DHC Working Group S. Jiang, Ed.
Internet-Draft Huawei Technologies Co., Ltd
Intended status: Standards Track D. Zhang
Expires: December 26, 2015 June 24, 2015
Secure DHCPv4
draft-jiang-dhc-sedhcpv4-01
Abstract
The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) enables
DHCPv4 servers to pass configuration parameters. It offers
configuration flexibility. If not being secured, DHCPv4 is
vulnerable to various attacks, particularly spoofing attacks. This
document analyzes the security issues of DHCPv4 and specifies a
Secure DHCPv4 mechanism for communications between DHCPv4 clients and
servers. This document provides a DHCPv4 client/server
authentication mechanism based on sender's public/private key pairs
or certificates with associated private keys. The DHCPv4 message
exchanges are protected by the signature option and the timestamp
option newly defined in this document.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 26, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language and Terminology . . . . . . . . . . . . 3
3. Security Overview of DHCPv4 . . . . . . . . . . . . . . . . . 3
4. Overview of Secure DHCPv4 Mechanism with Public Key . . . . . 4
4.1. New Components . . . . . . . . . . . . . . . . . . . . . 5
4.2. Support for Algorithm Agility . . . . . . . . . . . . . . 6
4.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 6
5. Extensions for Secure DHCPv4 . . . . . . . . . . . . . . . . 7
5.1. Public Key Option . . . . . . . . . . . . . . . . . . . . 7
5.2. Certificate Option . . . . . . . . . . . . . . . . . . . 8
5.3. Signature Option . . . . . . . . . . . . . . . . . . . . 9
5.4. Timestamp Option . . . . . . . . . . . . . . . . . . . . 11
5.5. Status Codes . . . . . . . . . . . . . . . . . . . . . . 11
6. Processing Rules and Behaviors . . . . . . . . . . . . . . . 12
6.1. Processing Rules of Sender . . . . . . . . . . . . . . . 12
6.2. Processing Rules of Recipient . . . . . . . . . . . . . . 13
6.3. Processing Rules of Relay Agent . . . . . . . . . . . . . 16
6.4. Timestamp Check . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
10. Change log [RFC Editor: Please remove] . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction
The Dynamic Host Configuration Protocol version 4 (DHCPv4, [RFC2131])
enables DHCPv4 servers to pass configuration parameters and offers
configuration flexibility. If not being secured, DHCPv4 is
vulnerable to various attacks, particularly spoofing attacks.
This document analyzes the security issues of DHCPv4 in details.
This document provides mechanisms for improving the security of
DHCPv4 between client and server:
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o the identity of a DHCPv4 message sender, which can be a DHCPv4
server or a client, can be verified by a recipient.
o the integrity of DHCPv4 messages can be checked by the recipient
of the message.
o anti-replay protection based on timestamps.
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.
The security mechanisms specified in this document is based on
sender's public/private key pairs or certificates with associated
private keys. It also integrates message signatures for the
integrity and timestamps for anti-replay. The sender authentication
procedure using certificates defined in this document depends on
deployed Public Key Infrastructure (PKI, [RFC5280]). However, the
deployment of PKI is out of the scope of this document.
Secure DHCPv4 is applicable in environments where physical security
on the link is not assured (such as over wireless) and attacks on
DHCPv4 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 DHCPv4
DHCPv4 is a client/server protocol that provides managed
configuration of devices. It enables a DHCPv4 server to
automatically configure relevant network parameters on clients.
Although [RFC3118] provide an optional DHCPv4 authentication
mechanism. It depends on the client's key that is "initially
distributed to the client through some out-of-band mechanism", and
"the DHCPv4 server MUST know the keys for all authorized clients",
Section 5.4 of [RFC3118]. However, [RFC3118] does not provides no
mechanism for distributing the client's key.
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For the keyed hash function, there are two key management mechanisms.
The first one is a key management done out of band, usually through
some manual process. The second approach is to use Public Key
Infrastructure (PKI).
As an example of the first approach, operators can set up a key
database for both servers and clients from which the client obtains a
key before running DHCPv4. Manual key distribution runs counter to
the goal of minimizing the configuration data needed at each host.
In comparison, the security mechanism defined in this document allows
the public key database on the client or sever 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 DHCPv4 Mechanism with Public Key
This document introduces a mechanism that uses public key signatures
as a mechanism for securing the DHCPv4 protocol. In order to enable
DHCPv4 clients and servers to perform mutual authentication without
previous key deployment, this solution provides a DHCPv4 client/
server authentication mechanism based on public/private key pairs
and, optionally, PKI certificates. The purpose of this design is to
make it easier to deploy DHCPv4 authentication and provides
protection of DHCPv4 message within the scope of whatever trust
relationship exists for the particular key used to authenticate the
message.
In this document, we introduce a public key option, a certificate
option, a signature option and a timestamp option with corresponding
verification mechanisms. A DHCPv4 message can include a public key
option, and carrying a digital signature and a timestamp option. The
signature can be verified using the supplied public key. The
recipient processes the payload of the DHCPv4 message only if the
validation is successful: the signature validates, and a trust
relationship exists for the key. Alternatively, a DHCPv4 message can
include a certificate option, and also carrying a digital signature
and a timestamp option. The signature can be verified by the
recipient. The recipient processes the payload of the DHCPv4 message
only if the validation is successful: the certificate validates, and
some trust relationship exists on the recipient for the provided
certificate. The end-to-end security protection can be
bidirectional, covering messages from servers to clients and from
clients to servers. Additionally, the optional timestamp mechanism
provides anti-replay protection.
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A trust relationship for a public key can be the result either of a
Trust-on-first-use (TOFU) policy, or a list of trusted keys
configured on the recipient.
A trust relationship for a certificate could also be treated either
as TOFU or configured in a list of trusted certificate authorities,
depending on the application.
TOFU can be used by a client to authenticate a server and its
messages. It can be deployed without a pre-established trust
relationship between the client and the server. It can be used for
all DHCPv4 messages, and the same single key can be used for all
clients since the server does not send a secret in plain text on the
wire. Overall this will provide a reasonable balance of easy
deployment and moderate level of security, as long as the risk of the
attack window on the first use is acceptable.
TOFU can also be used by a server to protect an existing DHCPv4
session with a particular client by preventing a malicious client
from hijacking the session. In this case the server does not even
have to store the client's public key or certificate after the
session; it only has to remember the public key during that
particular session and check if it can verify received messages with
that key. This type of authentication can be deployed without a pre-
established trust relationship.
If authentication has to be provided from the initial use, the Secure
DHCPv4 mechanism needs some infrastructure such as PKI so the
recipient of a public key or certificate can verify it securely. It
is currently a subject of further study how such an infrastructure
can be integrated to DHCPv4 in a way it makes the deployment easier.
The signature on a Secure DHCPv4 message can be expected to
significantly increase the size of the message. One example is
normal DHCPv4 message length plus a 1 KB for a X.509 certificate and
signature and 256 Byte for a signature. Packet fragments are highly
possible. In practise, the total length would be various in a large
range. Hence, deployment of Secure DHCPv4 should also consider the
issues of IP fragment, PMTU, etc.
4.1. New Components
The components of the solution specified in this document are as
follows:
o Servers and clients using public keys in their secure DHCPv4
messages generate a public/private key pair. A new DHCPv4 option
that carries the public key is defined.
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o Servers and clients that use certificates first generate a public/
private key pair and then obtain a public key certificate from a
Certificate Authority that signs the public key. Another new
DHCPv4 option is defined to carry the certificate.
o A signature generated using the private key which is used by the
receiver to verify the integrity of the DHCPv4 messages and then
the identity of the sender.
o A timestamp, to detect replayed packet. The secure DHCPv4 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 of this document.
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 among
different senders, and the different senders in a same administrative
domain may be allowed to use various algorithms simultaneously. It
is NOT RECOMMENDED that the same sender and recipient use various
algorithms in a single communication session.
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 an AlgorithmNotSupported status code
(defined in Section 5.5). Upon receiving this status code, the
client MAY resend the message protected with the mandatory algorithm
(defined in Section 5.3).
4.3. Applicability
By default, a secure DHCPv4 enabled client or server SHOULD start
with secure mode by sending secure DHCPv4 messages. If the recipient
is secure DHCPv4 enabled and the key or certificate authority is
trusted by the recipient, then their communication would be in secure
mode. In the scenario where the secure DHCPv4 enabled client and
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server fail to build up secure communication between them, the secure
DHCPv4 enabled client MAY choose to send unsecured DHCPv4 message
towards the server according to its local policies.
In the scenario where the recipient is a legacy DHCPv4 server that
does not support secure mechanism, the DHCPv4 server (for all of
known DHCPv4 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 such
messages. If the client accepts the unsecured messages from the
DHCPv4 server, the subsequent exchanges will be in the unsecured
mode.
In the scenario where a legacy client sends an unsecured message to a
secure DHCPv4 enabled server, there are two possibilities depending
on the server policy. If the server's policy requires the
authentication, an UnspecFail (value 1, [RFC6926]) 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 MAY fall back to the unsecured
DHCPv4 mode, and reply unsecured messages toward the client;
depending on the local policy, the server MAY continue to send the
secured reply messages with the consumption of computing resource.
The resources allocated for unsecured clients SHOULD be separated and
restricted.
These are all examples of how interactions can go, but there is
nothing to prevent clients from behaving adaptively in response to
secure messages from servers.
5. Extensions for Secure DHCPv4
This section describes the extensions to DHCPv4. Four new options
have been defined. The new options MUST be supported in the Secure
DHCPv4 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-code | option-len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. Public Key (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_PUBLIC_KEY (TBA1).
option-len Length of public key in octets.
Public Key A variable-length field containing a
SubjectPublicKeyInfo object specified in [RFC5280].
The SubjectPublicKeyInfo structure is comprised with
a public key and an AlgorithmIdentifier object
which is 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].
Note: when the option exceeds 255 octets in size (the maximum size of
a single option), multiple instances of the same option are generated
acording to [RFC3396]. And they should be put in sequential order in
the same DHCPv4 message.
5.2. Certificate Option
The Certificate option carries the public key certificate of the
client. The format of the Certificate 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-code | option-len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. Certificate (variable length) .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_CERTIFICATE (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, [RFC7296].
The support of X.509 certificate - Signature (4)
is mandatory.
Note: when the option exceeds 255 octets in size (the maximum size of
a single option), multiple instances of the same option are generated
acording to [RFC3396]. And they should be put in sequential order in
the same DHCPv4 message.
5.3. Signature Option
The Signature option allows a signature that is signed by the private
key to be attached to a DHCPv4 message. The Signature option could
be any place within the DHCPv4 message while it is logically created
after the entire DHCPv4 header and options, except for the
Authentication Option. It protects the entire DHCPv4 header and
options, including itself, except for the 'giaddr' field, the 'hops'
fields, the Authentication Option [RFC3118] and the Relay Agent
Information Option [RFC3046]. 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-code | option-len | HA-id | SA-id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Signature (variable length) |
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_SIGNATURE (TBA3).
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option-len 2 + 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
DHCPv4 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 DHCPv4 registry in IANA. The
support of RSASSA-PKCS1-v1_5 is mandatory. A
registry of the initial assigned values is defined
in Section 8.
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 DHCPv4 message header with the 'giaddr' and
'hops' fields are set to all 0. It is because
DHCPv4 relay agents may change their values.
2. All DHCPv4 options including the Signature
option (fill the signature field with zeroes)
except for the Authentication Option and the Relay
Agent Information Option, if there are any.
The signature field 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: when the option exceeds 255 octets in size (the maximum size of
a single option), multiple instances of the same option are generated
acording to [RFC3396]. And they should be put in sequential order in
the same DHCPv4 message.
Note: if both signature and authentication option are present,
signature option does not protect the Authentication Option. It is
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because both options need 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 auth option, the authors
chose not include authentication option in the signature. This
design allows the Authentication Option to be created after signature
has been calculated and filled with the valid message authentication
code (MAC).
5.4. Timestamp Option
The Timestamp option carries the current time on the sender. It adds
the anti-replay protection to the DHCPv4 messages. It is optional.
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-code | option-len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| Timestamp (64-bit) |
| |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code OPTION_TIMESTAMP (TBA4).
option-len 8, in octets.
Timestamp The current time of day (NTP-format timestamp
[RFC5905] in UTC (Coordinated Universal Time), a
64-bit unsigned fixed-point number, in seconds
relative to 0h on 1 January 1900.). It can reduce
the danger of replay attacks.
5.5. Status Codes
The following new status codes, see [RFC6926], are defined. They are
carried by the Status Code Option (value 151, [RFC6926]).
o AlgorithmNotSupported (TBD5): indicates that the DHCPv4 server
does not support algorithms that sender used.
o AuthenticationFail (TBD6): indicates that the DHCPv4 client fails
authentication check.
o TimestampFail (TBD7): indicates the message from DHCPv4 client
fails the timestamp check.
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o SignatureFail (TBD8): indicates the message from DHCPv4 client
fails the signature check.
6. Processing Rules and Behaviors
This section only covers the scenario where both DHCPv4 client and
server are secure enabled.
6.1. Processing Rules of Sender
The sender of a Secure DHCPv4 message could be a DHCPv4 server or a
DHCPv4 client.
The sender must have a public/private key pair in order to create
Secure DHCPv4 messages. The sender may also have a public key
certificate, which is signed by a CA assumed to be trusted by the
receipient, and its corresponding private key.
To support Secure DHCPv4, the Secure DHCPv4 enabled sender MUST
construct the DHCPv4 message following the rules defined in
[RFC2131].
A Secure DHCPv4 message sent by a DHCPv4 server or client MUST either
contain a Public Key option, which MUST be constructed as explained
in Section 5.1, or a Certificate option, which MUST be constructed as
explained in Section 5.2.
A Secure DHCPv4 message 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 DHCPv4 options except for the
'giaddr' field, the 'hops' fields, the Authentication Option and the
Relay Agent Information Option.
A Secure DHCPv4 message SHOULD contain one and only one Timestamp
option, which MUST be constructed as explained in Section 5.4. The
Timestamp field SHOULD be set to the current time, according to
sender's real time clock.
If the sender is a DHCPv4 server and also sends back Relay Agent
Information Options to relay agents, it MUST NOT include the Relay
Agent Information Options in the computation of signature, as defined
in Section 5.3.
If the sender is a DHCPv4 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:
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o Upon receiving an AlgorithmNotSupported error status code, the
client SHOULD resend the message protected with one of the
mandatory algorithms.
o Upon receiving an AuthenticationFail error status code, the client
is not able to build up the secure communication with the
recipient. However, there may be more than one DHCPv4 servers,
one of which may send AuthenticationFail and the other of which
may succeed. The client MAY use the AuthenticationFail as a hint
and switch to other public key certificate if it has another one;
but otherwise treat the message containing the status code as if
it had not been received. But it SHOULD NOT retry with the same
certificate. However, if the client decides to retransmit using
the same certificate after receiving AuthenticationFail, it MUST
NOT retransmit immediately. Section 4.1 of [RFC2131] has enforced
that "the client MUST adopt a retransmission strategy that
incorporates a randomized exponential backoff algorithm to
determine the delay between retransmissions."
o Upon receiving a TimestampFail error status code, the client MAY
resend the message with an adjusted timestamp according to the
returned clock from the DHCPv4 server. The client SHOULD NOT
change its own clock, but only compute an offset for the
communication session.
o Upon receiving a SignatureFail error status code, the client MAY
resend the message following normal retransmission routines.
6.2. Processing Rules of Recipient
The recipient of a Secure DHCPv4 message could be a DHCPv4 server or
a DHCPv4 client. In the failure cases, either DHCPv4 server or
client SHOULD NOT process the received message, and the server SHOULD
reply with a correspondent error status code, while the client
behaves as if no response had been received from that server. The
specific behavior depends on the configured local policy.
When receiving a DHCPv4 message, a Secure DHCPv4 enabled recipient
SHOULD discard any DHCPv4 messages that meet any of the following
conditions:
o the Signature option is absent,
o multiple Signature options are present,
o both the Public Key option and the Certificate option are absent,
o both the Public Key option and the Certificate option are present.
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In such failure, if the recipient is a DHCPv4 server, the server
SHOULD reply an UnspecFail error (value 1, [RFC6926]) status code.
If none of the Signature, Public Key or Certificate options is
present, the sender MAY be a legacy node or in unsecured mode, then,
the recipient MAY fall back to the unsecured DHCPv4 mode if its local
policy allows.
The recipient SHOULD first check the support of the hash and
signature algorithms that the sender used. If the check fails for a
client, the message SHOULD be dropped. If the check fails for a
server, the server SHOULD reply with an AlgorithmNotSupported error
status code, defined in Section 5.5, back to the client. If both
hash and signature algorithms are supported, the recipient then
checks the authority of this sender. The recipient SHOULD also use
the same algorithms in the return messages.
If a Public Key option is provided, 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 (TOFU). A local trust public key list is a data table
maintained by the recipient. It stores public keys from all senders
that are considered trustworthy.
If a Certificate option is provided, the recipient SHOULD validate
the certificate according to the rules defined in [RFC5280]. An
implementation may create a local trust certificate record for
verified certificates in order to avoid repeated verification
procedure in the future. A certificate that finds a match in the
local trust certificate list is treated as verified.
When the local policy of the recipient allows the use of TOFU, if a
Public Key option is provided but it is not found in the local trust
public key list, the recipient MAY accept the public key. The
recipient will normally store the key in the local list for
subsequent DHCPv4 sessions, but it may not necessarily have to do so
depending on the purpose of the authentication (see the case of
authenticating a client with TOFU described in Section 4).
The message that fails authentication check, either because the
certificate validation fails or because the public key is not
recognized, MUST be dropped. In such failure, the DHCPv4 server
SHOULD reply an AuthenticationFail error status code, defined in
Section 5.5, back to the client.
The recipient MAY choose to further process messages from a sender
when there is no matched public key. When a message is authenticated
using a key that has not previously been seen, the recipient may, if
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permitted by policy, treat the sender as trustworthy and record the
key for future use (i.e, TOFU).
At this point, the recipient has either recognized the authentication
of the sender, or decided to drop the message. The recipient MUST
now authenticate the sender by verifying the signature and checking
timestamp (see details in Section 6.4), if there is a Timestamp
option. 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.
Depending on server's local policy, the message without a Timestamp
option MAY be acceptable or rejected. If the server rejects such a
message, a TimestampFail error status code, defined in Section 5.5,
should be sent back to the client. The reply message that carries
the TimestampFail error status code SHOULD carry a timestamp option,
which indicates the server's clock for the client to use.
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 (if
there is a Timestamp option) are accepted as secured DHCPv4 messages
and continue to be handled for their contained DHCPv4 options as
defined in [RFC2131]. Messages that do not pass the above tests MUST
be discarded or treated as unsecured messages. In the case the
recipient is DHCPv4 server, the DHCPv4 server SHOULD reply a
SignatureFail error status code, defined in Section 5.5, for the
signature verification failure; or a TimestampFail error status code,
defined in Section 5.5, for the timestamp check failure, back to the
client.
Furthermore, the node that supports the verification of the Secure
DHCPv4 messages MAY impose additional constraints for the
verification. For example, it may impose limits on minimum and
maximum key lengths.
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.
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6.3. Processing Rules of Relay Agent
To support Secure DHCPv4, relay agents just need to follow the same
processing rules defined in [RFC2131]. There is nothing more the
relay agents have to do, either verify the messages from client or
server, or add any secure DHCPv4 options. Actually, by definition in
this document, relay agents SHOULD NOT add any secure DHCPv4 options.
6.4. Timestamp Check
In order to check the Timestamp option, defined in Section 5.4,
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.
To facilitate timestamp checking, each recipient SHOULD store the
following information for each sender, from which at least one
accepted secure DHCPv4 message is successfully verified (for both
timestamp check and signature verification):
o The receive time of the last received and accepted DHCPv4 message.
This is called RDlast.
o The timestamp in the last received and accepted DHCPv4 message.
This is called TSlast.
A verified (for both timestamp check and signature verification)
secure DHCPv4 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
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After the signature verification also succeeds, 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 DHCPv4 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
entries. The specific policy as to which entries are preferred over
others is left as an implementation decision.
An implementation MAY statefully record the latest timestamps from
senders. In such implementation, the timestamps MUST be strictly
monotonously increasing. This is reasonable given that DHCPv4
messages are rarely misordered.
7. Security Considerations
This document provides new security features to the DHCPv4 protocol.
Using public key based security mechanism and its verification
mechanism in DHCPv4 message exchanging provides the authentication
and data integrity protection. Timestamp mechanism provides anti-
replay function.
The Secure DHCPv4 mechanism is based on the pre-condition that the
recipient knows the public key of the sender or the sender's public
key certificate can be verified through a trust CA. Clients may
discard the DHCPv4 messages from unknown/unverified servers, which
may be fake servers; or may prefer DHCPv4 messages from known/
verified servers over unsigned messages or messages from unknown/
unverified servers. The pre-configuration operation also needs to be
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protected, which is out of scope. The deployment of PKI is also out
of scope.
When a recipient first encounters a new public key, it may also store
the key using a Trust On First Use policy. If the sender that used
that public key is in fact legitimate, then all future communication
with that sender can be protected by storing the public key. This
does not provide complete security, but it limits the opportunity to
mount an attack on a specific recipient to the first time it
communicates with a new sender.
When using TOFU, if the recipient automatically and unlimitedly
stores the public key, an attacker could force the recipient to
exhaust the storage by sending DHCPv4 messages with many different
keys. There are several possible ways to address this concern:
First, the new public key should only be stored after the signature
and timestamp validations succeed. It does not prevent the attack
itself, but will at least increase the cost of mounting the attack.
Another approach is that as long as a client recipient has an
uninterrupted connection to a particular network medium, it could
limit the number of keys that it will remember as a result of
messages received on that medium. Network events like a link state
transition would clear the counter, but there might also need to be a
counter based on absolute time. In addition, there should probably
be a mechanism for purging keys that have only been seen once after a
certain period.
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 DHCPv4 messages.
[RFC6273] has analyzed possible threats to the hash algorithms used
in SEND. Since the Secure DHCPv4 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 DHCPv4.
A server, whose local policy accepts messages without a Timestamp
option, may have to face the risk of replay attacks.
A window of vulnerability for replay attacks exists until the
timestamp expires. Secure DHCPv4 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
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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 DHCPv4 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 DHCPv4 nodes keep independently maintained
clocks or apply suitable security measures for the time
synchronization protocols.
One more consideration is that this protocol does reveal additional
client information in their certificate. It means less privacy. In
current practice, the client privacy and the client authentication
are mutually exclusive.
8. IANA Considerations
This document defines four new DHCPv4 [RFC2131] options. The IANA is
requested to assign values for these four options from the DHCPv4
Option Codes table of the DHCPv4 Parameters registry maintained in
http://www.iana.org/assignments/bootp-dhcp-parameters. The four
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 Timestamp Option (TBA4),described in Section 5.4.
The IANA is also requested to add two new registry tables to the
DHCPv4 Parameters registry maintained in
http://www.iana.org/assignments/bootp-dhcp-parameters. The two
tables are the Hash Algorithm for Secure DHCPv4 table and the
Signature Algorithm for Secure DHCPv4 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.
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Hash Algorithm for Secure DHCPv4. The values in this table are 8-bit
unsigned integers. The following initial values are assigned for
Hash Algorithm for Secure DHCPv4 in this document:
Name | Value | RFCs
-------------------+---------+--------------
SHA-256 | 0x01 | this document
SHA-512 | 0x02 | this document
Signature Algorithm for Secure DHCPv4. The values in this table are
8-bit unsigned integers. The following initial values are assigned
for Signature Algorithm for Secure DHCPv4 in this document:
Name | Value | RFCs
-------------------+---------+--------------
RSASSA-PKCS1-v1_5 | 0x01 | this document
IANA is requested to assign the following new DHCPv4 Status Codes,
defined in Section 5.5, in the DHCPv4 Parameters registry maintained
in http://www.iana.org/assignments/bootp-dhcp-parameters:
Code | Name | Reference
---------+-----------------------+--------------
TBD5 | AlgorithmNotSupported | this document
TBD6 | AuthenticationFail | this document
TBD7 | TimestampFail | this document
TBD8 | SignatureFail | this document
9. Acknowledgements
This document is developed based on the afforts from its brother
document Secure DHCPv6 [I-D.ietf-dhc-sedhcpv6]. Therefore, the
authors would like to thank these who helped to develop both
documents: 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, Fred Templin,
Robert Elz and other members of the IETF DHC working group for their
valuable comments.
This document was produced using the xml2rfc tool [RFC2629].
10. Change log [RFC Editor: Please remove]
draft-jiang-dhc-sedhcpv4-01: change according to the latest change in
Secure DHCPv6 during AD review and the comments received during IETF
92, 2015-06-18.
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draft-jiang-dhc-sedhcpv4-00: original version, this draft is largely
based on a mature counterpart, Secure DHCPv6, draft-ietf-dhc-secure-
dhcpv6. 2015-01-29.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
[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.
[RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the
Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
November 2002.
[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.
[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.
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[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC6926] Kinnear, K., Stapp, M., Desetti, R., Joshi, B., Russell,
N., Kurapati, P., and B. Volz, "DHCPv4 Bulk Leasequery",
RFC 6926, April 2013.
[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, October 2014.
11.2. Informative References
[I-D.ietf-dhc-sedhcpv6]
Jiang, S., Shen, S., Zhang, D., and T. Jinmei, "Secure
DHCPv6", draft-ietf-dhc-sedhcpv6-08 (work in progress),
June 2015.
[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
CN
Email: jiangsheng@huawei.com
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Dacheng Zhang
Beijing, 100095 100025
P.R. China
Email: dacheng.zhang@gmail.com
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