draft-huitema-dhc-anonymity-profile-00.txt

Network Working Group                                         C. Huitema
Internet-Draft                                                 Microsoft
Updates: 4361 (if approved)                            February 20, 2015
Intended status: Informational
Expires: August 24, 2015


                   Anonymity profile for DHCP clients
               draft-huitema-dhc-anonymity-profile-00.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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   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must



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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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Application domain  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Anonymity profile for DHCPv4  . . . . . . . . . . . . . . . .   4
     3.1.  Client IP address field . . . . . . . . . . . . . . . . .   5
     3.2.  Requested IP address option . . . . . . . . . . . . . . .   6
     3.3.  Client hardware address . . . . . . . . . . . . . . . . .   6
     3.4.  Client Identifier Option  . . . . . . . . . . . . . . . .   6
     3.5.  Host Name Option  . . . . . . . . . . . . . . . . . . . .   7
     3.6.  Client FQDN Option  . . . . . . . . . . . . . . . . . . .   7
     3.7.  UUID/GUID-based Client Identifier Option  . . . . . . . .   8
     3.8.  User and Vendor Class DHCP options  . . . . . . . . . . .   8
   4.  Anonymity profile for DHCPv6  . . . . . . . . . . . . . . . .   9
     4.1.  Client Identifier DHCPv6 Option . . . . . . . . . . . . .   9
     4.2.  Server Identifier Option  . . . . . . . . . . . . . . . .   9
     4.3.  Address assignment options  . . . . . . . . . . . . . . .  10
     4.4.  Option Request Option . . . . . . . . . . . . . . . . . .  10
       4.4.1.  Previous option values  . . . . . . . . . . . . . . .  10
     4.5.  Authentication Option . . . . . . . . . . . . . . . . . .  10
     4.6.  User and Vendor Class DHCPv6 options  . . . . . . . . . .  11
     4.7.  Client FQDN Option  . . . . . . . . . . . . . . . . . . .  11
   5.  Management considerations . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

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.




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   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.  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 keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2026].

2.  Application domain

   Mobile nodes can be tracked using multiple identifiers, the most
   prominent being MAC addresses.  For example, when devices use Wi-Fi
   connectivity, they place the MAC address in the header of all the
   packets that they transmit.  Standard implementation of Wi-Fi use
   unique 48 bit MAC addresses, assigned to the devices according to
   procedures defined by IEEE 802.  Even when the Wi-Fi packets are
   encrypted, the portion of the header containing the addresses will be
   sent in clear text.  Tracking devices can "listen to the airwaves" to
   find out what devices are transmitting near them.

   We can easily imagine that the MAC addresses can be correlated with
   other data, e.g., clear text names and cookies, to build a registry
   linking MAC addresses to the identity of devices' owners.  Once that
   correlation is done, tracking the MAC address is sufficient to track
   individual people, even when all application data sent from the
   devices is encrypted.  MAC addresses can also be correlated with IP
   addresses of devices, negating potential privacy benefits of IPv6
   "privacy" addresses.  Privacy advocates have some reason to be
   concerned.

   The obvious solution is to "randomize" the MAC address.  Before
   connecting to a particular network, the device replaces the MAC
   address with a randomly drawn 48 bit value.  MAC address
   randomization was successfully tried at the IETF in Honolulu in



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   November 2014 [IETFMACRandom].  However, we have to consider the
   linkage between MAC addresses, DHCP identifiers and IP addresses.

   From a privacy point of view, it is clear that MAC Addresses, IP
   addresses and DHCP identifiers shall evolve in synchrony.  For
   example, if the MAC address changes and the DHCP identifier stays
   constant, then it is really easy to correlate old and new MAC
   addresses, either by listening to DHCP traffic or by observing that
   the IP address remains constant, since it is tied to the DHCP
   identifier.  Conversely, if the DHCP identifier changes but the MAC
   address remains constant, the old and new identifiers and addresses
   can be correlated by listening to L2 traffic.  The procedures
   documented in the following sections construct DHCP identifiers from
   the current MAC address, automatically providing for this
   synchronization.

   Of course, there are downsides to randomizing MAC addresses and DHCP
   identifiers.  By definition, randomization will break management
   procedures that rely on tracking MAC addresses.  Even if this is not
   too much of a concern, we have to be worried about the frequency of
   MAC address randomization.  Suppose for example that many devices
   would get new random MAC addresses at short intervals, maybe every
   few minutes.  This would generate new DHCP requests in rapid
   succession, with a high risk of exhausting DHCPv4 address pools.
   Even with IPv6, there would still be a risk of increased neighbor
   discovery traffic, and bloating of various address tables.
   Implementers will have to be cautious when programming devices to use
   randomized MAC addresses.  They will have to carefully chose the
   frequency with which such addresses will be renewed.

   This document only provides guidelines for using DHCP when privacy is
   considered more important than network management.  We assume that
   the request for anonymity is materialized by the assignment of a
   randomized MAC address to the network interface.  Once that decision
   is made, the following guidelines will avoid leakage of identity in
   DHCP parameters or in assigned addresses.

   There may be rare situations where the clients want anonymity to
   attackers but not to their DHCP server.  These clients should still
   use MAC Address randomization to hide from observers, and some form
   of encrypted communication to the DHCP server.  This document does
   not study this scenario.

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




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   number and values of options.  The number of options depend on the
   specific DHCP message:

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

   REQUEST:  The anonymized REQUEST messages SHOULD contain the Message
      Type, Client Identifier, Host name, and Parameter Request List
      options.  If the message is in response to an OFFER, it SHOULD
      contain the corresponding Server Identifier option.  It SHOULD NOT
      contain any other option.

   DECLINE:  The anonymized DECLINE messages SHOULD contain the Message
      Type, Client Identifier and Server Identifier options.

   RELEASE:  The anonymized RELEASE messages SHOULD contain the Message
      Type, Client Identifier and Server Identifier options.

   INFORM:  The anonymized INFORM messages MUST contain the Message
      Type, Client Id, Host name, and Parameter Request List options.
      It SHOULD NOT contain any other option.

   Header fields and option values SHOULD be set in accordance with the
   DHCP specification, but some header fields and option values SHOULD
   be constructed per the following guidelines.

3.1.  Client IP address field

   Four bytes in the header of the DHCP messages carry the "Client IP
   address" (ciaddr) as defined in [RFC2131].  In DHCP, this field is
   used by the clients to indicate the address that they used
   previously, so that as much as possible the server can allocate them
   the same address.

   There is very little privacy implication of sending this address in
   the DHCP messages, except in one case, when connecting to a different
   network than the last network connected.  If the DHCP client somehow
   repeated the address used in a previous network attachment,
   monitoring services might use the information to tie the two network
   locations.  DHCP clients should ensure that the field is cleared when
   they know that the network attachment has changed, and in particular
   of the link layer address is reset by the device's administrator.








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3.2.  Requested IP address option

   The Requested IP address option (code 50) allows the client to
   request that a particular IP address be assigned.  The option is
   mandatory in some protocol messages per [RFC2131], for example when a
   client selects to use an address offered by a server.  However, this
   option is not mandatory in the DHCPDISCOVER message.  It is simply a
   convenience, an attempt to regain the same IP address that was used
   in a previous connection.  Doing so entails the risk of disclosing an
   IP address used by the client at a previous location, or with a
   different MAC Address.

   When using the anonymity profile, clients SHOULD NOT use the
   Requested IP address option in DHCPDISCOVER Messages.

3.3.  Client hardware address

   Sixteen bytes in the header of the DHCP messages carry the "Client
   hardware address" (chaddr) as defined in [RFC2131].  The presence of
   this address is necessary for the proper operation of the DHCP
   service.

   Hardware addresses, called "link layer address" in many RFCs, can be
   used to uniquely identify a device, especially if they follow the
   IEEE 802 recommendations.  These unique identifiers can be used by
   monitoring services to track the location of the device and its user.
   The only plausible defense is to somehow reset the hardware address
   to a random value when visiting an untrusted location, before
   transmitting anything at that location with the hardware address.  If
   the hardware address is reset to a new value, or randomized, the DHCP
   client SHOULD use the new randomized value in the DHCP messages.

3.4.  Client Identifier Option

   The client identifier option is defined in [RFC2132] with option code
   61.  It is discussed in details in [RFC4361].  The purpose of the
   client identifier option is to identify the client in a manner
   independent of the link layer address.  This is particularly useful
   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.



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   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 privacy considerations trump
   management considerations, DHCP clients MUST use client identifiers
   based solely on the link layer address that will be used in the
   underlying connection.  This will ensure that the DHCP client
   identifier does not leak any information that is not already
   available to entities monitoring the network connection.  It will
   also ensure that a strategy of randomizing the link layer address
   will not be nullified by DHCP options.

3.5.  Host Name Option

   The Host Name option is defined in [RFC2132] with option code 12.
   Depending on implementations, the option value can carry either a
   fully qualified domain name such as "node1984.example.com," or a
   simple host name such as "node1984."  The host name is commonly used
   by the DHCP server to identify the host, and also to automatically
   update the address of the host in local name services.

   Fully qualified domain names are obviously unique identifiers, but
   even simple host names can provide a significant amount of
   information on the identity of the device.  They are typically chosen
   to be unique in the context where the device is most often used.  If
   that context is wide enough, in a large company or in a big
   university, the host name will be a pretty good identifier of the
   device.  Monitoring services could use that information in
   conjunction with traffic analysis and quickly derive the identity of
   the device's owner.

   When privacy considerations trump management considerations, DHCP
   clients MUST always send a non-qualified host name instead of a fully
   qualified domain name, and SHOULD obfuscate the host name value, so
   it could not be linked to anything other than the link layer address.
   A simple solution would be to set the host name value to a
   hexadecimal representation of the link layer address that will be
   used in the underlying connection.

3.6.  Client FQDN Option

   The Client FQDN option is defined in [RFC4702] with option code 81.
   The option allows the DHCP clients to advertise to the DHCP server
   their fully qualified domain name (FQDN) such as



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   "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 privacy considerations trump management considerations, DHCP
   clients SHOULD NOT include the Client FQDN option in their DHCP
   requests.  Alternatively, they MAY include a special purpose FQDN
   using the same hostname as in the Host Name Option, with a suffix
   matching the connection-specific DNS suffix being advertised by that
   DHCP server.  Having a name in the DNS allows working with legacy
   systems that require one to be there, e.g., by verifying a forward
   and reverse lookup succeeds with the same result.

3.7.  UUID/GUID-based Client Identifier Option

   The UUID/GUID-based Client Machine Identifier option is defined in
   [RFC4578], with option code 97.  The option is part of a set of
   options for Intel Preboot eXecution Environment (PXE).  The purpose
   of the PXE system is to perform management functions on a device
   before its main OS is operational.  The Client Machine Identifier
   carries a 16-octet Globally Unique Identifier (GUID), which uniquely
   identifies the device.

   The PXE system is clearly designed for devices operating in a
   controlled environment, and its functions are not meant to be used by
   mobile nodes visiting untrusted networks.  If only for privacy
   reasons, nodes visiting untrusted networks MUST disable the PXE
   functions, and MUST NOT send the corresponding options.

3.8.  User and Vendor Class DHCP options

   Vendor identifying options are defined in [RFC2132] and [RFC3925].
   When using the anonymity profile, DHCP clients SHOULD NOT use the
   Vendor Specific Information option (code 43), the Vendor Class
   Identifier Option (60), the Vendor Class option (code 124), or the
   Vendor Specific Information option (code 125) as these options
   potentially reveal identifying information.








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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.

   When using the anonymity profile, DHCPv6 clients carefully select
   DHCPv6 options used in the various messages that they sent.  The list
   of options that are mandatory or optional for each message is
   specified in [RFC3315].  Some of these options have specific
   implications on anonymity.  The following sections provide guidance
   on the choice of option values when using the anonymity profile.

4.1.  Client Identifier DHCPv6 Option

   The client identifier option is defined in [RFC3315] with option code
   1.  The purpose of the client identifier option is to identify the
   client to the server.  The content of the option is a DHCP User ID
   (DUID) for which three formats are specified: Link-layer address plus
   time, Vendor-assigned unique ID based on Enterprise Number, Link-
   layer address.

   When using the anonymity profile, 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.

4.2.  Server Identifier Option

   When using the anonymity profile, DHCPv6 clients SHOULD use the
   Server Identifier Option (code 2) as specified in [RFC3315].  Clients
   MUST only include server identifier values that were received with
   the current MAC address, because reuse of old values discloses
   information that can be used to identify the client.








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4.3.  Address assignment options

   When using the anonymity profile, DHCPv6 clients might have to use
   SOLICIT or REQUEST messages to obtain IPv6 addresses through the DHCP
   server.  The clients SHOULD only use the options necessary to perform
   the requested DHCPv6 transactions, such as Identity Association for
   Non-temporary Addresses Option (code 3) or Identity Association for
   Temporary Addresses Option (code 4).

   The clients MAY use the IA Address Option (code 5) but need to
   balance the potential advantage of "address continuity" versus the
   potential risk of "previous address disclosure."  A potential
   solution is to remove all stored addresses when a MAC address
   changes, and to only use the IA Address option with addresses that
   have been explicitly assigned through the current MAC address.

4.4.  Option Request Option

   When using the anonymity profile, DHCPv6 clients SHOULD use the
   Option Request Option (code 6) as specified in [RFC3315].

4.4.1.  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.

4.5.  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.








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4.6.  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.

4.7.  Client FQDN Option

   The Client FQDN option is defined in [RFC4704] with option code 29.
   The option allows the DHCP clients to advertize to the DHCP their
   fully qualified domain name (FQDN) such as "mobile.example.com."
   When using the anonymity profile, DHCPv6 clients SHOULD NOT include
   the Client FQDN option in their DHCPv6 messages because it identifies
   the client.  As explained in Section 3.6 they MAY use a local-only
   FQDN by combining a host name derived from the link layer address and
   a suffix advertised by the local DHCP server.

5.  Management considerations

   The anonymity profile has the effect of hiding the client identity
   from the DHCP server.  This is not always desirable.  Some DHCP
   servers provide facilities like publishing names and addresses in the
   DNS, or ensuring that returning clients get reassigned the same
   address.  Implementers should be careful to only use the anonymity
   profile when privacy trumps management considerations.

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 early versions of
   this draft, notably Noel Anderson, Tushar Gupta, Gabriel Montenegro,
   Dave Thaler and Jun Wu.

9.  References








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9.1.  Normative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3925]  Littlefield, J., "Vendor-Identifying Vendor Options for
              Dynamic Host Configuration Protocol version 4 (DHCPv4)",
              RFC 3925, October 2004.

   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, February 2006.

   [RFC4578]  Johnston, M. and S. Venaas, "Dynamic Host Configuration
              Protocol (DHCP) Options for the Intel Preboot eXecution
              Environment (PXE)", RFC 4578, November 2006.

   [RFC4702]  Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
              Configuration Protocol (DHCP) Client Fully Qualified
              Domain Name (FQDN) Option", RFC 4702, October 2006.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, October 2006.

9.2.  Informative References

   [CNBC]     Weston, G., Greenwald, G., and R. Gallagher, "CBC News:
              CSEC used airport Wi-Fi to track Canadian travellers", Jan
              2014, <http://www.cbc.ca/news/politics/csec-used-airport-
              wi-fi-to-track-canadian-travellers-edward-snowden-
              documents-1.2517881>.

   [I-D.ietf-6man-default-iids]
              Gont, F., Cooper, A., Thaler, D., and W. Will,
              "Recommendation on Stable IPv6 Interface Identifiers",
              draft-ietf-6man-default-iids-02 (work in progress),
              January 2015.



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   [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-03 (work
              in progress), January 2015.

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

Author's Address

   Christian Huitema
   Microsoft
   Redmond, WA  98052
   U.S.A.

   Email: huitema@microsoft.com

































Huitema                  Expires August 24, 2015               [Page 13]