Internet DRAFT - draft-huitema-dhc-anonymity-profile
draft-huitema-dhc-anonymity-profile
Network Working Group C. Huitema
Internet-Draft Microsoft
Updates: 4361 (if approved) T. Mrugalski
Intended status: Standards Track ISC
Expires: October 9, 2015 S. Krishnan
Ericsson
April 7, 2015
Anonymity profile for DHCP clients
draft-huitema-dhc-anonymity-profile-02.txt
Abstract
Some DHCP options carry unique identifiers. These identifiers can
enable device tracking even if the device administrator takes care of
randomizing other potential identifications like link-layer addresses
or IPv6 addresses. The anonymity profile is designed for clients
that wish to remain anonymous to the visited network. The profile
provides guidelines on the composition of DHCP or DHCPv6 requests,
designed to minimize disclosure of identifying information. This
draft updates RFC4361.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 9, 2015.
Copyright Notice
Copyright (c) 2015 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements . . . . . . . . . . . . . . . . . . . . . . 3
2. Application domain . . . . . . . . . . . . . . . . . . . . . 3
2.1. MAC Address Randomization hypotheses . . . . . . . . . . 4
2.2. MAC Address Randomization and DHCP . . . . . . . . . . . 5
2.3. Radio fingerprinting . . . . . . . . . . . . . . . . . . 5
2.4. Operating system fingerprinting . . . . . . . . . . . . . 6
2.5. No anonymity profile identification . . . . . . . . . . . 6
2.6. Using the anonymity profiles . . . . . . . . . . . . . . 7
2.7. What about privacy for DHCP servers . . . . . . . . . . . 7
3. Anonymity profile for DHCPv4 . . . . . . . . . . . . . . . . 8
3.1. Client IP address field . . . . . . . . . . . . . . . . . 8
3.2. Requested IP address option . . . . . . . . . . . . . . . 9
3.3. Client hardware address . . . . . . . . . . . . . . . . . 9
3.4. Client Identifier Option . . . . . . . . . . . . . . . . 9
3.5. Host Name Option . . . . . . . . . . . . . . . . . . . . 10
3.6. Client FQDN Option . . . . . . . . . . . . . . . . . . . 11
3.7. UUID/GUID-based Client Identifier Option . . . . . . . . 11
3.8. User and Vendor Class DHCP options . . . . . . . . . . . 12
4. Anonymity profile for DHCPv6 . . . . . . . . . . . . . . . . 12
4.1. Do not send Confirm messages . . . . . . . . . . . . . . 12
4.2. Client Identifier DHCPv6 Option . . . . . . . . . . . . . 13
4.2.1. Anonymous Information-Request . . . . . . . . . . . . 13
4.3. Server Identifier Option . . . . . . . . . . . . . . . . 14
4.4. Address assignment options . . . . . . . . . . . . . . . 14
4.4.1. Obtain temporary addresses . . . . . . . . . . . . . 14
4.5. Option Request Option . . . . . . . . . . . . . . . . . . 15
4.5.1. Previous option values . . . . . . . . . . . . . . . 15
4.6. Authentication Option . . . . . . . . . . . . . . . . . . 16
4.7. User and Vendor Class DHCPv6 options . . . . . . . . . . 16
4.8. Client FQDN Option . . . . . . . . . . . . . . . . . . . 16
5. Operational Considerations . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Reports surfaced recently of systems that would monitor the wireless
connections of passengers at Canadian airports [CNBC]. We can assume
that these are either fragments or trial runs of a wider system that
would attempt to monitor Internet users as they roam through wireless
access points and other temporary network attachments. We can also
assume that privacy conscious users will attempt to evade this
monitoring, for example by ensuring that low level identifiers such
as link-layer addresses are "randomized," so that the devices do not
broadcast a unique identifier in every location that they visit.
Of course, link layer "MAC" addresses are not the only way to
identify a device. As soon as it connects to a remote network, the
device may use DHCP and DHCPv6 to obtain network parameters. The
analysis of DHCP and DHCPv6 options shows that parameters of these
protocols can reveal identifiers of the device, negating the benefits
of link-layer address randomization. This is documented in detail in
[I-D.ietf-dhc-dhcp-privacy] and [I-D.ietf-dhc-dhcpv6-privacy]. The
natural reaction is to restrict the number and values of such
parameters in order to minimize disclosure.
In the absence of a common standard, different system developers are
likely to implement this minimization of disclosure in different
ways. Monitoring entities could then use the differences to identify
the software version running on the device. The proposed anonymity
profile provides a common standard that minimizes information
disclosure, including the disclosure of implementation identifiers.
1.1. Requirements
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 [RFC2119].
2. Application domain
Mobile nodes can be tracked using multiple identifiers, the most
prominent being MAC addresses. For example, when devices use Wi-Fi
connectivity, they place the MAC address in the header of all the
packets that they transmit. Standard implementation of Wi-Fi use
unique 48 bit MAC addresses, assigned to the devices according to
procedures defined by IEEE 802. Even when the Wi-Fi packets are
encrypted, the portion of the header containing the addresses will be
sent in clear text. Tracking devices can "listen to the airwaves" to
find out what devices are transmitting near them.
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We can easily imagine that the MAC addresses can be correlated with
other data, e.g., clear text names and cookies, to build a registry
linking MAC addresses to the identity of devices' owners. Once that
correlation is done, tracking the MAC address is sufficient to track
individual people, even when all application data sent from the
devices is encrypted. MAC addresses can also be correlated with IP
addresses of devices, negating potential privacy benefits of IPv6
"privacy" addresses. Privacy advocates have some reason to be
concerned.
The obvious solution is to "randomize" the MAC address. Before
connecting to a particular network, the device replaces the MAC
address with a randomly drawn 48 bit value. MAC address
randomization was successfully tried at the IETF in Honolulu in
November 2014 [IETFMACRandom]. However, we have to consider the
linkage between MAC addresses, DHCP identifiers and IP addresses.
2.1. MAC Address Randomization hypotheses
There is not yet an established standard for randomizing MAC
addresses. Various prototypes have tried different strategies, such
as:
Per connection: Configure a random MAC address at the time of
connecting to a network, e.g. to specific Wi-Fi SSID, and keep it
for the duration of the connection.
Per network: Same as "per connection," but always use the same MAC
address for the same network -- different of course from the
addresses used in other networks.
Time interval: Change the MAC address at regular time intervals.
In practice, there are many reasons to keep the MAC address constant
for the duration of a link-layer connection, as in the "per
connection" or "per network" variants. On Wi-Fi networks, changing
the MAC address requires dropping the existing Wi-Fi connection and
then re-establishing it, which implies repeating the connection
process and associated procedures. The IP addresses will change,
which means that all required TCP connections will have to be re-
established. If the network access is provided through a NAT,
changing IP address also means that the NAT traversal procedures will
have to be restarted. This means a lot of disruption. At the same
time, an observer on the network will easily notice that a station
left, another came in just after that, and that the new one appears
to be communicating with pretty much the same set of IP addresses as
the old one. This provides for easy correlation.
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The anonymity profile pretty much assumes that the MAC address
randomization follows the "per connection" or "per network"
strategies, or a variant of the "time interval" strategy in which the
interval has about the same duration as the average connection.
2.2. MAC Address Randomization and DHCP
From a privacy point of view, it is clear that MAC Addresses, IP
addresses and DHCP identifiers shall evolve in synchrony. For
example, if the MAC address changes and the DHCP identifier stays
constant, then it is really easy to correlate old and new MAC
addresses, either by listening to DHCP traffic or by observing that
the IP address remains constant, since it is tied to the DHCP
identifier. Conversely, if the DHCP identifier changes but the MAC
address remains constant, the old and new identifiers and addresses
can be correlated by listening to L2 traffic. The procedures
documented in the following sections construct DHCP identifiers from
the current MAC address, automatically providing for this
synchronization.
The proposed anonymity profiles solve this synchronization issues by
deriving most identifiers from the MAC address, and generally by
making making sure that DHCP parameter values do not remain constant
after an address change.
2.3. Radio fingerprinting
MAC address randomization solves the trivial monitoring problem in
which someone just uses a Wi-Fi scanner and records the MAC addresses
seen on the air. DHCP anonymity solves the more elaborated scenario
in which someone monitor MAC addresses and identities used in DHCP at
the access point or DHCP server. But this are not the only ways to
track a mobile device.
Radio fingerprinting is a process that identifies a radio transmitter
by the unique "fingerprint" of its signal transmission, i.e., the
tiny differences caused by minute imperfections of the radio
transmission hardware. This can be applied to diverse types of
radios, including Wi-Fi as described for example in
[WiFiRadioFingerprinting]. No amount of MAC address randomization
will protect against such techniques. Protections may exist, but
they are outside the scope of the present document.
On the other hand, we should not renounce randomization just because
radio fingerprinting exists. The radio fingerprinting techniques are
harder to deploy than just recording MAC addresses with a scanner.
They can only track devices for which the fingerprint are known, and
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thus have a narrower scope of application than mass monitoring of
addresses and DHCP parameters.
2.4. Operating system fingerprinting
When a standard like DHCP allows for multiple options, different
implementers will make different choices for the options that they
support or the values they chose for the options. Conversely,
monitoring the options and values present in DHCP messages reveals
these differences and allows for "operating system fingerprinting,"
i.e., finding the type and version of software that a particular
device is running. Finding these versions provides some information
about the device identity, and thus goes against the goal of
anonymity.
The design of the anonymity profiles attempts to minimize the number
of options and the choice of values, in order to reduce the
possibilities of operating system fingerprinting.
2.5. No anonymity profile identification
Reviewers of the anonymity profiles have sometimes suggested adding
an option to explicitly identify the profiles as "using the anonymity
option." One suggestion is that if the client wishes to remain
anonymous, it would be good if the client told the server about that
in case the server is willing to co-operate. Another possibility
would be to use specific privacy-oriented construct, such as for
example a new type of DUID of temporary DUID that would be changing
over time.
This is not workable in a large number of cases as it is possible
that the network operator (or other entities that have access to the
operator's network) might be actively participating in surveillance
and anti-privacy, willingly or not. Declaring a preference for
anonymity is a bit like walking around with a Guy Fawkes mask. When
anonymity is required, it is generally not a good idea to stick out
of the crowd. Simply revealing the desire for privacy, could cause
the attacker to react by triggering additional surveillance or
monitoring mechanisms. Therefore we feel that it is preferable to
not disclose one's desire for privacy.
This preference leads to some important implications. In particular,
we make an effort to make the mitigation techniques difficult to
distinguish from regular client behaviors, if at all possible.
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2.6. Using the anonymity profiles
There are downsides to randomizing MAC addresses and DHCP
identifiers. By definition, randomization will break management
procedures that rely on tracking MAC addresses. Even if this is not
too much of a concern, we have to be worried about the frequency of
MAC address randomization. Suppose for example that many devices
would get new random MAC addresses at short intervals, maybe every
few minutes. This would generate new DHCP requests in rapid
succession, with a high risk of exhausting DHCPv4 address pools.
Even with IPv6, there would still be a risk of increased neighbor
discovery traffic, and bloating of various address tables.
Implementers will have to be cautious when programming devices to use
randomized MAC addresses. They will have to carefully chose the
frequency with which such addresses will be renewed.
This document only provides guidelines for using DHCP when clients
care about privacy and servers do not object. We assume that the
request for anonymity is materialized by the assignment of a
randomized MAC address to the network interface. Once that decision
is made, the following guidelines will avoid leakage of identity in
DHCP parameters or in assigned addresses.
There may be rare situations where the clients want anonymity to
attackers but not to their DHCP server. These clients should still
use MAC Address randomization to hide from observers, and some form
of encrypted communication to the DHCP server. This scenario is not
yet supported in this document.
2.7. What about privacy for DHCP servers
This document only provides recommendations for DHCP clients. The
main target are DHCP clients used in mobile devices. Such devices
are a tempting target for various monitoring systems, and providing
them with a simple anonymity solution is urgent. We can argue that
some mobile devices embed DHCP servers, and that providing solutions
for such devices is also quite important. Two plausible examples
would be a DHCP server for a car network, or a DHCP server for a
mobile hot spot. However, mobile servers get a lot of privacy
protection through the use of access control and link layer
encryption. Servers may disclose information to clients through
DHCP, but they normally only do that to clients that have passed the
link-layer access control and have been authorized to use the network
services. This arguably makes solving the server problem less urgent
than solving the client problem.
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The server part will be covered by the general mitigation work going
on in DHCP working group, following the analyses presented in
[I-D.ietf-dhc-dhcp-privacy] and [I-D.ietf-dhc-dhcpv6-privacy].
3. Anonymity profile for DHCPv4
Clients using the DHCPv4 anonymity profile limit the disclosure of
information by controlling the header parameters and by limiting the
number and values of options. The number of options depend on the
specific DHCP message:
DISCOVER: The anonymized DISCOVER messages MUST contain the Message
Type, Client Identifier, Host name, and Parameter Request List
options. It SHOULD NOT contain any other option.
REQUEST: The anonymized REQUEST messages SHOULD contain the Message
Type, Client Identifier, Host name, and Parameter Request List
options. If the message is in response to an OFFER, it SHOULD
contain the corresponding Server Identifier option. It SHOULD NOT
contain any other option.
DECLINE: The anonymized DECLINE messages SHOULD contain the Message
Type, Client Identifier and Server Identifier options.
RELEASE: The anonymized RELEASE messages SHOULD contain the Message
Type, Client Identifier and Server Identifier options.
INFORM: The anonymized INFORM messages MUST contain the Message
Type, Client Id, Host name, and Parameter Request List options.
It SHOULD NOT contain any other option.
Header fields and option values SHOULD be set in accordance with the
DHCP specification, but some header fields and option values SHOULD
be constructed per the following guidelines.
3.1. Client IP address field
Four bytes in the header of the DHCP messages carry the "Client IP
address" (ciaddr) as defined in [RFC2131]. In DHCP, this field is
used by the clients to indicate the address that they used
previously, so that as much as possible the server can allocate them
the same address.
There is very little privacy implication of sending this address in
the DHCP messages, except in one case, when connecting to a different
network than the last network connected. If the DHCP client somehow
repeated the address used in a previous network attachment,
monitoring services might use the information to tie the two network
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locations. DHCP clients should ensure that the field is cleared when
they know that the network attachment has changed, and in particular
of the link layer address is reset by the device's administrator.
The clients using the anonymity profile MUST NOT include in the
message a Client IP Address that has been obtained with a different
MAC address.
3.2. Requested IP address option
The Requested IP address option (code 50) allows the client to
request that a particular IP address be assigned. The option is
mandatory in some protocol messages per [RFC2131], for example when a
client selects to use an address offered by a server. However, this
option is not mandatory in the DHCPDISCOVER message. It is simply a
convenience, an attempt to regain the same IP address that was used
in a previous connection. Doing so entails the risk of disclosing an
IP address used by the client at a previous location, or with a
different MAC Address.
When using the anonymity profile, clients SHOULD NOT use the
Requested IP address option in DHCPDISCOVER Messages. They MUST use
the option when mandated by the DHCP protocol, for example in
DHCPREQUEST Messages.
3.3. Client hardware address
Sixteen bytes in the header of the DHCP messages carry the "Client
hardware address" (chaddr) as defined in [RFC2131]. The presence of
this address is necessary for the proper operation of the DHCP
service.
Hardware addresses, called "link layer address" in many RFCs, can be
used to uniquely identify a device, especially if they follow the
IEEE 802 recommendations. These unique identifiers can be used by
monitoring services to track the location of the device and its user.
The only plausible defense is to somehow reset the hardware address
to a random value when visiting an untrusted location, before
transmitting anything at that location with the hardware address. If
the hardware address is reset to a new value, or randomized, the DHCP
client SHOULD use the new randomized value in the DHCP messages.
3.4. Client Identifier Option
The client identifier option is defined in [RFC2132] with option code
61. It is discussed in details in [RFC4361]. The purpose of the
client identifier option is to identify the client in a manner
independent of the link layer address. This is particularly useful
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if the DHCP server is expected to assign the same address to the
client after a network attachment is swapped and the link layer
address changes. It is also useful when the same node issues
requests through several interfaces, and expects the DHCP server to
provide consistent configuration data over multiple interfaces.
The considerations for hardware independence and strong client
identity have an adverse effect on the privacy of mobile clients,
because the hardware-independent unique identifier obviously enables
very efficient tracking of the client's movements.
The recommendations in [RFC4361] are very strong, stating for example
that "DHCPv4 clients MUST NOT use client identifiers based solely on
layer two addresses that are hard-wired to the layer two device
(e.g., the Ethernet MAC address)." These strong recommendations are
in fact a tradeoff between ease of management and privacy, and the
tradeoff should depend on the circumstances.
In contradiction to [RFC4361], When using the anonymity profile, DHCP
clients MUST use client identifiers based solely on the link layer
address that will be used in the underlying connection. This will
ensure that the DHCP client identifier does not leak any information
that is not already available to entities monitoring the network
connection. It will also ensure that a strategy of randomizing the
link layer address will not be nullified by DHCP options.
3.5. Host Name Option
The Host Name option is defined in [RFC2132] with option code 12.
Depending on implementations, the option value can carry either a
fully qualified domain name such as "node1984.example.com," or a
simple host name such as "node1984." The host name is commonly used
by the DHCP server to identify the host, and also to automatically
update the address of the host in local name services.
Fully qualified domain names are obviously unique identifiers, but
even simple host names can provide a significant amount of
information on the identity of the device. They are typically chosen
to be unique in the context where the device is most often used. If
that context is wide enough, in a large company or in a big
university, the host name will be a pretty good identifier of the
device. Monitoring services could use that information in
conjunction with traffic analysis and quickly derive the identity of
the device's owner.
When using the anonymity profile, DHCP clients MAY avoid sending the
host name option. If they chose to send the option, DHCP clients
MUST always send a non-qualified host name instead of a fully
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qualified domain name, and MUST obfuscate the host name value, so it
could not be linked to anything other than the link layer address.
When obfuscating the host name, DHCP clients SHOULD set the host name
value to a hexadecimal representation of the link layer address that
will be used in the underlying connection. They MAY choose another
convention in rare cases, for example in multi-homed scenarios.
3.6. Client FQDN Option
The Client FQDN option is defined in [RFC4702] with option code 81.
The option allows the DHCP clients to advertise to the DHCP server
their fully qualified domain name (FQDN) such as
"mobile.example.com." This would allow the DHCP server to update in
the DNS the PTR record for the IP address allocated to the client.
Depending on circumstances, either the DHCP client or the DHCP server
could update in the DNS the A record for the FQDN of the client.
Obviously, this option uniquely identifies the client, exposing it to
the DHCP server or to anyone listening to DHCP traffic. In fact, if
the DNS record is updated, the location of the client becomes visible
to anyone with DNS lookup capabilities.
When using the anonymity profile, DHCP clients SHOULD NOT include the
Client FQDN option in their DHCP requests. Alternatively, they MAY
include a special purpose FQDN using the same hostname as in the Host
Name Option, with a suffix matching the connection-specific DNS
suffix being advertised by that DHCP server. Having a name in the
DNS allows working with legacy systems that require one to be there,
e.g., by verifying a forward and reverse lookup succeeds with the
same result.
3.7. UUID/GUID-based Client Identifier Option
The UUID/GUID-based Client Machine Identifier option is defined in
[RFC4578], with option code 97. The option is part of a set of
options for Intel Preboot eXecution Environment (PXE). The purpose
of the PXE system is to perform management functions on a device
before its main OS is operational. The Client Machine Identifier
carries a 16-octet Globally Unique Identifier (GUID), which uniquely
identifies the device.
The PXE system is clearly designed for devices operating in a
controlled environment, and its functions are not meant to be used by
mobile nodes visiting untrusted networks. If only for privacy
reasons, nodes visiting untrusted networks MUST disable the PXE
functions, and MUST NOT send the corresponding options.
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3.8. User and Vendor Class DHCP options
Vendor identifying options are defined in [RFC2132] and [RFC3925].
When using the anonymity profile, DHCP clients SHOULD NOT use the
Vendor Specific Information option (code 43), the Vendor Class
Identifier Option (60), the Vendor Class option (code 124), or the
Vendor Specific Information option (code 125) as these options
potentially reveal identifying information.
4. Anonymity profile for DHCPv6
DHCPv6 is typically used by clients in one of two scenarios: stateful
and stateless configuration. In the stateful scenario, clients use a
combination of SOLICIT, REQUEST, CONFIRM, RENEW, REBIND and RELEASE
messages to obtain addresses, and manage these addresses.
In the stateless scenario, clients configure addresses using a
combination of client managed identifiers and router-advertised
prefixes, without involving the DHCPv6 services. Different ways of
constructing these prefixes have different implications on privacy,
which are discussed in [I-D.ietf-6man-default-iids] and
[I-D.ietf-6man-ipv6-address-generation-privacy]. In the stateless
scenario, clients use DHCPv6 to obtain network configuration
parameters, through the INFORMATION-REQUEST message.
The choice between the stateful and stateless scenario depends of
flag and prefix options published by the "Router Advertisement"
messages of local routers, as specified in [RFC4861]. When these
options enable stateless address configuration hosts using the
anonymity profile SHOULD choose it over stateful address
configuration, because stateless configuration requires fewer
information disclosure than stateful configuration.
When using the anonymity profile, DHCPv6 clients carefully select
DHCPv6 options used in the various messages that they sent. The list
of options that are mandatory or optional for each message is
specified in [RFC3315]. Some of these options have specific
implications on anonymity. The following sections provide guidance
on the choice of option values when using the anonymity profile.
4.1. Do not send Confirm messages
The [RFC3315] requires clients to send a Confirm message when they
attach to a new link to verify whether the addressing and
configuration information they previously received is still valid.
This requirement was relaxed in [I-D.ietf-dhc-rfc3315bis]. When
these clients send Confirm messages, they include any IAs assigned to
the interface that may have moved to a new link, along with the
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addresses associated with those IAs. By examining the addresses in
the Confirm message an attacker can trivially identify the previous
point(s) of attachment.
Clients interested in protecting their privacy SHOULD NOT send
Confirm messages and instead directly try to acquire addresses on the
new link.
4.2. Client Identifier DHCPv6 Option
The client identifier option is defined in [RFC3315] with option code
1. The purpose of the client identifier option is to identify the
client to the server. The content of the option is a DHCP User ID
(DUID). One of the primary privacy concerns is that a client is
disclosing a stable identifier (the DUID) that can be use for
tracking and profiling. Three DUID formats are specified: Link-layer
address plus time, Vendor-assigned unique ID based on Enterprise
Number, Link-layer address.
When using the anonymity profile in conjunction with randomized MAC
addresses, DHCPv6 clients MUST use the DUID format number 3, Link-
layer address. The value of the Link-layer address should be that
currently assigned to the interface.
When using the anonymity profile without the benefit of randomized
MAC addresses, clients that want to protect their privacy SHOULD
generate a new randomized DUID-LLT every time they attach to a new
link or detect a possible link change event. The exact details are
left up to implementors, but there are several factors should be
taken into consideration. The DUID type SHOULD be set to 1 (DUID-
LLT). Hardware type SHOULD be set appropriately to the hardware
type. Time MAY be set to current time, but this will reveal the fact
that the DUID is newly generated. Implementors interested in hiding
this fact MAY use a time stamp from the past. e.g. a random timestamp
from the previous year could be a good value. In the most common
cases the link-layer address is based on MAC. The first three octets
are composed of the OUI (Organizationally Unique Identifier) that is
expected to have a value assigned to a real organization. See
[IEEE-OUI] for currently assigned values. Using a value that is
unassigned may disclose the fact that a DUID is randomized. Using a
value that belongs to a third party may have legal implications.
4.2.1. Anonymous Information-Request
According to [RFC3315], a DHCPv6 client typically includes its client
identifier in most of the messages it sends. There is one exception,
however. Client is allowed to omit its client identifier when
sending Information-Request.
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When using stateless DHCPv6, clients wanting to protect their privacy
SHOULD NOT include client identifiers in their Information-Request
messages. This will prevent the server from specifying client-
specific options if it is configured to do so, but the need for
anonymity precludes such options anyway.
4.3. Server Identifier Option
When using the anonymity profile, DHCPv6 clients SHOULD use the
Server Identifier Option (code 2) as specified in [RFC3315]. Clients
MUST only include server identifier values that were received with
the current MAC address, because reuse of old values discloses
information that can be used to identify the client.
4.4. Address assignment options
When using the anonymity profile, DHCPv6 clients might have to use
SOLICIT or REQUEST messages to obtain IPv6 addresses through the DHCP
server. The clients SHOULD only use the options necessary to perform
the requested DHCPv6 transactions, such as Identity Association for
Non-temporary Addresses Option (code 3) or Identity Association for
Temporary Addresses Option (code 4).
The clients MAY use the IA Address Option (code 5) but need to
balance the potential advantage of "address continuity" versus the
potential risk of "previous address disclosure." A potential
solution is to remove all stored addresses when a MAC address
changes, and to only use the IA Address option with addresses that
have been explicitly assigned through the current MAC address.
The interaction between prefix delegation and anonymity require
further study. For now, the simple solution is to avoid using prefix
delegation when striving for anonymity. When using the anonymity
profiles, clients SHOULD NOT use IA_PD, the prefix delegation form of
address assignment.
4.4.1. Obtain temporary addresses
[RFC3315] defines a special container (IA_TA) for requesting
temporary addresses. This is a good mechanism in principle, but
there are a number of issues associated with it. First, this is not
widely used feature, so clients depending solely on temporary
addresses may lock themselves out of service. Secondly, [RFC3315]
does not specify any renewal mechanisms for temporary addresses.
Therefore support for renewing temporary addresses may vary between
server implementations, including not being supported at all.
Finally, by requesting temporary addresses a client reveals its
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desire for privacy and potentially risks countermeasures as described
in Section 2.5.
Clients interested in their privacy SHOULD NOT use IA_TA. They
should simply send an IA_NA with a randomized IAID. This, along with
the mitigation technique discussed in Section 4.3, will ensure that a
client will get a new address that can be renewed and can be used as
long as needed. To get a new address, it can send Request message
with a new randomized IAID before releasing the other one. This will
cause the server to assign a new address, as it still has a valid
lease for the old IAID value. Once a new address is assigned, the
address obtained using the older IAID value can be released safely,
using the Release message or it may simply be allowed to time out.
This solution may not work if the server enforces specific policies,
e.g. only one address per client. If client does not succeed in
receiving a second address using a new IAID, it may release the first
one (using an old IAID) and then retry asking for a new address.
From the Operating System perspective, addresses obtained using this
technique SHOULD be treated as temporary as specified in [RFC4941].
4.5. Option Request Option
A DHCPv6 client may reveal other types of information, besides unique
identifiers. There are many ways a DHCPv6 client can perform certain
actions and the specifics can be used to fingerprint the client.
This may not reveal the identity of a client, but may provide
additional information, such as the device type, vendor type or OS
type and in some cases specific version.
One specific method used for fingerprinting utilizes the order in
which options are included in the message. Another related technique
utilizes the order in which option codes are included in an Option
Request Option (ORO).
The client willing to protect its privacy SHOULD randomize options
order before sending any DHCPv6 message. Such a client SHOULD also
randomly shuffle the option codes order in ORO.
4.5.1. Previous option values
According to [RFC3315], the client that includes an Option Request
Option in a Solicit or Request message MAY additionally include
instances of those options that are identified in the Option Request
option, with data values as hints to the server about parameter
values the client would like to have returned.
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When using the anonymity profile, clients SHOULD NOT include such
instances of options because old values might be used to identify the
client.
4.6. Authentication Option
The purpose of the Authentication option (code 11) is to authenticate
the identity of clients and servers and the contents of DHCP
messages. As such, the option can be used to identify the client,
and is incompatible with the stated goal of "client anonymity."
DHCPv6 clients that use the anonymity profile SHOULD NOT use the
authentication option. They MAY use it if they recognize that they
are operating in a trusted environment, e.g., in a work place
network.
4.7. User and Vendor Class DHCPv6 options
When using the anonymity profile, DHCPv6 clients SHOULD NOT use the
User Class option (code 15) or the Vendor Class option (code 16), as
these options potentially reveal identifying information.
4.8. Client FQDN Option
The Client FQDN option is defined in [RFC4704] with option code 29.
The option allows the DHCP clients to advertize to the DHCP their
fully qualified domain name (FQDN) such as "mobile.example.com."
When using the anonymity profile, DHCPv6 clients SHOULD NOT include
the Client FQDN option in their DHCPv6 messages because it identifies
the client. As explained in Section 3.6 they MAY use a local-only
FQDN by combining a host name derived from the link layer address and
a suffix advertised by the local DHCP server.
5. Operational Considerations
The anonymity profile has the effect of hiding the client identity
from the DHCP server. This is not always desirable. Some DHCP
servers provide facilities like publishing names and addresses in the
DNS, or ensuring that returning clients get reassigned the same
address. Implementers should be careful to only use the anonymity
profile when privacy trumps management considerations.
Clients using the anonymity profile in general consume more
resources. For example when they change MAC address and request for
a new IP, the old one is still marked as leased by the server.
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6. Security Considerations
The use of the anonymity profile does not change the security
considerations of the DHCPv4 or DHCPv6 protocols.
7. IANA Considerations
This draft does not require any IANA action.
8. Acknowledgments
The inspiration for this draft came from discussions in the Perpass
mailing list. Several people provided feedback on this draft,
notably Noel Anderson, Lorenzo Colitti, Stephen Farrell, Tushar
Gupta, Gabriel Montenegro, Marcin Siodelski, Dave Thaler and Jun Wu.
9. References
9.1. Normative References
[I-D.ietf-dhc-rfc3315bis]
Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., and T. Lemon, "Dynamic Host
Configuration Protocol for IPv6 (DHCPv6) bis", draft-ietf-
dhc-rfc3315bis-00 (work in progress), March 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for
Dynamic Host Configuration Protocol version 4 (DHCPv4)",
RFC 3925, October 2004.
[RFC4361] Lemon, T. and B. Sommerfeld, "Node-specific Client
Identifiers for Dynamic Host Configuration Protocol
Version Four (DHCPv4)", RFC 4361, February 2006.
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[RFC4702] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
Configuration Protocol (DHCP) Client Fully Qualified
Domain Name (FQDN) Option", RFC 4702, October 2006.
[RFC4704] Volz, B., "The Dynamic Host Configuration Protocol for
IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
Option", RFC 4704, October 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
9.2. Informative References
[CNBC] Weston, G., Greenwald, G., and R. Gallagher, "CBC News:
CSEC used airport Wi-Fi to track Canadian travellers", Jan
2014, <http://www.cbc.ca/news/politics/csec-used-airport-
wi-fi-to-track-canadian-travellers-edward-snowden-
documents-1.2517881>.
[I-D.ietf-6man-default-iids]
Gont, F., Cooper, A., Thaler, D., and W. Will,
"Recommendation on Stable IPv6 Interface Identifiers",
draft-ietf-6man-default-iids-02 (work in progress),
January 2015.
[I-D.ietf-6man-ipv6-address-generation-privacy]
Cooper, A., Gont, F., and D. Thaler, "Privacy
Considerations for IPv6 Address Generation Mechanisms",
draft-ietf-6man-ipv6-address-generation-privacy-04 (work
in progress), February 2015.
[I-D.ietf-dhc-dhcp-privacy]
Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
considerations for DHCP", draft-ietf-dhc-dhcp-privacy-00
(work in progress), February 2015.
[I-D.ietf-dhc-dhcpv6-privacy]
Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
considerations for DHCPv6", draft-ietf-dhc-
dhcpv6-privacy-00 (work in progress), February 2015.
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[IEEE-OUI]
IEEE, "Organizationally Unique Identifiers
http://www.ieee.org/netstorage/standards/oui.txt",
<http://www.ieee.org/netstorage/standards/oui.txt>.
[IETFMACRandom]
Zuniga, JC., "MAC Privacy", November 2014,
<http://www.ietf.org/blog/2014/11/mac-privacy/>.
[RFC4578] Johnston, M. and S. Venaas, "Dynamic Host Configuration
Protocol (DHCP) Options for the Intel Preboot eXecution
Environment (PXE)", RFC 4578, November 2006.
[WiFiRadioFingerprinting]
Brik, V., Banerjee, S., Gruteser, M., and S. Oh, "Wireless
Device Identification with Radiometric Signatures",
September 2008,
<http://www.winlab.rutgers.edu/~gruteser/papers/
brik_paradis.pdf>.
Authors' Addresses
Christian Huitema
Microsoft
Redmond, WA 98052
U.S.A.
Email: huitema@microsoft.com
Tomek Mrugalski
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
USA
Email: tomasz.mrugalski@gmail.com
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
Phone: +1 514 345 7900 x42871
Email: suresh.krishnan@ericsson.com
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