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Versions: 00 01 02 03 04 05 06 07 08 09                                 
Network Working Group                                      Sheng Jiang
Internet Draft                            Huawei Technologies Co., Ltd
Intended status: Informational                               Sean Shen
Expires: April 25, 2011                                          CNNIC
                                                             Tim Chown
                                             University of Southampton
                                                      October 21, 2010

             DHCPv6 and CGA Interaction: Problem Statement


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
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   This Internet-Draft will expire on April 25, 2011.

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   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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Internet-Draft   draft-ietf-csi-dhcpv6-cga-ps-06.txt      October 2010


   This document describes potential issues in the interaction between
   DHCPv6 and Cryptographically Generated Addresses (CGAs). Firstly, the
   scenario of using CGAs in DHCPv6 environments is discussed. Some
   operations are clarified for the interaction of DHCPv6 servers and
   CGA-associated hosts. We then also discuss how CGAs and DHCPv6 may
   have mutual benefits for each other, including using CGAs in DHCPv6
   operations to enhance its security features and using DHCPv6 to
   provide the CGA generation function.

   As an informational document, this document aims to analyze the
   possible interactions between CGAs and DHCPv6 from the functional
   perspective. This document does NOT propose/define any concrete
   solutions. Whether these possibilities are going to be defined as
   solutions or standards in the future is out of scope.

Table of Contents

   1. Introduction.................................................3
   2. Coexistence of DHCPv6 and CGA................................3
   3. What DHCPv6 can do for CGA...................................4
      3.1. Configuring CGA-relevant parameters using DHCPv6........4
      3.2. Computation Delegation of CGA generation using DHCPv6...5
   4. What CGA can do for DHCPv6...................................6
   5. Security Considerations......................................8
   6. IANA Considerations..........................................8
   7. Acknowledgements.............................................8
   8. Change Log [RFC Editor please remove]........................8
   9. References...................................................9
      9.1. Normative References....................................9
   Author's Addresses.............................................10

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1. Introduction

   The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315]
   can assign addresses statefully. Although there are other ways to
   assign IPv6 addresses [RFC4862, RFC5739], DHCPv6 is still useful when
   an administrator requires more control over address assignments to
   hosts. DHCPv6 can also be used to distribute other network
   configuration information.

   Cryptographically Generated Addresses (CGAs) [RFC3972] are IPv6
   addresses for which the interface identifiers are generated by
   computing a cryptographic one-way hash function from a public key and
   auxiliary parameters. Associated with public & private key pairs,
   CGAs are used in protocols, such as SEND [RFC3971] or SHIM6
   [RFC5533], to provide address validation and integrity protection in
   message exchanging.

   As an informational document, this document aims to analyze the
   possible interactions between CGAs and DHCPv6 from the functional
   perspective. This document does NOT propose/define any concrete
   solution. Whether these possibilities are going to be defined as
   solutions or standards in the future is out of scope.

   This document describes potential issues in the interaction between
   DHCPv6 and CGAs. Firstly, the scenario of using CGAs in DHCPv6
   environments is discussed. Some operations are clarified for the
   interaction of DHCPv6 servers and CGA-associated hosts. We then also
   discuss how CGAs and DHCPv6 may have mutual benefits for each other,
   including using CGAs in DHCPv6 operations to enhance its security
   features and using DHCPv6 to provide the CGA generation function.
   This document is designed to generate further discussion on the
   specifics of if/how the ideas in the document could be realized.

2. Coexistence of DHCPv6 and CGA

   CGAs can be used with IPv6 Stateless Address Configuration (SLAAC)
   [RFC4862]. The CGA-associated public key, which is also transported
   to the receiver, provides message origin validation and integrity
   protection without the need for negotiation and transportation of key

   CGAs were designed for SeND [RFC3971] and SeND is generally not used
   in the same environment as a DHCP server. However, after CGA has been
   defined, as an independent security property, many other CGA usages
   have been proposed and defined, such as SHIM6 [RFC5533], Enhanced
   Route Optimization for Mobile IPv6 [RFC4866], also using the CGA for

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   DHCP security purpose, analyzed in section 4 this document, etc. In
   these scenarios, CGAs may be used in DHCPv6-managed networks.

   In most cases, a CGA address is generated by the associated key pair
   owner, which normally is also the host that will use the CGA address,
   although the current CGA specifications do not consider or prohibit
   generation delegation. However, in a DHCPv6-managed network, hosts
   should use IPv6 global addresses only from DHCPv6 servers. This
   difference of roles needs to be carefully considered if there is a
   requirement to use CGAs in DHCPv6-managed environments.

   The current DHCPv6 specification [RFC3315] has a mechanism that could
   be used to allow a host to self-generate a CGA for use in a DHCPv6-
   managed environment, i.e. the DHCPv6 server can grant the use of
   host-generated CGA addresses on request from the client.

   Specifically, a node can request that a DHCPv6 server grants the use
   of a self-generated CGA by sending a DHCPv6 Request message. This
   DHCPv6 Request message contains an IA option including the CGA
   address. Depending on whether the CGA satisfies the CGA-related
   configuration parameters of the network, the DHCPv6 server can then
   send an acknowledgement to the node to either grant the use of the
   CGA or to indicate that the node must generate a new CGA with the
   correct CGA-related configuration parameters of the network. In the
   meantime the DHCPv6 server may log the requested address/host

3. What DHCPv6 can do for CGA

3.1. Configuring CGA-relevant parameters using DHCPv6

   In the current CGA specifications, it is not possible that network
   management to influence the CGA generation. Administrators may want
   to be able to configure parameters used to generate CGAs, for example
   to indicate hosts to use higher Sec-value CGAs. DHCPv6 could be used
   to assign subnet prefixes or other CGA-relevant parameters to CGA
   address owners. In some scenarios, the administrator may further want
   to enforce some parameters, in particular the necessary security-
   related parameters such as the SEC value.

   Depending on the scenario, the configuration information needed to
   generate CGAs (including a SEC value, a subnet prefix, a modifier, a
   public key, a Collision Count value and any Extension Fields) may be
   provided by either hosts or DHCPv6 servers. A DHCPv6 server might
   receive from hosts the configuration information customized by hosts,
   generate CGAs by using configuration information provided by both
   parties and deliver CGAs and their associated CGA Parameters data

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   structures to hosts. The details of such potential new methods need
   to be defined clearly in the solution specifications.

   When designing such solutions, the designer should thoroughly
   consider the impact on DHCPv6 model and the security of CGA usage. In
   order to be compatible with DHCPv6, the configuring procedure of CGA
   parameters should be compatible with the current DHCPv6 definition.
   When a DHCP6 server configures CGA parameters, integrity protection
   may be needed to avoid attacks, such as downgrade attack.

3.2. Computation Delegation of CGA generation using DHCPv6

   This document only analyzes the functional possibility of computation
   delegation of CGA generation using DHCPv6. Whether CGA generation
   could be delegated or whether DHCPv6 is the most suitable tool for
   computation delegation of CGA generation are primary out of scope.

   In the CGA generation procedure, the generation of the Modifier field
   of a CGA address is computationally intensive. This operation can
   lead to apparent slow performance and/or battery consumption problems
   for end hosts with limited computing ability and/or restricted
   battery power (e.g. mobile devices). As defined in [RFC3972], the
   modifier is a 128 unsigned integer that is selected so that the
   16*SEC leftmost bits of the second hash value, Hash2, are zero. The
   modifier is used during CGA generation to implement the hash
   extension and to enhance privacy by adding randomness to the CGA. The
   higher the number of bits required being 0, the more secure a CGA is
   against brute-force attacks. However, high number of bits also
   results in additional computational cost for the generation process,
   cost that could be deemed excessive. As an example, consider a Sec
   value equals 2, requesting the leftmost 32 bits of a SHA-1 Hash2 to
   be zero. For assuring this, a system has to generate in mean 2^32
   different modifiers, and perform the Hash2 operation to check the
   bits required to be 0. An estimation of the CPU power required to do
   this can be obtained as following: openSSL can perform in an Intel
   Core2-6300 on an Asus p5b-w motherboard close to 0.87 million of SHA-
   1 operations on 16 byte blocks per second. Since the input data of
   Hash2 operation is larger than 16 bytes, this value is an upper bound
   for the number of hash operations that can be performed for
   generating the modifier. Checking 2^32 different modifiers requires
   around 5000 seconds. A practice experimental on a platform with an
   Intel Duo2 (2.53GHz) workstation showed the results of average CGA
   generating time as below: when SEC=0, it took 100us; SEC=1, 60ms;
   SEC=2, 2000s (varies from 100~7000sec). The experiment was unable to
   be performed for SEC=3 or higher SEC values. Theoretically
   estimating, about 30000 hours are required to generate a SEC=3 CGA.

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   Sec was not designed as a way to burden current machines but to
   enable higher cost searches when machines get faster. That is, if
   generating a Sec>0 value is a big burden for our computers, host
   should probably still use Sec=0.

   A very low-power host might want to delegate its key and hash
   generation to a more general purpose computer.

   In such cases, a mechanism to delegate the computation of the
   modifier would be desirable. It is possible that the whole CGA
   generation procedure could be delegated to the DHCPv6 server. This
   would be especially useful for large SEC values.

   It may be possible to define a delegation operation that allows a
   client to pass computations to a DHCPv6 server, by introducing new
   DHCPv6 option(s). A node could thus initiate a DHCPv6 request to the
   DHCPv6 server requesting the computation of the Modifier or the CGA.
   The DHCPv6 server could then either compute the Modifier by itself,
   or redirect the computation requirement to another server. Once the
   DHCPv6 server generates (or obtains from the redirected computational
   server) the Modifier or the CGA address, it can respond to the node
   with the Modifier or the resulting address and the corresponding CGA
   Parameters data structure.

   Generating a key pair, which will be used to generate a CGA, also
   requires a notable computation, though this may only be issues on a
   very low-power host occasionally. Generation and distribution of a
   key pair can also be done by a DHCPv6 server. Of course, when
   designing these new functions, one should carefully consider the
   impact on security. However, the security considerations of specific
   solutions are out of scope of this document.

   New DHCPv6 options may be defined to support the interactions that
   are required when a DHCPv6 server generates a key pair for hosts.

4. What CGA can do for DHCPv6

   DHCPv6 is vulnerable to various attacks, e.g. fake address attacks
   where a 'rogue' DHCPv6 server responds with incorrect address
   information. A malicious rogue DHCPv6 server can also provide
   incorrect configuration to the client in order to divert the client
   to communicate with malicious services, like DNS or NTP. It may also
   mount a Denial of Service attack through mis-configuration of the
   client that causes all network communication from the client to fail.
   A rogue DHCPv6 server may also collect some critical information from

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   the client. Attackers may be able to gain unauthorized access to some
   resources, such as network access. See Section 23 [RFC3315].

   In the basic DHCPv6 specifications, regular IPv6 addresses are used.
   However, DHCPv6 servers, relay agents and clients could use host-
   based CGAs as their own addresses. A DHCPv6 message (from either a
   server, relay agent or client) with a CGA as source address can carry
   the CGA Parameters data structure and a digital signature. The
   receiver can verify both the CGA and signature, then process the
   payload of the DHCPv6 message only if the validation is successful. A
   CGA option with an address ownership proof mechanism and a signature
   option with a corresponding verification mechanism may be introduced
   into DHCPv6 protocol. With these two new options, the receiver of a
   DHCPv6 message can verify the sender address of the DHCPv6 message,
   which improves communication security of DHCPv6 messages. CGAs can be
   used for all DHCPv6 messages/processes as long as CGAs are available
   on the sender side.

   Using CGAs in DHCPv6 protocol can efficiently improve the security of
   DHCPv6. The address of a DHCPv6 message sender (which can be a DHCPv6
   server, a reply agent or a client) can be verified by a receiver.
   Also, the integrity of the sent data is provided if they are signed
   with the private key associated to the public key used to generate
   the CGA. The usage of CGA with pre-configured authorization, as
   introduced in next paragraph, can efficiently avoid the
   abovementioned attacks. It improves the communication security of
   DHCPv6 interactions. The usage of CGAs can also avoid DHCPv6's
   dependence on IPsec [RFC3315] in relay scenarios. This mechanism is
   applicable in environments where physical security on the link is not
   assured (such as over certain wireless infrastructures) or where
   available security mechanisms are not sufficient, and attacks on
   DHCPv6 are a concern.

   A CGA generated from an unauthorized public & private key pair can
   prove the source address ownership and provide data integrity
   protection. Furthermore, a CGA generated from a certified public &
   private key pair can also achieve authorization for DHCPv6 servers or
   relays, or on another direction, user authorization. The public keys
   may be pre-configured on both parties of communication or have a
   third party authority available for users to retrieve public keys.
   The public keys will be used for users to generate CGAs and verify
   CGAs and signatures. The pre-configuration can also include
   configuring more CGA parameters such as SEC value or more depending
   on policies. The pre-configuration can even be the whole CGA and
   related parameters, but in this case the address will be fixed. It
   may increase the vulnerability to, e.g., brute force attacks.

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5. Security Considerations

   As Section 4 of this document has discussed, CGAs can provide
   additional security features for DHCPv6. However, in defining
   solutions using DHCPv6 to configure CGAs, as suggested in Section 3
   of this document, careful consideration is required to evaluate
   whether the new mechanism introduces new security vulnerabilities.

   When DHCPv6 is used to manage CGAs, CGA relevant information is
   stored in a central repository, DHCPv6 server. It does not increase
   privacy risks. The CGA relevant information is only exposed to the
   network management plane. The privacy risks are not higher than other
   network managed entities, like normal IPv6 addresses managed by DHCP.
   Of course, the privacy risk is higher than using CGA with SLAAC, in
   which CGAs are fully host based.

   Without other pre-configured security mechanism, like PKI, using
   host-based CGA by DHCPv6 servers could not prevent attacks claiming
   to be a DHCPv6 server.

   This document does not contain a complete security analysis and any
   further work in this area should include such an analysis. Nobody
   should implement the techniques described in this document without
   conducting that more thorough analysis.

6. IANA Considerations

   There are no IANA considerations in this document.

7. Acknowledgements

   Useful comments were made by Marcelo Bagnulo, Alberto Garcia, Ted
   Lemon, Stephen Hanna, Russ Housley, Sean Turner, Tim Polk, Jari
   Arkko, David Harrington and other members of the IETF CSI working

8. Change Log [RFC Editor please remove]

   draft-jiang-csi-dhcpv6-cga-ps-00, original version, 2008-10-27

   draft-jiang-csi-dhcpv6-cga-ps-01, revised after comments at IETF 73,

   draft-jiang-csi-dhcpv6-cga-ps-02, revised after comments at CSI
   mailing list, 2009-06-17

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   draft-jiang-csi-dhcpv6-cga-ps-03, revised after comments at CSI
   mailing list, 2009-09-18

   draft-ietf-csi-dhcpv6-cga-ps-00, revised after comments at CSI
   mailing list and wg adoption call, 2009-10-12

   draft-ietf-csi-dhcpv6-cga-ps-01, revised after comments at IETF 76,

   draft-ietf-csi-dhcpv6-cga-ps-02, revised after comments received in
   CSI mail list, 2010-04-23

   draft-ietf-csi-dhcpv6-cga-ps-03, revised after comments received in
   CSI mail list, 2010-06-22

   draft-ietf-csi-dhcpv6-cga-ps-04, revised after comments received in
   CSI mail list, 2010-09-08

   draft-ietf-csi-dhcpv6-cga-ps-05, revised after IESG comments
   received, 2010-10-14

   draft-ietf-csi-dhcpv6-cga-ps-06, revised after IESG comments
   received, 2010-10-21

9. References

9.1. Normative References

   [RFC3315] R. Droms, Ed., J. Bound, B. Volz, T. Lemon, C. Perkins and
             M. Carney, "Dynamic Host Configure Protocol for IPv6", RFC
             3315, July 2003.

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

   [RFC3972] T. Aura, "Cryptographically Generated Address", RFC 3972,
             March 2005.

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

   [RFC4866] J. Arkko, C. Vogt and W. Haddad, "Enhanced Route
             Optimization for Mobile IPv6", RFC 4866, May 2007.

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

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Internet-Draft   draft-ietf-csi-dhcpv6-cga-ps-06.txt      October 2010

   [RFC5739] P. Eronen, J. Laganier, C. Madson, "IPv6 Configuration in
             Internet Key Exchange Protocol Version 2 (IKEv2)",
             RFC 5739, February 2010.

Author's Addresses

   Sheng Jiang
   Huawei Technologies Co., Ltd
   KuiKe Building, No.9 Xinxi Rd.,
   Shang-Di Information Industry Base, Hai-Dian District, Beijing 100085
   P.R. China
   Phone: 86-10-82836081
   Email: shengjiang@huawei.com

   Sean Shen
   4, South 4th Street, Zhongguancun
   Beijing 100190
   P.R. China
   Email: shenshuo@cnnic.cn

   Tim Chown
   University of Southampton
   Southampton, Hampshire SO17 1BJ
   United Kingdom
   Email: tjc@ecs.soton.ac.uk

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