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Versions: 00 01 02 03 04 rfc3634                         Standards Track
INTERNET-DRAFT                                          K. Luehrs
Dynamic Host Configuration Working Group                CableLabs
Expires December 2003                                   R. Woundy
                                                        Comcast Cable
                                                        J. Bevilacqua
                                                        YAS Corporation
                                                        N. Davoust
                                                        YAS Corporation
                                                        June 2003

                KDC Server Address Sub-option

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   reference material or to cite them other than as "work in progress."

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Copyright Notice

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


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

1.  Introduction

   A CCC DHCP Option code providing the 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 PS 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.

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   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 RFC 2119 [4].

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
   |  TBD |  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
   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

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Internet Draft   KDC Server Address Sub-option       June 2003

   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 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/

   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 certified) 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 insure 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

   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.

   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.

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Internet Draft   KDC Server Address Sub-option       June 2003

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 is requested to assign and
   register a sub-option code of the CCC option to the KDC Server
   Address sub-option.

5.  Normative References

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

   [2] "CableHome 1.1 Specification SP-CH1.1-I01-030418", CableLabs,
       April 2003, http://www.cablelabs.com/projects/cablehome/

   [3]  "CableHome 1.0 Specification SP-CH1.0-I04-030411", CableLabs,
        April 2003, http://www.cablelabs.com/projects/cablehome/

   [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

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

6.  Authors' Addresses

        Kevin Luehrs
        400 Centennial Parkway
        Louisville, CO 80027
        Phone:  (303) 661-9100
        EMail:  k.luehrs@cablelabs.com

        Richard Woundy
        Comcast Cable
        27 Industrial Drive
        Chelmsford, MA 01824
        Phone: (978) 244-4010
        EMail: richard_woundy@cable.comcast.com

        John Bevilacqua
        YAS Corporation
        300 Brickstone Square
        Andover, MA 01810
        Phone: (978) 749-9999
        EMail: john@yas.com

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Internet Draft   KDC Server Address Sub-option       June 2003

        Nancy Davoust
        YAS Corporation
        300 Brickstone Square
        Andover, MA 01810
        Phone: (978) 749-9999
        EMail: nancy@yas.com

7.  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 develop-
   ing 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 English.

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

   This document and the information contained herein is provided on an


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

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