draft-ietf-dhc-anonymity-profile-03.txt

Network Working Group                                         C. Huitema
Internet-Draft                                                 Microsoft
Intended status: Standards Track                            T. Mrugalski
Expires: March 4, 2016                                               ISC
                                                             S. Krishnan
                                                                Ericsson
                                                       September 1, 2015


                   Anonymity profile for DHCP clients
                draft-ietf-dhc-anonymity-profile-03.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
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   This Internet-Draft will expire on March 4, 2016.

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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Application domain  . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  MAC address Randomization hypotheses  . . . . . . . . . .   4
     2.2.  MAC address Randomization and DHCP  . . . . . . . . . . .   5
     2.3.  Radio fingerprinting  . . . . . . . . . . . . . . . . . .   5
     2.4.  Operating system fingerprinting . . . . . . . . . . . . .   6
     2.5.  No anonymity profile identification . . . . . . . . . . .   6
     2.6.  Using the anonymity profiles  . . . . . . . . . . . . . .   7
     2.7.  What about privacy for DHCP servers . . . . . . . . . . .   8
   3.  Anonymity profile for DHCPv4  . . . . . . . . . . . . . . . .   8
     3.1.  Option encoding and avoiding fingerprinting . . . . . . .   9
     3.2.  Client IP address field . . . . . . . . . . . . . . . . .   9
     3.3.  Requested IP address option . . . . . . . . . . . . . . .  10
     3.4.  Client hardware address . . . . . . . . . . . . . . . . .  10
     3.5.  Client Identifier Option  . . . . . . . . . . . . . . . .  11
     3.6.  Parameter Request List  . . . . . . . . . . . . . . . . .  11
     3.7.  Host Name Option  . . . . . . . . . . . . . . . . . . . .  12
     3.8.  Client FQDN Option  . . . . . . . . . . . . . . . . . . .  13
     3.9.  UUID/GUID-based Client Identifier Option  . . . . . . . .  13
     3.10. User and Vendor Class DHCP options  . . . . . . . . . . .  14
   4.  Anonymity profile for DHCPv6  . . . . . . . . . . . . . . . .  14
     4.1.  Option encoding and avoiding fingerprinting . . . . . . .  14
     4.2.  Do not send Confirm messages, unless really sure where  .  15
     4.3.  Client Identifier DHCPv6 Option . . . . . . . . . . . . .  15
       4.3.1.  Anonymous Information-Request . . . . . . . . . . . .  16
     4.4.  Server Identifier Option  . . . . . . . . . . . . . . . .  16
     4.5.  Address assignment options  . . . . . . . . . . . . . . .  16
       4.5.1.  Obtain temporary addresses  . . . . . . . . . . . . .  17
     4.6.  Option Request Option . . . . . . . . . . . . . . . . . .  17
       4.6.1.  Previous option values  . . . . . . . . . . . . . . .  18
     4.7.  Authentication Option . . . . . . . . . . . . . . . . . .  18
     4.8.  User and Vendor Class DHCPv6 options  . . . . . . . . . .  18
     4.9.  Client FQDN Option  . . . . . . . . . . . . . . . . . . .  18
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  19
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  Changes from previous versions  . . . . . . . . . . . . . . .  19



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   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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 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
   addresses, assigned to the devices according to procedures defined by



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   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].  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
   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,



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   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.

   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 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
   radios, including Wi-Fi as described for example in
   [WiFiRadioFingerprinting].  No amount of link-layer address




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   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.  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.




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   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 and servers do not object.  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
   not yet supported in 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:

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

   REQUEST:  The anonymized REQUEST messages SHOULD contain the Message
      Type, Client Identifier, Requested IP address 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 Identifier, and Parameter Request List options.  It
      SHOULD NOT contain any other option.




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   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 DISCOVER, REQUEST or
   INFORM messages is discussed in Section 3.7 and Section 3.8.

3.1.  Option encoding and 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.
   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 willing to protect its privacy SHOULD limit the subset of
   options sent in messages to the subset listed in the following
   sections.

   The client willing 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.

   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.






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   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 id 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.

   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 connects to a network when a
   lease is still valid.  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.

3.4.  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.




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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 DHCP options.

   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
   unique to the current link, using for example a combination of a
   local secret and an identifier of the connection.

3.6.  Parameter Request List

   The Parameter Request List (PRL) option is defined in [RFC2132] with
   option code 61.  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



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   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
   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
   SOULD 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



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   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.

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, 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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3.10.  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.

   The choice between the stateful and stateless scenario 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 choose it over 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.  Option encoding and 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



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

   The client willing 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.

   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.

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, Vendor-assigned unique ID
   based on Enterprise Number, 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,



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   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.  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.

4.3.1.  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.

   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.  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



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   solution is to remove all stored addresses when a link-layer address
   changes, and to only use the IA Address option with addresses that
   have been explicitly assigned through the current link-layer 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.

4.5.1.  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
   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.

   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 4.4, 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 the 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].

4.6.  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



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   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.

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




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   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.

   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, Lorenzo Colitti, Stephen Farrell, Nick Grifka,
   Tushar Gupta, 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.



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   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.

   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".



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   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.

10.  References

10.1.  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-01 (work in progress), July 2015.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.




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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>.

   [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>.

   [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>.

   [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-07 (work in progress), August
              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-07 (work
              in progress), June 2015.

   [I-D.ietf-dhc-dhcp-privacy]
              Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
              considerations for DHCPv4", draft-ietf-dhc-dhcp-privacy-01
              (work in progress), August 2015.

   [I-D.ietf-dhc-dhcpv6-privacy]
              Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
              considerations for DHCPv6", draft-ietf-dhc-
              dhcpv6-privacy-01 (work in progress), August 2015.

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



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   [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>.

   [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>.

   [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


   Tomek Mrugalski
   Internet Systems Consortium, Inc.
   950 Charter Street
   Redwood City, CA  94063
   USA

   Email: tomasz.mrugalski@gmail.com











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   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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