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Versions: 00 01 02 RFC 7943

Dynamic Host Configuration (dhc)                                 F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Informational                                    W. Liu
Expires: March 12, 2016                              Huawei Technologies
                                                       September 9, 2015


 A Method for Generating Semantically Opaque Interface Identifiers with
         Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
             draft-gont-dhcpv6-stable-privacy-addresses-00

Abstract

   This document describes a method for selecting IPv6 Interface
   Identifiers that can be employed by Dynamic Host Configuration
   Protocol for IPv6 (DHCPv6) servers when leasing non-temporary IPv6
   addresses to DHCPv6 clients.  This method is a DHCPv6 server side
   algorithm, that does not require any updates to the existing DHCPv6
   specifications.  The aforementioned method results in stable
   addresses within each subnet, even in the presence of multiple DHCPv6
   servers or DHCPv6 server reinstallments.  It 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
   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 March 12, 2016.

Copyright Notice

   Copyright (c) 2015 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Applicability and Design Goals  . . . . . . . . . . . . . . .   3
   3.  Method Specification  . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 describes a method for selecting IPv6 Interface
   Identifiers that can be employed by Dynamic Host Configuration
   Protocol for IPv6 (DHCPv6) servers when leasing non-temporary IPv6
   addresses to DHCPv6 clients (i.e., to be employed with IA_NA
   options).  This method is a DHCPv6 server side algorithm, that does
   not require any updates to the existing DHCPv6 specifications.  The
   aforementioned method has 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  Predicting 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,
      becomes very difficult.



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   The method specified in this document achieves the aforementioned
   properties 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 described 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].

2.  Applicability and Design Goals

   This document simply describes one possible approach for selecting
   IPv6 Interface Identifiers to be employed by Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6) servers when leasing non-
   temporary IPv6 addresses to DHCPv6 clients, with the following
   properties:

   o  The resulting IPv6 addresses remain stable within each subnet for
      the same network interface of the same client.

   o  The resulting IPv6 addresses cannot be predicted by an attacker
      without knowledge of a secret key.

   o  The resulting IPv6 addresses remain stable across DHCPv6 server
      re-installations, or even a database of previous DHCPv6 address
      leases is not available.

   o  The resulting IPv6 addresses remain stable when different DHCPv6
      servers (implementing this specification) are employed on the same
      network.

   The benefits of stable IPv6 addresses are discussed in
   [I-D.ietf-6man-ipv6-address-generation-privacy].  Providing address
   stability across server re-installations or when a database of
   previous DHCPv6 address leases is unavailable is of use not only when
   a DHCPv6 server must be re-installed or the address-lease database
   becomes corrupted, but is also of use when implementation constraints
   (e.g., a DHCPv6 server implementation on an embedded device) make it
   impossible for a DHCPv6 server implementation to maintain a database
   of previous DHCPv6 address leases.  Finally, [RFC7031] describes
   scenarios where multiple DHCPv6 servers are required to run in such a
   way as to provide increased availability in case of server failure;
   the algorithm specified in this document employs a (lightweight)
   calculated technique to (as opposed to e.g. state-sharing among
   DHCPv6 servers) to achieve address stability in such scenarios,
   without the need of an additional protocol to synchronize the lease
   databases of DHCPv6 servers.





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3.  Method Specification

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

   A DHCPv6 server implementing this specification must select the IPv6
   addresses to be leased with the following algorithm:

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

       RID = F(Prefix | Client_DUID | IAID | 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.  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].

       Prefix:
          The prefix employed for the local subnet.  If multiple servers
          operate on the same network to provide increased availability,
          all such DHCPv6 servers must be configured with the same
          Prefix.  It is the administrator's responsibility that the
          aforementioned requirement is met.

       |:
          An operator representing "concatenation".

       Client_DUID:
          The DUID value contained in the Client Identifier option
          received in the DHCPv6 client message.  The DUID can be
          treated as an array of 8-bit unsigned integers.

       IAID:



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          The IAID value contained in the IA_NA option received in the
          client message.  It must be interpreted as a 32-bit unsigned
          integer in network byte order.

       secret_key:
          A secret key configured by the DHCPv6 server administrator,
          which must not be known by the attacker.  It must be encoded
          as an array of 8-bit unsigned integers containing the ASCII
          codes corresponding to the secret key.  An implementation of
          this specification must provide an interface for viewing and
          changing the secret key.  All DHCPv6 servers leasing addresses
          from the same address range must employ the same secret key.

       Counter:
          A 32-bit unsigned integer in network byte order, that is
          employed to resolve address conflicts.  It must be initialized
          to 0.

   2.  A candidate IPv6 address (IPV6_ADDR) to be leased is obtained by
       concatenating as many bits as necessary from the RID value
       computed in the previous step (starting from the least
       significant bit) to the Prefix employed in the equation above.

   3.  The candidate IPv6 address from the previous step should be
       adjusted (if necessary) to the IPv6 address range from which the
       DHCPv6 server is expected to lease IPv6 addresses, as follows:

          if(IPV6_ADDR < IPV6_ADDR_LOW || IPV6_ADDR > IPV6_ADDR_HI){
               IPV6_ADDR = IPV6_ADDR_LOW + IPV6_ADDR % \
                           (IPV6_ADDR_HI - IPV6_ADDR_LOW + 1);
          }

       where:

       IPV6_ADDR:
          The candidate IPv6 address generated in the previous step..

       IPV6_ADDR_HI:
          An IPv6 address specifying the upper boundary of the IPv6
          address pool from which the DHCPv6 server leases IPv6
          addresses.  If an address range is not explicitly selected,
          IPV6_ADDR_HI must be set to the IPv6 address from Prefix (see
          the expression above) that has all of the bits of the
          Interface Identifier set to 1.

       IPV6_ADDR_LOW:
          An IPv6 address specifying the lower boundary of the IPv6
          address pool from which the DHCPv6 server leases IPv6



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          addresses.  If an address range is not explicitly selected,
          IPV6_ADDR_LOW must be set to the IPv6 address from Prefix (see
          the expression above) that has all of the bits of the
          Interface Identifier set to 0.

   4.  The Interface Identifier of the selected IPv6 address 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 IPv6 address should be computed with
       the updated Counter value.

   5.  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.

   Including the Prefix in the PRF computation causes the Interface
   Identifier to be different 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
   [I-D.ietf-6man-ipv6-address-generation-privacy].

   As required by [RFC3315], an IAID is associated with each of the
   client's network interfaces, and is consistent across restarts of the
   DHCPv6 client.

   The Counter parameter provides the means to intentionally cause this
   algorithm to produce different IPv6 addresses (all other parameters
   being the same).  This can be of use 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




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   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.

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
   RFC.

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, attacks that rely on predictable Interface
   IDs (such as [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
   [RFC3315].

6.  Acknowledgements

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

   The authors would like to thank Stephane Bortzmeyer, Tatuya Jinmei,
   Andre Kostur, Tomek Mrugalski, Hosnieh Rafiee, Jean-Francois
   Tremblay, Tina Tsou, and Bernie Volz, for providing valuable comments
   on earlier versions of this documents.

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



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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC5453]  Krishnan, S., "Reserved IPv6 Interface Identifiers",
              RFC 5453, DOI 10.17487/RFC5453, February 2009,
              <http://www.rfc-editor.org/info/rfc5453>.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <http://www.rfc-editor.org/info/rfc7136>.

7.2.  Informative References

   [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>.

   [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-07 (work
              in progress), June 2015.

   [I-D.ietf-opsec-ipv6-host-scanning]
              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", draft-ietf-opsec-ipv6-host-scanning-08 (work in
              progress), August 2015.



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

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <http://www.rfc-editor.org/info/rfc1321>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <http://www.rfc-editor.org/info/rfc6151>.

   [RFC7031]  Mrugalski, T. and K. Kinnear, "DHCPv6 Failover
              Requirements", RFC 7031, DOI 10.17487/RFC7031, September
              2013, <http://www.rfc-editor.org/info/rfc7031>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <http://www.rfc-editor.org/info/rfc7217>.

Authors' Addresses

   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


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

   Email: liushucheng@huawei.com







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