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 (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.
The procedures as described within this document allow the dynamic
reconfiguration of individual hosts.
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.
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.
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
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.
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.
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.
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
[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.
The authors would like to thank David Allan, Nortel, for the
constructive comments to these procedures.
9. Authors' Addresses
Alcatel Network Strategy Group
Francis Wellesplein 1, 2018 Antwerp, Belgium
Phone: +32 3 240 7890
Peter De Schrijver
Alcatel Broadband Networking Division
Veldkant 33b, 2550 Kontich, Belgium
Phone: +32 3 450 3322
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