Network Working Group R. Droms, Editor
Request for Comments: 3118 Cisco Systems
Category: Standards Track W. Arbaugh, Editor
University of Maryland
June 2001
Authentication for DHCP Messages
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document defines a new Dynamic Host Configuration Protocol
(DHCP) option through which authorization tickets can be easily
generated and newly attached hosts with proper authorization can be
automatically configured from an authenticated DHCP server. DHCP
provides a framework for passing configuration information to hosts
on a TCP/IP network. In some situations, network administrators may
wish to constrain the allocation of addresses to authorized hosts.
Additionally, some network administrators may wish to provide for
authentication of the source and contents of DHCP messages.
1. IntrodUCtion
DHCP [1] transports protocol stack configuration parameters from
centrally administered servers to TCP/IP hosts. Among those
parameters are an IP address. DHCP servers can be configured to
dynamically allocate addresses from a pool of addresses, eliminating
a manual step in configuration of TCP/IP hosts.
Some network administrators may wish to provide authentication of the
source and contents of DHCP messages. For example, clients may be
subject to denial of service attacks through the use of bogus DHCP
servers, or may simply be misconfigured due to unintentionally
instantiated DHCP servers. Network administrators may wish to
constrain the allocation of addresses to authorized hosts to avoid
denial of service attacks in "hostile" environments where the network
medium is not physically secured, such as wireless networks or
college residence halls.
This document defines a technique that can provide both entity
authentication and message authentication. The current protocol
combines the original Schiller-Huitema-Droms authentication mechanism
defined in a previous work in progress with the "delayed
authentication" proposal developed by Bill Arbaugh.
1.1 DHCP threat model
The threat to DHCP is inherently an insider threat (assuming a
properly configured network where BOOTP ports are blocked on the
enterprise's perimeter gateways.) Regardless of the gateway
configuration, however, the potential attacks by insiders and
outsiders are the same.
The attack specific to a DHCP client is the possibility of the
establishment of a "rogue" server with the intent of providing
incorrect configuration information to the client. The motivation
for doing so may be to establish a "man in the middle" attack or it
may be for a "denial of service" attack.
There is another threat to DHCP clients from mistakenly or
accidentally configured DHCP servers that answer DHCP client requests
with unintentionally incorrect configuration parameters.
The threat specific to a DHCP server is an invalid client
masquerading as a valid client. The motivation for this may be for
"theft of service", or to circumvent auditing for any number of
nefarious purposes.
The threat common to both the client and the server is the resource
"denial of service" (DoS) attack. These attacks typically involve
the exhaustion of valid addresses, or the exhaustion of CPU or
network bandwidth, and are present anytime there is a shared
resource. In current practice, redundancy mitigates DoS attacks the
best.
1.2 Design goals
These are the goals that were used in the development of the
authentication protocol, listed in order of importance:
1. Address the threats presented in Section 1.1.
2. Avoid changing the current protocol.
3. Limit state required by the server.
4. Limit complexity (complexity breeds design and implementation
errors).
1.3 Requirements Terminology
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [5].
1.4 DHCP Terminology
This document uses the following terms:
o "DHCP client"
A DHCP client or "client" is an Internet host using DHCP to
oBTain configuration parameters such as a network address.
o "DHCP server"
A DHCP server or "server" is an Internet host that returns
configuration parameters to DHCP clients.
2. Format of the authentication option
The following diagram defines the format of the DHCP authentication
option:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Length Protocol Algorithm
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RDM Replay Detection (64 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Replay cont.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Replay cont.
+-+-+-+-+-+-+-+-+
Authentication Information
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The code for the authentication option is 90, and the length field
contains the length of the protocol, RDM, algorithm, Replay Detection
fields and authentication information fields in octets.
The protocol field defines the particular technique for
authentication used in the option. New protocols are defined as
described in Section 6.
The algorithm field defines the specific algorithm within the
technique identified by the protocol field.
The Replay Detection field is per the RDM, and the authentication
information field is per the protocol in use.
The Replay Detection Method (RDM) field determines the type of replay
detection used in the Replay Detection field.
If the RDM field contains 0x00, the replay detection field MUST be
set to the value of a monotonically increasing counter. Using a
counter value such as the current time of day (e.g., an NTP-format
timestamp [4]) can reduce the danger of replay attacks. This method
MUST be supported by all protocols.
3. Interaction with Relay Agents
Because a DHCP relay agent may alter the values of the 'giaddr' and
'hops' fields in the DHCP message, the contents of those two fields
MUST be set to zero for the computation of any hash function over the
message header. Additionally, a relay agent may append the DHCP
relay agent information option 82 [7] as the last option in a message
to servers. If a server finds option 82 included in a received
message, the server MUST compute any hash function as if the option
were NOT included in the message without changing the order of
options. Whenever the server sends back option 82 to a relay agent,
the server MUST not include the option in the computation of any hash
function over the message.
4. Configuration token
If the protocol field is 0, the authentication information field
holds a simple configuration token:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Length 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 0 0 0 0 0 0 0 Replay Detection (64 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Replay cont.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Replay cont.
-+-+-+-+-+-+-+-+
Authentication Information
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The configuration token is an opaque, unencoded value known to both
the sender and receiver. The sender inserts the configuration token
in the DHCP message and the receiver matches the token from the
message to the shared token. If the configuration option is present
and the token from the message does not match the shared token, the
receiver MUST discard the message.
Configuration token may be used to pass a plain-text configuration
token and provides only weak entity authentication and no message
authentication. This protocol is only useful for rudimentary
protection against inadvertently instantiated DHCP servers.
DISCUSSION:
The intent here is to pass a constant, non-computed token such as
a plain-text password. Other types of entity authentication using
computed tokens such as Kerberos tickets or one-time passwords
will be defined as separate protocols.
5. Delayed authentication
If the protocol field is 1, the message is using the "delayed
authentication" mechanism. In delayed authentication, the client
requests authentication in its DHCPDISCOVER message and the server
replies with a DHCPOFFER message that includes authentication
information. This authentication information contains a nonce value
generated by the source as a message authentication code (MAC) to
provide message authentication and entity authentication.
This document defines the use of a particular technique based on the
HMAC protocol [3] using the MD5 hash [2].
5.1 Management Issues
The "delayed authentication" protocol does not attempt to address
situations where a client may roam from one administrative domain to
another, i.e., interdomain roaming. This protocol is focused on
solving the intradomain problem where the out-of-band exchange of a
shared secret is feasible.
5.2 Format
The format of the authentication request in a DHCPDISCOVER or a
DHCPINFORM message for delayed authentication is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Length 0 0 0 0 0 0 0 1 Algorithm
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RDM Replay Detection (64 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Replay cont.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Replay cont.
+-+-+-+-+-+-+-+-+
The format of the authentication information in a DHCPOFFER,
DHCPREQUEST or DHCPACK message for delayed authentication is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Length 0 0 0 0 0 0 0 1 Algorithm
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RDM Replay Detection (64 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Replay cont.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Replay cont. Secret ID (32 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
secret id cont HMAC-MD5 (128 bits) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following definitions will be used in the description of the
authentication information for delayed authentication, algorithm 1:
Replay Detection - as defined by the RDM field
K - a secret value shared between the source and
destination of the message; each secret has a
unique identifier (secret ID)
secret ID - the unique identifier for the secret value
used to generate the MAC for this message
HMAC-MD5 - the MAC generating function [3, 2].
The sender computes the MAC using the HMAC generation algorithm [3]
and the MD5 hash function [2]. The entire DHCP message (except as
noted below), including the DHCP message header and the options
field, is used as input to the HMAC-MD5 computation function. The
'secret ID' field MUST be set to the identifier of the secret used to
generate the MAC.
DISCUSSION:
Algorithm 1 specifies the use of HMAC-MD5. Use of a different
technique, such as HMAC-SHA, will be specified as a separate
protocol.
Delayed authentication requires a shared secret key for each
client on each DHCP server with which that client may wish to use
the DHCP protocol. Each secret key has a unique identifier that
can be used by a receiver to determine which secret was used to
generate the MAC in the DHCP message. Therefore, delayed
authentication may not scale well in an architecture in which a
DHCP client connects to multiple administrative domains.
5.3 Message validation
To validate an incoming message, the receiver first checks that the
value in the replay detection field is acceptable according to the
replay detection method specified by the RDM field. Next, the
receiver computes the MAC as described in [3]. The receiver MUST set
the 'MAC' field of the authentication option to all 0s for
computation of the MAC, and because a DHCP relay agent may alter the
values of the 'giaddr' and 'hops' fields in the DHCP message, the
contents of those two fields MUST also be set to zero for the
computation of the MAC. If the MAC computed by the receiver does not
match the MAC contained in the authentication option, the receiver
MUST discard the DHCP message.
Section 3 provides additional information on handling messages that
include option 82 (Relay Agents).
5.4 Key utilization
Each DHCP client has a key, K. The client uses its key to encode any
messages it sends to the server and to authenticate and verify any
messages it receives from the server. The client's key SHOULD be
initially distributed to the client through some out-of-band
mechanism, and SHOULD be stored locally on the client for use in all
authenticated DHCP messages. Once the client has been given its key,
it SHOULD use that key for all transactions even if the client's
configuration changes; e.g., if the client is assigned a new network
address.
Each DHCP server MUST know, or be able to obtain in a secure manner,
the keys for all authorized clients. If all clients use the same
key, clients can perform both entity and message authentication for
all messages received from servers. However, the sharing of keys is
strongly discouraged as it allows for unauthorized clients to
masquerade as authorized clients by obtaining a copy of the shared
key. To authenticate the identity of individual clients, each client
MUST be configured with a unique key. Appendix A describes a
technique for key management.
5.5 Client considerations
This section describes the behavior of a DHCP client using delayed
authentication.
5.5.1 INIT state
When in INIT state, the client uses delayed authentication as
follows:
1. The client MUST include the authentication request option in its
DHCPDISCOVER message along with a client identifier option [6] to
identify itself uniquely to the server.
2. The client MUST perform the validation test described in section
5.3 on any DHCPOFFER messages that include authentication
information. If one or more DHCPOFFER messages pass the
validation test, the client chooses one of the offered
configurations.
Client behavior if no DHCPOFFER messages include authentication
information or pass the validation test is controlled by local
policy in the client. According to client policy, the client MAY
choose to respond to a DHCPOFFER message that has not been
authenticated.
The decision to set local policy to accept unauthenticated
messages should be made with care. Accepting an unauthenticated
DHCPOFFER message can make the client vulnerable to spoofing and
other attacks. If local users are not eXPlicitly informed that
the client has accepted an unauthenticated DHCPOFFER message, the
users may incorrectly assume that the client has received an
authenticated address and is not subject to DHCP attacks through
unauthenticated messages.
A client MUST be configurable to decline unauthenticated messages,
and SHOULD be configured by default to decline unauthenticated
messages. A client MAY choose to differentiate between DHCPOFFER
messages with no authentication information and DHCPOFFER messages
that do not pass the validation test; for example, a client might
accept the former and discard the latter. If a client does accept
an unauthenticated message, the client SHOULD inform any local
users and SHOULD log the event.
3. The client replies with a DHCPREQUEST message that MUST include
authentication information encoded with the same secret used by
the server in the selected DHCPOFFER message.
4. If the client authenticated the DHCPOFFER it accepted, the client
MUST validate the DHCPACK message from the server. The client
MUST discard the DHCPACK if the message fails to pass validation
and MAY log the validation failure. If the DHCPACK fails to pass
validation, the client MUST revert to INIT state and returns to
step 1. The client MAY choose to remember which server replied
with a DHCPACK message that failed to pass validation and discard
subsequent messages from that server.
If the client accepted a DHCPOFFER message that did not include
authentication information or did not pass the validation test,
the client MAY accept an unauthenticated DHCPACK message from the
server.
5.5.2 INIT-REBOOT state
When in INIT-REBOOT state, the client MUST use the secret it used in
its DHCPREQUEST message to obtain its current configuration to
generate authentication information for the DHCPREQUEST message. The
client MAY choose to accept unauthenticated DHCPACK/DHCPNAK messages
if no authenticated messages were received. The client MUST treat
the receipt (or lack thereof) of any DHCPACK/DHCPNAK messages as
specified in section 3.2 of [1].
5.5.3 RENEWING state
When in RENEWING state, the client uses the secret it used in its
initial DHCPREQUEST message to obtain its current configuration to
generate authentication information for the DHCPREQUEST message. If
client receives no DHCPACK messages or none of the DHCPACK messages
pass validation, the client behaves as if it had not received a
DHCPACK message in section 4.4.5 of the DHCP specification [1].
5.5.4 REBINDING state
When in REBINDING state, the client uses the secret it used in its
initial DHCPREQUEST message to obtain its current configuration to
generate authentication information for the DHCPREQUEST message. If
client receives no DHCPACK messages or none of the DHCPACK messages
pass validation, the client behaves as if it had not received a
DHCPACK message in section 4.4.5 of the DHCP specification [1].
5.5.5 DHCPINFORM message
Since the client already has some configuration information, the
client may also have established a shared secret value, K, with a
server. Therefore, the client SHOULD use the authentication request
as in a DHCPDISCOVER message when a shared secret value exists. The
client MUST treat any received DHCPACK messages as it does DHCPOFFER
messages, see section 5.5.1.
5.5.6 DHCPRELEASE message
Since the client is already in the BOUND state, the client will have
a security association already established with the server.
Therefore, the client MUST include authentication information with
the DHCPRELEASE message.
5.6 Server considerations
This section describes the behavior of a server in response to client
messages using delayed authentication.
5.6.1 General considerations
Each server maintains a list of secrets and identifiers for those
secrets that it shares with clients and potential clients. This
information must be maintained in such a way that the server can:
* Identify an appropriate secret and the identifier for that secret
for use with a client that the server may not have previously
communicated with
* Retrieve the secret and identifier used by a client to which the
server has provided previous configuration information
Each server MUST save the counter from the previous authenticated
message. A server MUST discard any incoming message which fails the
replay detection check as defined by the RDM avoid replay attacks.
DISCUSSION:
The authenticated DHCPREQUEST message from a client in INIT-REBOOT
state can only be validated by servers that used the same secret
in their DHCPOFFER messages. Other servers will discard the
DHCPREQUEST messages. Thus, only servers that used the secret
selected by the client will be able to determine that their
offered configuration information was not selected and the offered
network address can be returned to the server's pool of available
addresses. The servers that cannot validate the DHCPREQUEST
message will eventually return their offered network addresses to
their pool of available addresses as described in section 3.1 of
the DHCP specification [1].
5.6.2 After receiving a DHCPDISCOVER message
The server selects a secret for the client and includes
authentication information in the DHCPOFFER message as specified in
section 5, above. The server MUST record the identifier of the
secret selected for the client and use that same secret for
validating subsequent messages with the client.
5.6.3 After receiving a DHCPREQUEST message
The server uses the secret identified in the message and validates
the message as specified in section 5.3. If the message fails to
pass validation or the server does not know the secret identified by
the 'secret ID' field, the server MUST discard the message and MAY
choose to log the validation failure.
If the message passes the validation procedure, the server responds
as described in the DHCP specification. The server MUST include
authentication information generated as specified in section 5.2.
5.6.4 After receiving a DHCPINFORM message
The server MAY choose to accept unauthenticated DHCPINFORM messages,
or only accept authenticated DHCPINFORM messages based on a site
policy.
When a client includes the authentication request in a DHCPINFORM
message, the server MUST respond with an authenticated DHCPACK
message. If the server does not have a shared secret value
established with the sender of the DHCPINFORM message, then the
server MAY respond with an unauthenticated DHCPACK message, or a
DHCPNAK if the server does not accept unauthenticated clients based
on the site policy, or the server MAY choose not to respond to the
DHCPINFORM message.
6. IANA Considerations
Section 2 defines a new DHCP option called the Authentication Option,
whose option code is 90.
This document specifies three new name spaces associated with the
Authentication Option, which are to be created and maintained by
IANA: Protocol, Algorithm and RDM.
Initial values assigned from the Protocol name space are 0 (for the
configuration token Protocol in section 4) and 1 (for the delayed
authentication Protocol in section 5). Additional values from the
Protocol name space will be assigned through IETF Consensus, as
defined in RFC2434 [8].
The Algorithm name space is specific to individual Protocols. That
is, each Protocol has its own Algorithm name space. The guidelines
for assigning Algorithm name space values for a particular protocol
should be specified along with the definition of a new Protocol.
For the configuration token Protocol, the Algorithm field MUST be 0.
For the delayed authentication Protocol, the Algorithm value 1 is
assigned to the HMAC-MD5 generating function as defined in section 5.
Additional values from the Algorithm name space for Algorithm 1 will
be assigned through IETF Consensus, as defined in RFC2434.
The initial value of 0 from the RDM name space is assigned to the use
of a monotonically increasing value as defined in section 2.
Additional values from the RDM name space will be assigned through
IETF Consensus, as defined in RFC2434.
7. References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC2131, March
1997.
[2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, April
1992.
[3] Krawczyk H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for
Message Authentication", RFC2104, February 1997.
[4] Mills, D., "Network Time Protocol (Version 3)", RFC1305, March
1992.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC2219, March 1997.
[6] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC2132, March 1997.
[7] Patrick, M., "DHCP Relay Agent Information Option", RFC3046,
January 2001.
[8] Narten, T. and H. Alvestrand, "Guidelines for Writing and IANA
Considerations Section in RFCs", BCP 26, RFC2434, October 1998.
8. Acknowledgments
Jeff Schiller and Christian Huitema developed the original version of
this authentication protocol in a terminal room BOF at the Dallas
IETF meeting, December 1995. One of the editors (Droms) transcribed
the notes from that discussion, which form the basis for this
document. The editors appreciate Jeff's and Christian's patience in
reviewing this document and its earlier drafts.
The "delayed authentication" mechanism used in section 5 is due to
Bill Arbaugh. The threat model and requirements in sections 1.1 and
1.2 come from Bill's negotiation protocol proposal. The attendees of
an interim meeting of the DHC WG held in June, 1998, including Peter
Ford, Kim Kinnear, Glenn Waters, Rob Stevens, Bill Arbaugh, Baiju
Patel, Carl Smith, Thomas Narten, Stewart Kwan, Munil Shah, Olafur
Gudmundsson, Robert Watson, Ralph Droms, Mike Dooley, Greg Rabil and
Arun Kapur, developed the threat model and reviewed several
alternative proposals.
The replay detection method field is due to Vipul Gupta.
Other input from Bill Sommerfield is gratefully acknowledged.
Thanks also to John Wilkins, Ran Atkinson, Shawn Mamros and Thomas
Narten for reviewing earlier drafts of this document.
9. Security Considerations
This document describes authentication and verification mechanisms
for DHCP.
9.1 Protocol vulnerabilities
The configuration token authentication mechanism is vulnerable to
interception and provides only the most rudimentary protection
against inadvertently instantiated DHCP servers.
The delayed authentication mechanism described in this document is
vulnerable to a denial of service attack through flooding with
DHCPDISCOVER messages, which are not authenticated by this protocol.
Such an attack may overwhelm the computer on which the DHCP server is
running and may exhaust the addresses available for assignment by the
DHCP server.
Delayed authentication may also be vulnerable to a denial of service
attack through flooding with authenticated messages, which may
overwhelm the computer on which the DHCP server is running as the
authentication keys for the incoming messages are computed.
9.2 Protocol limitations
Delayed authentication does not support interdomain authentication.
A real digital signature mechanism such as RSA, while currently
computationally infeasible, would provide better security.
10. Editors' Addresses
Ralph Droms
Cisco Systems
300 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-4733
EMail: rdroms@cisco.com
Bill Arbaugh
Department of Computer Science
University of Maryland
A.V. Williams Building
College Park, MD 20742
Phone: (301) 405-2774
EMail: waa@cs.umd.edu
Appendix A - Key Management Technique
To avoid centralized management of a list of random keys, suppose K
for each client is generated from the pair (client identifier [6],
subnet address, e.g., 192.168.1.0), which must be unique to that
client. That is, K = MAC(MK, unique-id), where MK is a secret master
key and MAC is a keyed one-way function such as HMAC-MD5.
Without knowledge of the master key MK, an unauthorized client cannot
generate its own key K. The server can quickly validate an incoming
message from a new client by regenerating K from the client-id. For
known clients, the server can choose to recover the client's K
dynamically from the client-id in the DHCP message, or can choose to
precompute and cache all of the Ks a priori.
By deriving all keys from a single master key, the DHCP server does
not need Access to clear text passwords, and can compute and verify
the keyed MACs without requiring help from a centralized
authentication server.
To avoid compromise of this key management system, the master key,
MK, MUST NOT be stored by any clients. The client SHOULD only be
given its key, K. If MK is compromised, a new MK SHOULD be chosen
and all clients given new individual keys.
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