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Versions: 00 01 03 04 05 06 07 08 09                     Standards Track
INTERNET-DRAFT                                        Jonathan Trostle
draft-ietf-cat-iakerb-09.txt                          Cisco Systems
Updates: RFC 1510, 1964                               Michael Swift
October   2002                                        University of WA
                                                      Bernard Aboba
                                                      Microsoft
                                                      Glen Zorn
                                                      Cisco Systems


Initial and Pass Through Authentication Using Kerberos V5 and the GSS-API
(IAKERB)
                     <draft-ietf-cat-iakerb-09.txt>




Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [5].

   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 March 2003. Please send comments to the
   authors.

1. Abstract

   This document defines extensions to the Kerberos protocol
   specification (RFC 1510 [1]) and GSSAPI Kerberos protocol mechanism
   (RFC 1964 [2]) that enables a 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 [6].

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 [8].

   (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. A compliant IAKERB proxy MUST implement the IAKERB proxy
   protocol.


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


                      Figure 1: IAKERB proxying

   The second protocol is the minimal messages protocol which is based
   on user-user authentication [4]; this protocol is targetted at
   environments where the number of messages, prior to key
   establishment, needs to be minimized. In the normal minimal messages
   protocol, 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. 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



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   proxy then uses this ticket to generate an 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).  This
   ticket is created by the IAKERB proxy and contains the same fields as
   the original service ticket that the proxy sent in the AP_REQ
   message, 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. Thus
   protected application data (e.g., GSS signed and wrapped messages)
   can flow before this final message is sent.

   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; the protocol now proceeds as in the TGS_REQ case described
   above with the IAKERB proxy including the client's TGT in the
   additional tickets field of the TGS_REQ message.

   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.

   For most IAKERB scenarios, such as when the client does not have an
   IP address, or cannot directly contact a KDC, the IAKERB proxy
   protocol should be adequate. 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.








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                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



5.  GSSAPI Encapsulation

   The mechanism ID for IAKERB proxy GSS-API Kerberos, in accordance
   with the mechanism proposed by SPNEGO [7] 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)}.

   NOTE: An IAKERB implementation does not require SPNEGO in order to
   achieve interoperability with other IAKERB peers. Two IAKERB
   implementations may interoperate in the same way that any two peers
   can interoperate using a pre-established GSSAPI mechanism.  The above
   OID's allow two SPNEGO peers to securely negotiate IAKERB from among
   a set of GSS mechanisms.

   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 [3]:






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   InitialContextToken ::= [APPLICATION 0] IMPLICIT SEQUENCE {
           thisMech        MechType
                   -- MechType is OBJECT IDENTIFIER
                   -- representing iakerb proxy or iakerb min messages
           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.



   Message          TOK_ID

   KRB_KDC_REQ      00 03

   KRB_KDC_REP      01 03

   We also define the token ID for the KRB_TKT_PUSH token (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



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   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:

   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 KRB_ERROR messages received from the KDC
      - encapsulate with GSS framing, using token ID 03 00 and the OID
        that corresponds to the stored protocol option
      - send to client (using the stored client IP address and port)
        (one possible exception is the KRB_ERR_RESPONSE_TOO_BIG error
        which can lead to a retry of the KDC_REQ message over the TCP
        transport by the server, instead of simply proxying the error
        to the client).

   4. For sending/receiving AP_REQ and AP_REP messages
      - process per RFC 1510 and RFC 1964; the created AP_REP message
        SHOULD include the subkey (with same etype as the session key)
        to facilitate use with other key derivation algorithms outside
        of [2]. The subkey SHOULD be created using locally generated



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        entropy as one of the inputs (in addition to other inputs
        such as the session key).

   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 KRB_ERROR messages
      - decapsulate by removing GSS framing (token ID 03 00)
      - process inner Kerberos message according to RFC 1510
        and possibly retry the request (time skew errors lead
        to retries in most existing Kerberos implementations)

   4. For sending/receiving AP_REQ and AP_REP messages
      - process per RFC 1510 and RFC 1964; the created AP_REQ
        message SHOULD include the subsession key in the
        authenticator field.

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 [7] MAY be used to
   securely negotiate between the protocols (and amongst other GSS
   mechanism 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 apply the Kerberos user-user authentication protocol [4] in this
   scenario (other work in this area includes the IETF work in progress
   effort to apply Kerberos user user authentication to DHCP
   authentication).

   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 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 either send a KRB_ERROR message back to the
   client, or it will send an initial context token consisting of the
   GSS header (minimal messages OID with a two byte token header 01 03),
   followed by an AS_REP message. The AS_REP message will contain the
   AP_REQ message in a padata field; the ticket in the AP_REQ is a



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   user-user ticket encrypted in the session key from the client's
   original TGT. We define the padata type PA-AP-REQ with type number
   25.  The corresponding padata value is the AP_REQ message without any
   GSS framing. For the IAKERB minimal messages AS option, the AP_REQ
   message authenticator MUST include the RFC 1964 [2] checksum.  The
   mutual-required and use-session-key flags are set in the ap-options
   field of the AP_REQ message.

   The protocol is complete in the KRB_ERROR case (from the server
   perspective, but the client should retry depending on the error
   type). If the IAKERB proxy receives an AS_REP message from the KDC,
   the IAKERB proxy will then obtain the client's TGT from the AS_REP
   message. 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 (e.g. on a KRB_ERR_RESPONSE_TOO_BIG error,
   switching to TCP from UDP). Ultimately, the IAKERB proxy either
   proxies a KRB_ERROR message to the client (after adding the GSS
   framing), sends one of the new GSS framed KRB_ERROR messages defined
   above, or it receives the TGS_REP message from the KDC and then
   creates the AP_REQ message according to RFC 1964 [2]. The IAKERB
   proxy then sends a GSS token containing the AS_REP message with the
   AP_REQ message in the padata field as described above. (Note:
   although the server sends the context token with the AP_REQ, the
   client is the initiator.) The IAKERB proxy MUST set both the mutual-
   required and use-session-key flags in the AP_REQ message in order to
   cause the client to authenticate as well. The authenticator SHOULD
   include the subsession key (containing locally added entropy).  The
   client will reply with the GSSAPI enscapsulated AP_REP message, if
   the IAKERB proxy's authentication succeeds (which SHOULD include the
   subkey field to facilitate use with other key derivation algorithms
   outside of [2]). 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 (see
   Figure 3):

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

   NOTE: The KRB_TKT_PUSH message must be encoded using ASN.1 DER.  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 from the
   AP_REQ ticket to the reverse ticket). Sending the reverse ticket



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   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.


                Client  --------> IAKERB proxy --------------------> KDC
                        AS_REQ                  AS_REQ

                Client            IAKERB proxy <-------------------- KDC
                                                AS_REP w/ client 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
                         AS_REP w/ AP_REQ in padata field

                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 3: IAKERB Minimal Messages Option: AS case



   (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
   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. NOTE: this realm could be the
   client's home realm, the proxy's realm, or an intermediate realm. 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



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   (with the MUTUAL AUTH flag set - see AS_REQ case).

   To summarize, here are the steps for the minimal messages TGS
   protocol:

   Client:
          (has TGT already for, or targetted at, realm X.ORG)
          sends TKT_PUSH message to server containing client's ticket
          for X.ORG (which could be a crossrealm TGT)

   Server:
          (has TGT already targetted at realm X.ORG)
          sends to KDC (where KDC has principal id = server name,
            server realm from client ticket) a TGS_REQ:
          TGT in TGS_REQ is server's TGT
          Additional ticket in TGS_REQ is client's TGT from TKT_PUSH
            message
          Server name in TGS_REQ (optional by rfc1510) is not present
          Server realm in TGS_REQ is realm in server's TGT - X.ORG

   KDC:
          Builds a ticket:
             Server name = client's name
             Client name = server's name, Client realm = server's realm
             Server realm = client's realm
             Encrypted with: session key from client's TGT (passed in
                additional tickets field)
          Build a TGS_REP
             Encrypted with session key from server's TGT
          Sends TGS_REP and ticket to server

   Server:
          Decrypts TGS_REP from KDC using session key from its TGT
          Constructs AP_REQ
              Ticket = ticket from KDC (which was encrypted with
                       client's TGT session key)
              authenticator clientname = server's name (matches
                clientname in AP-REQ ticket)
              authenticator clientrealm = server's realm
              subsession key in authenticator is present (same
              etype as the etype of the session key in the ticket)
              checksum in authenticator is the RFC 1964 checksum
              sequence number in authenticator is present (RFC 1964)
              ap-options has both use-session-key and mutual-required
              flags set
          Sends AP_REQ (with GSS-API framing) to client

   Client:
          Receives AP_REQ
          Decrypts ticket using session key from its TGT
          Verifies AP_REQ
          Builds AP_REP and sends to server (AP_REP SHOULD include
          subkey field to facilitate use with other key derivation
          algorithms outside of [2] e.g., [8] and its successors.



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          Some apps may have their own message protection key
          derivation algorithm and protected message format.
          AP_REP includes the sequence number per RFC 1964.)

   Server:
          Verifies AP-REP. Builds reverse ticket as described above
          and sends reverse ticket to client using the KRB_TKT_PUSH
          message. The reverse ticket is the same as the AP_REQ
          ticket except the client name, realm are switched with the
          server name, realm fields and it is encrypted in a secret
          key known to the IAKERB proxy.

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.

9. Security Considerations

   Similar to other network access protocols, IAKERB allows an
   unauthenticated client (possibly outside the security perimeter of an
   organization) to send messages that are proxied to interior servers.
   When combined with DNS SRV RR's for KDC lookup, there is the
   possibility that an attacker can send an arbitrary message to an
   interior server. There are several aspects to note here:

   (1) in many scenarios, compromise of the DNS lookup will require the
   attacker to already have access to the internal network. Thus the
   attacker would already be able to send arbitrary messages to interior
   servers. No new vulnerabilities are added in these scenarios.

   (2) in a scenario where DNS SRV RR's are being used to locate the
   KDC, IAKERB is being used, and an external attacker can modify DNS
   responses to the IAKERB proxy, there are several countermeasures to
   prevent arbitrary messages from being sent to internal servers:

   (a) KDC port numbers can be statically configured on the IAKERB
   proxy. In this case, the messages will always be sent to KDC's. For
   an organization that runs KDC's on a static port (usually port 88)
   and does not run any other servers on the same port, this
   countermeasure would be easy to administer and should be effective.

   (b) the proxy can do application level sanity checking and filtering.
   This countermeasure should eliminate many of the above attacks.

   (c) DNS security can be deployed. This countermeasure is probably
   overkill for this particular problem, but if an organization has



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   already deployed DNS security for other reasons, then it might make
   sense to leverage it here. Note that Kerberos could be used to
   protect the DNS exchanges.  The initial DNS SRV KDC lookup by the
   proxy will be unprotected, but an attack here is at most a denial of
   service (the initial lookup will be for the proxy's KDC to facilitate
   Kerberos protection of subsequent DNS exchanges between itself and
   the DNS server).

   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).

10. Acknowledgements

   We thank the Kerberos Working Group chair, Doug Engert, for his
   efforts in helping to progress this specification. We also thank Ken
   Raeburn for his comments and the other working group participants for
   their input.

11.  References

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

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

   [3] J. Linn, "Generic Security Service Application Program
       Interface Version 2, Update 1", RFC 2743.

   [4] D. Davis, R. Swick, "Workstation Services and Kerberos
       Authentication at Project Athena", Technical Memorandum TM-424,
       MIT Laboratory for Computer Science, February 1990.

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

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

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

   [8] Part 11: Wireless LAN Medium Access Control (MAC) and Physical
       Layer (PHY) Specifications, ANSI/IEEE Std. 802.11, 1999 Edition.





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INTERNET DRAFT               October 2002             Expires March 2003


12.  Author's Addresses

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

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

   Bernard Aboba
   Microsoft
   One Microsoft Way
   Redmond, Washington, 98052, U.S.A.
   Email: bernarda@microsoft.com
   Phone: (425) 706-6605

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

   This draft expires on March 31st, 2003.





























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