Internet DRAFT - draft-ietf-intarea-broadcast-consider
draft-ietf-intarea-broadcast-consider
Internet Engineering Task Force R. Winter
Internet-Draft University of Applied Sciences Augsburg
Intended status: Informational M. Faath
Expires: September 14, 2018 Conntac GmbH
F. Weisshaar
University of Applied Sciences Augsburg
March 13, 2018
Privacy considerations for protocols relying on IP broadcast and
multicast
draft-ietf-intarea-broadcast-consider-09
Abstract
A number of application-layer protocols make use of IP broadcasts or
multicast messages for functions such as local service discovery or
name resolution. Some of these functions can only be implemented
efficiently using such mechanisms. When using broadcasts or
multicast messages, a passive observer in the same broadcast/
multicast domain can trivially record these messages and analyze
their content. Therefore, designers of protocols that make use of
broadcast/multicast messages need to take special care when designing
their protocols.
Status of This Memo
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Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Types and usage of broadcast and multicast . . . . . . . 4
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Privacy considerations . . . . . . . . . . . . . . . . . . . 5
2.1. Message frequency . . . . . . . . . . . . . . . . . . . . 5
2.2. Persistent identifiers . . . . . . . . . . . . . . . . . 5
2.3. Anticipate user behavior . . . . . . . . . . . . . . . . 6
2.4. Consider potential correlation . . . . . . . . . . . . . 7
2.5. Configurability . . . . . . . . . . . . . . . . . . . . . 7
3. Operational considerations . . . . . . . . . . . . . . . . . 8
4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Other considerations . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
Broadcast and multicast messages have a large (and to the sender
unknown) receiver group by design. Because of that, these two
mechanisms are vital for a number of basic network functions such as
auto-configuration or link-layer address lookup. Also application
developers use broadcast/multicast messages to implement things such
as local service or peer discovery. It appears that an increasing
number of applications make use of it as suggested by experimental
results obtained on campus networks including the IETF meeting
network [TRAC2016]. This trend is not entirely surprising. As
[RFC0919] puts it, "The use of broadcasts [...] is a good base for
many applications". Broadcast and multicast functionality in a
subnetwork are therefore important as a lack thereof renders the
protocols relying on these mechanisms inoperable [RFC3819].
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Using broadcast/multicast can become problematic if the information
that is being distributed can be regarded as sensitive or when the
information that is distributed by multiple of these protocols can be
correlated in a way that sensitive data can be derived. This is
clearly true for any protocol, but broadcast/multicast is special in
at least two respects:
(a) The aforementioned large receiver group, consisting of receivers
unknown to the sender. This makes eavesdropping without special
privileges or a special location in the network trivial for
anybody in the same broadcast/multicast domain.
(b) Encryption is difficult when broadcast/multicast messages are
used, for instance because a non-trivial key management protocol
might be required. When encryption is not used, the content of
these messages is easily accessible, making it easy to spoof and
replay them.
Given the above, privacy protection for protocols based on broadcast
or multicast communication is significantly more difficult compared
to unicast communication and at the same time invading the privacy is
much easier.
Privacy considerations of IETF-specified protocols have received some
attention in the recent past (e.g. [RFC7721] or [RFC7819]). There
is also general guidance available for document authors on when and
how to include a privacy considerations section in their documents
and on how to evaluate the privacy implications of Internet protocols
[RFC6973]. RFC6973 also describes potential threats to privacy in
great detail and lists terminology that is also used in this
document. In contrast to RFC6973, this document contains a number of
privacy considerations especially for protocols that rely on
broadcast/multicast, intended to reduce the likelihood that a
broadcast/multicast protocol can be misused to collect sensitive data
about devices, users and groups of users in a broadcast/multicast
domain.
The above mentioned considerations particularly apply to protocols
designed outside the IETF - for two reasons. For one, non-standard
protocols will likely not receive operational attention and support
in making them more secure, e.g. what DHCP snooping does for DHCP.
But because these protocols are typically not documented, network
equipment does not provide similar features for them. The other
reason is that these protocols have been designed in isolation, where
a set of considerations to follow is useful in the absence of a
larger community providing feedback and expertise to improve the
protocol. In particular, carelessly designed protocols that use
broadcast/multicast can break privacy efforts at different layers of
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the protocol stack such as MAC address or IP address randomization
[RFC4941].
1.1. Types and usage of broadcast and multicast
In IPv4, two major types of broadcast addresses exist, the limited
broadcast which is defined as all-ones (255.255.255.255, defined in
section 5.3.5.1 of [RFC1812]) and the directed broadcast with the
given network prefix of an IP address and the host part of all-ones
(defined in section 5.3.5.2. of [RFC1812]). Broadcast packets are
received by all nodes in a subnetwork. Limited broadcasts never
transit a router. The same is true for directed broadcasts by
default, but routers may provide an option to do this [RFC2644].
IPv6 on the other hand does not provide broadcast addresses but
solely relies on multicast [RFC4291].
In contrast to broadcast addresses, multicast addresses represent an
identifier for a set of interfaces that can be a set different from
all nodes in the subnetwork. All interfaces that are identified by a
given multicast address receive packets destined towards that address
and are called a multicast group. In both IPv4 and IPv6, multiple
pre-defined multicast addresses exist. The ones most relevant for
this document are the ones with subnet scope. For IPv4, an IP prefix
is reserved for this purpose called the Local Network Control Block
(224.0.0.0/24, defined in section 4 of [RFC5771]). For IPv6, the
relevant multicast addresses are the two All Nodes Addresses, which
every IPv6-capable host is required to recognize as identifying
itself (see section 2.7.1 of [RFC4291]).
Typical usage of these addresses include local service discovery
(e.g. Multicast DNS (mDNS) [RFC6762] and Link-Local Multicast Name
Resolution (LLMNR) [RFC4795] make use of multicast),
autoconfiguration (e.g. DHCPv4 [RFC2131] uses broadcasts and DHCPv6
[RFC3315] uses multicast addresses) and other vital network services
such as address resolution or duplicate address detection. But
besides these core network functions, also applications make use of
broadcast and multicast functionality, often implementing proprietary
protocols. In sum, these protocols distribute a diverse set of
potentially privacy sensitive information to a large receiver group
and to be part of this receiver group, the only requirement is to be
on same subnetwork.
1.2. Requirements Language
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].
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2. Privacy considerations
There are a few obvious and a few not necessarily obvious things
designers of protocols utilizing broadcast/multicast should consider
in respect to the privacy implications of their protocol. Most of
these items are based on protocol behavior observed as part of
experiments on operational networks [TRAC2016].
2.1. Message frequency
Frequent broadcast/multicast traffic caused by an application can
give away user behavior and online connection times. This allows a
passive observer to potentially deduce a user's current activity
(e.g. a game) and it allows to create an online profile (i.e. times
the user is on the network). The higher the frequency of these
messages and the duration of time these messages are sent, the more
accurate this profile will be. Given that broadcasts/multicasts are
only visible in the same broadcast/multicast domain, these messages
also give the rough location of the user away (e.g. a campus or
building).
This behavior has e.g. been observed by a synchronization mechanism
of a popular application, where multiple messages have been sent per
minute via broadcast. Given this behavior, it is possible to record
a device's time on the network with a sub-minute accuracy given only
the traffic of this single application installed on the device. But
also services used for local name resolution in modern operating
systems utilize broadcast/multicast protocols (e.g. mDNS, LLMNR or
NetBIOS) to announce for example resources regularly which also allow
tracking the online time of a device.
If a protocol relies on frequent or periodic broadcast/multicast
messages, the frequency SHOULD be chosen conservatively, in
particular if the messages contain persistent identifiers (see next
subsection). Also, intelligent message suppression mechanisms such
as the ones employed in mDNS [RFC6762] SHOULD be implemented. The
lower the frequency of broadcast messages, the harder passive traffic
analysis and surveillance becomes.
2.2. Persistent identifiers
A few protocols that make use of broadcast/multicast messages
observed in the wild make use of persistent identifiers. This
includes the use of host names or more abstract persistent
identifiers such as a universally unique identifiers (UUID) or
similar. These IDs, which e.g. identify the installation of a
certain application might not change across updates of the software
and can therefore be extremely long lived. This allows a passive
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observer to track a user precisely if broadcast/multicast messages
are frequent. This is even true in case the IP and/or MAC address
changes. Such identifiers also allow two different interfaces (e.g.
WiFi and Ethernet) to be correlated to the same device. If the
application makes use of persistent identifiers for multiple
installations of the same application for the same user, this even
allows to infer that different devices belong to the same user.
The aforementioned broadcast messages from a synchronization
mechanism of a popular application also included a persistent
identifier in every broadcast. This identifier never changed after
the application was installed and it allowed to track a device even
when it changed its network interface or when it connected to a
different network.
Persistent IDs are considered bad practice in general for broadcast
and multicast communication, as persistent application layer IDs will
make efforts on lower layers to randomize identifiers (e.g.
[I-D.huitema-6man-random-addresses]) useless. When protocols that
make use of broadcast/multicast need to make use of IDs, these IDs
SHOULD be rotated frequently to make user tracking more difficult.
2.3. Anticipate user behavior
A large number of users name their device after themselves, either
using their first name, last name or both. Often a host name
includes the type, model or maker of a device, its function or it
includes language specific information. Based on data gathered
during experiments performed at IETF meetings and at a large campus
network, this appears currently to be prevalent user behavior
[TRAC2016]. For protocols using the host name as part of the
messages, this clearly will reveal personally identifiable
information to everyone on the local network. This information can
also be used to mount more sophisticated attacks, when e.g. the owner
of a device is identified (as an interesting target) or properties of
the device are known (e.g. known vulnerabilities). Host names are
also a type of persistent identifier and therefore the considerations
in Section 2.2 apply.
Some of the most commonly used operating systems include the name the
user chooses for the user account during the installation process as
part of the host name of the device. The name of the operating
system can also be included, revealing therefore two pieces of
information, which can be regarded as private information if the host
name is used in broadcast/multicast messages.
Where possible, the use of host names and other user-provided
information in protocols making use of broadcast/multicast SHOULD be
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avoided. An application might want to display the information it
will broadcast on the LAN at install/config time, so the user is at
least aware of the application's behavior. More host name
considerations can be found in [RFC8117]. More information on user
participation can be found in [RFC6973].
2.4. Consider potential correlation
A large number of services and applications make use of the
broadcast/multicast mechanism. That means there are various sources
of information that are easily accessible by a passive observer. In
isolation, the information these protocols reveal might seem
harmless, but given multiple such protocols, it might be possible to
correlate this information. E.g. a protocol that uses frequent
messages including a UUID to identify the particular installation
does not give the identity of the user away. But a single message
including the user's host name might just do that and it can be
correlated using e.g. the MAC address of the device's interface.
In the experiments described in [TRAC2016], it was possible to
correlate frequently sent broadcast messages that included a unique
identifier with other broadcast/multicast messages containing
usernames (e.g. mDNS, LLMNR or NetBIOS), but also relationships to
other users. This allowed to reveal the real identity of the users
of many devices but it also gave some information about their social
environment away.
A designer of a protocol that makes use of broadcast/multicast needs
to be aware of the fact that even if - in isolation - the information
a protocol leaks seems harmless, there might be ways to correlate
that information with information from other protocols to reveal
sensitive information about a user.
2.5. Configurability
A lot of applications and services relying on broadcast/multicast
protocols do not include the means to declare "safe" environments
(e.g. based on the SSID of a WiFi network and the MAC addresses of
the access points). E.g. a device connected to a public WiFi will
likely broadcast the same information as when connected to the home
network. It would be beneficial if certain behavior could be
restricted to "safe" environments.
A popular operating system e.g. allows the user to specify the trust
level of the network the device connects to, which for example
restricts specific system services (using broadcast/multicast
messages for their normal operation) to be used in trusted networks
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only. Such functionality could implemented as part of an
application.
An application developer making use of broadcasts/multicasts as part
of the application SHOULD make the broadcast feature, if possible,
configurable, so that potentially sensitive information does not leak
on public networks, where the threat to privacy is much larger.
3. Operational considerations
Besides changing end-user behavior, choosing sensible defaults as an
operating system vendor (e.g. for suggesting host names) and the
considerations for protocol designers mentioned in this document,
there is something that the network administrators/operators can do
to limit the above mentioned problems.
A feature commonly found on access points e.g. is to manage/filter
broadcast and multicast traffic. This will potentially break certain
applications or some of their functionality but will also protect the
users from potentially leaking sensitive information. Wireless
access points often provide finer-grained control beyond a simple on/
off switch for well-known protocols or provide mechanisms to manage
broadcast/multicast traffic intelligently using e.g. proxies (see
[I-D.ietf-mboned-ieee802-mcast-problems]). These mechanisms however
only work on standardized protocols.
4. Summary
Increasingly, applications rely on protocols that send and receive
broadcast and multicast messages. For some, broadcasts/multicasts
are the basis of their application logic, others use broadcasts/
multicasts to improve certain aspects of the application but are
fully functional in case broadcasts/multicasts fail. Irrespective of
the role of broadcast and multicast messages for the application, the
designers of protocols that make use of them should be very careful
in their protocol design because of the special nature of broadcast
and multicast.
It is not always possible to implement certain functionality via
unicast, but in case a protocol designer chooses to rely on
broadcast/multicast, the following should be carefully considered:
o IETF-specified protocols, such as mDNS [RFC6762], SHOULD be used
if possible as operational support might exist to protect against
the leakage of private information. Also, for some protocols
privacy extensions are being specified, which can be used if
implemented. E.g. for DNS-SD privacy extensions are documented in
[I-D.ietf-dnssd-privacy]
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o Using user-specified information inside broadcast/multicast
messages SHOULD be avoided, as users will often use personal
information or other information aiding attackers, in particular
if the user is unaware about how that information is being used
o The use of persistent IDs in messages SHOULD be avoided, as this
allows user tracking, correlation and potentially has a
devastating effect on other privacy protection mechanisms
o If one really must design a new protocol relying on broadcast/
multicast and cannot use an IETF-specified protocol, then:
* the protocol SHOULD be very conservative in how frequently it
sends messages as an effort in data minimization
* it SHOULD make use of mechanisms implemented in IETF-specified
protocols that can be helpful in privacy protection such as
message suppression in mDNS
* it SHOULD be designed in a way that information sent in
broadcast/multicast messages cannot be correlated with
information from other protocols using broadcast/multicast
* it SHOULD be possible to let the user configure "safe"
environments if possible (e.g. based on the SSID) to minimize
the risk of information leakage (e.g. a home network as opposed
to a public Wifi)
5. Other considerations
Besides privacy implications, frequent broadcasting also represents a
performance problem. In particular in certain wireless technologies
such as 802.11, broadcast and multicast are transmitted at a much
lower rate (the lowest common denominator rate) compared to unicast
and therefore have a much bigger impact on the overall available
airtime [I-D.ietf-mboned-ieee802-mcast-problems]. Further, it will
limit the ability for devices to go to sleep if frequent broadcasts
are being sent. A similar problem in respect to Router
Advertisements is addressed in
[I-D.ietf-v6ops-reducing-ra-energy-consumption]. In that respect
broadcasts/multicast can be used for another class of attacks that is
not related to privacy. The potential impact on network performance
should nevertheless be considered when designing a protocol that
makes use of broadcast/multicast.
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6. Acknowledgments
We would like to thank Eliot Lear, Joe Touch and Stephane Bortzmeyer
for their valuable input to this document.
This work was partly supported by the European Commission under grant
agreement FP7-318627 mPlane. Support does not imply endorsement.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
This document deals with privacy-related considerations of broadcast-
and multicast-based protocols. It contains advice for designers of
such protocols to minimize the leakage of privacy-sensitive
information. The intent of the advice is to make sure that
identities will remain anonymous and user tracking will be made
difficult.
It should be noted that certain applications could make use of
existing mechanisms to protect multicast traffic such as the ones
defined in [RFC5374]. Examples of such applications can be found in
Appendix A. of [RFC5374]. Given the required infrastructure and
assumptions about these applications and the security infrastructure,
many applications will not be able to make use of such mechanisms.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[I-D.huitema-6man-random-addresses]
Huitema, C., "Implications of Randomized Link Layers
Addresses for IPv6 Address Assignment", draft-huitema-
6man-random-addresses-03 (work in progress), March 2016.
[I-D.ietf-dnssd-privacy]
Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
SD", draft-ietf-dnssd-privacy-00 (work in progress),
October 2016.
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[I-D.ietf-mboned-ieee802-mcast-problems]
Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
Zuniga, "Multicast Considerations over IEEE 802 Wireless
Media", draft-ietf-mboned-ieee802-mcast-problems-01 (work
in progress), February 2018.
[I-D.ietf-v6ops-reducing-ra-energy-consumption]
Yourtchenko, A. and L. Colitti, "Reducing energy
consumption of Router Advertisements", draft-ietf-v6ops-
reducing-ra-energy-consumption-03 (work in progress),
November 2015.
[RFC0919] Mogul, J., "Broadcasting Internet Datagrams", STD 5, RFC
919, DOI 10.17487/RFC0919, October 1984,
<http://www.rfc-editor.org/info/rfc919>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<http://www.rfc-editor.org/info/rfc1812>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[RFC2644] Senie, D., "Changing the Default for Directed Broadcasts
in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644,
August 1999, <http://www.rfc-editor.org/info/rfc2644>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
[RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, DOI 10.17487/RFC3819, July 2004,
<http://www.rfc-editor.org/info/rfc3819>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795, DOI
10.17487/RFC4795, January 2007,
<http://www.rfc-editor.org/info/rfc4795>.
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[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<http://www.rfc-editor.org/info/rfc4941>.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008,
<http://www.rfc-editor.org/info/rfc5374>.
[RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI
10.17487/RFC5771, March 2010,
<http://www.rfc-editor.org/info/rfc5771>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, DOI
10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<http://www.rfc-editor.org/info/rfc7721>.
[RFC7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819,
April 2016, <http://www.rfc-editor.org/info/rfc7819>.
[RFC8117] Huitema, C., Thaler, D., and R. Winter, "Current Hostname
Practice Considered Harmful", RFC 8117, DOI 10.17487/
RFC8117, March 2017, <https://www.rfc-editor.org/info/
rfc8117>.
[TRAC2016]
Faath, M., Weisshaar, F., and R. Winter, "How Broadcast
Data Reveals Your Identity and Social Graph", 7th
International Workshop on TRaffic Analysis and
Characterization IEEE TRAC 2016, September 2016.
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Authors' Addresses
Rolf Winter
University of Applied Sciences Augsburg
Augsburg
DE
Email: rolf.winter@hs-augsburg.de
Michael Faath
Conntac GmbH
Augsburg
DE
Email: faath@conntac.net
Fabian Weisshaar
University of Applied Sciences Augsburg
Augsburg
DE
Email: fabian.weisshaar@hs-augsburg.de
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