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Network Working Group                                   R. Droms, Editor
INTERNET DRAFT                                       Bucknell University
Obsoletes: draft-ietf-dhc-authentication-05.txt                June 1998
                                                   Expires December 1998


                    Authentication for DHCP Messages
                 <draft-ietf-dhc-authentication-06.txt>

Status of this memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   To view the entire list of current Internet-Drafts, please check
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   (US West Coast).

Abstract

   The Dynamic Host Configuration Protocol (DHCP) [1] 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.  This
   document defines a new 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.

1. Introduction

   DHCP 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.




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   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.

1.1 Requirements

   Throughout this document, the words that are used to define the
   significance of particular requirements are capitalized.  These words
   are:

      o "MUST"

        This word or the adjective "REQUIRED" means that the
        item is an absolute requirement of this specification.

      o "MUST NOT"

        This phrase means that the item is an absolute prohibition
        of this specification.

      o "SHOULD"

        This word or the adjective "RECOMMENDED" means that there
        may exist valid reasons in particular circumstances to ignore
        this item, but the full implications should be understood and
        the case carefully weighed before choosing a different course.

      o "SHOULD NOT"

        This phrase means that there may exist valid reasons in
        particular circumstances when the listed behavior is acceptable
        or even useful, but the full implications should be understood
        and the case carefully weighed before implementing any behavior
        described with this label.








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      o "MAY"

        This word or the adjective "OPTIONAL" means that this item is
        truly optional.  One vendor may choose to include the item
        because a particular marketplace requires it or because it
        enhances the product, for example; another vendor may omit the
        same item.

1.2 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 of "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:


    +----------+----------+----------+
    |   Code   |  Length  | Protocol |
    +----------+----------+----------+-----------+---
    |                  Authentication information    ...
    +----------+----------+----------+-----------+---


   The code for the authentication option is TBD, and the length field
   contains the length of the protocol and authentication information
   fields in octets.  The protocol field defines the particular
   technique for authentication used in the option.

   This document defines two protocols in sections 3 and 4, encoded with
   protocol field values 0 and 1.  Protocol field values 2-254 are
   reserved for future use.  Other protocols may be defined according to
   the procedure described in section 5.

3. Protocol 0

   If the protocol field is 0, the authentication information field



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   holds a simple authentication token:


    +----------+----------+----------+
    |   Code   |   n+1    |    0     |
    +----------+----------+----------+-----------+------
    |   Authentication token (n octets) ...
    +----------+----------+----------+-----------+------


   The authentication token is an opaque, unencoded value known to both
   the sender and receiver.  The sender inserts the authentication token
   in the DHCP message and the receiver matches the token from the
   message to the shared token.  If the authentication option is present
   and the token from the message does not match the shared token, the
   receiver MUST discard the message.

   Protocol 0 may be used to pass a plain-text password and provides
   only weak entity authentication and no message authentication.  This
   protocol is useful for rudimentary protection against, e.g.,
   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.


4. Protocol 1

   If the protocol field is 1, the authentication information contains
   an encrypted value generated by the source as a message
   authentication code (MAC) to provide message authentication and
   entity authentication.

   This technique is based on the HMAC protocol [3] using the MD5 hash
   {2].












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4.1 Format

   The format of the authentication information for protocol 1 is:


    +----------+----------+----------+
    |   Code   |    n     |    1     |
    +----------+----------+----------+----------+-
    |               Counter (8 octets)            ...
    +----------+----------+----------+----------+-
    |               MAC                           ...
    +----------+----------+----------+----------+-

   The following definitions will be used in the description of the
   authentication information for protocol 1:

   K        - a secret value shared between the source and destination
              of the message
   Counter  - the value of a 64-bit monotonically increasing counter
   HMAC-MD5 - the MAC generating function as defined by [3] and [2]

   The sender computes the MAC as described in [3].  The 'counter' field
   of the authentication option MUST be set to the value of a
   monotonically increasing counter and the 'MAC' field of the
   authentication option MUST be set to all 0s for the computation of
   the MAC.  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
   message digest.  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.

   DISCUSSION:

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

      Protocol 1 requires a shared secret key for each client on each
      DHCP server with which that client may wish to use the DHCP
      protocol.  Therefore, protocol 1 may not scale well in an
      architecture in which a DHCP client may connect to multiple
      administrative domains.

4.2 Message validation

   To validate an incoming message, the receiver checks the 'counter'
   field and computes the MAC as described in [3]. If the 'counter'



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   field does not contain a value larger than the last value of
   'counter' used by the sender, the receiver MUST discard the incoming
   message. The receiver MUST set the 'MAC' field of the authentication
   option to all 0s for computation of the MAC. 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.

4.3 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 must be
   initially distributed to the client through some out-of-band
   mechanism, and must be stored locally on the client for use in all
   authenticated DHCP messages.  Once the client has been given its key,
   it may 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 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.
   Servers will be able to perform message authentication.  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. Definition of new authentication protocols

   The author of a new DHCP option will follow these steps to obtain
   acceptance of the protocol as a part of the DHCP Internet Standard:

   1. The author devises the new authentication protocol.
   2. The author documents the new protocol as an Internet Draft.
   3. The author submits the Internet Draft for review through the IETF
      standards process as defined in "Internet Official Protocol
      Standards" (STD 1).  The new protocol will be submitted for
      eventual acceptance as an Internet Standard.
   4. The new protocol progresses through the IETF standards process;
      the new option will be reviewed by the Dynamic Host Configuration
      Working Group (if that group still exists), or as an Internet
      Draft not submitted by an IETF working group.

   This procedure for defining new authentication protocols will ensure
   that:



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   * new options are reviewed for technical correctness and
     appropriateness, and
   * documentation for new options is complete and published.


6. References

   [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
       Bucknell University, October 1993.

   [2] Rivest, R., "The MD5 Message-Digest Algorithm",
       RFC-1321, April 1992.

   [3] Krawczyk H., M. Bellare and R. Canetti, "HMAC: Keyed-Hashing for
       Message Authentication" <draft-ietf-ipsec-hmac-md5-01.txt> (work in
       progress), August 1996.

   [4] Mills, D., "Network Time Protocol (Version 3)", RFC-1305, March
       1992.

7. Acknowledgments

   Jeff Schiller and Christian Huitema developed this scheme during a
   terminal room BOF at the Dallas IETF meeting, December 1995.  The
   author transcribed the notes from that discussion, which form the
   basis for this document.  The editor appreciates Jeff's and
   Christian's patience in reviewing this document and its earlier
   drafts.

   Thanks also to John Wilkins, Ran Atkinson and Shawn Mamros for
   reviewing this document, and to Thomas Narten for reviewing earlier
   drafts of this document.

8. Security considerations

   This document describes authentication and verification mechanisms
   for DHCP.

9. Author's address

   Ralph Droms
   Computer Science Department
   323 Dana Engineering
   Bucknell University
   Lewisburg, PA 17837

   Phone: (717) 524-1145
   EMail: droms@bucknell.edu



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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, subnet
   address), which must be unique to that client.  That is, K = MD5(MK,
   unique-id), where MK is a secret master key and MD5 is some encoding
   function.

   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.





































Droms                                                           [Page 8]


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