Internet Engineering Task Force                          Richard Johnson
Internet Draft                                               Kim Kinnear
Expiration: August 2003                                       Mark Stapp
File: draft-ietf-dhc-server-override-00.txt               Jay Kumarasamy
                                                     Cisco Systems, Inc.

                   DHCP Server-ID Override Suboption

                            February 5, 2003

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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-

   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

      The list of Internet-Draft Shadow Directories can be accessed at

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This memo defines a new suboption of the DHCP relay information
   option [6] which allows the DHCP relay to specify a new value for the
   Server-ID option, which is inserted by the DHCP Server.  In some
   cases it is convenient for the DHCP relay to act as the actual DHCP
   server such that DHCP RENEWAL requests will come to the relay instead
   of going to the server directly.  This gives the relay the
   opportunity to include the Relay Agent option with appropriate

Johnson, et. al.                                                [Page 1]

Internet Draft      DHCP Server-ID Override Suboption      February 2003

   suboptions even on RENEWAL messages.

   This new relay agent suboption allows the relay to tell the DHCP
   server what value to use in the Server-ID option [3].  If this
   suboption is not present, the server should build the Server-ID
   option in the normal fashion.

1.0 Introduction

   There are many situations where the DHCP relay is involved and can
   insert a relay agent option with appropriate suboptions easily into
   DHCP DISCOVER messages.  Once the lease has been granted, however,
   future DHCP RENEWAL messages are sent directly to the DHCP Server as
   specified in the Server-ID option.  This means that the relay may not
   see the DHCP RENEWAL messages (depending upon network topology) and
   thus can not provide the same relay agent option information in the
   RENEWAL messages.

   This new DHCP relay agent suboption, Server-ID override, allows the
   relay to tell the DHCP server what value to place into the Server-ID
   option.  Using this, the relay agent can force RENEWAL messages to
   come to it instead of the server.  The relay may then insert the
   relay agent option with appropriate suboptions and relay the request
   to the actual server.  In this fashion the DHCP server will be
   provided with the same relay agent information upon renewals (such as
   Circuit-ID, Remote-ID, Device Class, etc.) as was provided in the
   initial DISCOVER message.  In effect, this makes a RENEWAL into a

   This new suboption could also be used by the DHCP relay in order to
   allow the relay to appear as the actual DHCP server to the client.
   This has the advantage that the relay can more easily keep up-to-date
   information about leases granted, etc.

   In short, this new suboption allows the DHCPv4 relay to function in
   the same fashion as the DHCPv6 relay currently does.

1.1 Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [1].

2.0 Server-ID Override Suboption Definition

   The format of the suboption is:

      Code   Len    Overridden Server-ID address

Johnson, et. al.                                                [Page 2]

Internet Draft      DHCP Server-ID Override Suboption      February 2003

      | TBD |  n  | a1  | a2  | a3  | a4  |

   The option length (n) is 4.  The octets "a1" through "a4" specify the
   value which SHOULD be inserted into the Server-ID option by the DHCP
   Server upon reply.

   DHCP Servers SHOULD use this value as the value to insert into the
   Server-ID option ONLY when the protocol is in the SELECTING and
   REQUESTING and REBINDING states.  If this suboption appears in a DHCP
   request which is part of a lease RENEWAL, it SHOULD be ignored.

3.0 IANA Considerations


4.0 Acknowledgements

   This document is the result of work done within Cisco Systems.
   Thanks to Jay Kumarasamy, Kim Kinnear, and Mark Stapp for their work
   on this suboption definition and the other related work for which
   this is necessary.

5.0 Security Considerations

   Message authentication in DHCP for intradomain use where the out-of-
   band exchange of a shared secret is feasible is defined in RFC 3118
   [5].  Potential exposures to attack are discussed in section 7 of the
   DHCP protocol specification in RFC 2131 [2].

   The DHCP Relay Agent option depends on a trusted relationship between
   the DHCP relay agent and the server, as described in section 5 of RFC
   3046.  While the introduction of fraudulent relay-agent options can
   be prevented by a perimeter defense that blocks these options unless
   the relay agent is trusted, a deeper defense using the authentication
   option for relay agent options [4] SHOULD be deployed as well.

   If a rouge DHCP relay were inserted between the client and the
   server, it could redirect clients to it using this suboption.  This
   would allow such a system to later deny renew requests and thus force
   clients to discontinue use of their allocated address.  This
   interception, however, would need to be done during the initial
   DISCOVER and OFFER phase, since the suboption value SHOULD be ignored
   by the server during RENEWAL state.  Either DHCP Authentication [5]
   or DHCP Relay Agent option authentication [4] would address this

Johnson, et. al.                                                [Page 3]

Internet Draft      DHCP Server-ID Override Suboption      February 2003


   [1] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, BCP 14, March 1997.

   [2] Droms, R. "Dynamic Host Configuration Protocol", RFC 2131,
             March 1997.

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

   [4] Stapp, M. "The Authentication Suboption for the DHCP
       Relay Agent Option", draft-ietf-dhc-auth-suboption-00.txt,
       June 23, 2002

   [5] Droms, R. "Authentication for DHCP Messages", RFC 3118,
       June 2001

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

Author Information:

   Richard Johnson
   Jay Kumarasamy
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA 95134

   Phone: (408) 526-4000


   Kim Kinnear
   Mark Stapp
   Cisco Systems
   250 Apollo Drive
   Chelmsford, MA  01824

   Phone: (978) 244-8000


Johnson, et. al.                                                [Page 4]