Internet DRAFT - draft-bortzmeyer-dprive-rfc7626-bis
draft-bortzmeyer-dprive-rfc7626-bis
dprive S. Bortzmeyer
Internet-Draft AFNIC
Obsoletes: 7626 (if approved) S. Dickinson
Intended status: Informational Sinodun IT
Expires: July 19, 2019 January 15, 2019
DNS Privacy Considerations
draft-bortzmeyer-dprive-rfc7626-bis-02
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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 19, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The Alleged Public Nature of DNS Data . . . . . . . . . . 5
2.2. Data in the DNS Request . . . . . . . . . . . . . . . . . 5
2.2.1. Data in the DNS payload . . . . . . . . . . . . . . . 7
2.3. Cache Snooping . . . . . . . . . . . . . . . . . . . . . 7
2.4. On the Wire . . . . . . . . . . . . . . . . . . . . . . . 7
2.4.1. Unencrypted Transports . . . . . . . . . . . . . . . 7
2.4.2. Encrypted Transports . . . . . . . . . . . . . . . . 9
2.5. In the Servers . . . . . . . . . . . . . . . . . . . . . 10
2.5.1. In the Recursive Resolvers . . . . . . . . . . . . . 10
2.5.2. In the Authoritative Name Servers . . . . . . . . . . 12
2.5.3. Rogue Servers . . . . . . . . . . . . . . . . . . . . 13
2.5.4. Authentication of servers . . . . . . . . . . . . . . 13
2.5.5. Blocking of services . . . . . . . . . . . . . . . . 14
2.6. Re-identification and Other Inferences . . . . . . . . . 14
2.7. More Information . . . . . . . . . . . . . . . . . . . . 15
3. Actual "Attacks" . . . . . . . . . . . . . . . . . . . . . . 15
4. Legalities . . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
7. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 17
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
This document is an analysis of the DNS privacy issues, in the spirit
of Section 8 of [RFC6973].
The Domain Name System 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 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
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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 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.
Because DNS relies on caching heavily, the algorithm described just
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).
Almost all this DNS traffic is currently sent in clear (unencrypted).
At the time of writing there is increasing deployment of DNS-over-TLS
[RFC7858] and work underway on DoH [RFC8484]. There are a few cases
where there is some alternative channel encryption, for instance, in
an IPsec VPN, 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
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traffic as a possible privacy technique. Some encryption solutions
are only designed for TCP, not UDP.
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:
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.
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.
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].
2. Risks
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. Privacy risks for the holder of a zone (the
risk that someone gets the data) are discussed in [RFC5936] and
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[RFC5155]. Non-privacy risks (such as cache poisoning) are out of
scope.
2.1. 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 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.
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.
2.2. 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", because the port 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
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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
source address in an HTTP connection. It is now 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
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 a CGN or other NAT.) However, for both
IPv4 and IPv6 addresses, it's important to note that source addresses
are propagated with queries and comprise metadata about the host,
user, or application that originated them.
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2.2.1. 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).
2.3. 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 counter
measures have already been developed and deployed.
2.4. On the Wire
2.4.1. 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 there is
on-going work attempting to encrypt the SNI in the TLS handshake but
that this is a non-trivial problem [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
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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 computer is
configured. By order of increasing attack surface:
The recursive resolver can be on the end user's computer. 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 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
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 premises. For most
residential users and potentially other networks, the typical case
is for the end user's computer to be configured (typically
automatically through DHCP) with the addresses of 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. Some machines
may be configured to use public DNS resolvers such as those
operated today 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.
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2.4.2. Encrypted Transports
The use of encrypted transports directly mitigates passive
surveillance of the DNS payload, however there are still some privacy
attacks possible.
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.
There are some general examples, for example, certain studies have
highlighted that IP TTL or TCP Window sizes os-fingerprint [2] values
can be used to fingerprint client OS's or that various techniques can
be used to de-NAT DNS queries dns-de-nat [3].
The use of clear text transport options to decrease latency may also
identify a user e.g. using TCP Fast Open [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,
for example session resumption, the maximum number of messages to
send or 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.
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2.5. 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
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] [packetq-list] 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]).
2.5.1. 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. You may get an idea of
the data collected by reading the privacy policy of a big public
resolver, e.g., <https://developers.google.com/speed/public-dns/
privacy>.
2.5.1.1. Encrypted transports
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 2.4
use of session based encrypted transports (TCP/TLS) can expose
correlation data about users. Such concerns in the TCP/TLS layers
apply equally to DNS-over-TLS and DoH which both use TLS as the
underlying transport.
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2.5.1.2. DoH vs DNS-over-TLS
The proposed specification for DoH [RFC8484] includes a Privacy
Considerations section which highlights some of the 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.
DNS-over-TLS, 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 DNS-over-TLS from a privacy perspective at the cost of
using no identifiable headers, there might be others that provide
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feature rich data flows where the low-level origin of the DNS query
is easily identifiable.
Privacy focussed users should be aware of the potential for
additional client identifiers in DoH compared to DNS-over-TLS and may
want to only use DoH implementations that provide clear guidance on
what identifiers they add.
2.5.2. 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. 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
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
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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, 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.
2.5.3. Rogue Servers
The previous paragraphs discussed DNS privacy, assuming that all the
traffic was directed to the intended servers and that the potential
attacker was purely passive. But, in reality, we can have active
attackers redirecting the traffic, not to change it but just to
observe it.
For instance, a rogue DHCP server, or a trusted DHCP server that has
had its configuration altered by malicious parties, can direct you to
a rogue recursive resolver. Most of the time, it seems to be done to
divert traffic by providing lies for some domain names. But it could
be used just to capture the traffic and gather information about you.
Other attacks, besides using DHCP, are possible. The traffic from a
DNS client to a DNS server can be intercepted along its way from
originator to intended source, for instance, by transparent DNS
proxies in the network that will divert the traffic intended for a
legitimate DNS server. This rogue server can masquerade as the
intended server and respond with data to the client. (Rogue servers
that inject malicious data are possible, but it is a separate problem
not relevant to privacy.) A rogue server may respond correctly for a
long period of time, thereby foregoing detection. This may be done
for what could be claimed to be good reasons, such as optimization or
caching, but it leads to a reduction of privacy compared to if there
was no attacker present. 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]).
2.5.4. Authentication of servers
Both Strict mode for DNS-over-TLS and DoH require authentication of
the server and therefore as long as the authentication credentials
are obtained over a secure channel then using either of these
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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.
2.5.5. Blocking of services
User privacy can also be at risk if there is blocking (by local
network operators or more general mechanisms) of access to recursive
servers that offer encrypted transports. For example active blocking
of port 853 for DNS-over-TLS or of specific IP addresses (e.g.
1.1.1.1 or 2606:4700:4700::1111) could restrict the resolvers
available to the client. Similarly attacks on such services e.g.
DDoS could force users to switch to other services that do not offer
encrypted transports for DNS.
2.6. 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.
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.
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2.7. 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].
3. 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
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)) of the MORECOWBELL
surveillance program [morecowbell], which uses the DNS, both
passively and actively, to surreptitiously gather information about
the users, is another good example showing that the lack of privacy
protections in the DNS is actively exploited.
4. 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 [4] 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].
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5. 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].
6. 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. And thanks to the IESG members for
the last remarks.
7. Changelog
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 DNS-over-TLS and
DoH
o Add section on DNS payload
o Add section on authentication of servers
o Add section on blocking of services
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8. References
8.1. 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>.
[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>.
8.2. 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>.
[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/>.
[dns-footprint]
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>.
[grangeia.snooping]
Grangeia, L., "DNS Cache Snooping or Snooping the Cache
for Fun and Profit", February 2004,
<http://www.msit2005.mut.ac.th/msit_media/1_2551/nete4630/
materials/20080718130017Hc.pdf>.
[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-dprive-bcp-op]
Dickinson, S., Overeinder, B., Rijswijk-Deij, R., and A.
Mankin, "Recommendations for DNS Privacy Service
Operators", draft-ietf-dprive-bcp-op-01 (work in
progress), December 2018.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-17 (work
in progress), December 2018.
[I-D.ietf-tls-sni-encryption]
Huitema, C. and E. Rescorla, "Issues and Requirements for
SNI Encryption in TLS", draft-ietf-tls-sni-encryption-04
(work in progress), November 2018.
[morecowbell]
Grothoff, C., Wachs, M., Ermert, M., and J. Appelbaum,
"NSA's MORECOWBELL: Knell for DNS", GNUnet e.V., January
2015, <https://gnunet.org/morecowbell>.
[packetq] Dot SE, "PacketQ, a simple tool to make SQL-queries
against PCAP-files", 2011,
<https://github.com/dotse/packetq/wiki>.
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[packetq-list]
PacketQ, "PacketQ Mailing List",
<http://lists.iis.se/mailman/listinfo/packetq>.
[passive-dns]
Weimer, F., "Passive DNS Replication", April 2005,
<http://www.enyo.de/fw/software/dnslogger/#2>.
[pitfalls-of-dns-encrption]
Shulman, H., "Pretty Bad Privacy:Pitfalls of DNS
Encryption", <https://www.ietf.org/mail-archive/web/dns-
privacy/current/pdfWqAIUmEl47.pdf>.
[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>.
[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>.
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[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>.
[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>.
[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>.
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[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/uploads/tx_sidnpublications/
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>.
[yanbin-tsudik]
Yanbin, L. and G. Tsudik, "Towards Plugging Privacy Leaks
in the Domain Name System", October 2009,
<http://arxiv.org/abs/0910.2472>.
8.3. URIs
[1] https://lists.dns-oarc.net/pipermail/dns-
operations/2016-January/014141.html
[2] http://netres.ec/?b=11B99BD
[3] https://www.researchgate.net/publication/320322146_DNS-DNS_DNS-
based_De-NAT_Scheme
[4] 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
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Sara Dickinson
Sinodun IT
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
United Kingdom
Email: sara@sinodun.com
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