Internet Engineering Task Force (IETF) R. Winter

Internet Engineering Task Force (IETF) R. Winter
Request for Comments: 8386 University of Applied Sciences Augsburg
Category: Informational M. Faath
ISSN: 2070-1721 Conntac GmbH
F. Weisshaar
University of Applied Sciences Augsburg
May 2018

Privacy Considerations for
Protocols Relying on IP Broadcast or Multicast

Abstract

A number of application-layer protocols make use of IP broadcast 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 broadcast or multicast
messages, a passive observer in the same broadcast or multicast
domain can trivially record these messages and analyze their content.
Therefore, designers of protocols that make use of broadcast or
multicast messages need to take special care when designing their
protocols.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8386.

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Copyright Notice

Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................2
1.1. Types and Usage of Broadcast and Multicast .................4
1.2. Requirements Language ......................................5
2. Privacy Considerations ..........................................5
2.1. Message Frequency ..........................................5
2.2. Persistent Identifiers .....................................6
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. IANA Considerations ............................................10
7. Security Considerations ........................................10
8. References .....................................................10
8.1. Normative References ......................................10
8.2. Informative References ....................................10
Acknowledgments ...................................................13
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
autoconfiguration and 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

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[RFC919] puts it, "The use of broadcasts [...] is a good base for
many applications". Broadcast and multicast functionality in a
subnetwork is therefore important because a lack thereof renders the
protocols relying on these mechanisms inoperable [RFC3819].

Using broadcast/multicast can become problematic if the information
that is being distributed can be regarded as sensitive or if the
information that is distributed by multiple 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 consists 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, because, for instance, 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, invasion of privacy
is much easier.

Privacy considerations for IETF-specified protocols have received
some attention in the recent past (e.g., [RFC7721] and [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]. RFC 6973 also describes potential threats to
privacy in great detail and lists terminology that is also used in
this document. In contrast to RFC 6973, this document contains a
number of privacy considerations, especially for protocols that rely
on broadcast/multicast, that are intended to reduce the likelihood
that a broadcast- or multicast-based 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. First, non-standard
protocols will likely not receive operational attention and support
in making them more secure, e.g., what DHCP snooping does for DHCP.
Because these protocols are typically not documented, network
equipment does not provide similar features for them. Second, these

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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 the
protocol stack such as Media Access Control (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: limited
broadcast and directed broadcast. Section 5.3.5 of [RFC1812] defines
limited broadcast as all-ones (255.255.255.255) and defines directed
broadcast as the given network prefix of an IP address and the local
part of all-ones. 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 relies solely 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
called the "Local Network Control Block" (224.0.0.0/24, defined in
Section 4 of [RFC5771]) is reserved for this purpose. 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 includes 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. Aside
from these core network functions, applications also 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 the only requirement to be part of this receiver group is to be
on the same subnetwork.

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1.2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

2. Privacy Considerations

There are a few obvious and a few not necessarily obvious things that
designers of protocols utilizing broadcast/multicast should consider
in respect to the privacy implications for 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 to create an online profile (i.e., times the user
is on the network). This profile becomes more accurate as the
frequency of messages and the time duration over which they are sent
increases. Given that broadcast/multicast messages are only visible
in the same broadcast/multicast domain, these messages also give away
the rough location of the user (e.g., a campus or building).

This behavior has, for example, 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. Also, services used for local name resolution in modern
operating systems utilize broadcast- or multicast-based protocols
(e.g., mDNS, LLMNR, or NetBIOS) to announce, for example, resources
on a regular basis. This also allows tracking of the online times 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
Section 2.2). 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.

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2.2. Persistent Identifiers

A few protocols that make use of broadcast/multicast messages
observed in the wild also make use of persistent identifiers. This
includes the use of host names or more abstract persistent
identifiers such as a Universally Unique Identifiers (UUIDs) or
similar. These IDs, which, for example, 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 observer to track a user precisely if broadcast/multicast
messages are frequent. This is even true if the IP and/or MAC
address changes. Such identifiers also allow two different
interfaces (e.g., Wi-Fi 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 a passive observer 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, which allowed for the tracking of a
device even when it changed its network interface or when it
connected to a different network.

In general, persistent IDs are considered bad practice for broadcast
and multicast communication, as persistent application-layer IDs will
make efforts to randomize identifiers (e.g., [RANDOM-ADDR]) on lower
layers 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
language-specific information. Based on data gathered during
experiments performed at IETF meetings and at a large campus network,
this appears to be the currently 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, e.g., when 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; therefore, the considerations in Section 2.2
apply.

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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, therefore revealing two pieces of
information that 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
avoided. An application might want to display the information it
will broadcast on the LAN at install/config time, so that 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. For example, a protocol that uses
frequent messages including a UUID to identify the particular
installation does not give away the identity of the user. However, a
single message including the user's host name might do that, and it
can be correlated using, for example, 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); this revealed relationships
among users. This allowed the real identity of the users of many
devices to be revealed, and it also gave away some information about
their social environment.

A designer of a protocol that makes use of broadcast/multicast needs
to be aware of the fact that even if the information a protocol leaks
seems harmless in isolation, 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- or
multicast-based protocols do not include the means to declare "safe"
environments (e.g., based on the Service Set Identifier (SSID) of a

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Wi-Fi network and the MAC addresses of the access points). For
example, a device connected to a public Wi-Fi network will likely
broadcast the same information as when connected to the home network.
It would be beneficial if certain behaviors could be restricted to
"safe" environments.

For example, a popular operating system 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 only. Such functionality could be implemented as part of an
application.

An application developer making use of broadcast/multicast messages
as part of the application SHOULD, if possible, make the broadcast
feature 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
following 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 is the ability 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, for
example, proxies (see [MCAST-CONS]). However, these mechanisms only
work on standardized protocols.

4. Summary

Increasingly, applications rely on protocols that send and receive
broadcast and multicast messages. For some, broadcast/multicast
messages are the basis of their application logic; others use
broadcast/multicast messages to improve certain aspects of the
application but are fully functional in case broadcast/multicast
messages 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.

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It is not always possible to implement certain functionality via
unicast, but if 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; these can be used if
implemented. For example, for DNS-SD, privacy extensions are
documented in [DNSSD-PRIV].

o Using user-specified information inside broadcast/multicast
messages SHOULD be avoided, as users will often use personal
information or other information that aids 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 and correlation, and it potentially has a
devastating effect on other privacy-protection mechanisms.

o If one 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 such a way that information sent in
broadcast/multicast messages cannot be correlated with
information from other protocols using broadcast/multicast, and

* 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 Wi-Fi network).

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 [MCAST-CONS]. Further, it will limit the ability for devices

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to go to sleep if frequent broadcasts are being sent. A similar
problem in respect to Router Advertisements is addressed in
[RFC7772]. In that respect, broadcast/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.

6. IANA Considerations

This document has no IANA actions.

7. Security Considerations

This document deals with privacy-related considerations for
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.

To protect multicast traffic, certain applications can make use of
existing mechanisms, such as the ones defined in [RFC5374]. Examples
of such applications can be found in Appendix A of [RFC5374].
However, given the assumptions about these applications and the
required security infrastructure, many applications will not be able
to make use of such mechanisms.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2. Informative References

[DNSSD-PRIV]
Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
SD", Work in Progress, draft-ietf-dnssd-privacy-04, April
2018.

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RFC 8386 Broadcast/Multicast Privacy Considerations May 2018

[MCAST-CONS]
Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
Zuniga, "Multicast Considerations over IEEE 802 Wireless
Media", Work in Progress, draft-ietf-mboned-ieee802-mcast-
problems-01, February 2018.

[RANDOM-ADDR]
Huitema, C., "Implications of Randomized Link Layers
Addresses for IPv6 Address Assignment", Work in Progress,
draft-huitema-6man-random-addresses-03, March 2016.

[RFC919] Mogul, J., "Broadcasting Internet Datagrams", STD 5,
RFC 919, DOI 10.17487/RFC0919, October 1984,
<https://www.rfc-editor.org/info/rfc919>.

[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://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, <https://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, <https://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,
<https://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, <https://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,
<https://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,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc5771>.

[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc7721>.

[RFC7772] Yourtchenko, A. and L. Colitti, "Reducing Energy
Consumption of Router Advertisements", BCP 202, RFC 7772,
DOI 10.17487/RFC7772, February 2016,
<https://www.rfc-editor.org/info/rfc7772>.

[RFC7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819,
April 2016, <https://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>.

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[TRAC2016] Faath, M., Weisshaar, F., and R. Winter, "How Broadcast
Data Reveals Your Identity and Social Graph", Wireless
Communications and Mobile Computing Conference
(IWCMC), International Workshop on TRaffic Analysis and
Characterization (TRAC), DOI 10.1109/IWCMC.2016.7577084,
September 2016.

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.

Authors' Addresses

Rolf Winter
University of Applied Sciences Augsburg
Augsburg
Germany

Email: [email protected]

Michael Faath
Conntac GmbH
Augsburg
Germany

Email: [email protected]

Fabian Weisshaar
University of Applied Sciences Augsburg
Augsburg
Germany

Email: [email protected]

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