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
draft-ietf-csi-dhcpv6-cga-ps-09.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 ............................................ 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
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
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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
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.
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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
Email: jiangsheng@huawei.com
Sean Shen
CNNIC
4, South 4th Street, Zhongguancun
Beijing 100190
P.R. China
Email: shenshuo@cnnic.cn
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