Internet DRAFT - draft-bortzmeyer-perpass-dns-privacy
draft-bortzmeyer-perpass-dns-privacy
Network Working Group S. Bortzmeyer
Internet-Draft AFNIC
Intended status: Informational November 16, 2013
Expires: May 20, 2014
DNS privacy problem statement
draft-bortzmeyer-perpass-dns-privacy-01
Abstract
This document describes the privacy issues associated with the use of
the DNS by Internet users. It is intended to be a problem statement
and it does not prescribe solutions.
Discussions of the document should take place on the perpass mailing
list [perpass]
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 20, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
3. Risks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Data in the DNS request . . . . . . . . . . . . . . . . . 4
3.2. On the wire . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. In the servers . . . . . . . . . . . . . . . . . . . . . 6
3.3.1. In the resolvers . . . . . . . . . . . . . . . . . . 6
3.3.2. In the authoritative name servers . . . . . . . . . . 6
3.3.3. Rogue servers . . . . . . . . . . . . . . . . . . . . 7
4. Actual "attacks" . . . . . . . . . . . . . . . . . . . . . . 7
5. Legalities . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Possible technical solutions . . . . . . . . . . . . . . . . 8
6.1. On the wire . . . . . . . . . . . . . . . . . . . . . . . 8
6.1.1. Reducing the attack surface . . . . . . . . . . . . . 8
6.1.2. Encrypting the DNS traffic . . . . . . . . . . . . . 8
6.2. In the servers . . . . . . . . . . . . . . . . . . . . . 10
6.2.1. In the resolvers . . . . . . . . . . . . . . . . . . 10
6.2.2. In the authoritative name servers . . . . . . . . . . 10
6.2.3. Rogue servers . . . . . . . . . . . . . . . . . . . . 11
7. Security considerations . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 12
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 correct list.
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Let us start with a small reminder of the way the DNS works (with
some simplifications). A client, the stub resolver, issues a DNS
query to a server, the resolver (also called caching resolver or full
resolver). For instance, the query is "What are the AAAA records for
www.example.com?". AAAA is the qtype (Query Type) and
www.example.com the qname (Query Name). To get the answer, the
resolver will query first the root nameservers, which will, most of
the times, send a referral. Here, the referral will be to .com
nameservers. In turn, they will send a referral to the example.com
nameservers, which will provide 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" 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?". So, the
DNS leaks more information than it should.
Because the DNS uses caching heavily, not all questions are sent to
the authoritative name servers. If the stub resolver, a few seconds
later, ask 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.
Almost all the DNS queries are today sent over UDP, which have
practical consequences if someone thinks of encrypting this traffic.
I should be noted to that DNS resolvers often forward requests to a
bigger machine, 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.
We will use here the terminology of [RFC6973].
2. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Risks
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This draft is limited to 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]
and in [I-D.koch-perpass-dns-confidentiality]. Non-privacy risks
(such as cache poisoning) are out of scope.
3.1. Data in the DNS request
The DNS request includes many fields but two seem specially 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 question 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
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
if the qname embedding the software you use. For instance, some
BitTorrent clients query a SRV record for _bittorrent-
tracker._tcp.domain.example.
For the communication between the stub resolver and the resolver, the
source IP address is the one 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. 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.
3.2. 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 won't
be. When the other protocols will become more or more privacy-aware
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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 you are 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.
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.
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3.3. 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 and 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 thord-
parties for research or security purposes ("passive DNS
[passive-dns]").
3.3.1. In the resolvers
The resolvers see the entire traffic since there is typically no
caching before them. They are therefore well situated to observe the
traffic. To summary: your resolver knows a lot about you. The
resolver of a big IAP, or a big 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].
3.3.2. In the authoritative name servers
Unlike the resolvers, they are limited by caching. They see only a
part of the requests. For aggregated statistics ("what is the
percentage of LOC queries?"), it is sufficient but it may prevent an
observer to observe everything. Nevertheless, the authoritative name
servers see a random part of the traffic and it may be sufficient to
defeat some privacy expectations.
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Also, the end user has typically some legal/contractual link with the
resolver (he choosed the IAP, or he choosed 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 big TLD. The root sees the traffic for all the
TLD (and the huge amount of traffic for non-existing TLD) but a big
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 since the authoritative name server will
see the IP address of the end client, and not the address of a big
resolver shared by many users. This is no longer true if
[I-D.vandergaast-edns-client-subnet] is used because, in this case,
the authoritative name server sees the original IP address.
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. But 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.
3.3.3. Rogue servers
A rogue DHCP server can send 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.
4. Actual "attacks"
Many research papers about malware detection use DNS traffic to
detect "abnormal" bahaviour that can be traced back to the activity
of malware on infected machines. Yes, this reasearch 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].
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5. 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.
6. Possible technical solutions
We mention here only the solutions that could be deployed in the
current Internet. Disruptive solutions, like replacing the DNS with
a completely new resolution protocol, are interesting but are kept
for a future work. Remember that the focus of this document is on
describing the threats, not in detailing solutions. This section is
therefore non-normative and is NOT a technical specification of
solutions. For the same reason, there are not yet actual
recommendations in this document.
Raising seriously the bar against the eavesdropper will require
SEVERAL actions. Not one is decisive by itself but, together, they
can have an effect. The most important suggested here are:
qname minimization,
encryption of DNS traffic,
padding (sending random queries from time to time).
We detail some of these actions later, classified by the kind of
observer (on the wire, in a server, etc). Some actions will help
against several kinds of observers. For instance, padding, sending
gratuitous queries from time to time (queries where you're not
interested in the replies, just to disturb the analysis), is useful
against all sorts of observers. It is a costly technique, because it
increases the traffic on the network but it seriously blurs the
picture for the observer.
6.1. On the wire
6.1.1. Reducing the attack surface
See Section 6.2.1 since the solution described there apply against
on-the-wire eavesdropping as well as against observation by the
resolver.
6.1.2. Encrypting the DNS traffic
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To completely defeat an eavesdropper, there is only one solution:
encryption. But, from the end user point of view, even if you check
that your communication between your stub resolver and the resolver
is encrypted, you have no way to ensure that the communication
between the resolver and the autoritative name servers will be.
There are two different cases, communication between the stub
resolver and the resolver (no caching but only two parties so
solutions which rely on an agreement may work) and communication
between the resolver and the authoritative servers (less data because
of caching, but many parties involved, so any solution has to scale
well). Encrypting the "last mile", between the user's stub resolver
and the resolver may be sufficient since the biggest danger for
privacy is between the stub resolver and the resolver, because there
is no caching involved there.
The only encryption mechanism available for DNS which is today an
IETF standard is IPsec in ESP mode. It's deployment in the wide
Internet is very limited, for reasons which are out of scope here.
Still, it may be a solution for "the last mile" and, indeed, many VPN
solutions use it this way, encrypting the whole traffic, including
DNS to the safe resolver. In the IETF standards, a possible
candidate could be DTLS [RFC6347]. It enjoyed very little actual
deployment and its interaction with the DNS has never been
considered, studied or of course implemented. There are also non
standard encryption techniques like DNScrypt [dnscrypt] for the stub
resolver <-> resolver communication or DNScurve [dnscurve] for the
resolver <-> authoritative server communication. It seems today that
the possibility of massive encryption of DNS traffic is very remote.
Another solution would be to use more TCP for the queries, together
with TLS [RFC5246]. DNS can run over TCP and it provides a good way
to leverage the software and experience of the TLS world. There have
been discussions to use more TCP for the DNS, in light of reflection
attacks (based on the spoofing of the source IP address, which is
much more difficult with TCP). For instance, a stub resolver could
open a TCP connection with the resolver at startup and keep it open
to send queries and receive responses. The server would of course be
free to tear down these connections at will (when it is under stress,
for instance) and the client could reestablish them when necessary.
Remember that TLS sessions can survive TCP connections so there is no
need to restart the TLS negociation each time. This DNS-over-TLS-
over-TCP is already implemented in the Unbound resolver. It is safe
only if pipelining multiple questions over the same channel. Name
compression should also be disabled, or CRIME-style [crime] attacks
can apply.
Encryption alone does not guarantee perfect privacy, because of the
available metadata. For instance, the size of questions and
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responses, even encrypted, provide hints about what queries have been
sent. (DNScrypt uses random-length padding, and a 64 bytes block
size, to limit this risk, but this raises other issues, for instance
during amplification attacks.) Observing the periodicity of
encrypted questions/responses also discloses the TTL, which is yet
another hint about the queries. Non-cached responses are disclosing
the RTT between the resolver and authoritative servers. This is a
very useful indication to guess where authoritative servers are
located. Web pages are made of many resources, leading to multiple
requests, whose number and timing fingerprint which web site is being
browsed. So, observing encrypted traffic is not enough to recover
any plaintext queries, but is enough to answer the question "is one
of my employees browsing Facebook?".Finally, attackers can perform a
denial-of-service attack on possible targets, check if this makes a
difference on the encrypted traffic they observe, and infer what a
query was.
6.2. In the servers
6.2.1. In the resolvers
It does not seem there is a possible solution against a leaky
resolver. A resolver has to see the entire DNS traffic in clear.
The best approach to limit the problem is to have local resolvers
whose caching will limit the leak. Local networks should have a
local caching resolver (even if it forwards the unanswered questions
to a forwarder) and individual laptops can have their very own
resolver, too.
One mechanism to potentially mitigate on the wire attacks between
stub resolvers and caching resolvers is to determine if the network
location of the caching resolver can be moved closer to the end
user's computer (reducing the attack surface). As noted earlier in
Section 3.2, if an end user's computer is configured with a caching
resolver on the edge of the local network, an attacker would need to
gain access to that local network in order to successfully execute an
on the wire attack against the stub resolver. On the other hand, if
the end user's computer is configured to use a public DNS service as
the caching resolver, the attacker needs to simply get in the network
path between the end user and the public DNS server and so there is a
much greater opportunity for a successful attack. Configuring a
caching resolver closer to the end user can also reduce the
possibility of on the wire attacks.
6.2.2. In the authoritative name servers
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A possible solution would be to minimize the amount of data sent from
the resolver. When a resolver receives the query "What is the AAAA
record for www.example.com?", it sends to the root (assuming a cold
resolver, whose cache is empty) the very same question. Sending
"What are the NS records for .com?" would be sufficient (since it
will be the answer from the root anyway). To do so would be
compatible with the current DNS system and therefore could be
deployable, since it is an unilateral change to the resolvers.
To do so, the resolver needs to know the zone cut [RFC2181]. There
is not a zone cut at every label boundary. If we take the name
www.foo.bar.example, it is possible that there is a zone cut between
"foo" and "bar" but not between "bar" and "example". So, assuming
the resolver already knows the name servers of .example, when it
receives the query "What is the AAAA record of www.foo.bar.example",
it does not always know if the request should be sent to the name
servers of bar.example or to those of example. [RFC2181] suggest an
algorithm to find the zone cut, so resolvers may try it.
Note that DNSSEC-validating resolvers already have access to this
information, since they have to find the zone cut (the DNSKEY record
set is just below, the DS record set just above).
It can be noted that minimizing the amount of data sent also
partially addresses the case of a wire sniffer.
One should note that the behaviour suggested here (minimizing the
amount of data sent in qnames) is NOT forbidden by the [RFC1034]
(section 5.3.3) or [RFC1035] (section 7.2). Sending the full qname
to the authoritative name server is a tradition, not a protocol MUST.
Another note is that the answer to the NS query, unlike the referral
sent when the question is a full qname, is in the Answer section, not
in the Authoritative section. It has probably no practical
consequences.
6.2.3. Rogue servers
Traditional security measures (do not let malware change the system
configuration) are of course a must. A protection againt rogue
servers announced by DHCP could be to have a local resolver, and to
always use it, ignoring DHCP.
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7. Security considerations
Hey, man, the entire document is about security!
8. Acknowledgments
Thanks to Nathalie Boulvard and to the CENTR members for the original
work which leaded to this draft. Thanks to Olaf Kolkman, Francis
Dupont and Ondrej Sury for the interesting discussions. Thanks to
Dan York and Frank Denis for good written contributions.
9. References
9.1. 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.
9.2. 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.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[I-D.koch-perpass-dns-confidentiality]
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Koch, P., "Confidentiality Aspects of DNS Data,
Publication, and Resolution", draft-koch-perpass-dns-
confidentiality-00 (work in progress), November 2013.
[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.
[perpass] IETF, ., "The perpass mailing list", October 2013.
[dagon-malware]
Dagon, D., "Corrupted DNS Resolution Paths: The Rise of a
Malicious Resolution Authority", 2007.
[dns-footprint]
Stoner, E., "DNS footprint of malware", October 2010.
[darkreading-dns]
Lemos, R., "Got Malware? Three Signs Revealed In DNS
Traffic", May 2013.
[dnschanger]
Wikipedia, ., "DNSchanger", November 2011.
[dnscrypt]
Denis, F., "DNSCrypt", .
[dnscurve]
Bernstein, D., "DNScurve", .
[prism] NSA, ., "PRISM", 2007.
[crime] Rizzo, J. and T. Dong, "The CRIME attack against TLS",
2012.
[ditl] , "A Day in the Life of the Internet (DITL)", 2002.
[data-protection-directive]
, "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.
[passive-dns]
Weimer, F., "Passive DNS Replication", April 2005.
Author's Address
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Stephane Bortzmeyer
AFNIC
Immeuble International
Saint-Quentin-en-Yvelines 78181
France
Phone: +33 1 39 30 83 46
Email: bortzmeyer+ietf@nic.fr
URI: http://www.afnic.fr/
Bortzmeyer Expires May 20, 2014 [Page 14]
