Dynamic Host Configuration (dhc)                                 F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Standards Track                                  W. Liu
Expires: March 9, 2015                               Huawei Technologies
                                                       September 5, 2014

 A Method for Generating Semantically Opaque Interface Identifiers with
         Dynamic Host Configuration Protocol for IPv6 (DHCPv6)


   This document specifies a method for generating IPv6 Interface
   Identifiers to be used by Dynamic Host Configuration Protocol for
   IPv6 (DHCPv6) servers.  This method results in stable addresses
   within each subnet, even in the presence of multiple DHCPv6 servers
   or even DHCPv6 server reinstallments.  This method is a
   DHCPv6-variant of the method specified in RFC 7217 for IPv6 Stateless
   Address Autoconfiguration.

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
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   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 March 9, 2015.

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
   (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

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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Method Specification  . . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Stable IPv6 addresses tend to simplify event logging, trouble-
   shooting, enforcement of access controls and quality of service, etc.
   However, there are a number of scenarios in which a host employing
   the DHCPv6 protocol [RFC3315] may be assigned different IPv6
   addresses for the same interface within the same subnet over time.
   For example, this may happen when multiple servers operate on the
   same network to provide increased availability, but may also happen
   as a result of DHCPv6 server reinstallments and other scenarios.

   This document specifies a method to be employed by DHCPv6 servers for
   generating IPv6 addresses, with the following properties:

   o  The resulting IPv6 addresses remain stable within each subnet for
      the same network interface of the same client, even when different
      DHCPv6 servers (implementing this specification) are employed.

   o  It must be difficult for an outsider to predict the IPv6 addresses
      that will be generated by the method specified in this document,
      even with knowledge of the IPv6 addresses generated for other
      nodes within the same network.

   The method specified in this document achieves the aforementioned
   goals by means of a calculated technique as opposed to e.g. state-
   sharing among DHCPv6 servers . This approach has been already
   suggested in [RFC7031].  We note that the method specified in this
   document is essentially a DHCPv6-version of the "Method for
   Generating Semantically Opaque Interface Identifiers with IPv6
   Stateless Address Autoconfiguration (SLAAC)" specified in [RFC7217].

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2.  Terminology

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

3.  Method Specification

   DHCPv6 server implementations conforming to this specification MUST
   generate non-temporary IPv6 addresses using the algorithm specified
   in this section.

   Implementations conforming to this specification SHOULD provide the
   means for a system administrator to enable or disable the use of this
   algorithm for generating IPv6 addresses.

   Unless otherwise noted, all of the parameters included in the
   expression below MUST be included when generating an IPv6 address.

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

       RID = F(Prefix | Client_DUID | IA_NA | Counter | secret_key)


          Random (but stable) Identifier

          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.  The default algorithm to be
          employed for F() SHOULD be SHA-1 [FIPS-SHS].  An
          implementation MAY provide the means for selecting other other
          algorithms (e.g., SHA-256) for F().  Note: MD5 [RFC1321] is
          considered unacceptable for F() [RFC6151].

          An operator representing "concatenation".

          The prefix from which the DHCPv6 is assigning addresses (i.e.,
          the prefix representing the address pool).

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          The DUID value contained in the Client Identifier option
          received in the client message.

          The IAID value contained in the IA_NA option received in the
          client message.

          A variable that is employed to resolve address conflicts.  It
          MUST be initialized to 0.

          A secret key configured by the DHCPv6 server administrator,
          which MUST NOT be known by the attacker.  An implementation of
          this specification MUST provide an interface for viewing and
          changing the secret key.  All DHCPv6 servers leasing addresses
          from the same Prefix MUST employ the same secret key.

   2.  The Interface Identifier is 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 MUST be compared against the
       reserved IPv6 Interface Identifiers [RFC5453]
       [IANA-RESERVED-IID].  In the event that an unacceptable
       identifier has been generated, the Counter variable should be
       incremented by 1, and a new Interface ID should be computed with
       the updated Counter value.

   3.  The IPv6 address is finally obtained by concatenating the Prefix
       with the Interface Identifier obtained in the previous step.  If
       the resulting address is not available (e.g., there is a
       conflicting binding), the server should increment the Counter
       variable, and a new Interface ID and IPv6 address should be
       computed with the updated Counter value.

   This document requires that SHA-1 be the default function to be used
   for F(), such that, all other configuration parameters being the
   same, different implementations of this specification result in the
   same IPv6 addresses.

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   Including the Prefix in the PRF computation causes the Interface
   Identifier to for each address from a different prefix assigned to
   the same client.  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)

   As required by [RFC3315], an IAID is associated with each of the
   client's network interfaces, and is consistent across restarts of the
   DHCP client.

   The 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 address
   conflicts (e.g. the resulting address having a conflicting binding).

   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 DHCPv6 server (which we should expect), and the
   attacker can obtain enough material (i.e. addresses generated by the
   DHCPv6 server), 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.

   Providing a mechanism to display and change the secret_key is crucial
   for having different DHCPv6 servers produce the same IPv6 addresses,
   and for causing a replacement system 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 [RFC7136].  For example, the universal/local
   bit of Modified EUI-64 format identifiers is treated as any other bit
   of such identifier.

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4.  IANA Considerations

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

5.  Security Considerations

   The method specified in this document results in IPv6 Interface
   Identifiers (and hence IPv6 addresses) that do not follow any
   specific pattern.  Thus, address-scanning attacks
   [I-D.ietf-opsec-ipv6-host-scanning] are mitigated.

   The method specified in this document neither mitigates nor
   exacerbates the security considerations for DHCPv6 discussed in

6.  Acknowledgements

   This document is based on [RFC7217], authored by Fernando Gont.

   The authors would like to thank Ted Lemon, who kindly answered some
   DHCPv6-related questions.

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.

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

   [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.

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

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

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, February 2014.

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7.2.  Informative References

              FIPS, , "Secure Hash Standard (SHS)", Federal Information
              Processing Standards Publication 180-4, March 2012,

              Cooper, A., Gont, F., and D. Thaler, "Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              draft-ietf-6man-ipv6-address-generation-privacy-01 (work
              in progress), February 2014.

              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", draft-ietf-opsec-ipv6-host-scanning-04 (work in
              progress), June 2014.

              Reserved IPv6 Interface Identifiers, ,
              ipv6-interface-ids.xml", .

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

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

   [RFC7031]  Mrugalski, T. and K. Kinnear, "DHCPv6 Failover
              Requirements", RFC 7031, September 2013.

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

Authors' Addresses

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   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

   Will(Shucheng) Liu
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R. China

   Email: liushucheng@huawei.com

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