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RFC 3118

Authentication for DHCP Messages

Pages: 17
Group: DHC
Proposed STD

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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.


   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
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   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

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.
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   3. Limit state required by the server.
   4. Limit complexity (complexity breeds design and implementation

1.3 Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [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

   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.
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   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:
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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
   |     Code      |    Length     |0 0 0 0 0 0 0 0|0 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.


      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].
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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:
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   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.


      Algorithm 1 specifies the use of HMAC-MD5.  Use of a different
      technique, such as HMAC-SHA, will be specified as a separate

      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).
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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

   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

5.5.1 INIT state

   When in INIT state, the client uses delayed authentication as

   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

      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
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      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

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].
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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
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   *  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.


      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
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   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 RFC 2434 [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 RFC 2434.

   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 RFC 2434.

7. References

   [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March

   [2] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
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   [3] Krawczyk H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for
       Message Authentication", RFC 2104, February 1997.

   [4] Mills, D., "Network Time Protocol (Version 3)", RFC 1305, March

   [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", RFC 2219, March 1997.

   [6] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
       Extensions", RFC 2132, March 1997.

   [7] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
       January 2001.

   [8] Narten, T. and H. Alvestrand, "Guidelines for Writing and IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, 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.
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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.
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10. Editors' Addresses

   Ralph Droms
   Cisco Systems
   300 Apollo Drive
   Chelmsford, MA 01824

   Phone: (978) 244-4733

   Bill Arbaugh
   Department of Computer Science
   University of Maryland
   A.V. Williams Building
   College Park, MD 20742

   Phone: (301) 405-2774
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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.,, 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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