Internet Engineering Task Force (IETF) T. Wicinski, Ed.
Request for Comments: 9076
DNS Privacy Considerations
This document describes the privacy issues associated with the use of
the DNS by Internet users. It provides general observations about
typical current privacy practices. 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
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9076
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Table of Contents 1.
Risks in the DNS Data 4.1.
The Public Nature of DNS Data 4.2.
Data in the DNS Request 4.2.1.
Data in the DNS Payload 4.3.
Cache Snooping 5.
Risks on the Wire 5.1.
Unencrypted Transports 5.2.
Encrypted Transports 6.
Risks in the Servers 6.1.
In the Recursive Resolvers 6.1.1.
Resolver Selection 6.1.2.
Active Attacks on Resolver Configuration 6.1.3.
Blocking of DNS Resolution Services 6.1.4.
Encrypted Transports and Recursive Resolvers 6.2.
In the Authoritative Name Servers 7.
Other Risks 7.1.
Re-identification and Other Inferences 7.2.
More Information 8.
Actual "Attacks" 9.
Security Considerations 11.
IANA Considerations 12.
Normative References 12.2.
Informative References Appendix A
. Updates since RFC 7626
This document is an analysis of the DNS privacy issues, in the spirit
of Section 8
The Domain Name System (DNS) is specified in [RFC1034
and many later RFCs, which have never been consolidated. It is one
of the most important infrastructure components of the Internet and
is 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
Let us begin with a simplified reminder of how the DNS works (see
]). A client, the stub resolver, issues a DNS query to
a server called the recursive resolver (also called caching resolver,
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
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 servers will then
return the answers. 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
], which will only send the relevant part of the question
to the upstream name server.
DNS relies heavily on caching, so the algorithm described above is
actually a bit more complicated, and not all questions are sent to
the authoritative name servers. If the stub resolver asks the
recursive resolver a few seconds later, "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
unencrypted. However, there is increasing deployment of DNS over TLS
] 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 is some
alternative channel encryption, for instance, in an IPsec VPN tunnel,
at least between the stub resolver and the resolver. Some recent
analysis on the service quality of encrypted DNS traffic can be found
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, although new solutions are still
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 for 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:
This is the domain name in the URL that the user typed, selected
from a bookmark, or chose by clicking on a hyperlink. Presumably,
this is what is of interest for the eavesdropper.
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 images, etc. In some cases, there can be dozens of
domain names in different contexts on a single web page.
These are the additional requests performed by the DNS service
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 send additional requests to turn the name
servers' names into IP addresses. Similarly, even if glue records
are returned, a careful recursive server will send tertiary
requests to verify the IP addresses of those records.
It can also be noted 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 significant
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.
Privacy-related terminology is from [RFC6973
]. This document
This document focuses mostly on the study of privacy risks for the
end user (the one performing DNS requests). The risks of pervasive
] are considered as well as risks coming from a
more focused surveillance. In this document, the term "end user" is
used as defined in [RFC8890
This document does not attempt a comparison of specific privacy
protections provided by individual networks or organizations; 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 [RFC5155
] and [RFC5936
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 considerations such as
cache poisoning) are also out of scope.
The privacy risks associated with the use of other protocols that
make use of DNS information are not considered here.
The following four sections outline the privacy considerations
associated with different aspects of the DNS for the end user. When
reading these sections, it needs to be kept in mind that many of the
considerations (for example, recursive resolver and transport
protocol) can be specific to the network context that a device is
using at a given point in time. A user may have many devices, and
each device might utilize many different networks (e.g., home, work,
public, or cellular) over a period of time or even concurrently. An
exhaustive analysis of the privacy considerations for an individual
user would need to take into account the set of devices used and the
multiple dynamic contexts of each device. This document does not
attempt such a complex analysis; instead, it presents an overview of
the various considerations that could form the basis of such an
4. Risks in the DNS Data
4.1. The Public Nature of DNS Data
It has been stated that "the data in the DNS is public". This
sentence makes sense for an Internet-wide lookup system, and 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
nonexistence). In other words: one needs to know what to ask for in
order to receive a response. There are many ways in which supposedly
"private" resources currently leak. A few examples are DNSSEC NSEC
zone walking [RFC4470
], passive DNS services [passive-dns], etc. The
zone transfer QTYPE [RFC5936
] is often blocked or restricted to
authenticated/authorized access to enforce this difference (and maybe
for other reasons).
Another difference between the DNS data and a particular DNS
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. However, the same is not
true of a single transaction or a sequence of transactions; those
transactions are not / should not be public. A single transaction
reveals both the originator of the query and the query contents; this
potentially leaks sensitive information about a specific user. A
typical example from outside the DNS world is that the website of
Alcoholics Anonymous is public but the fact that you visit it should
not be. Furthermore, the ability to link queries reveals information
about individual use patterns.
4.2. 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 they probably want 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 websites 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,
There are also some BitTorrent clients that query an SRV record for
Another important thing about the privacy of the QNAME is 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 exchanging
emails but not with whom. If your Mail User Agent (MUA) starts
looking up Pretty Good Privacy (PGP) keys in the DNS [RFC7929
privacy becomes a lot more important. And email is just an example;
there would be other really interesting uses for a more privacy-
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
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. The edns-client-subnet (ECS)
EDNS0 option [RFC7871
] is sometimes used (see one privacy analysis in
[denis-edns-client-subnet]). Sometimes the end user has a personal
recursive resolver on their machine. In both cases, the IP address
originating queries to the authoritative server 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
via the ECS option and comprise metadata about the host, user, or
application that originated them.
4.2.1. Data in the DNS Payload
At the time of writing, there are no standardized client identifiers
contained in the DNS payload itself (ECS, as described in [RFC7871
is widely used; however, [RFC7871
] is only an Informational RFC).
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), but they are not designed to
actively track users (like HTTP cookies).
There are anecdotal accounts of Media Access Control (MAC) addresses
operations/2016-January/014143.html) and even user names being
inserted in nonstandard EDNS(0) options [RFC6891
] for stub-to-
resolver communications to support proprietary functionality
implemented at the resolver (e.g., parental filtering).
4.3. 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
TTLs [grangeia.snooping]. Since this also is a reconnaissance
technique for subsequent cache poisoning attacks, some
countermeasures have already been developed and deployed
5. Risks on the Wire
5.1. Unencrypted Transports
For unencrypted transports, DNS traffic can be seen by an
eavesdropper like any other traffic. (DNSSEC, specified in
], explicitly excludes confidentiality from its goals.) So,
if an initiator starts an HTTPS communication with a recipient, the
HTTP traffic will be encrypted, but the DNS exchange prior to it will
not be. When other protocols become more and more privacy aware and
secured against surveillance (e.g., [RFC8446
]), 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 ongoing
work attempting to encrypt the Server Name Identification (SNI) in
the TLS handshake [RFC8744
], which is one of the last remaining non-
DNS cleartext identifiers of a connection target.
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
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:
* 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. The recursive resolver will expose data to authoritative
resolvers as discussed in Section 6.2
* The recursive resolver may be at the local network edge. For
many/most enterprise networks and for some residential networks,
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 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.
* The recursive resolver can be in the IAP network. For most
residential networks and potentially other networks, the typical
case is for the user's device to be configured (typically
automatically through DHCP or relay agent options) with the
addresses of the DNS proxy in the Customer Premises Equipment
(CPE), which in turn 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.
* The recursive resolver can be a public DNS service (or a privately
run DNS resolver hosted on the public Internet). Some machines
may be configured to use public DNS resolvers such as those
operated by Google Public DNS or OpenDNS. The 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
user's connection and the public DNS service. It can be noted
that if the user selects a single resolver with a small client
population (even when using an encrypted transport), it can
actually serve to aid tracking of that user as they move across
It is also noted that, typically, a device connected _only_ to a
modern cellular network is
* directly configured with only the recursive resolvers of the IAP
* afforded some level of protection against some types of
eavesdropping for all traffic (including DNS traffic) due to the
cellular network link-layer encryption.
The attack surface for this specific scenario is not considered here.
5.2. Encrypted Transports
The use of encrypted transports directly mitigates passive
surveillance of the DNS payload; however, some privacy attacks are
still 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 cleartext/observable features. These issues are not
specific to DNS, but DNS traffic is susceptible to these attacks when
using specific transports.
Some general examples exist; for example, certain studies highlight
that the OS fingerprint values (http://netres.ec/?b=11B99BD
) of IPv4
TTL, IPv6 Hop Limit, or TCP Window size can be used to fingerprint
client OSes or that various techniques can be used to de-NAT DNS
Note that even when using encrypted transports, the use of cleartext
transport options to decrease latency can provide correlation of a
user's connections, e.g., using TCP Fast Open [RFC7413
Implementations that support encrypted transports also commonly reuse
connections for multiple DNS queries to optimize performance (e.g.,
via DNS pipelining or HTTPS multiplexing). Default configuration
options for encrypted transports could, in principle, fingerprint a
specific client application. For example:
* TLS version or cipher suite selection
* session resumption
* the maximum number of messages to send and
* a maximum connection time before closing a connections and
If libraries or applications offer user configuration of such options
(e.g., [getdns]), then they could, in principle, help to identify a
specific user. Users may want to use only the defaults to avoid this
While there are known attacks on older versions of TLS, the most
recent recommendations [RFC7525
] and the development of TLS 1.3
] largely mitigate those.
Traffic analysis of unpadded encrypted traffic is also possible
[pitfalls-of-dns-encryption] because the sizes and timing of
encrypted DNS requests and responses can be correlated to unencrypted
DNS requests upstream of a recursive resolver.
6. Risks 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
forgotten in risk analysis. But, to quote [RFC6973
] again, "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]).
6.1. 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.
6.1.1. 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. The choice by a user to join a particular network (e.g.,
by physically plugging in a cable or selecting a network in an OS
dialogue) typically updates a number of system resources -- these can
include IP addresses, the availability of IPv4/IPv6, DHCP server, and
DNS resolver. These individual changes, including the change in DNS
resolver, are not normally communicated directly to the user by the
OS when the network is joined. The choice of network has
historically determined the default system DNS resolver selection;
the two are directly coupled in this model.
The vast majority of users do not change their default system DNS
settings and so implicitly accept the network settings for the DNS.
The network resolvers have therefore historically been the sole
destination for all of the DNS queries from a device. These
resolvers may have varied 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 the
All major OSes expose the system DNS settings and allow users to
manually override them if desired.
More recently, some networks and users have actively chosen to use a
large public resolver, e.g., Google Public DNS
). There can be many reasons: cost
considerations for network operators, better reliability, or anti-
censorship considerations are just a few. Such services typically do
collected by such operators by reading one, e.g., Google Public DNS -
Your Privacy (https://developers.google.com/speed/public-dns/
In general, as with many other protocols, issues around
centralization also arise with DNS. The picture is fluid with
several competing factors contributing, where these factors can also
vary by geographic region. These include:
* ISP outsourcing, including to third-party and public resolvers
* regional market domination by one or only a few ISPs
* applications directing DNS traffic by default to a limited subset
of resolvers (see Section 18.104.22.168
An increased proportion of the global DNS resolution traffic being
served by only a few entities means that the privacy considerations
for users are highly dependent on the privacy policies and practices
of those entities. Many of the issues around centralization are
discussed in [centralisation-and-data-sovereignty].
22.214.171.124. Dynamic Discovery of DoH and Strict DoT
While support for opportunistic DoT can be determined by probing a
resolver on port 853, there is currently no standardized discovery
mechanism for DoH and Strict DoT servers.
This means that clients that might want to dynamically discover such
encrypted services, and where users are willing to trust such
services, are not able to do so. At the time of writing, efforts to
provide standardized signaling mechanisms to discover the services
offered by local resolvers are in progress [DNSOP-RESOLVER]. Note
that an increasing number of ISPs are deploying encrypted DNS; for
example, see the Encrypted DNS Deployment Initiative [EDDI].
126.96.36.199. Application-Specific Resolver Selection
An increasing number of applications are offering application-
specific encrypted DNS resolution settings, rather than defaulting to
using only the system resolver. A variety of heuristics and
resolvers are available in different applications, including hard-
coded lists of recognized DoH/DoT servers.
Generally, users are not aware of application-specific DNS settings
and may not have control over those settings. To address these
limitations, users will only be aware of and have the ability to
control such settings if applications provide the following
* communicate the change clearly to users when the default
application resolver changes away from the system resolver
* provide configuration options to change the default application
resolver, including a choice to always use the system resolver
* provide mechanisms for users to locally inspect, selectively
forward, and filter queries (either via the application itself or
use of the system resolver)
Application-specific changes to default destinations for users' DNS
queries might increase or decrease user privacy; it is highly
dependent on the network context and the application-specific
default. This is an area of active debate, and the IETF is working
on a number of issues related to application-specific DNS settings.
6.1.2. 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, there can be active
attackers in the network.
The Internet Threat model, as described in [RFC3552
], assumes that
the attacker controls the network. Such an attacker can completely
control any insecure DNS resolution, both passively monitoring the
queries and responses and substituting their own responses. Even if
encrypted DNS such as DoH or DoT is used, unless the client has been
configured in a secure way with the server identity, an active
attacker can impersonate the server. This implies that opportunistic
modes of DoH/DoT as well as modes where the client learns of the DoH/
DoT server via in-network mechanisms such as DHCP are vulnerable to
attack. In addition, if the client is compromised, the attacker can
replace the DNS configuration with one of its own choosing.
6.1.3. Blocking of DNS Resolution Services
User privacy can also be at risk if there is blocking of access to
remote recursive servers that offer encrypted transports, e.g., 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 blocking of specific IP addresses could restrict the resolvers
available to the user. The extent of the risk to 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.
As a matter of policy, some recursive resolvers use their position in
the query path to selectively block access to certain DNS records.
This is a form of rendezvous-based blocking as described in Section 4.3
]. Such blocklists often include servers
known to be used for malware, bots, or other security risks. In
order to prevent circumvention of their blocking policies, some
networks also block access to resolvers with incompatible policies.
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.
6.1.4. Encrypted Transports and Recursive Resolvers
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 described in Section 5.2
use of session-based encrypted transports (TCP/TLS) can expose
correlation data about users.
188.8.131.52. DoH-Specific Considerations
DoH inherits the full privacy properties of the HTTPS stack and as a
consequence introduces new privacy considerations when compared with
DNS over UDP, TCP, or TLS [RFC7858
]. Section 8.2
describes the privacy considerations in the server of the DoH
A brief summary of some of the issues includes the following:
* HTTPS presents new considerations for correlation, such as
explicit HTTP cookies and implicit fingerprinting of the unique
set and ordering of HTTP request header fields.
* The User-Agent and Accept-Language request header fields often
convey specific information about the client version or locale.
* Utilizing the full set of HTTP features enables DoH to be more
than an HTTP tunnel, but it is at the cost of opening up
implementations to the full set of privacy considerations of HTTP.
* Implementations are advised to expose the minimal set of data
needed to achieve the desired feature set.
] specifically makes selection of HTTPS functionality vs.
privacy an implementation choice. 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 HTTP
headers, and there might be others that provide feature-rich data
flows where the low-level origin of the DNS query is easily
identifiable. Some implementations have, in fact, chosen to restrict
the use of the User-Agent header so that resolver operators cannot
identify the specific application that is originating the DNS
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.
6.2. In the Authoritative Name Servers
Unlike what happens for recursive resolvers, the 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
minimization [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 user often has some legal/contractual link with the
recursive resolver (they have chosen the IAP, or they have chosen to
use a given public resolver) while having no control and perhaps no
awareness of the role of the authoritative name servers and their
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
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 or may
not be honest; in any case, it will have to follow its local laws.
For example, requests to a given ccTLD may go to servers managed by
organizations outside of the ccTLD's country. 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 (https://www.alexa.com/topsites),
provider hosts 10% of the domains today. The ten most important DNS
providers together host one-third of all domains. With the control
(or the ability to sniff the traffic) of a few name servers, you can
gather a lot of information.
7. Other Risks
7.1. Re-identification and Other Inferences
An observer has access not only to the data they directly collect but
also to the results of various inferences about this data. The term
"observer" here is used very generally; for example, the observer
might passively observe cleartext DNS traffic or be in the network
that is actively attacking the user by redirecting 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 has a document [RFC7624
that considers such inference-based attacks in a more general
7.2. More Information
Useful background information can also be found in [tor-leak]
(regarding the risk of privacy leaks through DNS) and in a few
academic papers: [yanbin-tsudik], [castillo-garcia],
[fangming-hori-sakurai], and [federrath-fuchs-herrmann-piosecny].
8. 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
greater good, but technically it is a privacy attack and it
demonstrates the power of the observation of DNS traffic. See
[dns-footprint], [dagon-malware], and [darkreading-dns].
Passive DNS services [passive-dns] allow reconstruction of the data
of sometimes an entire zone. Well-known passive DNS services keep
only the DNS responses and not the source IP address of the client,
precisely for privacy reasons. Other passive DNS services may not be
so careful. And there are still potential problems with revealing
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 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
quantumtheory-hacking-tactics/) 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.
To our knowledge, there are no specific privacy laws for DNS data in
any country. Interpreting general privacy laws, like the European
Union's [data-protection-directive] or GDPR (https://gdpr.eu/tag/
gdpr/), in the context of DNS traffic data is not an easy task, and
there is no known court precedent. See an interesting analysis in
10. 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
11. IANA Considerations
This document has no IANA actions.
12.1. Normative References
] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034
, DOI 10.17487/RFC1034
, November 1987,
] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035
, DOI 10.17487/RFC1035
November 1987, <https://www.rfc-editor.org/info/rfc1035
] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973
, July 2013,
] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258
, DOI 10.17487/RFC7258
12.2. Informative References
Vinot, N., "Vie privée: et le DNS alors? [Privacy: what
about DNS?]", February 2015,
ISC, "DNS Cache snooping - should I be concerned?",
October 2018, <https://kb.isc.org/docs/aa-00482
Castillo-Perez, S. and J. Garcia-Alfaro, "Anonymous
Resolution of DNS Queries", Lecture Notes in Computer
Science, Vol. 5332, DOI 10.1007/978-3-540-88873-4_5, 2008,
De Filippi, P. and S. McCarthy, "Cloud Computing:
Centralization and Data Sovereignty", European Journal of
Law and Technology, Vol. 3, No. 2, October 2012,
Dagon, D., "Corrupted DNS Resolution Paths: The Rise of a
Malicious Resolution Authority", ISC/OARC Workshop, 2007,
Lemos, R., "Got Malware? Three Signs Revealed In DNS
Traffic", May 2013,
European Parliament, "Directive 95/46/EC of the European
Parliament and of the Council of 24 October 1995 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. 31-50, November 1995,
Castro, S., Wessels, D., Fomenkov, M., and K. Claffy, "A
Day at the Root of the Internet", ACM SIGCOMM Computer
Communication Review, Vol. 38, No. 5,
DOI 10.1145/1452335.1452341, October 2008,
Denis, F., "Security and privacy issues of edns-client-
subnet", August 2013,
[ditl] CAIDA, "A Day in the Life of the Internet (DITL)",
Orevi, L., Herzberg, A., Zlatokrilov, H., and D. Sigron,
"DNS-DNS: DNS-based De-NAT Scheme", January 2017,
Stoner, E., "DNS Footprint of Malware", OARC Workshop,
October 2010, <https://www.dns-oarc.net/files/workshop-
Lu, C., Liu, B., Li, Z., Hao, S., Duan, H., Zhang, M.,
Leng, C., Liu, Y., Zhang, Z., and J. Wu, "An End-to-End,
Large-Scale Measurement of DNS-over-Encryption: How Far
Have We Come?", IMC '19: Proceedings of the Internet
Measurement Conference, pp. 22-35,
DOI 10.1145/3355369.3355580, October 2019,
[dnsmezzo] Bortzmeyer, S., "DNSmezzo", <http://www.dnsmezzo.net/
Sood, P., Arends, R., and P. Hoffman, "DNS Resolver
Information Self-publication", Work in Progress, Internet-
Draft, draft-ietf-dnsop-resolver-information-01, 11
February 2020, <https://datatracker.ietf.org/doc/html/
Huitema, C., Dickinson, S., and A. Mankin, "Specification
of DNS over Dedicated QUIC Connections", Work in Progress,
Internet-Draft, draft-ietf-dprive-dnsoquic-03, 12 July
[EDDI] EDDI, "Encrypted DNS Deployment Initiative",
Zhao, F., Hori, Y., and K. Sakurai, "Analysis of Privacy
Disclosure in DNS Query", MUE '07: Proceedings of the 2007
International Conference on Multimedia and Ubiquitous
Engineering, pp. 952-957, DOI 10.1109/MUE.2007.84,
ISBN 0-7695-2777-9, April 2007,
Federrath, H., Fuchs, K.-P., Herrmann, D., and C.
Piosecny, "Privacy-Preserving DNS: Analysis of Broadcast,
Range Queries and Mix-based Protection Methods", ESORICS
2011, pp. 665-683, DOI 10.1007/978-3-642-23822-2_36,
ISBN 978-3-642-23822-2, 2011, <https://svs.informatik.uni-
[getdns] "getdns", <https://getdnsapi.net
Grangeia, L., "Cache Snooping or Snooping the Cache for
Fun and Profit", 2005,
Herrmann, D., Gerber, C., Banse, C., and H. Federrath,
"Analyzing Characteristic Host Access Patterns for Re-
Identification of Web User Sessions", Lecture Notes in
Computer Science, Vol. 7127,
DOI 10.1007/978-3-642-27937-9_10, 2012, <https://epub.uni-
Grothoff, C., Wachs, M., Ermert, M., and J. Appelbaum,
"NSA's MORECOWBELL: Knell for DNS", January 2015, <https:/
[packetq] DNS-OARC, "A tool that provides a basic SQL-frontend to
PCAP-files", Release 1.4.3, commit 29a8288, October 2020,
Weimer, F., "Passive DNS Replication", 17th Annual FIRST
Conference, April 2005,
Shulman, H., "Pretty Bad Privacy: Pitfalls of DNS
Encryption", WPES '14: Proceedings of the 13th Workshop on
Privacy in the Electronic Society, pp. 191-200,
DOI 10.1145/2665943.2665959, November 2014,
[prism] Wikipedia, "PRISM (surveillance program)", July 2015,
] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552
, July 2003,
] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC 4033
, DOI 10.17487/RFC4033
, March 2005,
] Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
and DNSSEC On-line Signing", RFC 4470
, April 2006,
] 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,
] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936
, DOI 10.17487/RFC5936
, June 2010,
] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269
, June 2011,
] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891
, April 2013,
] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413
, DOI 10.17487/RFC7413
, December 2014,
] 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
] 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
, August 2015,
] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626
, August 2015,
] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms", RFC 7721
, DOI 10.17487/RFC7721
, March 2016,
] Barnes, R., Cooper, A., Kolkman, O., Thaler, D., and E.
Nordmark, "Technical Considerations for Internet Service
Blocking and Filtering", RFC 7754
, DOI 10.17487/RFC7754
March 2016, <https://www.rfc-editor.org/info/rfc7754
] Bortzmeyer, S., "DNS Query Name Minimisation to Improve
Privacy", RFC 7816
, DOI 10.17487/RFC7816
, March 2016,
] 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
] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871
, May 2016,
] Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
Cookies", RFC 7873
, DOI 10.17487/RFC7873
, May 2016,
] Wouters, P., "DNS-Based Authentication of Named Entities
(DANE) Bindings for OpenPGP", RFC 7929
, August 2016,
] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446
, DOI 10.17487/RFC8446
, August 2018,
] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484
, DOI 10.17487/RFC8484
, October 2018,
] 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
] Huitema, C., "Issues and Requirements for Server Name
Identification (SNI) Encryption in TLS", RFC 8744
, July 2020,
] Nottingham, M., "The Internet is for End Users", RFC 8890
, August 2020,
] Dickinson, S., Overeinder, B., van Rijswijk-Deij, R., and A.
Mankin, "Recommendations for DNS Privacy Service
Operators", BCP 232, RFC 8932
, DOI 10.17487/RFC8932
October 2020, <https://www.rfc-editor.org/info/rfc8932
] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000
, May 2021,
de Vries, W., "Making the DNS More Private with QNAME
Minimisation", April 2019,
Hesselman, C., Jansen, J., Wullink, M., Vink, K., and M.
Simon, "A privacy framework for 'DNS big data'
applications", November 2014,
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-
[tor-leak] Tor, "Tor FAQs: I keep seeing these warnings about SOCKS
and DNS information leaks. Should I worry?",
Yanbin, L. and G. Tsudik, "Towards Plugging Privacy Leaks
in Domain Name System", June 2010,
Appendix A. Updates since RFC 7626
Many references were updated. Discussions of encrypted transports,
including DoT and DoH, and sections on DNS payload, authentication of
servers, and blocking of services were added. With the publishing of
] on QNAME minimization, text, references, and initial
attempts to measure deployment were added to reflect this. The text
and references on the Snowden revelations were updated.
The "Risks Overview" section was changed to "Scope" to help clarify
the risks being considered. Text on cellular network DNS, blocking,
and security was added. Considerations for recursive resolvers were
collected and placed together. A discussion on resolver selection
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,
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.
Sara Dickinson and Stephane Bortzmeyer were the original authors of
the document, and their contribution to the initial draft of this
document is greatly appreciated.
Tim Wicinski (editor)
Elkins, WV 26241
United States of America