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Network Working Group                                          K. LuehrsRequest for Comments: 3634                                     CableLabsCategory: Standards Track                                      R. Woundy                                                           Comcast Cable                                                           J. Bevilacqua                                                              N. Davoust                                                         YAS Corporation                                                           December 2003Key Distribution Center (KDC) Server Address Sub-option forthe Dynamic Host Configuration Protocol (DHCP)CableLabs Client Configuration (CCC) OptionStatus 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 inBCP 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 insection 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 clientLuehrs, 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 inBCP-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 20036.  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 2001Luehrs, et al.              Standards Track                     [Page 5]

RFC 3634             KDC Server Address Sub-option         December 20037.  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.comLuehrs, et al.              Standards Track                     [Page 6]

RFC 3634             KDC Server Address Sub-option         December 20038.  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]