Forcerenew Nonce Authentication
A way to authenticate DHCPv4 FORCERENEW messages with a per-client nonce, without distributing keys out of band.
Miles, D., Dec, W., Bristow, J., and R. Maglione, "Forcerenew Nonce Authentication", RFC 6704, DOI 10.17487/RFC6704, August 2012, <https://www.rfc-editor.org/info/rfc6704>.
@misc{rfc6704,
series = {Request for Comments},
number = 6704,
howpublished = {RFC 6704},
publisher = {RFC Editor},
doi = {10.17487/RFC6704},
url = {https://www.rfc-editor.org/info/rfc6704},
author = {D. Miles and W. Dec and J. Bristow and R. Maglione},
title = {{Forcerenew Nonce Authentication}},
pagetotal = 12,
year = 2012,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) D. Miles
Request for Comments: 6704 Google
Updates: 3203 W. Dec
Category: Standards Track Cisco Systems
ISSN: 2070-1721 J. Bristow
Swisscom Schweiz AG
R. Maglione
Telecom Italia
August 2012
Forcerenew Nonce Authentication
Abstract
Dynamic Host Configuration Protocol (DHCP) FORCERENEW allows for the
reconfiguration of a single host by forcing the DHCP client into a
Renew state on a trigger from the DHCP server. In the Forcerenew
Nonce Authentication protocol, the server sends a nonce to the client
in the initial DHCP ACK that is used for subsequent validation of a
FORCERENEW message. This document updates RFC 3203.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6704.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Requirements Language ...........................................3
3. Message Authentication ..........................................3
3.1. Forcerenew Nonce Authentication ............................3
3.1.1. Forcerenew Nonce Protocol Capability Option .........4
3.1.2. Forcerenew Nonce Authentication Protocol ............6
3.1.3. Server Considerations for Forcerenew Nonce
Authentication ......................................8
3.1.4. Client Considerations for Forcerenew Nonce
Authentication ......................................9
4. IANA Considerations ............................................10
5. Security Considerations ........................................10
5.1. Protocol Vulnerabilities ..................................11
6. Acknowledgements ...............................................11
7. Normative References ...........................................11
1. Introduction
The DHCP reconfigure extension defined in [RFC3203] is a useful
mechanism allowing dynamic reconfiguration of a single host triggered
by the DHCP server. Its application is currently limited by a
requirement that a Forcerenew message is always authenticated using
procedures as described in [RFC3118]. Authentication for DHCP
[RFC3118] is mandatory for FORCERENEW; however, as it is currently
defined, [RFC3118] requires distribution of constant token or shared-
secret out-of-band to DHCP clients.
The motivation for making authentication mandatory in DHCP FORCERENEW
was to prevent an off-network attacker from taking advantage of DHCP
FORCERENEW to accurately predict the timing of a DHCP renewal.
Without DHCP FORCERENEW, DHCP renewal timing is under the control of
the client, and an off-network attacker has no way of predicting when
it will happen, since it doesn't have access to the exchange between
the DHCP client and DHCP server.
However, the requirement to use the DHCP authentication described in
[RFC3118] is more stringent than is required for this use case and
has limited adoption of DHCP FORCERENEW. [RFC3315] defines an
authentication protocol using a nonce to prevent off-network
attackers from successfully causing clients to renew. Since the off-
network attacker doesn't have access to the nonce, it can't trick the
client into renewing at a time of its choosing.
This document defines extensions to Authentication for DHCPv4
Messages [RFC3118] to create a new authentication protocol for DHCPv4
FORCERENEW [RFC3203] messages; this method does not require out-of-
band key distribution to DHCP clients. The Forcerenew Nonce is
exchanged between server and client on initial DHCP ACK and is used
for verification of any subsequent FORCERENEW message. This document
updates [RFC3203].
2. Requirements Language
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 [RFC2119].
3. Message Authentication
The Forcerenew message MUST be authenticated using either [RFC3118]
or the proposed Forcerenew Nonce Authentication protocol.
3.1. Forcerenew Nonce Authentication
The Forcerenew Nonce Authentication protocol provides protection
against misconfiguration of a client caused by a Forcerenew message
sent by a malicious DHCP server. In this protocol, a DHCP server
sends a Forcerenew Nonce to the client in the initial exchange of
DHCP messages. The client records the Forcerenew Nonce for use in
authenticating subsequent Forcerenew messages from that server. The
server then includes a Hashed Message Authentication Code (HMAC)
computed from the Forcerenew nonce in subsequent Forcerenew messages.
Both the Forcerenew Nonce sent from the server to the client and the
HMAC in subsequent Forcerenew messages are carried as the
Authentication information in a DHCP Authentication option. The
format of the Authentication information is defined in the following
section.
The Forcerenew Nonce Authentication protocol is used (initiated by
the server) only if the client and server are not using the
authentication mechanism specified in [RFC3118] and the client and
server have negotiated to use the Forcerenew Nonce Authentication
protocol.
3.1.1. Forcerenew Nonce Protocol Capability Option
A DHCP client indicates DHCP Forcerenew Nonce Protocol capability by
including a FORCERENEW_NONCE_CAPABLE (145) option in DHCP Discover
and Request messages sent to the server.
A DHCP server that does not support Forcerenew Nonce Authentication
protocol authentication SHOULD ignore the FORCERENEW_NONCE_CAPABLE
(145) option. A DHCP server indicates DHCP Forcerenew Nonce Protocol
preference by including a FORCERENEW_NONCE_CAPABLE (145) option in
any DHCP Offer messages sent to the client.
A DHCP client MUST NOT send DHCP messages with authentication options
where the protocol value is Forcerenew Nonce Authentication.
The FORCERENEW_NONCE_CAPABLE option contains code 145, length n, and
a sequence of algorithms the client supports:
Code Len Algorithms
+-----+-----+----+----+----+
| 145 | n | A1 | A2 | A3 | ....
+-----+-----+----+----+----+
Figure 1: FORCERENEW_NONCE_CAPABLE Option
In this document, Section 3.1.2 defines the Forcerenew Nonce
Authentication protocol for algorithm equal to 1 and type equal to 2;
future documents will specify the other values for algorithm !=1 and
type !=2, allowing a different algorithm to be used with shorter/
longer values.
The client would indicate that it supports the functionality by
inserting the FORCERENEW_NONCE_CAPABLE option in the DHCP Discover
and Request messages. If the server supports Forcerenew nonce
authentication and requires Forcerenew nonce authentication, it will
insert the FORCERENEW_NONCE_CAPABLE option in the DHCPOFFER.
Server Client Server
(not selected) (selected)
v v v
| | |
| Begins initialization |
| | |
| _____________/|\____________ |
|/DHCPDISCOVER | DHCPDISCOVER \|
| w/FORCERENEW- | w/FORCERENEW- |
| NONCE-CAPABLE | NONCE-CAPABLE |
| | |
Determines | Determines
configuration | configuration
| | |
|\ | /|
| \__________ | _________/ |
| DHCPOFFER \ | /DHCPOFFER |
|w/FORCERENEW \ | /w/FORCERENEW|
|NONCE-CAPABLE \| /NONCE-CAPABLE|
| | |
| Collects replies |
| | |
| Selects configuration |
| | |
| _____________/|\____________ |
|/ DHCPREQUEST | DHCPREQUEST\ |
| w/Forcerenew- | w/Forcerenew- |
| Nonce-Capable | Nonce-Capable |
| | |
| | Commits configuration
| | |
| |Creates 128-bit Forcerenew Nonce
| | |
| | _____________/|
| |/ DHCPACK |
| | w/Auth-Proto= |
| | Forcerenew- |
| | Nonce |
| | |
|Client stores Forcerenew Nonce |
| | |
| Initialization complete |
| | |
. . .
. . .
| | |
| Forcerenew |
| | _____________/|
| |/ DHCPFORCE |
| | w/Auth-Proto= |
| | Forcerenew- |
| | Digest(HMAC)|
| | |
| Client checks HMAC digest |
| using stored Forcerenew Nonce |
| | |
| |\____________ |
| | DHCPREQUEST\ |
| | w/FORCERENEW- |
| | NONCE-CAPABLE |
| | |
| | Commits configuration
| | |
| |Creates 128-bit Forcerenew Nonce
| | |
| | _____________/|
| |/ DHCPACK |
| | w/Auth-Proto= |
| | Forcerenew- |
| | Nonce |
| | |
| | |
| | |
. . .
. . .
| | |
| Graceful shutdown |
| | |
| |\ ____________ |
| | DHCPRELEASE \|
| | |
| | Discards lease
| | |
v v v
Figure 2: Timeline Diagram of Messages Exchanged between DHCP Client
and Servers Using the Forcerenew Nonce Authentication Protocol
3.1.2. Forcerenew Nonce Authentication Protocol
The Forcerenew Nonce Authentication protocol makes use of both the
DHCP authentication option defined in [RFC3118] reusing the option
format and of the Reconfigure Key Authentication Protocol defined in
[RFC3315].
The following diagram defines the format of the DHCP authentication
option:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Protocol | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RDM | Replay Detection (64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Replay cont. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Replay cont. | |
+-+-+-+-+-+-+-+-+ |
| |
| Authentication Information |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Format of the DHCP Authentication Option
The following fields are set in an DHCP authentication option for the
Forcerenew Nonce Authentication protocol.
Code: 90 (Authentication) per [RFC3118]
Length: contains the length of the protocol
Protocol: 3 (Reconfigure Key) per [RFC3118]
Algorithm: 1 (HMAC-MD5) per [RFC3118] and [RFC3315]
Replay Detection: per the Replay Detection Method (RDM)
Replay Detection Method (RDM): 0
Authentication Information: specified below
The format of the Authentication Information for the Forcerenew Nonce
Authentication Protocol is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Value (128 bits) |
+-+-+-+-+-+-+-+-+ |
. .
. .
. +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Format of the Authentication Information
Type: The type of data in Value field carried in this option:
1 Forcerenew nonce Value (used in ACK message)
2 HMAC-MD5 digest of the message (Forcerenew message)
Value: The message authentication code generated by applying MD5
to the DHCP message
3.1.3. Server Considerations for Forcerenew Nonce Authentication
The use of Forcerenew Nonce Authentication protocol is dependent on
the client indicating its capability through the
FORCERENEW_NONCE_CAPABLE (145) DHCP option in any DHCP Discover or
Request messages. The DHCP Discovery or Request message from the
client MUST contain the FORCERENEW_NONCE_CAPABLE (145) option if the
Forcerenew Nonce Protocol is to be used by the server. The absence
of the FORCERENEW_NONCE_CAPABLE (145) option indicates to the server
that the Forcerenew Nonce Authentication protocol is not supported;
thus, the server MUST NOT include a Forcerenew Nonce Protocol
Authentication option in the DHCP ACK.
The server indicates its support of the Forcerenew Nonce
Authentication protocol by including the DHCP
FORCERENEW_NONCE_CAPABLE (145) option in the DHCPOFFER. The server
SHOULD NOT include this option unless the client has indicated its
capability in a DHCP Discovery message. The presence of the
FORCERENEW_NONCE_CAPABLE (145) option in the DHCP offer may be used
by clients to prefer DHCP servers that are Forcerenew Nonce
Authentication protocol capable over those servers that do not
support such capability.
If a capable server receives a DISCOVER or REQUEST (any type) that
indicates the client is capable, and the server has no previous nonce
recorded, it MUST generate a nonce and include it in the ACK.
The server selects a Forcerenew Nonce for a client only during
Request/ACK message exchange. The server records the Forcerenew
nonce and transmits that nonce to the client in an Authentication
option in the DHCP ACK message.
The server SHOULD NOT include the nonce in an ACK when responding to
a renew unless a new nonce was generated. This minimizes the number
of times the nonce is sent over the wire.
If the server to which the DHCP Request message was sent at time T1
has not responded, the client enters the REBINDING state and attempts
to contact any server. The new Server receiving the DHCP message
MUST generate a new nonce.
The Forcerenew nonce is 128 bits long, and it MUST be a
cryptographically strong random or pseudo-random number that cannot
easily be predicted. The nonce is embedded as a 128-bit value of the
Authentication information where type is set to 1 (Forcerenew nonce
Value).
To provide authentication for a Forcerenew message, the server
selects a replay detection value according to the RDM selected by the
server and computes an HMAC-MD5 of the Forcerenew message, based on
the procedure specified in Section 21.5 of [RFC3315], using the
Forcerenew Nonce for the client. The server computes the HMAC-MD5
over the entire DHCP Forcerenew message, including the Authentication
option; the HMAC-MD5 field in the Authentication option is set to
zero for the HMAC-MD5 computation
3.1.4. Client Considerations for Forcerenew Nonce Authentication
A client that supports this mechanism MUST indicate Forcerenew nonce
Capability by including the FORCERENEW_NONCE_CAPABLE (145) DHCP
option defined in Section 3.1.1 in all DHCP Discover and Request
messages. DHCP servers that support Forcerenew nonce Protocol
authentication MUST include the FORCERENEW_NONCE_CAPABLE (145) DHCP
option in all DHCP Offers, allowing the client to use this capability
in selecting DHCP servers should multiple Offers arrive.
The client MUST validate the DHCP ACK message contains a Forcerenew
Nonce in a DHCP authentication option. If the server has indicated
capability for Forcerenew Nonce Authentication protocol in the DHCP
OFFER and the subsequent ACK received by the client while in the
selecting state omits a valid DHCP authentication option for the
Forcerenew Nonce Authentication protocol, the client MUST discard the
message and return to the INIT state.
The client MUST record the Forcerenew Nonce from any valid ACK it
receives, if the ACK contains one.
To authenticate a Forcerenew message, the client computes an HMAC-
MD5, based on the procedure specified in Section 21.5 of [RFC3315],
over the DHCP Forcerenew message (after setting the HMAC-MD5 field in
the Authentication option to zero), using the Forcerenew Nonce
received from the server. If this computed HMAC-MD5 matches the
value in the Authentication option, the client accepts the FORCERENEW
message.
4. IANA Considerations
IANA has assigned the following new DHCPv4 option code from the
registry "BOOTP Vendor Extensions and DHCP Options" maintained at
http://www.iana.org/assignments/bootp-dhcp-parameters:
Tag: 145
Name: FORCERENEW_NONCE_CAPABLE
Data length: 1
Description: Forcerenew Nonce Capable
Reference: this document
5. 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 [RFC3118] or the mechanism described in
this document.
The mechanism in [RFC3315] for DHCPv6, which this document mirrors
for DHCPv4, uses a nonce to prevent an off-link attacker from
successfully triggering a renewal on a client by sending
DHCPFORCERENEW; since the attacker is off-link, it doesn't have the
nonce, and can't force a renewal.
An on-link attacker can always simply watch the DHCP renewal message
go out and respond to it, so this mechanism is useless for preventing
on-link attacks; hence, the security of the nonce in the case of on-
link attacks isn't relevant. Therefore, HMAC-MD5 is, by definition,
adequate for the purpose, and there is no need for an extensible HMAC
mechanism. FORCERENEW messages failing the authentication should be
silently discarded by the client.
5.1. Protocol Vulnerabilities
The mechanism described in this document is vulnerable to a denial-
of-service (DoS) 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.
The mechanism described provides protection against the use of a
FORCERENEW message by a malicious DHCP server to mount a DoS or man-
in-the-middle attack on a client. This protocol can be compromised
by an attacker that can intercept the initial message in which the
DHCP server sends the nonce to the client.
6. Acknowledgements
This contribution is based on work by Vitali Vinokour. Major
sections of this document use modified text from [RFC3315]. The
authors wish to thank Ted Lemon, Matthew Ryan, and Bernie Volz for
their support.
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
[RFC3203] T'Joens, Y., Hublet, C., and P. De Schrijver, "DHCP
reconfigure extension", RFC 3203, December 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
Authors' Addresses
David Miles
Google
EMail: davidmiles@google.com
Wojciech Dec
Cisco Systems
Haarlerbergpark Haarlerbergweg 13-19
Amsterdam, NOORD-HOLLAND 1101 CH
Netherlands
EMail: wdec@cisco.com
James Bristow
Swisscom Schweiz AG
Zentweg 9
Bern, 3050,
Switzerland
EMail: James.Bristow@swisscom.com
Roberta Maglione
Telecom Italia
Via Reiss Romoli 274
Torino 10148
Italy
EMail: roberta.maglione@telecomitalia.it
AI ELI5
On this page
§1Why FORCERENEW Needs Lighter Authentication
The DHCP reconfigure extension in RFC 3203 lets a server trigger reconfiguration of a single host. It requires every Forcerenew message to be authenticated using RFC 3118, which in turn needs a constant token or shared secret to be distributed to clients out of band.
The reason for mandatory authentication is to stop an off-network attacker from predicting when a renewal will happen. Without FORCERENEW, renewal timing is under the client's control and the attacker cannot see the exchange.
The RFC argues that the RFC 3118 requirement is more stringent than this use case needs and has limited adoption of FORCERENEW. RFC 6704 borrows the nonce idea from DHCPv6 (RFC 3315) and defines a new authentication protocol for DHCPv4 FORCERENEW that needs no out-of-band key distribution. It updates RFC 3203.
The old rule was like needing a secret handshake taught to everyone in advance. This one lets the server hand out the secret on the spot.
§2How the Nonce Protocol Works
A Forcerenew message MUST now be authenticated using either RFC 3118 or the Forcerenew Nonce Authentication protocol. The new protocol protects a client against misconfiguration caused by a Forcerenew message from a malicious DHCP server.
The server sends a Forcerenew Nonce to the client in the initial exchange of DHCP messages. The client records it. The server then includes a Hashed Message Authentication Code (HMAC) computed from the nonce in later Forcerenew messages. Both the nonce and the HMAC are carried as authentication information in a DHCP Authentication option.
The protocol is used only if the client and server are not using the RFC 3118 mechanism and have negotiated to use it.
The server gives you a secret word once. After that, every 'please check in' note has a stamp that only someone who knows the word can make.
§3Negotiating Support
A client signals support by including a FORCERENEW_NONCE_CAPABLE option (code 145) in its DHCP Discover and Request messages. The option holds the code, a length n and a list of algorithms the client supports. This document defines algorithm 1 with type 2; other values are left to future documents.
A server that does not support the protocol SHOULD ignore the option. A server that does support it and requires it puts the same option in its DHCPOFFER. The server SHOULD NOT include the option unless the client indicated capability, and a client may prefer capable servers when several offers arrive. A capable server MUST include the option in all offers, and a client that supports the mechanism MUST include it in all Discover and Request messages.
If the client's message lacks the option, the server MUST NOT include a Forcerenew Nonce authentication option in the DHCP ACK. A client MUST NOT send DHCP messages with authentication options whose protocol value is Forcerenew Nonce Authentication.
Before using the secret word, both sides say 'I know this trick'. If one side does not, they skip it.
- DHCPDISCOVERIncludes FORCERENEW_NONCE_CAPABLE.
- DHCPOFFERIncludes FORCERENEW_NONCE_CAPABLE.
- DHCPREQUESTIncludes FORCERENEW_NONCE_CAPABLE.
- DHCPACKCarries a 128-bit Forcerenew Nonce; the client stores it.
- ForcerenewCarries an HMAC digest; the client checks it with the stored nonce.
- DHCPREQUESTRenewal, again with FORCERENEW_NONCE_CAPABLE.
- DHCPACKCarries a nonce again.
- DHCPRELEASEGraceful shutdown; the lease is discarded.
§4The Authentication Option Format
The protocol reuses the DHCP authentication option format from RFC 3118 and the Reconfigure Key Authentication Protocol from RFC 3315. The option fields are set as follows: Code 90 (Authentication), Length (the length of the protocol), Protocol 3 (Reconfigure Key), Algorithm 1 (HMAC-MD5), Replay Detection Method (RDM) 0, and a Replay Detection value set according to the RDM.
The Authentication Information begins with a Type byte followed by a 128-bit Value. Type 1 means the Forcerenew nonce value, used in the ACK message. Type 2 means the HMAC-MD5 digest of the Forcerenew message.
The note has labelled boxes. One box says what kind of secret stuff is inside, and another holds the secret word or the stamp.
- 90Code — Authentication option, per RFC 3118.
- 3Protocol — Reconfigure Key, per RFC 3118.
- 1Algorithm — HMAC-MD5.
- 0RDM — Replay Detection Method used by this protocol.
- 1Type 1 — Forcerenew nonce value, used in the ACK message.
- 2Type 2 — HMAC-MD5 digest of the Forcerenew message.
§5Server and Client Behavior
If a capable server gets a DISCOVER or REQUEST from a capable client and has no nonce recorded yet, it MUST generate one and put it in the ACK. The server picks a nonce only during the Request/ACK exchange, records it and sends it in an Authentication option. It SHOULD NOT repeat the nonce in an ACK to a renew unless a new one was generated. If the client moves to the REBINDING state and a new server answers, that server MUST generate a new nonce.
The nonce is 128 bits long and MUST be a cryptographically strong random or pseudo-random number that cannot easily be predicted. To authenticate a Forcerenew message, the server picks a replay detection value according to its RDM and computes an HMAC-MD5 over the entire message, following Section 21.5 of RFC 3315, with the HMAC field set to zero during the calculation.
The client MUST check that the ACK contains a Forcerenew Nonce in an authentication option. If the server advertised capability in its OFFER but the ACK omits a valid option, the client MUST discard the message and return to the INIT state. The client MUST record the nonce from any valid ACK that has one. On a Forcerenew message it computes the same HMAC-MD5 with the stored nonce and accepts the message only if the result matches.
The server makes up a hard-to-guess secret word and tells it to you. Later you check that the stamp on a message matches what that word would make. If it does not match, you ignore the message.
§6Security Considerations
FORCERENEW can be used in some networks to snoop and spoof traffic, so every FORCERENEW message MUST be authenticated, either with RFC 3118 or with this mechanism. As in DHCPv6, the nonce stops an off-link attacker, who does not have it, from forcing a renewal.
An on-link attacker can simply watch the renewal go out and answer it, so the mechanism does not help against on-link attacks. The RFC concludes HMAC-MD5 is adequate for the purpose and no extensible HMAC mechanism is needed. Messages that fail authentication should be silently discarded by the client.
Known weaknesses: a flood of bogus FORCERENEW messages can overwhelm the client device with authentication work, and the protocol can be compromised by an attacker who can intercept the initial message in which the server sends the nonce.
The secret word keeps strangers far away from tricking you. It cannot help against someone standing right next to you who hears the word, and a crowd shouting fake notes can still tire you out.
§7IANA Considerations
IANA assigned a new DHCPv4 option code from the registry 'BOOTP Vendor Extensions and DHCP Options': tag 145, name FORCERENEW_NONCE_CAPABLE, data length 1, description 'Forcerenew Nonce Capable', with this document as the reference.
The group that keeps the official list of option numbers wrote down number 145 for this feature.