Network Working Group                                         C. Huitema
Internet-Draft                                                 Microsoft
Intended status: Standards Track                            July 1, 2015
Expires: January 2, 2016

   Implications of Randomized Link Layers Addresses for IPv6 Address


   Hosts may assign random link-layer addresses to network interfaces in
   an attempt to increase privacy and reduce trackability.  Careless
   assignment of IPv6 addresses may negate the privacy advantages of
   random link-layer addresses.  We propose simple solutions to ensure
   that IPv6 addresses do change whenever the link layer addresses

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Randomized link-layer addresses . . . . . . . . . . . . . . .   3
     2.1.  Randomized link-layer address format  . . . . . . . . . .   3
     2.2.  Link-layer address life time  . . . . . . . . . . . . . .   4
   3.  Considerations on IPv6 address assignment . . . . . . . . . .   4
     3.1.  IEEE-identifier-based IIDs  . . . . . . . . . . . . . . .   4
     3.2.  Static, manually configured IIDs  . . . . . . . . . . . .   5
     3.3.  Constant, semantically opaque IIDs  . . . . . . . . . . .   5
     3.4.  Stable, semantically opaque IIDs  . . . . . . . . . . . .   5
     3.5.  Temporary IIDs  . . . . . . . . . . . . . . . . . . . . .   6
     3.6.  DHCPv6 generation of IIDs . . . . . . . . . . . . . . . .   6
     3.7.  Transition/co-existence technologies  . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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 like
   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.  After connecting to a link, the host will try to
   obtain IPv6 addresses for that link.  There are multiple ways to
   assign these addresses.  The privacy implications of various
   assignment methods are studied in
   [I-D.ietf-6man-ipv6-address-generation-privacy], but this study does
   not fully take into account the effect of link-layer address

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   The purpose of this document is to provide guidance to implementers,
   so they chose address assignment methods that are compatible with
   link layer address randomization.  This document is complementary to
   [I-D.ietf-dhc-anonymity-profile], which specifies how to use DHCPv6
   in conjunction with randomized link-layer addresses.

1.1.  Requirements

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

2.  Randomized link-layer addresses

   Mobile nodes can be tracked using multiple identifiers, the most
   prominent being the 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.

   The obvious solution is to "randomize" the MAC address.  Before
   connecting to a particular network, the device replaces the MAC
   address with a randomly drawn 48 bit value.  MAC address
   randomization was successfully tried at the IETF in Honolulu in
   November 2014 [IETFMACRandom].  However, we have to consider the
   linkage between MAC addresses and IPv6 addresses.

   From a privacy point of view, it is clear that MAC Addresses and IPv6
   addresses and DHCP identifiers shall evolve in synchrony.  For
   example, if the MAC address changes and the IID portion of the IPv6
   address stays constant, then it is really easy to correlate old and
   new MAC address.  Conversely, if the IID changes but the MAC address
   remains constant, the old and new identifiers and addresses can be
   correlated by listening to the link's traffic.

2.1.  Randomized link-layer address format

   At the time of this writing, there is no standard way to construct
   randomized link layer addresses, but many implementations use the
   following algorithm for IEEE 802 48 bit MACs:

      Set the the "u" (universal/local) bit to 1 (local).

      Set the the "g" (individual/group) bit to 0 (individual).

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      Pick random values for all the other bits.

2.2.  Link-layer address life time

   This document makes the hypothesis that randomized link layer
   addresses are chosen just prior to the connection to a link.  Hosts
   are expected to maintain the same link-layer address for the duration
   of the connection.

   There are circumstances where a host may decide to reset its link
   layer address while maintaining an attachment to a link.  For
   example, a host Ethernet interface may remain "plugged in" while the
   interface driver is reset to use a new MAC address.  These conditions
   will be considered equivalent to disconnecting and then reconnecting
   with a new link layer address.  The previously used IPv6 addresses
   will be discarded, and a new set of addreses will be assigned.

   There are circonstances where a host may decide to reconnect to a
   particular link using the same link-layer address as for a previous
   attachment.  In this case, the assignment algorithm will normally
   result in assigning the same IPv6 address as in the previous session,
   except under exceptional circumstances such as resetting the "secret
   key" used in [RFC7217].

3.  Considerations on IPv6 address assignment

   Several IPv6 address assignment methods have been defined over time.
   We review here these methods in light of link layer address
   randomization, using the same nomenclature as

3.1.  IEEE-identifier-based IIDs

   IEEE-identifier-based IIDs could be derived from randomized link
   layer ID, using the algorithm specified in Appendix A of [RFC4291].

   If the IIDs are constructed using the random link layer addresses,
   and if the random link layer addresses are constructed using the
   algorithm specified in Section 2.1, then the issues described in
   section 3 of [I-D.ietf-6man-ipv6-address-generation-privacy] are
   somewhat mitigated, but many concerns remain.  The correlation over
   time still be possible for the lifetime of the link layer address,
   and the location tracking will only be mitigated if link layer
   addresses do change with location.

   In addition to the lifetime and location tracking concerns, there is
   also a "scope" issue with IEEE-identifier-based IIDs.  The practice
   will export the link-layer address value to all places where the IPv6

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   address is used.  This increase the potential "surface" for privacy
   attacks, and is not desirable.

   There is a small probability of collision between IIDs derived from
   random link layer addresses and IIDs obtained through the sematically
   opaque, cryptographically generated, or temporary assignment methods.
   The "u" bit is set to global for globally assigned link layer
   addresses, but set to "local" for both random link layer addresses
   and for IIDs derived through some random process.  The collision risk
   is however very small, and may not be a practical concern.

3.2.  Static, manually configured IIDs

   Because static, manually configured IIDs are stable, both correlation
   and location tracking are possible for the life of the address.
   Using randomized link-local addresses doesn't change that.

   In practice, static assignment and link-layer address randomization
   address different scenarios.  Static assignments are typically used
   for static hosts, while randomization is typically used for mobile

3.3.  Constant, semantically opaque IIDs

   This address assignment method allows correlation and location
   tracking because the IID is constant across IPv6 links and time.
   Using randomized link-local addresses doesn't change that.  In fact,
   the constant values allow for correlation between the random link-
   local address and the host's identity, removing most of privacy value
   of random link-layer addresses.

   Section 4.3 of [I-D.ietf-6man-ipv6-address-generation-privacy]
   addresses the general case of systems generating constant IID using
   the algorithms specified in [RFC4941], mentioning the implementation
   of this algorithm in Windows.  Tests on the Windows platform show
   that the "constant" IIDs do in fact change if the link layer address
   is changed to a random value, and thus do in fact preserve the
   privacy value of random link-layer addresses.

3.4.  Stable, semantically opaque IIDs

   [RFC7217] specifies an algorithm that generates, for each network
   interface, a unique random IID per IPv6 link.  The privacy properties
   of that algorithm depends on the specific source of the "Net_Iface"
   chosen by the implementer.

   Most sources for the Net_IFace parameter listed in Appendix A of
   [RFC7217] will result in stable identifiers, independent of the link-

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   layer address.  This will enable tracking over time of a host that
   repeatedly visits the same location, despite any attempts by the host
   to use different random link-layer address values.  In fact, the
   stable IIDs will enable correlation of different link-layer addresses
   to the same host identity.

   Tracking over time is prevented if the Net_IFace parameter is set to
   the current link layer address.  In that case, the stable addresses
   will have exactly the same lifetime as the link-layer identifiers.
   This SHOULD be the default solution for mobile hosts.

   Some hosts are static by nature.  This is for example the case of
   servers.  For such hosts, address stability is probably more
   important than preventing tracking over time.  Such hosts should
   probably not attempt to configure random link layer addresses.  They
   MAY want use a more stable sources for the Net_IFace than the link
   address programmed in the network interface card, as explained in

3.5.  Temporary IIDs

   As stated in [I-D.ietf-6man-ipv6-address-generation-privacy], "a host
   that uses only a temporary address mitigates all four threats.  Its
   activities may only be correlated for the lifetime a single temporary
   address."  There is however a condition.  If the lifetime of the
   temporary address exceeds the lifetime of the random link layer
   address, then correlation of successive link-layer addresses become
   possible, effectively enabling a form of tracking.

   If a host uses both temporary and stable addresses, the privacy
   properties are those of the particular stable addresses.  This is
   also true is a host uses temporary addresses and configure but doen't
   use a stable address.  The address configuration will require
   performing duplicate address detection, generating at least a few
   packets on the local links.  Observing this packets, an on-link
   attacker can correlate the link-layer address with the stable
   address.  If the stable address includes a constant identifier, then
   the benefits of using rnadom link-local addresses will be negated.

3.6.  DHCPv6 generation of IIDs

   When using DHCPv6 in conjunction with random link layer addresses,
   implementers SHOULD follow the recommendations of

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3.7.  Transition/co-existence technologies

   Transition technologies typically embed an IPv4 address in a
   specifically formatted IPv6 address.  Tracking over time becomes
   possible if the IPv4 address has a longer lifetime than the random
   link-layer address.

   To mitigate the potential tracking issues with embedded IPv4
   addresses, hosts using random link-local addresses SHOULD implement
   the DHCPv4 profile specified in [I-D.ietf-dhc-anonymity-profile].

4.  Security Considerations

   This whole document concerns the privacy and security properties of
   different IPv6 address generation mechanisms.

5.  IANA Considerations

   This draft does not require any IANA action.

6.  Acknowledgments

   The inspiration for this draft came from the authors of
   [I-D.ietf-6man-ipv6-address-generation-privacy], Alissa Cooper,
   Fernando Gont, and Dave Thaler.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217, April 2014.

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7.2.  Informative References

   [CNBC]     Weston, G., Greenwald, G., and R. Gallagher, "CBC News:
              CSEC used airport Wi-Fi to track Canadian travellers", Jan
              2014, <

              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.

              Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              profile for DHCP clients", draft-ietf-dhc-anonymity-
              profile-00 (work in progress), May 2015.

              Zuniga, JC., "MAC Privacy", November 2014,

Author's Address

   Christian Huitema
   Redmond, WA  98052


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