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PROPOSED STANDARD
Errata Exist
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.
















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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



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   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.




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   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.









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                     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           |



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                       |               | _____________/|
                       |               |/ 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].




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   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














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   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.




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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 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



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   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



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   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.













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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


















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