Internet DRAFT - draft-intarea-broadcast-consider
draft-intarea-broadcast-consider
Internet Engineering Task Force R. Winter
Internet-Draft M. Faath
Intended status: Informational F. Weisshaar
Expires: May 4, 2017 University of Applied Sciences Augsburg
October 31, 2016
Privacy considerations for IP broadcast and multicast protocol designers
draft-intarea-broadcast-consider-02
Abstract
A number of application-layer protocols make use of IP broadcasts or
multicast messages for functions like 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, broadcast/multicast protocol designers
need to take special care when designing their protocols.
Status of This Memo
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This Internet-Draft will expire on May 4, 2017.
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Copyright (c) 2016 IETF Trust and the persons identified as the
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Privacy considerations . . . . . . . . . . . . . . . . . . . 3
2.1. Message frequency . . . . . . . . . . . . . . . . . . . . 4
2.2. Persistent identifiers . . . . . . . . . . . . . . . . . 4
2.3. Anticipate user behavior . . . . . . . . . . . . . . . . 5
2.4. Consider potential correlation . . . . . . . . . . . . . 5
2.5. Configurability . . . . . . . . . . . . . . . . . . . . . 6
3. Operational considerations . . . . . . . . . . . . . . . . . 7
4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Other considerations . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. Informative References . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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. Application developers use broadcast/multicast
messages to implement things like local service or peer discovery and
it appears that an increasing number of applications make use of it.
And, as RFC 919 [RFC0919] puts it, "The use of broadcasts [...] is a
good base for many applications".
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 broadcast/multicast domain.
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(b) Encryption is more difficult when broadcast/multicast messages,
leaving content of these messages in the clear and making it
easier 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. RFC 7721 [RFC7721] or RFC 7919
[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 broadcast/multicast protocol designers
that are intended to reduce the likelihood that a broadcast/multicast
protocol can be misused to collect sensitive data about devices,
users and groups of users on a broadcast/multicast domain. These
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 such as e.g. DHCP snooping does for DHCP because they
typically are not documented. 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. In particular, carelessly designed
broadcast/multicast protocols can break privacy efforts at different
layers of the protocol stack such as MAC address or IP address
randomization [RFC4941].
1.1. 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 RFC 2119 [RFC2119].
2. Privacy considerations
There are a few obvious and a few not necessarily obvious things
designers of broadcast/multicast protocols 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].
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2.1. Message frequency
Frequent broadcast/multicast traffic caused by an application can
give user behavior and online times away. 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, 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 their shares regularly and allow a
tracking of 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 traffic
analysis and surveillance becomes.
2.2. Persistent identifiers
A few broadcast/multicast protocols observed in the wild make use of
persistent identifiers. This includes the use of host names or more
abstract persistent identifiers such as a UUID or similar. These
IDs, which e.g. identify the installation of a certain application
might not change across updates of the software and are therefore
extremely long lived. This allows a passive 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
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identifier in every broadcast. This identifier did never change
after the application was installaed and allowed to track a device
even when it changed its network interface or when it connected to a
different network.
If a broadcast/multicast protocol relies on IDs to be transmitted, it
SHOULD be considered if frequent ID rotations are possible in order
to make user tracking more difficult. 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 or at least much 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
includes language specific information. Based on gathered data, 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).
A popular operating system vendor includes 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 is also
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 broadcast/multicast protocols SHOULD be avoided. If
only a persistent ID is needed, this can be generated. 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 [I-D.ietf-intarea-hostname-practice]. More information on
user participation can be found in RFC 6973 [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
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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 broadcast protocol designer should 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 other
broadcast protocol information to reveal sensitive information about
a user.
2.5. Configurability
A lot of applications and services using 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 untrusted
networks. 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 thread to privacy is much larger.
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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 are things that the network administrators/operators can do to
limit the above mentioned problems.
A feature not uncommonly found on access points e.g. is to 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.
4. Summary
Increasingly, applications rely on 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 broad- 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
o Avoid using user-specified information inside broadcast/multicast
messages 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 Avoid persistent IDs in messages as this allows user tracking,
correlation and potentially has a devastating effect on other
privacy protection mechanisms
o If you really must use a broadcast/multicast protocol and cannot
use an IETF-specified protocol, then:
* Be very conservative in how frequently you send messages as an
effort in data minimization
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* Seek advice from IETF-specifies protocols such as message
suppression in mDNS
* Try to design the protocol in a way that the information cannot
be correlated with other information in broadcast/multicast
messages
* Let the user configure safe environments if possible (e.g.
based on the SSID)
[Note: discussions on this document should be take place on the
Intarea mailing list of the IETF. Subscription:
https://www.ietf.org/mailman/listinfo/int-area, Mailing list archive:
https://www.ietf.org/mail-archive/web/int-area/current/maillist.html]
5. Other considerations
Besides the privacy implications of frequent broadcasting, it 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 which could leads to jitter, delayed packets,
packet loss and retransmissions and therefore impact application
performance. 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 can be used for another class of attacks that not related
to privacy. The potential impact on network performance should
nevertheless be considered by broadcast protocol designers.
6. Acknowledgments
We would like to thank Eliot Lear and Stephane Bortzmeyer for their
input.
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.
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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.
9. 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-intarea-hostname-practice]
Huitema, C. and D. Thaler, "Current Hostname Practice
Considered Harmful", draft-ietf-intarea-hostname-
practice-00 (work in progress), October 2015.
[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>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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>.
[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>.
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[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>.
[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.
Authors' Addresses
Rolf Winter
University of Applied Sciences Augsburg
Augsburg
DE
Email: rolf.winter@hs-augsburg.de
Michael Faath
University of Applied Sciences Augsburg
Augsburg
DE
Email: michael.faath@hs-augsburg.de
Fabian Weisshaar
University of Applied Sciences Augsburg
Augsburg
DE
Email: fabian.weisshaar@hs-augsburg.de
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