Internet DRAFT - draft-ietf-dhc-forcerenew-nonce
draft-ietf-dhc-forcerenew-nonce
dhc D. Miles
Internet-Draft Google
Updates: 3203 (if approved) W. Dec
Intended status: Standards Track Cisco Systems
Expires: December 16, 2012 J. Bristow
Swisscom Schweiz AG
R. Maglione
Telecom Italia
June 14, 2012
Forcerenew Nonce Authentication
draft-ietf-dhc-forcerenew-nonce-07
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 Forcerenew Nonce
Authentication 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 16, 2012.
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
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publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Message authentication . . . . . . . . . . . . . . . . . . . . 3
3.1. Forcerenew Nonce Authentication . . . . . . . . . . . . . 4
3.1.1. Forcerenew Nonce Protocol Capability Option . . . . . 4
3.1.2. Forcerenew Nonce Protocol . . . . . . . . . . . . . . 7
3.1.3. Server considerations for Forcerenew Nonce
Authentication . . . . . . . . . . . . . . . . . . . . 8
3.1.4. Client considerations for Forcerenew Nonce
Authentication . . . . . . . . . . . . . . . . . . . . 9
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6.1. Protocol vulnerabilities . . . . . . . . . . . . . . . . . 11
7. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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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 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.
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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.
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 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(<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 contains code <TDB>, length n and
a sequence of algorithms the client supports:
Code Len Algorithms
+-----+-----+----+----+----+
| TBD | n | A1 | A2 | A3 | ....
+-----+-----+----+----+----+
This document, in section Section 3.1.2, defines the Forcerenew Nonce
Protocol for algorithm equal to 1 and type equal to 2; future
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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 DHCP Offer message.
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- |
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| | 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
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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 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 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following fields are set in an DHCP authentication option for the
Forcerenew Nonce Authentication Protocol:
code 90
length field contains the length of the protocol
protocol 3
algorithm 1
Replay Detection field is per the Replay Detection Method (RDM)
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Replay Detection Method (RDM) 0
Authentication Information: specified below
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 Forcerenew nonce 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(<TBD>)
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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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 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,
based on the procedure specified in section 21.5 of [RFC3315], 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
A client that supports this mechanism MUST indicate Forcerenew nonce
Capability by including the FORCERENEW_NONCE_CAPABLE(<TBD>) 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(<TBD>) 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
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Nonce in a DHCP authentication option. If the server has indicated
capability for Forcerenew Nonce Protocol authentication 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 Protocol, the client MUST discard the message and
return to the INIT stat
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. 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,
Matthew Ryan 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:
Tag: TBD
Name: FORCERENEW_NONCE_CAPABALE
Data lenght: 1
Description: Forcerenew Nonce Capable
Reference: this document
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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 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, and 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.
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. 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.,
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and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
Authors' Addresses
David Miles
Google
Phone:
Fax:
Email:
URI:
Wojciech Dec
Cisco Systems
Haarlerbergpark Haarlerbergweg 13-19
Amsterdam, NOORD-HOLLAND 1101 CH
Netherlands
Phone:
Fax:
Email: wdec@cisco.com
URI:
James Bristow
Swisscom Schweiz AG
Zentweg 9
Bern, 3050,
Switzerland
Phone:
Fax:
Email: James.Bristow@swisscom.com
URI:
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Roberta Maglione
Telecom Italia
Via Reiss Romoli 274
Torino 10148
Italy
Phone:
Email: roberta.maglione@telecomitalia.it
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