Network Working Group Sheng Jiang
Sean Shen
Internet Draft Huawei Technologies Co., Ltd
Expires: January 2009 July 4th, 2008
Secure DHCPv6 Using CGAs
draft-jiang-dhc-secure-dhcpv6-00.txt
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Abstract
The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) enables
DHCP servers to pass configuration parameters. It offers
configuration flexibility. If not secured, DHCPv6 is vulnerable to
various attacks particularly fake attack. This document analyzes
the security issues of DHCPv6 and specifies security mechanisms,
mainly using CGAs.
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Table of Contents
1. Introduction................................................2
2. Terminology.................................................3
3. Security Overview of DHCPv6..................................3
4. Secure DHCPv6 Overview.......................................4
4.1. New Components.........................................5
4.2. Support for algorithm agility...........................5
5. Secure DHCPv6 Options........................................5
5.1. CGA Parameter Option....................................5
5.2. Signature Option........................................6
6. Processing Rules and Behaviors...............................9
6.1. Processing Rules of Sender..............................9
6.2. Processing Rules of Receiver............................9
7. Security Considerations.....................................10
8. IANA Considerations........................................11
9. References.................................................12
9.1. Normative References...................................12
9.2. Informative References.................................12
Author's Addresses............................................13
Intellectual Property Statement................................13
Disclaimer of Validity........................................14
Copyright Statement...........................................14
1. Introduction
The Dynamic Host Configuration Protocol for IPv6 (DHCPv6 [RFC3315])
enables DHCP servers to pass configuration parameters. It offers
configuration flexibility. If not secured, DHCPv6 is vulnerable to
various attacks particularly fake attack.
This document analyzes the security issues of DHCPv6. This document
is aiming to provide mechanisms for improving the security of DHCPv6,
thus the address of a DHCP message sender, which can be a DHCP server,
a reply agent or a client, is able to be verified by a receiver. It
improves communication security of DHCPv6 interaction. The security
mechanisms specified in this document is mainly based on the
Cryptographically Generated Addresses (CGA [RFC3972]).
Secure DHCPv6 is applicable in environments where physical security
on the link is not assured (such as over wireless) or where available
security mechanisms are not sufficient, and attacks on DHCPv6 are a
concern.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Security Overview of DHCPv6
DHCPv6 is a client/server protocol that provides managed and stateful
configuration of devices. It enables DHCPv6 server to auto-configure
relevant network parameters on clients through the DHCPv6 message
exchanging mechanisms. In the basic DHCPv6 specifications [RFC3315],
security of DHCPv6 message can be improved in a few aspects.
In the basic DHCPv6 specifications, regular IPv6 addresses are used.
It is possible for a malicious attacker to use a fake address to
spoof or launch an attack.
"One attack specific to a DHCP client is the establishment of a
malicious server with the intent of providing incorrect configuration
information to the client. The motivation for doing so may be to
mount a 'man in the middle' attack that causes the client to
communicate with a malicious server instead of a valid server for
some service such as DNS or NTP. The malicious server may also mount
a denial of service attack through mis-configuration of the client
that causes all network communication from the client to fail."
[RFC3315]
"A DHCP client may also be subject to attack through the receipt of a
Reconfigure message from a malicious server that causes the client to
obtain incorrect configuration information from that server."
[RFC3315]
Fake servers can also provide clients with partially correct
information that allows the attacker to route traffic through certain
host where critical information can be collected. This becomes
important to detect and prevent when encrypted traffic is allowed to
pass through firewalls. Clients can be configured with bogus data, so
that they will assume that the network is down.
Once servers start updating DNS and other directory services,
attackers may spoof DHCP servers to register incorrect information in
those services.
Another possible attack is that attackers may be able to gain
unauthorized access to some resources, such as network access.
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The basic DHCPv6 specifications achieve message origin authentication
and message integrity via an authentication option with a symmetric
key pair. For the key of the hash function, there are two key
management mechanisms. Firstly, the key management is out of band,
usually manual, i.e. operators set up key database for both server
and client before running DHCPv6. Usually multiple keys are deployed
once a time and key id is used to specify which key is used. Secondly,
a DHCPv6 server sends a reconfigure key to the client in the initial
exchange of DHCPv6 messages for future use, in this case security is
not guaranteed because this key is transmitted in plaintext. In
either way, the security of key itself is in question mark.
Communication between a server and a relay agent, and communication
between relay agents, can be secured through the use of IPSec, as
described in section 21.1 in [RFC3315]. However, IPSec is quite
complicated. A simpler security mechanism may have better deploy
ability. Furthermore, the manual configuration and static keys are
potential issue makers. Relay agents may require other security
mechanisms besides IPSec.
4. Secure DHCPv6 Overview
To solve the abovementioned security issues, we introduce CGAs into
DHCPv6. CGAs are introduced in [RFC3972]. "CGAs are IPv6 addresses
for which the interface identifier is generated by computing a
cryptographic one-way hash function from a public key and auxiliary
parameters. The binding between the public key and the address can be
verified by re-computing the hash value and by comparing the hash
with the interface identifier. Messages sent from an IPv6 address can
be protected by attaching the public key and auxiliary parameters and
by signing the message with the corresponding private key. The
protection works without a certification authority or any security
infrastructure."
In this document, a CGA option with an address ownership proof
mechanism and a signature option with a corresponding verification
mechanism are introduced. With them, the receiver of a DHCP message
can verify the sender address of the DHCP message, which improves
communication security of DHCP messages. By using the signature
option, the verification of data integrity and replay protections can
also be achieved without the authentication option.
This documentation focuses on using CGAs to secure the DHCPv6
protocol. It assumes the sender, which uses CGAs, has self-generated
or been configured CGAs. The CGA configuration in the DHCPv6 network
is out of scope and specified in [CGA-config].
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4.1. New Components
The components of the solution specified in this document are as
follows:
- CGAs are used to make sure that the sender of a DHCPv6 message
is the "owner" of the claimed address. A public-private key
pair has been generated by a node itself or configured before
it can claim an address. A new DHCPv6 option, the CGA Parameter
Option, is used to carry the public key and associated
parameters.
- Public key signatures protect the integrity of the messages and
authenticate the identity of their sender. The authority of a
public key is established either with the authorization
delegation process, by using certificates, or through the
address ownership proof mechanism, by using CGAs, or with both.
4.2. Support for algorithm agility
Hash functions are the fundamental of security mechanisms, including
CGAs in this document. "...they have two security properties: to be
one way and collision free." "The recent attacks have demonstrated
that one of those security properties is not true."[RFC4270]
Following the approach recommended by [RFC4270] and [new-hashes], our
analysis shows none of these attacks are currently doable. However,
these attacks indicate the possibility of future real-world attacks.
Therefore, we have to take into account that future attacks will be
improved and provide a support for multiple hash algorithms. Our
mechanisms, in this document, support not only hash algorithm agility
but also signature algorithm agility.
The support for hash agility within CGAs has been defined in
[RFC4982]. The usage of CGAs in this document should also obey
[RFC4982], too.
5. Secure DHCPv6 Options
The options defined in this section MUST be supported, if the node
has been configured to use Secure DHCPv6.
5.1. CGA Parameter Option
The CGA option allows the verification of the sender's CGAs. The
format of the CGA option is described as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_CGA_PARAMETER | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. CGA Parameters (variable length) .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_CGA_PARAMETER (TBA1).
option-len Length of CGA Parameters in octets.
CGA Parameters A variable-length field containing the CGA
Parameters data structure described in Section 4
of [RFC3972]. This specification requires that
the public key found from the CGA Parameters
field in the CGA option MUST be that referred by
the Key Hash field in the Signature option.
Packets received with two different keys MUST be
silently discarded. Note that a future extension
may provide a mechanism allowing the owner of an
address and the signer to be different parties.
5.2. Signature Option
The Signature option allows public key-based signatures to be
attached to DHCPv6 messages. The Signature option SHOULD be the last
option in order to protect all DHCPv6 options. Any options after the
Signature option will be ignored. The format of the Signature option
is described as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_SIGNATURE | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HA-id | SA-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HA-id-KH | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (64-bit) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Key Hash (128-bit) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Signature (variable length) .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_SIGNATURE (TBA2).
option-len 32 + Length of signature field in octets.
HA-id Hash Algorithm id. The hash algorithm is used
for computing the signature result. RSA
signature [RSA] with SHA-1 [sha-1] is adopted.
In order to provide hash algorithm agility, SHA-
1 is assigned an initial value 0x0000 in this
document.
SA-id Signature Algorithm id. The signature algorithm
is used for computing the signature result. RSA
signature with RSASSA-PKCS1-v1_5 algorithm is
adopted. In order to provide algorithm agility,
RSASSA_PKCS1-v1_5 is assigned an initial value
0x0000 in this document.
HA-id-KH Hash Algorithm id for Key Hash. Hash algorithm
used for producing the Key Hash field in the
Signature option. SHA-1 is adopted. In order to
provide hash algorithm agility, SHA-1 is
assigned an initial value 0x0000 in this
document.
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Reserved A 16-bit field reserved for future use. The
value MUST be initialized to zero by the sender,
and MUST be ignored by the receiver.
Timestamp The current time of day (NTP-format timestamp
[RFC1305], a 64-bit unsigned fixed-point number,
in seconds relative to 0h on 1 January 1900.).
It can reduce the danger of replay attacks.
Key Hash A 128-bit field containing the most significant
(leftmost) 128 bits of a SHA-1 hash of the
public key used for constructing the signature.
The SHA-1 hash is taken over the presentation
used in the Public Key field of the CGA
Parameters data structure carried in the CGA
option. Its purpose is to associate the
signature to a particular key known by the
receiver. Such a key can either be stored in the
certificate cache of the receiver or be received
in the CGA option in the same message.
Signature A variable-length field containing a digital
signature. The signature value is computed with
the hash algorithm and the signature algorithm,
as described in HA-id and SA-id. The signature
constructed by using the sender's private key
over the following sequence of octets:
1. The 128-bit CGA Message Type tag value for
Secure DHCPv6, 0x81be a1eb 0021 ce7e caa9 4090
0665 d2e0 02c2. (The tag value has been
generated randomly by the editor of this
specification.).
2. The 128-bit Source Address field from the IP
header.
3. The 128-bit Destination Address field from
the IP header.
4. The DHCPv6 message header.
5. All DHCPv6 options preceding the Signature
option.
6. The content between the option-len field and
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the signature field in this Signature option, in
the format described above.
6. Processing Rules and Behaviors
6.1. Processing Rules of Sender
A DHCPv6 node, which could be a server, relay agent or client, can be
configured to send Secure DHCPv6 messages only if CGAs have been
configured on it.
The node MUST record the following configuration information:
CGA parameters Any information required to construct CGAs, as
described in [RFC3972].
Keypair A public-private key pair. The public key used
for constructing the signature MUST be the same
in CGA parameters.
CGA flag A flag that indicates whether CGA is used or not.
If a node has been configured to use Secure DHCPv6, the node MUST
send a message using a CGA, which be constructed as specified in
Section 4 of [RFC3972], as the source address unless they are sent
with the unspecified source address. In the message, both the CGA
option and the Signature option MUST be present in all DHCPv6
messages. The CGA Parameter field in the CGA option is filled
according to the rules presented above and in [RFC3972]. The public
key in the field is taken from the configuration used to generate the
CGA, typically from a data structure associated with the source
address. The Signature option MUST be constructed as explained in
Section 5.2 and be the last DHCPv6 option.
6.2. Processing Rules of Receiver
The node that supports the verification of the Secure DHCPv6 messages
MUST record the following configuration information:
Minbits The minimum acceptable key length for public
keys used in the generation of CGAs. The default
SHOULD be 1024 bits. Implementations MAY also
set an upper limit for the amount of computation
needed when verifying packets that use these
security associations. Any implementation should
follow prudent cryptographic practice in
determining the appropriate key lengths.
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On a node that has been configured to use Secure DHCPv6, DHCPv6
message without either the CGA option or the Signature option MUST be
treated as unsecured. Note the Secure DHCPv6 nodes MAY simply discard
the unsecured messages.
The receiving node MUST verify the source address of the packet by
using the algorithm described in Section 5 of [RFC3974]. The inputs
to the algorithm are the source address, as used in IP header, and
the CGA Parameters field.
If the CGA verification is successful, the recipient proceeds with a
more time-consuming cryptographic check of the signature. Note that
even if the CGA verification succeeds, no claims about the validity
of the use can be made until the signature has been checked.
The receiving node MUST verify the Signature option as follows: the
receiver MUST ignore any options that come after the Signature option.
The Key Hash field MUST indicate the use of a known public key,
either one learned from a preceding CGA option in the same message,
or one known by other means. The signature field verification MUST
show that the signature has been calculated as specified in the
previous section.
Only the messages that get through both CGA and signature
verifications are accepted as secured DHCPv6 messages and continue to
be handled for their contained DHCPv6 options.
Messages that do not pass all the above tests MUST be silently
discarded if the host has been configured to accept only secured
DHCPv6 messages. The messages MAY be accepted if the host has been
configured to accept both secured and unsecured messages but MUST be
treated as an unsecured message. The receiver MAY also otherwise
silently discard packets.
7. Security Considerations
This document provides new security features to the DHCPv6 protocol.
DHCPv6 nodes without CGAs or the DHCPv6 messages that use unspecific
addresses cannot be protected.
Downgrade attacks cannot be avoided if nodes are configured to accept
both secured and unsecured messages. A future specification may
provide a mechanism on how to treat unsecured DHCPv6 messages. One
simple solution may be that Secure DHCPv6 is mandated on all servers,
reply agents and clients if a certain link has been deployed Secure
DHCPv6.
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8. IANA Considerations
This document defines two new DHCPv6 [RFC3315] options, which must be
assigned Option Type values within the option numbering space for
DHCPv6 messages:
The CGA Parameter Option (TBA1), described in Section 5.1.
The Signature Option (TBA2), described in Section 5.2.
This document defines three new registries that have been created and
are maintained by IANA. Initial values for these registries are given
below. Future assignments are to be made through Standards Action
[RFC2434]. Assignments for each registry consist of a name, a value
and a RFC number where the registry is defined.
Hash Algorithm id(HA-id). The values in this name space are 16-bit
unsigned integers. The following initial values are assigned for HA-
id in this document:
Name | Value | RFCs
-------------------+---------+------------
SHA-1 | 0x0000 | this document
Signature Algorithm (SA-id). The values in this name space are 16-bit
unsigned integers. The following initial values are assigned for SA-
id in this document:
Name | Value | RFCs
-------------------+---------+------------
SHA-1 | 0x0000 | this document
Hash Algorithm id for Key Hash (HA-id-KH). The values in this name
space are 16-bit unsigned integers. The following initial values are
assigned for HA-id-KH in this document:
Name | Value | RFCs
-------------------+---------+------------
RSASSA-PKCS1-v1_5 | 0x0000 | this document
This document defines a new 128-bit value under the CGA Message Type
[RFC3972] namespace, 0x81be a1eb 0021 ce7e caa9 4090 0665 d2e0 02c2.
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9. References
9.1. Normative References
[RFC1305] D. Mills, "Network Time Protocol (Version 3) Specification,
Implementation and Analysis", RFC1305, March, 1992.
[RFC2462] S. Thomson, T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC2462, December 1998.
[RFC3315] R. Droms, et al., "Dynamic Host Configure Protocol for
IPv6", RFC3315, July 2003.
[RFC3972] T. Aura, "Cryptographically Generated Address", RFC3972,
March 2005.
[RFC4270] P. Hoffman, B. Schneier, "Attacks on Cryptographic Hashed
in Internet Protocols", RFC 4270, November 2005.
[RFC4982] M. Bagnulo, J. Arkko, "Support for Multiple Hash Algorithms
in Cryptographically Generated Addresses (CGAs)", RFC4982,
July 2007.
[CGA-Conf]S.Jiang, et al., "Requirements for configuring
Cryptographically Generated Addresses (CGA) and overview on
RA and DHCPv6-based solutions", working in progress.
[new-hashes] S.Bellovin, E. Rescorla, "Deploying a New Hash
Algorithm", November 2005.
[RSA] RSA Laboratories, "RSA Encryption Standard, Version 2.1",
PKCS 1, November 2002.
[sha-1] National Institute of Standards and Technology, "Secure
Hash Standard", FIBS PUB 180-1, April 1995,
<http://www.itl.nist.gov/fipspubs/fip180-1.htm>.
9.2. Informative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC2119, March 1997.
[RFC2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, April 1998.
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Author's Addresses
Sheng Jiang
Huawei Technologies Co., Ltd
QuiKe Building, No.9 Xinxi Rd.,
Shang-Di Information Industry Base,
Hai-Dian District, Beijing, P.R. China
100085
Phone: 86-10-82836774
Email: shengjiang@huawei.com
Sean Shen
Huawei Technologies Co., Ltd
QuiKe Building, No.9 Xinxi Rd.,
Shang-Di Information Industry Base,
Hai-Dian District, Beijing, P.R. China
100085
Phone: 86-10-82836072
Email: sshen@huawei.com
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