Network Working Group                                      Sheng Jiang
Internet Draft                            Huawei Technologies Co., Ltd
Intended status: Informational                               Sean Shen
Expires: September 16, 2012                                      CNNIC
                                                        March 12, 2012

            Analysis of Possible DHCPv6 and CGA Interactions


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   This document analyzes the possible interactions between DHCPv6 and
   Cryptographically Generated Addresses (CGAs), and gives
   recommendations on whether or not these interactions should be
   developed as solutions.

Table of Contents

   1. Introduction ................................................ 3
   2. Coexistence of DHCPv6 and CGA ............................... 3
   3. Configuring CGA-relevant parameters using DHCPv6 ............ 4
   4. Using CGA to Protect DHCPv6 ................................. 5
   5. Computation Delegation of CGA generation .................... 6
   6. Conclusion .................................................. 7
   7. Security Considerations ..................................... 8
   8. IANA Considerations ......................................... 8
   9. Acknowledgements ............................................ 9
   10. Diff from last IESG review (2010-10) [RFC Editor please remove]
            ....................................................... 9
   11. References ................................................ 10
      11.1. Normative References ................................. 10
   Author's Addresses ............................................ 11

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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 also used when
   network administrators require more control over address assignments
   or management to hosts. DHCPv6 can also be used to distribute other
   network configuration information from network to hosts.

   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 analyzes the possible
   interactions between DHCPv6 and Cryptographically Generated
   Addresses (CGAs), and gives recommendations and reasons whether
   these possibilities should be developed as solutions or be declined
   in the future. This document itself does NOT define any concrete

   Firstly, the scenario of using CGAs in DHCPv6 environments is
   discussed. Then, configuring CGA-relevant parameters using DHCPv6 is
   discussed. Although CGA generation delegation is considered not
   suitable for DHCPv6, it is also analyzed. Security considerations
   for proposed interactions are examined.

2. Coexistence of DHCPv6 and CGA

   CGAs were designed for SeND [RFC3971]. 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 materials. 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], etc. In these scenarios, CGAs may be used in DHCPv6-
   managed networks.

   A CGA address is generated by a host that owns the associated key
   pair. However, hosts in DHCPv6-managed network get their addresses

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   from DHCPv6 servers. For a DHCPv6-managed network, CGA owners could
   be declined network access.

   Although the current DHCPv6 specification [RFC3315] has a mechanism
   that allow a host to request the assignment of a self-generated
   address from DHCPv6 servers, "DHCPv6 says nothing about details of
   temporary addresses like lifetimes, how clients use temporary
   addresses, rules for generating successive temporary addresses, etc."
   (quoted from Section 12 [RFC3315]. There is no existing operation to
   allow DHCPv6 servers to decline the host-requested address and to
   reply with information to generate a new address.

   New DHCPv6 options could be defined to allow DHCPv6 servers to
   decline requested-CGAs, to inform the host about why the address has
   been declined, and to give information needed to construct an
   acceptable CGA.

   Specifically, a node could 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 a
   suggested CGA-related configuration parameters of the network. In
   the meantime the DHCPv6 server may log the requested address/host
   combination, which completes CGA registration operation.

3. Configuring CGA-relevant parameters using DHCPv6

   In the current CGA specifications, it is not possible for network
   management to influence the CGA generation. Administrators may want
   to be able to configure or suggest parameters used to generate CGAs.
   For example, if a network only accepts the network access requests
   from hosts that use CGAs with Sec value 1 or higher for security
   reasons, this information should be able to propagate to hosts.

   The CGA associated Parameters used to generate a CGA includes
   several parameters [RFC3972]:

      - a Public Key. The key pair is generated by CGA owner. For
      security reasons, the key pair, more specifically the private key,
      should not be transported through networks. Centrally
      managed/generated key is conflict with primary CGA concept.
      Therefore, a mechanism to configure/suggest a value is not

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      - a Prefix. The prefix can be obtained though Router
      Advertisement messages of neighbor discovery protocol. DHCPv6 may
      provide another mechanism to propagate the prefix information to
      the host. This may enable the CGA usage scenarios without ND

      - a 3-bit security parameter Sec. It is possible that networks
      request hosts to use CGAs with high Sec value for secure access.
      However, it is dangerous to allow network to enforce hosts to
      generate new CGAs with high Sec value, particularly the
      generation with high Sec value is extremely computational
      consumption, as analyzed in Section 5. A reasonable compromise
      could be the network gives suggested information for Sec value,
      only when the access requests from host are declined for low Sec

      - a Modifier. This is generated during the CGA generation
      procedure. Therefore, a mechanism to configure/suggest a value is
      not analyzed.

      - a Collision Count value. This is generated during the CGA
      generation procedure. Therefore, a mechanism to configure/suggest
      a value is not analyzed.

      - any Extension Fields that could be used. So far, there is no
      concrete use case for this parameter. If new Extension Fields are
      defined in the future, whether they are suitable to network-
      managed configuration should be carefully analyzed based on the
      specific case.

4. Using CGA to Protect 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 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, a relay agent or a client) with a CGA as source address can

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   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 would
   allow 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 ownership 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. It improves the communication security of
   DHCPv6 interactions. The usage of CGAs combining with signature
   verification 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.

   The usage of CGAs can prove the source address ownership and provide
   data integrity protection. Furthermore, CGAs of DHCPv6 servers may
   be pre-notified to hosts. Then, hosts can decline the DHCPv6
   messages from other servers. But in this case the address will be
   fixed. It may increase the vulnerability to, e.g., brute force
   attacks against. The pre-notification operation also needs to be
   protected, which is out of scope.

5. Computation Delegation of CGA generation

   As analyzed in this section, CGA generation may be computationally
   intensive when Sec value is set to be high. However, DHCPv6 servers
   are normally not computationally powerful enough to also generate
   CGAs for all of its hosts.. Particularly, a DHCPv6 server is
   architecturally designed to serve thousands hosts simultaneously.

   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

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   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.

   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.

   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. This would be especially useful for large SEC values.

   However, DHCPv6 servers are not suitable to serve such computational
   delegation requests from thousands clients. Correspondently, the
   security analysis of CGA generation delegation and key generation
   delegation are out of scope.

6. Conclusion

   This document analyzed the possible interactions between CGA and
   DHCPv6. The analysis has determined that a few interactions are not
   worth pursuing including: enforcing CGA Sec value, using DHCPv6 to
   manage CGAs, using DHCPv6 to assign certificates or centrally
   generated key pair, using DHCPv6 for delegating CGA generation or
   key generation, etc.

   This document suggests a few possible interactions be investigated:

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      - allowing DHCPv6 servers to decline requested-CGAs and reply
      with Prefix or Sec values to generate an appropriate CGAs,

      - using CGA addresses for interactions between DHCPv6
      servers/relays and clients

7. Security Considerations

   Allowing DHCPv6 servers to decline the requested-CGA and reply with
   information to generate an appropriate CGA might actually increase
   network access flexibility. This might also benefit the network
   security too.

   Prefix is information that can be advertised. However, if DHCPv6
   propagates the prefix to hosts, then attackers have another way to
   propagate bogus prefixes. This can waste hosts' resources. DHCPv6
   snooping, DHCPv6 authentication and DHCPv6 server using CGAs can
   help to prevent or discover bogus prefixes.

   When propagating the Sec value from the DHCPv6 server to host, it is
   only returned if the DHCPv6 declines the requested-CGA. For security
   reasons, networks should not enforce any CGA parameters. Enforcing
   CGA parameters could allow malicious attackers to attack hosts by
   forcing them to perform computationally intensive operations.
   Networks can suggest the Sec value, but hosts need not heed the
   suggestion. However, if the hosts do not follow the suggestion, then
   the network might deny network services, including access services.

   Using CGA as source addresses of DHCPv6 servers, relays or, also in
   DHCPv6 message exchanging provides the source address ownership
   verification and data integrity protection.

   IF a DHCPv6 server rejected a client CGA based on a certain Sec
   value, it should not suggest a new Sec value either equal or lower
   than that rejected Sec value.

   Without other pre-configured security mechanism, like pre-notified
   DHCPv6 server address, using host-based CGA by DHCPv6 servers could
   not prevent attacks claiming to be a DHCPv6 server. Alternatively,
   IPsec may be used, but it is a heavier security mechanism.

8. IANA Considerations

   There are no IANA considerations in this document.

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9. Acknowledgements

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

10. Diff from last IESG review (2010-10) [RFC Editor please remove]

   Added CGA-DHCP co-existing scenarios, as the second paragraph in
   Section 2.

   Added the need for a DHCPv6 address registration operation in
   Section 2.

   Rewrite the CGA parameters configuration section. Analyze the
   requirements per CGA parameters.

   Added statement that network should not enforce the CGA parameters,
   but may suggest.

   Removed misleading words that linked CGA with PKI.

   Removed misleading words central managed CGA.

   Removed the combination of CGA and an external
   authentication/authorization, since it is conflict with primary CGA

   Removed the possible operations that DHCPv6 server may assign
   certificates or centrally generated key pair.

   Added statement that CGA generation delegation is not suitable for
   DHCPv6 servers.

   Added a conclusion section so that the message from this document is
   clearly summarized.

   Rewrite the security consideration section. Only focus on proposed

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11. References

11.1. 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.

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

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Author's Addresses

   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus
   No.156 Beiqing Road
   Hai-Dian District, Beijing  100095
   P.R. China
   Phone: 86-10-82882681

   Sean Shen
   4, South 4th Street, Zhongguancun
   Beijing 100190
   P.R. China

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