IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Standards Track                         August 19,                       September 3, 2013
Expires: February 20, March 7, 2014

 A Method for Generating Semantically Opaque Interface Identifiers with
            IPv6 Stateless Address Autoconfiguration (SLAAC)


   This document specifies a method for generating IPv6 Interface
   Identifiers to be used with IPv6 Stateless Address Autoconfiguration
   (SLAAC), such that addresses configured using this method are stable
   within each subnet, but the Interface Identifier changes when hosts
   move from one network to another.  This method is meant to be an
   alternative to generating Interface Identifiers based on hardware
   address (e.g., using IEEE identifiers), such that the benefits of
   stable addresses can be achieved without sacrificing the privacy of
   users.  The method specified in this document applies to all prefixes
   a host may be employing, including link-local, global, and unique-
   local addresses.

Status of this Memo

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

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   This Internet-Draft will expire on February 20, March 7, 2014.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Design goals . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Algorithm specification  . . . . . . . . . . . . . . . . . . .  9
   4.  Resolving Duplicate Address Detection (DAD) conflicts  . . . . 14
   5.  Specified Constants  . . . . . . . . . . . . . . . . . . . . . 15
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Possible sources for the Net_Iface parameter  . . . . 23
     A.1.  Interface Index  . . . . . . . . . . . . . . . . . . . . . 23
     A.2.  Interface Name . . . . . . . . . . . . . . . . . . . . . . 23
     A.3.  Link-layer Addresses . . . . . . . . . . . . . . . . . . . 23
     A.4.  Logical Network Service Identity . . . . . . . . . . . . . 24
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25

1.  Introduction

   [RFC4862] specifies Stateless Address Autoconfiguration (SLAAC) for
   IPv6 [RFC2460], which typically results in hosts configuring one or
   more "stable" addresses composed of a network prefix advertised by a
   local router, and an Interface Identifier (IID) that typically embeds
   a hardware address (e.g., using IEEE identifiers) [RFC4291].

      Cryptographically Generated Addresses (CGAs) [RFC3972] are yet
      another method for generating Interface Identifiers, which bind a
      public signature key to an IPv6 address in the SEcure Neighbor
      Discovery (SEND) [RFC3971] protocol.

   Generally, the traditional SLAAC addresses are thought to simplify
   network management, since they simplify Access Control Lists (ACLs)
   and logging.  However, they have a number of drawbacks:

   o  since the resulting Interface Identifiers do not vary over time,
      they allow correlation of node activities within the same network,
      thus negatively affecting the privacy of users.

   o  since the resulting Interface Identifiers are constant across
      networks, the resulting IPv6 addresses can be leveraged to track
      and correlate the activity of a node across multiple networks
      (e.g. track and correlate the activities of a typical client
      connecting to the public Internet from different locations), thus
      negatively affecting the privacy of users.

   o  since embedding the underlying link-layer address in the Interface
      Identifier will result in specific address patterns, such patterns
      may be leveraged by attackers to reduce the search space when
      performing address scanning attacks.  For example, the IPv6
      addresses of all nodes manufactured by the same vendor (at a given
      time frame) will likely contain the same IEEE Organizationally
      Unique Identifier (OUI) in the Interface Identifier.

   o  embedding the underlying link-layer address in the Interface
      Identifier leaks device-specific information that could be
      leveraged to launch device-specific attacks.

   o  embedding the underlying link-layer address in the Interface
      Identifier means that replacement of the underlying interface
      hardware will result in a change of the IPv6 address(es) assigned
      to that interface.

   [I-D.cooper-6man-ipv6-address-generation-privacy] provides additional
   details regarding how these vulnerabilities could be exploited, and
   the extent to which the method discussed in this document mitigates

   The "Privacy Extensions for Stateless Address Autoconfiguration in
   IPv6" [RFC4941] (henceforth referred to as "temporary addresses")
   were introduced to complicate the task of eavesdroppers and other
   information collectors (e.g.  IPv6 addresses in web server logs or
   email headers, etc.) to correlate the activities of a node, and
   basically result in temporary (and random) Interface Identifiers.
   These temporary addresses are generated in addition to the
   traditional IPv6 addresses based on IEEE identifiers, with the
   "temporary addresses" being employed for "outgoing communications",
   and the traditional SLAAC addresses being employed for "server"
   functions (i.e., receiving incoming connections).

      It should be noted that temporary addresses can be challenging in
      a number of areas.  For example, from a network-management point
      of view, they tend to increase the complexity of event logging,
      trouble-shooting, enforcement of access controls and quality of
      service, etc.  As a result, some organizations disable the use of
      temporary addresses even at the expense of reduced privacy
      [Broersma].  Temporary addresses may also result in increased
      implementation complexity, which might not be possible or
      desirable in some implementations (e.g., some embedded devices).

      In scenarios in which temporary addresses are deliberately not
      used (possibly for any of the aforementioned reasons), all a host
      is left with is the stable addresses that have been generated
      using e.g.  IEEE identifiers.  In such scenarios, it may still be
      desirable to have addresses that mitigate address scanning
      attacks, and that at the very least do not reveal the node's
      identity when roaming from one network to another -- without
      complicating the operation of the corresponding networks.

   However, even with "temporary addresses" in place, a number of issues
   remain to be mitigated.  Namely,

   o  since "temporary addresses" [RFC4941] do not eliminate the use of
      fixed identifiers for server-like functions, they only partially
      mitigate host-tracking and activity correlation across networks
      (see [I-D.cooper-6man-ipv6-address-generation-privacy] for some
      example attacks that are still possible with temporary addresses).

   o  since "temporary addresses" [RFC4941] do not replace the
      traditional SLAAC addresses, an attacker can still leverage
      patterns in those addresses to greatly reduce the search space for
      "alive" nodes [Gont-DEEPSEC2011] [CPNI-IPv6]

   Hence, there is a motivation to improve the properties of "stable"
   addresses regardless of whether temporary addresses are employed or

   We note that attackers can employ a plethora of probing techniques
   [I-D.ietf-opsec-ipv6-host-scanning] to exploit the aforementioned
   issues.  Some of them (such as the use of ICMPv6 Echo Request and
   ICMPv6 Echo Response packets) could mitigated by a personal firewall
   at the target host.  For other vectors, such listening to ICMPv6
   "Destination Unreachable, Address Unreachable" (Type 1, Code 3) error
   messages referring to the target addresses
   [I-D.ietf-opsec-ipv6-host-scanning], there is nothing a host can do
   (e.g., a personal firewall at the target host would not be able to
   mitigate this probing technique).

   This document specifies a method to generate Interface Identifiers
   that are stable/constant for each network interface within each
   subnet, but that change as hosts move from one network to another,
   thus keeping the "stability" properties of the Interface Identifiers
   specified in [RFC4291], while still mitigating address-scanning
   attacks and preventing correlation of the activities of a node as it
   moves from one network to another.

   The method specified in this document is a orthogonal to the use of
   "temporary" addresses [RFC4941], since it is meant to improve the
   security and privacy properties of the stable addresses that are
   employed along with the aforementioned "temporary" addresses.  In
   scenarios in which "temporary addresses" are employed, implementation
   of the mechanism described in this document (in replacement of stable
   addresses based on e.g.  IEEE identifiers) would mitigate address-
   scanning attacks and also mitigate the remaining vectors for
   correlating host activities based on the node's constant (i.e. stable
   across networks) Interface Identifiers.  On the other hand, for nodes
   that currently disable "temporary addresses" [RFC4941] for some of
   the reasons described earlier in this document, implementation of
   this mechanism will result in stable privacy-enhanced addresses which
   address some of the concerns related to addresses that embed IEEE
   identifiers [RFC4291], and which mitigate IPv6 address-scanning

   We note that this method is incrementally deployable, since it does
   not pose any interoperability implications when deployed on networks
   where other nodes do not implement or employ it.  Additionally, we
   note that this document does not update or modify IPv6 StateLess
   Address Auto-Configuration (SLAAC) [RFC4862] itself, but rather only
   specifies an alternative algorithm to generate Interface Identifiers.
   Therefore, the usual address lifetime properties (as specified in the
   corresponding Prefix Information Options) apply when IPv6 addresses
   are generated as a result of employing the algorithm specified in
   this document with SLAAC [RFC4862].  Additionally, from the point of
   view of renumbering, we note that these addresses behave like the
   traditional IPv6 addresses (that embed a hardware address) resulting
   from SLAAC [RFC4862].

   While the method specified in this document is meant to be used with
   SLAAC, this does not preclude the same algorithm from being used with
   other address configuration mechanisms, such as DHCPv6 [RFC3315] or
   manual address configuration.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Design goals

   This document specifies a method for selecting Interface Identifiers
   to be used with IPv6 SLAAC, with the following goals:

   o  The resulting Interface Identifiers remain constant/stable for
      each prefix used with SLAAC within each subnet.  That is, the
      algorithm generates the same Interface Identifier when configuring
      an address (for the same interface) belonging to the same prefix
      within the same subnet.

   o  The resulting Interface Identifiers do change when addresses are
      configured for different prefixes.  That is, if different
      autoconfiguration prefixes are used to configure addresses for the
      same network interface card, the resulting Interface Identifiers
      must be (statistically) different.  This means that, given two
      addresses produced by the method specified in this document, it
      must be difficult for an attacker tell whether the addresses have
      been generated/used by the same node.

   o  It must be difficult for an outsider to predict the Interface
      Identifiers that will be generated by the algorithm, even with
      knowledge of the Interface Identifiers generated for configuring
      other addresses.

   o  Depending on the specific implementation approach (see Section 3
      and Appendix A), the resulting Interface Identifiers may be
      independent of the underlying hardware (e.g. link-layer address).
      This means that e.g. replacing a Network Interface Card (NIC) will
      not have the (generally undesirable) effect of changing the IPv6
      addresses used for that network interface.

   o  The method specified in this document is meant to be an
      alternative to producing IPv6 addresses based on e.g.  IEEE
      identifiers (as specified in [RFC2464]).  It is meant to be
      employed for all of the stable (i.e. non-temporary) IPv6 addresses
      configured with SLAAC for a given interface, including global,
      link-local, and unique-local IPv6 addresses.

   We note that of use of the algorithm specified in this document is
   (to a large extent) orthogonal to the use of "temporary addresses"
   [RFC4941].  When employed along with "temporary addresses", the
   method specified in this document will mitigate address-scanning
   attacks and correlation of node activities across networks (see
   [I-D.cooper-6man-ipv6-address-generation-privacy] and [IAB-PRIVACY]).
   On the other hand, hosts that do not implement/use "temporary
   addresses" but employ the method specified in this document would, at
   the very least, mitigate the host-tracking and address scanning
   issues discussed in the previous section.

3.  Algorithm specification

   IPv6 implementations conforming to this specification MUST generate
   Interface Identifiers using the algorithm specified in this section
   in replacement of any other algorithms used for generating "stable"
   addresses with SLAAC (such as those specified in [RFC2464]).
   However, implementations conforming to this specification MAY employ
   the algorithm specified in [RFC4941] to generate temporary addresses
   in addition to the addresses generated with the algorithm specified
   in this document.  The method specified in this document MUST be
   employed for generating the Interface Identifiers with SLAAC for all
   the stable addresses of a given interface, including IPv6 global,
   link-local, and unique-local addresses.

      This means that this document does not formally obsolete or
      deprecate any of the existing algorithms to generate Interface
      Identifiers (e.g. such as that specified in [RFC2464]).  However,
      those IPv6 implementations that employ this specification MUST
      generate all of their "stable" addresses as specified in this

   Implementations conforming to this specification SHOULD provide the
   means for a system administrator to enable or disable the use of this
   algorithm for generating Interface Identifiers.

   Unless otherwise noted, all of the parameters included in the
   expression below MUST be included when generating an Interface

   1.  Compute a random (but stable) identifier with the expression:

       RID = F(Prefix, Net_Iface, Network_ID, DAD_Counter, secret_key)


          Random (but stable) Interface Identifier

          A pseudorandom function (PRF) that is not computable from the
          outside (without knowledge of the secret key), which should
          produce an output of at least 64 bits.The PRF could be
          implemented as a cryptographic hash of the concatenation of
          each of the function parameters.

          The prefix to be used for SLAAC, as learned from an ICMPv6
          Router Advertisement message, or the link-local IPv6 unicast

          An implementation-dependent stable identifier associated with
          the network interface for which the RID is being generated.
          An implementation MAY provide a configuration option to select
          the source of the identifier to be used for the Net_Iface
          parameter.  A discussion of possible sources for this value
          (along with the corresponding trade-offs) can be found in
          Appendix A.

          Some network specific data that identifies the subnet to which
          this interface is attached.  For example the IEEE 802.11
          Service Set Identifier (SSID) corresponding to the network to
          which this interface is associated.  This parameter is

          A counter that is employed to resolve Duplicate Address
          Detection (DAD) conflicts.  It MUST be initialized to 0, and
          incremented by 1 for each new tentative address that is
          configured as a result of a DAD conflict.  Implementations
          that record DAD_Counter in non-volatile memory for each
          {Prefix, Net_Iface, Network_ID} tuple MUST initialize
          DAD_Counter to the recorded value if such an entry exists in
          non-volatile memory).  See Section 4 for additional details.

          A secret key that is not known by the attacker.  The secret
          key MUST be initialized at operating system installation time
          to a pseudo-random number (see [RFC4086] for randomness
          requirements for security).  An implementation MAY provide the
          means for the the system administrator to change or display
          the secret key.

   2.  The Interface Identifier is finally obtained by taking as many
       bits from the RID value (computed in the previous step) as
       necessary, starting from the least significant bit.

          We note that [RFC4291] requires that, the Interface IDs of all
          unicast addresses (except those that start with the binary
          value 000) be 64-bit long.  However, the method discussed in
          this document could be employed for generating Interface IDs
          of any arbitrary length, albeit at the expense of reduced
          entropy (when employing Interface IDs smaller than 64 bits).

       The resulting Interface Identifier should be compared against the
       Subnet-Router Anycast [RFC4291] and the Reserved Subnet Anycast
       Addresses [RFC2526], and against those Interface Identifiers
       already employed in an address of the same network interface and
       the same network prefix.  In the event that an unacceptable
       identifier has been generated, this situation should be handled
       in the same way as the case of duplicate addresses (see
       Section 4).

   This document does not require the use of any specific PRF for the
   function F() above, since the choice of such PRF is usually a trade-
   off between a number of properties (processing requirements, ease of
   implementation, possible intellectual property rights, etc.), and
   since the best possible choice for F() might be different for
   different types of devices (e.g. embedded systems vs. regular
   servers) and might possibly change over time.

   Note that the result of F() in the algorithm above is no more secure
   than the secret key.  If an attacker is aware of the PRF that is
   being used by the victim (which we should expect), and the attacker
   can obtain enough material (i.e. addresses configured by the victim),
   the attacker may simply search the entire secret-key space to find
   matches.  To protect against this, the secret key should be of a
   reasonable length.  Key lengths of at least 128 bits should be
   adequate.  The secret key is initialized at system installation time
   to a pseudo-random number, thus allowing this mechanism to be
   enabled/used automatically, without user intervention.

   Including the SLAAC prefix in the PRF computation causes the
   Interface Identifier to vary across each prefix (link-local, global,
   etc.) employed by the node and, as consequently, also across
   networks.  This mitigates the correlation of activities of multi-
   homed nodes (since each of the corresponding addresses will employ a
   different Interface ID), host-tracking (since the network prefix will
   change as the node moves from one network to another), and any other
   attacks that benefit from predictable Interface Identifiers (such as
   address scanning attacks).

   The Net_Iface is a value that identifies the network interface for
   which an IPv6 address is being generated.  The following properties
   are required for the Net_Iface parameter:

   o  it MUST be constant across system bootstrap sequences and other
      network events (e.g., bringing another interface up or down)

   o  it MUST be different for each network interface simultaneously in

   Since the stability of the addresses generated with this method
   relies on the stability of all arguments of F(), it is key that the
   Net_Iface be constant across system bootstrap sequences and other
   network events.  Additionally, the Net_Iface must uniquely identify
   an interface within the node, such that two interfaces connecting to
   the same network do not result in duplicate addresses.  Different
   types of operating systems might benefit from different stability
   properties of the Net_Iface parameter.  For example, a client-
   oriented operating system might want to employ Net_Iface identifiers
   that are attached to the underlying network interface card (NIC),
   such that a removable NIC always gets the same IPv6 address,
   irrespective of the system communications port to which it is
   attached.  On the other hand, a server-oriented operating system
   might prefer Net_Iface identifiers that are attached to system slots/
   ports, such that replacement of a network interface card does not
   result in an IPv6 address change.  Appendix A discusses possible
   sources for the Net_Iface, along with their pros and cons.

   Including the optional Network_ID parameter when computing the RID
   value above would cause the algorithm to produce a different
   Interface Identifier when connecting to different networks, even when
   configuring addresses belonging to the same prefix.  This means that
   a host would employ a different Interface Identifier as it moves from
   one network to another even for IPv6 link-local addresses or Unique
   Local Addresses (ULAs).  In those scenarios where the Network_ID is
   unknown to the attacker, including this parameter might help mitigate
   attacks where a victim node connects to the same subnet as the
   attacker, and the attacker tries to learn the Interface Identifier
   used by the victim node for a remote network (see Section 7 for
   further details).

   The DAD_Counter parameter provides the means to intentionally cause
   this algorithm produce a different IPv6 addresses (all other
   parameters being the same).  This could be necessary to resolve
   Duplicate Address Detection (DAD) conflicts, as discussed in detail
   in Section 4.

   Finally, we note that all of the bits in the resulting Interface IDs
   are treated as "opaque" bits.  For example, the universal/local bit
   of Modified EUI-64 format identifiers is treated as any other bit of
   such identifier.  In theory, this might result in Duplicate Address
   Detection (DAD) failures that would otherwise not be encountered.
   However, this is not deemed as a real issue, because of the following

   o  The interface IDs of all addresses (except those of addresses that
      that start with the binary value 000) are 64-bit long.  Since the
      method specified in this document results in random Interface IDs,
      the probability of DAD failures is very small.

   o  Real world data indicates that MAC address reuse is far more
      common than assumed [HDMoore].  This means that even IPv6
      addresses that employ (allegedly) unique identifiers (such as IEEE
      identifiers) might result in DAD failures, and hence
      implementations should be prepared to gracefully handle such

   o  Since some popular and widely-deployed operating systems (such as
      Microsoft Windows) do not employ unique hardware identifiers for
      the Interface IDs of their stable addresses, reliance on such
      unique identifiers is more reduced in the deployed world (fewer
      deployed systems rely on them for the avoidance of address

4.  Resolving Duplicate Address Detection (DAD) conflicts

   If as a result of performing Duplicate Address Detection (DAD)
   [RFC4862] a host finds that the tentative address generated with the
   algorithm specified in Section 3 is a duplicate address, it SHOULD
   resolve the address conflict by trying a new tentative address as

   o  DAD_Counter is incremented by 1.

   o  A new Interface Identifier is generated with the algorithm
      specified in Section 3, using the incremented DAD_Counter value.

   This procedure may be repeated a number of times until the address
   conflict is resolved.  Hosts SHOULD try at least IDGEN_RETRIES (see
   Section 5) tentative addresses if DAD fails for successive generated
   addresses, in the hopes of resolving the address conflict.  We also
   note that hosts MUST limit the number of tentative addresses that are
   tried (rather than indefinitely try a new tentative address until the
   conflict is resolved).

   In those (unlikely) scenarios in which duplicate addresses are
   detected and in which the order in which the conflicting nodes
   configure their addresses may vary (e.g., because they may be
   bootstrapped in different order), the algorithm specified in this
   section for resolving DAD conflicts could lead to addresses that are
   not stable within the same subnet.  In order to mitigate this
   potential problem, nodes MAY record the DAD_Counter value employed
   for a specific {Prefix, Net_Iface, Network_ID} tuple in non-volatile
   memory, such that the same DAD_Counter value is employed when
   configuring an address for the same Prefix and subnet at any other
   point in time.

   In the event that a DAD conflict cannot be solved (possibly after
   trying a number of different addresses), address configuration would
   fail.  In those scenarios, nodes MUST NOT automatically fall back to
   employing other algorithms for generating Interface Identifiers.

5.  Specified Constants

   This document specifies the following constant:

      defaults to 3.

6.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an

7.  Security Considerations

   This document specifies an algorithm for generating Interface
   Identifiers to be used with IPv6 Stateless Address Autoconfiguration
   (SLAAC), as an alternative to e.g.  Interface Identifiers that embed
   IEEE identifiers (such as those specified in [RFC2464]).  When
   compared to such identifiers, the identifiers specified in this
   document have a number of advantages:

   o  They prevent trivial host-tracking, since when a host moves from
      one network to another the network prefix used for
      autoconfiguration and/or the Network ID (e.g., IEEE 802.11 SSID)
      will typically change, and hence the resulting Interface
      Identifier will also change (see

   o  They mitigate address-scanning techniques which leverage
      predictable Interface Identifiers (e.g., known Organizationally
      Unique Identifiers) [I-D.ietf-opsec-ipv6-host-scanning].

   o  They may result in IPv6 addresses that are independent of the
      underlying hardware (i.e. the resulting IPv6 addresses do not
      change if a network interface card is replaced) if an appropriate
      source for Net_Iface (Section 3) is employed.

   o  They prevent the information leakage produced by embedding
      hardware addresses in the Interface Identifier (which could be
      exploited to launch device-specific attacks).

   o  Since the method specified in this document will result in
      different Interface Identifiers for each configured address,
      knowledge/leakage of the Interface Identifier employed for one
      stable address of will not negatively affect the security/privacy
      of other stable addresses configured for other prefixes (whether
      at the same time or at some other point in time).

   In scenarios in which an attacker can connect to the same subnet as a
   victim node, the attacker might be able to learn the Interface
   Identifier employed by the victim node for an arbitrary prefix, by
   simply sending a forged Router Advertisement [RFC4861] for that
   prefix, and subsequently learning the corresponding address
   configured by the victim node (either listening to the Duplicate
   Address Detection packets, or to any other traffic that employs the
   newly configured address).  We note that a number of factors might
   limit the ability of an attacker to successfully perform such an

   o  First-Hop security mechanisms such as RA-Guard [RFC6105]
      [I-D.ietf-v6ops-ra-guard-implementation] could prevent the forged
      Router Advertisement from reaching the victim node

   o  If the victim implementation includes the (optional) Network_ID
      parameter for computing F() (see Section 3), and the Network_ID
      employed by the victim for a remote network is unknown to the
      attacker, the Interface Identifier learned by the attacker would
      differ from the one used by the victim when connecting to the
      legitimate network.

   In any case, we note that at the point in which this kind of attack
   becomes a concern, a host should consider employing Secure Neighbor
   Discovery (SEND) [RFC3971] to prevent an attacker from illegitimately
   claiming authority for a network prefix.

   We note that this algorithm is meant to be an alternative to
   Interface Identifiers such as those specified in [RFC2464], but is
   not meant as an alternative to temporary Interface Identifiers (such
   as those specified in [RFC4941]).  Clearly, temporary addresses may
   help to mitigate the correlation of activities of a node within the
   same network, and may also reduce the attack exposure window (since
   temporary addresses are short-lived when compared to the addresses
   generated with the method specified in this document).  We note that
   implementation of this algorithm would still benefit those hosts
   employing "temporary addresses", since it would mitigate host-
   tracking vectors still present when such addresses are used (see
   [I-D.cooper-6man-ipv6-address-generation-privacy]), and would also
   mitigate address-scanning techniques that leverage patterns in IPv6
   addresses that embed IEEE identifiers.

   Finally, we note that the method described in this document addresses
   some of the privacy concerns arising from the use of IPv6 addresses
   that embed IEEE identifiers, without the use of temporary addresses,
   thus possibly offering an interesting trade-off for those scenarios
   in which the use of temporary addresses is not feasible.

8.  Acknowledgements

   The algorithm specified in this document has been inspired by Steven
   Bellovin's work ([RFC1948]) in the area of TCP sequence numbers.

   The author would like to thank (in alphabetical order) Mikael
   Abrahamsson, Ran Atkinson, Karl Auer, Steven Bellovin, Matthias
   Bethke, Ben Campbell, Brian Carpenter, Tassos Chatzithomaoglou, Tim
   Chown, Alissa Cooper, Dominik Elsbroek, Brian Haberman, Bob Hinden,
   Christian Huitema, Ray Hunter, Jouni Korhonen, Eliot Lear, Jong-Hyouk
   Lee, Andrew McGregor, Tom Petch, Michael Richardson, Mark Smith, Dave
   Thaler, Ole Troan, James Woodyatt, and He Xuan, for providing
   valuable comments on earlier versions of this document.

   This document is based on the technical report "Security Assessment
   of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by
   Fernando Gont on behalf of the UK Centre for the Protection of
   National Infrastructure (CPNI).

   Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
   their continued support.

9.  References

9.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, March 1999.

   [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.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.

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

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

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

   [RFC6105]  Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
              Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
              February 2011.

9.2.  Informative References

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, December 1998.

              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", draft-ietf-opsec-ipv6-host-scanning-02 (work in
              progress), July 2013.

              Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)",
              draft-ietf-v6ops-ra-guard-implementation-07 (work in
              progress), November 2012.

              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              draft-cooper-6man-ipv6-address-generation-privacy-00 (work
              in progress), July 2013.

   [HDMoore]  HD Moore, "The Wild West",  Louisville, Kentucky, U.S.A.
              September 25-29, 2012,

              Gont, "Results of a Security Assessment of the Internet
              Protocol version 6 (IPv6)",  DEEPSEC 2011 Conference,
              Vienna, Austria, November 2011, <http://

              Gont, "Recent Advances in IPv6 Security",  BRUCON 2012
              Conference, Ghent, Belgium, September 2012, <http://

              Broersma, R., "IPv6 Everywhere: Living with a Fully IPv6-
              enabled environment",  Australian IPv6 Summit 2010,
              Melbourne, VIC Australia, October 2010, <http://

              IAB, "Privacy and IPv6 Addresses",  July 2011, <http://

              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).

Appendix A.  Possible sources for the Net_Iface parameter

   The following subsections describe a number of possible sources for
   the Net_Iface parameter employed by the F() function in Section 3.
   The choice of a specific source for this value represents a number of
   trade-offs, which may vary from one implementation to another.

A.1.  Interface Index

   The Interface Index [RFC3493] [RFC3542] of an interface uniquely
   identifies an interface within a node.  However, these identifiers
   might or might not have the stability properties required for the
   Net_Iface value employed by this method.  For example, the Interface
   Index might change upon removal or installation of a network
   interface (typically one with a smaller value for the Interface
   Index, when such a naming scheme is used), or when network interfaces
   happen to be initialized in a different order.  We note that some
   implementations are known to provide configuration knobs to set the
   Interface Index for a given interface.  Such configuration knobs
   could be employed to prevent the Interface Index from changing (e.g.
   as a result of the removal of a network interface).

A.2.  Interface Name

   The Interface Name (e.g., "eth0", "em0", etc) tends to be more stable
   than the underlying Interface Index, since such stability is
   required/desired when interface names are employed in network
   configuration (firewall rules, etc.).  The stability properties of
   Interface Names depend on implementation details, such as what is the
   namespace used for Interface Names.  For example, "generic" interface
   names such as "eth0" or "wlan0" will generally be invariant with
   respect to network interface card replacements.  On the other hand,
   vendor-dependent interface names such as "rtk0" or the like will
   generally change when a network interface card is replaced with one
   from a different vendor.

   We note that Interface Names might still change when network
   interfaces are added or removed once the system has been bootstrapped
   (for example, consider Universal Serial Bus-based network interface
   cards which might be added or removed once the system has been

A.3.  Link-layer Addresses

   Link-layer addresses typically provide for unique identifiers for
   network interfaces; although, for obvious reasons, they generally
   change when a network interface card is replaced.  In scenarios where
   neither Interface Indexes nor Interface Names have the stability
   properties specified in Section 3 for Net_Iface, an implementation
   might want to employ the link-layer address of the interface for the
   Net_Iface parameter, albeit at the expense of making the
   corresponding IPv6 addresses dependent on the underlying network
   interface card (i.e., the corresponding IPv6 address would typically
   change upon replacement of the underlying network interface card).

A.4.  Logical Network Service Identity

   Host operating systems with a conception of logical network service
   identity, distinct from network interface identity or index, may keep
   a Universally Unique Identifier (UUID) [RFC4122] or similar
   identifier with the stability properties appropriate for use as the
   Net_Iface parameter.

Author's Address

   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com