





Internet Engineering Task Force (IETF)                         S. Sakane
Request for Comments: 6784                                 Cisco Systems
Category: Standards Track                                    M. Ishiyama
ISSN: 2070-1721                                      Toshiba Corporation
                                                           November 2012


                      Kerberos Options for DHCPv6

Abstract

   This document defines four new options for the Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6).  These options are used to
   carry configuration information for Kerberos.

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/rfc6784.























Sakane & Ishiyama            Standards Track                    [Page 1]

RFC 6784               Kerberos Options for DHCPv6         November 2012


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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Introduction ....................................................3
   2. Conventions Used in This Document ...............................4
   3. Kerberos Options ................................................4
      3.1. Kerberos Principal Name Option .............................4
      3.2. Kerberos Realm Name Option .................................5
      3.3. Kerberos Default Realm Name Option .........................6
      3.4. Kerberos KDC Option ........................................6
   4. Client and Server Operation .....................................7
      4.1. KDC Discovery for a Client .................................8
   5. IANA Considerations .............................................8
   6. Security Considerations .........................................9
   7. Acknowledgments .................................................9
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10
   Appendix A. An Example of the Operation of the Client .............11






Sakane & Ishiyama            Standards Track                    [Page 2]

RFC 6784               Kerberos Options for DHCPv6         November 2012


1.  Introduction

   Kerberos Version 5 [RFC4120] is a trusted third-party authentication
   system.  Each organization wishing to use Kerberos establishes its
   own "realm", and each client is registered as part of that realm.  At
   least one Key Distribution Center (KDC) is required for the operation
   of a Kerberos realm.

   When a client wishes to communicate with, and be authenticated to, a
   Kerberos application server (also a client of the KDC), the client
   identifies itself, and its realm, to the KDC and acquires a
   credential from the KDC.  The client then presents the credential to
   the Kerberos application server, which can use the credential to
   authenticate the client.  The client needs to know at least one IP
   address for a KDC in order to initiate this process.

   One example of the application of this protocol is as follows.  A
   student might want to use a shared, public workstation, one that is
   not configured for Kerberos.  If there is a mechanism for the
   workstation to obtain a realm name and IP address for a KDC, then a
   student need only input a user-id and pass phrase to be able to use
   Kerberos.

   The Kerberos V5 specification [RFC4120] defines the use of DNS SRV
   records [RFC2782] for KDC discovery.  Some systems, such as
   industrial systems, do not use DNS.  Such systems already have their
   own name spaces and their own name resolution systems, including
   preconfigured mapping tables for devices, and do not use Fully
   Qualified Domain Names.  However, many of these systems do use DHCP.

   Adding a DNS server to such systems may decrease the reliability of
   the system and increase the management cost.  In such an environment,
   another mechanism is needed to provide an IP address for the KDC.
   For the PacketCable Architecture [PCARCH], RFC 3634 [RFC3634] defines
   the KDC Server Address sub-option for the DHCPv4 CableLabs Client
   Configuration option.  However, a mechanism is still needed to
   provide a realm name and an IPv6 address -- one that does not depend
   on any external architecture.

   This document defines a Kerberos option for DHCPv6 that provides a
   realm name and/or a list of KDC IP addresses.  This option does not
   replace or modify any of the existing methods for obtaining this
   information.








Sakane & Ishiyama            Standards Track                    [Page 3]

RFC 6784               Kerberos Options for DHCPv6         November 2012


2.  Conventions Used in This Document

   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 RFC 2119 [RFC2119].

   It is assumed that the readers are familiar with the terms and
   concepts described in DHCPv6 [RFC3315], Kerberos V5 [RFC4120], and
   DNS SRV [RFC2782].

3.  Kerberos Options

   This document defines four DHCPv6 configuration parameters for
   Kerberos.

      Kerberos Principal Name Option

      Kerberos Realm Name Option

      Kerberos Default Realm Name Option

      Kerberos KDC Option

   This section describes the format of each option and the usage of
   each field in that option.

   These options, except for the Kerberos KDC Option, MUST NOT appear
   more than once in a DHCPv6 message.

3.1.  Kerberos Principal Name Option

   The Kerberos Principal Name Option carries the name of a Kerberos
   principal.  This is sent by the client to the DHCPv6 server, which
   MAY use it to select a specific set of configuration parameters,
   either for a client or for a Kerberos application server.
















Sakane & Ishiyama            Standards Track                    [Page 4]

RFC 6784               Kerberos Options for DHCPv6         November 2012


   The format of the Kerberos Principal Name Option is:

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   OPTION_KRB_PRINCIPAL_NAME   |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      :                                                               :
      :                        principal-name                         :
      :                       (variable length)                       :
      :                                                               :
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  option-code (16 bits): OPTION_KRB_PRINCIPAL_NAME (75)

   o  option-len (16 bits): length of the principal-name field.

   o  principal-name (variable): a client principal name.  The encoding
      of the principal-name field MUST conform to the definition of
      "PrincipalName" in Section 5.2.2 of RFC 4120 [RFC4120].

3.2.  Kerberos Realm Name Option

   The Kerberos Realm Name Option carries a Kerberos realm name.  A
   DHCPv6 client uses this option to specify to a DHCPv6 server which
   realm the client wants to access.

   The format of the Kerberos Realm Name Option is:

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     OPTION_KRB_REALM_NAME     |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      :                                                               :
      :                          realm-name                           :
      :                       (variable length)                       :
      :                                                               :
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  option-code (16 bits): OPTION_KRB_REALM_NAME (76)

   o  option-len (16 bits): the length of the realm-name field in
      octets.

   o  realm-name (variable): a realm-name.  The encoding of the
      realm-name field MUST conform to the definition of "Realm" in
      Section 5.2.2 of RFC 4120 [RFC4120].



Sakane & Ishiyama            Standards Track                    [Page 5]

RFC 6784               Kerberos Options for DHCPv6         November 2012


3.3.  Kerberos Default Realm Name Option

   The Kerberos Default Realm Name Option is used to specify a default
   realm name for the Kerberos system.  A DHCPv6 server uses this option
   to specify the default realm name to both clients and Kerberos
   application servers.

   The option-code of this option is OPTION_KRB_DEFAULT_REALM_NAME (77).
   The format and usage of the option-len and realm-name fields are
   identical to those for the Kerberos Realm Name Option.

3.4.  Kerberos KDC Option

   The Kerberos KDC Option is used to provide configuration information
   about a KDC.

   The format of the Kerberos KDC Option is:

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         OPTION_KRB_KDC        |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Priority            |            Weight             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Transport Type|          Port Number          |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
      |                                                               |
      |                                                               |
      |                       KDC IPv6 address        +---------------+
      |                                               |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               :
      :                                                               :
      :                          realm-name                           :
      :                       (variable length)                       :
      :                                                               :
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  option-code (16 bits): OPTION_KRB_KDC (78)

   o  option-len (16 bits): 23 + the length of the realm-name field in
      octets.

   o  Priority (16 bits): see the description of the Weight field.







Sakane & Ishiyama            Standards Track                    [Page 6]

RFC 6784               Kerberos Options for DHCPv6         November 2012


   o  Weight (16 bits): the Priority and Weight fields provide a hint to
      the client as to which KDC to select.  The usage of the Priority
      and Weight values MUST follow the specification for DNS SRV
      [RFC2782].

   o  Transport Type (8 bits): The Transport Type specifies the
      transport protocol used for Kerberos.  Kerberos [RFC4120] defines
      UDP and TCP transports.  Exchanges over TCP are further described
      in [RFC5021], while the transport of Kerberos over Transport Layer
      Security (TLS) is described in [RFC6251].

   The transport type is defined below.

        Value    Transport Type
        ----     --------------
        0        Reserved
        1        UDP
        2        TCP
        3        TLS
        4-254    Unassigned
        255      Reserved

   o  Port Number (16 bits): the port number on which the KDC listens.

   o  KDC IPv6 address (128 bits): the IPv6 address of the KDC.

   o  realm-name (variable): the name of the realm for which the
      specified KDC provides service.  The encoding of the realm-name
      field MUST conform to the definition of "Realm" in Section 5.2.2
      of RFC 4120 [RFC4120].

4.  Client and Server Operation

   This section describes the operations of the client and server.  It
   assumes that the client has been configured with a principal name.

   If a client requires a realm name, the client sends a DHCPv6 Option
   Request Option (ORO) specifying the Kerberos Default Realm Name
   Option.  The DHCPv6 server responds with a Reply message containing a
   Kerberos Default Realm Name Option.

   If a client requires configuration parameters for a KDC, the client
   sends a DHCPv6 ORO specifying the Kerberos KDC Option.  The client
   MAY include a Kerberos Principal Name Option.  The client MAY include
   a Kerberos Realm Name Option.

   The DHCPv6 server replies with one or more sets of configuration
   parameters for a Kerberos KDC.  If the client has specified either a



Sakane & Ishiyama            Standards Track                    [Page 7]

RFC 6784               Kerberos Options for DHCPv6         November 2012


   Kerberos Principal Name Option or a Kerberos Realm Name Option, then
   the DHCPv6 server MAY use those parameters to select specific sets of
   configuration parameters.

   Where the server replies with more than one set of configuration
   parameters, the usage of the Priority and Weight fields by the client
   MUST follow the specification for DNS SRV [RFC2782].

   The client MAY include other options with data values as hints to the
   DHCPv6 server about parameter values the client would like to have
   returned; this is specified in Section 18.1.5 of RFC 3315 [RFC3315].

4.1.  KDC Discovery for a Client

   When a client implements both the DNS method defined by
   Section 7.2.3.2 of [RFC4120] and the DHCP method defined by this
   document, the choice of method is determined by local policy.  The
   administrator of the realm usually defines the method as part of the
   configuration of the client before the client is installed.

   When no criteria have been specified and the client could get the
   Kerberos information from either the DNS server or the DHCPv6 server,
   then the information from DNS SHOULD be preferred.

5.  IANA Considerations

   IANA has assigned four option codes from the DHCPv6 Option Codes
   registry for the following:

      75 OPTION_KRB_PRINCIPAL_NAME

      76 OPTION_KRB_REALM_NAME

      77 OPTION_KRB_DEFAULT_REALM_NAME

      78 OPTION_KRB_KDC

   IANA has created the Kerberos Message Transport Types sub-registry,
   under the Kerberos Parameters registry.  The initial entries are
   described in Section 3.4.

   The assignment of future entries is by "IETF Review" policy as
   described in BCP 26 [RFC5226].  Per that policy, a document specifies
   the symbolic name of such entries, which are assigned numeric codes
   by IANA once publication is approved.






Sakane & Ishiyama            Standards Track                    [Page 8]

RFC 6784               Kerberos Options for DHCPv6         November 2012


6.  Security Considerations

   The security considerations in RFC 3315 [RFC3315] apply.

   DHCPv6 messages can be modified in transit.  If an adversary modifies
   the response from a DHCPv6 server or injects its own response, a
   client may be led into contacting a malicious KDC.  Both cases are
   categorized as a Denial-of-Service (DoS) attack.  However, a
   malicious KDC does not know the shared key and so is unable to
   proceed any further with the exchange.  If a client receives a
   response from such a KDC, the client can use the shared key to detect
   that the message originates from a malicious KDC.

   A shared, unconfigured workstation may obtain its KDC information,
   and default realm, via DHCPv6.  Such a workstation may not have a
   host or other service key, and thus it may be unable to validate the
   Ticket-Granting Ticket issued by the KDC.  A modified DHCPv6 response
   would then result in the workstation talking to a malicious KDC, and
   the workstation would not be able to detect that this has happened.
   This in turn could allow access by unauthorized users.

   To minimize potential vulnerabilities, a client SHOULD use DHCPv6
   authentication as defined in Section 21 of RFC 3315 [RFC3315].

   Kerberos information may be manually configured on the client before
   requesting information from DHCPv6.  Manual configuration of the
   device SHOULD be preferred to configuration via the DHCPv6 server.

7.  Acknowledgments

   The authors are very grateful to Nobuo Okabe and Shigeya Suzuki.
   They contributed the explanation as to why DNS is inappropriate for
   some industry networks.  Ted Lemon made many suggestions to improve
   DHCP aspects of this specification.  Ken'ichi Kamada and Yukiyo
   Akisada contributed to the initial work on this document.  Tom Petch
   helped to improve the readability of this document.  The authors also
   thank Jeffrey Hutzelman, Kazunori Miyazawa, Kensuke Hosoya, Nicolas
   Williams, Nobumichi Ozoe, Sam Hartman, and Stephen Farrell.  They
   made valuable comments and suggestions.












Sakane & Ishiyama            Standards Track                    [Page 9]

RFC 6784               Kerberos Options for DHCPv6         November 2012


8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [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.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [RFC5021]  Josefsson, S., "Extended Kerberos Version 5 Key
              Distribution Center (KDC) Exchanges over TCP", RFC 5021,
              August 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

8.2.  Informative References

   [PCARCH]   CableLabs, "PacketCable 1.0(TM) Architecture Framework
              Technical Report", December 1999,
              <http://www.packetcable.com/downloads/specs/
              pkt-tr-arch-v01-991201.pdf>.

   [RFC3634]  Luehrs, K., Woundy, R., Bevilacqua, J., and N. Davoust,
              "Key Distribution Center (KDC) Server Address Sub-option
              for the Dynamic Host Configuration Protocol (DHCP)
              CableLabs Client Configuration (CCC) Option", RFC 3634,
              December 2003.

   [RFC6251]  Josefsson, S., "Using Kerberos Version 5 over the
              Transport Layer Security (TLS) Protocol", RFC 6251,
              May 2011.








Sakane & Ishiyama            Standards Track                   [Page 10]

RFC 6784               Kerberos Options for DHCPv6         November 2012


Appendix A.  An Example of the Operation of the Client

   When no criteria have been specified and the client could get the
   Kerberos information from either the DNS server or the DHCPv6 server,
   then the information from DNS SHOULD be preferred.  The following is
   an informational guide for the client in such an environment.

                               No Resp. or
               +------------+  DNS Info. +-----------+ No Resp.
     Start --> | Ask DHCP(1)| ---------> | Ask DNS(3)| ------>
               +------------+            +-----------+     Terminate(4)
                /          \                      \
      Only KRB /            \ DNS and              \ KRB Info.
        Info. /              \ KRB Info.            \
             /                \                      \
            |                  |                       |
            |                  V                       |
            V     No Ans.  +-----------+  KRB Info.    V
       Use Info. <-------- | Ask DNS(6)| ---------> Use Info.
       from DHCP           +-----------+            from DNS
       (2), (7)                                     (5), (8)

        Abbreviations:
          Resp.: Response
          Info.: Information
          KRB  : Kerberos

   1) Initially, the client requests both DNS and Kerberos information
      from the DHCPv6 server.

   2) If the DHCPv6 server replies with Kerberos information and not
      with DNS information, then the client uses that information.

   3) If the DHCPv6 server does not reply or replies with only DNS
      information, then the client requests Kerberos information from
      the DNS server.

   4) If the client gets no response or no Kerberos information from
      the DNS server, then the client terminates the process.

   5) If the client gets Kerberos information from the DNS server, then
      the client uses that information.

   6) If, as the result of (1), the DHCPv6 server replies with both DNS
      and Kerberos information, then the client requests Kerberos
      information from the DNS server.





Sakane & Ishiyama            Standards Track                   [Page 11]

RFC 6784               Kerberos Options for DHCPv6         November 2012


   7) If the client gets no response from the DNS server, then the
      client uses the Kerberos information from the DHCPv6 server.

   8) If, as the result of (6), the DNS server replies with Kerberos
      information, then the client uses the information from the DNS
      server and not that from the DHCPv6 server.

Authors' Addresses

   Shoichi Sakane
   Cisco Systems
   9-7-1 Akasaka
   Minato-ku, Tokyo  107-6227
   Japan

   EMail: ssakane@cisco.com


   Masahiro Ishiyama
   Toshiba Corporation
   1, Komukai-toshiba-cho, Saiwai-ku,
   Kawasaki, Kanagawa  212-8582
   Japan

   EMail: masahiro.ishiyama@toshiba.co.jp


























Sakane & Ishiyama            Standards Track                   [Page 12]

