Network Working Group                                         Y. T'Joens
Request for Comments: 3203                                     C. Hublet
Category: Standards Track                                        Alcatel
                                                         P. De Schrijver
                                                           December 2001

                       DHCP reconfigure extension

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 extensions to DHCP (Dynamic Host Configuration
   Protocol) to allow dynamic reconfiguration of a single host triggered
   by the DHCP server (e.g., a new IP address and/or local configuration
   parameters).  This is achieved by introducing a unicast FORCERENEW
   message which forces the client to the RENEW state.  The behaviour
   for hosts using the DHCP INFORM message to obtain configuration
   information is also described.

1. Introduction

   The procedures as described within this document allow the dynamic
   reconfiguration of individual hosts.

1.1 Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2. DHCP force renew

   This section describes the FORCERENEW message extension.

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2.1 Terminology

   DHCP client : host to be reconfigured using DHCP.

   DHCP server : server which configured the DHCP client.

2.2 Force renew procedures

   The DHCP server sends a unicast FORCERENEW message to the client.
   Upon receipt of the unicast FORCERENEW message, the client will
   change its state to the RENEW state, and will then try to renew its
   lease according to normal DHCP procedures.  If the server wants to
   assign a new IP address to the client, it will reply to the DHCP
   REQUEST with a DHCP NAK.  The client will then go back to the init
   state and broadcast a DHCP DISCOVER message.  The server can now
   assign a new IP address to the client by replying with a DHCP OFFER.
   If the FORCERENEW message is lost, the DHCP server will not receive a
   DHCP REQUEST from the client and it should retransmit the FORCERENEW
   message using an exponential backoff algorithm.  Depending on the
   bandwidth of the network between server and client, the server should
   choose a delay.  This delay grows exponentially as retransmissions
   fail.  The amount of retransmissions should be limited.

   The procedures described above assume the server to send a unicast
   FORCERENEW message to the client.  Receipt of a multicast FORCERENEW
   message by the client should be silently discarded.

   It can be that a client has obtained a network address through some
   other means (e.g., manual configuration) and has used a DHCP INFORM
   request to obtain other local configuration parameters.  Such clients
   should respond to the receipt of a unicast FORCERENEW message with a
   new DHCP INFORM request so as to obtain a potential new set of local
   configuration parameters.  Note that the usage of these procedures
   are limited to the set of options that are eligible for configuration
   by DHCP and should not override manually configured parameters.

   Note further that usage of the FORCERENEW message to reconfigure a
   client address or local configuration parameters can lead to the
   interruption of active sessions, and that as such these  procedures
   should be used in controlled circumstances.

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2.3 Example usage

2.3.1 Embedded DHCP clients

   The autoconfiguration of home gateways (more generically Network
   Termination equipment) for public networking purposes can be achieved
   through means of DHCP, as described in [DSL_autoconf].  In order to
   allow service changes or service interruption, the FORCERENEW message
   can trigger the home gateway to contact the DHCP server, prior to the
   expiry of the lease.

2.3.2 Hospitality service scenario

   In self provisioned networks, e.g., hotel rooms, the hotel owned DHCP
   server can hand out limited use IP addresses, that allows the
   customer to consume local services or select external services from a
   web browser interface.  In order to allow external services through
   other service providers, e.g., global internet services or enterprise
   VPN services, the DHCP server can trigger the client to ask for a new
   DHCP initialization session so as to obtain e.g., a globally routed
   IP address.

2.3.3 Network renumbering

   Under tightly controlled conditions, the FORCERENEW procedures can be
   used to brute force the renumbering of entire subnets, client per
   client, under control of a DHCP server.

2.4 Rationale

   The approach as described in this document has a number of
   advantages.  It does not require new states to be added to the DHCP
   client implementation.  This minimizes the amount of code to be
   changed.  It also allows lease RENEWAL to be driven by the server,
   which can be used to optimize network usage or DHCP server load.

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3. Extended DHCP state diagram

+--------+             +------+
| Init / |         +-->+ Init +<---------------+-------------------+
| Reboot |         |   +--+---+                |                   |
+---+----+     DHCPNAK/ -/Send DHCPDISCOVER    |                   |
    |          Restart    |     (broadcast)    |                   |
    |              |      v   v-------------+  |                   |
 -/Send DHCPREQUEST| +----+------+    DHCPOFFER/DHCPDECLINE        |
    |   (broadcast)| | Selecting |----------+  |                   |
    v              | +----+------+             |                   |
+---+----+         |   DHCPOFFER/DHCPREQUEST   |                   |
| Reboot +---------+  (broadcast)              |                   |
+---+----+                v                    |                   |
    |                +----+-------+            DHCPNAK /halt network
    |                + Requesting |            |       lease expired
   DHCPACK/          +----+-------+            |                   |
   Record lease           |                    |                   |
   set timers         DHCPACK/Record lease     |                   |
    |                     v   Set T1 & T2      |                   |
    |                  +--+----+DHCPFORCE  +---+---+          +----+---+
    +----------------->+ Bound +---------->+ Renew +--------->+ Rebind |
                       +--+-+--+T1 expires +-+-+---+T2 expires+----+---+
                          ^     /DHCPREQUEST | |    /broadcast     |
                       DHCPACK    to leasing | |    DHCPREQUEST    |
                          |        server    | |                   |

4. Message layout

   The FORCERENEW message makes use of the normal DHCP message layout
   with the introduction of a new DHCP message type.  DHCP option 53
   (DHCP message type) is extended with a new value: DHCPFORCERENEW (9)

5. IANA Considerations

   The new value for DHCP option 53 (DHCP message type) to indicate a
   DHCPFORCERENEW message is 9.

6. Security Considerations

   As in some network environments FORCERENEW can be used to snoop and
   spoof traffic, the FORCERENEW message MUST be authenticated using the
   procedures as described in [DHCP-AUTH].  FORCERENEW messages failing
   the authentication should be silently discarded by the client.

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6.1 Protocol vulnerabilities

   The mechanism described in this document is vulnerable to a denial of
   service attack through flooding a client with bogus FORCERENEW
   messages.  The calculations involved in authenticating the bogus
   FORECERENEW messages may overwhelm the device on which the client is

7. References

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

   [DHCP-AUTH]    Droms, R. and W. Arbaugh, "Authentication for DHCP
                  Messages", RFC 3118, June 2001.

   [DSL_autoconf] Technical Report TR-044, "Auto-Configuration for Basic
                  Internet (IP-based) Services", DSL Forum, November

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

8. Acknowledgements

   The authors would like to thank David Allan, Nortel, for the
   constructive comments to these procedures.

9. Authors' Addresses

   Yves T'joens
   Alcatel Network Strategy Group
   Francis Wellesplein 1, 2018 Antwerp, Belgium
   Phone: +32 3 240 7890

   Peter De Schrijver
   Mind NV
   Vaartkom 11
   3000 Leuven

   Alcatel Broadband Networking Division
   Veldkant 33b, 2550 Kontich, Belgium
   Phone: +32 3 450 3322

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RFC 3203               DHCP reconfigure extension          December 2001

10. Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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