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Versions: 00 01 02 03

Network Working Group                                           R. Hibbs
Internet-Draft                                  Richard Barr Hibbs, P.E.
Expires: October 29, 2004                                       C. Smith
                                                          C & C Catering
                                                                 B. Volz
                                                     Cisco Systems, Inc.
                                                                M. Zohar
                                        IBM T. J. Watson Research Center
                                                          April 30, 2004


 Dynamic Host Configuration Protocol for IPv4 (DHCPv4) Threat Analysis
                  draft-ietf-dhc-v4-threat-analysis-02

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   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
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on October 29, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   DHCPv4 (RFC 2131) is a stable, widely used protocol for configuration
   of host systems in a TCP/IPv4 network. It did not provide for
   authentication of clients and servers, nor did it provide for data
   confidentiality. This is reflected in the original "Security
   Considerations" section of RFC 2131, which identifies a few threats



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   and leaves development of any defenses against those threats to
   future work. Beginning in about 1995 DHCP security began to attract
   attention from the Internet community, eventually resulting in the
   publication of RFC 3118 in 2001. Although RFC 3118 was a mandatory
   prerequisite for the DHCPv4 Reconfigure Extension, RFC 3203, it has
   had no known usage by any commercial or private implementation since
   its adoption. The DHC Working Group has adopted a work item for 2003
   to review and modify or replace RFC 3118 to afford a workable, easily
   deployed security mechanism for DHCPv4. This memo provides a
   comprehensive threat analysis of the Dynamic Host Configuration
   Protocol for use both as RFC 2131 advances from Draft Standard to
   Full Standard and to support our chartered work improving the
   acceptance and deployment of RFC 3118.






































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1   Issues for Consideration . . . . . . . . . . . . . . . . .  4
     1.2   Assumptions  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3   Scope of this Memo . . . . . . . . . . . . . . . . . . . .  4
     1.4   Use of Key Words . . . . . . . . . . . . . . . . . . . . .  5
   2.  General Threats to DHCPv4  . . . . . . . . . . . . . . . . . .  5
     2.1   Denial-of-Service Attacks  . . . . . . . . . . . . . . . .  5
       2.1.1   Refusal to Configure Clients . . . . . . . . . . . . .  5
       2.1.2   Impersonating Clients  . . . . . . . . . . . . . . . .  5
       2.1.3   Flooding . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2   Client Misconfiguration  . . . . . . . . . . . . . . . . .  6
     2.3   Theft of Network Service . . . . . . . . . . . . . . . . .  6
     2.4   Packet Insertion, Deletion, or Modification  . . . . . . .  7
   3.  Weaknesses of RFC 3118 . . . . . . . . . . . . . . . . . . . .  7
     3.1   Key Exposure . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2   Key Distribution . . . . . . . . . . . . . . . . . . . . .  7
     3.3   Replay attacks . . . . . . . . . . . . . . . . . . . . . .  8
     3.4   Protocol Agreement Difficulties  . . . . . . . . . . . . .  8
   4.  DHCPv4 Security Requirements . . . . . . . . . . . . . . . . .  8
     4.1   Environments . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2   Capabilities . . . . . . . . . . . . . . . . . . . . . . .  8
     4.3   Musings on the Key Distribution Problem  . . . . . . . . .  9
     4.4   Data Confidentiality . . . . . . . . . . . . . . . . . . . 10
       4.4.1   "Public" Data in DHCP Packets  . . . . . . . . . . . . 10
       4.4.2   Protecting Data in DHCP Options  . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.1   01 Draft . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.2   02 Draft . . . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   9.1   Normative References . . . . . . . . . . . . . . . . . . . . 12
   9.2   Informative References . . . . . . . . . . . . . . . . . . . 13
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
       Intellectual Property and Copyright Statements . . . . . . . . 15













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1.  Introduction

   DHCPv4 as defined in RFC 1541 [RFC1541] and RFC 2131 [RFC2131] does
   not provide any form of communication security, confidentiality, data
   integrity, or peer entity authentication.

   A design team was formed at IETF-55 in Atlanta in November 2002 to
   look at DHCPv4 and RFC 3118 [RFC3118] to document security
   requirements for DHCPv4. RFC 3118 defines the current security
   mechanisms for DHCPv4.

   Unfortunately, RFC 3118 has neither been implemented nor deployed to
   date. There is widespread feeling that its current restriction to
   manual keying of clients limits its deployment. The DHC Working Group
   seeks to rectify this situation by defining security mechanisms for
   DHCPv4 that have better deployment properties.

1.1  Issues for Consideration

   Specific issues to be considered include:
   o  Improved key management and scalability.
   o  Security for messages passed between relay agents and servers.
   o  The increased usage of DHCPv4 on insecure (e.g., wireless) and
      public LANs.
   o  The need for clients to be able to authenticate servers, without
      simultaneously requiring client authentication by the server.

1.2  Assumptions

   This document assumes that the reader is familiar with both the base
   DHCPv4 protocol as defined in RFC 2131 and the DHCPv4 authentication
   extension as defined in RFC 3118, and does not attempt to provide a
   tutorial on either.

1.3  Scope of this Memo

   This document confines its analysis to DHCPv4, as defined in RFC 2131
   and RFC 2132 [RFC2132] and DHCP Authentication, as defined in RFC
   3118.

   Excluded from our analysis are:
   o  Securing messages between relay agents and servers: work is
      already underway on this, see [auth-subopt] and [relay-ipsec].
   o  DHCP Reconfigure Extension (FORCERENEW), RFC 3203 [RFC3203]: the
      authors believe it is appropriate to put the onus to provide the
      analysis on those who are interested in moving that work forward.
   o  DHCP Failover Protocol, as defined in [failover]: the
      server-to-server protocol used differs significantly from DHCP.



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   o  DHCP-DNS Interaction, as defined in [fqdn]: securing communication
      between DHCP servers and DNS servers is a DNS update security
      issue and therefore out of scope for the DHC working group.
   o  DHCPv6, as defined in RFC 3315 [RFC3315]: while we believe that
      authentication techniques developed for DHCPv4 would generally be
      applicable to DHCPv6, there are fundamental differences between
      the two protocols and RFC 3118 specifies DHCPv4-style message and
      options formats, so we have chosen to concentrate on DHCPv4.
   o  DHCP Lease Query, as defined in [leasequery]: because of lack of
      maturity.

1.4  Use of Key Words

   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 RFC 2119 [RFC2119].

2.  General Threats to DHCPv4

   These are the classes of threats to the base DHCPv4 protocol. Not all
   of these are DHCP-specific, nor can all the concerns listed be
   addressed by DHCP authentication.

2.1  Denial-of-Service Attacks

2.1.1  Refusal to Configure Clients

   A rogue DHCP server can refuse to configure clients by responding
   with either partial information (i.e., missing the IP address, yet
   containing other information) or a non-routable (or otherwise bad) IP
   address. Or, the server may respond to DHCPDISCOVER messages (with
   DHCPOFFER messages) but then ignore the subsequent client DHCPREQUEST
   messages. This may cause a client to repeatedly fail to be
   configured, though clients could take steps to ensure that they
   subsequently ignore such servers for a period of time.

2.1.2  Impersonating Clients

   A rogue client can impersonate a client or many clients, by using
   another client's client identifer (client identifier option) and/or
   hardware address (chaddr) or by generating these identifiers. This
   may be done to:
   o  Obtain addresses when hardware address or client identifier
      restrictions (lists) are configured into the site's server through
      some mechanism not described in RFC 2131. Some sites may use such
      a mechanism to restrict the clients that are allowed addresses. A
      rogue client listens to DHCPv4 traffic and captures a few chaddrs
      or client identifiers and starts using them.



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   o  Simulate many clients to consume all available addresses. The
      rogue client may either hold on to these addresses (until the
      leases expire) or decline the addresses (by sending a DHCPDECLINE)
      in the hopes that the server will remove the declined address from
      use for a longer period of time.
   o  Create havoc on the subnet by injecting fake messages on behalf of
      other clients that prematurely release (DHCPRELEASE) or decline
      (DHCPDECLINE) their addresses. A rogue client listens to DHCPv4
      traffic and gleans client identity and address information and
      uses this information to inject fake messages.

2.1.3  Flooding

   A rogue client can flood the network with (near-) continuous DHCPv4
   request messages thereby consuming processing resources and network
   bandwidth.

   We mention this attack only for completeness, as there is little or
   nothing that a DHCP server can do to prevent such an attack and the
   client could just as well send messages of other protocols; and we
   will not discuss it further.

2.2  Client Misconfiguration

   Rogue servers may give out bad configuration information (such as
   fake gateways or DNS servers), or relay agents or other network
   elements may alter packets between a client and server, to cause the
   client to be misconfigured, or to enable future man-in-the-middle
   attacks. This category is usually part of another attack, be it theft
   of service, business espionage, or business interruption including
   denial of service.

2.3  Theft of Network Service

   By "theft of network service" we mean the taking of an unused address
   for network access or the use of an assigned address not belonging to
   the client, in contrast with "client masquerading" (Section 2.1.2)
   which refers specifically to the use of a legitimate client's chaddr
   or client identifier.

   Instantiation of an unauthorized client for purposes of using network
   resources or services is only partially preventable using
   client-server authentication techniques. We mention this attack only
   for completeness, as there is little or nothing that a DHCP server,
   itself, can do to prevent such an attack. Additional host and
   application security is required to prevent theft of service, and
   such layer 5 and higher functions are declared out of scope for this
   analysis.



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2.4  Packet Insertion, Deletion, or Modification

   If a client (or server or relay agent) is known to crash or shut down
   when invalid packets of some type are sent, this could be simply
   another type of denial of service attack. Likewise, simply deleting
   certain packet types (DHCPREQUEST to renew or rebind a lease) would
   eventually result in client lease expiration, a denial of service
   attack. A rogue relay agent or other host would typically use packet
   insertion and deletion to interrupt service. In a different vein, the
   modification of packets in the DHCP exchange may be used to
   facilitate many different types of attacks on either client or
   server. For example, a DHCPACK could be modified to a DHCPNAK,
   thereby denying the client a lease.

3.  Weaknesses of RFC 3118

   An authentication mechanism for DHCPv4 protocol messages was
   developed in RFC 3118, proposing two basic authentication mechanisms
   and the means for extending the repertoire of methods as needed. The
   configuration token method (protocol 0) relies on exchanging
   clear-text authentication tokens between as yet unconfigured DHCPv4
   clients and DHCPv4 servers. It is also vulnerable to message
   interception. Delayed authentication (protocol 1) focuses on solving
   the intradomain authentication problem where the out-of-band exchange
   of a shared secret is feasible.

3.1  Key Exposure

   The configuration token protocol, protocol 0, utilizes clear-text
   authentication tokens (i.e., passwords), providing only weak entity
   authentication and no message authentication. This protocol is
   vulnerable to interception and provides only the most rudimentary
   protection against inadvertently instantiated DHCP servers. It also
   leaks the key before knowing whether the server supports protocol 0.

3.2  Key Distribution

   Both protocols 0 and 1 suffer from the lack of a means to easily,
   quickly, and reliably distribute authentication tokens used in the
   protocols. In many environments, some existing key distribution
   mechanism is presumed to be trusted and reliable, with strong
   administrative procedures and a security-conscious user population in
   place, leaving only the selection and specification of an appropriate
   cryptographic algorithm as a concern of the protocol designer.

   Relying on such out-of-band methods to distribute and manage tens or
   hundreds of thousands of tokens is a significant barrier to the
   widespread implementation of either protocol 0 or 1.



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   Key distribution presents a significant system management challenge
   that is in marked contrast with DHCP itself, a protocol that has been
   successfully deployed in environments that make few demands or
   assumptions. If we are to hope for similarly successful deployment of
   authentication for DHCP, a means for mitigating (if not eliminating)
   these difficulties must be offered.

3.3  Replay attacks

   Since the configuration token protocol, protocol 0, passes a
   clear-text authentication token, the token would be visible to any
   host on the same subnet. Delayed authentication, protocol 1, is not
   susceptible to replay attacks since it contains a nonce value
   generated by the source and a message authentication code (MAC) which
   provides both message and entity authentication.

3.4  Protocol Agreement Difficulties

   An a priori agreement is presumed to have taken place between client
   and server on the authentication protocol to use. No mechanism is
   provided to allow for the discovery of supported protocols, nor is
   there a facility for negotiation. The only way to express non-support
   of a protocol is by failing to respond.

4.  DHCPv4 Security Requirements

   DHCPv4 was developed in an era when computers were primarily used in
   business and university environments. Security was either not a
   concern or was addressed by controlling physical access stemming from
   the belief that intrusion into critical systems was most likely to
   come from an external source. Now, with wireless access points and
   ubiquitous networking, physical access control mechanisms are no
   longer sufficient, and security and privacy issues are a major
   concern.

4.1  Environments

   The following environments can be expected for DHCPv4
   implementations:
   o  Network size from a few hosts to hundreds of thousands of hosts.
   o  Network topology from a single subnet to Class-A networks.
   o  Network location from a single room to international dispersion.
   o  Wired, broadcast wireless, and directional wireless media.
   o  Movement between different media and networks.

4.2  Capabilities

   The following are essential elements of DHCPv4 security:



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   o  Clients MUST be able to authenticate servers (to prevent
      misconfigured clients and assure that the correct servers are
      being contacted).
   o  Servers MUST be able to authenticate clients (use of hardware
      addresses and client-IDs provides no real security but is all that
      is easily possible today). Better mechanisms are needed for
      servers to identify clients to which they will offer service (to
      prevent IP address pool depletion, for example).
   o  Administrators MUST be able to choose between four authentication
      paradigms:
      *  No authentication required.
      *  Mutual authentication required.
      *  Client authenticates server.
      *  Server authenticates client.
   o  Integrity of DHCP packet exchanges MUST be assured.

   Not all capabilities may be needed or desired in all situations.

4.3  Musings on the Key Distribution Problem

   The authors believe that only by addressing scalability issues with
   key distribution can RFC 3118 achieve wide deployment. While it is
   not our intention to describe solutions in this document, we admit
   that we find the model used today by browsers and secure web servers
   attractive: trusted root certificates are distributed with the client
   implementation (web browser); users have the ability to extend the
   certificates that they will accept, install their own certificates
   (should client identification be required), and choose which
   certificate to present to servers requesting the client's identity.

   Analogously, DHCPv4 servers could make use of certificates just as
   web servers do, while DHCPv4 clients could be distributed with
   appropriate certificate authority certificates (trust anchors).
   Self-signed certificates are, of course, a possibility, should an
   organization wish full control over its domain of trust.

   Should this path be pursued, we believe that certificate revocation
   will be a major problem to confront, just as it is in the browser/
   web server environment today. Revocation of client certificates
   (which we believe would occur, on the whole, much more frequently
   than revocation of server certificates), would require only ordinary
   care in certificate validation by the DHCP server.

   Revocation of server certificates is more complex because of the
   difficulty updating client configurations, as well as the inability
   of clients to rely on certificate revocation lists while in the
   process of performing IP address and configuration management.




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4.4  Data Confidentiality

   Data Confidentiality was not provided for in the original DHCP
   protocol as defined in RFC 2131 or any of the subsequent RFCs.
   Historically, DHCP was mainly used to assign IP addresses and return
   configuration options such as the local gateway and DNS information.

   Over time the DHCP protocol has evolved, deployments are extending
   beyond physically secure intranets to public networks in hotspots,
   cafes, airports, and at home over broadband. And we are seeing an
   accompanying proliferation of new configuration options.

   DHCP has, in fact, become so successful that it is now used to
   transport a great deal of configuration data that could be obtained
   in a variety of other ways. It is certainly possible that a client or
   server might wish to reveal some of these data only to a
   properly-authenticated peer.

4.4.1  "Public" Data in DHCP Packets

   We assume that any information that may be gleaned directly from the
   network using, for example, Ethernet promiscuous mode is not
   confidential. It could be argued that over time more and more
   communication will be switched, encrypted, or secured at the physical
   layer, so that less information could easily be gleaned from the
   network traffic.

   Taking encryption into consideration, the IP packet payload might be
   encrypted, but not the IP header itself since it is required for
   packet routing. As a result none of the IP header fields are
   confidential. IP addresses included in the header are therefore not
   confidential. Similarly, the hardware addresses are also not
   confidential.

   Although the IP packet payload (which would include the UDP or TCP
   header) might normally be encrypted, some protocols have made
   explicit decisions not to encrypt their exchanges, declaring their
   data public. DNS is such a protocol [dns-threats]. Thus we may also
   treat DNS domain and server information as public.

   Commonly-used routing protocols such as BGP (RFC 1771) [RFC1771], RIP
   (RFC 1721) [RFC1721], and router discovery (RFC 1256) [RFC1256] also
   normally send advertisements in the clear and we therefore extend our
   definition of public DHCP data to include routing information
   options.






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4.4.2  Protecting Data in DHCP Options

   Some DHCP options (e.g., relay agent options, RFC 3046 [RFC3046]) or
   option families (site or vendor options) admit no analysis because
   the data carried by them may be of unknown sensitivity. Analysis of
   their confidentiality requirements must be done by their users.

   Other DHCPv4 options contain opaque data, not subject to
   interpretation by a DHCPv4 server.  With no RFC-based definition of
   the data content of these options, we can offer no opinion of the
   sensitivity of the data, and leave any confidentiality treatment to
   other work.

   Should some data require confidentiality, it may be possible to
   exploit the "public" data above to allow a two-step configuration
   process in which sufficient client configuration is first obtained by
   the normal DHCPDISCOVER/OFFER/REQUEST/ACK exchange, and private data
   subsequently transmitted over a secure communications channel (such
   as IPsec) using DHCPINFORM.

5.  IANA Considerations

   None known.

6.  Security Considerations

   This entire memo presents a threat analysis of the DHCPv4 protocol.

7.  Acknowledgements

   This document is the result of work undertaken the by DHCP working
   group, beginning at 55th IETF meeting in Atlanta. The authors would
   also like to acknowledge contributions from others not directly
   involved in writing this memo, including John Beatty and Vipul Gupta
   of Sun Microsystems, and Ralph Droms of Cisco Systems. And Bernard
   Aboba and Mark Stapp for their careful reviews and suggestions.

8.  Change Log

   This section summaries the changes made to this Internet-Draft as it
   has evolved.

8.1  01 Draft

   No significant changes were made:
   o  Updated author information.
   o  Converted to using xml2rfc to generate the draft.




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   o  Removed unused references.
   o  Added the Change Log section.

8.2  02 Draft

   The following changes were made:
   o  Updated author information.
   o  Added text to 1.3 to exclude security for messages passed between
      relay agents and servers as there are two Internet-Drafts on this
      subject.
   o  Reworded several sections, particularily in section 2.
   o  Revised and renamed section 2.1.2; now includes more attacks.
   o  Revised section 2.1.3.
   o  Minor revisions to section 3, 3.2, and 3.2.
   o  Added more to section 4.4.2.
   o  Other minor insertions, deletions, and modifications based on
      comments from Bernard Aboba and Mark Stapp and to otherwise
      improve the document.

9.  References

9.1  Normative References

   [RFC1256]  Deering, S., "ICMP Router Discovery Messages", RFC 1256,
              September 1991.

   [RFC1541]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              1541, October 1993.

   [RFC1721]  Malkin, G., "RIP Version 2 Protocol Analysis", RFC 1721,
              November 1994.

   [RFC1771]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.

   [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.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option", RFC
              3046, January 2001.

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP



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              Messages", RFC 3118, June 2001.

9.2  Informative References

   [RFC3203]  T'Joens, Y., Hublet, C. and P. De Schrijver, "DHCP
              reconfigure extension", RFC 3203, December 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and
              M. Carney, "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6)", RFC 3315, July 2003.

   [auth-subopt]
              Stapp, M. and T. Lemon, "The Authentication Suboption for
              the DHCP Relay Agent Option",
              draft-ietf-dhc-auth-suboption-03 (work in progress),
              February 2004.

   [dns-threats]
              Atkins, D. and R. Austein, "Threat Analysis Of The Domain
              Name System", draft-ietf-dnsext-dns-threats-06 (work in
              progress), February 2004.

   [failover]
              Droms, R. and K. Kinnear, "DHCP Failover Protocol",
              draft-ietf-dhc-failover-12 (work in progress), December
              2003.

   [fqdn]     Stapp, M. and Y. Rekhter, "The DHCP Client FQDN Option",
              draft-ietf-dhc-fqdn-option-06 (work in progress), October
              2003.

   [leasequery]
              Woundy, R., "DHCP Lease Query",
              draft-ietf-dhc-leasequery-07 (work in progress), March
              2004.

   [relay-ipsec]
              Droms, R., "Authentication of DHCP Relay Agent Options
              Using IPsec", draft-ietf-dhc-relay-agent-ipsec-01 (work in
              progress), November 2003.











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Authors' Addresses

   Richard Barr Hibbs
   Richard Barr Hibbs, P.E.
   952 Sanchez Street
   San Francisco, California  94114-3362
   USA

   Phone: +1 415 648 3920
   Fax:   +1 415 648 9017
   EMail: rbhibbs@pacbell.net


   Carl Smith
   C & C Catering
   1121 Holly St
   Alameda, California  94502
   USA

   EMail: islandia@alumni.ucsd.edu


   Bernard Volz
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719
   USA

   Phone: +1 978 936 0382
   EMail: volz@cisco.com


   Mimi Zohar
   IBM T. J. Watson Research Center
   19 Skyline Drive
   Hawthorne, New York  10532-2134
   USA

   Phone: +1 914 784 7606
   EMail: zohar@us.ibm.com











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Internet-Draft              Threat Analysis                   April 2004


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