DHCP FORCERENEW: DHCP reconfigure extension
Lets a DHCP server tell one client to renew its lease now, instead of waiting to be asked.
T'Joens, Y., Hublet, C., and P. De Schrijver, "DHCP reconfigure extension", RFC 3203, DOI 10.17487/RFC3203, December 2001, <https://www.rfc-editor.org/info/rfc3203>.
@misc{rfc3203,
series = {Request for Comments},
number = 3203,
howpublished = {RFC 3203},
publisher = {RFC Editor},
doi = {10.17487/RFC3203},
url = {https://www.rfc-editor.org/info/rfc3203},
author = {Y. T'Joens and C. Hublet and P. De Schrijver},
title = {{DHCP reconfigure extension}},
pagetotal = 6,
year = 2001,
month = dec,
} RFC text
Contents
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.
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.
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.
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
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
10. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Reconfiguring a Host from the Server
This RFC defines extensions to DHCP that allow dynamic reconfiguration of a single host, triggered by the DHCP server. The change might be a new IP address, new local configuration parameters, or both. It does this by adding a unicast FORCERENEW message that forces the client into the RENEW state. The RFC also says how hosts that use the DHCP INFORM message should behave.
The key words MUST, SHOULD and so on are used as in RFC 2119.
Normally you have to ask the lunch lady for a new tray. Here the lunch lady can walk over and tell one kid to come and ask again.
§2.2Force Renew Procedures
The server sends a unicast FORCERENEW to the client. On receipt, the client changes to the RENEW state and renews its lease with normal DHCP procedures. If the server wants the client to take a new address, it answers the client's DHCP REQUEST with a DHCP NAK. The client then goes back to the init state and broadcasts a DHCP DISCOVER, and the server can reply with an OFFER carrying the new address.
If the FORCERENEW is lost, the server sees no REQUEST from the client. It should then retransmit using an exponential backoff, choosing a delay that suits the bandwidth between server and client, and it should limit the number of retransmissions.
A multicast FORCERENEW should be silently discarded by the client. A client that got its address some other way (manual configuration, for example) and only uses INFORM to get other parameters should answer a unicast FORCERENEW with a new INFORM. It should not override manually configured parameters.
Reconfiguring a client this way can interrupt active sessions, so the procedures should be used in controlled circumstances.
The helper taps one computer on the shoulder, and the computer asks again for its settings. If the helper wants it to change address, the helper says no to the old one, and the computer starts over.
- FORCERENEW (unicast)Client moves to the RENEW state.
- DHCP REQUESTNormal lease renewal.
- DHCP NAKSent when the server wants to assign a new address.
- DHCP DISCOVER (broadcast)Client is back in the init state.
- DHCP OFFERCarries the new IP address.
§2.3Example Uses and Rationale
The RFC gives three example uses. Embedded clients such as home gateways can be told to contact the server before their lease expires, which allows service changes or interruptions. In a hotel, the server can hand out limited-use addresses first and later trigger a new initialization so the guest gets a globally routed address. Under tightly controlled conditions, FORCERENEW can renumber whole subnets, client by client.
The stated advantages are that no new states are needed in the client, which minimizes code changes, and that lease renewal can be driven by the server, which can optimize network usage or server load.
A hotel gives you a basic pass at first. Later it can ask you to come back to the desk to get a better one.
§4Message Layout and the Extended State Diagram
FORCERENEW uses the normal DHCP message layout. The only change is a new value for DHCP option 53 (DHCP message type): DHCPFORCERENEW, value 9. IANA is asked to record that value.
The extended state diagram in the RFC adds one arrow to the DHCP client's states. A client in the Bound state can move to Renew either when timer T1 expires or on receiving the DHCPFORCERENEW message, and the rest of the diagram is unchanged.
It is the same envelope as always, with a new sticker on it that says: please renew.
§6Security Considerations
FORCERENEW could be used to snoop on and spoof traffic in some network environments. So the message MUST be authenticated using the procedures in RFC 3118, and messages that fail authentication should be silently discarded by the client.
The mechanism is still open to a denial of service attack: a flood of bogus FORCERENEW messages can make a client spend so much effort on authentication checks that the device is overwhelmed.
Only a real helper should be allowed to tap you on the shoulder. But if a crowd of fakes taps you all day, you get too tired to do anything else.