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Versions: 00 01 02 draft-ietf-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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   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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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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  . . . . . . . . . . . . . . . . . . . . . . . .   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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