Network Working Group Sheng Jiang
Internet Draft Huawei Technologies Co., Ltd
Intended status: Informational Sean Shen
Expires: November 30, 2011 CNNIC
Tim Chown
University of Southampton
May 30, 2011
DHCPv6 and CGA Interaction: Problem Statement
draft-ietf-csi-dhcpv6-cga-ps-07.txt
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Abstract
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.
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
also discussed per parameters. Using CGA to protect DHCPv6 is
recommended though the concrete protocol is not defined in this
document. 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], also using the CGA for DHCPv6 security purpose, analyzed
in section 4 this document, etc. In these scenarios, CGAs may be used
in DHCPv6-managed networks.
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A CGA address is generated by a host that owns the associated key
pair. However, in a DHCPv6-managed network, hosts should get their
addresses from DHCPv6 servers. This may result that a DHCPv6-managed
network declines the network access requests from CGA owners.
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 requested CGA and reply
suggestion in order to generate a new address accordingly.
New DHCPv6 options may be defined to support DHCPv6 servers to
decline the requested CGA, notify the hosts the reason and give
suggestion information in order to generate a new CGA accordingly.
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 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 that 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. Hosts
may generate such CGA in order to get access or use network services.
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. Central
managed/generated key is conflict with primary CGA concept.
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Therefore, there are no requirements for network to
configure/suggest it.
- 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, there are no requirements for network to
configure/suggest it.
- a Collision Count value. This is generated during the CGA
generation procedure. Therefore, there are no requirements for
network to configure/suggest it.
- 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
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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 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 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 may decline the DHCPv6 messages
from other servers, which may be fake servers. But in this case the
address will be fixed. It may increase the vulnerability to, e.g.,
brute force attacks. 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 to take such heavy burden,
too. 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
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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.
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 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. A few interactions has been declined by the analysis,
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 may be defined in
the future:
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- allowing DHCPv6 servers to decline the requested CGA and reply
suggestion in order to generate a new CGA accordingly,
- using DHCPv6 to propagate prefix to hosts,
- propagate the suggested Sec value to hosts,
- DHCPv6 servers, relays or clients use CGA addresses as source
addresses, also in DHCPv6 message exchanging.
7. Security Considerations
By allowing DHCPv6 servers to decline the requested CGA and reply
suggestion in order to generate a new CGA accordingly, is actually
increase the network access flexibility. This may also benefit the
network security too.
Prefix is information that should be advertised. However, the new
mechanism using DHCPv6 to propagate prefix to hosts gives attackers
another way to propagate bogus prefixes, which may waste hosts
resources. DHCPv6 snooping, DHCPv6 authentication and DHCPv6 server
using CGAs can help to prevent or discover bogus prefixes.
The suggested Sec value is only replied to the host when requested
CGA is declined by the DHCPv6 server. For security reasons, networks
should NOT enforce any CGA parameters. Otherwise, malicious attackers
may use this enforcement to attack hosts. Networks may suggest
certain CGA parameters, but host does not have to follow. However, if
the hosts not follow, they may not be able to access part or full
network 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.
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.
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, Jari
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Arkko, David Harrington 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
Huawei Building, No.3 Xinxi Rd.,
Hai-Dian District, Beijing 100085
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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