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Network Working Group                                         Y. T'JoensRequest for Comments: 3203                                     C. HubletCategory: Standards Track                                        Alcatel                                                         P. De Schrijver                                                                    Mind                                                           December 2001DHCP reconfigure extensionStatus 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 inRFC 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 20012.1 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 20012.3 Example usage2.3.1 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 20013. 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.T'Joens, et al.             Standards Track                     [Page 4]

RFC 3203               DHCP reconfigure extension          December 20016.1 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",RFC2131, 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.beT'Joens, et al.             Standards Track                     [Page 5]

RFC 3203               DHCP reconfigure extension          December 200110. 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]