <xref target="RFC7816"/>.</t>
<t>Honor a SOURCE PREFIX-LENGTH set to 0 in a query containing the EDNS(0)
Client Subnet (ECS) option (<xref target="RFC7871"/> Section 7.1.2).</t>
</list>
</t>
<t>Optimizations:
</t>
<t>
<list style="symbols">
<t>As per Section 2 of <xref target="RFC7871"/> the server should either:
<list style="symbols">
<t>not use the ECS option in upstream queries at all, or</t>
<t>offer alternative services, one that sends ECS and one that does not.</t>
</list></t>
</list>
</t>
<t>If operators do offer a service that sends the ECS options upstream they should
use the shortest prefix that is operationally feasible and ideally
use a policy of allowlisting upstream servers to send ECS to in order to
reduce data leakage. Operators should make clear in any policy statement what
prefix length they actually send and the specific policy used.
</t>
<t>Allowlisting 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 allowlisting to incur significant operational overhead
compared to dynamic detection of ECS support on authoritative servers.
</t>
<t>Additional options:
</t>
<t>
<list style="symbols">
<t>Aggressive Use of DNSSEC-Validated Cache <xref target="RFC8198"/> and <xref target="RFC8020"/>
(NXDOMAIN: There Really Is Nothing Underneath) to reduce the number of queries
to authoritative servers to increase privacy.</t>
<t>Run a copy of the root zone on loopback <xref target="RFC8806"/> to avoid making queries to
the root servers that might leak information.</t>
</list>
</t>
</section>

<section anchor="client-query-obfuscation" title="Client query obfuscation">
<t>Additional options:
</t>
<t>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.
<xref target="Pitfalls-of-DNS-Encryption"/>. For example, a resolver with a very small
community of users risks exposing data in this way and ought to 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.
</t>
<t>At the time of writing there are no standardized or widely recognized techniques
to perform such obfuscation or bulk pre-fetches.
</t>
<t>Another technique that particularly small operators may consider is forwarding
local traffic to a larger resolver (with a privacy policy that aligns with their
own practices) over an encrypted protocol so that the upstream queries are
obfuscated among those of the large resolver.
</t>
</section>

<section anchor="data-sharing" title="Data sharing">
<t><xref target="RFC6973"/> Threats:
</t>
<t>
<list style="symbols">
<t>Surveillance.</t>
<t>Stored data compromise.</t>
<t>Correlation.</t>
<t>Identification.</t>
<t>Secondary use.</t>
<t>Disclosure.</t>
</list>
</t>
<t>DNS Privacy Threats:
</t>
<t>
<list style="symbols">
<t>Contravention of legal requirements not to process user data.</t>
</list>
</t>
<t>Mitigations:
</t>
<t>Operators should not share identifiable data with third-parties.
</t>
<t>If operators choose to share identifiable data with third-parties in specific
circumstance they should publish the terms under which data is shared.
</t>
<t>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. This can benefit not only the operator (through
inclusion in novel research) but also the wider Internet community. See the policy published by SURFnet <xref target="SURFnet-policy"/> on data sharing for research as
an example.
</t>
</section>
</section>
</section>

<section anchor="recursive-operator-privacy-statement-rps" title="Recursive operator Privacy Statement (RPS)">
<t>To be compliant with this Best Common Practices document, a DNS recursive
operator SHOULD publish a Recursive operator Privacy Statement (RPS). Adopting the
outline, and including the headings in the order provided, is a benefit to
persons comparing RPSs from multiple operators.
</t>
<t><xref target="current-policy-and-privacy-statements"/> provides a comparison of some existing
policy and privacy statements.
</t>

<section anchor="outline-of-an-rps" title="Outline of an RPS">
<t>The contents of <xref target="policy"/> and <xref target="practice"/> are non-normative, other than the
order of the headings. Material under each topic is present to assist the
operator developing their own RPS and:
</t>
<t>
<list style="symbols">
<t>Relates <spanx style="emph">only</spanx> to matters around to the technical operation of DNS privacy services, and not on any other matters.</t>
<t>Does not attempt to offer an exhaustive list for the contents of an RPS.</t>
<t>Is not intended to form the basis of any legal/compliance documentation.</t>
</list>
</t>
<t><xref target="example-rps"/> provides an example (also non-normative) of an RPS
statement for a specific operator scenario.
</t>

<section anchor="policy" title="Policy">
<t>
<list style="numbers">
<t>Treatment of IP addresses. Make an explicit statement that IP addresses are treated as personal data.</t>
<t>Data collection and sharing. Specify clearly what data (including IP addresses)
is:
<list style="symbols">
<t>Collected and retained by the operator, and for what period it is retained.</t>
<t>Shared with partners.</t>
<t>Shared, sold, or rented to third-parties.

<vspace/></t>
</list>
and in each case whether it is aggregated, pseudonymized, or anonymized and
the conditions of data transfer. Where possible provide details of the
techniques used for the above data minimizations.</t>
<t>Exceptions. Specify any exceptions to the above, for example, technically
malicious or anomalous behavior.</t>
<t>Associated entities. Declare and explicitly enumerate any partners,
third-party affiliations, or sources of funding.</t>
<t>Correlation. Whether user DNS data is correlated or combined with any other
personal information held by the operator.</t>
<t>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.
<list style="symbols">
<t>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 servers).</t>
<t>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.</t>
<t>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.</t>
<t>Specify if any replies are being filtered out or altered for any other
reason, including commercial ones.</t>
</list></t>
</list>
</t>
</section>

<section anchor="practice" title="Practice">
<t>[NOTE FOR RFC EDITOR: Please update this section to use letters for the
sub-bullet points instead of numbers. This was not done during review because
the markdown tool used to write the document did not support it.]
</t>
<t>Communicate the current operational practices of the service.
</t>
<t>
<list style="numbers">
<t>Deviations. Specify any temporary or permanent deviations from the policy for
operational reasons.</t>
<t>Client facing capabilities. With reference to each subsection of
<xref target="on-the-wire-between-client-and-server"/> provide specific details of which
capabilities (transport, DNSSEC, padding, etc.) are provided on which client
facing addresses/port combination or DoH URI template. For
<xref target="authentication-of-dns-privacy-services"/>, clearly specify which specific
authentication mechanisms are supported for each endpoint that offers DoT:
<list style="numbers">
<t>The authentication domain name to be used (if any).</t>
<t>The SPKI pin sets to be used (if any) and policy for rolling keys.


<vspace/></t>
</list></t>
<t>Upstream capabilities. With reference to section
<xref target="data-sent-onwards-from-the-server"/> provide specific details of which
capabilities are provided upstream for data sent to authoritative servers.</t>
<t>Support. Provide contact/support information for the service.</t>
<t>Data Processing. This section can optionally communicate links to and the
high level contents of any separate statements the operator has published which
cover applicable data processing legislation or agreements with regard to the
location(s) of service provision.</t>
</list>
</t>
</section>
</section>

<section anchor="enforcementaccountability" title="Enforcement/accountability">
<t>Transparency reports may help with building user trust that operators adhere to
their policies and practices.
</t>
<t>Independent monitoring or analysis could be performed where possible of:
</t>
<t>
<list style="symbols">
<t>ECS, QNAME minimization, EDNS(0) padding, etc.</t>
<t>Filtering.</t>
<t>Uptime.</t>
</list>
</t>
<t>This is by analogy with several TLS or website analysis tools that are
currently available e.g., <xref target="SSL-Labs"/> or
<xref target="Internet.nl"/>.
</t>
<t>Additionally operators could choose to engage the services of a third party auditor to verify their compliance with their published RPS.
</t>
</section>
</section>

<section anchor="iana-considerations" title="IANA considerations">
<t>None
</t>
</section>

<section anchor="security-considerations" title="Security considerations">
<t>Security considerations for DNS over TCP are given in <xref target="RFC7766"/>, many of which
are generally applicable to session based DNS. Guidance on operational requirements for DNS over TCP are also available in [I-D.dnsop-dns-tcp-requirements]. Security considerations for DoT are given in [RFC7858] and <xref target="RFC8310"/>, those for DoH in [RFC8484].
</t>
<t>Security considerations for DNSSEC are given in <xref target="RFC4033"/>, <xref target="RFC4034"/> and <xref target="RFC4035"/>.
</t>
</section>

<section anchor="acknowledgements" title="Acknowledgements">
<t>Many thanks to Amelia Andersdotter for a very thorough review of the first draft
of this document and Stephen Farrell for a thorough review at WGLC and for
suggesting the inclusion of an example RPS. 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, Jon Reed, Lorenzo Colitti for comments at the mic. Thanks to
Loganaden Velvindron for useful updates to the text.
</t>
<t>Sara Dickinson thanks the Open Technology Fund for a grant to support the work
on this document.
</t>
</section>

<section anchor="contributors" title="Contributors">
<t>The below individuals contributed significantly to the document:
</t>
<t>John Dickinson
<vspace/>
Sinodun Internet Technologies
<vspace/>
Magdalen Centre
<vspace/>
Oxford Science Park
<vspace/>
Oxford OX4 4GA
<vspace/>
United Kingdom
</t>
<t>Jim Hague
<vspace/>
Sinodun Internet Technologies
<vspace/>
Magdalen Centre
<vspace/>
Oxford Science Park
<vspace/>
Oxford OX4 4GA
<vspace/>
United Kingdom
</t>
</section>

<section anchor="changelog" title="Changelog">
<t>draft-ietf-dprive-bcp-op-13
</t>
<t>
<list style="symbols">
<t>Minor edits</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-12
</t>
<t>
<list style="symbols">
<t>Change DROP to RPS throughout</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-11
</t>
<t>
<list style="symbols">
<t>Improve text around use of normative language</t>
<t>Fix section 5.1.3.2 bullets</t>
<t>Improve text in 6.1.2. item 2.</t>
<t>Rework text of 6.1.2. item 5 and update example DROP</t>
<t>Various editorial improvements</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-10
</t>
<t>
<list style="symbols">
<t>Remove direct references to draft-ietf-dprive-rfc7626-bis, instead have one
general reference RFC7626</t>
<t>Clarify that the DROP statement outline is non-normative and add some further
qualifications about content</t>
<t>Update wording on data sharing to remove explicit discussion of consent</t>
<t>Move table in section 5.2.3 to an appendix</t>
<t>Move section 6.2 to an appendix</t>
<t>Corrections to references, typos and editorial updates from initial IESG
comments.</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-09
</t>
<t>
<list style="symbols">
<t>Fix references so they match the correct section numbers in draft-ietf-dprive-rfc7626-bis-05</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-08
</t>
<t>
<list style="symbols">
<t>Address IETF Last call comments.</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-07
</t>
<t>
<list style="symbols">
<t>Editorial changes following AD review.</t>
<t>Change all URIs to Informational References.</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-06
</t>
<t>
<list style="symbols">
<t>Final minor changes from second WGLC.</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-05
</t>
<t>
<list style="symbols">
<t>Remove some text on consent:
<list style="symbols">
<t>Paragraph 2 in section 5.3.3</t>
<t>Item 6 in the DROP Practice statement (and example)</t>
</list></t>
<t>Remove .onion and TLSA options</t>
<t>Include ACME as a reference for certificate management</t>
<t>Update text on session resumption usage</t>
<t>Update section 5.2.4 on client fingerprinting</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-04
</t>
<t>
<list style="symbols">
<t>Change DPPPS to DROP (DNS Recursive Operator Privacy) statement</t>
<t>Update structure of DROP slightly</t>
<t>Add example DROP statement</t>
<t>Add text about restricting access to full logs</t>
<t>Move table in section 5.2.3 from SVG to inline table</t>
<t>Fix many editorial and reference nits</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-03
</t>
<t>
<list style="symbols">
<t>Add paragraph about operational impact</t>
<t>Move DNSSEC requirement out of the Appendix into main text as a privacy threat
that should be mitigated</t>
<t>Add TLS version/Cipher suite as tracking threat</t>
<t>Add reference to Mozilla TRR policy</t>
<t>Remove several TODOs and QUESTIONS.</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-02
</t>
<t>
<list style="symbols">
<t>Change 'open resolver' for 'public resolver'</t>
<t>Minor editorial changes</t>
<t>Remove recommendation to run a separate TLS 1.3 service</t>
<t>Move TLSA to purely a optimization in Section 5.2.1</t>
<t>Update reference on minimal DoH headers.</t>
<t>Add reference on user switching provider after service issues in Section 5.1.4</t>
<t>Add text in Section 5.1.6 on impact on operators.</t>
<t>Add text on additional threat to TLS proxy use (Section 5.1.7)</t>
<t>Add reference in Section 5.3.1 on example policies.</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-01
</t>
<t>
<list style="symbols">
<t>Many minor editorial fixes</t>
<t>Update DoH reference to RFC8484 and add more text on DoH</t>
<t>Split threat descriptions into ones directly referencing RFC6973 and other DNS Privacy threats</t>
<t>Improve threat descriptions throughout</t>
<t>Remove reference to the DNSSEC TLS Chain Extension draft until new version submitted.</t>
<t>Clarify use of allowlisting for ECS</t>
<t>Re-structure the DPPPS, add Result filtering section.</t>
<t>Remove the direct inclusion of privacy policy comparison, now just reference dnsprivacy.org and an example of such work.</t>
<t>Add an appendix briefly discussing DNSSEC</t>
<t>Update affiliation of 1 author</t>
</list>
</t>
<t>draft-ietf-dprive-bcp-op-00
</t>
<t>
<list style="symbols">
<t>Initial commit of re-named document after adoption to replace
draft-dickinson-dprive-bcp-op-01</t>
</list>
</t>
<t>





















</t>
</section>

</middle>
<back>
<references title="Normative References">
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4033.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5280.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6973.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7457.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7525.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7766.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7816.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7828.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7830.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7858.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7871.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8020.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8198.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8310.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8467.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8484.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8490.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8499.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8806.xml"?>
</references>
<references title="Informative References">
<reference anchor='Bloom-filter'
           target='http://dl.ifip.org/db/conf/im/im2019/189282.pdf'>
    <front>
        <title>Privacy-Conscious Threat Intelligence Using DNSBLOOM</title>
          <author initials='R.' surname='van Rijswijk-Deij'> </author>
          <author initials='G.' surname='Rijnders'> </author>
          <author initials='M.' surname='Bomhoff'> </author>
          <author initials='L.' surname='Allodi'> </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='Brenker-and-Arnes'
           target='https://pdfs.semanticscholar.org/7b34/12c951cebe71cd2cddac5fda164fb2138a44.pdf'>
    <front>
        <title>CIRCUMVENTING IP-ADDRESS PSEUDONYMIZATION</title>
          <author initials='T.' surname='Brekne'> </author>
          <author initials='A.' surname='Arnes'> </author>
        <date year='2005'/>
    </front>
</reference>
<reference anchor='Crypto-PAn' target='https://github.com/CESNET/ipfixcol/tree/master/base/src/intermediate/anonymization/Crypto-PAn'>
    <front>
        <title>Crypto-PAn</title>
        <author>
            <organization>CESNET</organization>
        </author>
        <date year='2015'/>
    </front>
</reference>
<reference anchor='DNS-Privacy-not-so-private'
 target='https://petsymposium.org/2018/files/hotpets/4-siby.pdf'>
    <front>
        <title>DNS Privacy not so private: the traffic analysis perspective.</title>
        <author initials='S.' surname='Silby'> </author>
        <author initials='M.' surname='Juarez'> </author>
        <author initials='N.' surname='Vallina-Rodriguez'> </author>
        <author initials='C.' surname='Troncosol'> </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='DoH-resolver-policy' 
   target='https://wiki.mozilla.org/Security/DOH-resolver-policy'>
    <front>
        <title>Security/DOH-resolver-policy</title>
        <author>
            <organization>Mozilla</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='Geolocation-Impact-Assessement' 
          target='https://support.google.com/analytics/answer/2763052?hl=en'>
    <front>
        <title>Anonymize IP Geolocation Accuracy Impact Assessment</title>
        <author>
            <organization>Conversion Works</organization>
        </author>
        <date year='2017'/>
    </front>
</reference>
<reference anchor='Harvan'
           target='http://mharvan.net/talks/noms-ip_anon.pdf'>
    <front>
        <title>Prefix- and Lexicographical-order-preserving IP Address Anonymization</title>
          <author initials='M.' surname='Harvan'> </author>
        <date year='2006'/>
    </front>
</reference>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.bellis-dnsop-xpf.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-dnsop-dns-tcp-requirements.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-httpbis-bcp56bis.xml"?>
<reference anchor='IP-Anonymization-in-Analytics' target='https://support.google.com/analytics/answer/2763052?hl=en'>
    <front>
        <title>IP Anonymization in Analytics</title>
        <author>
            <organization>Google</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='ISC-Knowledge-database-on-cache-snooping' 
   target='https://kb.isc.org/docs/aa-00482'>
    <front>
        <title>DNS Cache snooping - should I be concerned?</title>
        <author>
            <organization>ISC Knowledge Database</organization>
        </author>
        <date year='2018'/>
    </front>
</reference>
<reference anchor='Internet.nl' target='https://internet.nl'>
    <front>
        <title>Internet.nl Is Your Internet Up To Date?</title>
        <author>
            <organization>Internet.nl</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='MAC-address-EDNS' target='https://lists.dns-oarc.net/pipermail/dns-operations/2016-January/014143.html'>
    <front>
        <title>Embedding MAC address in DNS requests for selective filtering IDs</title>
        <author>
           <organization>DNS-OARC mailing list</organization>
        </author>
        <date year='2016'/>
    </front>
</reference>
<reference anchor='Passive-Observations-of-a-Large-DNS'
           target='http://tma.ifip.org/2018/wp-content/uploads/sites/3/2018/06/tma2018_paper30.pdf'>
    <front>
        <title>Passive Observations of a Large DNS Service: 2.5 Years in the Life of Google</title>
          <author initials='W. B.' surname='de Vries'> </author>
          <author initials='R.' surname='van Rijswijk-Deij'> </author>
          <author initials='P.' surname='de Boer'> </author>
          <author initials='A.' surname='Pras'> </author>
        <date year='2018'/>
    </front>
</reference>
<reference anchor='Pitfalls-of-DNS-Encryption'
 target='https://dl.acm.org/citation.cfm?id=2665959'>
    <front>
        <title>Pretty Bad Privacy: Pitfalls of DNS Encryption</title>
        <author initials='H.' surname='Shulman' fullname='Haya Shulman'>
            <organization>Fachbereich Informatik, Technische Universität Darmstadt</organization>
        </author>
        <date year='2014'/>
    </front>
</reference>
<reference anchor='PowerDNS-dnswasher' target='https://github.com/PowerDNS/pdns/blob/master/pdns/dnswasher.cc'>
    <front>
        <title>dnswasher</title>
        <author>
            <organization>PowerDNS</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4034.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4035.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5077.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6235.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6265.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7626.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7873.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8027.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8094.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8404.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8446.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8555.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8618.xml"?>
<reference anchor='Ramaswamy-and-Wolf'
           target='http://www.ecs.umass.edu/ece/wolf/pubs/ton2007.pdf'>
    <front>
        <title>High-Speed Prefix-Preserving IP Address Anonymization for Passive Measurement Systems</title>
          <author initials='R.' surname='Ramaswamy'> </author>
          <author initials='T.' surname='Wolf'> </author>
        <date year='2007'/>
    </front>
</reference>
<reference anchor='SSL-Labs' target='https://www.ssllabs.com/ssltest/'>
    <front>
        <title>SSL Server Test</title>
        <author>
            <organization>SSL Labs</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='SURFnet-policy' 
   target='https://surf.nl/datasharing'>
    <front>
        <title>SURFnet Data Sharing Policy</title>
        <author>
            <organization>SURFnet</organization>
        </author>
        <date year='2016'/>
    </front>
</reference>
<reference anchor='TCPdpriv' target='http://ita.ee.lbl.gov/html/contrib/tcpdpriv.html'>
    <front>
        <title>TCPdpriv</title>
        <author>
            <organization>Ipsilon Networks, Inc.</organization>
        </author>
        <date year='2005'/>
    </front>
</reference>
<reference anchor='Xu-et-al.'
           target='http://an.kaist.ac.kr/~sbmoon/paper/intl-journal/2004-cn-anon.pdf'>
    <front>
        <title>Prefix-preserving IP address anonymization: measurement-based security evaluation and a new cryptography-based scheme</title>
          <author initials='J.' surname='Fan'> </author>
          <author initials='J.' surname='Xu'> </author>
          <author initials='M. H.' surname='Ammar'> </author>
          <author initials='S. B.' surname='Moon'> </author>
        <date year='2004'/>
    </front>
</reference>
<reference anchor='dnsdist' target='https://dnsdist.org'>
    <front>
        <title>dnsdist Overview</title>
        <author>
            <organization>PowerDNS</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='dnstap' target='http://dnstap.info'>
    <front>
        <title>DNSTAP</title>
        <author>
            <organization>dnstap.info</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='dot-ALPN'
target='https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids'>
<front>
<title>TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs</title>
<author>
 <organization>IANA (iana.org)</organization>
</author>
<date year='2020'/>
</front>
</reference>
<reference anchor='haproxy' target='https://www.haproxy.org/'>
    <front>
        <title>HAPROXY</title>
        <author>
            <organization>haproxy.org</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='ipcipher1'
           target='https://medium.com/@bert.hubert/on-ip-address-encryption-security-analysis-with-respect-for-privacy-dabe1201b476'>
    <front>
        <title>On IP address encryption: security analysis with respect for privacy</title>
          <author initials='B.' surname='Hubert'> </author>
        <date year='2017'/>
    </front>
</reference>
<reference anchor='ipcipher2'
           target='https://github.com/PowerDNS/ipcipher'>
    <front>
        <title>ipcipher</title>
        <author>
            <organization>PowerDNS</organization>
        </author>
        <date year='2017'/>
    </front>
</reference>
<reference anchor='ipcrypt'
           target='https://github.com/veorq/ipcrypt'>
    <front>
        <title>ipcrypt: IP-format-preserving encryption</title>
        <author>
            <organization>veorq</organization>
        </author>
        <date year='2015'/>
    </front>
</reference>
<reference anchor='ipcrypt-analysis'
           target='https://www.ietf.org/mail-archive/web/cfrg/current/msg09494.html'>
    <front>
        <title>Analysis of ipcrypt?</title>
          <author initials='J.' surname='Aumasson'> </author>
        <date year='2018'/>
    </front>
</reference>
<reference anchor='nginx' target='https://nginx.org/'>
    <front>
        <title>NGINX</title>
        <author>
            <organization>nginx.org</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='pcap' target='http://www.tcpdump.org/'>
    <front>
        <title>PCAP</title>
        <author>
            <organization>tcpdump.org</organization>
        </author>
        <date year='2016'/>
    </front>
</reference>
<reference anchor='policy-comparison' 
   target='https://dnsprivacy.org/wiki/display/DP/Comparison+of+policy+and+privacy+statements+2019'>
    <front>
        <title>Comparison of policy and privacy statements 2019</title>
        <author>
            <organization>dnsprivacy.org</organization>
        </author>
        <date year='2019'/>
    </front>
</reference>
<reference anchor='stunnel' target='https://kb.isc.org/article/AA-01386/0/DNS-over-TLS.html'>
    <front>
        <title>DNS-over-TLS</title>
        <author>
            <organization>ISC Knowledge Database</organization>
        </author>
        <date year='2018'/>
    </front>
</reference>
<reference anchor='van-Dijkhuizen-et-al.'
           target='https://doi.org/10.1145/3182660'>
    <front>
        <title>A Survey of Network Traffic Anonymisation Techniques and Implementations</title>
          <author initials='N.' surname='Van Dijkhuizen '> </author>
          <author initials='J.' surname='Van Der Ham'> </author>
        <date year='2018'/>
    </front>
</reference>
</references>

<section anchor="documents" title="Documents">
<t>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.
</t>

<section anchor="potential-increases-in-dns-privacy" title="Potential increases in DNS privacy">
<t>These documents are limited in scope to communications between stub
clients and recursive resolvers:
</t>
<t>
<list style="symbols">
<t>'Specification for DNS over Transport Layer Security (TLS)' <xref target="RFC7858"/>.</t>
<t>'DNS over Datagram Transport Layer Security (DTLS)' <xref target="RFC8094"/>. Note that this
document has the Category of Experimental.</t>
<t>'DNS Queries over HTTPS (DoH)' <xref target="RFC8484"/>.</t>
<t>'Usage Profiles for DNS over TLS and DNS over DTLS' <xref target="RFC8310"/>.</t>
<t>'The EDNS(0) Padding Option' <xref target="RFC7830"/> and 'Padding Policy for EDNS(0)'
<xref target="RFC8467"/>.</t>
</list>
</t>
<t>These documents apply to recursive and authoritative DNS but are relevant when
considering the operation of a recursive server:
</t>
<t>
<list style="symbols">
<t>'DNS Query Name minimization to Improve Privacy' <xref target="RFC7816"/>.</t>
</list>
</t>
</section>

<section anchor="potential-decreases-in-dns-privacy" title="Potential decreases in DNS privacy">
<t>These documents relate to functionality that could provide increased tracking of
user activity as a side effect:
</t>
<t>
<list style="symbols">
<t>'Client Subnet in DNS Queries' <xref target="RFC7871"/>.</t>
<t>'Domain Name System (DNS) Cookies' <xref target="RFC7873"/>).</t>
<t>'Transport Layer Security (TLS) Session Resumption without Server-Side State'
<xref target="RFC5077"/> referred to here as simply TLS session resumption.</t>
<t><xref target="RFC8446"/> Appendix C.4 describes Client Tracking Prevention in TLS 1.3</t>
<t>'A DNS Packet Capture Format' <xref target="RFC8618"/>.</t>
<t>Passive DNS <xref target="RFC8499"/>.</t>
<t>Section 8 of <xref target="RFC8484"/> outlines the privacy considerations of DoH. Note that
(while that document advises exposing the minimal set of data needed to
achieve the desired feature set) depending on the specifics of a DoH
implementation there may be increased identification and tracking compared to
other DNS transports.</t>
</list>
</t>
</section>

<section anchor="related-operational-documents" title="Related operational documents">
<t>
<list style="symbols">
<t>'DNS Transport over TCP - Implementation Requirements' <xref target="RFC7766"/>.</t>
<t>'Operational requirements for DNS over TCP'
<xref target="I-D.ietf-dnsop-dns-tcp-requirements"/>.</t>
<t>'The edns-tcp-keepalive EDNS0 Option' <xref target="RFC7828"/>.</t>
<t>'DNS Stateful Operations' <xref target="RFC8490"/>.</t>
</list>
</t>
</section>
</section>

<section anchor="ip-address-techniques" title="IP address techniques">
<t>The following table presents a high level comparison of various techniques
employed or under development in 2019, and classifies them according to
categorization of technique and other properties. Both the specific techniques
and the categorisations are described in more detail in the following sections.
The list of techniques includes the main techniques in current use, but does not
claim to be comprehensive.
</t>
<texttable title="Table 1: Classification of techniques
">
<ttcol align="left">Categorization/Property</ttcol>
<ttcol align="center">GA</ttcol>
<ttcol align="center">d</ttcol>
<ttcol align="center">TC</ttcol>
<ttcol align="center">C</ttcol>
<ttcol align="center">TS</ttcol>
<ttcol align="center">i</ttcol>
<ttcol align="center">B</ttcol>

<c>Anonymization</c><c>X</c><c>X</c><c>X</c><c></c><c></c><c></c><c>X</c>
<c>Pseudoanonymization</c><c></c><c></c><c></c><c>X</c><c>X</c><c>X</c><c></c>
<c>Format preserving</c><c>X</c><c>X</c><c>X</c><c>X</c><c>X</c><c>X</c><c></c>
<c>Prefix preserving</c><c></c><c></c><c>X</c><c>X</c><c>X</c><c></c><c></c>
<c>Replacement</c><c></c><c></c><c>X</c><c></c><c></c><c></c><c></c>
<c>Filtering</c><c>X</c><c></c><c></c><c></c><c></c><c></c><c></c>
<c>Generalization</c><c></c><c></c><c></c><c></c><c></c><c></c><c>X</c>
<c>Enumeration</c><c></c><c>X</c><c></c><c></c><c></c><c></c><c></c>
<c>Reordering/Shuffling</c><c></c><c></c><c>X</c><c></c><c></c><c></c><c></c>
<c>Random substitution</c><c></c><c></c><c>X</c><c></c><c></c><c></c><c></c>
<c>Cryptographic permutation</c><c></c><c></c><c></c><c>X</c><c>X</c><c>X</c><c></c>
<c>IPv6 issues</c><c></c><c></c><c></c><c></c><c>X</c><c></c><c></c>
<c>CPU intensive</c><c></c><c></c><c></c><c>X</c><c></c><c></c><c></c>
<c>Memory intensive</c><c></c><c></c><c>X</c><c></c><c></c><c></c><c></c>
<c>Security concerns</c><c></c><c></c><c></c><c></c><c></c><c>X</c><c></c>
</texttable>
<t>Legend of techniques: GA = Google Analytics, d = dnswasher, TC = TCPdpriv, C = CryptoPAn, TS = TSA, i = ipcipher, B = Bloom filter
</t>
<t>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.
</t>
<t>For example, a common goal is that distributed packet captures must be in an
existing data format such as PCAP <xref target="pcap"/> or C-DNS <xref target="RFC8618"/> 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.,
<xref target="Brenker-and-Arnes"/> have observed that, as the entropy in an IPv4 address is
limited, if an attacker can
</t>
<t>
<list style="symbols">
<t>ensure packets are captured by the target and</t>
<t>send forged traffic with arbitrary source and destination addresses to that target and</t>
<t>obtain a de-identified log of said traffic from that target</t>
</list>
</t>
<t>any format-preserving pseudonymization is vulnerable to an attack along the
lines of a cryptographic chosen plaintext attack.
</t>

<section anchor="categorization-of-techniques" title="Categorization of techniques">
<t>Data minimization methods may be categorized by the processing used and the
properties of their outputs. The following builds on the categorization
employed in <xref target="RFC6235"/>:
</t>
<t>
<list style="symbols">
<t>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.</t>
<t>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 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.</t>
<t>Replacement. A one-to-one replacement of a field to a new value of the same
type, for example, using a regular expression.</t>
<t>Filtering. Removing or replacing data in a field. Field
data can be overwritten, often with zeros, either partially (truncation or reverse truncation) or
completely (black-marker anonymization).</t>
<t>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
(&gt;=1024) or non-ephemeral (&gt;1024). Another example, precision degradation,
reduces the accuracy of e.g., a numeric value or a timestamp.</t>
<t>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 distance.</t>
<t>Reordering/shuffling. Preserving the original data, but rearranging its order,
often in a random manner.</t>
<t>Random substitution. As replacement, but using randomly generated replacement
values.</t>
<t>Cryptographic permutation. Using a permutation function, such as a hash
function or cryptographic block cipher, to generate a replacement
de-identified value.</t>
</list>
</t>
</section>

<section anchor="specific-techniques" title="Specific techniques">

<section anchor="google-analytics-nonprefix-filtering" title="Google Analytics non-prefix filtering">
<t>Since May 2010, Google Analytics has provided a facility <xref target="IP-Anonymization-in-Analytics"/> 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 <xref target="Geolocation-Impact-Assessement"/>
which suggest that the impact of
this on reducing the accuracy of determining the user's location from 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%.
</t>
<t>Anonymization: Format-preserving, Filtering (trucation).
</t>
</section>

<section anchor="dnswasher" title="dnswasher">
<t>Since 2006, PowerDNS have included a de-identification tool dnswasher
<xref target="PowerDNS-dnswasher"/> 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 0.0.0.1, the second to
0.0.0.2 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.
</t>
<t>Anonymization: Format-preserving, Enumeration.
</t>
</section>

<section anchor="prefixpreserving-map" title="Prefix-preserving map">
<t>Used in <xref target="TCPdpriv"/>,
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 TCPdpriv 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.
</t>
<t>Anonymization: Format-preserving, prefix preservation (general).
</t>
</section>

<section anchor="cryptographic-prefixpreserving-pseudonymization" title="Cryptographic Prefix-Preserving Pseudonymization">
<t>Cryptographic prefix-preserving pseudonymization was originally proposed as an
improvement to the prefix-preserving map implemented in TCPdpriv, described in
<xref target="Xu-et-al."/> and implemented in the <xref target="Crypto-PAn"/> tool.
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 <xref target="Harvan"/>. 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
mitigated to some extent (for IPv4, at least) by pre-calculating results for
some number of prefix bits.
</t>
<t>Pseudonymization: Format-preserving, prefix preservation (general).
</t>
</section>

<section anchor="tophash-subtreereplicated-anonymization" title="Top-hash Subtree-replicated Anonymization">
<t>Proposed in <xref target="Ramaswamy-and-Wolf"/>,
Top-hash Subtree-replicated Anonymization (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.
</t>
<t>Pseudonymization: Format-preserving, prefix preservation (general).
</t>
</section>

<section anchor="ipcipher" title="ipcipher">
<t>A recently-released proposal from PowerDNS, ipcipher
<xref target="ipcipher1"/> <xref target="ipcipher2"/>  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 <xref target="ipcrypt"/> suitable for 32bit
block lengths. However, the author of ipcrypt has since indicated <xref target="ipcrypt-analysis"/> that it has
low security, and further analysis has revealed it is vulnerable to attack.
</t>
<t>Pseudonymization: Format-preserving, cryptographic permutation.
</t>
</section>

<section anchor="bloom-filters" title="Bloom filters">
<t>van Rijswijk-Deij et al.
have recently described work using Bloom filters <xref target="Bloom-filter"/> 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 approach cannot be used to regenerate traffic, and so cannot be
used with tools used to process live traffic.
</t>
<t>Anonymized: Generalization.
</t>
</section>
</section>
</section>

<section anchor="current-policy-and-privacy-statements" title="Current policy and privacy statements">
<t>A tabular comparison of policy and privacy statements from various DNS
Privacy service operators based loosely on the proposed RPS structure can
be found at <xref target="policy-comparison"/>. The analysis is based on the data
available in December 2019.
</t>
<t>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.
</t>
<t>It is also noted that Mozilla have published a DoH resolver policy
<xref target="DoH-resolver-policy"/>, which describes the minimum set of policy
requirements that a party must satisfy to be considered as a potential
partner for Mozilla’s Trusted Recursive Resolver (TRR) program.
</t>
</section>

<section anchor="example-rps" title="Example RPS">
<t>The following example RPS is very loosely based on some elements of
published privacy statements for some public resolvers, with additional fields
populated to illustrate the what the full contents of an RPS might
look like. This should not be interpreted as
</t>
<t>
<list style="symbols">
<t>having been reviewed or approved by any operator in any way</t>
<t>having any legal standing or validity at all</t>
<t>being complete or exhaustive</t>
</list>
</t>
<t>This is a purely hypothetical example of an RPS to outline example
contents - in this case for a public resolver operator providing a basic DNS
Privacy service via one IP address and one DoH URI with security based
filtering. It does aim to meet minimal compliance as specified in
<xref target="recommendations-for-dns-privacy-services"/>.
</t>

<section anchor="policy-1" title="Policy">
<t>
<list style="numbers">
<t>Treatment of IP addresses. Many nations classify IP addresses as personal
data, and we take a conservative approach in treating IP addresses as personal
data in all jurisdictions in which our systems reside.</t>
<t>Data collection and sharing.
<list style="numbers">
<t>IP addresses. Our normal course of data management does
not have any IP address information or other personal data logged to disk or
transmitted out of the location in which the query was received. We may
aggregate certain counters to larger network block levels for statistical
collection purposes, but those counters do not maintain specific IP address
data nor is the format or model of data stored capable of being
reverse-engineered to ascertain what specific IP addresses made what
queries.</t>
<t>Data collected in logs. We do keep some generalized location information
(at the city/metropolitan area level) so that we can conduct debugging and
analyze abuse phenomena. We also use the collected information for the
creation and sharing of telemetry (timestamp, geolocation, number of hits,
first seen, last seen) for contributors, public publishing of general
statistics of system use (protections, threat types, counts, etc.)
When you use our DNS Services, here is the full list of items that are included in our logs:
<list style="symbols">
<t>Request domain name, e.g., example.net</t>
<t>Record type of requested domain, e.g., A, AAAA, NS, MX, TXT, etc.</t>
<t>Transport protocol on which the request arrived, i.e. UDP, TCP, DoT,
<vspace/>
DoH</t>
<t>Origin IP general geolocation information: i.e. geocode, region ID,
city ID, and metro code</t>
<t>IP protocol version – IPv4 or IPv6</t>
<t>Response code sent, e.g., SUCCESS, SERVFAIL, NXDOMAIN, etc.</t>
<t>Absolute arrival time using a precision in ms</t>
<t>Name of the specific instance that processed this request</t>
<t>IP address of the specific instance to which this request was
addressed (no relation to the requestor’s IP address)</t>
</list>
We may keep the following data as summary information, including all the
above EXCEPT for data about the DNS record requested:
<list style="symbols">
<t>Currently-advertised BGP-summarized IP prefix/netmask of apparent
client origin</t>
<t>Autonomous system number (BGP ASN) of apparent client origin</t>
</list>
All the above data may be kept in full or partial form in permanent
archives.</t>
<t>Sharing of data.
Except as described in this document, we do not intentionally share,
sell, or rent individual personal information associated with the
requestor (i.e. source IP address or any other information that can
positively identify the client using our infrastructure) with anyone
without your consent.
We generate and share high level anonymized aggregate statistics
including threat metrics on threat type, geolocation, and if available,
sector, as well as other vertical metrics including performance metrics
on our DNS Services (i.e. number of threats blocked, infrastructure
uptime) when available with our threat intelligence (TI) partners,
academic researchers, or the public.
Our DNS Services share anonymized data on specific domains queried
(records such as domain, timestamp, geolocation, number of hits, first
seen, last seen) with our threat intelligence partners. Our DNS Services
also builds, stores, and may share certain DNS data streams which store
high level information about domain resolved, query types, result codes,
and timestamp. These streams do not contain IP address information of
requestor and cannot be correlated to IP address or other personal data.
We do not and never will share any of its data with marketers, nor will
it use this data for demographic analysis.</t>
</list></t>
<t>Exceptions. There are exceptions to this storage model: In the event of
actions or observed behaviors which we deem malicious or anomalous, we may
utilize more detailed logging to collect more specific IP address data in the
process of normal network defence and mitigation. This collection and
transmission off-site will be limited to IP addresses that we determine are
involved in the event.</t>
<t>Associated entities. Details of our Threat Intelligence partners can be found
at our website page (insert link).</t>
<t>Correlation of Data. We do not correlate or combine information from our logs
with any personal information that you have provided us for other services, or
with your specific IP address.</t>
<t>Result filtering.
<list style="numbers">
<t>Filtering. We utilise cyber threat intelligence about malicious domains
from a variety of public and private sources and blocks access to those
malicious domains when your system attempts to contact them. An NXDOMAIN is
returned for blocked sites.
<list style="numbers">
<t>Censorship. We will not provide a censoring component and will limit our
actions solely to the blocking of malicious domains around phishing,
malware, and exploit kit domains.</t>
<t>Accidental blocking. We implement allowlisting algorithms to make sure
legitimate domains are not blocked by accident. However, in the rare case of
blocking a legitimate domain, we work with the users to quickly allowlist
that domain. Please use our support form (insert link) if you believe we are
blocking a domain in error.</t>
</list></t>
</list></t>
</list>
</t>
</section>

<section anchor="practice-1" title="Practice">
<t>
<list style="numbers">
<t>Deviations from Policy. None in place since (insert date).</t>
<t>Client facing capabilities.
<list style="numbers">
<t>We offer UDP and TCP DNS on port 53 on (insert IP address)</t>
<t>We offer DNS over TLS as specified in RFC7858 on (insert IP address). It
is available on port 853 and port 443. We also implement RFC7766.
<list style="numbers">
<t>The DoT authentication domain name used is (insert domain name).</t>
<t>We do not publish SPKI pin sets.</t>
</list></t>
<t>We offer DNS over HTTPS as specified in RFC8484 on (insert URI template).</t>
<t>Both services offer TLS 1.2 and TLS 1.3.</t>
<t>Both services pad DNS responses according to RFC8467.</t>
<t>Both services provide DNSSEC validation.

<vspace/></t>
</list></t>
<t>Upstream capabilities.
<list style="numbers">
<t>Our servers implement QNAME minimization.</t>
<t>Our servers do not send ECS upstream.</t>
</list></t>
<t>Support. Support information for this service is available at (insert link).</t>
<t>Data Processing. We operate as the legal entity (insert entity) registered in
(insert country); as such we operate under (insert country/region) law. Our
separate statement regarding the specifics of our data processing policy,
practice, and agreements can be found here (insert link).</t>
</list>
</t>
</section>
</section>

</back>
</rfc>