dprive S. Dickinson
Internet-Draft Sinodun IT
Intended status: Best Current Practice B. Overeinder
Expires: January 9, 2020 R. van Rijswijk-Deij
NLnet Labs
A. Mankin
Salesforce
July 8, 2019
Recommendations for DNS Privacy Service Operators
draft-ietf-dprive-bcp-op-03
Abstract
This document presents operational, policy and security
considerations for DNS operators who choose to offer DNS Privacy
services. With these recommendations, the operator can make
deliberate decisions regarding which services to provide, and how the
decisions and alternatives impact the privacy of users.
This document also presents a framework to assist writers of DNS
Privacy Policy and Practices Statements (analogous to DNS Security
Extensions (DNSSEC) Policies and DNSSEC Practice Statements described
in [RFC6841]).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 9, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Privacy related documents . . . . . . . . . . . . . . . . . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Recommendations for DNS privacy services . . . . . . . . . . 6
5.1. On the wire between client and server . . . . . . . . . . 7
5.1.1. Transport recommendations . . . . . . . . . . . . . . 7
5.1.2. Authentication of DNS privacy services . . . . . . . 8
5.1.3. Protocol recommendations . . . . . . . . . . . . . . 9
5.1.4. DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.5. Availability . . . . . . . . . . . . . . . . . . . . 11
5.1.6. Service options . . . . . . . . . . . . . . . . . . . 12
5.1.7. Impact on Operators . . . . . . . . . . . . . . . . . 12
5.1.8. Limitations of using a pure TLS proxy . . . . . . . . 13
5.2. Data at rest on the server . . . . . . . . . . . . . . . 13
5.2.1. Data handling . . . . . . . . . . . . . . . . . . . . 13
5.2.2. Data minimization of network traffic . . . . . . . . 14
5.2.3. IP address pseudonymization and anonymization methods 15
5.2.4. Pseudonymization, anonymization or discarding of
other correlation data . . . . . . . . . . . . . . . 16
5.2.5. Cache snooping . . . . . . . . . . . . . . . . . . . 17
5.3. Data sent onwards from the server . . . . . . . . . . . . 17
5.3.1. Protocol recommendations . . . . . . . . . . . . . . 17
5.3.2. Client query obfuscation . . . . . . . . . . . . . . 18
5.3.3. Data sharing . . . . . . . . . . . . . . . . . . . . 19
6. DNS privacy policy and practice statement . . . . . . . . . . 19
6.1. Recommended contents of a DPPPS . . . . . . . . . . . . . 20
6.1.1. Policy . . . . . . . . . . . . . . . . . . . . . . . 20
6.1.2. Practice . . . . . . . . . . . . . . . . . . . . . . 21
6.2. Current policy and privacy statements . . . . . . . . . . 22
6.3. Enforcement/accountability . . . . . . . . . . . . . . . 22
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Security considerations . . . . . . . . . . . . . . . . . . . 23
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 23
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 23
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12. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . 27
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Appendix A. Documents . . . . . . . . . . . . . . . . . . . . . 30
A.1. Potential increases in DNS privacy . . . . . . . . . . . 30
A.2. Potential decreases in DNS privacy . . . . . . . . . . . 30
A.3. Related operational documents . . . . . . . . . . . . . . 31
Appendix B. IP address techniques . . . . . . . . . . . . . . . 31
B.1. Google Analytics non-prefix filtering . . . . . . . . . . 32
B.2. dnswasher . . . . . . . . . . . . . . . . . . . . . . . . 33
B.3. Prefix-preserving map . . . . . . . . . . . . . . . . . . 33
B.4. Cryptographic Prefix-Preserving Pseudonymisation . . . . 33
B.5. Top-hash Subtree-replicated Anonymisation . . . . . . . . 34
B.6. ipcipher . . . . . . . . . . . . . . . . . . . . . . . . 34
B.7. Bloom filters . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
The Domain Name System (DNS) is at the core of the Internet; almost
every activity on the Internet starts with a DNS query (and often
several). However the DNS was not originally designed with strong
security or privacy mechanisms. A number of developments have taken
place in recent years which aim to increase the privacy of the DNS
system and these are now seeing some deployment. This latest
evolution of the DNS presents new challenges to operators and this
document attempts to provide an overview of considerations for
privacy focused DNS services.
In recent years there has also been an increase in the availability
of "public resolvers" [I-D.ietf-dnsop-terminology-bis] which users
may prefer to use instead of the default network resolver because
they offer a specific feature (e.g. good reachability, encrypted
transport, strong privacy policy, filtering (or lack of), etc.).
These open resolvers have tended to be at the forefront of adoption
of privacy related enhancements but it is anticipated that operators
of other resolver services will follow.
Whilst protocols that encrypt DNS messages on the wire provide
protection against certain attacks, the resolver operator still has
(in principle) full visibility of the query data and transport
identifiers for each user. Therefore, a trust relationship exists.
The ability of the operator to provide a transparent, well
documented, and secure privacy service will likely serve as a major
differentiating factor for privacy conscious users if they make an
active selection of which resolver to use.
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It should also be noted that the choice of a user to configure a
single resolver (or a fixed set of resolvers) and an encrypted
transport to use in all network environments has both advantages and
disadvantages. For example the user has a clear expectation of which
resolvers have visibility of their query data however this resolver/
transport selection may provide an added mechanism to track them as
they move across network environments. Commitments from operators to
minimize such tracking are also likely to play a role in user
selection of resolvers.
More recently the global legislative landscape with regard to
personal data collection, retention, and pseudonymization has seen
significant activity. It is an untested area that simply using a DNS
resolution service constitutes consent from the user for the operator
to process their query data. The impact of recent legislative
changes on data pertaining to the users of both Internet Service
Providers and public DNS resolvers is not fully understood at the
time of writing.
This document has two main goals:
o To provide operational and policy guidance related to DNS over
encrypted transports and to outline recommendations for data
handling for operators of DNS privacy services.
o To introduce the DNS Privacy Policy and Practice Statement (DPPPS)
and present a framework to assist writers of this document. A
DPPPS is a document that an operator can publish outlining their
operational practices and commitments with regard to privacy
thereby providing a means for clients to evaluate the privacy
properties of a given DNS privacy service. In particular, the
framework identifies the elements that should be considered in
formulating a DPPPS. This document does not, however, define a
particular Policy or Practice Statement, nor does it seek to
provide legal advice or recommendations as to the contents.
A desired operational impact is that all operators (both those
providing resolvers within networks and those operating large anycast
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.
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
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statement. Readers are advised to seek out any errata or updates
that apply to this document.
2. Scope
"DNS Privacy Considerations" [I-D.bortzmeyer-dprive-rfc7626-bis]
describes the general privacy issues and threats associated with the
use of the DNS by Internet users and much of the threat analysis here
is lifted from that document and from [RFC6973]. However this
document is limited in scope to best practice considerations for the
provision of DNS privacy services by servers (recursive resolvers) to
clients (stub resolvers or forwarders). Privacy considerations
specifically from the perspective of an end user, or those for
operators of authoritative nameservers are out of scope.
This document includes (but is not limited to) considerations in the
following areas (taken from [I-D.bortzmeyer-dprive-rfc7626-bis]):
1. Data "on the wire" between a client and a server
2. Data "at rest" on a server (e.g. in logs)
3. Data "sent onwards" from the server (either on the wire or shared
with a third party)
Whilst the issues raised here are targeted at those operators who
choose to offer a DNS privacy service, considerations for areas 2 and
3 could equally apply to operators who only offer DNS over
unencrypted transports but who would like to align with privacy best
practice.
3. Privacy related documents
There are various documents that describe protocol changes that have
the potential to either increase or decrease the privacy of the DNS.
Note this does not imply that some documents are good or bad, better
or worse, just that (for example) some features may bring functional
benefits at the price of a reduction in privacy and conversely some
features increase privacy with an accompanying increase in
complexity. A selection of the most relevant documents are listed in
Appendix A for reference.
4. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
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14 [RFC2119] and [RFC8174] when, and only when, they appear in all
capitals, as shown here.
DNS terminology is as described in [I-D.ietf-dnsop-terminology-bis]
with one modification: we restate the clause in the original
definition of Privacy-enabling DNS server in [RFC8310] to include the
requirement that a DNS over (D)TLS server should also offer at least
one of the credentials described in Section 8 and implement the
(D)TLS profile described in Section 9 of [RFC8310].
Other Terms:
o DPPPS: DNS Privacy Policy and Practice Statement, see Section 6.
o DNS privacy service: The service that is offered via a privacy-
enabling DNS server and is documented either in an informal
statement of policy and practice with regard to users privacy or a
formal DPPPS.
5. Recommendations for DNS privacy services
We describe two classes of threats:
o 'Privacy Considerations for Internet Protocols' [RFC6973] Threats
* Privacy terminology, threats to privacy and mitigations as
described in Sections 3, 5 and 6 of [RFC6973].
o DNS Privacy Threats
* These are threats to the users and operators of DNS privacy
services that are not directly covered by [RFC6973]. These may
be more operational in nature such as certificate management or
service availability issues.
We describe three classes of actions that operators of DNS privacy
services can take:
o Threat mitigation for well understood and documented privacy
threats to the users of the service and in some cases to the
operators of the service.
o Optimization of privacy services from an operational or management
perspective
o Additional options that could further enhance the privacy and
usability of the service
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This document does not specify policy only best practice, however for
DNS Privacy services to be considered compliant with these best
practice guidelines they SHOULD implement (where appropriate) all:
o Threat mitigations to be minimally compliant
o Optimizations to be moderately compliant
o Additional options to be maximally compliant
5.1. On the wire between client and server
In this section we consider both data on the wire and the service
provided to the client.
5.1.1. Transport recommendations
[RFC6973] Threats:
o Surveillance:
* Passive surveillance of traffic on the wire
[I-D.bortzmeyer-dprive-rfc7626-bis] Section 2.4.2.
DNS Privacy Threats:
o Active injection of spurious data or traffic
Mitigations:
A DNS privacy service can mitigate these threats by providing service
over one or more of the following transports
o DNS-over-TLS [RFC7858] and [RFC8310]
o DoH [RFC8484]
It is noted that a DNS privacy service can also be provided over DNS-
over-DTLS [RFC8094], however this is an Experimental specification
and there are no known implementations at the time of writing.
It is also noted that DNS privacy service might be provided over
IPSec, DNSCrypt or VPNs. However, use of these transports for DNS
are not standardized and any discussion of best practice for
providing such a service is out of scope for this document.
Whilst encryption of DNS traffic can protect against active injection
this does not diminish the need for DNSSEC, see Section 5.1.4.
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5.1.2. Authentication of DNS privacy services
[RFC6973] Threats:
o Surveillance:
* Active attacks that can redirect traffic to rogue servers
[I-D.bortzmeyer-dprive-rfc7626-bis] Section 2.5.3.
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 DNS-over-TLS clients that select a 'Strict Privacy' usage
profile [RFC8310] (to mitigate the threat of active attack on the
client) require the ability to authenticate the DNS server. To
enable this, DNS privacy services that offer DNS-over-TLS should
provide credentials in the form of either X.509 certificates or SPKI
pinsets.
When offering DoH [RFC8484], HTTPS requires authentication of the
server as part of the protocol.
NOTE: At this time the reference to the TLS DNSSEC chain extension
draft has been removed as it is no longer considered an active TLS WG
document.
Optimizations:
DNS privacy services can also consider the following capabilities/
options:
o As recommended in [RFC8310] providing DANE TLSA records for the
nameserver
* In particular, the service could provide TLSA records such that
authenticating solely via the PKIX infrastructure can be
avoided.
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.
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It is noted that SPKI pinset management is described in [RFC7858] but
that key pinning mechanisms in general have fallen out of favor
operationally for various reasons such as the logistical overhead of
rolling keys.
DNS Privacy Threats:
o Invalid certificates, resulting in an unavailable service.
o Mis-identification of a server by a client e.g. typos in URLs or
authentication domain names
Mitigations:
It is recommended that operators:
o Follow the guidance in Section 6.5 of [RFC7525] with regards to
certificate revocation
o Choose a short, memorable authentication name for the service
o Automate the generation and publication of certificates
o Monitor certificates to prevent accidental expiration of
certificates
5.1.3. Protocol recommendations
5.1.3.1. DNS-over-TLS
DNS Privacy Threats:
o Known attacks on TLS such as those described in [RFC7457]
o Traffic analysis, for example: Pitfalls of DNS Encryption [1]
o Potential for client tracking via transport identifiers
o Blocking of well known ports (e.g. 853 for DNS-over-TLS)
Mitigations:
In the case of DNS-over-TLS, TLS profiles from Section 9 and the
Countermeasures to DNS Traffic Analysis from section 11.1 of
[RFC8310] provide strong mitigations. This includes but is not
limited to:
o Adhering to [RFC7525]
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o Implementing only (D)TLS 1.2 or later as specified in [RFC8310]
o Implementing EDNS(0) Padding [RFC7830] using the guidelines in
[RFC8467]
o Clients should not be required to use TLS session resumption
[RFC5077] or Domain Name System (DNS) Cookies [RFC7873].
o A DNS-over-TLS privacy service on both port 853 and 443. This
practice may not be possible if e.g. the operator deploys DoH on
the same IP address.
Optimizations:
o Concurrent processing of pipelined queries, returning responses as
soon as available, potentially out of order as specified in
[RFC7766]. This is often called 'OOOR' - out-of-order responses.
(Providing processing performance similar to HTTP multiplexing)
o Management of TLS connections to optimize performance for clients
using either
* [RFC7766] and EDNS(0) Keepalive [RFC7828] and/or
* DNS Stateful Operations [I-D.ietf-dnsop-session-signal]
Additional options that providers may consider:
o Offer a .onion [RFC7686] service endpoint
5.1.3.2. DoH
DNS Privacy Threats:
o Known attacks on TLS such as those described in [RFC7457]
o Traffic analysis, for example: DNS Privacy not so private: the
traffic analysis perspective [2]
o Potential for client tracking via transport identifiers
Mitigations:
o Clients must be able to forego the use of HTTP Cookies [RFC6265]
and still use the service
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o Clients should not be required to include any headers beyond the
absolute minimum to obtain service from a DoH server. (See
Section 6.1 of [I-D.ietf-httpbis-bcp56bis].)
5.1.4. DNSSEC
DNS Privacy Threats:
o Users may be directed to bogus IP addresses for e.g. websites
where they might reveal personal information to attackers.
Mitigations:
o All DNS privacy services must offer a DNS privacy service that
performs DNSSEC validation. In addition they must be able to
provide the DNSSEC 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 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.
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 DNS Roadblock avoidance.
5.1.5. Availability
DNS Privacy Threats:
o A failed DNS privacy service could force the user to switch
providers, fallback to cleartext or accept no DNS service for the
outage.
Mitigations:
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A DNS privacy service must be engineered for high availability.
Particular care should to be taken to protect DNS privacy services
against denial-of-service attacks, as experience has shown that
unavailability of DNS resolving because of attacks is a significant
motivation for users to switch services. See, for example
Section IV-C of Passive Observations of a Large DNS Service: 2.5
Years in the Life of Google [3].
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:
o Unfairly disadvantaging users of the privacy service with respect
to the services available. This could force the user to switch
providers, fallback to cleartext or accept no DNS service for the
outage.
Mitigations:
A DNS privacy service should deliver the same level of service as
offered on un-encrypted channels in terms of such options as
filtering (or lack thereof), DNSSEC validation, etc.
5.1.7. Impact on Operators
DNS Privacy Threats:
o Increased use of encryption impacts operator ability to manage
their network [RFC8404]
Many monitoring solutions for DNS traffic rely on the plain text
nature of this traffic and work by intercepting traffic on the wire,
either using a separate view on the connection between clients and
the resolver, or as a separate process on the resolver system that
inspects network traffic. Such solutions will no longer function
when traffic between clients and resolvers is encrypted. There are,
however, legitimate reasons for operators to inspect DNS traffic,
e.g. to monitor for network security threats. Operators may
therefore need to invest in alternative means of monitoring that
relies on either the resolver software directly, or exporting DNS
traffic from the resolver using e.g. dnstap [4].
Optimization:
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When implementing alternative means for traffic monitoring, operators
of a DNS privacy service should consider using privacy conscious
means to do so (see, for example, the discussion on the use of Bloom
Filters in the #documents appendix in this document).
5.1.8. Limitations of using a pure TLS proxy
DNS Privacy Threats:
o Limited ability to manage or monitor incoming connections using
DNS specific techniques
o Misconfiguration of the target server could lead to data leakage
if the proxy to target server path is not encrypted.
Optimization:
Some operators may choose to implement DNS-over-TLS using a TLS proxy
(e.g. nginx [5], haproxy [6] or stunnel [7]) in front of a DNS
nameserver because of proven robustness and capacity when handling
large numbers of client connections, load balancing capabilities and
good tooling. Currently, however, because such proxies typically
have no specific handling of DNS as a protocol over TLS or DTLS using
them can restrict traffic management at the proxy layer and at the
DNS server. For example, all traffic received by a nameserver behind
such a proxy will appear to originate from the proxy and DNS
techniques such as ACLs, RRL or DNS64 will be hard or impossible to
implement in the nameserver.
Operators may choose to use a DNS aware proxy such as dnsdist [8]
which offer custom options (similar to that proposed in
[I-D.bellis-dnsop-xpf]) to add source information to packets to
address this shortcoming. It should be noted that such options
potentially significantly increase the leaked information in the
event of a misconfiguration.
5.2. Data at rest on the server
5.2.1. Data handling
[RFC6973] Threats:
o Surveillance
o Stored data compromise
o Correlation
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o Identification
o Secondary use
o Disclosure
Other Threats
o Contravention of legal requirements not to process user data?
Mitigations:
The following are common activities for DNS service operators and in
all cases should be minimized or completely avoided if possible for
DNS privacy services. If data is retained it should be encrypted and
either aggregated, pseudonymized or anonymized whenever possible. In
general the principle of data minimization described in [RFC6973]
should be applied.
o Transient data (e.g. that is used for real time monitoring and
threat analysis which might be held only memory) should be
retained for the shortest possible period deemed operationally
feasible.
o The retention period of DNS traffic logs should be only those
required to sustain operation of the service and, to the extent
that such exists, meet regulatory requirements.
o DNS privacy services should not track users except for the
particular purpose of detecting and remedying technically
malicious (e.g. DoS) or anomalous use of the service.
o Data access should be minimized to only those personnel who
require access to perform operational duties.
Optimizations:
o 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 may also include
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revealing other confidential information, for example on the
structure of an internal corporate network.
The problem of effectively ensuring that DNS traffic logs contain no
or minimal privacy-sensitive information is not one that currently
has a generally agreed solution or any Standards to inform this
discussion. This section presents and overview of current techniques
to simply provide reference on the current status of this work.
Research into data minimization techniques (and particularly IP
address pseudonymization/anonymization) was sparked in the late
1990s/early 2000s, partly driven by the desire to share significant
corpuses of traffic captures for research purposes. Several
techniques reflecting different requirements in this area and
different performance/resource tradeoffs emerged over the course of
the decade. Developments over the last decade have been both a
blessing and a curse; the large increase in size between an IPv4 and
an IPv6 address, for example, renders some techniques impractical,
but also makes available a much larger amount of input entropy, the
better to resist brute force re-identification attacks that have
grown in practicality over the period.
Techniques employed may be broadly categorized as either
anonymization or pseudonymization. The following discussion uses the
definitions from [RFC6973] Section 3, with additional observations
from van Dijkhuizen et al. [9]
o Anonymization. To enable anonymity of an individual, there must
exist a set of individuals that appear to have the same
attribute(s) as the individual. To the attacker or the observer,
these individuals must appear indistinguishable from each other.
o Pseudonymization. The true identity is deterministically replaced
with an alternate identity (a pseudonym). When the
pseudonymization schema is known, the process can be reversed, so
the original identity becomes known again.
In practice there is a fine line between the two; for example, how to
categorize a deterministic algorithm for data minimization of IP
addresses that produces a group of pseudonyms for a single given
address.
5.2.3. IP address pseudonymization and anonymization methods
As [I-D.bortzmeyer-dprive-rfc7626-bis] makes clear, the big privacy
risk in DNS is connecting DNS queries to an individual and the major
vector for this in DNS traffic is the client IP address.
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There is active discussion in the space of effective pseudonymization
of IP addresses in DNS traffic logs, however there seems to be no
single solution that is widely recognized as suitable for all or most
use cases. There are also as yet no standards for this that are
unencumbered by patents. The following table presents a high level
comparison of various techniques employed or under development today
and classifies them according to categorization of technique and
other properties. The list of techniques includes the main
techniques in current use, but does not claim to be comprehensive.
Appendix B provides a more detailed survey of these techniques and
definitions for the categories and properties listed below.
Figure showing comparison of IP address techniques (SVG) [10]
The choice of which method to use for a particular application will
depend on the requirements of that application and consideration of
the threat analysis of the particular situation.
For example, a common goal is that distributed packet captures must
be in an existing data format such as PCAP [pcap] or C-DNS
[I-D.ietf-dnsop-dns-capture-format] that can be used as input to
existing analysis tools. In that case, use of a format-preserving
technique is essential. This, though, is not cost-free - several
authors (e.g. Brenker & Arnes [11]) have observed that, as the
entropy in an IPv4 address is limited, given a de-identified log from
a target, if an attacker is capable of ensuring packets are captured
by the target and the attacker can send forged traffic with arbitrary
source and destination addresses to that target, any format-
preserving pseudonymization is vulnerable to an attack along the
lines of a cryptographic chosen plaintext attack.
5.2.4. Pseudonymization, anonymization or discarding of other
correlation data
DNS Privacy Threats:
o IP TTL/Hoplimit can be used to fingerprint client OS
o TLS version/Cipher suite combinations can be used to fingerprint
the client application or TLS library
o Tracking of TCP sessions
o Tracking of TLS sessions and session resumption mechanisms
o Resolvers _might_ receive client identifiers e.g. MAC addresses
in EDNS(0) options - some CPE devices are known to add them.
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o HTTP headers
Mitigations:
o Data minimization or discarding of such correlation data.
5.2.5. Cache snooping
[RFC6973] Threats:
o Surveillance:
* Profiling of client queries by malicious third parties
Mitigations:
o See ISC Knowledge database on cache snooping [12] for an example
discussion on defending against cache snooping
5.3. Data sent onwards from the server
In this section we consider both data sent on the wire in upstream
queries and data shared with third parties.
5.3.1. Protocol recommendations
[RFC6973] Threats:
o Surveillance:
* Transmission of identifying data upstream.
Mitigations:
As specified in [RFC8310] for DNS-over-TLS but applicable to any DNS
Privacy services the server should:
o Implement QNAME minimization [RFC7816]
o Honor a SOURCE PREFIX-LENGTH set to 0 in a query containing the
EDNS(0) Client Subnet (ECS) option and not send an ECS option in
upstream queries.
Optimizations:
o The server should either
* not use the ECS option in upstream queries at all, or
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* offer alternative services, one that sends ECS and one that
does not.
If operators do offer a service that sends the ECS options upstream
they should use the shortest prefix that is operationally feasible
(NOTE: the authors believe they will be able to add a reference for
advice here soon) and ideally use a policy of whitelisting upstream
servers to send ECS to in order to minimize data leakage. Operators
should make clear in any policy statement what prefix length they
actually send and the specific policy used.
Whitelisting has the benefit that not only does the operator know
which upstream servers can use ECS but also allows the operator to
decide which upstream servers apply privacy policies that the
operator is happy with. However some operators consider whitelisting
to incur significant operational overhead compared to dynamic
detection of ECS on authoritative servers.
Additional options:
o Aggressive Use of DNSSEC-Validated Cache [RFC8198] to reduce the
number of queries to authoritative servers to increase privacy.
o Run a copy of the root zone on loopback [RFC7706] to avoid making
queries to the root servers that might leak information.
5.3.2. Client query obfuscation
Additional options:
Since queries from recursive resolvers to authoritative servers are
performed using cleartext (at the time of writing), resolver services
need to consider the extent to which they may be directly leaking
information about their client community via these upstream queries
and what they can do to mitigate this further. Note, that even when
all the relevant techniques described above are employed there may
still be attacks possible, e.g. [Pitfalls-of-DNS-Encryption]. For
example, a resolver with a very small community of users risks
exposing data in this way and OUGHT obfuscate this traffic by mixing
it with 'generated' traffic to make client characterization harder.
The resolver could also employ aggressive pre-fetch techniques as a
further measure to counter traffic analysis.
At the time of writing there are no standardized or widely recognized
techniques to perform such obfuscation or bulk pre-fetches.
Another technique that particularly small operators may consider is
forwarding local traffic to a larger resolver (with a privacy policy
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that aligns with their own practices) over an encrypted protocol so
that the upstream queries are obfuscated among those of the large
resolver.
5.3.3. Data sharing
[RFC6973] Threats:
o Surveillance
o Stored data compromise
o Correlation
o Identification
o Secondary use
o Disclosure
DNS Privacy Threats:
o Contravention of legal requirements not to process user data
Mitigations:
Operators should not provide identifiable data to third-parties
without explicit consent from clients (we take the stance here that
simply using the resolution service itself does not constitute
consent).
Even when consent is granted operators should employ data
minimization techniques such as those described in Section 5.2.1 if
data is shared with third-parties.
Operators should consider including specific guidelines for the
collection of aggregated and/or anonymized data for research
purposes, within or outside of their own organization. This can
benefit not only the operator (through inclusion in novel research)
but also the wider Internet community. See SURFnet's policy [13] on
data sharing for research as an example.
6. DNS privacy policy and practice statement
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6.1. Recommended contents of a DPPPS
6.1.1. Policy
1. Make an explicit statement that IP addressses are treated as PII
2. State if IP addresses are being logged
3. Specify clearly what data (including whether it is aggregated,
pseudonymized or anonymized and the conditions of data transfer)
is:
* Collected and retained by the operator, and for what period it
is retained
* Shared with partners
* Shared, sold or rented to third-parties
4. Specify any exceptions to the above, for example technically
malicious or anomalous behavior
5. Declare any partners, third-party affiliations or sources of
funding
6. Whether user DNS data is correlated or combined with any other
personal information held by the operator
7. Result filtering. This section should explain whether the
operator filters, edits or alters in any way the replies that it
receives from the authoritative servers for each DNS zone, before
forwarding them to the clients. For each category listed below,
the operator should also specify how the filtering lists are
created and managed, whether it employs any third-party sources
for such lists, and which ones.
* Specify if any replies are being filtered out or altered for
network and computer security reasons (e.g. preventing
connections to malware-spreading websites or botnet control
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
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* Specify if any replies are being filtered out or altered for
any other reason, including commercial ones
6.1.2. Practice
This section should explain the current operational practices of the
service.
1. Specify any temporary or permanent deviations from the policy for
operational reasons
2. With reference to section Section 5 provide specific details of
which capabilities are provided on which client facing addresses
and ports
3. Specify the authentication name to be used (if any) and if TLSA
records are published (including options used in the TLSA
records)
4. Specify the SPKI pinsets to be used (if any) and policy for
rolling keys
5. Provide contact/support information for the service
6. Jurisdiction. This section should communicate the applicable
jurisdictions and law enforcement regimes under which the service
is being provided.
* Specify the entity or entities that will control the data and
be responsible for their treatment, and their legal place of
business
* Specify, either directly or by pointing to the applicable
privacy policy, the relevant privacy laws that apply to the
treatment of the data, the rights that users enjoy in regard
to their own personal information that is treated by the
service, and how they can contact the operator to enforce them
* Specify the countries in which the servers handling the DNS
requests and the data are located (if the operator applies a
geolocation policy so that requests from certain countries are
only served by certain servers, this should be specified as
well)
* Specify whether the operator has any agreement in place with
law enforcement agencies, or other public and private parties
dealing with security and intelligence, to give them access to
the servers and/or to the data
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7. Describe how consent is obtained from the user of the DNS privacy
service differentiating
* Uninformed users for whom this trust relationship is implicit
* Privacy-conscious users, that make an explicit trust choice
(this may prove relevant in the context of e.g. the GDPR as it
relates to consent)
6.2. Current policy and privacy statements
A tabular comparison of existing policy and privacy statements from
various DNS Privacy service operators based on the proposed DPPPS
structure can be found on dnsprivacy.org [14].
We note that the existing set of policies vary widely in style,
content and detail and it is not uncommon for the full text for a
given operator to equate to more than 10 pages of moderate font sized
A4 text. It is a non-trivial task today for a user to extract a
meaningful overview of the different services on offer.
It is also noted that Mozilla have published a Security/DoH-resolver
policy [15], 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.
6.3. Enforcement/accountability
Transparency reports may help with building user trust that operators
adhere to their policies and practices.
Independent monitoring or analysis could be performed where possible
of:
o ECS, QNAME minimization, EDNS(0) padding, etc.
o Filtering
o Uptime
This is by analogy with e.g. several TLS or website analysis tools
that are currently available e.g. SSL Labs [16] or Internet.nl [17].
Additionally operators could choose to engage the services of a third
party auditor to verify their compliance with their published DPPPS.
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7. IANA considerations
None
8. Security considerations
Security considerations for DNS-over-TCP are given in [RFC7766], many
of which are generally applicable to session based DNS.
TODO: e.g. New issues for DoS defence, server admin policies
9. Acknowledgements
Many thanks to Amelia Andersdotter for a very thorough review of the
first draft of this document. Thanks to John Todd for discussions on
this topic, and to Stephane Bortzmeyer, Puneet Sood and Vittorio
Bertola for review. Thanks to Daniel Kahn Gillmor, Barry Green, Paul
Hoffman, Dan York, John Reed, Lorenzo Colitti for comments at the
mic. Thanks to Loganaden Velvindron for useful updates to the text.
Sara Dickinson thanks the Open Technology Fund for a grant to support
the work on this document.
10. Contributors
The below individuals contributed significantly to the document:
John Dickinson
Sinodun Internet Technologies
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
United Kingdom
Jim Hague
Sinodun Internet Technologies
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
United Kingdom
11. Changelog
draft-ietf-dprive-bcp-op-03
o Add paragraph about operational impact
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o Move DNSSEC requirement out of the Appendix into main text as a
privacy threat that should be mitigated
o Add TLS version/Cipher suite as tracking threat
o Add reference to Mozilla TRR policy
o Remove several TODOs and QUESTIONS.
draft-ietf-dprive-bcp-op-02
o Change 'open resolver' for 'public resolver'
o Minor editorial changes
o Remove recommendation to run a separate TLS 1.3 service
o Move TLSA to purely a optimisation in Section 5.2.1
o Update reference on minimal DoH headers.
o Add reference on user switching provider after service issues in
Section 5.1.4
o Add text in Section 5.1.6 on impact on operators.
o Add text on additional threat to TLS proxy use (Section 5.1.7)
o Add reference in Section 5.3.1 on example policies.
draft-ietf-dprive-bcp-op-01
o Many minor editorial fixes
o Update DoH reference to RFC8484 and add more text on DoH
o Split threat descriptions into ones directly referencing RFC6973
and other DNS Privacy threats
o Improve threat descriptions throughout
o Remove reference to the DNSSEC TLS Chain Extension draft until new
version submitted.
o Clarify use of whitelisting for ECS
o Re-structure the DPPPS, add Result filtering section.
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o Remove the direct inclusion of privacy policy comparison, now just
reference dnsprivacy.org and an example of such work.
o Add an appendix briefly discussing DNSSEC
o Update affiliation of 1 author
draft-ietf-dprive-bcp-op-00
o Initial commit of re-named document after adoption to replace
draft-dickinson-dprive-bcp-op-01
12. References
12.1. Normative References
[I-D.ietf-dnsop-session-signal]
Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
draft-ietf-dnsop-session-signal-20 (work in progress),
December 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[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>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011, <https://www.rfc-
editor.org/info/rfc6265>.
[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>.
[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>.
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[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>.
[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>.
[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>.
[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>.
[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>.
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[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>.
12.2. Informative References
[I-D.bellis-dnsop-xpf]
Bellis, R., Dijk, P., and R. Gacogne, "DNS X-Proxied-For",
draft-bellis-dnsop-xpf-04 (work in progress), March 2018.
[I-D.bortzmeyer-dprive-rfc7626-bis]
Bortzmeyer, S. and S. Dickinson, "DNS Privacy
Considerations", draft-bortzmeyer-dprive-rfc7626-bis-02
(work in progress), January 2019.
[I-D.ietf-dnsop-dns-capture-format]
Dickinson, J., Hague, J., Dickinson, S., Manderson, T.,
and J. Bond, "C-DNS: A DNS Packet Capture Format", draft-
ietf-dnsop-dns-capture-format-10 (work in progress),
December 2018.
[I-D.ietf-dnsop-dns-tcp-requirements]
Kristoff, J. and D. Wessels, "DNS Transport over TCP -
Operational Requirements", draft-ietf-dnsop-dns-tcp-
requirements-04 (work in progress), June 2019.
[I-D.ietf-dnsop-terminology-bis]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", draft-ietf-dnsop-terminology-bis-14 (work in
progress), September 2018.
[I-D.ietf-httpbis-bcp56bis]
Nottingham, M., "Building Protocols with HTTP", draft-
ietf-httpbis-bcp56bis-08 (work in progress), November
2018.
[pcap] tcpdump.org, "PCAP", 2016, <http://www.tcpdump.org/>.
[Pitfalls-of-DNS-Encryption]
Shulman, H., "Pretty Bad Privacy: Pitfalls of DNS
Encryption", 2014, <https://www.ietf.org/mail-archive/web/
dns-privacy/current/pdfWqAIUmEl47.pdf>.
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[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>.
[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>.
[RFC6841] Ljunggren, F., Eklund Lowinder, AM., and T. Okubo, "A
Framework for DNSSEC Policies and DNSSEC Practice
Statements", RFC 6841, DOI 10.17487/RFC6841, January 2013,
<https://www.rfc-editor.org/info/rfc6841>.
[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>.
[RFC7686] Appelbaum, J. and A. Muffett, "The ".onion" Special-Use
Domain Name", RFC 7686, DOI 10.17487/RFC7686, October
2015, <https://www.rfc-editor.org/info/rfc7686>.
[RFC7706] Kumari, W. and P. Hoffman, "Decreasing Access Time to Root
Servers by Running One on Loopback", RFC 7706,
DOI 10.17487/RFC7706, November 2015, <https://www.rfc-
editor.org/info/rfc7706>.
[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>.
[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>.
12.3. URIs
[1] https://www.ietf.org/mail-archive/web/dns-privacy/current/
pdfWqAIUmEl47.pdf
[2] https://petsymposium.org/2018/files/hotpets/4-siby.pdf
[3] http://tma.ifip.org/2018/wp-content/uploads/sites/3/2018/06/
tma2018_paper30.pdf
[4] http://dnstap.info
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[5] https://nginx.org/
[6] https://www.haproxy.org/
[7] https://kb.isc.org/article/AA-01386/0/DNS-over-TLS.html
[8] https://dnsdist.org
[9] https://doi.org/10.1145/3182660
[10] https://github.com/Sinodun/draft-dprive-bcp-op/blob/master/
draft-00/ip_techniques_table.svg
[11] https://pdfs.semanticscholar.org/7b34/12c951cebe71cd2cddac5fda16
4fb2138a44.pdf
[12] https://kb.isc.org/docs/aa-00482
[13] https://surf.nl/datasharing
[14] https://dnsprivacy.org/wiki/display/DP/
Comparison+of+policy+and+privacy+statements
[15] https://wiki.mozilla.org/Security/DOH-resolver-policy
[16] https://www.ssllabs.com/ssltest/
[17] https://internet.nl
[18] https://support.google.com/analytics/answer/2763052?hl=en
[19] https://www.conversionworks.co.uk/blog/2017/05/19/anonymize-ip-
geo-impact-test/
[20] https://github.com/edmonds/pdns/blob/master/pdns/dnswasher.cc
[21] http://ita.ee.lbl.gov/html/contrib/tcpdpriv.html
[22] http://an.kaist.ac.kr/~sbmoon/paper/intl-journal/2004-cn-
anon.pdf
[23] https://www.cc.gatech.edu/computing/Telecomm/projects/cryptopan/
[24] http://mharvan.net/talks/noms-ip_anon.pdf
[25] http://www.ecs.umass.edu/ece/wolf/pubs/ton2007.pdf
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[26] https://medium.com/@bert.hubert/on-ip-address-encryption-
security-analysis-with-respect-for-privacy-dabe1201b476
[27] https://github.com/PowerDNS/ipcipher
[28] https://github.com/veorq/ipcrypt
[29] https://www.ietf.org/mail-archive/web/cfrg/current/msg09494.html
[30] http://dl.ifip.org/db/conf/im/im2019/189282.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 of the document.
A.1. Potential increases in DNS privacy
These documents are limited in scope to communications between stub
clients and recursive resolvers:
o 'Specification for DNS over Transport Layer Security (TLS)'
[RFC7858], referred to here as 'DNS-over-TLS'.
o 'DNS over Datagram Transport Layer Security (DTLS)' [RFC8094],
referred to here as 'DNS-over-DTLS'. Note that this document has
the Category of Experimental.
o 'DNS Queries over HTTPS (DoH)' [RFC8484] referred to here as DoH.
o 'Usage Profiles for DNS over TLS and DNS over DTLS' [RFC8310]
o 'The EDNS(0) Padding Option' [RFC7830] and 'Padding Policy for
EDNS(0)' [RFC8467]
These documents apply to recursive to authoritative DNS but are
relevant when considering the operation of a recursive server:
o 'DNS Query Name minimization to Improve Privacy' [RFC7816]
referred to here as 'QNAME minimization'
A.2. Potential decreases in DNS privacy
These documents relate to functionality that could provide increased
tracking of user activity as a side effect:
o 'Client Subnet in DNS Queries' [RFC7871]
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o 'Domain Name System (DNS) Cookies' [RFC7873])
o 'Transport Layer Security (TLS) Session Resumption without Server-
Side State' [RFC5077] referred to here as simply TLS session
resumption.
o 'A DNS Packet Capture Format' [I-D.ietf-dnsop-dns-capture-format]
o Passive DNS [I-D.ietf-dnsop-terminology-bis]
Note that depending on the specifics of the implementation [RFC8484]
may also provide increased tracking.
A.3. Related operational documents
o 'DNS Transport over TCP - Implementation Requirements' [RFC7766]
o 'Operational requirements for DNS-over-TCP'
[I-D.ietf-dnsop-dns-tcp-requirements]
o 'The edns-tcp-keepalive EDNS0 Option' [RFC7828]
o 'DNS Stateful Operations' [I-D.ietf-dnsop-session-signal]
Appendix B. IP address techniques
Data minimization methods may be categorized by the processing used
and the properties of their outputs. The following builds on the
categorization employed in [RFC6235]:
o Format-preserving. Normally when encrypting, the original data
length and patterns in the data should be hidden from an attacker.
Some applications of de-identification, such as network capture
de-identification, require that the de-identified data is of the
same form as the original data, to allow the data to be parsed in
the same way as the original.
o Prefix preservation. Values such as IP addresses and MAC
addresses contain prefix information that can be valuable in
analysis, e.g. manufacturer ID in MAC addresses, subnet in IP
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.
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o Replacement. A one-to-one replacement of a field to a new value
of the same type, for example using a regular expression.
o Filtering. Removing (and thus truncating) or replacing data in a
field. Field data can be overwritten, often with zeros, either
partially (grey marking) or completely (black marking).
o Generalization. Data is replaced by more general data with
reduced specificity. One example would be to replace all TCP/UDP
port numbers with one of two fixed values indicating whether the
original port was ephemeral (>=1024) or non-ephemeral (>1024).
Another example, precision degradation, reduces the accuracy of
e.g. a numeric value or a timestamp.
o Enumeration. With data from a well-ordered set, replace the first
data item data using a random initial value and then allocate
ordered values for subsequent data items. When used with
timestamp data, this preserves ordering but loses precision and
distance.
o Reordering/shuffling. Preserving the original data, but
rearranging its order, often in a random manner.
o Random substitution. As replacement, but using randomly generated
replacement values.
o Cryptographic permutation. Using a permutation function, such as
a hash function or cryptographic block cipher, to generate a
replacement de-identified value.
B.1. Google Analytics non-prefix filtering
Since May 2010, Google Analytics has provided a facility [18] 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 [19] 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%.
Anonymization: Format-preserving, Filtering (grey marking).
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B.2. dnswasher
Since 2006, PowerDNS have included a de-identification tool dnswasher
[20] 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.
Anonymization: Format-preserving, Enumeration.
B.3. Prefix-preserving map
Used in TCPdpriv [21], this algorithm stores a set of original and
anonymised IP address pairs. When a new IP address arrives, it is
compared with previous addresses to determine the longest prefix
match. The new address is anonymized by using the same prefix, with
the remainder of the address anonymized with a random value. The use
of a random value means that TCPdrpiv is not deterministic; different
anonymized values will be generated on each run. The need to store
previous addresses means that TCPdpriv has significant and unbounded
memory requirements, and because of the need to allocated anonymized
addresses sequentially cannot be used in parallel processing.
Anonymization: Format-preserving, prefix preservation (general).
B.4. Cryptographic Prefix-Preserving Pseudonymisation
Cryptographic prefix-preserving pseudonymisation was originally
proposed as an improvement to the prefix-preserving map implemented
in TCPdpriv, described in Xu et al. [22] and implemented in the
Crypto-PAn tool [23]. Crypto-PAn is now frequently used as an
acronym for the algorithm. Initially it was described for IPv4
addresses only; extension for IPv6 addresses was proposed in Harvan &
Schoenwaelder [24] and implemented in snmpdump. This uses a
cryptographic algorithm rather than a random value, and thus
pseudonymity is determined uniquely by the encryption key, and is
deterministic. It requires a separate AES encryption for each output
bit, so has a non-trivial calculation overhead. This can be
mitigated to some extent (for IPv4, at least) by pre-calculating
results for some number of prefix bits.
Pseudonymization: Format-preserving, prefix preservation (general).
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B.5. Top-hash Subtree-replicated Anonymisation
Proposed in Ramaswamy & Wolf [25], Top-hash Subtree-replicated
Anonymisation (TSA) originated in response to the requirement for
faster processing than Crypto-PAn. It used hashing for the most
significant byte of an IPv4 address, and a pre-calculated binary tree
structure for the remainder of the address. To save memory space,
replication is used within the tree structure, reducing the size of
the pre-calculated structures to a few Mb for IPv4 addresses.
Address pseudonymization is done via hash and table lookup, and so
requires minimal computation. However, due to the much increased
address space for IPv6, TSA is not memory efficient for IPv6.
Pseudonymization: Format-preserving, prefix preservation (general).
B.6. ipcipher
A recently-released proposal from PowerDNS [26], ipcipher [27] 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 [28]
suitable for 32bit block lengths. However, the author of ipcrypt has
since indicated [29] that it has low security, and further analysis
has revealed it is vulnerable to attack.
Pseudonymization: Format-preserving, cryptographic permutation.
B.7. Bloom filters
van Rijswijk-Deij et al. [30] have recently described work using
Bloom filters to categorize query traffic and record the traffic as
the state of multiple filters. The goal of this work is to allow
operators to identify so-called Indicators of Compromise (IOCs)
originating from specific subnets without storing information about,
or be able to monitor the DNS queries of an individual user. By
using a Bloom filter, it is possible to determine with a high
probability if, for example, a particular query was made, but the set
of queries made cannot be recovered from the filter. Similarly, by
mixing queries from a sufficient number of users in a single filter,
it becomes practically impossible to determine if a particular user
performed a particular query. Large numbers of queries can be
tracked in a memory-efficient way. As filter status is stored, this
approach cannot be used to regenerate traffic, and so cannot be used
with tools used to process live traffic.
Anonymized: Generalization.
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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
Amsterdam 1098 XH
The Netherlands
Email: benno@nlnetLabs.nl
Roland M. van Rijswijk-Deij
NLnet Labs
Science Park 400
Amsterdam 1098 XH
The Netherlands
Email: roland@nlnetLabs.nl
Allison Mankin
Salesforce
Email: allison.mankin@gmail.com
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