wdiff rfc8932v1.txt rfc8932.txt

Internet Engineering Task Force (IETF) S. Dickinson
Request for Comments: 8932 Sinodun IT
BCP: 232 B. Overeinder
Category: Best Current Practice R. van Rijswijk-Deij
ISSN: 2070-1721 NLnet Labs
A. Mankin
Salesforce
October 2020

Recommendations for DNS Privacy Service Operators

Abstract

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, as
well as understanding how the those decisions and the alternatives impact
the privacy of users.

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 RFC 6841.

Status of This Memo

This memo documents an Internet Best Current Practice.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8932.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction
2. Scope
3. Privacy-Related Documents
4. Terminology
5. Recommendations for DNS Privacy Services
5.1. On the Wire between Client and Server
5.1.1. Transport Recommendations
5.1.2. Authentication of DNS Privacy Services
5.1.3. Protocol Recommendations
5.1.4. DNSSEC
5.1.5. Availability
5.1.6. Service Options
5.1.7. Impact of Encryption on Monitoring by DNS Privacy
Service Operators
5.1.8. Limitations of Fronting a DNS Privacy Service with a
Pure TLS Proxy
5.2. Data at Rest on the Server
5.2.1. Data Handling
5.2.2. Data Minimization of Network Traffic
5.2.3. IP Address Pseudonymization and Anonymization Methods
5.2.4. Pseudonymization, Anonymization, or Discarding of Other
Correlation Data
5.2.5. Cache Snooping
5.3. Data Sent Onwards from the Server
5.3.1. Protocol Recommendations
5.3.2. Client Query Obfuscation
5.3.3. Data Sharing
6. Recursive Operator Privacy Statement (RPS)
6.1. Outline of an RPS
6.1.1. Policy
6.1.2. Practice
6.2. Enforcement/Accountability
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Documents
A.1. Potential Increases in DNS Privacy
A.2. Potential Decreases in DNS Privacy
A.3. Related Operational Documents
Appendix B. IP Address Techniques
B.1. Categorization of Techniques
B.2. Specific Techniques
B.2.1. Google Analytics Non-Prefix Filtering
B.2.2. dnswasher
B.2.3. Prefix-Preserving Map
B.2.4. Cryptographic Prefix-Preserving Pseudonymization
B.2.5. Top-Hash Subtree-Replicated Anonymization
B.2.6. ipcipher
B.2.7. Bloom Filters
Appendix C. Current Policy and Privacy Statements
Appendix D. Example RPS
D.1. Policy
D.2. Practice
Acknowledgements
Contributors
Authors' Addresses

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 that aim to increase the privacy of the DNS,
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" [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.

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.

It should also be noted that there are both advantages and
disadvantages to a user choosing to configure a single resolver (or a
fixed set of resolvers) and an encrypted transport to use in all
network environments. 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 for
tracking 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.

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 Section 5.3.3 describes
some mitigations of data-sharing risk.

This document has two main goals:

* 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.

* 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 that 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 of an RPS.

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 differences in
operator policy -- rather than gross disparities in privacy concerns.

Community insight [or judgment?] (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.

2. Scope

"DNS Privacy Considerations" [RFC7626] describes the general privacy
issues and threats associated with the use of the DNS by Internet
users; much of the threat analysis here is lifted from that document
and [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.

This document includes (but is not limited to) considerations in the
following areas:

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 otherwise like to align with
privacy best practice.

3. Privacy-Related Documents

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 that 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 is listed in Appendix A for reference.

4. Terminology

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

DNS terminology is as described in [RFC8499], except with one modification:
we restate the clause in regard to
the original definition of privacy-enabling DNS server in [RFC8310] to include Section 6 of
[RFC8499]. In this document we use the requirement that full definition of a DNS over
(D)TLS privacy-enabling DNS server as given in [RFC8310], i.e., that
such a server should also offer at least one of the credentials
described in Section 8 of [RFC8310] and implement the (D)TLS profile
described in Section 9 of [RFC8310].

Other Terms:

RPS: Recursive operator Privacy Statement; see Section 6.

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.

5. Recommendations for DNS Privacy Services

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.

We describe two classes of threats:

* Threats described in [RFC6973], "Privacy Considerations for
Internet Protocols"

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

* 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:

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

* Optimization of privacy services from an operational or management
perspective.

* 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:

* Threat mitigations to be minimally compliant.

* Optimizations to be moderately compliant.

* Additional options to be maximally compliant.

The rest of this document does not use normative language but instead
refers only to the three differing classes of action that correspond
to the three named levels of compliance stated above. However,
compliance (to the indicated level) remains a normative requirement.

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:

Threats described in [RFC6973]:
Surveillance:
Passive surveillance of traffic on the wire.

DNS Privacy Threats:
Active injection of spurious data or traffic.

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

* DNS over TLS (DoT) [RFC7858] [RFC8310].

* DNS over HTTPS (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
DNSCrypt [DNSCrypt], 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.

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 Section 5.1.4.

5.1.2. Authentication of DNS Privacy Services

[RFC6973] Threats:

Threats described in [RFC6973]:
Surveillance:
Active attacks on client resolver configuration.

Mitigations:
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 DoT, 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 DoT need to provide
credentials that will be accepted by the client's trust model, in
the form of either X.509 certificates [RFC5280] or Subject Public
Key Info (SPKI) pin sets [RFC8310].

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

Server operators should also follow the best practices with regard
to certificate revocation, as described in [RFC7525].

5.1.2.1. 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
operators.

It is noted that SPKI pin-set pin set 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:
* Invalid certificates, resulting in an unavailable service,
which might force a user to fallback fall back to cleartext.

* Misidentification of a server by a client -- e.g., typos in DoH
URL templates [RFC8484] or authentication domain names
[RFC8310] that accidentally direct clients to attacker-
controlled servers.

Mitigations:
It is recommended that operators:

* Follow the guidance in Section 6.5 of [RFC7525] with regard to
certificate revocation.

* Automate the generation, publication, and renewal of
certificates. For example, Automatic Certificate Management
Environment (ACME) [RFC8555] provides a mechanism to actively
manage certificates through automation and has been implemented
by a number of certificate authorities.

* Monitor certificates to prevent accidental expiration of
certificates.

* Choose a short, memorable authentication domain name for the
service.

5.1.3. Protocol Recommendations

5.1.3.1. DoT

DNS Privacy Threats:
* Known attacks on TLS, such as those described in [RFC7457].

* Traffic analysis, for example: [Pitfalls-of-DNS-Encryption]. [Pitfalls-of-DNS-Encryption]
(focused on DoT).

* Potential for client tracking via transport identifiers.

* Blocking of well-known ports (e.g., 853 for DoT).

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

* Adhering to [RFC7525].

* Implementing only (D)TLS 1.2 or later, as specified in
[RFC8310].

* Implementing Extension Mechanisms for DNS (EDNS(0)) Padding
[RFC7830] using the guidelines in [RFC8467] or a successor
specification.

* 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 [RFC5077]
with TLS 1.2 (Section 2.2 of [RFC8446]) or Domain Name System
(DNS) Cookies [RFC7873].

* 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" Application-Layer Protocol Negotiation (ALPN)
value [dot-ALPN].

Optimizations:
* 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).

* Management of TLS connections to optimize performance for
clients using [RFC7766] and EDNS(0) Keepalive [RFC7828]

Additional Options:
Management of TLS connections to optimize performance for clients
using DNS Stateful Operations [RFC8490].

5.1.3.2. DoH

DNS Privacy Threats:
* Known attacks on TLS, such as those described in [RFC7457].

* Traffic analysis, for example: [DNS-Privacy-not-so-private]. [DNS-Privacy-not-so-private]
(focused on DoH).

* Potential for client tracking via transport identifiers.

Mitigations:
* Clients must be able to forgo the use of HTTP cookies [RFC6265]
and still use the service.

* Use of HTTP/2 padding and/or EDNS(0) padding, as described in
Section 9 of [RFC8484].

* 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 [BUILD-W-HTTP].)

5.1.4. DNSSEC

DNS Privacy Threats:
Users may be directed to bogus IP addresses that, 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 whose TLS authentication can
somehow be subverted.

Mitigations:
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
Resource Records (RRs) to the client so that it can perform its
own validation.

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.

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 nonvalidating client)
that the data provided by the DNS privacy service was actually DNSSEC
authenticated. As with cleartext, DNS cleartext DNS, the user is still solely
trusting the Authentic Data (AD) bit (if present) set by the
resolver.

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 that supports
DNSSEC has a far simpler task in terms of DNSSEC roadblock avoidance
[RFC8027].

5.1.5. Availability

DNS Privacy Threats:
A failed failing DNS privacy service could force the user to switch
providers, fallback fall back to cleartext, or accept no DNS service for
the duration of the outage.

Mitigations:
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 (DoS) 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].

Techniques such as those described in Section 10 of [RFC7766] can
be of use to operators to defend against such attacks.

5.1.6. Service Options

DNS Privacy Threats:
Unfairly disadvantaging users of the privacy service with respect
to the services available. This could force the user to switch
providers, fallback fall back to cleartext, or accept no DNS service for
the duration of the outage.

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

5.1.7. Impact of Encryption on Monitoring by DNS Privacy Service
Operators

DNS Privacy Threats:
Increased use of encryption can impact a DNS privacy service
operator's ability to monitor traffic and therefore manage their
DNS servers [RFC8404].

Many monitoring solutions for DNS traffic rely on the plaintext
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 need to inspect DNS traffic -- e.g.,
to monitor for network security threats. Operators may therefore
need to invest in an alternative means of monitoring that relies on
either the resolver software directly, or exporting DNS traffic from
the resolver using, for example, [dnstap].

Optimization:
When implementing alternative means for traffic monitoring,
operators of a DNS privacy service should consider using privacy-
conscious means to do so. See Section 5.2 for more details on
data handling and the discussion on the use of Bloom Filters in
Appendix B.

5.1.8. Limitations of Fronting a DNS Privacy Service with a Pure TLS
Proxy

DNS Privacy Threats:
* Limited ability to manage or monitor incoming connections using
DNS-specific techniques.

* 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.

Optimization:
Some operators may choose to implement DoT using a TLS proxy
(e.g., [nginx], [haproxy], or [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 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 Access Control Lists (ACLs),
Response Rate Limiting (RRL), or DNS64 [RFC6147] will be hard or
impossible to implement in the nameserver.

Operators may choose to use a DNS-aware proxy, such as [dnsdist],
that offers custom options (similar to those proposed in
[DNS-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:

Threats described in [RFC6973]:
* Surveillance.

* Stored-data compromise.

* Correlation.

* Identification.

* Secondary use.

* Disclosure.

Other Threats
* Contravention of legal requirements not to process user data.

Mitigations:
The following are recommendations relating to common activities
for DNS service operators; 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 [RFC6973]
should be applied.

* Transient data (e.g., data 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.

* The retention period of DNS traffic logs should be only as long
as is required to sustain operation of the service and meet
regulatory requirements, to the extent that they exist.

* 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.

* 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.

Optimizations:
* Consider use of full-disk encryption for logs and data-capture
storage.

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
can be used to link a record to an individual, but it may also
include 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 an 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 trade-offs 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].

* 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.

* 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, it is
difficult to categorize a deterministic algorithm for data
minimization of IP addresses that produces a group of pseudonyms for
a single given address.

5.2.3. IP Address Pseudonymization and Anonymization Methods

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.

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.

Appendix B provides a more detailed survey of various techniques
employed or under development in 2019. 2020.

5.2.4. Pseudonymization, Anonymization, or Discarding of Other
Correlation Data

DNS Privacy Threats:
* Fingerprinting of the client OS via various means, including:
IP TTL/Hoplimit, TCP parameters (e.g., window size, Explicit
Congestion Notification (ECN) support, selective acknowledgment
(SACK)), OS-specific DNS query patterns (e.g., for network
connectivity, captive portal detection, or OS-specific
updates).

* Fingerprinting of the client application or TLS library by, for
example, HTTP headers (e.g., User-Agent, Accept, Accept-
Encoding), TLS version/Cipher-suite combinations, or other
connection parameters.

* Correlation of queries on multiple TCP sessions originating
from the same IP address.

* Correlating of queries on multiple TLS sessions originating
from the same client, including via session-resumption
mechanisms.

* Resolvers _might_ receive client identifiers -- e.g., Media
Access Control (MAC) addresses in EDNS(0) options. Some
customer premises equipment (CPE) devices are known to add them
[MAC-address-EDNS].

Mitigations:
* Data minimization or discarding of such correlation data.

5.2.5. Cache Snooping

[RFC6973] Threats:

Threats described in [RFC6973]:
Surveillance:
Profiling of client queries by malicious third parties.

Mitigations:
See [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 nonrecursive
queries.

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:

Threats described in [RFC6973]:
Surveillance:
Transmission of identifying data upstream.

Mitigations:
As specified in [RFC8310] for DoT but applicable to any DNS
privacy services, the
The server should:

* implement QNAME minimization [RFC7816].

* honor a SOURCE PREFIX-LENGTH set to 0 in a query containing the
EDNS(0) Client Subnet (ECS) option ([RFC7871], Section 7.1.2).
This is as specified in [RFC8310] for DoT but applicable to any
DNS privacy service.

Optimizations:
As per Section 2 of [RFC7871], the server should either:

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

* 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
and ideally use a policy of allowlisting upstream servers to which to
send ECS 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.

Allowlisting has the benefit that not only does the operator know
which upstream servers can use ECS, but also the operator can 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.

Additional options:

* "Aggressive Use of DNSSEC-Validated Cache" [RFC8198] and
"NXDOMAIN: There Really Is Nothing Underneath" [RFC8020] to reduce
the number of queries to authoritative servers to increase
privacy.

* Run a local copy of the root zone on loopback [RFC8806] 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 to obfuscate this traffic by
mixing it with "generated" traffic to make client characterization
harder. The resolver could also employ aggressive prefetch
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 prefetches.

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.

5.3.3. Data Sharing

[RFC6973] Threats:

Threats described in [RFC6973]:
* Surveillance.

* Stored-data compromise.

* Correlation.

* Identification.

* Secondary use.

* Disclosure.

DNS Privacy Threats:
Contravention of legal requirements not to process user data.

Mitigations:
Operators should not share identifiable data with third parties.

If operators choose to share identifiable data with third parties
in specific circumstances, they should publish the terms under
which data is shared.

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 [SURFnet-policy] on data sharing for research
as an example.

6. Recursive Operator Privacy Statement (RPS)

To be compliant with this Best Current Practice 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.

Appendix C provides a comparison of some existing policy and privacy
statements.

6.1. Outline of an RPS

The contents of Sections 6.1.1 and 6.1.2 are non-normative, other
than the order of the headings. Material under each topic is present
to assist the operator developing their own RPS. This material:

* Relates _only_ to matters around the technical operation of DNS
privacy services, and no other matters.

* Does not attempt to offer an exhaustive list for the contents of
an RPS.

* Is not intended to form the basis of any legal/compliance
documentation.

Appendix D provides an example (also non-normative) of an RPS
statement for a specific operator scenario.

6.1.1. Policy

1. Treatment of IP addresses. Make an explicit statement that IP
addresses are treated as personal data.

2. Data collection and sharing. Specify clearly what data
(including IP addresses) is:

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

* Shared with partners.

* Shared, sold, or rented to third parties.

In each case, specify whether data is aggregated, pseudonymized,
or anonymized and the conditions of data transfer. Where
possible provide details of the techniques used for the above
data minimizations.

3. Exceptions. Specify any exceptions to the above -- for example,
technically malicious or anomalous behavior.

4. Associated entities. Declare and explicitly enumerate any
partners, third-party affiliations, or sources of funding.

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

6. 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
servers).

* 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.

* 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

Communicate the current operational practices of the service.

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

2. Client-facing capabilities. With reference to each subsection of
Section 5.1, provide specific details of which capabilities
(transport, DNSSEC, padding, etc.) are provided on which client-
facing addresses/port combination or DoH URI template. For
Section 5.1.2, clearly specify which specific authentication
mechanisms are supported for each endpoint that offers DoT:

a. The authentication domain name to be used (if any).

b. The SPKI pin sets to be used (if any) and policy for rolling
keys.

3. Upstream capabilities. With reference to Section 5.3, provide
specific details of which capabilities are provided upstream for
data sent to authoritative servers.

4. Support. Provide contact/support information for the service.

5. Data Processing. This section can optionally communicate links
to, and the high-level contents of, any separate statements the
operator has published that cover applicable data-processing
legislation or agreements with regard to the location(s) of
service provision.

6.2. Enforcement/Accountability

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

Where possible, independent monitoring or analysis could be performed
of:

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

* Filtering.

* Uptime.

This is by analogy with several TLS or website-analysis tools that
are currently available -- e.g., [SSL-Labs] or [Internet.nl].

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

7. IANA Considerations

This document has no IANA actions.

8. Security Considerations

Security considerations for DNS over TCP are given in [RFC7766], many
of which are generally applicable to session-based DNS. Guidance on
operational requirements for DNS over TCP are also available in
[DNS-OVER-TCP]. Security considerations for DoT are given in
[RFC7858] and [RFC8310], and those for DoH in [RFC8484].

Security considerations for DNSSEC are given in [RFC4033], [RFC4034],
and [RFC4035].

9. References

9.1. Normative References

[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-editor.org/info/rfc2119>.

[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,
<https://www.rfc-editor.org/info/rfc4033>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[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-editor.org/info/rfc6973>.

[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, <https://www.rfc-editor.org/info/rfc7457>.

[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, <https://www.rfc-editor.org/info/rfc7525>.

[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,
<https://www.rfc-editor.org/info/rfc7766>.

[RFC7816] Bortzmeyer, S., "DNS Query Name Minimisation to Improve
Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
<https://www.rfc-editor.org/info/rfc7816>.

[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-editor.org/info/rfc7828>.

[RFC7830] Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
DOI 10.17487/RFC7830, May 2016,
<https://www.rfc-editor.org/info/rfc7830>.

[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, <https://www.rfc-editor.org/info/rfc7858>.

[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-editor.org/info/rfc7871>.

[RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
November 2016, <https://www.rfc-editor.org/info/rfc8020>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
July 2017, <https://www.rfc-editor.org/info/rfc8198>.

[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-editor.org/info/rfc8310>.

[RFC8467] Mayrhofer, A., "Padding Policies for Extension Mechanisms
for DNS (EDNS(0))", RFC 8467, DOI 10.17487/RFC8467,
October 2018, <https://www.rfc-editor.org/info/rfc8467>.

[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.

[RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
RFC 8490, DOI 10.17487/RFC8490, March 2019,
<https://www.rfc-editor.org/info/rfc8490>.

[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.

[RFC8806] Kumari, W. and P. Hoffman, "Running a Root Server Local to
a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,
<https://www.rfc-editor.org/info/rfc8806>.

9.2. Informative References

[Bloom-filter]
van Rijswijk-Deij, R., Rijnders, G., Bomhoff, M., and L.
Allodi, "Privacy-Conscious Threat Intelligence Using
DNSBLOOM", IFIP/IEEE International Symposium on Integrated
Network Management (IM2019), 2019,
<http://dl.ifip.org/db/conf/im/im2019/189282.pdf>.

[Brekne-and-Arnes]
Brekne, T. and A. Årnes, "Circumventing IP-address
pseudonymization", Communications and Computer Networks,
2005, <https://pdfs.semanticscholar.org/7b34/12c951cebe71c
d2cddac5fda164fb2138a44.pdf>.

[BUILD-W-HTTP]
Nottingham, M., "Building Protocols with HTTP", Work in
Progress, Internet-Draft, draft-ietf-httpbis-bcp56bis-09,
1 November 2019, <https://tools.ietf.org/html/draft-ietf-
httpbis-bcp56bis-09>.

[Crypto-PAn]
CESNET, "Crypto-PAn", commit 636b237, March 2015,
<https://github.com/CESNET/ipfixcol/tree/master/base/src/
intermediate/anonymization/Crypto-PAn>.

[DNS-OVER-TCP]
Kristoff, J. and D. Wessels, "DNS Transport over TCP -
Operational Requirements", Work in Progress, Internet-
Draft, draft-ietf-dnsop-dns-tcp-requirements-06, 6 May
2020, <https://tools.ietf.org/html/draft-ietf-dnsop-dns-
tcp-requirements-06>.

[DNS-Privacy-not-so-private]
Silby, S., Juarez, M., Vallina-Rodriguez, N., and C.
Troncoso, "DNS Privacy not so private: the traffic
analysis perspective.", Privacy Enhancing
Technologies Symposium, 2018,
<https://petsymposium.org/2018/files/hotpets/4-siby.pdf>.

[DNS-XPF] Bellis, R., Dijk, P. V., and R. Gacogne, "DNS X-Proxied-
For", Work in Progress, Internet-Draft, draft-bellis-
dnsop-xpf-04, 5 March 2018,
<https://tools.ietf.org/html/draft-bellis-dnsop-xpf-04>.

[DNSCrypt] "DNSCrypt - Official Project Home Page",
<https://www.dnscrypt.org>.

[dnsdist] PowerDNS, "dnsdist Overview", <https://dnsdist.org>.

[dnstap] "dnstap", <https://dnstap.info>.

[DoH-resolver-policy]
Mozilla, "Security/DOH-resolver-policy", 2019,
<https://wiki.mozilla.org/Security/DOH-resolver-policy>.

[dot-ALPN] IANA, "Transport Layer Security (TLS) Extensions: TLS
Application-Layer Protocol Negotiation (ALPN) Protocol
IDs", <https://www.iana.org/assignments/tls-extensiontype-
values>.

[Geolocation-Impact-Assessment]
Conversion Works, "Anonymize IP Geolocation Accuracy
Impact Assessment", 19 May 2017,
<https://support.google.com/analytics/
answer/2763052?hl=en>.
<https://www.conversionworks.co.uk/blog/2017/05/19/
anonymize-ip-geo-impact-test/>.

[haproxy] "HAProxy - The Reliable, High Performance TCP/HTTP Load
Balancer", <https://www.haproxy.org/>.

[Harvan] Harvan, M., "Prefix- and Lexicographical-order-preserving
IP Address Anonymization", IEEE/IFIP Network Operations
and Management Symposium, DOI 10.1109/NOMS.2006.1687580,
2006, <http://mharvan.net/talks/noms-ip_anon.pdf>.

[Internet.nl]
Internet.nl, "Internet.nl Is Your Internet Up To Date?",
2019, <https://internet.nl>.

[IP-Anonymization-in-Analytics]
Google, "IP Anonymization in Analytics", 2019,
<https://support.google.com/analytics/
answer/2763052?hl=en>.

[ipcipher1]
Hubert, B., "On IP address encryption: security analysis
with respect for privacy", Medium, 7 May 2017,
<https://medium.com/@bert.hubert/on-ip-address-encryption-
security-analysis-with-respect-for-privacy-dabe1201b476>.

[ipcipher2]
PowerDNS, "ipcipher", commit fd47abe, 13 February 2018,
<https://github.com/PowerDNS/ipcipher>.

[ipcrypt] veorq, "ipcrypt: IP-format-preserving encryption",
commit 8cc12f9, 6 July 2015,
<https://github.com/veorq/ipcrypt>.

[ipcrypt-analysis]
Aumasson, J-P., "Subject: Re: [Cfrg] Analysis of
ipcrypt?", message to the Cfrg mailing list, 22 February
2018, <https://mailarchive.ietf.org/arch/msg/cfrg/
cFx5WJo48ZEN-a5cj_LlyrdN8-0/>.

[ISC-Knowledge-database-on-cache-snooping]
Goldlust, S. and C. Almond, "DNS Cache snooping - should I
be concerned?", ISC Knowledge Database, 15 October 2018,
<https://kb.isc.org/docs/aa-00482>.

[MAC-address-EDNS]
Hubert, B., "Embedding MAC address in DNS requests for
selective filtering", DNS-OARC mailing list, 25 January
2016, <https://lists.dns-oarc.net/pipermail/dns-
operations/2016-January/014143.html>.

[nginx] nginx.org, "nginx news", 2019, <https://nginx.org/>.

[Passive-Observations-of-a-Large-DNS]
de Vries, W. B., van Rijswijk-Deij, R., de Boer, P-T., and
A. Pras, "Passive Observations of a Large DNS Service: 2.5
Years in the Life of Google",
DOI 10.23919/TMA.2018.8506536, 2018,
<http://tma.ifip.org/2018/wp-
content/uploads/sites/3/2018/06/tma2018_paper30.pdf>.

[pcap] The Tcpdump Group, "Tcpdump & Libpcap", 2020,
<https://www.tcpdump.org/>.

[Pitfalls-of-DNS-Encryption]
Shulman, H., "Pretty Bad Privacy: Pitfalls of DNS
Encryption", Proceedings of the 13th Workshop on Privacy
in the Electronic Society, pp. 191-200,
DOI 10.1145/2665943.2665959, November 2014,
<https://dl.acm.org/citation.cfm?id=2665959>.

[policy-comparison]
Dickinson, S., "Comparison of policy and privacy
statements 2019", DNS Privacy Project, 18 December 2019,
<https://dnsprivacy.org/wiki/display/DP/
Comparison+of+policy+and+privacy+statements+2019>.

[PowerDNS-dnswasher]
PowerDNS, "dnswasher", commit 050e687, 24 April 2020,
<https://github.com/PowerDNS/pdns/blob/master/pdns/
dnswasher.cc>.

[Ramaswamy-and-Wolf]
Ramaswamy, R. and T. Wolf, "High-Speed Prefix-Preserving
IP Address Anonymization for Passive Measurement Systems",
DOI 10.1109/TNET.2006.890128, 2007,
<http://www.ecs.umass.edu/ece/wolf/pubs/ton2007.pdf>.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.

[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, <https://www.rfc-editor.org/info/rfc5077>.

[RFC6147] Bagnulo, M., Sullivan, A., Matthews, P., and I. van
Beijnum, "DNS64: DNS Extensions for Network Address
Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
DOI 10.17487/RFC6147, April 2011,
<https://www.rfc-editor.org/info/rfc6147>.

[RFC6235] Boschi, E. and B. Trammell, "IP Flow Anonymization
Support", RFC 6235, DOI 10.17487/RFC6235, May 2011,
<https://www.rfc-editor.org/info/rfc6235>.

[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.

[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<https://www.rfc-editor.org/info/rfc7626>.

[RFC7873] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
<https://www.rfc-editor.org/info/rfc7873>.

[RFC8027] Hardaker, W., Gudmundsson, O., and S. Krishnaswamy,
"DNSSEC Roadblock Avoidance", BCP 207, RFC 8027,
DOI 10.17487/RFC8027, November 2016,
<https://www.rfc-editor.org/info/rfc8027>.

[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-editor.org/info/rfc8094>.

[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-editor.org/info/rfc8404>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.

[RFC8618] Dickinson, J., Hague, J., Dickinson, S., Manderson, T.,
and J. Bond, "Compacted-DNS (C-DNS): A Format for DNS
Packet Capture", RFC 8618, DOI 10.17487/RFC8618, September
2019, <https://www.rfc-editor.org/info/rfc8618>.

[SSL-Labs] SSL Labs, "SSL Server Test", 2019,
<https://www.ssllabs.com/ssltest/>.

[stunnel] Goldlust, S., Almond, C., and F. Dupont, "DNS over TLS",
ISC Knowledge Database", 1 November 2018,
<https://kb.isc.org/article/AA-01386/0/DNS-over-TLS.html>.

[SURFnet-policy]
Baartmans, C., van Wynsberghe, A., van Rijswijk-Deij, R.,
and F. Jorna, "SURFnet Data Sharing Policy", June 2016,
<https://surf.nl/datasharing>.

[TCPdpriv]

[tcpdpriv] Ipsilon Networks, Inc., "TCPdpriv", 2005,
<http://ita.ee.lbl.gov/html/contrib/tcpdpriv.html>. "TCPDRIV - Program for Eliminating
Confidential Information from Traces", 2004,
<http://fly.isti.cnr.it/software/tcpdpriv/>.

[van-Dijkhuizen-et-al]
Van Dijkhuizen, N. and J. Van Der Ham, "A Survey of
Network Traffic Anonymisation Techniques and
Implementations", ACM Computing Surveys,
DOI 10.1145/3182660, May 2018,
<https://doi.org/10.1145/3182660>.

[Xu-et-al] Fan, J., Xu, J., Ammar, M.H., and S.B. Moon, "Prefix-
preserving IP address anonymization: measurement-based
security evaluation and a new cryptography-based scheme",
DOI 10.1016/j.comnet.2004.03.033, 2004,
<http://an.kaist.ac.kr/~sbmoon/paper/intl-journal/2004-cn-
anon.pdf>.

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 characteristics of the document. any document with
regard to potential increases or decreases in DNS privacy.

A.1. Potential Increases in DNS Privacy

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

* "Specification for DNS over Transport Layer Security (TLS)"
[RFC7858].

* "DNS over Datagram Transport Layer Security (DTLS)" [RFC8094].
Note that this document has the category of Experimental.

* "DNS Queries over HTTPS (DoH)" [RFC8484].

* "Usage Profiles for DNS over TLS and DNS over DTLS" [RFC8310].

* "The EDNS(0) Padding Option" [RFC7830] and "Padding Policies for
Extension Mechanisms for DNS (EDNS(0))" [RFC8467].

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

* "DNS Query Name Minimisation to Improve Privacy" [RFC7816].

A.2. Potential Decreases in DNS Privacy

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

* "Client Subnet in DNS Queries" [RFC7871].

* "Domain Name System (DNS) Cookies" [RFC7873]).

* "Transport Layer Security (TLS) Session Resumption without Server-
Side State" [RFC5077], referred to here as simply TLS session
resumption.

* [RFC8446], Appendix C.4 describes client tracking prevention in
TLS 1.3

* "Compacted-DNS (C-DNS): A Format for DNS Packet Capture"
[RFC8618].

* Passive DNS [RFC8499].

* Section 8 of [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.

A.3. Related Operational Documents

* "DNS Transport over TCP - Implementation Requirements" [RFC7766].

* "DNS Transport over TCP - Operational Requirements"
[DNS-OVER-TCP].

* "The edns-tcp-keepalive EDNS0 Option" [RFC7828].

* "DNS Stateful Operations" [RFC8490].

Appendix B. IP Address Techniques

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 categorizations 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.

+===========================+====+===+====+===+====+===+===+
| Categorization/Property | GA | d | TC | C | TS | i | B |
+===========================+====+===+====+===+====+===+===+
| Anonymization | X | X | X | | | | X |
+---------------------------+----+---+----+---+----+---+---+
| Pseudoanonymization Pseudonymization | | | | X | X | X | |
+---------------------------+----+---+----+---+----+---+---+
| Format preserving | X | X | X | X | X | X | |
+---------------------------+----+---+----+---+----+---+---+
| Prefix preserving | | | X | X | X | | |
+---------------------------+----+---+----+---+----+---+---+
| Replacement | | | X | | | | |
+---------------------------+----+---+----+---+----+---+---+
| Filtering | X | | | | | | |
+---------------------------+----+---+----+---+----+---+---+
| Generalization | | | | | | | X |
+---------------------------+----+---+----+---+----+---+---+
| Enumeration | | X | | | | | |
+---------------------------+----+---+----+---+----+---+---+
| Reordering/Shuffling | | | X | | | | |
+---------------------------+----+---+----+---+----+---+---+
| Random substitution | | | X | | | | |
+---------------------------+----+---+----+---+----+---+---+
| Cryptographic permutation | | | | X | X | X | |
+---------------------------+----+---+----+---+----+---+---+
| IPv6 issues | | | | | X | | |
+---------------------------+----+---+----+---+----+---+---+
| CPU intensive | | | | X | | | |
+---------------------------+----+---+----+---+----+---+---+
| Memory intensive | | | X | | | | |
+---------------------------+----+---+----+---+----+---+---+
| Security concerns | | | | | | X | |
+---------------------------+----+---+----+---+----+---+---+

Table 1: Classification of Techniques

Legend of techniques:

GA = Google Analytics
d = dnswasher
TC = TCPdpriv
C = CryptoPAn
TS = TSA
i = ipcipher
B = Bloom filter

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 Compacted-DNS
(C-DNS) [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.,
[Brekne-and-Arnes]) have observed that, as the entropy in an IPv4
address is limited, if an attacker can

* ensure packets are captured by the target and

* send forged traffic with arbitrary source and destination
addresses to that target and

* obtain a de-identified log of said traffic from that target,

any format-preserving pseudonymization is vulnerable to an attack
along the lines of a cryptographic chosen-plaintext attack.

B.1. Categorization of 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]:

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.

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, or subnet in IP addresses.
Prefix preservation ensures that prefixes are de-identified
consistently; for example, 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.

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

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).

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 nonephemeral (>1024).
Another example, precision degradation, reduces the accuracy of,
for example, a numeric value or a timestamp.

Enumeration. With data from a well-ordered set, replace the first
data item item's 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.

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

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

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

B.2. Specific Techniques

B.2.1. Google Analytics Non-Prefix Filtering

Since May 2010, Google Analytics has provided a facility
[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 [Geolocation-Impact-Assessment] that 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%.

Anonymization: Format-preserving, Filtering (truncation).

B.2.2. dnswasher

Since 2006, PowerDNS have has included a de-identification tool, dnswasher
[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 de-
identified address therefore depends on the order that addresses
arrive in the input, and when running over a large amount of data,
the address translation tables can grow to a significant size.

Anonymization: Format-preserving, Enumeration.

B.2.3. Prefix-Preserving Map

Used in [TCPdpriv], [tcpdpriv], this algorithm stores a set of original and
anonymized 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 requirements. The need to allocate anonymized addresses
sequentially means that TCPdpriv cannot be used in parallel
processing.

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

B.2.4. Cryptographic Prefix-Preserving Pseudonymization

Cryptographic prefix-preserving pseudonymization was originally
proposed as an improvement to the prefix-preserving map implemented
in TCPdpriv, described in [Xu-et-al] and implemented in the
[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 [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 and so has a nontrivial calculation overhead. This can be
mitigated to some extent (for IPv4, at least) by precalculating
results for some number of prefix bits.

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

B.2.5. Top-Hash Subtree-Replicated Anonymization

Proposed in [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 precalculated 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 precalculated structures to a few Mb megabytes 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).

B.2.6. ipcipher

A recently released proposal from PowerDNS, ipcipher [ipcipher1]
[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
[ipcrypt] suitable for 32-bit block lengths. However, the author of
ipcrypt has since indicated [ipcrypt-analysis] that it has low
security, and further analysis has revealed it is vulnerable to
attack.

Pseudonymization: Format-preserving, cryptographic permutation.

B.2.7. Bloom Filters

van Rijswijk-Deij et al. have recently described work using Bloom
Filters [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 being 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.

Anonymized: Generalization.

Appendix C. Current Policy and Privacy Statements

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 [policy-comparison]. The analysis is based
on the data available in December 2019.

We note that the existing 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 (A4 size) of text in a moderate-sized
font. It is a nontrivial task today for a user to extract a
meaningful overview of the different services on offer.

It is also noted that Mozilla have has published a DoH resolver policy
[DoH-resolver-policy] that 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.

Appendix D. Example RPS

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 what the full contents of
an RPS might look like. This should not be interpreted as

* having been reviewed or approved by any operator in any way

* having any legal standing or validity at all

* being complete or exhaustive

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 Section 5.

D.1. Policy

1. 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.

2. Data collection and sharing.

a. 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.

b. 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:

* Request Requested domain name -- e.g., example.net

* Record type of requested domain -- e.g., A, AAAA, NS, MX,
TXT, etc.

* Transport protocol on which the request arrived -- i.e.,
UDP, TCP, DoT, DoH

* Origin IP general geolocation information -- i.e.,
geocode, region ID, city ID, and metro code

* IP protocol version -- IPv4 or IPv6

* Response code sent -- e.g., SUCCESS, SERVFAIL, NXDOMAIN,
etc.

* Absolute arrival time using a precision in ms

* Name of the specific instance that processed this request

* IP address of the specific instance to which this request
was addressed (no relation to the requestor's IP address)

We may keep the following data as summary information,
including all the above EXCEPT for data about the DNS record
requested:

* Currently advertised BGP-summarized IP prefix/netmask of
apparent client origin

* Autonomous system number (BGP ASN) of apparent client
origin

All the above data may be kept in full or partial form in
permanent archives.

c. 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 service 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 the IP address
information of the requestor and cannot be correlated to IP
address or other personal data. We do not and never will
share any of the requestor's data with marketers, nor will we
use this data for demographic analysis.

3. Exceptions. There are exceptions to this storage model: In the
event of actions or observed behaviors that we deem malicious or
anomalous, we may utilize more detailed logging to collect more
specific IP address data in the process of normal network defense
and mitigation. This collection and transmission off-site will
be limited to IP addresses that we determine are involved in the
event.

4. Associated entities. Details of our Threat Intelligence partners
can be found at our website page (insert link).

5. 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.

6. Result filtering.

a. Filtering. We utilize cyber-threat intelligence about
malicious domains from a variety of public and private
sources and block access to those malicious domains when your
system attempts to contact them. An NXDOMAIN is returned for
blocked sites.

i. 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.

ii. 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.

D.2. Practice

1. Deviations from Policy. None in place since (insert date).

2. Client-facing capabilities.

a. We offer UDP and TCP DNS on port 53 on (insert IP address)

b. We offer DNS over TLS as specified in RFC 7858 on (insert IP
address). It is available on port 853 and port 443. We also
implement RFC 7766.

i. The DoT authentication domain name used is (insert
domain name).

ii. We do not publish SPKI pin sets.

c. We offer DNS over HTTPS as specified in RFC 8484 on (insert
URI template).

d. Both services offer TLS 1.2 and TLS 1.3.

e. Both services pad DNS responses according to RFC 8467.

f. Both services provide DNSSEC validation.

3. Upstream capabilities.

a. Our servers implement QNAME minimization.

b. Our servers do not send ECS upstream.

4. Support. Support information for this service is available at
(insert link).

5. 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).

Acknowledgements

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 Working Group Last Call and for suggesting the inclusion of
an example RPS. Thanks to John Todd for discussions on this topic,
and to Stéphane Bortzmeyer, Puneet Sood, and Vittorio Bertola for
review. Thanks to Daniel Kahn Gillmor, Barry Green, Paul Hoffman,
Dan York, Jon Reed, and 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.

Contributors

The below individuals contributed significantly to the document:

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

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

Authors' Addresses

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

Email: sara@sinodun.com

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

Email: benno@nlnetLabs.nl

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

Email: roland@nlnetLabs.nl

Allison Mankin
Salesforce.com, Inc.
Salesforce Tower
415 Mission Street, 3rd Floor
San Francisco, CA 94105
United States of America

Email: allison.mankin@gmail.com