dprive S. Dickinson
Internet-Draft Sinodun IT
Intended status: Best Current Practice B. Overeinder
Expires: January 17, 2019 NLnet Labs
R. van Rijswijk-Deij
SURFnet bv
A. Mankin
Salesforce
July 16, 2018
Recommendations for DNS Privacy Service Operators
draft-dickinson-dprive-bcp-op-01
Abstract
This document presents operational, policy and security
considerations for DNS operators who choose to offer DNS Privacy
services. With the recommendations, the operator can make deliberate
decisions which services to provide, and how the decisions and
alternatives impact the privacy of users.
This document also presents a framework to assist writers of DNS
Privacy Policy and Practices Statements (analogous to DNS Security
Extensions (DNSSEC) Policies and DNSSEC Practice Statements described
in [RFC6841]).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 17, 2019.
Dickinson, et al. Expires January 17, 2019 [Page 1]
Internet-Draft DNS Privacy Service Recommendations July 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Privacy related documents . . . . . . . . . . . . . . . . . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
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. Availability . . . . . . . . . . . . . . . . . . . . 10
5.1.5. Service options . . . . . . . . . . . . . . . . . . . 11
5.1.6. Limitations of using a pure TLS proxy . . . . . . . . 11
5.2. Data at rest on the server . . . . . . . . . . . . . . . 12
5.2.1. Data handling . . . . . . . . . . . . . . . . . . . . 12
5.2.2. Data minimization of network traffic . . . . . . . . 13
5.2.3. IP address pseudonymization and anonymization methods 14
5.2.4. Pseudonymization, anonymization or discarding of
other correlation data . . . . . . . . . . . . . . . 14
5.2.5. Cache snooping . . . . . . . . . . . . . . . . . . . 15
5.3. Data sent onwards from the server . . . . . . . . . . . . 15
5.3.1. Protocol recommendations . . . . . . . . . . . . . . 15
5.3.2. Client query obfuscation . . . . . . . . . . . . . . 16
5.3.3. Data sharing . . . . . . . . . . . . . . . . . . . . 17
6. DNS privacy policy and practice statement . . . . . . . . . . 17
6.1. Recommended contents of a DPPPS . . . . . . . . . . . . . 18
6.2. Current policy and privacy statements . . . . . . . . . . 19
6.2.1. Quad9 . . . . . . . . . . . . . . . . . . . . . . . . 19
6.2.2. Cloudflare . . . . . . . . . . . . . . . . . . . . . 19
6.2.3. Google . . . . . . . . . . . . . . . . . . . . . . . 20
6.2.4. OpenDNS . . . . . . . . . . . . . . . . . . . . . . . 20
6.2.5. Comparison . . . . . . . . . . . . . . . . . . . . . 20
Dickinson, et al. Expires January 17, 2019 [Page 2]
Internet-Draft DNS Privacy Service Recommendations July 2018
6.3. Enforcement/accountability . . . . . . . . . . . . . . . 20
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Security considerations . . . . . . . . . . . . . . . . . . . 21
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 23
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Appendix A. Documents . . . . . . . . . . . . . . . . . . . . . 26
A.1. Potential increases in DNS privacy . . . . . . . . . . . 26
A.2. Potential decreases in DNS privacy . . . . . . . . . . . 27
A.3. Related operational documents . . . . . . . . . . . . . . 27
Appendix B. IP address techniques . . . . . . . . . . . . . . . 28
B.1. Google Analytics non-prefix filtering . . . . . . . . . . 29
B.2. dnswasher . . . . . . . . . . . . . . . . . . . . . . . . 29
B.3. Prefix-preserving map . . . . . . . . . . . . . . . . . . 29
B.4. Cryptographic Prefix-Preserving Pseudonymisation . . . . 30
B.5. Top-hash Subtree-replicated Anonymisation . . . . . . . . 30
B.6. ipcipher . . . . . . . . . . . . . . . . . . . . . . . . 30
B.7. Bloom filters . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
[NOTE: This document is submitted to the IETF for initial review and
for feedback on the best forum for future versions of this document.
Initial considerations for DoH [I-D.ietf-doh-dns-over-https] are
included here in anticipation of that draft progressing to be an RFC
but further analysis is required.]
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 focussed DNS services.
In recent years there has also been an increase in the availability
of "open 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
Dickinson, et al. Expires January 17, 2019 [Page 3]
Internet-Draft DNS Privacy Service Recommendations July 2018
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.
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 users
selection of resolver.
More recently the global legislative landscape with regard to
personal data collection, retention, and pseudonymization has seen
significant activity with differing requirements active in different
jurisdictions. For example the user of a service and the service
itself may be in jurisdictions with conflicting legislation. 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 DNS
open 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
Dickinson, et al. Expires January 17, 2019 [Page 4]
Internet-Draft DNS Privacy Service Recommendations July 2018
particular Policy or Practice Statement, nor does it seek to
provide legal advice or recommendations as to the contents.
Community insight [or judgment?] about operational practices can
change quickly, and experience shows that a Best Current Practice
(BCP) document about privacy and security is a point-in-time
statement. Readers are advised to seek out any errata or updates
that apply to this document.
2. Scope
"DNS Privacy Considerations" [I-D.bortzmeyer-dprive-rfc7626-bis]
describes the general privacy issues and threats associated with the
use of the DNS by Internet users and much of the threat analysis here
is lifted from that document and from [RFC6873]. 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.
Dickinson, et al. Expires January 17, 2019 [Page 5]
Internet-Draft DNS Privacy Service Recommendations July 2018
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.
Privacy terminology is as described in Section 3 of [RFC6973].
DNS terminology is as described in [I-D.ietf-dnsop-terminology-bis]
with one modification: we use the definition of Privacy-enabling DNS
server taken from [RFC8310]:
o Privacy-enabling DNS server: A DNS server (most likely a full-
service resolver) that implements DNS-over-TLS [RFC7858], and may
optionally implement DNS-over-DTLS [RFC8094]. The server should
also offer at least one of the credentials described in Section 8
and implement the (D)TLS profile described in Section 9.
TODO: Update the definition of Privacy-enabling DNS server in
[I-D.ietf-dnsop-terminology-bis] to be complete and also include DoH,
then reference that here.
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 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
Dickinson, et al. Expires January 17, 2019 [Page 6]
Internet-Draft DNS Privacy Service Recommendations July 2018
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
TODO: Some of the threats listed in the following sections are taken
directly from Section 5 of RFC6973, some are just standalone
descriptions, we need to go through all of them and see if we can use
the RFC6973 threats where possible and make them consistent.
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
Threats:
o Surveillance: Passive surveillance of traffic on the wire
o Intrusion: 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]
o DoH [I-D.ietf-doh-dns-over-https]
Additional options:
o A DNS privacy service can also be provided over DNS-over-DTLS
[RFC8094], however note that this is an Experimental
specification.
It is 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
service is out of scope for this document.
Dickinson, et al. Expires January 17, 2019 [Page 7]
Internet-Draft DNS Privacy Service Recommendations July 2018
5.1.2. Authentication of DNS privacy services
Threats:
o Surveillance and Intrusion: Active attacks that can redirect
traffic to rogue servers
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, SPKI
pinsets or TLSA records.
When offering DoH [I-D.ietf-doh-dns-over-https], HTTPS requires
authentication of the server as part of the protocol.
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.
o Implementing [I-D.ietf-tls-dnssec-chain-extension]
* This can decrease the latency of connection setup to the server
and remove the need for the client to perform meta-queries to
obtain and validate the DANE records.
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.
Dickinson, et al. Expires January 17, 2019 [Page 8]
Internet-Draft DNS Privacy Service Recommendations July 2018
It is noted that SPKI pinset management is described in [RFC7858] but
that key pinning mechanisms in general have fallen out of favour
operationally for various reasons.
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 Choose a short, memorable authentication name for their service
o Automate the generation and publication of certificates
o Monitor certificates to prevent accidental expiration of
certificates
TODO: Could we provide references for certificate management best
practice, for example Section 6.5 of RFC7525?
5.1.3. Protocol recommendations
5.1.3.1. DNS-over-TLS
Threats:
o Known attacks on TLS (TODO: add a reference)
o Traffic analysis (TODO: add a reference)
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]
Dickinson, et al. Expires January 17, 2019 [Page 9]
Internet-Draft DNS Privacy Service Recommendations July 2018
o Implementing only (D)TLS 1.2 or later as specified in [RFC8310]
o Implementing EDNS(0) Padding [RFC7830] using the guidelines in
[I-D.ietf-dprive-padding-policy]
o Clients should not be required to use TLS session resumption
[RFC5077], Domain Name System (DNS) Cookies [RFC7873].
o A DNS-over-TLS privacy service on both port 853 and 443. We note
that this practice may require revision when DoH becomes more
widely deployed, because of the potential use of the same ports
for two incompatible types of service.
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
TODO: Fill this in, a lot of overlap with DNS-over-TLS but we need to
address DoH specific ones if possible.
Mitigations:
o Clients should not be required to use HTTP Cookies [RFC6265].
o Clients should not be required to include any headers beyond the
absolute minimum to obtain service from a DoH server.
5.1.4. Availability
Threats:
Dickinson, et al. Expires January 17, 2019 [Page 10]
Internet-Draft DNS Privacy Service Recommendations July 2018
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:
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.
TODO: Add reference to ongoing research on this topic.
5.1.5. Service options
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
offered on un-encrypted channels in terms of such options as
filtering (or lack of), DNSSEC validation, etc.
5.1.6. Limitations of using a pure TLS proxy
Optimization:
Some operators may choose to implement DNS-over-TLS using a TLS proxy
(e.g. nginx [1], haproxy [2] or stunnel [3]) 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.
Dickinson, et al. Expires January 17, 2019 [Page 11]
Internet-Draft DNS Privacy Service Recommendations July 2018
5.2. Data at rest on the server
5.2.1. Data handling
Threats:
o Surveillance
o Stored data compromise
o Correlation
o Identification
o Secondary use
o Disclosure
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 personal who require
access to perform operational duties.
Dickinson, et al. Expires January 17, 2019 [Page 12]
Internet-Draft DNS Privacy Service Recommendations July 2018
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
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. [4]
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.
Dickinson, et al. Expires January 17, 2019 [Page 13]
Internet-Draft DNS Privacy Service Recommendations July 2018
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.
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. This 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) [5]
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 [6]) have observed that, as the
entropy in a 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
Threats:
o IP TTL/Hoplimit can be used to fingerprint client OS
o Tracking of TCP sessions
o Tracking of TLS sessions and session resumption mechanisms
Dickinson, et al. Expires January 17, 2019 [Page 14]
Internet-Draft DNS Privacy Service Recommendations July 2018
o Resolvers _might_ receive client identifiers e.g. MAC addresses
in EDNS(0) options - some CPE devices are known to add them.
o HTTP headers
Mitigations:
o Data minimization or discarding of such correlation data
TODO: More analysis here.
5.2.5. Cache snooping
Threats:
o Profiling of client queries by malicious third parties
Mitigations:
TODO: Describe techniques to defend against cache snooping
5.3. Data sent onwards from the server
In this section we consider both data sent on the wire in upstream
queries and data shared with third parties.
5.3.1. Protocol recommendations
Threats:
o 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 Honour 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
Dickinson, et al. Expires January 17, 2019 [Page 15]
Internet-Draft DNS Privacy Service Recommendations July 2018
* 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.
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 preform 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
that aligns with their own practices) over an encrypted protocol so
that the upstream queries are obfuscated among those of the large
resolver.
Dickinson, et al. Expires January 17, 2019 [Page 16]
Internet-Draft DNS Privacy Service Recommendations July 2018
5.3.3. Data sharing
Threats:
o Surveillance
o Stored data compromise
o Correlation
o Identification
o Secondary use
o Disclosure
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.
TODO: More on data for research vs operations... how to still
motivate operators to share anonymized data?
TODO: Guidelines for when consent is granted?
TODO: Applies to server data handling too.. could operators offer
alternatives services one that implies consent for data processing,
one that doesn't?
6. DNS privacy policy and practice statement
Dickinson, et al. Expires January 17, 2019 [Page 17]
Internet-Draft DNS Privacy Service Recommendations July 2018
6.1. Recommended contents of a DPPPS
1 Policy
1.1 Recommendations. This section should explain, with reference to
section Section 5 of this document which recommendations the DNS
privacy service employs.
1.2 Data handling. This section should explain, with reference to
section Section 5.2 of this document the policy for gathering and
disseminating information collected by the DNS privacy service.
1.2.1 Specify clearly what data (including whether it is aggregated,
pseudonymized or anonymized) is:
1.2.1.1 Collected and retained by the operator (and for how long)
1.2.1.2 Shared with partners
1.2.1.3 Shared, sold or rented to third-parties
1.2.2 Specify any exceptions to the above, for example technically
malicious or anomalous behaviour
1.2.3 Declare any partners, third-party affiliations or sources of
funding
1.2.4 Whether user DNS data is correlated or combined with any other
personal information held by the operator
2 Practice. This section should explain the current operational
practices of the service.
2.1 Specify any temporary or permanent deviations from the policy for
operational reasons
2.2 With reference to section Section 5.1 provide specific details of
which capabilities are provided on which address and ports
2.3 With reference to section Section 5.3 provide specific details of
which capabilities are employed for upstream traffic from the server
2.4 Specify the authentication name to be used (if any) and if TLSA
records are published (including options used in the TLSA records)
2.5 Specify the SPKI pinsets to be used (if any) and policy for
rolling keys
Dickinson, et al. Expires January 17, 2019 [Page 18]
Internet-Draft DNS Privacy Service Recommendations July 2018
2.6 Provide a contact email address for the service
6.2. Current policy and privacy statements
NOTE: An analysis of these statements will clearly only provide a
snapshot at the time of writing. It is included in this version of
the draft to provide a basis for the assessment of the contents of
the DPPPS and is expected to be removed or substantially re-worked in
a future version.
6.2.1. Quad9
UDP/TCP and TLS (port 853) service provided on two addresses:
o 'Secure': 9.9.9.9, 149.112.112.112, 2620:fe::fe, 2620:fe::9
o 'Unsecured': 9.9.9.10, 149.112.112.10, 2620:fe::10
Policy:
o <https://www.quad9.net/policy/>
o <https://www.quad9.net/privacy/>
o <https://www.quad9.net/faq/>
6.2.2. Cloudflare
UDP/TCP and TLS (port 853) service provided on 1.1.1.1, 1.0.0.1,
2606:4700:4700::1111 and 2606:4700:4700::1001.
Policy:
o <https://developers.cloudflare.com/1.1.1.1/commitment-to-privacy/
privacy-policy/privacy-policy/>
DoH provided on: <https://cloudflare-dns.com/dns-query>
Policy:
o <https://developers.cloudflare.com/1.1.1.1/commitment-to-privacy/
privacy-policy/firefox/>
Tor endpoint: <https://dns4torpnlfs2ifuz2s2yf3fc7rdmsbhm6rw75euj35pac
6ap25zgqad.onion>.
Dickinson, et al. Expires January 17, 2019 [Page 19]
Internet-Draft DNS Privacy Service Recommendations July 2018
6.2.3. Google
UDP/TCP service provided on 8.8.8.8, 8.8.4.4, 2001:4860:4860::8888
and 2001:4860:4860::8844.
Policy: <https://developers.google.com/speed/public-dns/privacy>
6.2.4. OpenDNS
UDP/TCP service provided on 208.67.222.222 and 208.67.220.220 (no
IPv6).
We could find no specific privacy policy for the DNS resolution, only
a general one from Cisco that seems focussed on websites.
Policy: <https://www.cisco.com/c/en/us/about/legal/privacy-full.html>
6.2.5. Comparison
The following tables provides a high-level comparison of the policy
and practice statements above and also some observations of practice
measured at dnsprivacy.org [7]. The data is not exhaustive and has
not been reviewed or confirmed by the operators.
A question mark indicates no clear statement or data could be located
on the issue. A dash indicates the category is not applicable to the
service.
Table showing comparison of operators policies [8]
Table showing comparison of operators practices [9]
NOTE: Review and correction of any inaccuracies in the table would be
much appreciated.
6.3. Enforcement/accountability
Transparency reports may help with building user trust that operators
adhere to their policies and practices.
Independent monitoring should be performed where possible of:
o ECS, QNAME minimization, EDNS(0) padding, etc.
o Filtering
o Uptime
Dickinson, et al. Expires January 17, 2019 [Page 20]
Internet-Draft DNS Privacy Service Recommendations July 2018
7. IANA considerations
None
8. Security considerations
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 also to John Todd for
discussions on this topic, and to Stephane Bortzmeyer for review.
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-dickinson-dprive-bcp-op-01
o Update reference to RFC7626 to draft-bortzmeyer-rfc7626-bis
o Fix a few typos
draft-dickinson-dprive-bcp-op-00
Name change to add dprive. Differences to draft-dickinson-bcp-op-00:
o Reworked the Terminology, Introduction and Scope
Dickinson, et al. Expires January 17, 2019 [Page 21]
Internet-Draft DNS Privacy Service Recommendations July 2018
o Added Document section
o Reworked the Recommendations section to describe threat
mitigations, optimizations and other options. Split the
recommendations up into 3 subsections: on the wire, at rest and
upstream
o Added much more information on data handling and IP address
pseudonymization and anonymization
o Added more details and comparison of some existing policy/privacy
policies
o Applied virtually all of Amelia Andersdotter's suggested changes.
draft-dickinson-bcp-op-00
o Initial commit
12. References
12.1. Normative References
[I-D.ietf-dnsop-terminology-bis]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", draft-ietf-dnsop-terminology-bis-11 (work in
progress), July 2018.
[I-D.ietf-doh-dns-over-https]
Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", draft-ietf-doh-dns-over-https-12 (work in
progress), June 2018.
[I-D.ietf-dprive-padding-policy]
Mayrhofer, A., "Padding Policy for EDNS(0)", draft-ietf-
dprive-padding-policy-05 (work in progress), April 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>.
Dickinson, et al. Expires January 17, 2019 [Page 22]
Internet-Draft DNS Privacy Service Recommendations July 2018
[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>.
[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>.
[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>.
[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>.
12.2. Informative References
[I-D.bortzmeyer-dprive-rfc7626-bis]
Bortzmeyer, S. and S. Dickinson, "DNS Privacy
Considerations", draft-bortzmeyer-dprive-rfc7626-bis-00
(work in progress), July 2018.
Dickinson, et al. Expires January 17, 2019 [Page 23]
Internet-Draft DNS Privacy Service Recommendations July 2018
[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-07 (work in progress), May
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-02 (work in progress), May 2018.
[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-11 (work in progress),
July 2018.
[I-D.ietf-tls-dnssec-chain-extension]
Shore, M., Barnes, R., Huque, S., and W. Toorop, "A DANE
Record and DNSSEC Authentication Chain Extension for TLS",
draft-ietf-tls-dnssec-chain-extension-07 (work in
progress), March 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>.
[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>.
[RFC6873] Salgueiro, G., Gurbani, V., and A. Roach, "Format for the
Session Initiation Protocol (SIP) Common Log Format
(CLF)", RFC 6873, DOI 10.17487/RFC6873, February 2013,
<https://www.rfc-editor.org/info/rfc6873>.
[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>.
Dickinson, et al. Expires January 17, 2019 [Page 24]
Internet-Draft DNS Privacy Service Recommendations July 2018
[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>.
[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>.
[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>.
[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>.
[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://nginx.org/
[2] https://www.haproxy.org/
[3] https://kb.isc.org/article/AA-01386/0/DNS-over-TLS.html
[4] https://doi.org/10.1145/3182660
[5] https://github.com/Sinodun/draft-dprive-bcp-op/blob/master/draft-
01/ip_techniques_table.svg
[6] https://pdfs.semanticscholar.org/7b34/12c951cebe71cd2cddac5fda164
fb2138a44.pdf
[7] https://dnsprivacy.org/jenkins/job/dnsprivacy-monitoring/
Dickinson, et al. Expires January 17, 2019 [Page 25]
Internet-Draft DNS Privacy Service Recommendations July 2018
[8] https://github.com/Sinodun/draft-dprive-bcp-op/blob/master/draft-
01/policy_table.svg
[9] https://github.com/Sinodun/draft-dprive-bcp-op/blob/master/draft-
01/practice_table.svg
[10] https://support.google.com/analytics/answer/2763052?hl=en
[11] https://www.conversionworks.co.uk/blog/2017/05/19/anonymize-ip-
geo-impact-test/
[12] https://github.com/edmonds/pdns/blob/master/pdns/dnswasher.cc
[13] http://ita.ee.lbl.gov/html/contrib/tcpdpriv.html
[14] http://an.kaist.ac.kr/~sbmoon/paper/intl-journal/2004-cn-
anon.pdf
[15] https://www.cc.gatech.edu/computing/Telecomm/projects/cryptopan/
[16] http://mharvan.net/talks/noms-ip_anon.pdf
[17] https://medium.com/@bert.hubert/on-ip-address-encryption-
security-analysis-with-respect-for-privacy-dabe1201b476
[18] https://github.com/PowerDNS/ipcipher
[19] https://github.com/veorq/ipcrypt
[20] https://www.ietf.org/mail-archive/web/cfrg/current/msg09494.html
[21] https://tnc18.geant.org/core/presentation/127
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'.
Dickinson, et al. Expires January 17, 2019 [Page 26]
Internet-Draft DNS Privacy Service Recommendations July 2018
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)' [I-D.ietf-doh-dns-over-https]
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)' [I-D.ietf-dprive-padding-policy]
These documents apply to recursive to authoritative DNS but are
relevant when considering the operation of a recursive server:
o 'DNS Query Name minimization to Improve Privacy' [RFC7816]
referred to here as 'QNAME minimization'
A.2. Potential decreases in DNS privacy
These documents relate to functionality that could provide increased
tracking of user activity as a side effect:
o 'Client Subnet in DNS Queries' [RFC7871]
o 'Domain Name System (DNS) Cookies' [RFC7873])
o 'Transport Layer Security (TLS) Session Resumption without Server-
Side State' [RFC5077] referred to here as simply TLS session
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
[I-D.ietf-doh-dns-over-https] 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]
Dickinson, et al. Expires January 17, 2019 [Page 27]
Internet-Draft DNS Privacy Service Recommendations July 2018
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.
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.
Dickinson, et al. Expires January 17, 2019 [Page 28]
Internet-Draft DNS Privacy Service Recommendations July 2018
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 [10] 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 [11] 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).
B.2. dnswasher
Since 2006, PowerDNS have included a de-identification tool dnswasher
[12] 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 [13], 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).
Dickinson, et al. Expires January 17, 2019 [Page 29]
Internet-Draft DNS Privacy Service Recommendations July 2018
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. [14] and implemented in the
Crypto-PAn tool [15]. 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 [16] 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).
B.5. Top-hash Subtree-replicated Anonymisation
Proposed in Ramaswamy & Wolf, 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 [17], ipcipher [18] 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 [19]
suitable for 32bit block lengths. However, the author of ipcrypt has
since indicated [20] that it has low security, and further analysis
has revealed it is vulnerable to attack.
Pseudonymization: Format-preserving, cryptographic permutation.
Dickinson, et al. Expires January 17, 2019 [Page 30]
Internet-Draft DNS Privacy Service Recommendations July 2018
B.7. Bloom filters
van Rijswijk-Deij et al. [21] 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.
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
SURFnet bv
PO Box 19035
Utrecht 3501 DA Utrecht
The Netherlands
Email: roland.vanrijswijk@surfnet.nl
Dickinson, et al. Expires January 17, 2019 [Page 31]
Internet-Draft DNS Privacy Service Recommendations July 2018
Allison Mankin
Salesforce
Email: allison.mankin@gmail.com
Dickinson, et al. Expires January 17, 2019 [Page 32]
