dprive S. Bortzmeyer
Internet-Draft AFNIC
Obsoletes: 7626 (if approved) S. Dickinson
Intended status: Informational Sinodun IT
Expires: May 21, 2020 November 18, 2019
DNS Privacy Considerations
draft-ietf-dprive-rfc7626-bis-03
Abstract
This document describes the privacy issues associated with the use of
the DNS by Internet users. It is intended to be an analysis of the
present situation and does not prescribe solutions. This document
obsoletes RFC 7626.
Status of This Memo
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This Internet-Draft will expire on May 21, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. The Alleged Public Nature of DNS Data . . . . . . . . . . 5
3.2. Data in the DNS Request . . . . . . . . . . . . . . . . . 6
3.2.1. Data in the DNS payload . . . . . . . . . . . . . . . 7
3.3. Cache Snooping . . . . . . . . . . . . . . . . . . . . . 7
3.4. On the Wire . . . . . . . . . . . . . . . . . . . . . . . 8
3.4.1. Unencrypted Transports . . . . . . . . . . . . . . . 8
3.4.2. Encrypted Transports . . . . . . . . . . . . . . . . 9
3.5. In the Servers . . . . . . . . . . . . . . . . . . . . . 10
3.5.1. In the Recursive Resolvers . . . . . . . . . . . . . 11
3.5.2. In the Authoritative Name Servers . . . . . . . . . . 15
3.6. Re-identification and Other Inferences . . . . . . . . . 16
3.7. More Information . . . . . . . . . . . . . . . . . . . . 17
4. Actual "Attacks" . . . . . . . . . . . . . . . . . . . . . . 17
5. Legalities . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
9. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . 21
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
This document is an analysis of the DNS privacy issues, in the spirit
of Section 8 of [RFC6973].
The Domain Name System (DNS) is specified in [RFC1034], [RFC1035],
and many later RFCs, which have never been consolidated. It is one
of the most important infrastructure components of the Internet and
often ignored or misunderstood by Internet users (and even by many
professionals). Almost every activity on the Internet starts with a
DNS query (and often several). Its use has many privacy implications
and this document is an attempt at a comprehensive and accurate list.
Let us begin with a simplified reminder of how the DNS works (See
also [RFC8499]). A client, the stub resolver, issues a DNS query to
a server, called the recursive resolver (also called caching resolver
or full resolver or recursive name server). Let's use the query
"What are the AAAA records for www.example.com?" as an example. AAAA
is the QTYPE (Query Type), and www.example.com is the QNAME (Query
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Name). (The description that follows assumes a cold cache, for
instance, because the server just started.) The recursive resolver
will first query the root name servers. In most cases, the root name
servers will send a referral. In this example, the referral will be
to the .com name servers. The resolver repeats the query to one of
the .com name servers. The .com name servers, in turn, will refer to
the example.com name servers. The example.com name server will then
return the answer. The root name servers, the name servers of .com,
and the name servers of example.com are called authoritative name
servers. It is important, when analyzing the privacy issues, to
remember that the question asked to all these name servers is always
the original question, not a derived question. The question sent to
the root name servers is "What are the AAAA records for
www.example.com?", not "What are the name servers of .com?". By
repeating the full question, instead of just the relevant part of the
question to the next in line, the DNS provides more information than
necessary to the name server. In this simplified description,
recursive resolvers do not implement QNAME minimization as described
in [RFC7816], which will only send the relevant part of the question
to the upstream name server.
Because DNS relies on caching heavily, the algorithm described above
is actually a bit more complicated, and not all questions are sent to
the authoritative name servers. If a few seconds later the stub
resolver asks the recursive resolver, "What are the SRV records of
_xmpp-server._tcp.example.com?", the recursive resolver will remember
that it knows the name servers of example.com and will just query
them, bypassing the root and .com. Because there is typically no
caching in the stub resolver, the recursive resolver, unlike the
authoritative servers, sees all the DNS traffic. (Applications, like
web browsers, may have some form of caching that does not follow DNS
rules, for instance, because it may ignore the TTL. So, the
recursive resolver does not see all the name resolution activity.)
It should be noted that DNS recursive resolvers sometimes forward
requests to other recursive resolvers, typically bigger machines,
with a larger and more shared cache (and the query hierarchy can be
even deeper, with more than two levels of recursive resolvers). From
the point of view of privacy, these forwarders are like resolvers,
except that they do not see all of the requests being made (due to
caching in the first resolver).
At the time of writing, almost all this DNS traffic is currently sent
in clear (i.e., unencrypted). However there is increasing deployment
of DNS-over-TLS (DoT) [RFC7858] and DNS-over-HTTPS (DoH) [RFC8484],
particularly in mobile devices, browsers, and by providers of anycast
recursive DNS resolution services. There are a few cases where there
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is some alternative channel encryption, for instance, in an IPsec VPN
tunnel, at least between the stub resolver and the resolver.
Today, almost all DNS queries are sent over UDP [thomas-ditl-tcp].
This has practical consequences when considering encryption of the
traffic as a possible privacy technique. Some encryption solutions
are only designed for TCP, not UDP and new solutions are still
emerging [I-D.ietf-quic-transport].
Another important point to keep in mind when analyzing the privacy
issues of DNS is the fact that DNS requests received by a server are
triggered by different reasons. Let's assume an eavesdropper wants
to know which web page is viewed by a user. For a typical web page,
there are three sorts of DNS requests being issued:
o Primary request: this is the domain name in the URL that the user
typed, selected from a bookmark, or chose by clicking on an
hyperlink. Presumably, this is what is of interest for the
eavesdropper.
o Secondary requests: these are the additional requests performed by
the user agent (here, the web browser) without any direct
involvement or knowledge of the user. For the Web, they are
triggered by embedded content, Cascading Style Sheets (CSS),
JavaScript code, embedded images, etc. In some cases, there can
be dozens of domain names in different contexts on a single web
page.
o Tertiary requests: these are the additional requests performed by
the DNS system itself. For instance, if the answer to a query is
a referral to a set of name servers, and the glue records are not
returned, the resolver will have to do additional requests to turn
the name servers' names into IP addresses. Similarly, even if
glue records are returned, a careful recursive server will do
tertiary requests to verify the IP addresses of those records.
It can be noted also that, in the case of a typical web browser, more
DNS requests than strictly necessary are sent, for instance, to
prefetch resources that the user may query later or when
autocompleting the URL in the address bar. Both are a big privacy
concern since they may leak information even about non-explicit
actions. For instance, just reading a local HTML page, even without
selecting the hyperlinks, may trigger DNS requests.
For privacy-related terms, we will use the terminology from
[RFC6973].
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2. Scope
This document focuses mostly on the study of privacy risks for the
end user (the one performing DNS requests). We consider the risks of
pervasive surveillance [RFC7258] as well as risks coming from a more
focused surveillance.
This document does not attempt a comparison of specific privacy
protections provided by individual networks or organisations, it
makes only general observations about typical current practices.
Privacy risks for the holder of a zone (the risk that someone gets
the data) are discussed in [RFC5936] and [RFC5155].
Privacy risks for recursive operators (including access providers and
operators in enterprise networks) such as leakage of private
namespaces or blocklists are out of scope for this document.
Non-privacy risks (e.g security related concerns such as cache
poisoning) are also out of scope.
The privacy risks associated with the use of other protocols, e.g.,
unencrypted TLS SNI extensions or HTTPS destination IP address
fingerprinting are not considered here.
3. Risks
3.1. The Alleged Public Nature of DNS Data
It has long been claimed that "the data in the DNS is public". While
this sentence makes sense for an Internet-wide lookup system, there
are multiple facets to the data and metadata involved that deserve a
more detailed look. First, access control lists (ACLs) and private
namespaces notwithstanding, the DNS operates under the assumption
that public-facing authoritative name servers will respond to "usual"
DNS queries for any zone they are authoritative for without further
authentication or authorization of the client (resolver). Due to the
lack of search capabilities, only a given QNAME will reveal the
resource records associated with that name (or that name's non-
existence). In other words: one needs to know what to ask for, in
order to receive a response. The zone transfer QTYPE [RFC5936] is
often blocked or restricted to authenticated/authorized access to
enforce this difference (and maybe for other reasons).
Another differentiation to be considered is between the DNS data
itself and a particular transaction (i.e., a DNS name lookup). DNS
data and the results of a DNS query are public, within the boundaries
described above, and may not have any confidentiality requirements.
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However, the same is not true of a single transaction or a sequence
of transactions; that transaction is not / should not be public. A
typical example from outside the DNS world is: the web site of
Alcoholics Anonymous is public; the fact that you visit it should not
be.
3.2. Data in the DNS Request
The DNS request includes many fields, but two of them seem
particularly relevant for the privacy issues: the QNAME and the
source IP address. "source IP address" is used in a loose sense of
"source IP address + maybe source port number", because the port
number is also in the request and can be used to differentiate
between several users sharing an IP address (behind a Carrier-Grade
NAT (CGN), for instance [RFC6269]).
The QNAME is the full name sent by the user. It gives information
about what the user does ("What are the MX records of example.net?"
means he probably wants to send email to someone at example.net,
which may be a domain used by only a few persons and is therefore
very revealing about communication relationships). Some QNAMEs are
more sensitive than others. For instance, querying the A record of a
well-known web statistics domain reveals very little (everybody
visits web sites that use this analytics service), but querying the A
record of www.verybad.example where verybad.example is the domain of
an organization that some people find offensive or objectionable may
create more problems for the user. Also, sometimes, the QNAME embeds
the software one uses, which could be a privacy issue. For instance,
_ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org.
There are also some BitTorrent clients that query an SRV record for
_bittorrent-tracker._tcp.domain.example.
Another important thing about the privacy of the QNAME is the future
usages. Today, the lack of privacy is an obstacle to putting
potentially sensitive or personally identifiable data in the DNS. At
the moment, your DNS traffic might reveal that you are doing email
but not with whom. If your Mail User Agent (MUA) starts looking up
Pretty Good Privacy (PGP) keys in the DNS [RFC7929], then privacy
becomes a lot more important. And email is just an example; there
would be other really interesting uses for a more privacy-friendly
DNS.
For the communication between the stub resolver and the recursive
resolver, the source IP address is the address of the user's machine.
Therefore, all the issues and warnings about collection of IP
addresses apply here. For the communication between the recursive
resolver and the authoritative name servers, the source IP address
has a different meaning; it does not have the same status as the
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source address in an HTTP connection. It is typically the IP address
of the recursive resolver that, in a way, "hides" the real user.
However, hiding does not always work. Sometimes EDNS(0) Client
subnet [RFC7871] is used (see its privacy analysis in
[denis-edns-client-subnet]). Sometimes the end user has a personal
recursive resolver on her machine. In both cases, the IP address is
as sensitive as it is for HTTP [sidn-entrada].
A note about IP addresses: there is currently no IETF document that
describes in detail all the privacy issues around IP addressing in
general, although [RFC7721] does discuss privacy considerations for
IPv6 address generation mechanisms. In the meantime, the discussion
here is intended to include both IPv4 and IPv6 source addresses. For
a number of reasons, their assignment and utilization characteristics
are different, which may have implications for details of information
leakage associated with the collection of source addresses. (For
example, a specific IPv6 source address seen on the public Internet
is less likely than an IPv4 address to originate behind an address
sharing scheme.) However, for both IPv4 and IPv6 addresses, it is
important to note that source addresses are propagated with queries
and comprise metadata about the host, user, or application that
originated them.
3.2.1. Data in the DNS payload
At the time of writing there are no standardized client identifiers
contained in the DNS payload itself (ECS [RFC7871] while widely used
is only of Category Informational).
DNS Cookies [RFC7873] are a lightweight DNS transaction security
mechanism that provides limited protection against a variety of
increasingly common denial-of-service and amplification/forgery or
cache poisoning attacks by off-path attackers. It is noted, however,
that they are designed to just verify IP addresses (and should change
once a client's IP address changes), they are not designed to
actively track users (like HTTP cookies).
There are anecdotal accounts of MAC addresses [1] and even user names
being inserted in non-standard EDNS(0) options for stub to resolver
communications to support proprietary functionality implemented at
the resolver (e.g., parental filtering).
3.3. Cache Snooping
The content of recursive resolvers' caches can reveal data about the
clients using it (the privacy risks depend on the number of clients).
This information can sometimes be examined by sending DNS queries
with RD=0 to inspect cache content, particularly looking at the DNS
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TTLs [grangeia.snooping]. Since this also is a reconnaissance
technique for subsequent cache poisoning attacks, some counter
measures have already been developed and deployed.
3.4. On the Wire
3.4.1. Unencrypted Transports
For unencrypted transports, DNS traffic can be seen by an
eavesdropper like any other traffic. (DNSSEC, specified in
[RFC4033], explicitly excludes confidentiality from its goals.) So,
if an initiator starts an HTTPS communication with a recipient, while
the HTTP traffic will be encrypted, the DNS exchange prior to it will
not be. When other protocols will become more and more privacy-aware
and secured against surveillance (e.g., [RFC8446],
[I-D.ietf-quic-transport]), the use of unencrypted transports for DNS
may become "the weakest link" in privacy. It is noted that at the
time of writing there is on-going work attempting to encrypt the SNI
in the TLS handshake [I-D.ietf-tls-sni-encryption].
An important specificity of the DNS traffic is that it may take a
different path than the communication between the initiator and the
recipient. For instance, an eavesdropper may be unable to tap the
wire between the initiator and the recipient but may have access to
the wire going to the recursive resolver, or to the authoritative
name servers.
The best place to tap, from an eavesdropper's point of view, is
clearly between the stub resolvers and the recursive resolvers,
because traffic is not limited by DNS caching.
The attack surface between the stub resolver and the rest of the
world can vary widely depending upon how the end user's device is
configured. By order of increasing attack surface:
o The recursive resolver can be on the end user's device. In
(currently) a small number of cases, individuals may choose to
operate their own DNS resolver on their local machine. In this
case, the attack surface for the connection between the stub
resolver and the caching resolver is limited to that single
machine.
o The recursive resolver may be at the local network edge. For
many/most enterprise networks and for some residential users, the
caching resolver may exist on a server at the edge of the local
network. In this case, the attack surface is the local network.
Note that in large enterprise networks, the DNS resolver may not
be located at the edge of the local network but rather at the edge
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of the overall enterprise network. In this case, the enterprise
network could be thought of as similar to the Internet Access
Provider (IAP) network referenced below.
o The recursive resolver can be in the IAP network. For most
residential users and potentially other networks, the typical case
is for the end user's device to be configured (typically
automatically through DHCP or RA options) with the addresses of
the DNS proxy in the CPE, which in turns points to the DNS
recursive resolvers at the IAP. The attack surface for on-the-
wire attacks is therefore from the end user system across the
local network and across the IAP network to the IAP's recursive
resolvers.
o The recursive resolver can be a public DNS service. Some machines
may be configured to use public DNS resolvers such as those
operated by Google Public DNS or OpenDNS. The end user may have
configured their machine to use these DNS recursive resolvers
themselves -- or their IAP may have chosen to use the public DNS
resolvers rather than operating their own resolvers. In this
case, the attack surface is the entire public Internet between the
end user's connection and the public DNS service.
It is also noted that typically a device connected _only_ to a modern
cellular network is
o directly configured with only the recursive resolvers of the IAP
and
o all traffic (including DNS) between the device and the cellular
network is encrypted following an encryption profile edited by the
Third Generation Partnership Project (3GPP [2]).
The attack surface for this specific scenario is not considered here.
3.4.2. Encrypted Transports
The use of encrypted transports directly mitigates passive
surveillance of the DNS payload, however there are still some privacy
attacks possible. This section enumerates the residual privacy risks
to an end user when an attacker can passively monitor encrypted DNS
traffic flows on the wire.
These are cases where user identification, fingerprinting or
correlations may be possible due to the use of certain transport
layers or clear text/observable features. These issues are not
specific to DNS, but DNS traffic is susceptible to these attacks when
using specific transports.
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There are some general examples, for example, certain studies have
highlighted that IPv4 TTL, IPv6 Hop Limit, or TCP Window sizes os-
fingerprint [3] values can be used to fingerprint client OS's or that
various techniques can be used to de-NAT DNS queries dns-de-nat [4].
The use of clear text transport options to optimize latency may also
identify a user, e.g., using TCP Fast Open with TLS 1.2 [RFC7413].
More specifically, (since the deployment of encrypted transports is
not widespread at the time of writing) users wishing to use encrypted
transports for DNS may in practice be limited in the resolver
services available. Given this, 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 can actually serve to
identify the user as one that desires privacy and can provide an
added mechanism to track them as they move across network
environments.
Users of encrypted transports are also highly likely to re-use
sessions for multiple DNS queries to optimize performance (e.g., via
DNS pipelining or HTTPS multiplexing). Certain configuration options
for encrypted transports could also in principle fingerprint a user
or client application. For example:
o TLS version or cipher suite selection
o session resumption
o the maximum number of messages to send or
o a maximum connection time before closing a connections and re-
opening.
Whilst there are known attacks on older versions of TLS the most
recent recommendations [RFC7525] and developments [RFC8446] in this
area largely mitigate those.
Traffic analysis of unpadded encrypted traffic is also possible
[pitfalls-of-dns-encrption] because the sizes and timing of encrypted
DNS requests and responses can be correlated to unencrypted DNS
requests upstream of a recursive resolver.
3.5. In the Servers
Using the terminology of [RFC6973], the DNS servers (recursive
resolvers and authoritative servers) are enablers: they facilitate
communication between an initiator and a recipient without being
directly in the communications path. As a result, they are often
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forgotten in risk analysis. But, to quote again [RFC6973], "Although
[...] enablers may not generally be considered as attackers, they may
all pose privacy threats (depending on the context) because they are
able to observe, collect, process, and transfer privacy-relevant
data." In [RFC6973] parlance, enablers become observers when they
start collecting data.
Many programs exist to collect and analyze DNS data at the servers --
from the "query log" of some programs like BIND to tcpdump and more
sophisticated programs like PacketQ [packetq] and DNSmezzo
[dnsmezzo]. The organization managing the DNS server can use this
data itself, or it can be part of a surveillance program like PRISM
[prism] and pass data to an outside observer.
Sometimes, this data is kept for a long time and/or distributed to
third parties for research purposes [ditl] [day-at-root], security
analysis, or surveillance tasks. These uses are sometimes under some
sort of contract, with various limitations, for instance, on
redistribution, given the sensitive nature of the data. Also, there
are observation points in the network that gather DNS data and then
make it accessible to third parties for research or security purposes
("passive DNS" [passive-dns]).
3.5.1. In the Recursive Resolvers
Recursive Resolvers see all the traffic since there is typically no
caching before them. To summarize: your recursive resolver knows a
lot about you. The resolver of a large IAP, or a large public
resolver, can collect data from many users.
3.5.1.1. Resolver Selection
Given all the above considerations, the choice of recursive resolver
has direct privacy considerations for end users. Historically, end
user devices have used the DHCP-provided local network recursive
resolver, which may have strong, medium, or weak privacy policies
depending on the network. Privacy policies for these servers may or
may not be available and users need to be aware that privacy
guarantees will vary with network.
More recently some networks and end users have actively chosen to use
a large public resolver instead, e.g., Google Public DNS [5],
Cloudflare [6], or Quad9 [7]. There can be many reasons: cost
considerations for network operators, better reliability or anti-
censorship considerations are just a few. Such services typically do
provide a privacy policy and the end user can get an idea of the data
collected by such operators by reading one e.g., Google Public DNS -
Your Privacy [8].
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Even more recently some applications have announced plans to deploy
application-specific DNS settings which might be enabled by default.
For example, current proposals by Firefox [firefox] revolve around a
default based on the geographic region, using a pre-configured list
of large public resolver services which offer DoH, combined with non-
standard probing and signalling mechanism to disable DoH in
particular networks. Whereas Chrome [chrome] is experimenting with
using DoH to the DHCP-provided resolver if it is on a list of DoH-
compatible providers. At the time of writing, efforts to provide
standardized signalling mechanisms for applications to discover the
services offered by local resolvers are in progress
[I-D.ietf-dnsop-resolver-information].
If applications enable application-specific DNS settings without
properly informing the user of the change (or do not provide an
option for user configuration of the application's recursive
resolver) there is a potential privacy issue; depending on the
network context and the application default, the application might
use a recursive server that provides less privacy protection than the
default network-provided server without the user's full knowledge.
Users that are fully aware of an application specific DNS setting may
want to actively override any default in favour of their chosen
recursive resolver.
There are also concerns that, should the trend towards using large
public resolvers increase, this will itself provide a privacy
concern, due to a small number of operators having visibility of the
majority of DNS requests globally and the potential for aggregating
data across services about a user. Additionally the operating
organisation of the resolver may be in a different legal jurisdiction
than the user, which creates further privacy concerns around legal
protections of and access to the data collected by the operator.
At the time of writing the deployment models for DNS are evolving,
their implications are complex and extend beyond the scope of this
document. They are the subject of much other work including
[I-D.livingood-doh-implementation-risks-issues], the IETF ADD mailing
list [9] and the Encrypted DNS Deployment Initiative [10].
3.5.1.2. Active Attacks on Resolver Configuration
The previous section discussed DNS privacy, assuming that all the
traffic was directed to the intended servers (i.e those that would be
used in the absence of an active attack) and that the potential
attacker was purely passive. But, in reality, we can have active
attackers in the network redirecting the traffic, not just to observe
it but also potentially change it.
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For instance, a DHCP server controlled by an attacker can direct you
to a recursive resolver also controlled by that attacker. Most of
the time, it seems to be done to divert traffic in order to also
direct the user to a web server controlled by the attacker. However
it could be used just to capture the traffic and gather information
about you. Similarly, attacks on NDP/ARP might be used to re-direct
DNS queries to attacker controlled servers.
Other attacks, besides using DHCP, are possible. The cleartext
traffic from a DNS client to a DNS server can be intercepted along
its way from originator to intended source, for instance, by
transparent attacker controlled DNS proxies in the network that will
divert the traffic intended for a legitimate DNS server. This server
can masquerade as the intended server and respond with data to the
client. (Attacker controlled servers that inject malicious data are
possible, but it is a separate problem not relevant to privacy.) A
server controlled by an attacker may respond correctly for a long
period of time, thereby foregoing detection.
Also, malware like DNSchanger [dnschanger] can change the recursive
resolver in the machine's configuration, or the routing itself can be
subverted (for instance, [ripe-atlas-turkey]).
3.5.1.3. Blocking of User Selected Services
User privacy can also be at risk if there is blocking (by local
network operators or more general mechanisms) of access to remote
recursive servers that offer encrypted transports when the local
resolver does not offer encryption and/or has very poor privacy
policies. For example, active blocking of port 853 for DoT or of
specific IP addresses could restrict the resolvers available to the
user. The extent of the risk to end user privacy is highly dependent
on the specific network and user context; a user on a network that is
known to perform surveillance would be compromised if they could not
access such services, whereas a user on a trusted network might have
no privacy motivation to do so.
In some cases, networks might block access to remote resolvers for
security reasons, for example to cripple malware and bots or to
prevent data exfiltration methods that use encrypted DNS
communications as transport. In these cases, if the network fully
respects user privacy in other ways (i.e. encrypted DNS and good
data handling policies) the block can serve to further protect user
privacy by ensuring such security precautions.
It is also noted that attacks on remote resolver services, e.g., DDoS
could force users to switch to other services that do not offer
encrypted transports for DNS.
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3.5.1.4. Authentication of Servers
Both DoH and Strict mode for DoT [RFC8310] require authentication of
the server and therefore as long as the authentication credentials
are obtained over a secure channel then using either of these
transports defeats the attack of re-directing traffic to rogue
servers. Of course attacks on these secure channels are also
possible, but out of the scope of this document.
3.5.1.5. Encrypted Transports
3.5.1.5.1. DoT and DoH
Use of encrypted transports does not reduce the data available in the
recursive resolver and ironically can actually expose more
information about users to operators. As mentioned in Section 3.4
use of session based encrypted transports (TCP/TLS) can expose
correlation data about users. Such concerns in the TCP/TLS layers
apply equally to DoT and DoH which both use TLS as the underlying
transport, some examples are:
o fingerprinting based on TLS version and/or cipher suite selection
o user tracking via session resumption in TLS 1.2
3.5.1.5.2. DoH Specific Considerations
Section 8 of [RFC8484] highlights some of the privacy consideration
differences between HTTP and DNS. As a deliberate design choice DoH
inherits the privacy properties of the HTTPS stack and as a
consequence introduces new privacy concerns when compared with DNS
over UDP, TCP or TLS [RFC7858]. The rationale for this decision is
that retaining the ability to leverage the full functionality of the
HTTP ecosystem is more important than placing specific constraints on
this new protocol based on privacy considerations (modulo limiting
the use of HTTP cookies).
In analyzing the new issues introduced by DoH it is helpful to
recognize that there exists a natural tension between
o the wide practice in HTTP to use various headers to optimize HTTP
connections, functionality and behaviour (which can facilitate
user identification and tracking)
o and the fact that the DNS payload is currently very tightly
encoded and contains no standardized user identifiers.
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DoT, for example, would normally contain no client identifiers above
the TLS layer and a resolver would see only a stream of DNS query
payloads originating within one or more connections from a client IP
address. Whereas if DoH clients commonly include several headers in
a DNS message (e.g., user-agent and accept-language) this could lead
to the DoH server being able to identify the source of individual DNS
requests not only to a specific end user device but to a specific
application.
Additionally, depending on the client architecture, isolation of DoH
queries from other HTTP traffic may or may not be feasible or
desirable. Depending on the use case, isolation of DoH queries from
other HTTP traffic may or may not increase privacy.
The picture for privacy considerations and user expectations for DoH
with respect to what additional data may be available to the DoH
server compared to DNS over UDP, TCP or TLS is complex and requires a
detailed analysis for each use case. In particular the choice of
HTTPS functionality vs privacy is specifically made an implementation
choice in DoH and users may well have differing privacy expectations
depending on the DoH use case and implementation.
At the extremes, there may be implementations that attempt to achieve
parity with DoT from a privacy perspective at the cost of using no
identifiable headers, there might be others that provide feature rich
data flows where the low-level origin of the DNS query is easily
identifiable.
Privacy focused users should be aware of the potential for additional
client identifiers in DoH compared to DoT and may want to only use
DoH client implementations that provide clear guidance on what
identifiers they add.
3.5.2. In the Authoritative Name Servers
Unlike what happens for recursive resolvers, observation capabilities
of authoritative name servers are limited by caching; they see only
the requests for which the answer was not in the cache. For
aggregated statistics ("What is the percentage of LOC queries?"),
this is sufficient, but it prevents an observer from seeing
everything. Similarly the increasing deployment of QNAME
minimisation [ripe-qname-measurements] reduces the data visible at
the authoritative name server. Still, the authoritative name servers
see a part of the traffic, and this subset may be sufficient to
violate some privacy expectations.
Also, the end user typically has some legal/contractual link with the
recursive resolver (he has chosen the IAP, or he has chosen to use a
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given public resolver), while having no control and perhaps no
awareness of the role of the authoritative name servers and their
observation abilities.
As noted before, using a local resolver or a resolver close to the
machine decreases the attack surface for an on-the-wire eavesdropper.
But it may decrease privacy against an observer located on an
authoritative name server. This authoritative name server will see
the IP address of the end client instead of the address of a big
recursive resolver shared by many users.
This "protection", when using a large resolver with many clients, is
no longer present if ECS [RFC7871] is used because, in this case, the
authoritative name server sees the original IP address (or prefix,
depending on the setup).
As of today, all the instances of one root name server, L-root,
receive together around 50,000 queries per second. While most of it
is "junk" (errors on the Top-Level Domain (TLD) name), it gives an
idea of the amount of big data that pours into name servers. (And
even "junk" can leak information; for instance, if there is a typing
error in the TLD, the user will send data to a TLD that is not the
usual one.)
Many domains, including TLDs, are partially hosted by third-party
servers, sometimes in a different country. The contracts between the
domain manager and these servers may or may not take privacy into
account. Whatever the contract, the third-party hoster may be honest
or not but, in any case, it will have to follow its local laws. So,
requests to a given ccTLD may go to servers managed by organizations
outside of the ccTLD's country. End users may not anticipate that,
when doing a security analysis.
Also, it seems (see the survey described in [aeris-dns]) that there
is a strong concentration of authoritative name servers among
"popular" domains (such as the Alexa Top N list). For instance,
among the Alexa Top 100K [11], one DNS provider hosts today 10% of
the domains. The ten most important DNS providers host together one
third of the domains. With the control (or the ability to sniff the
traffic) of a few name servers, you can gather a lot of information.
3.6. Re-identification and Other Inferences
An observer has access not only to the data he/she directly collects
but also to the results of various inferences about this data. The
term 'observer' here is used very generally, it might be one that is
passively observing cleartext DNS traffic, one in the network that is
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actively attacking the user by re-directing DNS resolution, or it
might be a local or remote resolver operator.
For instance, a user can be re-identified via DNS queries. If the
adversary knows a user's identity and can watch their DNS queries for
a period, then that same adversary may be able to re-identify the
user solely based on their pattern of DNS queries later on regardless
of the location from which the user makes those queries. For
example, one study [herrmann-reidentification] found that such re-
identification is possible so that "73.1% of all day-to-day links
were correctly established, i.e., user u was either re-identified
unambiguously (1) or the classifier correctly reported that u was not
present on day t+1 any more (2)." While that study related to web
browsing behavior, equally characteristic patterns may be produced
even in machine-to-machine communications or without a user taking
specific actions, e.g., at reboot time if a characteristic set of
services are accessed by the device.
For instance, one could imagine that an intelligence agency
identifies people going to a site by putting in a very long DNS name
and looking for queries of a specific length. Such traffic analysis
could weaken some privacy solutions.
The IAB privacy and security program also have a work in progress
[RFC7624] that considers such inference-based attacks in a more
general framework.
3.7. More Information
Useful background information can also be found in [tor-leak] (about
the risk of privacy leak through DNS) and in a few academic papers:
[yanbin-tsudik], [castillo-garcia], [fangming-hori-sakurai], and
[federrath-fuchs-herrmann-piosecny].
4. Actual "Attacks"
A very quick examination of DNS traffic may lead to the false
conclusion that extracting the needle from the haystack is difficult.
"Interesting" primary DNS requests are mixed with useless (for the
eavesdropper) secondary and tertiary requests (see the terminology in
Section 1). But, in this time of "big data" processing, powerful
techniques now exist to get from the raw data to what the
eavesdropper is actually interested in.
Many research papers about malware detection use DNS traffic to
detect "abnormal" behavior that can be traced back to the activity of
malware on infected machines. Yes, this research was done for the
good, but technically it is a privacy attack and it demonstrates the
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power of the observation of DNS traffic. See [dns-footprint],
[dagon-malware], and [darkreading-dns].
Passive DNS systems [passive-dns] allow reconstruction of the data of
sometimes an entire zone. They are used for many reasons -- some
good, some bad. Well-known passive DNS systems keep only the DNS
responses, and not the source IP address of the client, precisely for
privacy reasons. Other passive DNS systems may not be so careful.
And there is still the potential problems with revealing QNAMEs.
The revelations from the Edward Snowden documents, which were leaked
from the National Security Agency (NSA) provide evidence of the use
of the DNS in mass surveillance operations [morecowbell]. For
example the MORECOWBELL surveillance program, which uses a dedicated
covert monitoring infrastructure to actively query DNS servers and
perform HTTP requests to obtain meta information about services and
to check their availability. Also the QUANTUMTHEORY [12] project
which includes detecting lookups for certain addresses and injecting
bogus replies is another good example showing that the lack of
privacy protections in the DNS is actively exploited.
5. Legalities
To our knowledge, there are no specific privacy laws for DNS data, in
any country. Interpreting general privacy laws like
[data-protection-directive] or GDPR [13] applicable in the European
Union in the context of DNS traffic data is not an easy task, and we
do not know a court precedent here. See an interesting analysis in
[sidn-entrada].
6. Security Considerations
This document is entirely about security, more precisely privacy. It
just lays out the problem; it does not try to set requirements (with
the choices and compromises they imply), much less define solutions.
Possible solutions to the issues described here are discussed in
other documents (currently too many to all be mentioned); see, for
instance, 'Recommendations for DNS Privacy Operators'
[I-D.ietf-dprive-bcp-op].
7. IANA Considerations
This document makes no requests of the IANA.
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8. Acknowledgments
Thanks to Nathalie Boulvard and to the CENTR members for the original
work that led to this document. Thanks to Ondrej Sury for the
interesting discussions. Thanks to Mohsen Souissi and John Heidemann
for proofreading and to Paul Hoffman, Matthijs Mekking, Marcos Sanz,
Tim Wicinski, Francis Dupont, Allison Mankin, and Warren Kumari for
proofreading, providing technical remarks, and making many
readability improvements. Thanks to Dan York, Suzanne Woolf, Tony
Finch, Stephen Farrell, Peter Koch, Simon Josefsson, and Frank Denis
for good written contributions. Thanks to Vittorio Bertola and
Mohamed Boucadair for a detailed review of the -bis. And thanks to
the IESG members for the last remarks.
9. Changelog
draft-ietf-dprive-rfc7626-bis-03
o Address 2 minor nits (typo in section 3.4.1 and adding an IANA
section)
o Minor updates from AD review
draft-ietf-dprive-rfc7626-bis-02
o Numerous editorial corrections thanks to Mohamed Boucadair and
* Minor additions to Scope section
* New text on cellular network DNS
o Additional text from Vittorio Bertola on blocking and security
draft-ietf-dprive-rfc7626-bis-01
o Re-structure section 3.5 (was 2.5)
* Collect considerations for recursive resolvers together
* Re-work several sections here to clarify their context (e.g.,
'Rogue servers' becomes 'Active attacks on resolver
configuration')
* Add discussion of resolver selection
o Update text and old reference on Snowdon revelations.
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o Add text on and references to QNAME minimisation RFC and
deployment measurements
o Correct outdated references
o Clarify scope by adding a Scope section (was Risks overview)
o Clarify what risks are considered in section 3.4.2
draft-ietf-dprive-rfc7626-bis-00
o Rename after WG adoption
o Use DoT acronym throughout
o Minor updates to status of deployment and other drafts
draft-bortzmeyer-dprive-rfc7626-bis-02
o Update various references and fix some nits.
draft-bortzmeyer-dprive-rfc7626-bis-01
o Update reference for dickinson-bcp-op to draft-dickinson-dprive-
bcp-op
draft-borztmeyer-dprive-rfc7626-bis-00:
Initial commit. Differences to RFC7626:
o Update many references
o Add discussions of encrypted transports including DoT and DoH
o Add section on DNS payload
o Add section on authentication of servers
o Add section on blocking of services
10. References
10.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[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>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
10.2. Informative References
[aeris-dns]
Vinot, N., "Vie privee: et le DNS alors?", (In French),
2015, <https://blog.imirhil.fr/vie-privee-et-le-dns-
alors.html>.
[castillo-garcia]
Castillo-Perez, S. and J. Garcia-Alfaro, "Anonymous
Resolution of DNS Queries", 2008,
<http://deic.uab.es/~joaquin/papers/is08.pdf>.
[chrome] Baheux, , "Experimenting with same-provider DNS-over-HTTPS
upgrade", September 2019,
<https://blog.chromium.org/2019/09/experimenting-with-
same-provider-dns.html>.
[dagon-malware]
Dagon, D., "Corrupted DNS Resolution Paths: The Rise of a
Malicious Resolution Authority", ISC/OARC Workshop, 2007,
<https://www.dns-oarc.net/files/workshop-2007/Dagon-
Resolution-corruption.pdf>.
[darkreading-dns]
Lemos, R., "Got Malware? Three Signs Revealed In DNS
Traffic", InformationWeek Dark Reading, May 2013,
<http://www.darkreading.com/analytics/security-monitoring/
got-malware-three-signs-revealed-in-dns-traffic/d/
d-id/1139680>.
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[data-protection-directive]
European Parliament, "Directive 95/46/EC of the European
Pariament and of the council on the protection of
individuals with regard to the processing of personal data
and on the free movement of such data", Official Journal L
281, pp. 0031 - 0050, November 1995, <http://eur-
lex.europa.eu/LexUriServ/
LexUriServ.do?uri=CELEX:31995L0046:EN:HTML>.
[day-at-root]
Castro, S., Wessels, D., Fomenkov, M., and K. Claffy, "A
Day at the Root of the Internet", ACM SIGCOMM Computer
Communication Review, Vol. 38, Number 5,
DOI 10.1145/1452335.1452341, October 2008,
<http://www.sigcomm.org/sites/default/files/ccr/
papers/2008/October/1452335-1452341.pdf>.
[denis-edns-client-subnet]
Denis, F., "Security and privacy issues of edns-client-
subnet", August 2013, <https://00f.net/2013/08/07/edns-
client-subnet/>.
[ditl] CAIDA, "A Day in the Life of the Internet (DITL)", 2002,
<http://www.caida.org/projects/ditl/>.
[]
Stoner, E., "DNS Footprint of Malware", OARC Workshop,
October 2010, <https://www.dns-oarc.net/files/workshop-
201010/OARC-ers-20101012.pdf>.
[dnschanger]
Wikipedia, "DNSChanger", October 2013,
<https://en.wikipedia.org/w/
index.php?title=DNSChanger&oldid=578749672>.
[dnsmezzo]
Bortzmeyer, S., "DNSmezzo", 2009,
<http://www.dnsmezzo.net/>.
[fangming-hori-sakurai]
Fangming, Z., Hori, Y., and K. Sakurai, "Analysis of
Privacy Disclosure in DNS Query", 2007 International
Conference on Multimedia and Ubiquitous Engineering (MUE
2007), Seoul, Korea, ISBN: 0-7695-2777-9, pp. 952-957,
DOI 10.1109/MUE.2007.84, April 2007,
<http://dl.acm.org/citation.cfm?id=1262690.1262986>.
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[federrath-fuchs-herrmann-piosecny]
Federrath, H., Fuchs, K., Herrmann, D., and C. Piosecny,
"Privacy-Preserving DNS: Analysis of Broadcast, Range
Queries and Mix-based Protection Methods", Computer
Security ESORICS 2011, Springer, page(s) 665-683,
ISBN 978-3-642-23821-5, 2011, <https://svs.informatik.uni-
hamburg.de/publications/2011/2011-09-14_FFHP_PrivacyPreser
vingDNS_ESORICS2011.pdf>.
[firefox] Deckelmann, , "What's next in making Encrypted DNS-over-
HTTPS the Default", September 2019,
<https://blog.mozilla.org/futurereleases/2019/09/06/whats-
next-in-making-dns-over-https-the-default/>.
[grangeia.snooping]
Grangeia, L., "DNS Cache Snooping or Snooping the Cache
for Fun and Profit", 2005,
<https://www.semanticscholar.org/paper/Cache-Snooping-or-
Snooping-the-Cache-for-Fun-and-
1-Grangeia/9b22f606e10b3609eafbdcbfc9090b63be8778c3>.
[herrmann-reidentification]
Herrmann, D., Gerber, C., Banse, C., and H. Federrath,
"Analyzing Characteristic Host Access Patterns for Re-
Identification of Web User Sessions",
DOI 10.1007/978-3-642-27937-9_10, 2012, <http://epub.uni-
regensburg.de/21103/1/Paper_PUL_nordsec_published.pdf>.
[I-D.ietf-dnsop-resolver-information]
Sood, P., Arends, R., and P. Hoffman, "DNS Resolver
Information Self-publication", draft-ietf-dnsop-resolver-
information-00 (work in progress), August 2019.
[I-D.ietf-dprive-bcp-op]
Dickinson, S., Overeinder, B., Rijswijk-Deij, R., and A.
Mankin, "Recommendations for DNS Privacy Service
Operators", draft-ietf-dprive-bcp-op-05 (work in
progress), October 2019.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-23 (work
in progress), September 2019.
[I-D.ietf-tls-sni-encryption]
Huitema, C. and E. Rescorla, "Issues and Requirements for
SNI Encryption in TLS", draft-ietf-tls-sni-encryption-09
(work in progress), October 2019.
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[I-D.livingood-doh-implementation-risks-issues]
Livingood, J., Antonakakis, M., Sleigh, B., and A.
Winfield, "Centralized DNS over HTTPS (DoH) Implementation
Issues and Risks", draft-livingood-doh-implementation-
risks-issues-04 (work in progress), September 2019.
[morecowbell]
Grothoff, C., Wachs, M., Ermert, M., and J. Appelbaum,
"NSA's MORECOWBELL: Knell for DNS", GNUnet e.V., January
2015, <https://pdfs.semanticscholar.org/2610/2b99bdd6a258a
98740af8217ba8da8a1e4fa.pdf>.
[packetq] DNS-OARC, "PacketQ, a simple tool to make SQL-queries
against PCAP-files", 2011, <https://github.com/DNS-OARC/
PacketQ>.
[passive-dns]
Weimer, F., "Passive DNS Replication", April 2005,
<https://www.first.org/conference/2005/papers/florian-
weimer-slides-1.pdf>.
[pitfalls-of-dns-encrption]
Shulman, H., "Pretty Bad Privacy:Pitfalls of DNS
Encryption", <https://dl.acm.org/citation.cfm?id=2665959>.
[prism] Wikipedia, "PRISM (surveillance program)", July 2015,
<https://en.wikipedia.org/w/index.php?title=PRISM_(surveil
lance_program)&oldid=673789455>.
[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>.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<https://www.rfc-editor.org/info/rfc5155>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.
[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
DOI 10.17487/RFC6269, June 2011, <https://www.rfc-
editor.org/info/rfc6269>.
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[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[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>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015, <https://www.rfc-
editor.org/info/rfc7624>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.
[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>.
[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>.
[RFC7929] Wouters, P., "DNS-Based Authentication of Named Entities
(DANE) Bindings for OpenPGP", RFC 7929,
DOI 10.17487/RFC7929, August 2016, <https://www.rfc-
editor.org/info/rfc7929>.
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[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[ripe-atlas-turkey]
Aben, E., "A RIPE Atlas View of Internet Meddling in
Turkey", March 2014,
<https://labs.ripe.net/Members/emileaben/a-ripe-atlas-
view-of-internet-meddling-in-turkey>.
[ripe-qname-measurements]
University of Twente, "Making the DNS More Private with
QNAME Minimisation", April 2019,
<https://labs.ripe.net/Members/wouter_de_vries/make-dns-a-
bit-more-private-with-qname-minimisation>.
[sidn-entrada]
Hesselman, C., Jansen, J., Wullink, M., Vink, K., and M.
Simon, "A privacy framework for 'DNS big data'
applications", November 2014,
<https://www.sidnlabs.nl/downloads/
yBW6hBoaSZe4m6GJc_0b7w/2211058ab6330c7f3788141ea19d3db7/
SIDN_Labs_Privacyraamwerk_Position_Paper_V1.4_ENG.pdf>.
[thomas-ditl-tcp]
Thomas, M. and D. Wessels, "An Analysis of TCP Traffic in
Root Server DITL Data", DNS-OARC 2014 Fall Workshop,
October 2014, <https://indico.dns-
oarc.net/event/20/session/2/contribution/15/material/
slides/1.pdf>.
[tor-leak]
Tor, "DNS leaks in Tor", 2013,
<https://www.torproject.org/docs/
faq.html.en#WarningsAboutSOCKSandDNSInformationLeaks>.
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Internet-Draft DNS Privacy Considerations November 2019
[yanbin-tsudik]
Yanbin, L. and G. Tsudik, "Towards Plugging Privacy Leaks
in the Domain Name System", October 2009,
<http://arxiv.org/abs/0910.2472>.
10.3. URIs
[1] https://lists.dns-oarc.net/pipermail/dns-
operations/2016-January/014141.html
[2] https://www.3gpp.org
[3] http://netres.ec/?b=11B99BD
[4] https://www.researchgate.net/publication/320322146_DNS-DNS_DNS-
based_De-NAT_Scheme
[5] https://developers.google.com/speed/public-dns
[6] https://developers.cloudflare.com/1.1.1.1/setting-up-1.1.1.1/
[7] https://www.quad9.net
[8] https://developers.google.com/speed/public-dns/privacy
[9] https://mailarchive.ietf.org/arch/browse/static/add
[10] https://www.encrypted-dns.org
[11] https://www.alexa.com/topsites
[12] https://theintercept.com/document/2014/03/12/nsa-gchqs-
quantumtheory-hacking-tactics/
[13] https://www.eugdpr.org/the-regulation.html
Authors' Addresses
Stephane Bortzmeyer
AFNIC
1, rue Stephenson
Montigny-le-Bretonneux
France 78180
Email: bortzmeyer+ietf@nic.fr
Bortzmeyer & Dickinson Expires May 21, 2020 [Page 27]
Internet-Draft DNS Privacy Considerations November 2019
Sara Dickinson
Sinodun IT
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
United Kingdom
Email: sara@sinodun.com
Bortzmeyer & Dickinson Expires May 21, 2020 [Page 28]