DHC Working Group S. Jiang, Ed.
Internet-Draft Huawei Technologies Co., Ltd
Intended status: Standards Track S. Shen
Expires: December 21, 2014 CNNIC
D. Zhang
Huawei Technologies Co., Ltd
T. Jinmei
WIDE Project
June 19, 2014
Secure DHCPv6 with Public Key
draft-ietf-dhc-sedhcpv6-03
Abstract
The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) enables
DHCPv6 servers to pass configuration parameters. It offers
configuration flexibility. If not secured, DHCPv6 is vulnerable to
various attacks, particularly spoofing attacks. This document
analyzes the security issues of DHCPv6 and specifies a Secure DHCPv6
mechanism for communication between DHCPv6 clients and DHCPv6
servers. This mechanism is based on public/private key pairs. The
authority of the sender may depend on either pre-configuration
mechanism or Public Key Infrastructure.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 21, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language and Terminology . . . . . . . . . . . . 3
3. Security Overview of DHCPv6 . . . . . . . . . . . . . . . . . 3
4. Overview of Secure DHCPv6 Mechanism with Public Key . . . . . 4
4.1. New Components . . . . . . . . . . . . . . . . . . . . . 6
4.2. Support for algorithm agility . . . . . . . . . . . . . . 6
4.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 7
5. Extensions for Secure DHCPv6 . . . . . . . . . . . . . . . . 7
5.1. Public Key Option . . . . . . . . . . . . . . . . . . . . 7
5.2. Certificate Option . . . . . . . . . . . . . . . . . . . 8
5.3. Signature Option . . . . . . . . . . . . . . . . . . . . 9
5.4. Status Codes . . . . . . . . . . . . . . . . . . . . . . 10
6. Processing Rules and Behaviors . . . . . . . . . . . . . . . 11
6.1. Processing Rules of Sender . . . . . . . . . . . . . . . 11
6.2. Processing Rules of Recipient . . . . . . . . . . . . . . 12
6.3. Processing Rules of Relay Agent . . . . . . . . . . . . . 14
6.4. Timestamp Check . . . . . . . . . . . . . . . . . . . . . 14
7. Deployment Consideration . . . . . . . . . . . . . . . . . . 16
7.1. Authentication on a client . . . . . . . . . . . . . . . 16
7.2. Authentication on a server . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
11. Change log [RFC Editor: Please remove] . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
12.1. Normative References . . . . . . . . . . . . . . . . . . 21
12.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
The Dynamic Host Configuration ProtocoFl for IPv6 (DHCPv6, [RFC3315])
enables DHCPv6 servers to pass configuration parameters. It offers
configuration flexibility. If not secured, DHCPv6 is vulnerable to
various attacks, particularly spoofing attacks.
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This document analyzes the security issues of DHCPv6 in details.
This document provides mechanisms for improving the security of
DHCPv6 between client and server:
o the identity of a DHCPv6 message sender, which can be a DHCPv6
server or a client, can be verified by a recipient.
o the integrity of DHCPv6 messages can be checked by the recipient
of the message.
o anti-replay protection based on timestamp checking.
Note: this secure mechanism in this document does not protect the
relay-relevant options, either added by a relay agent toward a server
or added by a server toward a relay agent, are considered less
vulnerable, because they are only transported within operator
networks. Communication between a server and a relay agent, and
communication between relay agents, may be secured through the use of
IPsec, as described in section 21.1 in [RFC3315].
The security mechanisms specified in this document is based on self-
generated public/private key pairs. It also integrates timestamps
for anti-replay. The authentication procedure defined in this
document may depend on either deployed Public Key Infrastructure
(PKI, [RFC5280]) or pre-configured sender's public key. However, the
deployment of PKI or pre-configuration is out of the scope.
Secure DHCPv6 is applicable in environments where physical security
on the link is not assured (such as over wireless) and attacks on
DHCPv6 are a concern.
2. Requirements Language and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119] when they appear in ALL CAPS. When these words are not in
ALL CAPS (such as "should" or "Should"), they have their usual
English meanings, and are not to be interpreted as [RFC2119] key
words.
3. Security Overview of DHCPv6
DHCPv6 is a client/server protocol that provides managed
configuration of devices. It enables DHCPv6 server to automatically
configure relevant network parameters on clients. In the basic
DHCPv6 specification [RFC3315], security of DHCPv6 message can be
improved.
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The basic DHCPv6 specifications can optionally authenticate the
origin of messages and validate the integrity of messages using an
authentication option with a symmetric key pair. [RFC3315] relies on
pre-established secret keys. For any kind of meaningful security,
each DHCPv6 client would need to be configured with its own secret
key; [RFC3315] provides no mechanism for doing this.
For the key of the hash function, there are two key management
mechanisms. Firstly, the key management is done out of band, usually
through some manual process. For example, operators can set up a key
database for both servers and clients which the client obtains a key
before running DHCPv6.
Manual key distribution runs counter to the goal of minimizing the
configuration data needed at each host.
[RFC3315] provides an additional mechanism for preventing off-network
timing attacks using the Reconfigure message: the Reconfigure Key
authentication method. However, this method provides no message
integrity or source integrity check. This key is transmitted in
plaintext.
In comparison, the public/private key security mechanism allows the
keys to be generated by the sender, and allows the public key
database on the recipient to be populated opportunistically or
manually, depending on the degree of confidence desired in a specific
application. PKI security mechanism is simpler in the local key
management respect.
4. Overview of Secure DHCPv6 Mechanism with Public Key
In order to enable a DHCPv6 client and a server mutually authenticate
each other without previous key deployment, this document introduces
the use of public/private key pair mechanism into DHCPv6, also with
timestamp. The authority of the sender may depend on either pre-
configuration mechanism or PKI. By combining with the signatures,
sender identity can be verified and messages protected.
This document introduces a Secure DHCPv6 mechanism that uses a
public/private key pair to secure the DHCPv6 protocol. In order to
enable DHCPv6 clients and DHCPv6 servers to perform mutual
authentication, this solution provides two public key based
mechanisms with different security strengths. One is stronger and
only the certificate signed by a trusted CA or preconfigured public
key can be accepted. The other one, called as leap of faith (LoF)
mechanism, is relatively weak. It allows a client/server pair that
lacks essential trust relationship to build up their trust
relationship at run time for subsequent exchanges based on faith.
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This design simplifies the precondition of deploying DHCPv6
authentication and provides limited protection of DHCPv6 message.
In the proposed solution, either public/private key pairs or
certificates can be used in authentication. When using public/
private key pairs directly, the public key of the sender is pre-
shared with the recipient, either opportunistically or through a
manual process. When using certificates, the sender has a
certificate for its public key, signed by a CA that is trusted by the
recipient. It is possible for the same public key to be used with
different recipients in both modes.
In this document, we introduce a public key option, a certificate
option and a signature option with a corresponding verification
mechanism. Timestamp is integrated into signature options. A DHCPv6
message (from a server or a client), with either a public key or
certificate option, and carrying a digital signature, can be verified
by the recipient for both the timestamp and authentication, then
process the payload of the DHCPv6 message only if the validation is
successful. Because the sender can be a DHCPv6 server or a client,
the end-to-end security protection can be from DHCPv6 servers to
clients or from clients to DHCPv6 servers.
The recipient may choose to further process the message from a sender
for which no authentication information exists, either non-matched
public key or certificate cannot be verified. By recording the
public key or unverifiable certificate that was used by the sender,
when the first time it is seen, the recipient can make a leap of
faith that the sender is trustworthy. If no evidence to the contrary
surfaces, the recipient can then validate the sender as trustworthy
when it subsequently sees the same public key or certificate used to
sign messages from the same sender. In opposite, once the recipient
has determined that it is being attacked, it can either forget that
sender, or remember that sender in a blacklist and drop further
packets associated with that sender.
This improves communication security of DHCPv6 messages.
Secure DHCPv6 messages are commonly large. IP fragments [RFC2460]
are highly possible. Hence, deployment of Secure DHCPv6 should also
consider the issues of IP fragment, PMTU, etc. Also, if there are
firewalls between secure DHCPv6 clients and secure DHCPv6 servers, it
is RECOMMENDED that the firewalls are configureed to pass ICMP Packet
Too Big messages [RFC4443].
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4.1. New Components
The components of the solution specified in this document are as
follows:
o The node generates a public/private key pair. A DHCPv6 option is
defined that carries the public key.
The node may also obtain a certificate from a Certificate
Authority that can be used to establish the trustworthiness of the
node. Another option is defined to carry the certificate.
Because the certificate contains the public key, there is never a
need to send both options at the same time.
o A signature generated using the private key that protects the
integrity of the DHCPv6 messages and authenticates the identity of
the sender.
o A timestamp, to detect and prevent packet replay. The secure
DHCPv6 nodes need to meet some accuracy requirements and be synced
to global time, while the timestamp checking mechanism allows a
configurable time value for clock drift. The real time provision
is out of scope.
4.2. Support for algorithm agility
Hash functions are used to provide message integrity checks. In
order to provide a means of addressing problems that may emerge in
the future with existing hash algorithms, as recommended in
[RFC4270], this document provides a mechanism for negotiating the use
of more secure hashes in the future.
In addition to hash algorithm agility, this document also provides a
mechanism for signature algorithm agility.
The support for algorithm agility in this document is mainly a
unilateral notification mechanism from sender to recipient. A
recipient MAY support various algorithms simultaneously, and the
differenet senders in a same administrative domain may be allowed to
use various algorithms simultaneously.
If the recipient does not support the algorithm used by the sender,
it cannot authenticate the message. In the client-to-server case,
the server SHOULD reply with a AlgorithmNotSupported status code
(defined in Section 5.4). Upon receiving this status code, the
client MAY resend the message protected with the mandatory algorithm
(defined in Section 5.3).
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4.3. Applicability
By default, a secure DHCPv6 enabled client SHOULD start with secure
mode by sending secure DHCPv6 messages. If the recipient is secure
DHCPv6 enabled server, their communication would be in secure mode.
In the scenario where the secure DHCPv6 enabled client and server
fail to build up secure communication between them, the secure DHCPv6
enabled client MAY choose to send unsecured DHCPv6 message towards
the server.
A secure DHCPv6 enabled server MAY also provide services for
unsecured clients. In such case, the resources allocated for
unsecured clients SHOULD be separated and restricted, in order to
protect against bidding down attacks.
In the scenario where the recipient is a legacy DHCPv6 server that
does not support secure mechanism, the DHCPv6 server (for all of
known DHCPv6 implementations) would just omit or disregard unknown
options (secure options defined in this document) and still process
the known options. The reply message would be unsecured, of course.
It is up to the local policy of the client whether to accept the
messages. If the client accepts the unsecured messages from the
DHCPv6 server, the subsequent exchanges will be in the unsecured
mode.
In the scenario where a legacy client sends an unsecured message to a
secure DHCPv6 enabled server, there are two possibilities depending
on the server policy. If the server's policy requires the
authentication, an UnspecFail (value 1, [RFC3315]) error status code,
SHOULD be returned. In such case, the client cannot build up the
connection with the server. If the server has been configured to
support unsecured clients, the server would fall back to the
unsecured DHCPv6 mode, and reply unsecured messages toward the
client. The resources allocated for unsecured clients SHOULD be
separated and restricted.
5. Extensions for Secure DHCPv6
This section extends DHCPv6. Three new options have been defined.
The new options MUST be supported in the Secure DHCPv6 message
exchange.
5.1. Public Key Option
The Public Key option carries the public key of the sender. The
format of the Public Key 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_PK_PARAMETER | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Public Key (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_PK_PARAMETER (TBA1).
option-len Length of public key in octets.
Public Key A variable-length field containing public key and
identify the algorithm with which the key is used
(e.g., RSA, DSA, or Diffie-Hellman). The algorithm
is identified using the AlgorithmIdentifier structure
specified in section 4.1.1.2, [RFC5280]. The object
identifiers for the supported algorithms and the
methods for encoding the public key materials
(public key and parameters) are specified in
[RFC3279], [RFC4055], and [RFC4491].
5.2. Certificate Option
The Certificate option carries the certificate of the sender. The
format of the Certificate option is described as follows:
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_CERT_PARAMETER | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Certificate (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_CERT_PARAMETER (TBA2).
option-len Length of certificate in octets.
Certificate A variable-length field containing certificate. The
encoding of certificate and certificate data MUST
be in format as defined in Section 3.6, [RFC5996].
The support of X.509 certificate is mandatory. The
length of a certificate is various.
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5.3. Signature Option
The Signature option allows public key-based signatures to be
attached to a DHCPv6 message. The Signature option could be any
place within the DHCPv6 message. It protects the entire DHCPv6
header and options, including itself, except for the Authentication
Option. The format of the Signature option is described as follows:
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 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Timestamp (64-bit) |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. Signature (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_SIGNATURE (TBA3).
option-len 10 + Length of Signature field in octets.
HA-id Hash Algorithm id. The hash algorithm is used for
computing the signature result. This design is
adopted in order to provide hash algorithm agility.
The value is from the Hash Algorithm for Secure
DHCPv6 registry in IANA. The support of SHA-256 is
mandatory. A registry of the initial assigned values
is defined in Section 8.
SA-id Signature Algorithm id. The signature algorithm is
used for computing the signature result. This
design is adopted in order to provide signature
algorithm agility. The value is from the Signature
Algorithm for Secure DHCPv6 registry in IANA. The
support of RSASSA-PKCS1-v1_5 is mandatory. A
registry of the initial assigned values is defined
in Section 8.
Timestamp The current time of day (NTP-format timestamp
[RFC5905] in UTC (Coordinated Universal Time), a
64-bit unsigned fixed-point number, in seconds
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relative to 0h on 1 January 1900.). It can reduce
the danger of replay attacks.
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
protects the following sequence of octets:
1. The DHCPv6 message header.
2. All DHCPv6 options including the Signature
option (fill the signature field with zeroes)
except for the Authentication Option.
The signature filed MUST be padded, with all 0, to
the next octet boundary if its size is not an even
multiple of 8 bits. The padding length depends on
the signature algorithm, which is indicated in the
SA-id field.
Note: if both signature and authentication option are presented,
signature option does not protect the Authentication Option. It
allows to be created after signature has been calculated and filled
with the valid signature. It is because both needs to apply hash
algorithm to whole message, so there must be a clear order and there
could be only one last-created option. In order to avoid update
[RFC3315] because of changing auth option, the authors chose not
include authentication option in the signature.
5.4. Status Codes
o AlgorithmNotSupported (TBD4): indicates that the DHCPv6 server
does not support algorithms that sender used.
o AuthFailNotSupportLoF (TBD5): indicates that the DHCPv6 client
fails authentication check and the DHCPv6 server does not support
the leaf of faith mode
o AuthFailSupportLoF (TBD6): indicates that the DHCPv6 client fails
authentication check. Although the DHCPv6 server does support the
leaf of faith, its list that stores public keys or unverifiable
certificates in the leap of faith mode currently exceeds.
o TimestampFail (TBD7): indicates the message from DHCPv6 client
fails the timstamp check.
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o SignatureFail (TBD8): indicates the message from DHCPv6 client
fails the signature check.
6. Processing Rules and Behaviors
This section only covers the scenario where both DHCPv6 client and
DHCPv6 server are secure enabled.
6.1. Processing Rules of Sender
The sender of a Secure DHCPv6 message could be a DHCPv6 server or a
DHCPv6 client.
The node must have a public/private key pair in order to create
Secure DHCPv6 messages. The node may have a certificate which is
signed by a CA trusted by both sender and recipient.
To support secure DHCPv6, the secure DHCPv6 enabled sender MUST
construct the DHCPv6 message following the rules defined in
[RFC3315].
A Secure DHCPv6 message, except for Relay-forward and Relay-reply
messages, MUST contain either a Public Key or a Certificate option,
which MUST be constructed as explained in Section 5.1 or Section 5.2.
A Secure DHCPv6 message, except for Relay-forward and Relay-reply
messages, MUST contain one and only one Signature option, which MUST
be constructed as explained in Section 5.3. It protects the message
header and all DHCPv6 options except for the Authentication Option.
Within the Signature option the Timestamp field SHOULD be set to the
current time, according to sender's real time clock.
A Relay-forward and relay-reply message MUST NOT contain any
additional Public Key or Certificate option or Signature Option,
aside from those present in the innermost encapsulated message from
the client or server.
If the sender is a DHCPv6 client, in the failure cases, it receives a
Reply message with an error status code. The error status code
indicates the failure reason on the server side. According to the
received status code, the client MAY take follow-up action:
o Upon receiving a AlgorithmNotSupported error status code, the
client MAY resend the message protected with the mandatory
algorithms.
o Upon receiving an AuthFailNotSupportLoF error status code, the
client is not able to build up the secure communication with the
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recipient. The client MAY switch to other certificate or public
key if it has. But it SHOULD NOT retry with the same certificate/
public-key. It MAY retry with the same certificate/public-key
following normal retransmission routines defined in [RFC3315].
o Upon receiving an AuthFailSupportLoF error status code, the client
is not able to build up the secure communication with the
recipient. The client MAY switch to other certificate or public
key if it has. The client MAY retry with the same certificate/
public-key following normal retransmission routines defined in
[RFC3315].
o Upon receiving a TimestampFail error status code, the client MAY
fall back to unsecured mode.
o Upon receiving a SignatureFail error status code, the client MAY
resend the message following normal retransmission routines
defined in [RFC3315].
6.2. Processing Rules of Recipient
The recipient of a Secure DHCPv6 message could be a DHCPv6 server or
a DHCPv6 client. In the failure cases, both DHCPv6 server and client
SHOULD NOT process received message, and the server SHOULD reply a
correspondent error status code, while the client does nothing. The
specific behavior depends on the configured local policy.
When receiving a DHCPv6 message, except for Relay-Forward and Relay-
Reply messages, a secure DHCPv6 enabled recipient SHOULD discard the
DHCPv6 message if the Signature option is absent, or multiple
Signature option is presented, or both the Public Key and Certificate
options are absent, or both the Public Key and Certificate option are
presented. In such failure, the DHCPv6 server SHOULD reply an
UnspecFail (value 1, [RFC3315]) error status code. If all three
options are absent, the sender MAY be legacy node or in unsecured
mode, then, the recipient MAY fall back to the unsecured DHCPv6 mode
if its local policy allows.
The recipient SHOULD first check the support of algorithms that
sender used. If all algorithms are supported, the recipient then
checks the authority of this sender. If not, the message is dropped.
In such failure, the DHCPv6 server SHOULD reply a
AlgorithmNotSupported error status code, defined in Section 5.4, back
to the client.
If the sender uses certificate, the recipient SHOULD validate the
sender's certificate following the rules defined in [RFC5280]. An
implementation may create a local trust certificate record for a
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verified certificate in order to avoid repeated verification
procedure in the future. A sender certificate that finds a match in
the local trust certificate list is treated as verified. A fast
search index may be created for this list.
If the sender uses a public key, the recipient SHOULD validate it by
finding a matching public key from the local trust public key list,
which is pre-configured or recorded from previous communications. A
local trust public key list is a data table maintained by the
recipient. It restores public keys from all trustworthy senders. A
fast search index may be created for this list.
The recipient may choose to further process the message from a sender
for which no authentication information exists, either non-matched
public key or certificate cannot be verified. By recording the
public key or unverifiable certificate that was used by the sender,
when the first time it is seen, the recipient can make a leap of
faith (LoF) that the sender is trustworthy. If no evidence to the
contrary surfaces, the recipient can then validate the sender as
trustworthy for subsequent message exchanges. In opposite, once the
recipient has determined that it is being attacked, it can either
forget that key, or remember that key in a blacklist and drop further
packets associated with that key.
If recipient does not support the leap of faith mode, the message
that fails authentication check MUST be dropped. In such failure,
the DHCPv6 server SHOULD reply an AuthFailNotSupportLoF error status
code, defined in Section 5.4, back to the client.
On the recipient that supports the leap of faith mode, the number of
cached public keys or unverifiable certificates MAY be limited in
order to protect against resource exhaustion attacks. If the
recipient's list that stores public keys or unverifiable certificates
in the leap of faith mode exceeds, the message that fails
authentication check MUST be dropped. In such failure, the DHCPv6
server SHOULD reply an AuthFailNotSupportLoF error status code,
defined in Section 5.4, back to the client. The resource releasing
policy against exceeding situations is out of scope. Giving the
complexity, the key rollover mechanism is out of scope of this
document.
At this point, the recipient has either recognized the authentication
of the sender, or decided to attempt a leap of faith. The recipient
MUST now authenticate the sender by verifying the Signature and
checking timestamp (see details in Section 6.4). The order of two
procedures is left as an implementation decision. It is RECOMMENDED
to check timestamp first, because signature verification is much more
computationally expensive.
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The signature field verification MUST show that the signature has
been calculated as specified in Section 5.3. Only the messages that
get through both the signature verifications and timestamp check are
accepted as secured DHCPv6 messages and continue to be handled for
their contained DHCPv6 options as defined in [RFC3315]. Messages
that do not pass the above tests MUST be discarded or treated as
unsecured messages. In the case the recipient is DHCPv6 server, the
DHCPv6 server SHOULD reply a SignatureFail error status code, defined
in Section 5.4, for the signature verification failure, or a
TimestampFail error status code, defined in Section 5.4, for the
timestamp check failure, back to the client.
Furthermore, the node that supports the verification of the Secure
DHCPv6 messages MAY record the following information:
Minbits The minimum acceptable key length for public keys. An upper
limit MAY also be set for the amount of computation needed when
verifying packets that use these security associations. The
appropriate lengths SHOULD be set according to the signature
algorithm and also following prudent cryptographic practice. For
example, minimum length 1024 and upper limit 2048 may be used for
RSA [RSA].
A Relay-forward or Relay-reply message with any Public Key,
Certificate or the Signature option is invalid. The message MUST be
discarded silently.
6.3. Processing Rules of Relay Agent
To support Secure DHCPv6, relay agents just need to follow the same
processing rules defined in [RFC3315]. There is nothing more the
relay agents have to do, either verify the messages from client or
server, or add any secure DHCPv6 options. Actually, by definition in
this document, relay agents SHOULD NOT add any secure DHCPv6 options.
6.4. Timestamp Check
Recipients SHOULD be configured with an allowed timestamp Delta
value, a "fuzz factor" for comparisons, and an allowed clock drift
parameter. The recommended default value for the allowed Delta is
300 seconds (5 minutes); for fuzz factor 1 second; and for clock
drift, 0.01 second.
Note: the Timestamp mechanism is based on the assumption that
communication peers have roughly synchronized clocks, with certain
allowed clock drift. So, accurate clock is not necessary. If one
has a clock too far from the current time, the timestamp mechanism
would not work.
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To facilitate timestamp checking, each recipient SHOULD store the
following information for each sender, from which at least one
accepted secure DHCPv6 message is successfully verified (for both
timestamp check and signature verification):
o The receive time of the last received and accepted DHCPv6 message.
This is called RDlast.
o The timestamp in the last received and accepted DHCPv6 message.
This is called TSlast.
A verified (for both timestamp check and signature verification)
secure DHCPv6 message initiates the update of the above variables in
the recipient's record.
Recipients MUST check the Timestamp field as follows:
o When a message is received from a new peer (i.e., one that is not
stored in the cache), the received timestamp, TSnew, is checked,
and the message is accepted if the timestamp is recent enough to
the reception time of the packet, RDnew:
-Delta < (RDnew - TSnew) < +Delta
After the signature verification also successes, the RDnew and
TSnew values SHOULD be stored in the cache as RDlast and TSlast.
o When a message is received from a known peer (i.e., one that
already has an entry in the cache), the timestamp is checked
against the previously received Secure DHCPv6 message:
TSnew + fuzz > TSlast + (RDnew - RDlast) x (1 - drift) - fuzz
If this inequality does not hold or RDnew < RDlast, the recipient
SHOULD silently discard the message. If, on the other hand, the
inequality holds, the recipient SHOULD process the message.
Moreover, if the above inequality holds and TSnew > TSlast, the
recipient SHOULD update RDlast and TSlast after the signature
verification also successes. Otherwise, the recipient MUST NOT
update RDlast or TSlast.
An implementation MAY use some mechanism such as a timestamp cache to
strengthen resistance to replay attacks. When there is a very large
number of nodes on the same link, or when a cache filling attack is
in progress, it is possible that the cache holding the most recent
timestamp per sender will become full. In this case, the node MUST
remove some entries from the cache or refuse some new requested
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entries. The specific policy as to which entries are preferred over
others is left as an implementation decision.
7. Deployment Consideration
This document defines two levels of authentication: full
authentication based on certificate or pre-shared key verification
and weaker authentication based on leap-of-faith (LoF). As a
mechanism, both levels can be applied on servers and clients.
Depending on the details of expected threats and other constraints,
some cases may have limited applicability. This section discusses
such details.
7.1. Authentication on a client
For clients, DHCP authentication generally means authenticating the
server (the sender of DHCP messages) and verifying message integrity.
This is satisfied with full authentication. Due to the configuration
overhead, however, full authentication may not always be feasible.
It would still be viable in a controlled environment with skilled
staff, such as a corporate intranet.
If LoF is used, message integrity is provided but there is a chance
for the client to incorrectly trust a malicious server at the
beginning of the first session with the server (and therefore keep
trusting it thereafter). But LoF guarantees the subsequent messages
are sent by the same server that sent the public key, and therefore
narrows the attack scope. This may make sense if the network can be
reasonably considered secure and requesting pre-configuration is
deemed to be infeasible. A small home network would be an example of
such cases.
For environments that are neither controlled nor really trustworthy,
such as a network cafe, full authentication wouldn't be feasible due
to configuration overhead, while pure LoF, i.e. silently trusting the
server at the first time, would be too insecure. But some
middleground might be justified, such as requiring human intervention
at the point of LoF.
7.2. Authentication on a server
As for authentication on a server, there are several different
scenarios to consider, each of which has different applicability
issues.
A server may have to selectively serve a specific client or deny
specific clients depending on the identify of the client. This will
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require full authentication, since if the server allows LoF any
malicious user can pretend to be a new legitimate client. Also, the
use of certification wouldn't be feasible in this case, since it's
less likely for all such clients to have valid (and generally
different) certificates. So the applicable case may be limited, but
a controlled environment with skilled staff and a specifically
expected set of clients such as a corporate intranet may still find
it useful and viable.
A server can prevent an attack on the DHCP session with an existing
client from a malicious client, e.g., by sending a bogus Release
message: the server would remember the original client's public key
at the beginning of the DHCP session and authenticate subsequent
messages (and their sender). Neither full authentication nor LoF is
needed for this purpose, since the server does not have to trust the
public key itself. So this can be generally used for any usage of
DHCP.
A server can prevent an attack by a malicious client that pretends to
be a valid past client and tries to establish a new DHCP session
(whether this is a real security threat may be a subject of debate,
but this is probably at least annoying). This is similar to the
first scenario, but full authentication may not necessarily be
required; since the purpose is to confirm a returning client has the
same identify as a valid past client, the server only has to remember
the client's public key at the first time. So LoF can be used at the
risk of allowing a malicious client to mount this attack before the
initial session with a valid client. An uncontrolled, but reasonably
reliable network like a home network may use this defense with LoF.
8. Security Considerations
This document provides new security features to the DHCPv6 protocol.
Using public key based security mechanism and its verification
mechanism in DHCPv6 message exchanging provides the authentication
and data integrity protection. Timestamp mechanism provides anti-
replay function.
The Secure DHCPv6 mechanism is based on the pre-condition that the
recipient knows the public key of senders or the sender's certificate
can be verified through a trust CA. It prevents DHCPv6 server
spoofing. The clients may discard the DHCPv6 messages from unknown/
unverified servers, which may be fake servers; or may prefer DHCPv6
messages from known/verified servers over unsigned messages or
messages from unknown/unverified servers. The pre-configuration
operation also needs to be protected, which is out of scope. The
deployment of PKI is also out of scope.
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However, when a DHCPv6 client first encounters a new public key or a
new unverifiable certificate, it can make a leap of faith. If the
DHCPv6 server that used that public key or unverifiable certificate
is in fact legitimate, then all future communication with that DHCPv6
server can be protected by storing the public key or unverifiable
certificate. This does not provide complete security, but it limits
the opportunity to mount an attack on a specific DHCPv6 client to the
first time it communicates with a new DHCPv6 server. The number of
cached public keys or unverifiable certificates MUST be limited in
order to protect the DHCPv6 server against resource exhaustion
attacks.
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.
[RFC6273] has analyzed possible threats to the hash algorithms used
in SEND. Since the Secure DHCPv6 defined in this document uses the
same hash algorithms in similar way to SEND, analysis results could
be applied as well: current attacks on hash functions do not
constitute any practical threat to the digital signatures used in the
signature algorithm in the Secure DHCPv6.
A window of vulnerability for replay attacks exists until the
timestamp expires. Secure DHCPv6 nodes are protected against replay
attacks as long as they cache the state created by the message
containing the timestamp. The cached state allows the node to
protect itself against replayed messages. However, once the node
flushes the state for whatever reason, an attacker can re-create the
state by replaying an old message while the timestamp is still valid.
In addition, the effectiveness of timestamps is largely dependent
upon the accuracy of synchronization between communicating nodes.
However, how the two communicating nodes can be synchronized is out
of scope of this work.
Attacks against time synchronization protocols such as NTP [RFC5905]
may cause Secure DHCPv6 nodes to have an incorrect timestamp value.
This can be used to launch replay attacks, even outside the normal
window of vulnerability. To protect against these attacks, it is
recommended that Secure DHCPv6 nodes keep independently maintained
clocks or apply suitable security measures for the time
synchronization protocols.
9. IANA Considerations
This document defines three new DHCPv6 [RFC3315] options. The IANA
is requested to assign values for these three options from the DHCPv6
Option Codes table of the DHCPv6 Parameters registry maintained in
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http://www.iana.org/assignments/dhcpv6-parameters. The three options
are:
The Public Key Option (TBA1), described in Section 5.1.
The Certificate Option (TBA2), described in Section 5.2.
The Signature Option (TBA3), described in Section 5.3.
The IANA is also requested to add two new registry tables to the
DHCPv6 Parameters registry maintained in
http://www.iana.org/assignments/dhcpv6-parameters. The two tables
are the Hash Algorithm for Secure DHCPv6 table and the Signature
Algorithm for Secure DHCPv6 table.
Initial values for these registries are given below. Future
assignments are to be made through Standards Action [RFC5226].
Assignments for each registry consist of a name, a value and a RFC
number where the registry is defined.
Hash Algorithm for Secure DHCPv6. The values in this table are 8-bit
unsigned integers. The following initial values are assigned for
Hash Algorithm for Secure DHCPv6 in this document:
Name | Value | RFCs
-------------------+---------+--------------
SHA-1 | 0x01 | this document
SHA-256 | 0x02 | this document
SHA-512 | 0x03 | this document
Signature Algorithm for Secure DHCPv6. The values in this table are
8-bit unsigned integers. The following initial values are assigned
for Signature Algorithm for Secure DHCPv6 in this document:
Name | Value | RFCs
-------------------+---------+--------------
RSASSA-PKCS1-v1_5 | 0x01 | this document
IANA is requested to assign the following new DHCPv6 Status Codes,
defined in Section 5.4, in the DHCPv6 Parameters registry maintained
in http://www.iana.org/assignments/dhcpv6-parameters:
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Code | Name | Reference
---------+-----------------------+--------------
TBD4 | AlgorithmNotSupported | this document
TBD5 | AuthFailNotSupportLoF | this document
TBD6 | AuthFailSupportLoF | this document
TBD7 | TimestampFail | this document
TBD8 | SignatureFail | this document
10. Acknowledgments
The authors would like to thank Bernie Volz, Ted Lemon, Ralph Droms,
Jari Arkko, Sean Turner, Stephen Kent, Thomas Huth, David Schumacher,
Francis Dupont, Tomek Mrugalski, Gang Chen, Qi Sun, Suresh Krishnan,
Tatuya Jinmei and other members of the IETF DHC working groups for
their valuable comments.
This document was produced using the xml2rfc tool [RFC2629].
11. Change log [RFC Editor: Please remove]
draft-ietf-dhc-sedhcpv6-03: addressed comments from WGLC. Added a
new section "Deployment Consideration". Corrected the Public Key
Field in the Public Key Option. Added considation for large DHCPv6
message transmission. Added TimestampFail error code. Refined the
retransmission rules. 2014-06-18.
draft-ietf-dhc-sedhcpv6-02: addressed comments (applicability
statement, redesign the error codes and their logic) from IETF89 DHC
WG meeting and volunteer reviewers. 2014-04-14.
draft-ietf-dhc-sedhcpv6-01: addressed comments from IETF88 DHC WG
meeting. Moved Dacheng Zhang from acknowledgement to be co-author.
2014-02-14.
draft-ietf-dhc-sedhcpv6-00: adopted by DHC WG. 2013-11-19.
draft-jiang-dhc-sedhcpv6-02: removed protection between relay agent
and server due to complexity, following the comments from Ted Lemon,
Bernie Volz. 2013-10-16.
draft-jiang-dhc-sedhcpv6-01: update according to review comments from
Ted Lemon, Bernie Volz, Ralph Droms. Separated Public Key/
Certificate option into two options. Refined many detailed
processes. 2013-10-08.
draft-jiang-dhc-sedhcpv6-00: original version, this draft is a
replacement of draft-ietf-dhc-secure-dhcpv6, which reached IESG and
dead because of consideration regarding to CGA. The authors followed
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the suggestion from IESG making a general public key based mechanism.
2013-06-29.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use in
the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC 4055,
June 2005.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4491] Leontiev, S. and D. Shefanovski, "Using the GOST R
34.10-94, GOST R 34.10-2001, and GOST R 34.11-94
Algorithms with the Internet X.509 Public Key
Infrastructure Certificate and CRL Profile", RFC 4491, May
2006.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
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[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010.
12.2. Informative References
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC4270] Hoffman, P. and B. Schneier, "Attacks on Cryptographic
Hashes in Internet Protocols", RFC 4270, November 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6273] Kukec, A., Krishnan, S., and S. Jiang, "The Secure
Neighbor Discovery (SEND) Hash Threat Analysis", RFC 6273,
June 2011.
[RSA] RSA Laboratories, "RSA Encryption Standard, Version 2.1,
PKCS 1", November 2002.
Authors' Addresses
Sheng Jiang (editor)
Huawei Technologies Co., Ltd
Q14, Huawei Campus, No.156 Beiqing Road
Hai-Dian District, Beijing, 100095
P.R. China
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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Dacheng Zhang
Huawei Technologies Co., Ltd
Q14, Huawei Campus, No.156 Beiqing Road
Hai-Dian District, Beijing, 100095
P.R. China
Email: zhangdacheng@huawei.com
Tatuya Jinmei
WIDE Project
Japan
Email: jinmei@wide.ad.jp
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