draft-ietf-intarea-broadcast-consider-09.txt   rfc8386.txt 
Internet Engineering Task Force R. Winter Internet Engineering Task Force (IETF) R. Winter
Internet-Draft University of Applied Sciences Augsburg Request for Comments: 8386 University of Applied Sciences Augsburg
Intended status: Informational M. Faath Category: Informational M. Faath
Expires: September 14, 2018 Conntac GmbH ISSN: 2070-1721 Conntac GmbH
F. Weisshaar F. Weisshaar
University of Applied Sciences Augsburg University of Applied Sciences Augsburg
March 13, 2018 May 2018
Privacy considerations for protocols relying on IP broadcast and Privacy Considerations for
multicast Protocols Relying on IP Broadcast or Multicast
draft-ietf-intarea-broadcast-consider-09
Abstract Abstract
A number of application-layer protocols make use of IP broadcasts or A number of application-layer protocols make use of IP broadcast or
multicast messages for functions such as local service discovery or multicast messages for functions such as local service discovery or
name resolution. Some of these functions can only be implemented name resolution. Some of these functions can only be implemented
efficiently using such mechanisms. When using broadcasts or efficiently using such mechanisms. When using broadcast or multicast
multicast messages, a passive observer in the same broadcast/ messages, a passive observer in the same broadcast or multicast
multicast domain can trivially record these messages and analyze domain can trivially record these messages and analyze their content.
their content. Therefore, designers of protocols that make use of Therefore, designers of protocols that make use of broadcast or
broadcast/multicast messages need to take special care when designing multicast messages need to take special care when designing their
their protocols. protocols.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." 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.
This Internet-Draft will expire on September 14, 2018. 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.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction ....................................................2
1.1. Types and usage of broadcast and multicast . . . . . . . 4 1.1. Types and Usage of Broadcast and Multicast .................4
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.2. Requirements Language ......................................5
2. Privacy considerations . . . . . . . . . . . . . . . . . . . 5 2. Privacy Considerations ..........................................5
2.1. Message frequency . . . . . . . . . . . . . . . . . . . . 5 2.1. Message Frequency ..........................................5
2.2. Persistent identifiers . . . . . . . . . . . . . . . . . 5 2.2. Persistent Identifiers .....................................6
2.3. Anticipate user behavior . . . . . . . . . . . . . . . . 6 2.3. Anticipate User Behavior ...................................6
2.4. Consider potential correlation . . . . . . . . . . . . . 7 2.4. Consider Potential Correlation .............................7
2.5. Configurability . . . . . . . . . . . . . . . . . . . . . 7 2.5. Configurability ............................................7
3. Operational considerations . . . . . . . . . . . . . . . . . 8 3. Operational Considerations ......................................8
4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Summary .........................................................8
5. Other considerations . . . . . . . . . . . . . . . . . . . . 9 5. Other Considerations ............................................9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 6. IANA Considerations ............................................10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations ........................................10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. References .....................................................10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References ......................................10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10 8.2. Informative References ....................................10
9.2. Informative References . . . . . . . . . . . . . . . . . 10 Acknowledgments ...................................................13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses ................................................13
1. Introduction 1. Introduction
Broadcast and multicast messages have a large (and to the sender Broadcast and multicast messages have a large (and, to the sender,
unknown) receiver group by design. Because of that, these two unknown) receiver group by design. Because of that, these two
mechanisms are vital for a number of basic network functions such as mechanisms are vital for a number of basic network functions such as
auto-configuration or link-layer address lookup. Also application autoconfiguration and link-layer address lookup. Also, application
developers use broadcast/multicast messages to implement things such developers use broadcast/multicast messages to implement things such
as local service or peer discovery. It appears that an increasing as local service or peer discovery. It appears that an increasing
number of applications make use of it as suggested by experimental number of applications make use of it as suggested by experimental
results obtained on campus networks including the IETF meeting results obtained on campus networks, including the IETF meeting
network [TRAC2016]. This trend is not entirely surprising. As network [TRAC2016]. This trend is not entirely surprising. As
[RFC0919] puts it, "The use of broadcasts [...] is a good base for
[RFC919] puts it, "The use of broadcasts [...] is a good base for
many applications". Broadcast and multicast functionality in a many applications". Broadcast and multicast functionality in a
subnetwork are therefore important as a lack thereof renders the subnetwork is therefore important because a lack thereof renders the
protocols relying on these mechanisms inoperable [RFC3819]. protocols relying on these mechanisms inoperable [RFC3819].
Using broadcast/multicast can become problematic if the information Using broadcast/multicast can become problematic if the information
that is being distributed can be regarded as sensitive or when the that is being distributed can be regarded as sensitive or if the
information that is distributed by multiple of these protocols can be information that is distributed by multiple protocols can be
correlated in a way that sensitive data can be derived. This is correlated in a way that sensitive data can be derived. This is
clearly true for any protocol, but broadcast/multicast is special in clearly true for any protocol, but broadcast/multicast is special in
at least two respects: at least two respects:
(a) The aforementioned large receiver group, consisting of receivers (a) The aforementioned large receiver group consists of receivers
unknown to the sender. This makes eavesdropping without special unknown to the sender. This makes eavesdropping without special
privileges or a special location in the network trivial for privileges or a special location in the network trivial for
anybody in the same broadcast/multicast domain. anybody in the same broadcast/multicast domain.
(b) Encryption is difficult when broadcast/multicast messages are (b) Encryption is difficult when broadcast/multicast messages are
used, for instance because a non-trivial key management protocol used, because, for instance, a non-trivial key management
might be required. When encryption is not used, the content of protocol might be required. When encryption is not used, the
these messages is easily accessible, making it easy to spoof and content of these messages is easily accessible, making it easy
replay them. to spoof and replay them.
Given the above, privacy protection for protocols based on broadcast Given the above, privacy protection for protocols based on broadcast
or multicast communication is significantly more difficult compared or multicast communication is significantly more difficult compared
to unicast communication and at the same time invading the privacy is to unicast communication, and at the same time, invasion of privacy
much easier. is much easier.
Privacy considerations of IETF-specified protocols have received some Privacy considerations for IETF-specified protocols have received
attention in the recent past (e.g. [RFC7721] or [RFC7819]). There some attention in the recent past (e.g., [RFC7721] and [RFC7819]).
is also general guidance available for document authors on when and There is also general guidance available for document authors on when
how to include a privacy considerations section in their documents and how to include a privacy considerations section in their
and on how to evaluate the privacy implications of Internet protocols documents and on how to evaluate the privacy implications of Internet
[RFC6973]. RFC6973 also describes potential threats to privacy in protocols [RFC6973]. RFC 6973 also describes potential threats to
great detail and lists terminology that is also used in this privacy in great detail and lists terminology that is also used in
document. In contrast to RFC6973, this document contains a number of this document. In contrast to RFC 6973, this document contains a
privacy considerations especially for protocols that rely on number of privacy considerations, especially for protocols that rely
broadcast/multicast, intended to reduce the likelihood that a on broadcast/multicast, that are intended to reduce the likelihood
broadcast/multicast protocol can be misused to collect sensitive data that a broadcast- or multicast-based protocol can be misused to
about devices, users and groups of users in a broadcast/multicast collect sensitive data about devices, users, and groups of users in a
domain. broadcast/multicast domain.
The above mentioned considerations particularly apply to protocols The above-mentioned considerations particularly apply to protocols
designed outside the IETF - for two reasons. For one, non-standard designed outside the IETF for two reasons. First, non-standard
protocols will likely not receive operational attention and support protocols will likely not receive operational attention and support
in making them more secure, e.g. what DHCP snooping does for DHCP. in making them more secure, e.g., what DHCP snooping does for DHCP.
But because these protocols are typically not documented, network Because these protocols are typically not documented, network
equipment does not provide similar features for them. The other equipment does not provide similar features for them. Second, these
reason is that these protocols have been designed in isolation, where protocols have been designed in isolation, where a set of
a set of considerations to follow is useful in the absence of a considerations to follow is useful in the absence of a larger
larger community providing feedback and expertise to improve the community providing feedback and expertise to improve the protocol.
protocol. In particular, carelessly designed protocols that use In particular, carelessly designed protocols that use broadcast/
broadcast/multicast can break privacy efforts at different layers of multicast can break privacy efforts at different layers of the
the protocol stack such as MAC address or IP address randomization protocol stack such as Media Access Control (MAC) address or IP
[RFC4941]. address randomization [RFC4941].
1.1. Types and usage of broadcast and multicast 1.1. Types and Usage of Broadcast and Multicast
In IPv4, two major types of broadcast addresses exist, the limited In IPv4, two major types of broadcast addresses exist: limited
broadcast which is defined as all-ones (255.255.255.255, defined in broadcast and directed broadcast. Section 5.3.5 of [RFC1812] defines
section 5.3.5.1 of [RFC1812]) and the directed broadcast with the limited broadcast as all-ones (255.255.255.255) and defines directed
given network prefix of an IP address and the host part of all-ones broadcast as the given network prefix of an IP address and the local
(defined in section 5.3.5.2. of [RFC1812]). Broadcast packets are part of all-ones. Broadcast packets are received by all nodes in a
received by all nodes in a subnetwork. Limited broadcasts never subnetwork. Limited broadcasts never transit a router. The same is
transit a router. The same is true for directed broadcasts by true for directed broadcasts by default, but routers may provide an
default, but routers may provide an option to do this [RFC2644]. option to do this [RFC2644]. IPv6, on the other hand, does not
IPv6 on the other hand does not provide broadcast addresses but provide broadcast addresses but relies solely on multicast [RFC4291].
solely relies on multicast [RFC4291].
In contrast to broadcast addresses, multicast addresses represent an In contrast to broadcast addresses, multicast addresses represent an
identifier for a set of interfaces that can be a set different from 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 all nodes in the subnetwork. All interfaces that are identified by a
given multicast address receive packets destined towards that address given multicast address receive packets destined towards that address
and are called a multicast group. In both IPv4 and IPv6, multiple and are called a "multicast group". In both IPv4 and IPv6, multiple
pre-defined multicast addresses exist. The ones most relevant for pre-defined multicast addresses exist. The ones most relevant for
this document are the ones with subnet scope. For IPv4, an IP prefix this document are the ones with subnet scope. For IPv4, an IP prefix
is reserved for this purpose called the Local Network Control Block called the "Local Network Control Block" (224.0.0.0/24, defined in
(224.0.0.0/24, defined in section 4 of [RFC5771]). For IPv6, the Section 4 of [RFC5771]) is reserved for this purpose. For IPv6, the
relevant multicast addresses are the two All Nodes Addresses, which relevant multicast addresses are the two All Nodes Addresses, which
every IPv6-capable host is required to recognize as identifying every IPv6-capable host is required to recognize as identifying
itself (see section 2.7.1 of [RFC4291]). itself (see Section 2.7.1 of [RFC4291]).
Typical usage of these addresses include local service discovery Typical usage of these addresses includes local service discovery
(e.g. Multicast DNS (mDNS) [RFC6762] and Link-Local Multicast Name (e.g., Multicast DNS (mDNS) [RFC6762] and Link-Local Multicast Name
Resolution (LLMNR) [RFC4795] make use of multicast), Resolution (LLMNR) [RFC4795] make use of multicast),
autoconfiguration (e.g. DHCPv4 [RFC2131] uses broadcasts and DHCPv6 autoconfiguration (e.g., DHCPv4 [RFC2131] uses broadcasts, and DHCPv6
[RFC3315] uses multicast addresses) and other vital network services [RFC3315] uses multicast addresses), and other vital network services
such as address resolution or duplicate address detection. But such as address resolution or duplicate address detection. Aside
besides these core network functions, also applications make use of from these core network functions, applications also make use of
broadcast and multicast functionality, often implementing proprietary broadcast and multicast functionality, often implementing proprietary
protocols. In sum, these protocols distribute a diverse set of protocols. In sum, these protocols distribute a diverse set of
potentially privacy sensitive information to a large receiver group potentially privacy-sensitive information to a large receiver group,
and to be part of this receiver group, the only requirement is to be and the only requirement to be part of this receiver group is to be
on same subnetwork. on the same subnetwork.
1.2. Requirements Language 1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "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 2. Privacy Considerations
There are a few obvious and a few not necessarily obvious things There are a few obvious and a few not necessarily obvious things that
designers of protocols utilizing broadcast/multicast should consider designers of protocols utilizing broadcast/multicast should consider
in respect to the privacy implications of their protocol. Most of in respect to the privacy implications for their protocol. Most of
these items are based on protocol behavior observed as part of these items are based on protocol behavior observed as part of
experiments on operational networks [TRAC2016]. experiments on operational networks [TRAC2016].
2.1. Message frequency 2.1. Message Frequency
Frequent broadcast/multicast traffic caused by an application can Frequent broadcast/multicast traffic caused by an application can
give away user behavior and online connection times. This allows a give away user behavior and online connection times. This allows a
passive observer to potentially deduce a user's current activity 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 (e.g., a game) and to create an online profile (i.e., times the user
the user is on the network). The higher the frequency of these is on the network). This profile becomes more accurate as the
messages and the duration of time these messages are sent, the more frequency of messages and the time duration over which they are sent
accurate this profile will be. Given that broadcasts/multicasts are increases. Given that broadcast/multicast messages are only visible
only visible in the same broadcast/multicast domain, these messages in the same broadcast/multicast domain, these messages also give away
also give the rough location of the user away (e.g. a campus or the rough location of the user (e.g., a campus or building).
building).
This behavior has e.g. been observed by a synchronization mechanism This behavior has, for example, been observed by a synchronization
of a popular application, where multiple messages have been sent per mechanism of a popular application, where multiple messages have been
minute via broadcast. Given this behavior, it is possible to record sent per minute via broadcast. Given this behavior, it is possible
a device's time on the network with a sub-minute accuracy given only to record a device's time on the network with a sub-minute accuracy
the traffic of this single application installed on the device. But given only the traffic of this single application installed on the
also services used for local name resolution in modern operating device. Also, services used for local name resolution in modern
systems utilize broadcast/multicast protocols (e.g. mDNS, LLMNR or operating systems utilize broadcast- or multicast-based protocols
NetBIOS) to announce for example resources regularly which also allow (e.g., mDNS, LLMNR, or NetBIOS) to announce, for example, resources
tracking the online time of a device. 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 If a protocol relies on frequent or periodic broadcast/multicast
messages, the frequency SHOULD be chosen conservatively, in messages, the frequency SHOULD be chosen conservatively, in
particular if the messages contain persistent identifiers (see next particular if the messages contain persistent identifiers (see
subsection). Also, intelligent message suppression mechanisms such Section 2.2). Also, intelligent message suppression mechanisms such
as the ones employed in mDNS [RFC6762] SHOULD be implemented. The as the ones employed in mDNS [RFC6762] SHOULD be implemented. The
lower the frequency of broadcast messages, the harder passive traffic lower the frequency of broadcast messages, the harder passive traffic
analysis and surveillance becomes. analysis and surveillance becomes.
2.2. Persistent identifiers 2.2. Persistent Identifiers
A few protocols that make use of broadcast/multicast messages A few protocols that make use of broadcast/multicast messages
observed in the wild make use of persistent identifiers. This observed in the wild also make use of persistent identifiers. This
includes the use of host names or more abstract persistent includes the use of host names or more abstract persistent
identifiers such as a universally unique identifiers (UUID) or identifiers such as a Universally Unique Identifiers (UUIDs) or
similar. These IDs, which e.g. identify the installation of a similar. These IDs, which, for example, identify the installation of
certain application might not change across updates of the software a certain application, might not change across updates of the
and can therefore be extremely long lived. This allows a passive software and can therefore be extremely long lived. This allows a
observer to track a user precisely if broadcast/multicast messages passive observer to track a user precisely if broadcast/multicast
are frequent. This is even true in case the IP and/or MAC address messages are frequent. This is even true if the IP and/or MAC
changes. Such identifiers also allow two different interfaces (e.g. address changes. Such identifiers also allow two different
WiFi and Ethernet) to be correlated to the same device. If the interfaces (e.g., Wi-Fi and Ethernet) to be correlated to the same
application makes use of persistent identifiers for multiple device. If the application makes use of persistent identifiers for
installations of the same application for the same user, this even multiple installations of the same application for the same user,
allows to infer that different devices belong to 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 The aforementioned broadcast messages from a synchronization
mechanism of a popular application also included a persistent mechanism of a popular application also included a persistent
identifier in every broadcast. This identifier never changed after identifier in every broadcast. This identifier never changed after
the application was installed and it allowed to track a device even the application was installed, which allowed for the tracking of a
when it changed its network interface or when it connected to a device even when it changed its network interface or when it
different network. connected to a different network.
Persistent IDs are considered bad practice in general for broadcast In general, persistent IDs are considered bad practice for broadcast
and multicast communication, as persistent application layer IDs will and multicast communication, as persistent application-layer IDs will
make efforts on lower layers to randomize identifiers (e.g. make efforts to randomize identifiers (e.g., [RANDOM-ADDR]) on lower
[I-D.huitema-6man-random-addresses]) useless. When protocols that layers useless. When protocols that make use of broadcast/multicast
make use of broadcast/multicast need to make use of IDs, these IDs need to make use of IDs, these IDs SHOULD be rotated frequently to
SHOULD be rotated frequently to make user tracking more difficult. make user tracking more difficult.
2.3. Anticipate user behavior 2.3. Anticipate User Behavior
A large number of users name their device after themselves, either A large number of users name their device after themselves, either
using their first name, last name or both. Often a host name 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 the type, model, or maker of a device, its function, or
includes language specific information. Based on data gathered language-specific information. Based on data gathered during
during experiments performed at IETF meetings and at a large campus experiments performed at IETF meetings and at a large campus network,
network, this appears currently to be prevalent user behavior this appears to be the currently prevalent user behavior [TRAC2016].
[TRAC2016]. For protocols using the host name as part of the For protocols using the host name as part of the messages, this
messages, this clearly will reveal personally identifiable clearly will reveal personally identifiable information to everyone
information to everyone on the local network. This information can on the local network. This information can also be used to mount
also be used to mount more sophisticated attacks, when e.g. the owner more sophisticated attacks, e.g., when the owner of a device is
of a device is identified (as an interesting target) or properties of identified (as an interesting target) or properties of the device are
the device are known (e.g. known vulnerabilities). Host names are known (e.g., known vulnerabilities). Host names are also a type of
also a type of persistent identifier and therefore the considerations persistent identifier; therefore, the considerations in Section 2.2
in Section 2.2 apply. apply.
Some of the most commonly used operating systems include the name the Some of the most commonly used operating systems include the name the
user chooses for the user account during the installation process as user chooses for the user account during the installation process as
part of the host name of the device. The name of the operating part of the host name of the device. The name of the operating
system can also be included, revealing therefore two pieces of system can also be included, therefore revealing two pieces of
information, which can be regarded as private information if the host information that can be regarded as private information if the host
name is used in broadcast/multicast messages. name is used in broadcast/multicast messages.
Where possible, the use of host names and other user-provided Where possible, the use of host names and other user-provided
information in protocols making use of broadcast/multicast SHOULD be information in protocols making use of broadcast/multicast SHOULD be
avoided. An application might want to display the information it avoided. An application might want to display the information it
will broadcast on the LAN at install/config time, so the user is at will broadcast on the LAN at install/config time, so that the user is
least aware of the application's behavior. More host name at least aware of the application's behavior. More host name
considerations can be found in [RFC8117]. More information on user considerations can be found in [RFC8117]. More information on user
participation can be found in [RFC6973]. participation can be found in [RFC6973].
2.4. Consider potential correlation 2.4. Consider Potential Correlation
A large number of services and applications make use of the A large number of services and applications make use of the
broadcast/multicast mechanism. That means there are various sources broadcast/multicast mechanism. That means there are various sources
of information that are easily accessible by a passive observer. In of information that are easily accessible by a passive observer. In
isolation, the information these protocols reveal might seem isolation, the information these protocols reveal might seem
harmless, but given multiple such protocols, it might be possible to harmless, but given multiple such protocols, it might be possible to
correlate this information. E.g. a protocol that uses frequent correlate this information. For example, a protocol that uses
messages including a UUID to identify the particular installation frequent messages including a UUID to identify the particular
does not give the identity of the user away. But a single message installation does not give away the identity of the user. However, a
including the user's host name might just do that and it can be single message including the user's host name might do that, and it
correlated using e.g. the MAC address of the device's interface. can be correlated using, for example, the MAC address of the device's
interface.
In the experiments described in [TRAC2016], it was possible to In the experiments described in [TRAC2016], it was possible to
correlate frequently sent broadcast messages that included a unique correlate frequently sent broadcast messages that included a unique
identifier with other broadcast/multicast messages containing identifier with other broadcast/multicast messages containing
usernames (e.g. mDNS, LLMNR or NetBIOS), but also relationships to usernames (e.g. mDNS, LLMNR, or NetBIOS); this revealed relationships
other users. This allowed to reveal the real identity of the users among users. This allowed the real identity of the users of many
of many devices but it also gave some information about their social devices to be revealed, and it also gave away some information about
environment away. their social environment.
A designer of a protocol that makes use of broadcast/multicast needs 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 to be aware of the fact that even if the information a protocol leaks
a protocol leaks seems harmless, there might be ways to correlate seems harmless in isolation, there might be ways to correlate that
that information with information from other protocols to reveal information with information from other protocols to reveal sensitive
sensitive information about a user. information about a user.
2.5. Configurability 2.5. Configurability
A lot of applications and services relying on broadcast/multicast A lot of applications and services relying on broadcast- or
protocols do not include the means to declare "safe" environments multicast-based protocols do not include the means to declare "safe"
(e.g. based on the SSID of a WiFi network and the MAC addresses of environments (e.g., based on the Service Set Identifier (SSID) of a
the access points). E.g. a device connected to a public WiFi will Wi-Fi network and the MAC addresses of the access points). For
likely broadcast the same information as when connected to the home example, a device connected to a public Wi-Fi network will likely
network. It would be beneficial if certain behavior could be broadcast the same information as when connected to the home network.
restricted to "safe" environments. It would be beneficial if certain behaviors could be restricted to
"safe" environments.
A popular operating system e.g. allows the user to specify the trust For example, a popular operating system allows the user to specify
level of the network the device connects to, which for example the trust level of the network the device connects to, which, for
restricts specific system services (using broadcast/multicast example, restricts specific system services (using broadcast/
messages for their normal operation) to be used in trusted networks multicast messages for their normal operation) to be used in trusted
only. Such functionality could implemented as part of an networks only. Such functionality could be implemented as part of an
application. application.
An application developer making use of broadcasts/multicasts as part An application developer making use of broadcast/multicast messages
of the application SHOULD make the broadcast feature, if possible, as part of the application SHOULD, if possible, make the broadcast
configurable, so that potentially sensitive information does not leak feature configurable so that potentially sensitive information does
on public networks, where the threat to privacy is much larger. not leak on public networks where the threat to privacy is much
larger.
3. Operational considerations 3. Operational Considerations
Besides changing end-user behavior, choosing sensible defaults as an Besides changing end-user behavior, choosing sensible defaults as an
operating system vendor (e.g. for suggesting host names) and the operating system vendor (e.g., for suggesting host names), and
considerations for protocol designers mentioned in this document, following the considerations for protocol designers mentioned in this
there is something that the network administrators/operators can do document, there is something that the network administrators/
to limit the above mentioned problems. operators can do to limit the above-mentioned problems.
A feature commonly found on access points e.g. is to manage/filter A feature commonly found on access points is the ability to manage/
broadcast and multicast traffic. This will potentially break certain filter broadcast and multicast traffic. This will potentially break
applications or some of their functionality but will also protect the certain applications or some of their functionality but will also
users from potentially leaking sensitive information. Wireless protect the users from potentially leaking sensitive information.
access points often provide finer-grained control beyond a simple on/ Wireless access points often provide finer-grained control beyond a
off switch for well-known protocols or provide mechanisms to manage simple on/off switch for well-known protocols or provide mechanisms
broadcast/multicast traffic intelligently using e.g. proxies (see to manage broadcast/multicast traffic intelligently using, for
[I-D.ietf-mboned-ieee802-mcast-problems]). These mechanisms however example, proxies (see [MCAST-CONS]). However, these mechanisms only
only work on standardized protocols. work on standardized protocols.
4. Summary 4. Summary
Increasingly, applications rely on protocols that send and receive Increasingly, applications rely on protocols that send and receive
broadcast and multicast messages. For some, broadcasts/multicasts broadcast and multicast messages. For some, broadcast/multicast
are the basis of their application logic, others use broadcasts/ messages are the basis of their application logic; others use
multicasts to improve certain aspects of the application but are broadcast/multicast messages to improve certain aspects of the
fully functional in case broadcasts/multicasts fail. Irrespective of application but are fully functional in case broadcast/multicast
the role of broadcast and multicast messages for the application, the messages fail. Irrespective of the role of broadcast and multicast
designers of protocols that make use of them should be very careful messages for the application, the designers of protocols that make
in their protocol design because of the special nature of broadcast use of them should be very careful in their protocol design because
and multicast. of the special nature of broadcast and multicast.
It is not always possible to implement certain functionality via It is not always possible to implement certain functionality via
unicast, but in case a protocol designer chooses to rely on unicast, but if a protocol designer chooses to rely on broadcast/
broadcast/multicast, the following should be carefully considered: multicast, the following should be carefully considered:
o IETF-specified protocols, such as mDNS [RFC6762], SHOULD be used o IETF-specified protocols, such as mDNS [RFC6762], SHOULD be used
if possible as operational support might exist to protect against if possible as operational support might exist to protect against
the leakage of private information. Also, for some protocols the leakage of private information. Also, for some protocols,
privacy extensions are being specified, which can be used if privacy extensions are being specified; these can be used if
implemented. E.g. for DNS-SD privacy extensions are documented in implemented. For example, for DNS-SD, privacy extensions are
[I-D.ietf-dnssd-privacy] documented in [DNSSD-PRIV].
o Using user-specified information inside broadcast/multicast o Using user-specified information inside broadcast/multicast
messages SHOULD be avoided, as users will often use personal messages SHOULD be avoided, as users will often use personal
information or other information aiding attackers, in particular information or other information that aids attackers, in
if the user is unaware about how that information is being used 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 o The use of persistent IDs in messages SHOULD be avoided, as this
allows user tracking, correlation and potentially has a allows user tracking and correlation, and it potentially has a
devastating effect on other privacy protection mechanisms devastating effect on other privacy-protection mechanisms.
o If one really must design a new protocol relying on broadcast/ o If one must design a new protocol relying on broadcast/multicast
multicast and cannot use an IETF-specified protocol, then: and cannot use an IETF-specified protocol, then:
* the protocol SHOULD be very conservative in how frequently it * the protocol SHOULD be very conservative in how frequently it
sends messages as an effort in data minimization sends messages as an effort in data minimization,
* it SHOULD make use of mechanisms implemented in IETF-specified * it SHOULD make use of mechanisms implemented in IETF-specified
protocols that can be helpful in privacy protection such as protocols that can be helpful in privacy protection, such as
message suppression in mDNS message suppression in mDNS,
* it SHOULD be designed in a way that information sent in * it SHOULD be designed in such a way that information sent in
broadcast/multicast messages cannot be correlated with broadcast/multicast messages cannot be correlated with
information from other protocols using broadcast/multicast information from other protocols using broadcast/multicast, and
* it SHOULD be possible to let the user configure "safe" * it SHOULD be possible to let the user configure "safe"
environments if possible (e.g. based on the SSID) to minimize environments if possible (e.g., based on the SSID) to minimize
the risk of information leakage (e.g. a home network as opposed the risk of information leakage (e.g., a home network as
to a public Wifi) opposed to a public Wi-Fi network).
5. Other considerations 5. Other Considerations
Besides privacy implications, frequent broadcasting also represents a Besides privacy implications, frequent broadcasting also represents a
performance problem. In particular in certain wireless technologies performance problem. In particular, in certain wireless technologies
such as 802.11, broadcast and multicast are transmitted at a much such as 802.11, broadcast and multicast are transmitted at a much
lower rate (the lowest common denominator rate) compared to unicast lower rate (the lowest common denominator rate) compared to unicast
and therefore have a much bigger impact on the overall available and therefore have a much bigger impact on the overall available
airtime [I-D.ietf-mboned-ieee802-mcast-problems]. Further, it will airtime [MCAST-CONS]. Further, it will limit the ability for devices
limit the ability for devices to go to sleep if frequent broadcasts to go to sleep if frequent broadcasts are being sent. A similar
are being sent. A similar problem in respect to Router problem in respect to Router Advertisements is addressed in
Advertisements is addressed in [RFC7772]. In that respect, broadcast/multicast can be used for
[I-D.ietf-v6ops-reducing-ra-energy-consumption]. In that respect another class of attacks that is not related to privacy. The
broadcasts/multicast can be used for another class of attacks that is potential impact on network performance should nevertheless be
not related to privacy. The potential impact on network performance considered when designing a protocol that makes use of broadcast/
should nevertheless be considered when designing a protocol that multicast.
makes use of broadcast/multicast.
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 6. IANA Considerations
This memo includes no request to IANA. This document has no IANA actions.
8. Security Considerations 7. Security Considerations
This document deals with privacy-related considerations of broadcast- This document deals with privacy-related considerations for
and multicast-based protocols. It contains advice for designers of broadcast- and multicast-based protocols. It contains advice for
such protocols to minimize the leakage of privacy-sensitive designers of such protocols to minimize the leakage of privacy-
information. The intent of the advice is to make sure that sensitive information. The intent of the advice is to make sure that
identities will remain anonymous and user tracking will be made identities will remain anonymous and user tracking will be made
difficult. difficult.
It should be noted that certain applications could make use of To protect multicast traffic, certain applications can make use of
existing mechanisms to protect multicast traffic such as the ones existing mechanisms, such as the ones defined in [RFC5374]. Examples
defined in [RFC5374]. Examples of such applications can be found in of such applications can be found in Appendix A of [RFC5374].
Appendix A. of [RFC5374]. Given the required infrastructure and However, given the assumptions about these applications and the
assumptions about these applications and the security infrastructure, required security infrastructure, many applications will not be able
many applications will not be able to make use of such mechanisms. to make use of such mechanisms.
9. References 8. References
9.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References [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>.
[I-D.huitema-6man-random-addresses] 8.2. Informative References
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] [DNSSD-PRIV]
Huitema, C. and D. Kaiser, "Privacy Extensions for DNS- Huitema, C. and D. Kaiser, "Privacy Extensions for DNS-
SD", draft-ietf-dnssd-privacy-00 (work in progress), SD", Work in Progress, draft-ietf-dnssd-privacy-04, April
October 2016. 2018.
[I-D.ietf-mboned-ieee802-mcast-problems] [MCAST-CONS]
Perkins, C., McBride, M., Stanley, D., Kumari, W., and J. Perkins, C., McBride, M., Stanley, D., Kumari, W., and J.
Zuniga, "Multicast Considerations over IEEE 802 Wireless Zuniga, "Multicast Considerations over IEEE 802 Wireless
Media", draft-ietf-mboned-ieee802-mcast-problems-01 (work Media", Work in Progress, draft-ietf-mboned-ieee802-mcast-
in progress), February 2018. problems-01, February 2018.
[I-D.ietf-v6ops-reducing-ra-energy-consumption] [RANDOM-ADDR]
Yourtchenko, A. and L. Colitti, "Reducing energy Huitema, C., "Implications of Randomized Link Layers
consumption of Router Advertisements", draft-ietf-v6ops- Addresses for IPv6 Address Assignment", Work in Progress,
reducing-ra-energy-consumption-03 (work in progress), draft-huitema-6man-random-addresses-03, March 2016.
November 2015.
[RFC0919] Mogul, J., "Broadcasting Internet Datagrams", STD 5, RFC [RFC919] Mogul, J., "Broadcasting Internet Datagrams", STD 5,
919, DOI 10.17487/RFC0919, October 1984, RFC 919, DOI 10.17487/RFC0919, October 1984,
<http://www.rfc-editor.org/info/rfc919>. <https://www.rfc-editor.org/info/rfc919>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995, RFC 1812, DOI 10.17487/RFC1812, June 1995,
<http://www.rfc-editor.org/info/rfc1812>. <https://www.rfc-editor.org/info/rfc1812>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC [RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
2131, DOI 10.17487/RFC2131, March 1997, RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>. <https://www.rfc-editor.org/info/rfc2131>.
[RFC2644] Senie, D., "Changing the Default for Directed Broadcasts [RFC2644] Senie, D., "Changing the Default for Directed Broadcasts
in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644, in Routers", BCP 34, RFC 2644, DOI 10.17487/RFC2644,
August 1999, <http://www.rfc-editor.org/info/rfc2644>. August 1999, <https://www.rfc-editor.org/info/rfc2644>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>. 2003, <https://www.rfc-editor.org/info/rfc3315>.
[RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D., [RFC3819] Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89, Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, DOI 10.17487/RFC3819, July 2004, RFC 3819, DOI 10.17487/RFC3819, July 2004,
<http://www.rfc-editor.org/info/rfc3819>. <https://www.rfc-editor.org/info/rfc3819>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <http://www.rfc-editor.org/info/rfc4291>. 2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local [RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795, DOI Multicast Name Resolution (LLMNR)", RFC 4795,
10.17487/RFC4795, January 2007, DOI 10.17487/RFC4795, January 2007,
<http://www.rfc-editor.org/info/rfc4795>. <https://www.rfc-editor.org/info/rfc4795>.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
<http://www.rfc-editor.org/info/rfc4941>. <https://www.rfc-editor.org/info/rfc4941>.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast [RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008, Protocol", RFC 5374, DOI 10.17487/RFC5374, November 2008,
<http://www.rfc-editor.org/info/rfc5374>. <https://www.rfc-editor.org/info/rfc5374>.
[RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for [RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
10.17487/RFC5771, March 2010, DOI 10.17487/RFC5771, March 2010,
<http://www.rfc-editor.org/info/rfc5771>. <https://www.rfc-editor.org/info/rfc5771>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013, DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>. <https://www.rfc-editor.org/info/rfc6762>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, DOI Considerations for Internet Protocols", RFC 6973,
10.17487/RFC6973, July 2013, DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>. <https://www.rfc-editor.org/info/rfc6973>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms", Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016, RFC 7721, DOI 10.17487/RFC7721, March 2016,
<http://www.rfc-editor.org/info/rfc7721>. <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 [RFC7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819, Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819,
April 2016, <http://www.rfc-editor.org/info/rfc7819>. April 2016, <https://www.rfc-editor.org/info/rfc7819>.
[RFC8117] Huitema, C., Thaler, D., and R. Winter, "Current Hostname [RFC8117] Huitema, C., Thaler, D., and R. Winter, "Current Hostname
Practice Considered Harmful", RFC 8117, DOI 10.17487/ Practice Considered Harmful", RFC 8117,
RFC8117, March 2017, <https://www.rfc-editor.org/info/ DOI 10.17487/RFC8117, March 2017,
rfc8117>. <https://www.rfc-editor.org/info/rfc8117>.
[TRAC2016] [TRAC2016] Faath, M., Weisshaar, F., and R. Winter, "How Broadcast
Faath, M., Weisshaar, F., and R. Winter, "How Broadcast Data Reveals Your Identity and Social Graph", Wireless
Data Reveals Your Identity and Social Graph", 7th Communications and Mobile Computing Conference
International Workshop on TRaffic Analysis and (IWCMC), International Workshop on TRaffic Analysis and
Characterization IEEE TRAC 2016, September 2016. 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 Authors' Addresses
Rolf Winter Rolf Winter
University of Applied Sciences Augsburg University of Applied Sciences Augsburg
Augsburg Augsburg
DE Germany
Email: rolf.winter@hs-augsburg.de Email: rolf.winter@hs-augsburg.de
Michael Faath Michael Faath
Conntac GmbH Conntac GmbH
Augsburg Augsburg
DE Germany
Email: faath@conntac.net Email: faath@conntac.net
Fabian Weisshaar Fabian Weisshaar
University of Applied Sciences Augsburg University of Applied Sciences Augsburg
Augsburg Augsburg
DE Germany
Email: fabian.weisshaar@hs-augsburg.de Email: fabian.weisshaar@hs-augsburg.de
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