Network Working Group S. Bortzmeyer
Internet-Draft AFNIC
Intended status: Informational October 26, 2014
Expires: April 29, 2015
DNS privacy considerations
draft-ietf-dprive-problem-statement-00
Abstract
This document describes the privacy issues associated with the use of
the DNS by Internet users. It is intended to be mostly an analysis
of the present situation, in the spirit of section 8 of [RFC6973] and
it does not prescribe solutions.
Discussions of the document should take place on the DPRIVE working
group mailing list [dprive].
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 April 29, 2015.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The alleged public nature of DNS data . . . . . . . . . . 4
2.2. Data in the DNS request . . . . . . . . . . . . . . . . . 4
2.3. Cache snooping . . . . . . . . . . . . . . . . . . . . . 6
2.4. On the wire . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. In the servers . . . . . . . . . . . . . . . . . . . . . 7
2.5.1. In the resolvers . . . . . . . . . . . . . . . . . . 7
2.5.2. In the authoritative name servers . . . . . . . . . . 8
2.5.3. Rogue servers . . . . . . . . . . . . . . . . . . . . 9
3. Actual "attacks" . . . . . . . . . . . . . . . . . . . . . . 9
4. Legalities . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Security considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
The Domain Name System is specified in [RFC1034] and [RFC1035]. It
is one of the most important infrastructure components of the
Internet and one of the most often ignored or misunderstood. Almost
every activity on the Internet starts with a DNS query (and often
several). Its use has many privacy implications and we try to give
here a comprehensive and accurate list.
Let us begin with a simplified reminder of how the DNS works. A
client, the stub resolver, issues a DNS query to a server, the
resolver (also called caching resolver or full resolver or recursive
name server). Let's use the query "What are the AAAA records for
www.example.com?" as an example. AAAA is the qtype (Query type), and
www.example.com is the qname (Query Name). The resolver will first
query the root nameservers. In most cases, the root nameservers will
send a referral. In this example, the referral will be to .com
nameservers. The .com nameserver, in turn, will refer to the
example.com nameservers. The example.com nameserver will then return
the answer. The root name servers, the name servers of .com and
those 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. Unlike what many "DNS for dummies"
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articles say, 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 nameserver.
Because the DNS uses caching heavily, not all questions are sent to
the authoritative name servers. If the stub resolver, a few seconds
later, asks to the resolver "What are the SRV records of _xmpp-
server._tcp.example.com?", the 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 resolver, unlike the authoritative servers, sees
everything.
Today, almost all DNS queries are sent over UDP. This has practical
consequences, when considering the encryption of this traffic: some
encryption solutions are only designed for TCP, not UDP.
It should be noted that DNS resolvers sometimes forward requests to
bigger machines, with a larger and more shared cache, the forwarders.
From the point of view of privacy, forwarders are like resolvers,
except that the caching in the resolver before them decreases the
amount of data they can see.
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
server. Let's assume the eavesdropper want to know which Web page is
visited by a user. For a typical Web page displayed by the user,
there are three sorts of DNS requests:
Primary request: this is the domain name that the user typed or
selected from a bookmark or choosed by clicking on an hyperklink.
Presumably, this is what is of interest for the eavesdropper.
Secondary requests: these are the 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
included content, CSS sheets, JavaScript code, embedded images,
etc. In some cases, there can be dozens of domain names in a
single page.
Tertiary requests: these are the 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 is not returned,
the resolver will have to do tertiary requests to turn name
servers' named into IP addresses.
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For privacy-related terms, we will use here the terminology of
[RFC6973].
2. Risks
This draft focuses mostly on the study of privacy risks for the end-
user (the one performing DNS requests). Privacy risks for the holder
of a zone (the risk that someone gets the data) are discussed in
[RFC5936]. 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 data and meta data that deserve a more
detailed look. First, access control lists and private name spaces
nonwithstanding, 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, more dubious 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 sequence of
transactions; that data 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 + may be source port", because the port is also in
the request and can be used to sort out several users sharing an IP
address (CGN for instance).
The qname is the full name sent by the original user. It gives
information about what the user does ("What are the MX records of
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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 revealing). Some qnames are more sensitive than
others. For instance, querying the A record of google-analytics.com
reveals very little (everybody visits Web sites which use Google
Analytics) but querying the A record of www.verybad.example where
verybad.example is the domain of an illegal or very offensive
organization may create more problems for the user. Another example
is when the qname embeds the software one uses. For instance,
_ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org. Or
some BitTorrent clients that query a 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 who with. If your MUA starts looking up PGP keys in the DNS
[I-D.wouters-dane-openpgp] then privacy becomes a lot more important.
And email is just an example; there will be other really interesting
uses for a more privacy-friendly DNS.
For the communication between the stub resolver and the 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 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 a
HTTP connection. It is now the IP address of the resolver which, in
a way "hides" the real user. However, it does not always work.
Sometimes [I-D.vandergaast-edns-client-subnet] is used (see its
privacy analysis in [denis-edns-client-subnet]). Sometimes the end
user has a personal resolver on her machine. In that case, the IP
address is as sensitive as it is for HTTP.
A note about IP addresses: there is currently no IETF document which
describes in detail the privacy issues of IP addressing. In the mean
time, 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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2.3. Cache snooping
The content of resolvers can reveal data about the clients using it.
This information can sometimes be examined by sending DNS queries
with RD=0 to inspect cache content, particularly looking at the DNS
TTLs. 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
DNS traffic can be seen by an eavesdropper like any other traffic.
It is typically not encrypted. (DNSSEC, specified in [RFC4033]
explicitely excludes confidentiality from its goals.) So, if an
initiator starts a HTTPS communication with a recipient, while the
HTTP traffic will be encrypted, the DNS exchange prior to it will not
be. When the other protocols will become more or more privacy-aware
and secured against surveillance, the DNS risks to become "the
weakest link" in privacy.
What also makes the DNS traffic different is that it may take a
different path than the communication between the initiator and the
recipient. For instance, an eavesdropper may be unable to tap the
wire between the initiator and the recipient but may have access to
the wire going to the resolver, or to the authoritative name servers.
The best place, from an eavesdropper's point of view, is clearly
between the stub resolvers and the resolvers, because he 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 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 stub resolver to caching resolver connection is limited to
that single machine.
The resolver can be in the IAP (Internet Access Provider) 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 resolver 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 resolvers.
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The resolver may also 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 IAP network referenced above.
The resolver can be a public DNS service. Some end users may be
configured to use public DNS resolvers such as those operated by
Google Public DNS or OpenDNS. The end user may have configured their
machine to use these DNS resolvers themselves - or their IAP may
choose 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.
2.5. In the servers
Using the terminology of [RFC6973], the DNS servers (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] 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 attacker.
Sometimes, these data are kept for a long time and/or distributed to
third parties, for research purposes [ditl], for security analysis,
or for surveillance tasks. Also, there are observation points in the
network which 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 resolvers
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Resolvers see all the traffic since there is typically no caching
before them. They are, therefore, well situated to observe the
traffic. To summarize: your 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 [1].
2.5.2. In the authoritative name servers
Unlike resolvers, authoritative name servers are limited by caching;
they see only a part of the requests. For aggregated statistics
("What is the percentage of LOC queries?"), this is sufficient; but
it may prevent 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 has typically some legal/contractual link with the
resolver (he has chosen the IAP, or he has chosen to use a given
public resolver), while he is often not even aware of the role of the
authoritative name servers and their observation abilities.
It is an interesting question whether the privacy issues are bigger
in the root or in a large TLD. The root sees the traffic for all the
TLDs (and the huge amount of traffic for non-existing TLD), but a
large TLD has less caching before it.
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
resolver shared by many users. A possible solution is to have a
local resolver and to forward the cache misses to a big resolver.
This "protection", when using a large resolver with many clients, is
no longer present if [I-D.vandergaast-edns-client-subnet] 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 20,000 queries per second. While most of it
is junk (errors on the TLD name), it gives an idea of the amount of
big data which pours into name servers.
Many domains, including TLD, are partially hosted by third-party
servers, sometimes in a different country. The contracts between the
domain manager and these servers may or may not take privacy into
account. Whatever the contract, the third-party hoster may be honest
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or not but, in any case, it will have to follow its local laws. It
may be surprising for an end-user that requests to a given ccTLD may
go to servers managed by organisations outside of the country.
2.5.3. Rogue servers
A rogue DHCP server can direct you to a rogue resolver. Most of the
times, 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. Same thing for malwares like
DNSchanger[dnschanger] which changes the resolver in the machine's
configuration.
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) second 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 you're actually
interested in.
Many research papers about malware detection use DNS traffic to
detect "abnormal" behaviour 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. It is used for many reasons, some good,
some bad. It is an example of privacy issue even when no source IP
address is kept.
4. Legalities
To our knowledge, there are no specific privacy laws for DNS data.
Interpreting general privacy laws like [data-protection-directive]
(European Union) in the context of DNS traffic data is not an easy
task and it seems there is no court precedent here.
5. Security considerations
This document is entirely about security, more precisely privacy. A
document on requirments for DNS privacy is [I-D.hallambaker-dnse].
Possible solutions to the issues described here are discussed in
[I-D.ietf-dnsop-qname-minimisation] (qname minimization), in
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[I-D.bortzmeyer-dnsop-privacy-sol] (local caching resolvers,
gratuitous queries), [I-D.hzhwm-start-tls-for-dns] (encryption of
traffic), in [I-D.wijngaards-dnsop-confidentialdns] (encryption also)
or in many other documents (there are many proposals to encrypt the
DNS). Attempts have been made to encrypt the resource record data
[I-D.timms-encrypt-naptr].
6. Acknowledgments
Thanks to Nathalie Boulvard and to the CENTR members for the original
work which leaded to this draft. Thanks to Ondrej Sury for the
interesting discussions. Thanks to Mohsen Souissi for proofreading
and to Warren Kumari for proofreading and many readability
improvements. Thanks to Dan York, Suzanne Woolf, Tony Finch, Peter
Koch and Frank Denis for good written contributions.
7. References
7.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July
2013.
7.2. Informative References
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5936] Lewis, E. and A. Hoenes, "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, June 2010.
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[I-D.vandergaast-edns-client-subnet]
Contavalli, C., Gaast, W., Leach, S., and E. Lewis,
"Client Subnet in DNS Requests", draft-vandergaast-edns-
client-subnet-02 (work in progress), July 2013.
[I-D.bortzmeyer-dnsop-privacy-sol]
Bortzmeyer, S., "Possible solutions to DNS privacy
issues", draft-bortzmeyer-dnsop-privacy-sol-00 (work in
progress), December 2013.
[I-D.ietf-dnsop-qname-minimisation]
Bortzmeyer, S., "DNS query name minimisation to improve
privacy", draft-ietf-dnsop-qname-minimisation-00 (work in
progress), October 2014.
[I-D.wijngaards-dnsop-confidentialdns]
Wijngaards, W. and G. Wiley, "Confidential DNS", draft-
wijngaards-dnsop-confidentialdns-01 (work in progress),
March 2014.
[I-D.timms-encrypt-naptr]
Timms, B., Reid, J., and J. Schlyter, "IANA Registration
for Encrypted ENUM", draft-timms-encrypt-naptr-01 (work in
progress), July 2008.
[I-D.hzhwm-start-tls-for-dns]
Zi, Z., Zhu, L., Heidemann, J., Mankin, A., and D.
Wessels, "Starting TLS over DNS", draft-hzhwm-start-tls-
for-dns-00 (work in progress), February 2014.
[I-D.hallambaker-dnse]
Hallam-Baker, P., "DNS Privacy and Censorship: Use Cases
and Requirements.", draft-hallambaker-dnse-01 (work in
progress), May 2014.
[I-D.wouters-dane-openpgp]
Wouters, P., "Using DANE to Associate OpenPGP public keys
with email addresses", draft-wouters-dane-openpgp-02 (work
in progress), February 2014.
[dprive] IETF, ., "The DPRIVE working group", March 2014,
<http://www.ietf.org/mail-archive/web/dns-privacy/>.
[dnsop] IETF, ., "The DNSOP working group", October 2013,
<http://www.ietf.org/mail-archive/web/dnsop/>.
[denis-edns-client-subnet]
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Denis, F., "Security and privacy issues of edns-client-
subnet", August 2013, <https://00f.net/2013/08/07/edns-
client-subnet/>.
[dagon-malware]
Dagon, D., "Corrupted DNS Resolution Paths: The Rise of a
Malicious Resolution Authority", 2007, <https://www.dns-
oarc.net/files/workshop-2007/Dagon-Resolution-
corruption.pdf>.
[]
Stoner, E., "DNS footprint of malware", October 2010,
<https://www.dns-oarc.net/files/workshop-201010/OARC-
ers-20101012.pdf>.
[darkreading-dns]
Lemos, R., "Got Malware? Three Signs Revealed In DNS
Traffic", May 2013, <http://www.darkreading.com/monitoring
/got-malware-three-signs-revealed-in-dns/240154181>.
[dnschanger]
Wikipedia, ., "DNSchanger", November 2011,
<http://en.wikipedia.org/wiki/DNSChanger>.
[dnscrypt]
Denis, F., "DNSCrypt", , <http://dnscrypt.org/>.
[dnscurve]
Bernstein, D., "DNScurve", , <http://dnscurve.org/>.
[packetq] Dot SE, ., "PacketQ, a simple tool to make SQL-queries
against PCAP-files", 2011, <https://github.com/dotse/
packetq/wiki>.
[dnsmezzo]
Bortzmeyer, S., "DNSmezzo", 2009,
<http://www.dnsmezzo.net/>.
[prism] NSA, ., "PRISM", 2007, <http://en.wikipedia.org/wiki/
PRISM_%28surveillance_program%29>.
[crime] Rizzo, J. and T. Dong, "The CRIME attack against TLS",
2012,
<http://en.wikipedia.org/wiki/CRIME_(security_exploit)>.
[ditl] CAIDA, ., "A Day in the Life of the Internet (DITL)",
2002, <http://www.caida.org/projects/ditl/>.
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[data-protection-directive]
Europe, ., "European directive 95/46/EC on the protection
of individuals with regard to the processing of personal
data and on the free movement of such data", November
1995, <http://eur-lex.europa.eu/LexUriServ/
LexUriServ.do?uri=CELEX:31995L0046:EN:HTML>.
[passive-dns]
Weimer, F., "Passive DNS Replication", April 2005,
<http://www.enyo.de/fw/software/dnslogger/#2>.
[tor-leak]
Tor, ., "DNS leaks in Tor", 2013, <https://
trac.torproject.org/projects/tor/wiki/doc/TorFAQ#Ikeepseei
ngthesewarningsaboutSOCKSandDNSandinformationleaks.ShouldI
worry>.
Author's Address
Stephane Bortzmeyer
AFNIC
1, rue Stephenson
Montigny-le-Bretonneux 78180
France
Phone: +33 1 39 30 83 46
Email: bortzmeyer+ietf@nic.fr
URI: http://www.afnic.fr/
Bortzmeyer Expires April 29, 2015 [Page 13]
