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rfc:rfc3634

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.

Abstract

 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
 [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
 +------+-----+-----+-----+-----+-----+-----+-----+--
 |  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-
 option.

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
 Director.

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,
      http://www.cablelabs.com/projects/cablehome/specifications/.
 [3]  "CableHome 1.0 Specification", CableLabs,
      http://www.cablelabs.com/projects/cablehome/specifications/.
 [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
 CableLabs
 858 Coal Creek Circle
 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
 Nancy Davoust
 YAS Corporation
 300 Brickstone Square
 Andover, MA 01810
 Phone: (978) 749-9999
 EMail: nancy@yas.com

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
 English.
 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Luehrs, et al. Standards Track [Page 7]

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