Network Working Group                                          K. Luehrs
Request for Comments: 3634                                     CableLabs
Category: Standards Track                                      R. Woundy
                                                           Comcast Cable
                                                           J. Bevilacqua
                                                              N. Davoust
                                                         YAS Corporation
                                                           December 2003

      Key Distribution Center (KDC) Server Address Sub-option for
            the Dynamic Host Configuration Protocol (DHCP)
              CableLabs Client Configuration (CCC) Option

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This document defines a new sub-option for the CableLabs Client
   Configuration (CCC) Dynamic Host Configuration Protocol (DHCP) option
   code for conveying the network addresses of Key Distribution Center
   (KDC) servers.

1.  Introduction

   A CableLabs Client Configuration (CCC) Dynamic Host Configuration
   Protocol (DHCP) Option code providing the Key Distribution Center
   (KDC) server address will be needed for CableHome-compliant
   residential gateways configured to use Kerberos for authentication as
   the first step in establishing a secure SNMPv3 link between the
   Portal Service (PS) logical element [2,3] in residential gateways,
   and the SNMP entity in the cable operator's data network.

   The CCC DHCP option code will be used to address specific needs of
   CableLabs client devices during their configuration processes.  This
   document proposes a sub-option for the CCC DHCP option.

Luehrs, et al.              Standards Track                     [Page 1]

RFC 3634             KDC Server Address Sub-option         December 2003

   Configuration of a class of CableLabs client devices described in [2]
   and [3] will require a DHCP sub-option to provide the client with the
   network address of a KDC server in the cable operator's data network.

   The class of devices assumed in [2] and [3] is unlike the class of
   devices considered in [1], which perform a DNS lookup of the Kerberos
   Realm name to find the KDC server network address.

   This document proposes a sub-option of the CCC DHCP option code for
   use with CableLabs client devices.  The proposed sub-option encodes
   an identifier for the network address of each of one or more Key
   Distribution Center servers with which the CableLabs client device
   exchanges security information.

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
   this document are to be interpreted as described in BCP 14, RFC 2119

2.  Key Distribution Center IP Address Sub-option

   CableHome specifications will specify the Key Distribution Center
   network address encoding as a sub-option of the CCC DHCP Option code.
   This field will be used to inform the client device of the network
   address of one or more Key Distribution Center servers.

   The encoding of the KDC Server Address sub-option will adhere to the
   format of an IPv4 address.  The minimum length for this option is 4
   octets, and the length MUST always be a multiple of 4.  If multiple
   KDC Servers are listed, they MUST be listed in decreasing order of
   priority.  The format of the KDC Server Address sub-option of the CCC
   option code is as shown below:

    SubOpt  Len      Address 1               Address 2
   |  10  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...

3.  Security Considerations

   This document relies upon the DHCP protocol [5] for authentication
   and security, i.e., it does not provide security in excess of what
   DHCP is (or will be) providing.  Potential exposures to attack in the
   DHCP protocol are discussed in section 7 of the DHCP protocol
   specification [5] and in Authentication for DHCP Messages [6].

   The CCC option can be used to misdirect network traffic by providing
   incorrect DHCP server addresses, incorrect provisioning server
   addresses, and incorrect Kerberos realm names to a CableLabs client

Luehrs, et al.              Standards Track                     [Page 2]

RFC 3634             KDC Server Address Sub-option         December 2003

   device.  This misdirection can lead to several threat scenarios.  A
   Denial of Service (DoS) attack can result from address information
   being simply invalid.  A man-in-the-middle attack can be mounted by
   providing addresses to a potential snooper.  A malicious service
   provider can steal customers from the customer selected service
   provider, by altering the Kerberos realm designation.

   These threats are mitigated by several factors.

   Within the cable delivery architecture required by CableLabs'
   PacketCable, DOCSIS, and CableHome specifications, the DHCP client is
   connected to a network through a cable modem and the Cable Modem
   Termination System (CMTS).  The CMTS is explicitly configured with a
   set of DHCP servers to which DHCP requests are forwarded.  Further, a
   correctly configured CMTS will only allow downstream traffic from
   specific IP addresses/ ranges.

   Assuming that server addresses were successfully spoofed to the point
   that a malicious client device was able to contact a KDC, the client
   device must still present valid certificates to the KDC before being
   service enabled.  Given the computational overhead of the certificate
   validation process, this situation could present a DoS opportunity.

   It is possible for a malicious (although certificate enabled) service
   provider to redirect a customer from the customer's selected service
   provider.  It is assumed that all service providers permitted onto an
   access providers network are trusted entities that will cooperate to
   ensure peaceful coexistence.  If a service provider is found to be
   redirecting customers, this should be handled as an administrative
   matter between the access provider and the service provider.

   Another safeguard that can be taken by service providers to limit
   their exposure to their KDC server(s) is to configure their network
   so that the KDC(s) reside on a separate subnetwork.

   Service providers can further protect their KDC server(s) by placing
   a firewall in front of the KDC(s) only allowing connections needed
   for its current provisioning processes.  The IP temporary addresses
   given the client devices from the DHCP server could be sent directly
   to the firewall from the DHCP server to open a hole for Kerberos
   messages only for those particular IP addresses for a short period of
   time.  If this was used it would be recommended that service
   providers authenticate their DHCP server to the KDC as well.  This
   could be done via password authentication rather than digital
   certificate due to the co-location of the DHCP server to the KDC.

Luehrs, et al.              Standards Track                     [Page 3]

RFC 3634             KDC Server Address Sub-option         December 2003

   Finally, Kerberos requires mutual client-server authentication.
   Therefore, the client device must authenticate itself with its
   digital certificate and the KDC is required to authenticate it to the
   client device.  If a hacker tries to redirect the client device by
   replacing the service provider-configured KDC Server Address sub-
   option with another IP address, it is not likely to be a valid
   service provider's KDC server and authentication will fail.

4.  IANA Considerations

   The KDC Server Address sub-option described in this document is
   intended to be a sub-option of the CableLabs Client Configuration
   (CCC) option described in [1].  IANA has assigned and registered
   sub-option code 10 of the CCC option to the KDC Server Address sub-

5.  Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

Luehrs, et al.              Standards Track                     [Page 4]

RFC 3634             KDC Server Address Sub-option         December 2003

6.  Normative References

   [1]  Beser, B. and P. Duffy, "DHCP Option for CableLabs Client
        Configuration", RFC 3495, March 2003.

   [2]  "CableHome 1.1 Specification", CableLabs,

   [3]  "CableHome 1.0 Specification", CableLabs,

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [5]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [6]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
        RFC 3118, June 2001

Luehrs, et al.              Standards Track                     [Page 5]

RFC 3634             KDC Server Address Sub-option         December 2003

7.  Authors' Addresses

   Kevin Luehrs
   858 Coal Creek Circle
   Louisville, CO 80027

   Phone: (303) 661-9100

   Richard Woundy
   Comcast Cable
   27 Industrial Drive
   Chelmsford, MA 01824

   Phone: (978) 244-4010

   John Bevilacqua
   YAS Corporation
   300 Brickstone Square
   Andover, MA 01810

   Phone: (978) 749-9999

   Nancy Davoust
   YAS Corporation
   300 Brickstone Square
   Andover, MA 01810

   Phone: (978) 749-9999

Luehrs, et al.              Standards Track                     [Page 6]

RFC 3634             KDC Server Address Sub-option         December 2003

8.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Luehrs, et al.              Standards Track                     [Page 7]