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Versions: 00 01                                                         
Network Working Group                                   William A. Arbaugh
INTERNET DRAFT                                        Angelos D. Keromytis
                                                University of Pennsylvania
                                                             November 1997
                                                          Expires May 1998

                    DHCP Continuation Option Code

Status of this memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
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   The Dynamic Host Configuration Protocol (DHCP) provides a framework
   for passing configuration information to hosts on a TCP/IP network.
   Currently options are limited to an information size of 256 bytes
   because of the one-octet size of the length field. This document
   defines a new option that permits the continuation of the previous
   option information.

1. Introduction

   The Dynamic Host Configuration Protocol (DHCP) [1] provides a
   framework for passing configuration information to hosts on a TCP/IP
   network.  Configuration parameters and other control information are
   carried in tagged data items that are stored in the 'options' field
   of the DHCP message.  The data items themselves are also called

   Each option is assigned a one-octet option code and an one-octet size
   field. The one-octet size field limits the information contained in
   an option to 256 bytes.  While there exist options that permit the use
   of the sname and file fields of the header, these options only add an
   additional 192 bytes when the fields are not in use. This document

Arbaugh/Keromytis                                                 [Page 1]

DRAFT              DHCP Continuation Option Code             November 1997

   describes a new DHCP option for continuing the information from the
   previous option. This option MUST not appear as the first option in
   a message.  The option preceding this one MUST have a size of 256

2. Definition of option [TBD]

   Option code [TBD] indicates that the data contained in the option is
   a continuation of the previous option.

    Code   Len  option code   Data...
   | TBD | XXX | Continuation of previous option data

   The example below shows how the option would work with a hypothetical
   authentication option that requires more than 255 bytes of information.

    Code   Len  option Data...
   | 90  | 256 | 04  | d1    d2    d4     ... d255
    Code   Len  Data...
   | TBD | 20  | d257 d258  d259  d260    ... d276

4. References

   [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
       Bucknell University, March 1997.

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

5. Security Considerations

   DHCP currently provides no authentication or security mechanisms.
   Potential exposures to attack are discussed in section 7 of the DHCP
   protocol specification [1].  One of the reasons for this definition is
   to provide support for the exchange of public key certificates are
   which usually larger than 256 bytes.

Arbaugh/Keromytis                                                 [Page 2]

DRAFT              DHCP Continuation Option Code             November 1997

6. Author's Address

   William A. Arbaugh
   Angelos D. Keromytis
   Distributed Systems Lab -- 102 Moore
   Department of Computer and Information Sciences
   University of Pennsylvania
   200 South 33rd St.
   Philadelphia, PA. 19104-6389

   Email: {waa, angelos}@dsl.cis.upenn.edu

Arbaugh/Keromytis                                                 [Page 3]