softwire D. Miles
Internet-Draft Alcatel-Lucent
Intended status: Standards Track W. Dec
Expires: June 23, 2012 Cisco Systems
J. Bristow
Swisscom Schweiz AG
R. Maglione
Telecom Italia
December 21, 2011
Forcerenew Nonce Authentication
draft-ietf-dhc-forcerenew-nonce-03
Abstract
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 Forcerenew Nonce Authentication the server sends a nonce
with the client on the initial DHCP ACK that is used for subsequent
validation of a Forcerenew message.
Status of this Memo
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This Internet-Draft will expire on June 23, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
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 Protocol . . . . . . . . . . . . . . 6
3.1.3. Server considerations for Forcerenew Nonce
Authentication . . . . . . . . . . . . . . . . . . . . 7
3.1.4. Client considerations for Forcerenew Nonce
Authentication . . . . . . . . . . . . . . . . . . . . 8
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Protocol vulnerabilities . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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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 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 mandatory authentication was
originally motivated by a legitimate security concern whereby in some
network environments a FORCERENEW message can be spoofed. However,
in some networks native security mechanisms already provide
sufficient protection against spoofing of DHCP traffic. An example
of such network is a Broadband Forum TR-101 [TR-101i2] compliant
access network. In such environments the mandatory coupling between
FORCERENEW and DHCP Authentication [RFC3118] can be relaxed. This
document defines extensions to Authentication for DHCP(v4) 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.
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 an HMAC computed from the Forcerenew nonce in
subsequent Forcerenew messages.
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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 protocol is used (initiated by the server) only
if the client and server are not using any other authentication
protocol 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(<TBD>) option in DHCP Discover
and Request messages sent to the server.
A DHCP server that does not support Forcerenew Nonce Protocol
authentication should ignore the FORCERENEW_NONCE_CAPABLE(<TBD>)
option. A DHCP server indicates DHCP Forcerenew Nonce Protocol
preference by including a FORCERENEW_NONCE_CAPABLE(<TBD>) 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(<TBD>).
The FORCERENEW_NONCE_CAPABLE option is a zero length option with code
of <TDB> and format as follows:
Code Len
+-----+-----+
| TBD | 0 |
+-----+-----+
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 is configured to require Forcerenew nonce
authentication, it will insert the FORCERENEW_NONCE_CAPABLE option in
the DHCP Offer message.
Server Client Server
(not selected) (selected)
v v v
| | |
| Begins initialization |
| | |
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| _____________/|\____________ |
|/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 |
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| 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
3.1.2. Forcerenew Nonce Protocol
The Forcerenew Nonce Protocol makes use of both the DHCP
authentication option defined in [RFC3118] re-using the option format
and of the Reconfigure Key Authentication Protocol defined in
[RFC3315].
The following fields are set in an DHCP authentication option for the
Forcerenew Nonce Authentication Protocol:
protocol 3
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algorithm 1
RDM 0
The format of the Authentication information for the Forcerenew Nonce
Authentication Protocol is:
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) |
+-+-+-+-+-+-+-+-+ |
. .
. .
. +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Type of data in Value field carried in this option:
1 Reconfigure Key value (used in ACK message)
2 HMAC-MD5 digest of the message (FORCERENEW message)
Value Data as defined by field
3.1.3. Server considerations for Forcerenew Nonce Authentication
The use of Forcerenew Nonce Protocol is dependent on the client
indicating its capability through the FORCERENEW_NONCE_CAPABLE(<TBD>)
DHCP option in any DHCP Discover or Request messages. The DHCP
Discovery or Request message from the client MUST contain the
FORCERENEW_NONCE_CAPABLE(<TBD>) option if the Forcerenew Nonce
Protocol is to be used by the server. The absence of the
FORCERENEW_NONCE_CAPABLE(<TBD>) option indicates to the server that
the Forcerenew Nonce Authentication protocol is not supported and
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 Protocol
authentication by including the DHCP FORCERENEW_NONCE_CAPABLE(<TBDP>)
option in the DHCP Offer message. 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(<TDB>) option in the DHCP offer may be used
by clients to prefer Forcerenew nonce Protocol authentication-capable
DHCP servers over those servers which do not support such capability.
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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 Forcerenew nonce is 128 bits long, and MUST be a
cryptographically strong random or pseudo-random number that cannot
easily be predicted. The nonce is imbedded 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 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. The server includes the HMAC-MD5
in the authentication information field in an Authentication option
included in the Forcerenew message sent to the client with type set
to 2 (HMAC-MD5 digest).
3.1.4. Client considerations for Forcerenew Nonce Authentication
The client MUST indicate Forcerenew nonce Capability by including the
FORCERENEW_NONCE_CAPABLE(<TBD>) DHCP option (Section 2.1.1) in all
DHCP Discover and Request messages. DHCP servers that support
Forcerenew nonce Protocol authentication MUST include the DHCP
Forcerenew Nonce protocol authentication option in DHCP Offers with
type set to zero(0), allowing the client to use this capability in
selecting DHCP servers should multiple Offers arrive.
A DHCP server has indicates its support through the inclusion of the
FORCERENEW_NONCE_CAPABLE(<TBD>) option in the DHCP Offer. 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 Protocol authentication in the DHCP Offer and a
subsequent Ack omits a valid DHCP authentication option for the
Forcerenew Nonce Protocol, the client MUST send a DHCP Decline
message and return to the DHCP Init state.
The client will receive a Forcerenew Nonce from the server in the
initial DHCP Ack message from the server. The client records the
Forcerenew Nonce for use in authenticating subsequent Forcerenew
messages.
To authenticate a Forcerenew message, the client computes an HMAC-MD5
over the DHCP Forcerenew message, using the Forcerenew Nonce received
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from the server. If this computed HMAC-MD5 matches the value in the
Authentication option, the client accepts the Forcerenew message.
4. Acknowledgements
Comments are solicited and should be addressed to the DHC WG mailing
list (dhcwg@ietf.org) and/or the authors. This contribution is based
on work by Vitali Vinokour. Major sections of this draft use
modified text from [RFC3315]. The authors wish to thank Ted Lemon
and Bernie Volz for their support.
5. IANA Considerations
This document requests IANA to assign 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:
an option code of TBD1 for FORCERENEW_NONCE_CAPABALE
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 [RFC3118] or this proposal. In this
proposal any party able intercept the nonce exchange could
impersonate a server and thus offers no protection from man-in-the-
middle attacks. 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.
The mechanism described provides protection against the use of a
FORCERENEW message by a malicious DHCP server to mount a denial of
service 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.
7. References
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7.1. 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.
7.2. Informative References
[TR-101i2]
Anschutz, T., "Migration to Ethernet-Based Broadband
Aggregation Broadband Forum TR-101 Issue 2", July 2011.
Authors' Addresses
David Miles
Alcatel-Lucent
L3 / 215 Spring St
Melbourne, Victoria 3000,
Australia
Phone: +61 3 9664 3308
Fax:
Email: david.miles@alcatel-lucent.com
URI:
Wojciech Dec
Cisco Systems
Haarlerbergpark Haarlerbergweg 13-19
Amsterdam, NOORD-HOLLAND 1101 CH
Netherlands
Phone:
Fax:
Email: wdec@cisco.com
URI:
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James Bristow
Swisscom Schweiz AG
Zentweg 9
Bern, 3050,
Switzerland
Phone:
Fax:
Email: James.Bristow@swisscom.com
URI:
Roberta Maglione
Telecom Italia
Via Reiss Romoli 274
Torino 10148
Italy
Phone:
Email: roberta.maglione@telecomitalia.it
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