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
August 2012 Forcerenew Nonce Authentication
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.
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Table of Contents
1. Introduction ....................................................22. Requirements Language ...........................................33. Message Authentication ..........................................33.1. Forcerenew Nonce Authentication ............................33.1.1. Forcerenew Nonce Protocol Capability Option .........43.1.2. Forcerenew Nonce Authentication Protocol ............63.1.3. Server Considerations for Forcerenew Nonce
Authentication ......................................83.1.4. Client Considerations for Forcerenew Nonce
Authentication ......................................94. IANA Considerations ............................................105. Security Considerations ........................................105.1. Protocol Vulnerabilities ..................................116. Acknowledgements ...............................................117. Normative References ...........................................111. 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
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
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
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/
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.
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
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
4. IANA Considerations
IANA has assigned the following new DHCPv4 option code from the
registry "BOOTP Vendor Extensions and DHCP Options" maintained at
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
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.
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
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.
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