Network Working Group                                     Sheng Jiang
Internet Draft                                        Sam(Zhongqi) Xia
Intended status: Standards Track          Huawei Technologies Co., Ltd
Expires: May 30, 2011                                November 19, 2010

  Configuring Cryptographically Generated Addresses (CGA) using DHCPv6

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   A Cryptographically Generated Address is an IPv6 addresses binding
   with a public/private key pair. However, the current CGA
   specifications are lack of procedures to enable proper management of
   the usage of CGAs. This document defines the process using DHCPv6 to
   manage CGAs in detail. A new DHCPv6 option is defined accordingly.
   This document also analyses the configuration of the parameters,
   which are used to generate CGAs, using DHCPv6. Although the document
   does not define new DHCPv6 option to carry these parameters for
   various reasons, the configuration procedure is described.

Table of Contents

   1. Introduction................................................3
   2. Terminology.................................................3
   3. CGA Configure Process Using DHCPv6...........................3
      3.1. Configuration of the parameters required for the generation
      of CGA......................................................4
      3.2. Host requests CGA Approved to the DHCPv6 server..........5
   4. CGA Grant Option............................................7
   5. Security Considerations......................................7
   6. IANA Considerations.........................................8
   7. Acknowledgments.............................................8
   8. References..................................................8
      8.1. Normative References....................................8
      8.2. Informative References..................................9
   Author's Addresses............................................10

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1. Introduction

   Cryptographically Generated Addresses (CGA, [RFC3972]) provide means
   to verify the ownership of IPv6 addresses without requiring any
   security infrastructure such as a certification authority.

   CGAs were originally designed for SeND [RFC3971] and SeND is
   generally not used in the same environment as a Dynamic Host
   Configure Protocol for IPv6 (DHCPv6) [RFC3315] server. However, after
   CGA has been defined, as an independent security property, many other
   CGA usages have been proposed and defined, such as Site Multihoming
   by IPv6 Intermediation (SHIM6) [RFC5533], Enhanced Route Optimization
   for Mobile IPv6 [RFC4866], also using the CGA for DHCP security
   purpose [I-D.ietf-dhc-secure-dhcpv6], etc. The use of CGAs allows
   identity verification in different protocols. In these scenarios,
   CGAs may be used in DHCPv6-managed networks.

   As [I-D.ietf-csi-dhcpv6-cga-ps] analyses, in the current
   specifications, there is a lack of procedures to enable proper
   management of the usage of CGAs. Particularly, in a DHCPv6-managed
   network, a new DHCPv6 option is missed, therefore, the DHCPv6 server
   can NOT grant the use of host-generated CGA addresses on request from
   the client, or reject the CGA on the basis of a too-low sec value. In
   order to fill this gap, a new DHCPv6 option, CGA Grant Option, is
   defined in this document.

   This document also analyses the configuration of the parameters,
   which are used to generate CGAs, using DHCPv6. Although the document
   does not define new DHCPv6 option to carry these parameters for
   various reasons, the configuration procedure is described. The
   procedure works with existing options or future define options.

2. Terminology

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

3. CGA Configure Process Using DHCPv6

   The CGA specifications [RFC3972] define the procedure to generate a
   CGA. However, it assumes that hosts decide by itself or have been
   preconfigured all CGA relevant parameters. In reality, the network
   management MAY want to assign/enforcement some parameters to hosts;
   the network management MAY also manage the use of CGAs.

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   Among the mechanisms in which configuration parameters could be
   pushed to the end hosts and/or CGA related information sent back to a
   central administration, we discuss the stateful configuration
   mechanism based on DCHPv6 in this document. Other mechanisms may also
   provide similar functions, but out of scope.

   In this section, configuration CGA parameters and that a DHCPv6
   server grants the CGA usage are described in details.

3.1. Configuration of the parameters required for the generation of CGA

   Each CGA is associated with a CGA Parameters data structure, which is
   formed by all input parameters [RFC3972] except for Sec value that is
   embedded in the CGA. The CGA associated Parameters used to generate a
   CGA includes:

     - a Public Key,

     - a Subnet Prefix,

     - a 3-bit security parameter, Sec. Additionally, it should be noted
     that the hash algorithm to be used in the generation of the CGA is
     also defined by the Sec value [RFC4982],

     - any Extension Fields that could be used.

     - Note: the modifier and the Collision Count value in the CGA
     Parameter data structure are generated during the CGA generation
     process. They do NOT need to be configured.

   In a DHCPv6 managed network, a host may initiate a request for the
   relevant CGA configuration information needed to the DHCPv6 server.
   The server responds with the configuration information for the host.
   The Option Request Option, defined in Section 22.7 in [RFC3315], can
   be used for host to indicate which options the client requests from
   the server. For response, the requested Option should be included.
   The server MAY also initiatively push these parameters by attaching
   these option in the response messages which are initiated for other

     - The Public/Private key pair is generated by hosts themselves and
     considered not suitable for network transmission for security
     reasons. The configuration of the client key pair or certificate is
     out of scope.

     - Currently, there are convenient mechanisms for allowing an
     administrator to configure the subnet prefix for a host, by Router

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     Advertisement [RFC4861, RFC4862]. However, this does not suitable
     for the DHCP-managed network. To propagate the prefix through DHCP
     interactions, DHCPv6 Prefix Delegation Option [RFC3633] MAY be
     used. However, this option was designed to assign prefix block for
     routers. A new Prefix Assignment Option MAY need to be defined.
     Since alternative approach is existing and there are debates
     whether a new Prefix Assignment Option MAY is necessary, this
     document does not define it.

     - Although the network management MAY want to enforce or configure
     a Sec value to the hosts, it is considered as a very dangerous
     action. A malicious fake server may send out a high Sec value to
     attack clients giving the fact that generation a CGA with a high
     Sec value is very computational intensive [I-D.ietf-csi-dhcpv6-cga-
     ps]. Another risk is that a malicious server could propagate a Sec
     value providing less protection than intended by the network
     administrator, facilitating a brute force attack against the hash,
     or the selection of the weakest hash algorithm available for CGA
     definition. A recommendation Sec value is considered as confusion
     information. The receiving host is lack for information to make
     choose whether generates a CGA according to the recommendation or
     not. Therefore, the document does not define a DHCPv6 option to
     propagate the Sec value.

     - Although there is an optional Extension Fields in CGA Parameter
     data structure, there is NO any defined extension fields. If in the
     future, new Extension Fields in CGA Parameter data structure are
     defined, future specification may define correspondent DHCPv6
     options to carry these parameters.

   Upon reception of the CGA relevant parameters from DHCPv6 server, the
   end hosts SHOULD generate addresses compliant with the received
   parameters. If the parameters change, the end hosts SHOULD generate
   new addresses compliant with the parameters propagated.

3.2. Host requests CGA Approved to the DHCPv6 server

   A CGA address is generated by the associated key pair owner, normally
   an end host. However, in a DHCPv6-managed network, hosts should use
   IPv6 global addresses only from a DHCPv6 server. The process
   described below allows a host, also DHCPv6 client, uses self-
   generated CGAs in a DHCPv6-managed environment, by requesting the
   granting from a DHCPv6 server.

   The client sends a CGA, which is generated by itself, to a DHCPv6
   server, and requests the DHCP server to determine whether the
   generated CGA satisfies the requirements of the network

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   configuration, wherein the network configuration comprises a CGA
   security level set by the DHCP; and generates a new CGA if the
   generated CGA does not satisfy the requirements of the network

   Client initiation behavior

   In details, a DHCPv6 client SHOULD send a DHCPv6 Request message to
   initiate the CGA granting process.

   This DHCPv6 Request message MUST include an Option Request option,
   which requests the CGA Grant Option, defined in Section 4 in this
   document, to indicate the DHCPv6 server responses with the address
   granting decision. The CGA_Grant field in the embedded CGA Grant
   Option should be set all 1 (FFx).

   The client MUST include one or more IA Options, either IA_NA or IA
   _TA, in the Request message. Each IA Option MUST include one or more
   IA Address Options. CGAs are carried in the IA Address Options.

   Server behavior

   Upon reception of the Request message, the DHCPv6 server SHOULD
   verify whether the client's CGAs satisfy the CGA-related
   configuration parameters of the network. The DHCPv6 server SHOULD NOT
   handle the Request which the CGA Grant field is not all 1(FFx). The
   DHCPv6 server then send an acknowledgement, a Reply message, to the
   client to either grant the use of the CGA or decline the requested
   CGA. The CGA_Grant field SHOULD be set following the rule, defined in
   Section 4 in this document. When the requested CGA is declined, the
   DHCPv6 server MAY also recommend a Sec value to the client a using
   the CGA Grant option.

   In the meantime, the DHCPv6 server MAY log the requested CGA
   addresses. This information MAY later be used by other network
   functions, such as ACL.

   Client receiving behavior

   Upon reception of the acknowledgement from server, the client can
   legally use the granted CGAs. The client SHOULD silently drop any
   message that has the CGA Grant field set any other value, but F0x,
   00x~07x. If the server declines the requested CGA, the client MUST
   generate a new CGA. If the server replies with CGA-relevant
   parameters, the client MAY generate a new CGA accordingly.

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4. CGA Grant Option

   DHCPv6 CGA Grant Option is used to indicate the DHCPv6 client whether
   the requested address is granted or not. In the decline case, a
   recommended Sec value MAY be sent, too.

    0                  1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |       OPTION_ADDR_GRANT       |       option-len              |
   |   CGA Grant   |





       CGA Grant

         The CGA_Grant field sets all 1 (FFx) when a client requests
         granting from server. It sets F0x to indicate that the
         requested CGA is granted; it sets 00x to indicate that the
         requested Address is declined without any recommended Sec
         value. It sets 01x~07x to indicate that requested Address is
         declined and the recommended Sec value (value from 1~7).

   Note: On receiving the CGA Grant Option with reject information and
   recommended Sec value, the client MAY generate a new CGA with the
   recommended Sec value. If choosing not use the recommended Sec
   value, the client MAY take the risk that it is not able to use full
   network capabilities.

5. Security Considerations

   The mechanisms based on DHCPv6 are all vulnerable to attacks to the
   DHCP client. Proper use of DHCPv6 autoconfiguration facilities
   [RFC3315], such as AUTH option or Secure DHCP [I-D.ietf-dhc-secure-
   dhcpv6] can prevent these threats, provided that a configuration
   token is known to both the client and the server.

   Note that, as expected, it is not possible to provide secure
   configuration of CGA without a previous configuration of security

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   information at the client (either a trust anchor, a DHCPv6
   configuration token...). However, considering that the values of
   these elements could be shared by the hosts in the network segment,
   these security elements can be configured more easily in the end
   hosts than its addresses.

6. IANA Considerations

   This document defines two new DHCPv6 [RFC3315] options, which must be
   assigned Option Type values within the option numbering space for
   DHCPv6 messages:

   The DHCPv6 CGA Grant Option (TBA1), described in Section 4.

7. Acknowledgments

   The authors would like to thank Marcelo Bagnulo Braun and Alberto
   Garcia-Martinez from Universidad Carlos III de Madrid for been
   involved in the early requirement identification. Valuable comments
   from Bernie Volz, Ted Lemon, John Jason Brzozowski and Dujuan Gu,
   Huawei are appreciated.

8. Change Log [RFC Editor please remove]

   draft-jiang-dhc-cga-config-dhcpv6-02, remove Sec option according to
   IETF 79 discussion, 2010-11-19.

   draft-jiang-dhc-cga-config-dhcpv6-01, remove CGA generation
   delegation according to IETF 77 and mail list discussion, 2010-08-24.

   draft-jiang-dhc-cga-config-dhcpv6-00, original version, 2010-02-03.

9. References

9.1. Normative References

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

   [RFC3315] R. Droms, Ed., "Dynamic Host Configure Protocol for IPv6",
             RFC3315, July 2003.

   [RFC3633] O. Troan and R. Droms, "IPv6 Prefix Options for Dynamic
             Host Configuration Protocol (DHCP) version 6", RFC 3633,
             December 2003.

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

   [RFC3972] T. Aura, "Cryptographically Generated Address", RFC3972,
             March 2005.

   [RFC4861] T. Narten, et al., "Neighbor Discovery for IP version 6
             (IPv6)", RFC 4861, September 2007.

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

   [RFC4866] J. Arkko, C. Vogt and W. Haddad, "Enhanced Route
             Optimization for Mobile IPv6", RFC4866, May 2007.

   [RFC4982] M. Bagnulo, "Support for Multiple Hash Algorithms in
             Cryptographically Generated Addresses (CGAs) ", RFC4982,
             July 2007.

   [RFC5533] E. Nordmark and M. Bagnulo, "Shim6: Level 3 Multihoming
             Shim Protocol for IPv6" FRC 5533, June 2009.

9.2. Informative References

             S. Jiang, S. Shen and T. Chown, "DHCPv6 and CGA
             Interaction: Problem Statement", draft-ietf-csi-dhcpv6-cga-
             ps (work in progress), October, 2010.

             S. Jiang and S. Shen, "Secure DHCPv6 Using CGAs", draft-
             ietf-dhc-secure-dhcpv6 (work in progress), June 2010.

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Author's Addresses

   Sheng Jiang
   Huawei Technologies Co., Ltd
   Huawei Building, No.3 Xinxi Rd.,
   Shang-Di Information Industry Base, Hai-Dian District, Beijing 100085
   P.R. China

   Sam(Zhongqi) Xia
   Huawei Technologies Co., Ltd
   Huawei Building, No.3 Xinxi Rd.,
   Shang-Di Information Industry Base, Hai-Dian District, Beijing 100085
   P.R. China

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