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rfc:rfc3203

Network Working Group Y. T'Joens Request for Comments: 3203 C. Hublet Category: Standards Track Alcatel

                                                       P. De Schrijver
                                                                  Mind
                                                         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.

Abstract

 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

2. DHCP force renew

 This section describes the FORCERENEW message extension.

T'Joens, et al. Standards Track [Page 1] RFC 3203 DHCP reconfigure extension December 2001

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.

T'Joens, et al. Standards Track [Page 2] RFC 3203 DHCP reconfigure extension December 2001

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.

T'Joens, et al. Standards Track [Page 3] RFC 3203 DHCP reconfigure extension December 2001

3. Extended DHCP state diagram

+——–+ +——+

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.

T'Joens, et al. Standards Track [Page 4] RFC 3203 DHCP reconfigure extension December 2001

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
 running.

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
                2001
 [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
 EMail: yves.tjoens@alcatel.be
 Peter De Schrijver
 Mind NV
 Vaartkom 11
 3000 Leuven
 EMail: p2@mind.be
 Alcatel Broadband Networking Division
 Veldkant 33b, 2550 Kontich, Belgium
 Phone: +32 3 450 3322
 EMail: Christian.Hublet@alcatel.be

T'Joens, et al. Standards Track [Page 5] RFC 3203 DHCP reconfigure extension December 2001

10. Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

T'Joens, et al. Standards Track [Page 6]

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