draft-ietf-dprive-problem-statement-04.txt   draft-ietf-dprive-problem-statement-05.txt 
DNS PRIVate Exchange (dprive) Working Group S. Bortzmeyer DNS PRIVate Exchange (dprive) Working Group S. Bortzmeyer
Internet-Draft AFNIC Internet-Draft AFNIC
Intended status: Informational March 23, 2015 Intended status: Informational May 23, 2015
Expires: September 24, 2015 Expires: November 24, 2015
DNS privacy considerations DNS privacy considerations
draft-ietf-dprive-problem-statement-04 draft-ietf-dprive-problem-statement-05
Abstract Abstract
This document describes the privacy issues associated with the use of 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 the DNS by Internet users. It is intended to be an analysis of the
present situation and does not prescribe solutions. present situation and does not prescribe solutions.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
skipping to change at page 1, line 32 skipping to change at page 1, line 32
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 24, 2015. This Internet-Draft will expire on November 24, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The alleged public nature of DNS data . . . . . . . . . . 4 2.1. The alleged public nature of DNS data . . . . . . . . . . 5
2.2. Data in the DNS request . . . . . . . . . . . . . . . . . 5 2.2. Data in the DNS request . . . . . . . . . . . . . . . . . 5
2.3. Cache snooping . . . . . . . . . . . . . . . . . . . . . 6 2.3. Cache snooping . . . . . . . . . . . . . . . . . . . . . 6
2.4. On the wire . . . . . . . . . . . . . . . . . . . . . . . 6 2.4. On the wire . . . . . . . . . . . . . . . . . . . . . . . 7
2.5. In the servers . . . . . . . . . . . . . . . . . . . . . 8 2.5. In the servers . . . . . . . . . . . . . . . . . . . . . 8
2.5.1. In the recursive resolvers . . . . . . . . . . . . . 8 2.5.1. In the recursive resolvers . . . . . . . . . . . . . 9
2.5.2. In the authoritative name servers . . . . . . . . . . 8 2.5.2. In the authoritative name servers . . . . . . . . . . 9
2.5.3. Rogue servers . . . . . . . . . . . . . . . . . . . . 9 2.5.3. Rogue servers . . . . . . . . . . . . . . . . . . . . 10
2.6. Re-identification and other inferences . . . . . . . . . 10 2.6. Re-identification and other inferences . . . . . . . . . 11
3. Actual "attacks" . . . . . . . . . . . . . . . . . . . . . . 10 3. Actual "attacks" . . . . . . . . . . . . . . . . . . . . . . 11
4. Legalities . . . . . . . . . . . . . . . . . . . . . . . . . 11 4. Legalities . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Security considerations . . . . . . . . . . . . . . . . . . . 11 5. Security considerations . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12 7. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12 8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 12 8.2. Informative References . . . . . . . . . . . . . . . . . 13
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 16 8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction 1. Introduction
This document is an analysis of the DNS privacy issues, in the spirit This document is an analysis of the DNS privacy issues, in the spirit
of section 8 of [RFC6973]. of section 8 of [RFC6973].
The Domain Name System is specified in [RFC1034] and [RFC1035]. It The Domain Name System is specified in [RFC1034] and [RFC1035] and
is one of the most important infrastructure components of the many later RFCs, which have never been consolidated. It is one of
Internet and often ignored or misunderstood. Almost every activity the most important infrastructure components of the Internet and
on the Internet starts with a DNS query (and often several). Its use often ignored or misunderstood by Internet users (and even by many
has many privacy implications and this is an attempt at a professionals). Almost every activity on the Internet starts with a
comprehensive and accurate list. 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 Let us begin with a simplified reminder of how the DNS works. (See
also [I-D.hoffman-dns-terminology].) A client, the stub resolver, also [I-D.ietf-dnsop-dns-terminology].) A client, the stub resolver,
issues a DNS query to a server, called the recursive resolver (also issues a DNS query to a server, called the recursive resolver (also
called caching resolver or full resolver or recursive name server). called caching resolver or full resolver or recursive name server).
Let's use the query "What are the AAAA records for www.example.com?" 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 as an example. AAAA is the QTYPE (Query Type), and www.example.com
is the qname (Query Name). (The description which follows assume a is the QNAME (Query Name). (The description which follows assume a
cold cache, for instance because the server just started.) The cold cache, for instance because the server just started.) The
recursive resolver will first query the root nameservers. In most recursive resolver will first query the root nameservers. In most
cases, the root nameservers will send a referral. In this example, cases, the root nameservers will send a referral. In this example,
the referral will be to the .com nameservers. The resolver repeats the referral will be to the .com nameservers. The resolver repeats
the query to one of the .com nameservers. The .com nameservers, in the query to one of the .com nameservers. The .com nameservers, in
turn, will refer to the example.com nameservers. The example.com turn, will refer to the example.com nameservers. The example.com
nameserver will then return the answer. The root name servers, the nameserver will then return the answer. The root name servers, the
name servers of .com and the name servers of example.com are called name servers of .com and the name servers of example.com are called
authoritative name servers. It is important, when analyzing the authoritative name servers. It is important, when analyzing the
privacy issues, to remember that the question asked to all these name privacy issues, to remember that the question asked to all these name
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activity.) activity.)
It should be noted that DNS recursive resolvers sometimes forward It should be noted that DNS recursive resolvers sometimes forward
requests to other recursive resolvers, typically bigger machines, requests to other recursive resolvers, typically bigger machines,
with a larger and more shared cache (and the query hierarchy can be with a larger and more shared cache (and the query hierarchy can be
even deeper, with more than two levels of recursive resolvers). From even deeper, with more than two levels of recursive resolvers). From
the point of view of privacy, these forwarders are like resolvers, 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 except that they do not see all of the requests being made (due to
caching in the first resolver). caching in the first resolver).
All this DNS traffic is currently sent in clear (unencrypted), except Almost all this DNS traffic is currently sent in clear (unencrypted).
a few cases when the IP traffic is protected, for instance in an There are a few cases where there is some channel encryption, for
IPsec VPN. instance in an IPsec VPN, at least between the stub resolver and the
resolver.
Today, almost all DNS queries are sent over UDP. This has practical Today, almost all DNS queries are sent over UDP [thomas-ditl-tcp].
consequences when considering encryption of the traffic as a possible This has practical consequences when considering encryption of the
privacy technique. Some encryption solutions are only designed for traffic as a possible privacy technique. Some encryption solutions
TCP, not UDP. are only designed for TCP, not UDP.
Another important point to keep in mind when analyzing the privacy Another important point to keep in mind when analyzing the privacy
issues of DNS is the mix of many sort of DNS requests received by a issues of DNS is the fact that DNS requests received by a server were
server. Let's assume an eavesdropper wants to know which Web page is triggered by different reasons. Let's assume an eavesdropper wants
viewed by an user. For a typical Web page, there are three sorts of to know which Web page is viewed by a user. For a typical Web page,
DNS requests being issued: there are three sorts of DNS requests being issued:
Primary request: this is the domain name in the URL that the user Primary request: this is the domain name in the URL that the user
typed, selected from a bookmark or chose by clicking on an typed, selected from a bookmark or chose by clicking on an
hyperlink. Presumably, this is what is of interest for the hyperlink. Presumably, this is what is of interest for the
eavesdropper. eavesdropper.
Secondary requests: these are the additional requests performed by Secondary requests: these are the additional requests performed by
the user agent (here, the Web browser) without any direct the user agent (here, the Web browser) without any direct
involvement or knowledge of the user. For the Web, they are involvement or knowledge of the user. For the Web, they are
triggered by embedded content, CSS sheets, JavaScript code, triggered by embedded content, CSS sheets, JavaScript code,
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Tertiary requests: these are the additional requests performed by Tertiary requests: these are the additional requests performed by
the DNS system itself. For instance, if the answer to a query is 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 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 returned, the resolver will have to do additional requests to turn
name servers' names into IP addresses. Similarly, even if glue name servers' names into IP addresses. Similarly, even if glue
records are returned, a careful recursive server will do tertiary records are returned, a careful recursive server will do tertiary
requests to verify the IP addresses of those records. requests to verify the IP addresses of those records.
It can be noted also that, in the case of a typical Web browser, more It can be noted also that, in the case of a typical Web browser, more
DNS requests are sent, for instance to prefetch resources that the DNS requests than stricly necessary are sent, for instance to
user may query later, or when autocompleting the URL in the address prefetch resources that the user may query later, or when
bar (which obviously is a big privacy concern). autocompleting the URL in the address bar. It is a big privacy
concern since it 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 here the terminology of For privacy-related terms, we will use here the terminology of
[RFC6973]. [RFC6973].
2. Risks 2. Risks
This document focuses mostly on the study of privacy risks for the This document focuses mostly on the study of privacy risks for the
end-user (the one performing DNS requests). We consider the risks of end-user (the one performing DNS requests). We consider the risks of
pervasive surveillance ([RFC7258]) as well as risks coming from a pervasive surveillance ([RFC7258]) as well as risks coming from a
more focused surveillance. Privacy risks for the holder of a zone more focused surveillance. Privacy risks for the holder of a zone
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2.1. The alleged public nature of DNS data 2.1. The alleged public nature of DNS data
It has long been claimed that "the data in the DNS is public". While 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 this sentence makes sense for an Internet-wide lookup system, there
are multiple facets to the data and metadata involved that deserve a are multiple facets to the data and metadata involved that deserve a
more detailed look. First, access control lists and private more detailed look. First, access control lists and private
namespaces nonwithstanding, the DNS operates under the assumption namespaces nonwithstanding, the DNS operates under the assumption
that public facing authoritative name servers will respond to "usual" that public facing authoritative name servers will respond to "usual"
DNS queries for any zone they are authoritative for without further DNS queries for any zone they are authoritative for without further
authentication or authorization of the client (resolver). Due to the authentication or authorization of the client (resolver). Due to the
lack of search capabilities, only a given qname will reveal the lack of search capabilities, only a given QNAME will reveal the
resource records associated with that name (or that name's non- resource records associated with that name (or that name's non-
existence). In other words: one needs to know what to ask for, in existence). In other words: one needs to know what to ask for, in
order to receive a response. The zone transfer qtype [RFC5936] is order to receive a response. The zone transfer QTYPE [RFC5936] is
often blocked or restricted to authenticated/authorized access to often blocked or restricted to authenticated/authorized access to
enforce this difference (and maybe for other, more dubious reasons). enforce this difference (and maybe for other, more dubious reasons).
Another differentiation to be considered is between the DNS data Another differentiation to be considered is between the DNS data
itself and a particular transaction (i.e., a DNS name lookup). DNS 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 data and the results of a DNS query are public, within the boundaries
described above, and may not have any confidentiality requirements. described above, and may not have any confidentiality requirements.
However, the same is not true of a single transaction or sequence of However, the same is not true of a single transaction or sequence of
transactions; that transaction is not/should not be public. A transactions; that transaction is not/should not be public. A
typical example from outside the DNS world is: the Web site of typical example from outside the DNS world is: the Web site of
Alcoholics Anonymous is public; the fact that you visit it should not Alcoholics Anonymous is public; the fact that you visit it should not
be. be.
2.2. Data in the DNS request 2.2. Data in the DNS request
The DNS request includes many fields but two of them seem The DNS request includes many fields but two of them seem
particularly relevant for the privacy issues: the qname and the 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. "source IP address" is used in a loose sense of
"source IP address + maybe source port", because the port is also in "source IP address + maybe source port", because the port is also in
the request and can be used to sort out several users sharing an IP the request and can be used to differentiate between several users
address (behind a CGN for instance [RFC6269]). sharing an IP address (behind a CGN for instance [RFC6269]).
The qname is the full name sent by the user. It gives information 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?" 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, means he probably wants to send email to someone at example.net,
which may be a domain used by only a few persons and therefore very which may be a domain used by only a few persons and therefore very
revealing about communication relationships). Some qnames are more revealing about communication relationships). Some QNAMEs are more
sensitive than others. For instance, querying the A record of sensitive than others. For instance, querying the A record of a
google-analytics.com reveals very little (everybody visits Web sites well-known Web statistics domain reveals very little (everybody
which use Google Analytics) but querying the A record of visits Web sites which use this analytics service) but querying the A
www.verybad.example where verybad.example is the domain of an record of www.verybad.example where verybad.example is the domain of
organization that some people find offensive or objectionable, may an organization that some people find offensive or objectionable, may
create more problems for the user. Also, sometimes, the qname embeds create more problems for the user. Also, sometimes, the QNAME embeds
the software one uses, which could be a privacy issue. For instance, the software one uses, which could be a privacy issue. For instance,
_ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org. _ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org.
There are also some BitTorrent clients that query a SRV record for There are also some BitTorrent clients that query a SRV record for
_bittorrent-tracker._tcp.domain.example. _bittorrent-tracker._tcp.domain.example.
Another important thing about the privacy of the qname is the future Another important thing about the privacy of the QNAME is the future
usages. Today, the lack of privacy is an obstacle to putting usages. Today, the lack of privacy is an obstacle to putting
potentially sensitive or personally identifiable data in the DNS. At potentially sensitive or personally identifiable data in the DNS. At
the moment your DNS traffic might reveal that you are doing email but the moment your DNS traffic might reveal that you are doing email but
not with whom. If your MUA starts looking up PGP keys in the DNS not with whom. If your MUA starts looking up PGP keys in the DNS
[I-D.wouters-dane-openpgp] then privacy becomes a lot more important. [I-D.wouters-dane-openpgp] then privacy becomes a lot more important.
And email is just an example; there would be other really interesting And email is just an example; there would be other really interesting
uses for a more privacy-friendly DNS. uses for a more privacy-friendly DNS.
For the communication between the stub resolver and the recursive For the communication between the stub resolver and the recursive
resolver, the source IP address is the address of the user's machine. resolver, the source IP address is the address of the user's machine.
Therefore, all the issues and warnings about collection of IP Therefore, all the issues and warnings about collection of IP
addresses apply here. For the communication between the recursive addresses apply here. For the communication between the recursive
resolver and the authoritative name servers, the source IP address resolver and the authoritative name servers, the source IP address
has a different meaning; it does not have the same status as the has a different meaning; it does not have the same status as the
source address in a HTTP connection. It is now the IP address of the source address in a HTTP connection. It is now the IP address of the
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Therefore, all the issues and warnings about collection of IP Therefore, all the issues and warnings about collection of IP
addresses apply here. For the communication between the recursive addresses apply here. For the communication between the recursive
resolver and the authoritative name servers, the source IP address resolver and the authoritative name servers, the source IP address
has a different meaning; it does not have the same status as the has a different meaning; it does not have the same status as the
source address in a HTTP connection. It is now the IP address of the source address in a HTTP connection. It is now the IP address of the
recursive resolver which, in a way "hides" the real user. However, recursive resolver which, in a way "hides" the real user. However,
hiding does not always work. Sometimes hiding does not always work. Sometimes
[I-D.ietf-dnsop-edns-client-subnet] is used (see its privacy analysis [I-D.ietf-dnsop-edns-client-subnet] is used (see its privacy analysis
in [denis-edns-client-subnet]). Sometimes the end user has a in [denis-edns-client-subnet]). Sometimes the end user has a
personal recursive resolver on her machine. In both cases, the IP personal recursive resolver on her machine. In both cases, the IP
address is as sensitive as it is for HTTP. address is as sensitive as it is for HTTP [sidn-entrada].
A note about IP addresses: there is currently no IETF document which A note about IP addresses: there is currently no IETF document which
describes in detail all the privacy issues around IP addressing. In describes in detail all the privacy issues around IP addressing. In
the meantime, the discussion here is intended to include both IPv4 the meantime, the discussion here is intended to include both IPv4
and IPv6 source addresses. For a number of reasons their assignment and IPv6 source addresses. For a number of reasons their assignment
and utilization characteristics are different, which may have and utilization characteristics are different, which may have
implications for details of information leakage associated with the implications for details of information leakage associated with the
collection of source addresses. (For example, a specific IPv6 source collection of source addresses. (For example, a specific IPv6 source
address seen on the public Internet is less likely than an IPv4 address seen on the public Internet is less likely than an IPv4
address to originate behind a CGN or other NAT.) However, for both address to originate behind a CGN or other NAT.) However, for both
IPv4 and IPv6 addresses, it's important to note that source addresses IPv4 and IPv6 addresses, it's important to note that source addresses
are propagated with queries and comprise metadata about the host, are propagated with queries and comprise metadata about the host,
user, or application that originated them. user, or application that originated them.
2.3. Cache snooping 2.3. Cache snooping
The content of recursive resolvers' caches can reveal data about the The content of recursive resolvers' caches can reveal data about the
clients using it (the privacy risks depend on the number of clients). clients using it (the privacy risks depend on the number of clients).
This information can sometimes be examined by sending DNS queries This information can sometimes be examined by sending DNS queries
with RD=0 to inspect cache content, particularly looking at the DNS with RD=0 to inspect cache content, particularly looking at the DNS
TTLs. Since this also is a reconnaissance technique for subsequent TTLs [grangeia.snooping]. Since this also is a reconnaissance
cache poisoning attacks, some counter measures have already been technique for subsequent cache poisoning attacks, some counter
developed and deployed. measures have already been developed and deployed.
2.4. On the wire 2.4. On the wire
DNS traffic can be seen by an eavesdropper like any other traffic. DNS traffic can be seen by an eavesdropper like any other traffic.
It is typically not encrypted. (DNSSEC, specified in [RFC4033] It is typically not encrypted. (DNSSEC, specified in [RFC4033]
explicitly excludes confidentiality from its goals.) So, if an explicitly excludes confidentiality from its goals.) So, if an
initiator starts a HTTPS communication with a recipient, while the initiator starts a HTTPS communication with a recipient, while the
HTTP traffic will be encrypted, the DNS exchange prior to it will not 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 be. When other protocols will become more and more privacy-aware and
secured against surveillance, the DNS risks to become "the weakest secured against surveillance, the DNS may become "the weakest link"
link" in privacy. in privacy.
An important specificity of the DNS traffic is that it may take a An important specificity of the DNS traffic is that it may take a
different path than the communication between the initiator and the different path than the communication between the initiator and the
recipient. For instance, an eavesdropper may be unable to tap the recipient. For instance, an eavesdropper may be unable to tap the
wire between the initiator and the recipient but may have access to wire between the initiator and the recipient but may have access to
the wire going to the recursive resolver, or to the authoritative the wire going to the recursive resolver, or to the authoritative
name servers. name servers.
The best place to tap, from an eavesdropper's point of view, is The best place to tap, from an eavesdropper's point of view, is
clearly between the stub resolvers and the recursive resolvers, clearly between the stub resolvers and the recursive resolvers,
because traffic is not limited by DNS caching. because traffic is not limited by DNS caching.
The attack surface between the stub resolver and the rest of the 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 world can vary widely depending upon how the end user's computer is
configured. By order of increasing attack surface: configured. By order of increasing attack surface:
The recursive resolver can be on the end user's computer. In The recursive resolver can be on the end user's computer. In
(currently) a small number of cases, individuals may choose to (currently) a small number of cases, individuals may choose to
operate their own DNS resolver on their local machine. In this case operate their own DNS resolver on their local machine. In this
the attack surface for the connection between the stub resolver and case the attack surface for the connection between the stub
the caching resolver is limited to that single machine. resolver and the caching resolver is limited to that single
machine.
The recursive resolver may be at the local network edge. For many/ The recursive resolver may be at the local network edge. For
most enterprise networks and for some residential users the caching many/most enterprise networks and for some residential users the
resolver may exist on a server at the edge of the local network. In caching resolver may exist on a server at the edge of the local
this case the attack surface is the local network. Note that in network. In this case the attack surface is the local network.
large enterprise networks the DNS resolver may not be located at the Note that in large enterprise networks the DNS resolver may not be
edge of the local network but rather at the edge of the overall located at the edge of the local network but rather at the edge of
enterprise network. In this case the enterprise network could be the overall enterprise network. In this case the enterprise
thought of as similar to the IAP network referenced below. network could be thought of as similar to the IAP network
referenced below.
The recursive resolver can be in the IAP (Internet Access Provider) The recursive resolver can be in the IAP (Internet Access
premises. For most residential users and potentially other networks Provider) premises. For most residential users and potentially
the typical case is for the end user's computer to be configured other networks the typical case is for the end user's computer to
(typically automatically through DHCP) with the addresses of the DNS be configured (typically automatically through DHCP) with the
recursive resolvers at the IAP. The attack surface for on-the-wire addresses of the DNS recursive resolvers at the IAP. The attack
attacks is therefore from the end user system across the local surface for on-the-wire attacks is therefore from the end user
network and across the IAP network to the IAP's recursive resolvers. 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 The recursive resolver can be a public DNS service. Some machines
may be configured to use public DNS resolvers such as those operated may be configured to use public DNS resolvers such as those
by Google Public DNS or OpenDNS. The end user may have configured operated today by Google Public DNS or OpenDNS. The end user may
their machine to use these DNS recursive resolvers themselves - or have configured their machine to use these DNS recursive resolvers
their IAP may have chosen to use the public DNS resolvers rather than themselves - or their IAP may have chosen to use the public DNS
operating their own resolvers. In this case the attack surface is resolvers rather than operating their own resolvers. In this case
the entire public Internet between the end user's connection and the the attack surface is the entire public Internet between the end
public DNS service. user's connection and the public DNS service.
2.5. In the servers 2.5. In the servers
Using the terminology of [RFC6973], the DNS servers (recursive Using the terminology of [RFC6973], the DNS servers (recursive
resolvers and authoritative servers) are enablers: they facilitate resolvers and authoritative servers) are enablers: they facilitate
communication between an initiator and a recipient without being communication between an initiator and a recipient without being
directly in the communications path. As a result, they are often directly in the communications path. As a result, they are often
forgotten in risk analysis. But, to quote again [RFC6973], "Although forgotten in risk analysis. But, to quote again [RFC6973], "Although
[...] enablers may not generally be considered as attackers, they may [...] enablers may not generally be considered as attackers, they may
all pose privacy threats (depending on the context) because they are all pose privacy threats (depending on the context) because they are
skipping to change at page 9, line 27 skipping to change at page 9, line 44
authoritative name server. This authoritative name server will see authoritative name server. This authoritative name server will see
the IP address of the end client, instead of the address of a big the IP address of the end client, instead of the address of a big
recursive resolver shared by many users. recursive resolver shared by many users.
This "protection", when using a large resolver with many clients, is This "protection", when using a large resolver with many clients, is
no longer present if [I-D.ietf-dnsop-edns-client-subnet] is used no longer present if [I-D.ietf-dnsop-edns-client-subnet] is used
because, in this case, the authoritative name server sees the because, in this case, the authoritative name server sees the
original IP address (or prefix, depending on the setup). original IP address (or prefix, depending on the setup).
As of today, all the instances of one root name server, L-root, As of today, all the instances of one root name server, L-root,
receive together around 20,000 queries per second. While most of it receive together around 50,000 queries per second. While most of it
is junk (errors on the TLD name), it gives an idea of the amount of is "junk" (errors on the TLD name), it gives an idea of the amount of
big data which pours into name servers. big data which pours into name servers.
Many domains, including TLDs, are partially hosted by third-party Many domains, including TLDs, are partially hosted by third-party
servers, sometimes in a different country. The contracts between the servers, sometimes in a different country. The contracts between the
domain manager and these servers may or may not take privacy into domain manager and these servers may or may not take privacy into
account. Whatever the contract, the third-party hoster may be honest 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. It or not but, in any case, it will have to follow its local laws. So,
may be surprising for an end-user that requests to a given ccTLD may requests to a given ccTLD may go to servers managed by organizations
go to servers managed by organisations outside of the country. outside of the ccTLD's country. End-users may not anticipate that,
when doing a security analysis.
Also, it seems [aeris-dns] that there is a strong concentration of Also, it seems [aeris-dns] that there is a strong concentration of
authoritative name servers among "popular" domains (such as the Alexa authoritative name servers among "popular" domains (such as the Alexa
Top N list). For instance, among the Alexa Top 100k, one DNS Top N list). For instance, among the Alexa Top 100k, one DNS
provider hosts today 10 % of the domains. The ten most important DNS provider hosts today 10 % of the domains. The ten most important DNS
providers host together one third of the domains. With the control 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 (or the ability to sniff the traffic) of a few name servers, you can
gather a lot of information. gather a lot of information.
2.5.3. Rogue servers 2.5.3. Rogue servers
A rogue DHCP server, or a trusted DHCP server that has had its The previous paragraphs discussed DNS privacy, assuming that all the
configuration altered by malicious parties, can direct you to a rogue traffic was directed to the intended servers, and that the potential
recursive resolver. Most of the time, it seems to be done to divert attacker was purely passive. But, in reality, we can have active
traffic, by providing lies for some domain names. But it could be attackers, redirecting the traffic, not for changing it but just to
used just to capture the traffic and gather information about you. observe it.
Same thing for malware like DNSchanger [dnschanger] which changes the
recursive resolver in the machine's configuration, or with 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 transparent DNS proxies in the network that will divert the traffic
intended for a legitimate DNS server (for instance intended for a legitimate DNS server. This rogue server can
[turkey-googledns]). masquerade as the intended server and respond with data to the
client. (Rogue servers that inject malicious data are possible, but
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 were 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 [turkey-googledns]).
A practical consequence of this section is that solutions for DNS
privacy may have to address authentication of the server, not just
passive sniffing.
2.6. Re-identification and other inferences 2.6. Re-identification and other inferences
An observer has access not only to the data he/she directly collects An observer has access not only to the data he/she directly collects
but also to the results of various inferences about these data. but also to the results of various inferences about these data.
For instance, an user can be re-identified via DNS queries. If the 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 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 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 user solely based on their pattern of DNS queries later on regardless
of the location from which the user makes those queries. For of the location from which the user makes those queries. For
example, one study [herrmann-reidentification] found that such re- example, one study [herrmann-reidentification] found that such re-
identification is possible so that "73.1% of all day-to-day links identification is possible so that "73.1% of all day-to-day links
were correctly established, i.e. user u was either re-identified were correctly established, i.e. user u was either re-identified
unambiguously (1) or the classifier correctly reported that u was not 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 present on day t+1 any more (2)". While that study related to web
browsing behaviour, equally characteristic patterns may be produced browsing behaviour, equally characteristic patterns may be produced
skipping to change at page 11, line 10 skipping to change at page 11, line 51
of malware on infected machines. Yes, this research was done for the of malware on infected machines. Yes, this research was done for the
good; but, technically, it is a privacy attack and it demonstrates good; but, technically, it is a privacy attack and it demonstrates
the power of the observation of DNS traffic. See [dns-footprint], the power of the observation of DNS traffic. See [dns-footprint],
[dagon-malware] and [darkreading-dns]. [dagon-malware] and [darkreading-dns].
Passive DNS systems [passive-dns] allow reconstruction of the data of Passive DNS systems [passive-dns] allow reconstruction of the data of
sometimes an entire zone. They are used for many reasons, some good, sometimes an entire zone. They are used for many reasons, some good,
some bad. Well-known passive DNS systems keep only the DNS some bad. Well-known passive DNS systems keep only the DNS
responses, and not the source IP address of the client, precisely for responses, and not the source IP address of the client, precisely for
privacy reasons. Other passive DNS systems may not be so careful. privacy reasons. Other passive DNS systems may not be so careful.
And there is still the potential problems with revealing qnames. And there is still the potential problems with revealing QNAMEs.
The revelations (from the Edward Snowden documents, leaked from the The revelations (from the Edward Snowden documents, leaked from the
NSA) of the MORECOWBELL surveillance program [morecowbell], which NSA) of the MORECOWBELL surveillance program [morecowbell], which
uses the DNS, both passively and actively, to gather surreptitiously uses the DNS, both passively and actively, to surreptitiously gather
information about the users, is another good example that the lack of information about the users, is another good example showing that the
privacy protections in the DNS is actively exploited. lack of privacy protections in the DNS is actively exploited.
4. Legalities 4. Legalities
To our knowledge, there are no specific privacy laws for DNS data, in To our knowledge, there are no specific privacy laws for DNS data, in
any country. Interpreting general privacy laws like any country. Interpreting general privacy laws like
[data-protection-directive] (European Union) in the context of DNS [data-protection-directive] (European Union) in the context of DNS
traffic data is not an easy task and it seems there is no court traffic data is not an easy task and it seems there is no court
precedent here. precedent here. An interesting analysis is [sidn-entrada].
5. Security considerations 5. Security considerations
This document is entirely about security, more precisely privacy. It This document is entirely about security, more precisely privacy. It
just lays out the problem, it does not try to set requirements (with just lays out the problem, it does not try to set requirements (with
the choices and compromises they imply), much less to define the choices and compromises they imply), much less to define
solutions. A possible document on requirments for DNS privacy is solutions. A possible document on requirments for DNS privacy is
[I-D.hallambaker-dnse]. Possible solutions to the issues described [I-D.hallambaker-dnse]. Possible solutions to the issues described
here are discussed in other documents (currently too many to all be here are discussed in other documents (currently too many to all be
mentioned), see for instance [I-D.ietf-dnsop-qname-minimisation] for mentioned), see for instance [I-D.ietf-dnsop-qname-minimisation] for
skipping to change at page 11, line 47 skipping to change at page 12, line 40
6. Acknowledgments 6. Acknowledgments
Thanks to Nathalie Boulvard and to the CENTR members for the original Thanks to Nathalie Boulvard and to the CENTR members for the original
work which leaded to this document. Thanks to Ondrej Sury for the work which leaded to this document. Thanks to Ondrej Sury for the
interesting discussions. Thanks to Mohsen Souissi and John Heidemann interesting discussions. Thanks to Mohsen Souissi and John Heidemann
for proofreading, to Paul Hoffman, Matthijs Mekking, Marcos Sanz, Tim for proofreading, to Paul Hoffman, Matthijs Mekking, Marcos Sanz, Tim
Wicinski, Francis Dupont, Allison Mankin and Warren Kumari for Wicinski, Francis Dupont, Allison Mankin and Warren Kumari for
proofreading, technical remarks, and many readability improvements. proofreading, technical remarks, and many readability improvements.
Thanks to Dan York, Suzanne Woolf, Tony Finch, Stephen Farrell, Peter Thanks to Dan York, Suzanne Woolf, Tony Finch, Stephen Farrell, Peter
Koch and Frank Denis for good written contributions. Koch, Simon Josefsson and Frank Denis for good written contributions.
7. IANA considerations 7. IANA considerations
This document has no actions for IANA. This document has no actions for IANA.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
skipping to change at page 13, line 10 skipping to change at page 13, line 43
[I-D.ietf-dnsop-edns-client-subnet] [I-D.ietf-dnsop-edns-client-subnet]
Contavalli, C., Gaast, W., Lawrence, D., and W. Kumari, Contavalli, C., Gaast, W., Lawrence, D., and W. Kumari,
"Client Subnet in DNS Requests", draft-ietf-dnsop-edns- "Client Subnet in DNS Requests", draft-ietf-dnsop-edns-
client-subnet-00 (work in progress), November 2014. client-subnet-00 (work in progress), November 2014.
[I-D.iab-privsec-confidentiality-threat] [I-D.iab-privsec-confidentiality-threat]
Barnes, R., Schneier, B., Jennings, C., Hardie, T., Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann, Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A "Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", draft-iab-privsec- Threat Model and Problem Statement", draft-iab-privsec-
confidentiality-threat-04 (work in progress), March 2015. confidentiality-threat-06 (work in progress), May 2015.
[I-D.hallambaker-dnse] [I-D.hallambaker-dnse]
Hallam-Baker, P., "DNS Privacy and Censorship: Use Cases Hallam-Baker, P., "DNS Privacy and Censorship: Use Cases
and Requirements.", draft-hallambaker-dnse-02 (work in and Requirements.", draft-hallambaker-dnse-02 (work in
progress), November 2014. progress), November 2014.
[I-D.wouters-dane-openpgp] [I-D.wouters-dane-openpgp]
Wouters, P., "Using DANE to Associate OpenPGP public keys Wouters, P., "Using DANE to Associate OpenPGP public keys
with email addresses", draft-wouters-dane-openpgp-02 (work with email addresses", draft-wouters-dane-openpgp-02 (work
in progress), February 2014. in progress), February 2014.
skipping to change at page 13, line 32 skipping to change at page 14, line 20
[I-D.hzhwm-start-tls-for-dns] [I-D.hzhwm-start-tls-for-dns]
Zi, Z., Zhu, L., Heidemann, J., Mankin, A., and D. Zi, Z., Zhu, L., Heidemann, J., Mankin, A., and D.
Wessels, "Starting TLS over DNS", draft-hzhwm-start-tls- Wessels, "Starting TLS over DNS", draft-hzhwm-start-tls-
for-dns-01 (work in progress), July 2014. for-dns-01 (work in progress), July 2014.
[I-D.ietf-dnsop-qname-minimisation] [I-D.ietf-dnsop-qname-minimisation]
Bortzmeyer, S., "DNS query name minimisation to improve Bortzmeyer, S., "DNS query name minimisation to improve
privacy", draft-ietf-dnsop-qname-minimisation-02 (work in privacy", draft-ietf-dnsop-qname-minimisation-02 (work in
progress), March 2015. progress), March 2015.
[I-D.hoffman-dns-terminology] [I-D.ietf-dnsop-dns-terminology]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", draft-hoffman-dns-terminology-02 (work in Terminology", draft-ietf-dnsop-dns-terminology-01 (work in
progress), March 2015. progress), April 2015.
[dprive] IETF, DPRIVE., "The DPRIVE working group", March 2014, [dprive] IETF, DPRIVE., "The DPRIVE working group", March 2014,
<http://www.ietf.org/mail-archive/web/dns-privacy/>. <http://www.ietf.org/mail-archive/web/dns-privacy/>.
[denis-edns-client-subnet] [denis-edns-client-subnet]
Denis, F., "Security and privacy issues of edns-client- Denis, F., "Security and privacy issues of edns-client-
subnet", August 2013, <https://00f.net/2013/08/07/edns- subnet", August 2013, <https://00f.net/2013/08/07/edns-
client-subnet/>. client-subnet/>.
[dagon-malware] [dagon-malware]
skipping to change at page 14, line 36 skipping to change at page 15, line 26
against PCAP-files", 2011, against PCAP-files", 2011,
<https://github.com/dotse/packetq/wiki>. <https://github.com/dotse/packetq/wiki>.
[dnsmezzo] [dnsmezzo]
Bortzmeyer, S., "DNSmezzo", 2009, Bortzmeyer, S., "DNSmezzo", 2009,
<http://www.dnsmezzo.net/>. <http://www.dnsmezzo.net/>.
[prism] NSA, , "PRISM", 2007, <http://en.wikipedia.org/wiki/ [prism] NSA, , "PRISM", 2007, <http://en.wikipedia.org/wiki/
PRISM_%28surveillance_program%29>. PRISM_%28surveillance_program%29>.
[grangeia.snooping]
Grangeia, L., "DNS Cache Snooping or Snooping the Cache
for Fun and Profit", 2004,
<http://www.msit2005.mut.ac.th/msit_media/1_2551/nete4630/
materials/20080718130017Hc.pdf>.
[ditl] CAIDA, , "A Day in the Life of the Internet (DITL)", 2002, [ditl] CAIDA, , "A Day in the Life of the Internet (DITL)", 2002,
<http://www.caida.org/projects/ditl/>. <http://www.caida.org/projects/ditl/>.
[day-at-root] [day-at-root]
Castro, S., Wessels, D., Fomenkov, M., and K. Claffy, "A Castro, S., Wessels, D., Fomenkov, M., and K. Claffy, "A
Day at the Root of the Internet", 2008, Day at the Root of the Internet", 2008,
<http://www.sigcomm.org/sites/default/files/ccr/ <http://www.sigcomm.org/sites/default/files/ccr/
papers/2008/October/1452335-1452341.pdf>. papers/2008/October/1452335-1452341.pdf>.
[turkey-googledns] [turkey-googledns]
skipping to change at page 15, line 37 skipping to change at page 16, line 30
[castillo-garcia] [castillo-garcia]
Castillo-Perez, S. and J. Garcia-Alfaro, "Anonymous Castillo-Perez, S. and J. Garcia-Alfaro, "Anonymous
Resolution of DNS Queries", 2008, Resolution of DNS Queries", 2008,
<http://deic.uab.es/~joaquin/papers/is08.pdf>. <http://deic.uab.es/~joaquin/papers/is08.pdf>.
[fangming-hori-sakurai] [fangming-hori-sakurai]
Fangming, , Hori, Y., and K. Sakurai, "Analysis of Privacy Fangming, , Hori, Y., and K. Sakurai, "Analysis of Privacy
Disclosure in DNS Query", 2007, Disclosure in DNS Query", 2007,
<http://dl.acm.org/citation.cfm?id=1262690.1262986>. <http://dl.acm.org/citation.cfm?id=1262690.1262986>.
[thomas-ditl-tcp]
Thomas, M. and D. Wessels, "An Analysis of TCP Traffic in
Root Server DITL Data"", 2014, <https://indico.dns-
oarc.net/event/20/session/2/contribution/15/material/
slides/1.pdf>.
[federrath-fuchs-herrmann-piosecny] [federrath-fuchs-herrmann-piosecny]
Federrath, H., Fuchs, K., Herrmann, D., and C. Piosecny, Federrath, H., Fuchs, K., Herrmann, D., and C. Piosecny,
"Privacy-Preserving DNS: Analysis of Broadcast, Range "Privacy-Preserving DNS: Analysis of Broadcast, Range
Queries and Mix-Based Protection Methods", 2011, Queries and Mix-Based Protection Methods", 2011,
<https://svs.informatik.uni-hamburg.de/publications/2011/2 <https://svs.informatik.uni-hamburg.de/publications/2011/2
011-09-14_FFHP_PrivacyPreservingDNS_ESORICS2011.pdf>. 011-09-14_FFHP_PrivacyPreservingDNS_ESORICS2011.pdf>.
[aeris-dns] [aeris-dns]
Vinot, N., "[In French] Vie privee : et le DNS alors ?", Vinot, N., "[In French] Vie privee : et le DNS alors ?",
2015, <https://blog.imirhil.fr/vie-privee-et-le-dns- 2015, <https://blog.imirhil.fr/vie-privee-et-le-dns-
alors.html>. alors.html>.
[herrmann-reidentification] [herrmann-reidentification]
Herrmann, D., Gerber, C., Banse, C., and H. Federrath, Herrmann, D., Gerber, C., Banse, C., and H. Federrath,
"Analyzing characteristic host access patterns for re- "Analyzing characteristic host access patterns for re-
identification of web user sessions", 2012, identification of web user sessions", 2012,
<http://epub.uni-regensburg.de/21103/1/ <http://epub.uni-regensburg.de/21103/1/
Paper_PUL_nordsec_published.pdf>. Paper_PUL_nordsec_published.pdf>.
[sidn-entrada]
Hesselman, C., Jansen, J., Wullink, M., Vink, K., and M.
Simon, "A privacy framework for 'DNS big data'
applications", 2014,
<https://www.sidnlabs.nl/uploads/tx_sidnpublications/
SIDN_Labs_Privacyraamwerk_Position_Paper_V1.4_ENG.pdf>.
8.3. URIs 8.3. URIs
[1] https://developers.google.com/speed/public-dns/privacy [1] https://developers.google.com/speed/public-dns/privacy
Author's Address Author's Address
Stephane Bortzmeyer Stephane Bortzmeyer
AFNIC AFNIC
1, rue Stephenson 1, rue Stephenson
Montigny-le-Bretonneux 78180 Montigny-le-Bretonneux 78180
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