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Versions: (draft-gont-6man-stable-privacy-addresses) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 7217

IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Standards Track                        January 27, 2014
Expires: July 31, 2014


 A Method for Generating Semantically Opaque Interface Identifiers with
            IPv6 Stateless Address Autoconfiguration (SLAAC)
              draft-ietf-6man-stable-privacy-addresses-17

Abstract

   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
   addresses (e.g., IEEE LAN MAC addresses), 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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 31, 2014.

Copyright Notice

   Copyright (c) 2014 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



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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Relationship to Other standards . . . . . . . . . . . . . . .   5
   4.  Design goals  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Algorithm specification . . . . . . . . . . . . . . . . . . .   6
   6.  Resolving Duplicate Address Detection (DAD) conflicts . . . .  11
   7.  Specified Constants . . . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Possible sources for the Net_Iface parameter . . . .  18
     A.1.  Interface Index . . . . . . . . . . . . . . . . . . . . .  18
     A.2.  Interface Name  . . . . . . . . . . . . . . . . . . . . .  18
     A.3.  Link-layer Addresses  . . . . . . . . . . . . . . . . . .  19
     A.4.  Logical Network Service Identity  . . . . . . . . . . . .  19
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  19

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., an IEEE LAN MAC address) [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,



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      thus negatively affecting the privacy of users (see
      [I-D.ietf-6man-ipv6-address-generation-privacy] and
      [IAB-PRIVACY]).

   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
      [I-D.ietf-opsec-ipv6-host-scanning].  For example, the IPv6
      addresses of all nodes manufactured by the same vendor (within a
      given time frame) will likely contain the same IEEE
      Organizationally Unique Identifier (OUI) in the Interface
      Identifier.

   o  embedding the underlying hardware 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.ietf-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
   them.

   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 LAN MAC addresses, with the
   "temporary addresses" being employed for "outgoing communications",
   and the traditional SLAAC addresses being employed for "server"
   functions (i.e., receiving incoming connections).





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   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 typically been generated from
   the underlying hardware addresses.  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.ietf-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 SLAAC addresses to greatly reduce the search space for
      "alive" nodes [Gont-DEEPSEC2011] [CPNI-IPv6]
      [I-D.ietf-opsec-ipv6-host-scanning].

   Hence, there is a motivation to improve the properties of "stable"
   addresses regardless of whether temporary addresses are employed or
   not.

   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.






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2.  Terminology

   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 RFC 2119 [RFC2119].

3.  Relationship to Other standards

   The method specified in this document is 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 LAN MAC addresses) will 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],
   implementation of this mechanism would mitigate the host-tracking and
   address scanning issues discussed in Section 1.

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

4.  Design goals

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

   o  The resulting Interface Identifiers remain stable for each prefix
      used with SLAAC within each subnet for the same network interface.
      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 must 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.





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   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 5
      and Appendix A), the resulting Interface Identifiers may be
      independent of the underlying hardware (e.g. IEEE LAN MAC
      address).  This means that e.g. replacing a Network Interface Card
      (NIC) or adding links dynamically to a Link Aggregation Group
      (LAG) 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 hardware addresses
      (e.g. IEEE LAN MAC addresses, as specified in [RFC2464]).  That
      is, this document does not formally obsolete or deprecate any of
      the existing algorithms to generate Interface Identifiers.  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 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].

5.  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],
   [RFC2467], and [RFC2470]).  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




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   Identifiers with SLAAC for all the stable addresses, including IPv6
   global, link-local, and unique-local addresses.

   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
   Identifier.

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

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

       Where:

       RID:
          Random (but stable) Identifier

       F():
          A pseudorandom function (PRF) that MUST NOT be computable from
          the outside (without knowledge of the secret key).  F() MUST
          also be difficult to reverse, such that it resists attempts to
          obtain the secret_key, even when given samples of the output
          of F() and knowledge or control of the other input parameters.
          F() SHOULD produce an output of at least 64 bits.  F() could
          be implemented as a cryptographic hash of the concatenation of
          each of the function parameters.  SHA-1 [FIPS-SHS] and SHA-256
          are two possible options for F().  Note: MD5 [RFC1321] is
          considered unacceptable for F() [RFC6151].

       Prefix:
          The prefix to be used for SLAAC, as learned from an ICMPv6
          Router Advertisement message, or the link-local IPv6 unicast
          prefix [RFC4291].

       Net_Iface:
          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.

       Network_ID:




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          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.  Additionally, Simple DNA
          [RFC6059] describes ideas that could be leveraged to generate
          a Network_ID parameter.  This parameter is OPTIONAL.

       DAD_Counter:
          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 6 for additional details.

       secret_key:
          A secret key that is not known by the attacker.  The secret
          key MUST be initialized to a pseudo-random number (see
          [RFC4086] for randomness requirements for security) at
          operating system installation time or when the IPv6 protocol
          stack is initialized for the first time.  An implementation
          MAY provide the means for the the system administrator to
          display and change 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
       reserved IPv6 Interface Identifiers [RFC5453]
       [IANA-RESERVED-IID], 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 6).

   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-



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   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.

   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
   IPv6 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
      use

   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 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 causes 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



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   one network to another even for IPv6 link-local addresses or Unique
   Local Addresses (ULAs) [RFC4193].  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 9 for further details).

   The DAD_Counter parameter provides the means to intentionally cause
   this algorithm to 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 6.

   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 at
   least 128 bits.  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.  Providing a mechanism to
   display and change the secret_key would allow and administrator to
   cause a replaced system (with the same implementation of this
   document) to generate the same IPv6 addresses as the system being
   replaced.  We note that since the privacy of the scheme specified in
   this document relies on the secrecy of the secret_key parameter,
   implementations should constrain access to the secret_key parameter
   to the extent practicable (e.g., require superuser privileges to
   access it).  Furthermore, in order to prevent leakages of the
   secret_key parameter, it should not be used for any other purposes
   than being a parameter to the scheme specified in this document.

   We note that all of the bits in the resulting Interface IDs are
   treated as "opaque" bits [I-D.ietf-6man-ug].  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
   IPv6 address collisions and Duplicate Address Detection (DAD)
   failures that would otherwise not be encountered.  However, this is
   not deemed as a likely issue, because of the following
   considerations:

   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.



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   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
      LAN MAC addresses) might result in DAD failures, and hence
      implementations should be prepared to gracefully handle such
      occurrences.  Additionally, some virtualization technologies
      already employ hardware addresses that are randomly selected, and
      hence cannot be guaranteed to be unique
      [I-D.ietf-opsec-ipv6-host-scanning].

   o  Since some popular and widely-deployed operating systems (such as
      Microsoft Windows) do not embed hardware addresses in 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 collisions).

   Finally, that since different implementation are likely to use
   different values for the secret_key parameter, and may also employ
   different PRFs for F() and different sources for the Net_Iface
   parameter, the addresses generated by this scheme should not expected
   to be stable across different operating system installations.  For
   example, a host that is dual-boot or that is reinstalled may result
   in different IPv6 addresses for each operating system and/or
   installation.

6.  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 5 is a duplicate address, it SHOULD
   resolve the address conflict by trying a new tentative address as
   follows:

   o  DAD_Counter is incremented by 1.

   o  A new Interface Identifier is generated with the algorithm
      specified in Section 5, using the incremented DAD_Counter value.

   Hosts SHOULD introduce a random delay between 0 and IDGEN_DELAY
   seconds (see Section 7) before trying a new tentative address, to
   avoid lock-step behavior of multiple hosts.

   This procedure may be repeated a number of times until the address
   conflict is resolved.  Hosts SHOULD try at least IDGEN_RETRIES (see
   Section 7) 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




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   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.
   We note that the use of non-volatile memory is OPTIONAL, and hosts
   that do not implement this feature are still compliant to this
   protocol specification.

   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.

7.  Specified Constants

   This document specifies the following constant:

   IDGEN_RETRIES:
      defaults to 3.

   IDGEN_DELAY:
      defaults to 1 second.

8.  IANA Considerations

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

9.  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
   hardware addresses (such as those specified in [RFC2464], [RFC2467],
   and [RFC2470]).  When compared to such identifiers, the identifiers
   specified in this document have a number of advantages:





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   o  They prevent trivial host-tracking based on the IPv6 address,
      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
      [I-D.ietf-6man-ipv6-address-generation-privacy]).

   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 5) 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 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).

   We note that while some probing techniques (such as the use of ICMPv6
   Echo Request and ICMPv6 Echo Response packets) could be mitigated by
   a personal firewall at the target host, for other probing vectors,
   such as 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).  Hence, the method
   specified in this document is still of value for nodes that employ
   personal firewalls.

   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
   attack:



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   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 5), 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.ietf-6man-ipv6-address-generation-privacy]), and would also
   mitigate address-scanning techniques that leverage patterns in IPv6
   addresses that embed IEEE LAN MAC addresses.  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 LAN
   MAC addresses, 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.

10.  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, Stephen Farrell, Eric Gray,
   Brian Haberman, Bob Hinden, Christian Huitema, Ray Hunter, Jouni
   Korhonen, Suresh Krishnan, Eliot Lear, Jong-Hyouk Lee, Andrew
   McGregor, Thomas Narten, Simon Perreault, Tom Petch, Michael



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   Richardson, Vincent Roca, Mark Smith, Hannes Frederic Sowa, Martin
   Stiemerling, Dave Thaler, Ole Troan, Lloyd Wood, James Woodyatt, and
   He Xuan, for providing valuable comments on earlier versions of this
   document.

   Hannes Frederic Sowa produced a reference implementation of this
   specification for the Linux kernel.

   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).

11.  References

11.1.  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.

   [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.

   [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.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

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






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   [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.

   [RFC5453]  Krishnan, S., "Reserved IPv6 Interface Identifiers", RFC
              5453, February 2009.

   [I-D.ietf-6man-ug]
              Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", draft-ietf-6man-ug-06 (work in
              progress), December 2013.

11.2.  Informative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [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.

   [RFC2467]  Crawford, M., "Transmission of IPv6 Packets over FDDI
              Networks", RFC 2467, December 1998.

   [RFC2470]  Crawford, M., Narten, T., and S. Thomas, "Transmission of
              IPv6 Packets over Token Ring Networks", RFC 2470, December
              1998.

   [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.

   [RFC6059]  Krishnan, S. and G. Daley, "Simple Procedures for
              Detecting Network Attachment in IPv6", RFC 6059, November
              2010.



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

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, March 2011.

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

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

   [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-00 (work
              in progress), October 2013.

   [HDMoore]  HD Moore, , "The Wild West", Louisville, Kentucky, U.S.A,
              September 2012, <https://speakerdeck.com/hdm/derbycon-2012
              -the-wild-west>.

   [IANA-RESERVED-IID]
              Reserved IPv6 Interface Identifiers, ,
              "http://www.iana.org/assignments/ipv6-interface-ids/
              ipv6-interface-ids.xml", .

   [Gont-DEEPSEC2011]
              Gont, , "Results of a Security Assessment of the Internet
              Protocol version 6 (IPv6)", DEEPSEC 2011 Conference,
              Vienna, Austria, November 2011,
              <http://www.si6networks.com/presentations/deepsec2011/
              fgont-deepsec2011-ipv6-security.pdf>.

   [Broersma]
              Broersma, R., "IPv6 Everywhere: Living with a Fully
              IPv6-enabled environment", Australian IPv6 Summit 2010,
              Melbourne, VIC Australia, October 2010,
              <http://www.ipv6.org.au/10ipv6summit/talks/
              Ron_Broersma.pdf>.





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   [IAB-PRIVACY]
              IAB, , "Privacy and IPv6 Addresses", July 2011,
              <http://www.iab.org/wp-content/IAB-uploads/2011/07/
              IPv6-addresses-privacy-review.txt>.

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

   [FIPS-SHS]
              FIPS, , "Secure Hash Standard (SHS)", Federal Information
              Processing Standards Publication 180-4, March 2012,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

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 5.
   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



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   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
   bootstrapped).

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 5 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
   Argentina

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









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