Analysis of Possible DHCPv6 and CGA Interactions
draft-ietf-csi-dhcpv6-cga-ps-08

Network Working Group                                      Sheng Jiang 
Internet Draft                            Huawei Technologies Co., Ltd 
Intended status: Informational                               Sean Shen 
Expires: September 16, 2012                                      CNNIC 
                                                             Tim Chown 
                                             University of Southampton 
                                                         March 8, 2012 
                                    

            Analysis of Possible DHCPv6 and CGA Interactions 
                                    
                  draft-ietf-csi-dhcpv6-cga-ps-08.txt 

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Abstract 

   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 ............................................ 8 
   10. Diff from last IESG review (2010-10) [RFC Editor please remove]9 
   11. References ................................................. 9 
      11.1. 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 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 solutions. 

   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 
      analyzed. 

 
 
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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 attendance. 

      - 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 value. 

      - 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 
   carry the CGA Parameters data structure and a digital signature. The 
   receiver can verify both the CGA and signature, then process the 
 
 
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   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 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 
 
 
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   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: 

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

9. Acknowledgements 

   Useful comments were made by Marcelo Bagnulo, Alberto Garcia, Ted 
   Lemon, Stephen Hanna, Russ Housley, Sean Turner, Tim Polk, David 

 
 
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   Harrington, Jari Arkko 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 
   concept. 

   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 
   operations. 

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. 
 
 
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   [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. 

    

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 
   Email: jiangsheng@huawei.com 
    
   Sean Shen 
   CNNIC 
   4, South 4th Street, Zhongguancun 
   Beijing 100190 
   P.R. China 
   Email: shenshuo@cnnic.cn 
    
   Tim Chown 
   University of Southampton 
   Highfield 
   Southampton, Hampshire SO17 1BJ 
   United Kingdom 
   Email: tjc@ecs.soton.ac.uk 

 
 
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