draft-ietf-dhc-anonymity-profile-08.txt

Network Working Group                                         C. Huitema
Internet-Draft                                                 Microsoft
Intended status: Standards Track                            T. Mrugalski
Expires: August 22, 2016                                             ISC
                                                             S. Krishnan
                                                                Ericsson
                                                       February 19, 2016


                   Anonymity profile for DHCP clients
                draft-ietf-dhc-anonymity-profile-08.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.

Status of This Memo

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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 22, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
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   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 . . . . . . . . . . .   8
   3.  Anonymity profile for DHCPv4  . . . . . . . . . . . . . . . .   8
     3.1.  Avoiding fingerprinting . . . . . . . . . . . . . . . . .   9
     3.2.  Client IP address field . . . . . . . . . . . . . . . . .   9
     3.3.  Requested IP address option . . . . . . . . . . . . . . .  10
     3.4.  Client hardware address field . . . . . . . . . . . . . .  11
     3.5.  Client Identifier Option  . . . . . . . . . . . . . . . .  11
     3.6.  Parameter Request List Option . . . . . . . . . . . . . .  12
     3.7.  Host Name Option  . . . . . . . . . . . . . . . . . . . .  12
     3.8.  Client FQDN Option  . . . . . . . . . . . . . . . . . . .  13
     3.9.  UUID/GUID-based Client Identifier Option  . . . . . . . .  14
     3.10. User and Vendor Class DHCP options  . . . . . . . . . . .  14
   4.  Anonymity profile for DHCPv6  . . . . . . . . . . . . . . . .  14
     4.1.  Avoiding fingerprinting . . . . . . . . . . . . . . . . .  15
     4.2.  Do not send Confirm messages, unless really sure where  .  15
     4.3.  Client Identifier DHCPv6 Option . . . . . . . . . . . . .  16
       4.3.1.  Anonymous Information-Request . . . . . . . . . . . .  17
     4.4.  Server Identifier Option  . . . . . . . . . . . . . . . .  17
     4.5.  Address assignment options  . . . . . . . . . . . . . . .  17
       4.5.1.  Obtain temporary addresses  . . . . . . . . . . . . .  18
       4.5.2.  Prefix delegation . . . . . . . . . . . . . . . . . .  18
     4.6.  Option Request Option . . . . . . . . . . . . . . . . . .  19
       4.6.1.  Previous option values  . . . . . . . . . . . . . . .  19
     4.7.  Authentication Option . . . . . . . . . . . . . . . . . .  19
     4.8.  User and Vendor Class DHCPv6 options  . . . . . . . . . .  19
     4.9.  Client FQDN Option  . . . . . . . . . . . . . . . . . . .  20
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  20
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  Changes from previous versions  . . . . . . . . . . . . . . .  21



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   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     10.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   There have been reports 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 the same 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 link-layer addresses, a.k.a.  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 link-layer



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   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.

   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.  link-layer addresses can also be correlated
   with IP addresses of devices, negating potential privacy benefits of
   IPv6 "privacy" addresses.  Privacy advocates have reasons 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.  Link-layer address
   randomization was successfully tried at the IETF in Honolulu in
   November 2014 [IETFMACRandom] and in following meetings
   [IETFTrialsAndMore]; it is studied in the IEEE 802 EC Privacy
   Recommendation Study Group [IEEE802PRSG].  However, we have to
   consider the linkage between link-layer addresses, DHCP identifiers
   and IP addresses.

2.1.  MAC address Randomization hypotheses

   There is not yet an established standard for randomizing link-layer
   addresses.  Various prototypes have tried different strategies, such
   as:

   Per connection:  Configure a random link-layer 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 link-
      layer address for the same network -- different of course from the
      addresses used in other networks.

   Time interval:  Change the link-layer address at regular time
      intervals.

   In practice, there are many reasons to keep the link-layer 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 link-layer address requires dropping the existing Wi-Fi
   connection and then re-establishing it, which implies repeating the



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   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 the same set of IP addresses as the old one.
   This provides for easy correlation.

   The anonymity profile pretty much assumes that the link-layer 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 link-layer address, IP
   address and DHCP identifier shall evolve in synchrony.  For example,
   if the link-layer address changes and the DHCP identifier stays
   constant, then it is really easy to correlate old and new link-layer
   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 link-
   layer 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 link-layer address, automatically
   providing for this synchronization.

   The proposed anonymity profiles solve this synchronization issues by
   deriving most identifiers from the link-layer address, and generally
   by 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 link-layer addresses and identities used in
   DHCP at the access point or DHCP server.  But these 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



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   radios, including Wi-Fi as described for example in
   [WiFiRadioFingerprinting].  No amount of link-layer 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 link-layer addresses with a
   scanner.  They can only track devices for which the fingerprint are
   known, and 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 for a 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.  (See
   [GuyFawkesMask] for an explanation of this usage.)  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.



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   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.

2.6.  Using the anonymity profiles

   There are downsides to randomizing link-layer addresses and DHCP
   identifiers.  By definition, randomization will break management
   procedures that rely on tracking link-layer addresses.  Even if this
   is not too much of a concern, we have to be worried about the
   frequency of link-layer address randomization.  Suppose for example
   that many devices would get new random link-layer 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.  We assume that the request for anonymity is
   materialized by the assignment of a randomized link-layer 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 link-layer address randomization to hide from observers, and some
   form of encrypted communication to the DHCP server.  This scenario is
   out of scope for this document.

   To preserve anonymity, the clients need to not use stable values for
   the client identifiers.  This is clearly a tradeoff, because a stable
   client identifier guarantees that the client will receive consistent
   parameters over time.  An example is given in [RFC7618], where the
   client identifier is used to guarantee that the same client will
   always get the same combination of IP address and port range.  Static
   clients benefit most from stable parameters, and can often be already
   identified by physical connection layer parameters.  These static
   clients will normally not use the anonymity profile.  Mobile clients,
   in contrast, have the option of using the anonymity profile in
   conjunction with [RFC7618] if they are more concerned with privacy
   protection than with stable parameters.



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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.

   Server privacy issues are presented in [I-D.ietf-dhc-dhcp-privacy]
   and [I-D.ietf-dhc-dhcpv6-privacy].  Mitigation of these issues is
   left to further study.

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:

   DHCPDISCOVER:  The anonymized DHCPDISCOVER messages MUST contain the
      Message Type, MAY contain the Client Identifier, and MAY contain
      the Parameter Request List options.  It SHOULD NOT contain any
      other option.

   DHCPREQUEST:  The anonymized DHCPREQUEST messages MUST contain the
      Message Type, MAY contain the Client Identifier, and MAY contain
      the Parameter Request List options.  If the message is in response
      to a DHCPOFFER, it MUST contain the corresponding Server
      Identifier option and the Requested IP address.  If the message is
      not in response to a DHCPOFFER, it MAY contain a Requested IP
      address as explained in Section 3.3.  It SHOULD NOT contain any
      other option.

   DHCPDECLINE:  The anonymized DHCPDECLINE messages MUST contain the
      Message Type, Server Identifier, and Requested IP address options,
      and MAY contain the Client Identifier options.






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   DHCPRELEASE:  The anonymized DHCPRELEASE messages MUST contain the
      Message Type and Server Identifier options, and MAY contain the
      Client Identifier option.

   DHCPINFORM:  The anonymized DHCPINFORM messages MUST contain the
      Message Type, and MAY contain the Client Identifier 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.

   The inclusion of HostName and FQDN options in DHCPDISCOVER,
   DHCPREQUEST or DHCPINFORM messages is discussed in Section 3.7 and
   Section 3.8.

3.1.  Avoiding fingerprinting

   There are many choices for implementing DHCPv4 messages.  Clients can
   choose to transmit a specific set of options, pick particular
   encoding for these options, and transmit options in different orders.
   These choices can be use to fingerprint the client.

   The following sections provide guidance on the encoding of options
   and fields within the packets.  However, this guidance alone may not
   be sufficient to prevent fingerprinting from revealing information,
   such as the device type, vendor type or OS type and in some cases
   specific version, or from revealing whether the client is using the
   anonymity profile.

   The client intending to protect its privacy SHOULD limit the subset
   of options sent in messages to the subset listed in the remaining
   subsections.

   The client intending to protect its privacy SHOULD randomize options
   order before sending any DHCPv4 message.  If this random ordering
   cannot be implemented, the client MAY arrange options by increasing
   order of option codes.

3.2.  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.





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   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
   locations.  DHCP clients SHOULD ensure that the field is cleared when
   they know that the network attachment has changed, and in particular
   if 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
   link-layer address.

3.3.  Requested IP address option

   The Requested IP address option is defined in [RFC2132] with code 50.
   It 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 link-layer
   address.  The risk exists for all forms of IP addresses, public or
   private, as some private addresses may be used in a wide scope, e.g.
   when an Internet Service Provider is using Network Address
   Translation.

   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.

   There are scenarios in which a client connecting to a network
   remembers previously allocated address, i.e. is in the INIT-REBOOT
   state.  In that state, the client that is concerned with privacy
   SHOULD perform a complete four way handshake starting with
   DHCPDISCOVER to obtain a new address lease.  If the client can
   ascertain that this is exactly the same network to which it was
   previously connected, and if the link layer address did not change,
   the client MAY issue a DHCPREQUEST to try reclaim the current
   address.








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3.4.  Client hardware address field

   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.  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.

3.5.  Client Identifier Option

   The client identifier option is defined in [RFC2132] with option code
   61.  It is discussed in detail 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
   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.  One option would
   be to not transmit this option at all, but this may affect
   interoperability and will definitely mark the client as requesting
   anonymity, exposing it to the risks mentioned in Section 2.5.

   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 the Client Identifier
   Option.



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   There are usages of DHCP where the underlying connection is a point
   to point link, in which case there is no link layer address available
   to construct a non-revealing identifier.  If anonymity is desired in
   such networks, the client SHOULD pick a random identifier that is
   highly likely to be unique to the current link, using for example a
   combination of a local secret and an identifier of the connection.
   The algorithm for combining secret and identifiers described in
   section 5 of [RFC7217] solves a similar problem.  The criteria for
   the generation of random numbers are stated in [RFC4086].

3.6.  Parameter Request List Option

   The Parameter Request List (PRL) option is defined in [RFC2132] with
   option code 55.  It list the parameters requested from the server by
   the client.  Different implementations request different
   parameters.[RFC2132] specifies that "the client MAY list the options
   in order of preference."  It practice, this means that different
   client implementations will request different parameters, in
   different orders.

   The choice of option numbers and the specific ordering of option
   numbers in the Parameter Request List 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.

   The client willing to protect its privacy SHOULD only request a
   minimal number of options in PRL, and SHOULD also randomly shuffle
   the option codes order in PRL.  If this random ordering cannot be
   implemented, the client MAY order option codes order in PRL by
   increasing order of option codes.

3.7.  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



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   conjunction with traffic analysis and quickly derive the identity of
   the device's owner.

   When using the anonymity profile, DHCP clients SHOULD NOT send 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
   qualified domain name, and MUST obfuscate the host name value.

   There are many ways to obfuscate a host name.  The construction rules
   SHOULD guarantee that a different host name is generated each time
   the link-layer address changes and that the obfuscated host name will
   not reveal the underlying link layer address.  The construction
   SHOULD generate names that are unique enough to minimize collisions
   in the local link.  Clients MAY use the following algorithm: compute
   a secure hash of a local secret and of the link layer address that
   will be used in the underlying connection, and then use the
   hexadecimal representation of the first 6 bytes of the hash as the
   obfuscated host name.

   There is a potential downside to having a specific name pattern for
   hosts that require anonymity, as explained in Section 2.5.  For this
   reason, the above algorithm is just a suggestion.

3.8.  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.





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3.9.  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.  The main usage of the PXE system is to
   install a new version of the operating system through a high speed
   Ethernet connection.  The process is typically controlled from the
   user interface during the boot process.  Common sense seems to
   dictate that getting a new operating system from an unauthenticated
   server at an untrusted location is a really bad idea, and that even
   if the option was available users would not activate it.  In any
   case, the option is only used in the "pre boot" environment, and
   there is no reason to use it once the system is up and running.
   Nodes visiting untrusted networks MUST NOT send or use the PXE
   options.

3.10.  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.




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   The choice between the stateful and stateless scenarios depends on
   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 use stateless address configuration instead
   of stateful address configuration, because stateless configuration
   requires fewer information disclosures than stateful configuration.

   When using the anonymity profile, DHCPv6 clients carefully select
   DHCPv6 options used in the various messages that they send.  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.  Avoiding fingerprinting

   There are many choices for implementing DHCPv6 messages.  As
   explained in Section 3.1, these choices can be use to fingerprint the
   client.

   The following sections provide guidance on the encoding of options.
   However, this guidance alone may not be sufficient to prevent
   fingerprinting from revealing information, such as the device type,
   vendor type or OS type and in some cases specific version, or from
   revealing whether the client is using the anonymity profile.

   The client intending to protect its privacy SHOULD limit the subset
   of options sent in messages to the subset listed in the following
   sections.

   The client intending to protect its privacy SHOULD randomize options
   order before sending any DHCPv6 message.  If this random ordering
   cannot be implemented, the client MAY arrange options by increasing
   order of option codes.

4.2.  Do not send Confirm messages, unless really sure where

   [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 addresses
   associated with those IAs.  By examining the addresses in the Confirm
   message an attacker can trivially identify the previous point(s) of
   attachment.




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   Clients interested in protecting their privacy SHOULD NOT send
   Confirm messages and instead directly try to acquire addresses on the
   new link.  However, not sending confirm messages can result in
   connectivity hiatus in some scenarios, e.g. roaming between two
   access points in the same wireless network.  DHCPv6 clients that can
   verify that the previous link and the current link are part of the
   same network MAY send Confirm messages while still protecting their
   privacy.  Such link identification should happen before DHCPv6 is
   used, and thus cannot depend on the DHCPv6 information used in
   [RFC6059].  In practice, the most reliable detection of network
   attachment is through link layer security, e.g.  [IEEE8021X].

4.3.  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 Unique
   Identifier (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 in
   [RFC3315]: Link-layer address plus time (DUID-LLT), Vendor-assigned
   unique ID based on Enterprise Number, and Link-layer address.  A
   fourth type, DUID-UUID is defined in [RFC6355].

   When using the anonymity profile in conjunction with randomized link-
   layer 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
   link-layer 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.  Syntactically this
   identifier will conform to [RFC3315] but its content is meaningless.
   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.  The link address embedded in the LLT SHOULD be
   set to a randomized value.  Time SHOULD be set to a random timestamp
   from the previous year.  Time MAY be set to current time, but this
   will reveal the fact that the DUID is newly generated, and could
   provide information for device fingerprinting.  The criteria for
   generating highly unique random numbers are listed in [RFC4086].








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4.3.1.  Anonymous Information-Request

   According to [RFC3315], a DHCPv6 client 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.

   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.

4.4.  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 link-layer address, because reuse of old values discloses
   information that can be used to identify the client.

4.5.  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.  In DHCPv6, the collection of addresses assigned to a client
   is identified by an Identity Association (IA).Clients interested in
   privacy SHOULD request addresses using the Identity Association for
   Non-temporary Addresses Option (IA_NA, code 3).

   The IA_NA option includes an IAID parameter that identifies a unique
   identity association for the interface for which the Address is
   requested.  Clients interested in protecting their privacy MUST
   ensure that the IAID does not enable client identification.  They
   also need to conform to the requirement of [RFC3315] that the IAID
   for that IA MUST be consistent across restarts of the DHCP client.
   We interpret that as requiring that the IAID MUST be constant for the
   association, as long as the link layer address remains constant.

   Clients MAY meet the privacy, uniqueness and stability requirement of
   the IAID by constructing it as the combination of one byte encoding
   the interface number in the system, and the first three bytes of the
   link layer address.

   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 link-layer address



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   changes, and to only use the IA Address option with addresses that
   have been explicitly assigned through the current link-layer address.

4.5.1.  Obtain temporary addresses

   [RFC3315] defines a special container (IA_TA, code 4) 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
   a widely used feature, so clients depending solely on temporary
   addresses may lock themselves out of service.  Secondly, [RFC3315]
   does not specify any lifetime or lease length for temporary
   addresses.  Therefore support for renewing temporary addresses may
   vary between client implementations, including not being supported at
   all.  Finally, by requesting temporary addresses a client reveals its
   desire for privacy and potentially risks countermeasures as described
   in Section 2.5.

   Because of these Clients interested in their privacy SHOULD NOT use
   IA_TA.

   The addresses obtained according to Section 4.5 are temporary in
   nature, and will be discarded when the link layer address is changed.
   They thus meet most of the use cases of the temporary addresses
   defined in [RFC4941].  Clients interested in their privacy should not
   publish their IPv6 addresses in the DNS or otherwise associate them
   with name services, and thus do not normally need two classes of
   addresses, one public, one temporary.

   The use of mechanisms to allocate several IPv6 addresses to a client
   while preserving privacy is for further study.

4.5.2.  Prefix delegation

   The use of DHCPv6 address assignment option for Prefix Delegation is
   defined in [RFC3633].  Because current host OS implementations do not
   typically request prefixes, clients that wish to use DHCPv6 PD - just
   like clients that wish to use any DHCP or DHCPv6 option that is not
   currently widely used - should recognize that doing so will serve as
   a form of fingerprinting unless or until client use of DHCPv6 PD
   becomes more widespread.

   The anonymity properties of DHCPv6 Prefix Delegation, which use IA_PD
   identity associations, are similar to those of of DHCPv6 address
   assignment using IA_NA identity associations.  The IAID could
   potentially be used to identify the client, and a prefix hint sent in
   the IA_PD Prefix option could be used to track the client's previous
   location.  Clients that desire anonymity and never request more than
   one prefix SHOULD set the IAID value to zero, as authorized in



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   section 6 of [RFC3633], and SHOULD NOT document any previously
   assigned prefix in the IA_PD Prefix option.

4.6.  Option Request Option

   The Option Request Option (ORO) is defined in [RFC3315] with option
   code 6.  It specifies the options that the client is requesting from
   the server.  The choice of requested options and the order of
   encoding of these options in the ORO can be used to fingerprint the
   client.

   The client willing to protect its privacy SHOULD only request a
   minimal subset of options and SHOULD randomly shuffle the option
   codes order in ORO.  If this random ordering cannot be implemented,
   the client MAY order option codes in ORO by increasing order of
   option codes.

4.6.1.  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.

   When using the anonymity profile, clients SHOULD NOT include such
   instances of options because old values might be used to identify the
   client.

4.7.  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.

4.8.  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.






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4.9.  Client FQDN Option

   The Client FQDN option is defined in [RFC4704] with option code 29.
   The option allows the DHCP clients to advertise to the DHCP server
   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 3.8 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.

   Clients using the anonymity profile may be consuming more resources.
   For example when they change link-layer address and request for a new
   IP, the old one is still marked as leased by the server.

   Some DHCP servers will only give addresses to pre-registered MAC
   addresses, forcing clients to choose between remaining anonymous and
   obtaining connectivity.

   Implementers SHOULD provide a way for clients to control when the
   anonymity profile is used, and when standard behavior is preferred.
   Implementers MAY implement this control by tying use of the anonymity
   profile to that of link-layer address randomization.

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, Brian Carpenter, Lorenzo Colitti, Stephen




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   Farrell, Nick Grifka, Tushar Gupta, Brian Haberman, Gabriel
   Montenegro, Marcin Siodelski, Dave Thaler, Bernie Volz, and Jun Wu.

9.  Changes from previous versions

   The RFC Editor must ensure that this section is removed prior to RFC
   publication.

   Changes from draft-00 to draft-01:

   1.  In Section 2.6, added guidance when using [RFC7618].

   2.  In Section 3.5, added guidance for case when no link layer
       address is available.

   3.  In Section 3.7, changed the recommended mechanism for obfuscating
       host names, in order to avoid reveal the underlying link layer
       address.

   4.  In Section 4.2, added an exception to the "should not send
       confirm" recommendation, to account for the "good" use of Confirm
       when roaming between access points on the same network.

   Changes from draft-01 to draft-02:

   1.  In Section 3, checked the requirements of parameters in messages
       to ensure consistency with the main text.

   2.  In Section 3.5, added guidance for case when no link layer
       address is available.

   3.  In Section 3.7, specified that clients SHOULD NOT send the
       option, and that the optional obfuscation mechanism is just a
       suggestion.

   4.  Updated the text in Section 4.5.1 for temporary IPv6 address
       allocation.

   5.  Rewrote Section 5 on operational requirements for clarity.

   Changes from draft-02 to draft-03:

   1.  Removed the update of [RFC4361] since we are specifying when to
       use that RFC, but are not recommending any specific change.

   2.  Fixed a number of typos and nits.





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   3.  In Section 2.7, specified that mitigation of server privacy
       issues is left for further study.

   4.  Moved the guidance on avoiding fingerprinting to Section 3.1 and
       Section 4.1.

   5.  In Section 3.5, added text explaining why the client identifier
       option should still be sent, even when anonymity is desired.

   6.  Added Section 3.6 specifying the random ordering of requested
       option codes in the PRL parameter, with an alternative option for
       strict ordering.

   7.  Changed the requirement in Section 4.6 to allow "increasing order
       of option codes" as an alternative to "randomized order of
       options".

   8.  In Section 4.5.1 revised the language stating lack of support for
       renewing temporary addresses, as RFC 3315 does in fact specify a
       mechanism for doing so.

   Changes from draft-03 to draft-04 address comments received during
   Working Group Last Call:

   1.  In Section 3, tightened the normative language and the use of
       message codes.

   2.  In Section 3.3, clarified the reference to the INIT-REBOOT
       scenario.

   3.  Revised the writing of Section 4.5 for greater clarity.

   Changes from draft-04 to draft-05 address comments received after
   Working Group Last Call:

   1.  Changed the title of Section 4.1 to "Avoiding fingerprinting" to
       align with Section 3.1.

   2.  Fixed editing nits in Section 4.5, and added specification that
       the IAID is composed of the interface identifier and the first
       three bytes of the HW address.  This matches the implementation
       in Windows 10, and insures that variations will not be used to
       fingerprint the client software.

   3.  Dropping "This draft updates RFC4361" from the Abstract, since
       this draft does not actually update RFC4361.

   4.  Pruned the list of normative references.



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   Changes from draft-05 to draft-06 address comments received during AD
   evaluation

   1.  In Section 3.3, clarified that the requirement to not publish
       addresses from previous networks also applies to private
       addresses.

   2.  In Section 3.6, corrected the value of the option number to 55.

   3.  In Section 3.9, provided more guidance on disabling the PXE
       option.

   4.  In Section 4.2, provided guidance on network identification, with
       references to [RFC6059] and [IEEE8021X].

   5.  In Section 4.5, expanded the Identity Association (IA) acronym.

   6.  In Section 4.3, spelled out DUID-LLT and tightened the text to
       make randomized identifiers the recommended default.

   Changes from draft-06 to draft-07 address comments received during
   IETF last call

   1.  Added informative references to [RFC4086] and [RFC7217] in
       Section 3.5.

   2.  In Section 4.3, added precision that the generated DUID-LLT are
       meaningless, and added an informative reference to [RFC4086].

   3.  In Section 4.5.2, reworked the text to reflect the consensus from
       IPv6 experts and provide informed guidance on the use of the
       option if prefix delegation is required.

   4.  In Section 5, noticed servers that might mandate link layer
       address registration.

   Changes from draft-07 to draft-08 address comments received during
   IESG review

   1.  Corrected a number of typos and applied several minor editing
       suggestions.

   2.  Added reference to IEEE study group and other IETF experiments in
       Section 2.

   3.  Added reference to journal paper on Guy Fawkes mask in
       Section 2.5.




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   4.  Removed "if servers do not object" from appliction scope in
       Section 2.6.

   5.  Removed redondant text from Section 3.4.

   6.  In Section 3.5, say "will not be nullified by the Client
       Identifier Option" instead of "will not be nullified by DHCP
       options."

   7.  In Section 3.9, remove the qualification "if only for privacy
       reasons."

   8.  Clarified the preference for stateless address configuration in
       Section 4.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <http://www.rfc-editor.org/info/rfc2131>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              DOI 10.17487/RFC3633, December 2003,
              <http://www.rfc-editor.org/info/rfc3633>.

   [RFC4702]  Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
              Configuration Protocol (DHCP) Client Fully Qualified
              Domain Name (FQDN) Option", RFC 4702,
              DOI 10.17487/RFC4702, October 2006,
              <http://www.rfc-editor.org/info/rfc4702>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.



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   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <http://www.rfc-editor.org/info/rfc4941>.

10.2.  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>.

   [GuyFawkesMask]
              Nickelsburg, M., "A brief history of the Guy Fawkes mask",
              July 2013, <http://theweek.com/articles/463151/
              brief-history-guy-fawkes-mask>.

   [I-D.ietf-6man-default-iids]
              Gont, F., Cooper, A., Thaler, D., and S. LIU,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-10 (work in progress),
              February 2016.

   [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-08 (work
              in progress), September 2015.

   [I-D.ietf-dhc-dhcp-privacy]
              Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
              considerations for DHCP", draft-ietf-dhc-dhcp-privacy-04
              (work in progress), February 2016.

   [I-D.ietf-dhc-dhcpv6-privacy]
              Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
              considerations for DHCPv6", draft-ietf-dhc-
              dhcpv6-privacy-04 (work in progress), February 2016.

   [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-03 (work in progress), February 2016.






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   [IEEE8021X]
              IEEE Std 802.1X-2010, "IEEE Standards for Local and
              Metropolitan Area Networks: Port based Network Access
              Control", Feb 2010, <http://standards.ieee.org/getieee802/
              download/802.1X-2010.pdf>.

   [IEEE802PRSG]
              IEEE 802 EC PRSG, "IEEE 802 EC Privacy Recommendation
              Study Group", Dec 2015,
              <http://www.ieee802.org/PrivRecsg/>.

   [IETFMACRandom]
              Zuniga, JC., "MAC Privacy", November 2014,
              <http://www.ietf.org/blog/2014/11/mac-privacy/>.

   [IETFTrialsAndMore]
              Bernardos, CJ., Zuniga, JC., and P. O'Hanlon, "Wi-Fi
              Internet connectivity and privacy: hiding your tracks on
              the wireless Internet", October 2015,
              <http://www.it.uc3m.es/cjbc/papers/
              pdf/2015_bernardos_cscn_privacy.pdf>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <http://www.rfc-editor.org/info/rfc2132>.

   [RFC3925]  Littlefield, J., "Vendor-Identifying Vendor Options for
              Dynamic Host Configuration Protocol version 4 (DHCPv4)",
              RFC 3925, DOI 10.17487/RFC3925, October 2004,
              <http://www.rfc-editor.org/info/rfc3925>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, DOI 10.17487/RFC4361,
              February 2006, <http://www.rfc-editor.org/info/rfc4361>.

   [RFC4578]  Johnston, M. and S. Venaas, Ed., "Dynamic Host
              Configuration Protocol (DHCP) Options for the Intel
              Preboot eXecution Environment (PXE)", RFC 4578,
              DOI 10.17487/RFC4578, November 2006,
              <http://www.rfc-editor.org/info/rfc4578>.





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   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, DOI 10.17487/RFC4704, October 2006,
              <http://www.rfc-editor.org/info/rfc4704>.

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059,
              DOI 10.17487/RFC6059, November 2010,
              <http://www.rfc-editor.org/info/rfc6059>.

   [RFC6355]  Narten, T. and J. Johnson, "Definition of the UUID-Based
              DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355,
              DOI 10.17487/RFC6355, August 2011,
              <http://www.rfc-editor.org/info/rfc6355>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <http://www.rfc-editor.org/info/rfc7217>.

   [RFC7618]  Cui, Y., Sun, Q., Farrer, I., Lee, Y., Sun, Q., and M.
              Boucadair, "Dynamic Allocation of Shared IPv4 Addresses",
              RFC 7618, DOI 10.17487/RFC7618, August 2015,
              <http://www.rfc-editor.org/info/rfc7618>.

   [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










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   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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