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
draft-ietf-csi-dhcpv6-cga-ps-06.txt
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Abstract
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
materials.
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
combination.
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
group.
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,
2009-01-08
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,
2009-12-16
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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[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
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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