Internet DRAFT - draft-winfaa-broadcast-consider
draft-winfaa-broadcast-consider
Internet Engineering Task Force R. Winter
Internet-Draft M. Faath
Intended status: Informational F. Weisshaar
Expires: April 21, 2016 University of Applied Sciences Augsburg
October 19, 2015
Considerations for IP broadcast and multicast protocol designers
draft-winfaa-broadcast-consider-01
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
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This Internet-Draft will expire on April 21, 2016.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Design considerations . . . . . . . . . . . . . . . . . . . . 3
2.1. Message frequency . . . . . . . . . . . . . . . . . . . . 3
2.2. Persistent identifiers . . . . . . . . . . . . . . . . . 3
2.3. Anticipate user behaviour . . . . . . . . . . . . . . . . 4
2.4. Rember - You are not alone . . . . . . . . . . . . . . . 4
2.5. Configurability . . . . . . . . . . . . . . . . . . . . . 4
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. Normative References . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Additional Stuff . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
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.
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 really, but broadcast/multicast is
special in two respects: a) the aforementioned large receiver group
which makes it trivial for anybody on a LAN to collect the
information without special priviledges or a special location in the
network and b) encryption is more difficult when broadcasting/
multicasting messages. This draft documents a number of design
considerations for broadcast/multicast protocol designers that are
intended to reduce the likelyhood that a broadcast protocol can be
misused to collect sensitive data about devices, users and groups of
users on a LAN.
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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. Design considerations
There are a few obvious and a few not necessarily obvious things
designers of broadcast/multicast protocols should consider. Most of
these items are based on protocol behaviour observed as part of an
experiment on an operational network.
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 decuct 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).
If a protocol relies on frequent or periodic broadcast/multicast
messages, the frequency should be chosen conservatively, in
particular if the messages contain persisten identifiers.
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
e.g. identify the installation of a certain application and might
not change across updates of the software. 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 persitent 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 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.
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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
to currently be prevalent user behaviour. 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.
2.4. Rember - You are not alone
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 the protocol's information 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.
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3. IANA Considerations
This memo includes no request to IANA.
4. Security Considerations
TBD
5. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Appendix A. Additional Stuff
This becomes an Appendix.
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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