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INTERNET-DRAFT                                        Mike Swift
draft-ietf-cat-iakerb-07.txt                          University of WA
Updates: RFC 1510, 1964                               Jonathan Trostle
July 2001                                             Cisco Systems
                                                      Bernard Aboba
                                                      Microsoft
                                                      Glen Zorn
                                                      Cisco Systems


Extending the GSS Kerberos Mechanism for Initial Kerberos Authentication
(IAKERB)
                     <draft-ietf-cat-iakerb-07.txt>




Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [6].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This draft expires in January 2002. Please send comments to the
   authors.

1. Abstract

   This document defines extensions to the Kerberos protocol
   specification (RFC 1510 [1]) and GSSAPI Kerberos mechanism (RFC 1964
   [2]) that enables a RFC 1964 client to obtain Kerberos tickets for
   services where the KDC is not accessible to the client, but is
   accessible to the application server. Some common scenarios where
   lack of accessibility would occur are when the client does not have
   an IP address prior to authenticating to an access point, the client
   is unable to locate a KDC, or a KDC is behind a firewall. The
   document specifies two protocols to allow a client to exchange KDC
   messages (which are GSS encapsulated) with an IAKERB proxy instead of
   a KDC.



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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 RFC2119 [7].

3. Motivation

   When authenticating using Kerberos V5, clients obtain tickets from a
   KDC and present them to services. This method of operation works well
   in many situations, but is not always applicable. The following is a
   list of some of the scenarios that this proposal addresses:

   (1) The client must initially authenticate to an access point in
   order to gain full access to the network. Here the client may be
   unable to directly contact the KDC either because it does not have an
   IP address, or the access point packet filter does not allow the
   client to send packets to the Internet before it authenticates to the
   access point.

   (2) A KDC is behind a firewall so the client will send Kerberos
   messages to the IAKERB proxy which will transmit the KDC request and
   reply messages between the client and the KDC. (The IAKERB proxy is a
   special type of Kerberos application server that also relays KDC
   request and reply messages between a client and the KDC).

4. Overview

   This proposal specifies two protocols that address the above
   scenarios: the IAKERB proxy option and the IAKERB minimal messages
   option. In the IAKERB proxy option (see Figure 1) an application
   server called the IAKERB proxy acts as a protocol gateway and proxies
   Kerberos messages back and forth between the client and the KDC. The
   IAKERB proxy is also responsible for locating the KDC and may
   additionally perform other application proxy level functions such as
   auditing.


            Client <---------> IAKERB proxy <----------> KDC


                      Figure 1: IAKERB proxying


   The second protocol is the minimal messages protocol that extends the
   technique in [5]; this protocol is targetted at environments where
   the number of messages (prior to key establishment) needs to be
   minimized. Here the client sends its ticket granting ticket (TGT) to
   the IAKERB proxy (in a KRB_TKT_PUSH message) for the TGS case. The
   IAKERB proxy then sends a TGS_REQ to the KDC with the client's TGT in
   the additional tickets field of the TGS_REQ message. As a result, the
   returned ticket will list the client as the ticket's server
   principal, and will be encrypted with the session key from the
   client's TGT. The IAKERB proxy then uses this ticket to generate an



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   AP request that is sent to the client (see Figure 2). Thus mutual
   authentication is accomplished with three messages between the client
   and the IAKERB proxy versus four or more (the difference is larger if
   crossrealm operations are involved). Subsequent to mutual
   authentication and key establishment, the IAKERB proxy sends a ticket
   to the client (in a KRB_TKT_PUSH message) that contains the same
   fields as the original service ticket except the client and server
   names are reversed and it is encrypted in a long term key known to
   the IAKERB proxy. Its purpose is to enable fast subsequent re-
   authentication by the client to the application server (using the
   conventional AP request AP reply exchange) for subsequent sessions.
   In addition to minimizing the number of messages, a secondary goal is
   to minimize the number of bytes transferred between the client and
   the IAKERB proxy prior to mutual authentication and key
   establishment. Therefore, the final service ticket (the reverse
   ticket) is sent after mutual authentication and key establishment is
   complete, rather than as part of the initial AP_REQ from the IAKERB
   proxy to the client.

   The AS_REQ case for the minimal messages option is similar, where the
   client sends up the AS_REQ message and the IAKERB proxy forwards it
   to the KDC. The IAKERB proxy pulls the client TGT out of the AS_REP
   message and also forwards the AS_REP message back to the client. The
   protocol now proceeds as in the TGS_REQ case with the IAKERB proxy
   including the client's TGT in the additional tickets field of the
   TGS_REQ message.


                Client  --------> IAKERB proxy
                        TKT_PUSH (w/ TGT)

                Client            IAKERB proxy --------------------> KDC
                                                TGS_REQ with client
                                                TGT as additional TGT

                Client            IAKERB proxy <-------------------- KDC
                                                TGS_REP with service
                                                ticket

                Client <--------  IAKERB proxy                       KDC
                         AP_REQ

                Client -------->  IAKERB proxy                       KDC
                         AP_REP

           -------------------------------------------------------------
            post-key establishment and application data flow phase:

                Client <--------  IAKERB proxy                       KDC
                        TKT_PUSH (w/ticket targetted at IAKERB proxy
                               to enable fast subsequent authentication)


                 Figure 2: IAKERB Minimal Messages Option: TGS case



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   A compliant IAKERB proxy MUST implement the IAKERB proxy protocol,
   and MAY implement the IAKERB minimal message protocol. In general,
   the existing Kerberos paradigm where clients contact the KDC to
   obtain service tickets should be preserved where possible.

   If the client has a service ticket for the target server, needs to
   authenticate to the target server, and does not have direct
   connectivity with the target server, it should use the IAKERB proxy
   protocol. If the client needs to obtain a crossrealm TGT (and the
   conventional Kerberos protocol cannot be used), then the IAKERB proxy
   protocol must be used. In a scenario where the client does not have a
   service ticket for the target server, it is crucial that the number
   of messages between the client and the target server be minimized
   (especially if the client and target server are in different realms),
   and/or it is crucial that the number of bytes transferred between the
   client and the target server be minimized, then the client should
   consider using the minimal messages protocol. The reader should see
   the security considerations section regarding the minimal messages
   protocol.

5.  GSSAPI Encapsulation

   The mechanism ID for IAKERB proxy GSS-API Kerberos, in accordance
   with the mechanism proposed by SPNEGO [8] for negotiating protocol
   variations, is:  {iso(1) org(3) dod(6) internet(1) security(5)
   mechanisms(5) iakerb(10) iakerbProxyProtocol(1)}.  The proposed
   mechanism ID for IAKERB minimum messages GSS-API Kerberos, in
   accordance with the mechanism proposed by SPNEGO for negotiating
   protocol variations, is: {iso(1) org(3) dod(6) internet(1)
   security(5) mechanisms(5) iakerb(10)
   iakerbMinimumMessagesProtocol(2)}.

   The AS request, AS reply, TGS request, and TGS reply messages are all
   encapsulated using the format defined by RFC1964 [2].  This consists
   of the GSS-API token framing defined in appendix B of RFC1508 [3]:

   InitialContextToken ::= [APPLICATION 0] IMPLICIT SEQUENCE {
           thisMech        MechType
                   -- MechType is OBJECT IDENTIFIER
                   -- representing "Kerberos V5"
           innerContextToken ANY DEFINED BY thisMech
                   -- contents mechanism-specific;
                   -- ASN.1 usage within innerContextToken
                   -- is not required
        }

   The innerContextToken consists of a 2-byte TOK_ID field (defined
   below), followed by the Kerberos V5 KRB_AS_REQ, KRB_AS_REP,
   KRB_TGS_REQ, or KRB_TGS_REP messages, as appropriate. The TOK_ID
   field shall be one of the following values, to denote that the
   message is either a request to the KDC or a response from the KDC.






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   Message         TOK_ID

   KRB_KDC_REQ     00 03

   KRB_KDC_REP     01 03

   We also define the token ID for the KRB_TKT_PUSH message (defined
   below and used in the minimal messages variation):

   Message         TOK_ID

   KRB_TKT_PUSH    02 03

   For completeness, we list the other RFC 1964 defined token ID's here:

   Message         TOK_ID

   AP_REQ          01 00

   AP_REP          02 00

   KRB_ERROR       03 00

6.  The IAKERB proxy protocol

   The IAKERB proxy will proxy Kerberos KDC request, KDC reply, and
   KRB_ERROR messages back and forth between the client and the KDC as
   illustrated in Figure 1. Messages received from the client must first
   have the Kerberos GSS header (RFC1964 [2]) stripped off. The
   unencapsulated message will then be forwarded to a KDC. The IAKERB
   proxy is responsible for locating an appropriate KDC using the realm
   information in the KDC request message it received from the client.
   In addition, the IAKERB proxy SHOULD implement a retry algorithm for
   KDC requests over UDP (including selection of alternate KDC's if the
   initial KDC does not respond to its requests). For messages sent by
   the KDC, the IAKERB proxy encapsulates them with a Kerberos GSS
   header before sending them to the client.

   We define two new Kerberos error codes that allow the proxy to
   indicate the following error conditions to the client:

   (a) when the proxy is unable to obtain an IP address for a KDC in the
   client's realm, it sends the KRB_IAKERB_ERR_KDC_NOT_FOUND KRB_ERROR
   (80) message to the client.

   (b) when the proxy has an IP address for a KDC in the client realm,
   but does not receive a response from any KDC in the realm (including
   in response to retries), it sends the KRB_IAKERB_ERR_KDC_NO_RESPONSE
   KRB_ERROR (81) message to the client.

   To summarize, the sequence of steps for processing is as follows:

   Servers:




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   1. For received KDC_REQ messages (with token ID 00 03)
      - process GSS framing (check OID)
        if the OID is not one of the two OID's specified in the GSSAPI
        Encapsulation section above, then process according to mechanism
        defined by that OID (if the OID is recognized). The processing
        is outside the scope of this specification. Otherwise, strip
        off GSS framing.
      - find KDC for specified realm (if KDC IP address cannot be
        obtained, send a KRB_ERROR message with error code
        KRB_IAKERB_ERR_KDC_NOT_FOUND to the client).
      - send to KDC (storing client IP address, port, and indication
        whether IAKERB proxy option or minimal messages option is
        being used)
      - retry with same or another KDC if no response is received. If
        the retries also fail, send an error message with error code
        KRB_IAKERB_ERR_KDC_NO_RESPONSE to the client.

   2. For received KDC_REP messages
      - encapsulate with GSS framing, using token ID 01 03 and the OID
        that corresponds to the stored protocol option
      - send to client (using the stored client IP address and port)

   3. For received AP_REQ and AP_REP messages
      - process locally per RFC 1964

   Clients:

   1. For sending KDC_REQ messages
      - create AS_REQ or TGS_REQ message
      - encapsulate with GSS framing (token ID 00 03 and OID
        corresponding to the protocol option).
      - send to server

   2. For received KDC_REP messages
      - decapsulate by removing GSS framing (token ID 01 03)
      - process inner Kerberos message according to RFC 1510

   3. For received AP_REQ and AP_REP messages
      - process locally per RFC 1964

7. The IAKERB minimal messages protocol

   The client MAY initiate the IAKERB minimal messages variation when
   the number of messages must be minimized (the most significant
   reduction in the number of messages can occur when the client and the
   IAKERB proxy are in different realms). SPNEGO [8] may be used to
   securely negotiate between the protocols. A compliant IAKERB server
   MAY support the IAKERB minimal messages protocol.

   (a) AS_REQ case: (used when the client does not have a TGT)

   We extend the technique used in Hornstein [5]. The client indicates
   that the minimal message sub-protocol will be used by using the
   appropriate OID as described above. The client sends the GSS



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   encapsulated AS_REQ message to the IAKERB proxy, and the IAKERB proxy
   processes the GSS framing (as described above for the IAKERB proxy
   option) and forwards the AS_REQ message to the KDC.

   The IAKERB proxy will proxy the returned message (AS_REP or
   KRB_ERROR) from the KDC back to the client (after processing and
   removing the GSS framing). The protocol is complete in the KRB_ERROR
   case (from the server perspective, but the client should retry
   depending on the error type). In the AS_REP case, the IAKERB proxy
   will obtain the client's TGT from the AS_REP message before
   forwarding the AS_REP message to the client. The IAKERB proxy then
   sends a TGS_REQ message with the client's TGT in the additional
   tickets field to the client's KDC (ENC-TKT-IN-SKEY option).

   The IAKERB proxy MAY handle returned KRB_ERROR messages and retry the
   TGS request message. Ultimately, the IAKERB proxy either proxies a
   KRB_ERROR message to the client, or it sends a GSS Initial Context
   token containing an AP_REQ message to the client. (Note: although the
   server sends the initial context token, the client is the initiator.)
   The IAKERB proxy MUST set the MUTUAL AUTH flag in the Initial Context
   token in order to cause the client to authenticate as well. The
   client will reply with the GSSAPI enscapsulated AP_REP message, if
   the IAKERB proxy's authentication succeeds. If all goes well, then,
   in order to enable subsequent efficient client authentications, the
   IAKERB proxy will then send a final message of type KRB_TKT_PUSH
   containing a Kerberos ticket (the reverse ticket) that has the IAKERB
   client principal identifier in the client identifier field of the
   ticket and its own principal identity in the server identifier field
   of the ticket:

     KRB_TKT_PUSH :: = [APPLICATION 17] SEQUENCE {
       pvno[0]             INTEGER,  -- 5 (protocol version)
       msg-type[1]         INTEGER,  -- 17 (message type)
       ticket[2]           Ticket
     }

   The key used to encrypt the reverse ticket is a long term secret key
   chosen by the IAKERB proxy. The fields are identical to the AP_REQ
   ticket, except the client name will be switched with the server name,
   and the server realm will be switched with the client realm. (The one
   other exception is that addresses should not be copied unless the
   IAKERB proxy has included the client's address in the TGS_REQ message
   to the KDC). Sending the reverse ticket allows the client to
   efficiently initiate subsequent reauthentication attempts with a
   RFC1964 AP_REQ message. Note that the TKT_PUSH message is sent after
   mutual authentication and key establishment are complete.

   (b) TGS_REQ case: (used when the client has a TGT)

   The client indicates that the minimal messages sub-protocol will be
   used by using the appropriate OID as described above. The client
   initially sends a KRB_TKT_PUSH message (with the GSS header) to the
   IAKERB proxy in order to send it a TGT. The IAKERB proxy will obtain
   the client's TGT from the KRB_TKT_PUSH message and then proceed to



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   send a TGS_REQ message to a KDC where the realm of the KDC is equal
   to the realm from the server realm field in the TGT sent by the
   client in the KRB_TKT_PUSH message. The protocol then continues as in
   the minimal messages AS_REQ case described above (see Figure 2); the
   IAKERB proxy's TGS_REQ message contains the client's TGT in the
   additional tickets field (ENC-TKT-IN-SKEY option). The IAKERB proxy
   then receives the TGS_REP message from the KDC and then sends a RFC
   1964 AP_REQ message to the client (with the MUTUAL AUTH flag set -
   see AS_REQ case).

8. Addresses in Tickets

   In IAKERB, the machine sending requests to the KDC is the server and
   not the client. As a result, the client should not include its
   addresses in any KDC requests for two reasons. First, the KDC may
   reject the forwarded request as being from the wrong client. Second,
   in the case of initial authentication for a dial-up client, the
   client machine may not yet possess a network address. Hence, as
   allowed by RFC1510 [1], the addresses field of the AS and TGS
   requests SHOULD be blank and the caddr field of the ticket SHOULD
   similarly be left blank. One exception is in an AS request (where the
   request body is not integrity protected); the IAKERB proxy MAY add
   its own addresses and the addresses of the client to the AS request.

9. Combining IAKERB with Other Kerberos Extensions

   This protocol is usable with other proposed Kerberos extensions such
   as PKINIT (Public Key Cryptography for Initial Authentication in
   Kerberos [4]). In such cases, the messages which would normally be
   sent to the KDC are instead sent by the client application to the
   server, which then forwards them to a KDC.

10. Security Considerations

   In the minimal messages protocol option, the application server sends
   an AP_REQ message to the client. The ticket in the AP_REQ message
   SHOULD NOT contain authorization data since some operating systems
   may allow the client to impersonate the server and increase its own
   privileges. If the ticket from the server connotes any authorization,
   then the minimal messages protocol should not be used. Also, the
   minimal messages protocol may facilitate denial of service attacks in
   some environments; to prevent these attacks, it may make sense for
   the minimal messages protocol server to only accept a KRB_TGT_PUSH
   message on a local network interface (to ensure that the message was
   not sent from a remote malicious host).

11. Acknowledgements

   We thank Ken Raeburn for his helpful comments.

12.  References

   [1] J. Kohl, C. Neuman, "The Kerberos Network Authentication
       Service (V5)", RFC 1510.



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   [2] J. Linn, "The Kerberos Version 5 GSS-API Mechanism", RFC 1964.

   [3] J. Linn, "Generic Security Service Application Program Interface",
       RFC 2078.

   [4] B. Tung, C. Neuman, M. Hur, A. Medvinsky, S. Medvinsky, J. Wray,
       J. Trostle, "Public Key Cryptography for Initial Authentication in
       Kerberos", WORK IN PROGRESS Internet Draft
       draft-ietf-cat-kerberos-pkinit-12.txt.

   [5] K. Hornstein, T. Lemon, B. Aboba, J. Trostle, "DHCP Authentication
       via Kerberos V", WORK IN PROGRESS Internet Draft
       draft-hornstein-dhc-kerbauth-02.txt.

   [6] S. Bradner, "The Internet Standards Process -- Revision 3", BCP
       9, RFC 2026, October 1996.

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

   [8] E. Baize, D. Pinkas, "The Simple and Protected GSS-API Negotiation
       Mechanism," RFC 2478, December 1998.

12.  Author's Addresses

   Michael Swift
   University of Washington
   Seattle, WA
   Email: mikesw@cs.washington.edu

   Jonathan Trostle
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA 95134, U.S.A.
   Email: jtrostle@cisco.com
   Phone: (408) 527-6201

   Bernard Aboba
   Microsoft
   One Microsoft Way
   Redmond, Washington, 98052, U.S.A.
   Email: bernarda@microsoft.com

   Glen Zorn
   Cisco Systems
   Bellevue, WA U.S.A.
   Email: gwz@cisco.com
   Phone: (425) 468-0955

   This draft expires on January 31st, 2002.






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