Privacy considerations for IP broadcast and multicast protocol designers
draft-ietf-intarea-broadcast-consider-00
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| Document | Type | Active Internet-Draft (intarea WG) | |
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| Authors | Rolf Winter , Michael Faath , Fabian Weisshaar | ||
| Last updated | 2016-10-31 | ||
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draft-ietf-intarea-broadcast-consider-00
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-ietf-intarea-broadcast-consider-00
Abstract
A number of application-layer protocols make use of IP broadcasts or
multicast messages for functions such as local service discovery or
name resolution. Some of these functions can only be implemented
efficiently using such mechanisms. When using broadcasts or
multicast messages, a passive observer in the same broadcast 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 4, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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 . . . . . . . . . . . . . . . . . . . . 3
2.2. Persistent identifiers . . . . . . . . . . . . . . . . . 4
2.3. Anticipate user behaviour . . . . . . . . . . . . . . . . 4
2.4. Consider potential correlation . . . . . . . . . . . . . 5
2.5. Configurability . . . . . . . . . . . . . . . . . . . . . 5
3. Operational considerations . . . . . . . . . . . . . . . . . 5
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Informative References . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Broadcast and multicast messages have a large 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 [TRAC2016].
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
which makes it trivial for anybody on a LAN to collect the
information without special privileges or a special location in the
network and b) encryption is more difficult when broadcasting/
multicasting messages.
Privacy considerations of IETF-specified protocols have received some
attention in the recent past(e.g. [RFC7721] or
[I-D.ietf-dhc-dhcp-privacy]). This draft documents a number of
privacy considerations for broadcast/multicast protocol designers
that are intended to reduce the likelihood that a broadcast protocol
can be misused to collect sensitive data about devices, users and
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groups of users on a LAN. 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 behaviour observed as part of experiments on
operational networks [TRAC2016].
2.1. Message frequency
Frequent broadcast/multicast traffic caused by an application can
give user behaviour 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 are only
visible in the same broadcast domain, these messages also give the
rough location of the user away (e.g. a campus or building).
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. 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].
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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.
2.2. Persistent identifiers
A few broadcast/multicast protocols observed in the wild make use of
persistent identifiers. This includes the use of hostnames 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.
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 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 behaviour
A large number of users name their device after themselves, either
using their first name, last name or both. Often a hostname 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 behaviour [TRAC2016]. For protocols
using the hostname as part of the messages, this clearly will reveal
personally identifiable information to everyone on the local network.
Where possible, the use of hostnames 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 behaviour. More hostname
considerations can be found in [I-D.ietf-intarea-hostname-practice].
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2.4. Consider potential correlation
A large number of services and applications make use of the
broadcast/multicast mechanism. That means there are various sources
of information that are easily accessible by a passive observer. In
isolation, the information these protocols reveal might seem
harmless, but given multiple such protocols, it might be possible to
correlate this information. E.g. a protocol that uses frequent
messages including a UUID to identify the particular installation
does not give the identity of the user away. But a single message
including the user's hostname might just do that and it can be
correlated using e.g. the MAC address of the device's interface.
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 protocols do not
include the means to declare "safe" environments (e.g. based on the
SSID of a WiFi network). 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 behaviour could be
restricted to "safe" environments.
An application developer making use of broadcasts as part of the
application SHOULD make the broadcast feature, if possible,
configurable, so that potentially sensitive information does not leak
on public networks.
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.
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4. Acknowledgements
This work was partly supported by the European Commission under grant
agreement FP7-318627 mPlane. Support does not imply endorsement.
5. IANA Considerations
This memo includes no request to IANA.
6. 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.
7. 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-dhc-dhcp-privacy]
Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
considerations for DHCP", draft-ietf-dhc-dhcp-privacy-05
(work in progress), February 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.
[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>.
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[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[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>.
[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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