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RFC 2942 - Telnet Authentication: Kerberos Version 5


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Network Working Group                                            T. Ts'o
Request for Comments: 2942                              VA Linux Systems
Category: Standards Track                                 September 2000

               Telnet Authentication: Kerberos Version 5

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 (2000).  All Rights Reserved.

Abstract

   This document describes how Kerberos Version 5 [1] is used with the
   telnet protocol.   It describes an telnet authentication suboption to
   be used with the telnet authentication option [2].   This mechanism
   can also used to provide keying material to provide data
   confidentiality services in conjunction with the telnet encryption
   option [3].

1. Command Names and Codes

      Authentication Types

         KERBEROS_V5    2

      Sub-option Commands

         AUTH               0
         REJECT             1
         ACCEPT             2
         RESPONSE           3
         FORWARD            4
         FORWARD_ACCEPT     5
         FORWARD_REJECT     6

2.  Command Meanings

   IAC SB AUTHENTICATION IS <authentication-type-pair> AUTH <Kerberos V5
   KRB_AP_REQ message> IAC SE

      This is used to pass the Kerberos V5 [1] KRB_AP_REQ message to the
      remote side of the connection.  The first octet of the
      <authentication-type-pair> value is KERBEROS_V5, to indicate that
      Version 5 of Kerberos is being used.  The Kerberos V5
      authenticator in the KRB_AP_REQ message must contain a Kerberos V5
      checksum of the two-byte authentication type pair.  This checksum
      must be verified by the server to assure that the authentication
      type pair was correctly negotiated.  The Kerberos V5 authenticator
      must also include the optional subkey field, which shall be filled
      in with a randomly chosen key.  This key shall be used for
      encryption purposes if encryption is negotiated, and shall be used
      as the negotiated session key (i.e., used as keyid 0) for the
      purposes of the telnet encryption option; if the subkey is not
      filled in, then the ticket session key will be used instead.

      If data confidentiality services is desired the ENCRYPT_US-
      ING_TELOPT flag must be set in the authentication-type-pair as
      specified in [2].

   IAC SB AUTHENTICATION REPLY <authentication-type-pair> ACCEPT IAC SE

      This command indicates that the authentication was successful.

      If the AUTH_HOW_MUTUAL bit is set in the second octet of the
      authentication-type-pair, the RESPONSE command must be sent before
      the ACCEPT command is sent.

   IAC SB AUTHENTICATION REPLY <authentication-type-pair> REJECT
      <optional reason for rejection> IAC SE

      This command indicates that the authentication was not successful,
      and if there is any more data in the sub-option, it is an ASCII
      text message of the reason for the rejection.

   IAC SB AUTHENTICATION REPLY <authentication-type-pair> RESPONSE
   <KRB_AP_REP message> IAC SE

      This command is used to perform mutual authentication.  It is only
      used when the AUTH_HOW_MUTUAL bit is set in the second octet of
      the authentication-type-pair.  After an AUTH command is verified,
      a RESPONSE command is sent which contains a Kerberos V5 KRB_AP_REP
      message to perform the mutual authentication.

   IAC SB AUTHENTICATION <authentication-type-pair> FORWARD <KRB_CRED
   message> IAC SE

      This command is used to forward kerberos credentials for use by
      the remote session.  The credentials are passed as a Kerberos V5
      KRB_CRED message which includes, among other things, the forwarded
      Kerberos ticket and a session key associated with the ticket.
      Part of the KRB_CRED message is encrypted in the key previously
      exchanged for the telnet session by the AUTH suboption.

   IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_ACCEPT IAC
   SE

      This command indicates that the credential forwarding was
      successful.

   IAC SB AUTHENTICATION <authentication-type-pair> FORWARD_REJECT
      <optional reason for rejection> IAC SE

      This command indicates that the credential forwarding was not
      successful, and if there is any more data in the suboption, it is
      an ASCII text message of the reason for the rejection.

3.  Implementation Rules

   If the second octet of the authentication-type-pair has the AUTH_WHO
   bit set to AUTH_CLIENT_TO_SERVER, then the client sends the initial
   AUTH command, and the server responds with either ACCEPT or REJECT.
   In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the
   server will send a RESPONSE before it sends the ACCEPT.

   If the second octet of the authentication-type-pair has the AUTH_WHO
   bit set to AUTH_SERVER_TO_CLIENT, then the server sends the initial
   AUTH command, and the client responds with either ACCEPT or REJECT.
   In addition, if the AUTH_HOW bit is set to AUTH_HOW_MUTUAL, the
   client will send a RESPONSE before it sends the ACCEPT.

   The Kerberos principal used by the server will generally be of the
   form "host/<hostname>@realm".  That is, the first component of the
   Kerberos principal is "host"; the second component is the fully
   qualified lower-case hostname of the server; and the realm is the
   Kerberos realm to which the server belongs.

   Any Telnet IAC characters that occur in the KRB_AP_REQ or KRB_AP_REP
   messages, the KRB_CRED structure, or the optional rejection text
   string must be doubled as specified in [4].  Otherwise the following
   byte might be mis-interpreted as a Telnet command.

4.  Examples

   User "joe" may wish to log in as user "pete" on machine "foo".  If
   "pete" has set things up on "foo" to allow "joe" access to his
   account, then the client would send IAC SB AUTHENTICATION NAME "pete"
   IAC SE IAC SB AUTHENTICATION IS KERBEROS_V5 AUTH <KRB_AP_REQ_MESSAGE>
   IAC SE

   The server would then authenticate the user as "joe" from the
   KRB_AP_REQ_MESSAGE, and if the KRB_AP_REQ_MESSAGE was accepted by
   Kerberos, and if "pete" has allowed "joe" to use his account, the
   server would then continue the authentication sequence by sending a
   RESPONSE (to do mutual authentication, if it was requested) followed
   by the ACCEPT.

   If forwarding has been requested, the client then sends IAC SB
   AUTHENTICATION IS KERBEROS_V5 CLIENT|MUTUAL FORWARD <KRB_CRED
   structure with credentials to be forwarded> IAC SE.  If the server
   succeeds in reading the forwarded credentials, the server sends
   FORWARD_ACCEPT else, a FORWARD_REJECT is sent back.

       Client                           Server
                                        IAC DO AUTHENTICATION
       IAC WILL AUTHENTICATION

       [ The server is now free to request authentication information.
         ]

                                        IAC SB AUTHENTICATION SEND
                                        KERBEROS_V5 CLIENT|MUTUAL
                                        KERBEROS_V5 CLIENT|ONE_WAY IAC
                                        SE

       [ The server has requested mutual Version 5 Kerberos
         authentication.  If mutual authentication is not supported,
         then the server is willing to do one-way authentication.

         The client will now respond with the name of the user that it
         wants to log in as, and the Kerberos ticket.  ]

       IAC SB AUTHENTICATION NAME
       "pete" IAC SE
       IAC SB AUTHENTICATION IS
       KERBEROS_V5 CLIENT|MUTUAL AUTH
       <KRB_AP_REQ message> IAC SE

       [ Since mutual authentication is desired, the server sends across
         a RESPONSE to prove that it really is the right server.  ]

                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V5 CLIENT|MUTUAL
                                        RESPONSE <KRB_AP_REP message>
                                        IAC SE

       [ The server responds with an ACCEPT command to state that the
         authentication was successful.  ]

                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V5 CLIENT|MUTUAL ACCEPT
                                        IAC SE

       [ If so requested, the client now sends the FORWARD command to
         forward credentials to the remote site.  ]

       IAC SB AUTHENTICATION IS KER-
       BEROS_V5 CLIENT|MUTUAL
       FORWARD <KRB_CRED message> IAC
       SE

       [ The server responds with a FORWARD_ACCEPT command to state that
         the credential forwarding was successful.  ]

                                        IAC SB AUTHENTICATION REPLY
                                        KERBEROS_V5 CLIENT|MUTUAL
                                        FORWARD_ACCEPT IAC SE

5. Security Considerations

   The selection of the random session key in the Kerberos V5
   authenticator is critical, since this key will be used for encrypting
   the telnet data stream if encryption is enabled.  It is strongly
   advised that the random key selection be done using cryptographic
   techniques that involve the Kerberos ticket's session key.  For
   example, using the current time, encrypting it with the ticket
   session key, and then correcting for key parity is a strong way to
   generate a subsession key, since the ticket session key is assumed to
   be never disclosed to an attacker.

   Care should be taken before forwarding a user's Kerberos credentials
   to the remote server.  If the remote server is not trustworthy, this
   could result in the user's credentials being compromised.  Hence, the
   user interface should not forward credentials by default; it would be
   far safer to either require the user to explicitly request
   credentials forwarding for each connection, or to have a trusted list
   of hosts for which credentials forwarding is enabled, but to not
   enable credentials forwarding by default for all machines.

   The IAC SB AUTHENTICATION NAME name IAC SE message is unprotected in
   this protocol.  Its contents should be verified by a secure method
   after authentication completes before it is used.

6. IANA Considerations

   The authentication type KERBEROS_V5 and its associated suboption
   values are registered with IANA.  Any suboption values used to extend
   the protocol as described in this document must be registered with
   IANA before use.  IANA is instructed not to issue new suboption
   values without submission of documentation of their use.

7. Acknowledgments

   This document was originally written by Dave Borman of Cray Research,
   Inc.  Theodore Ts'o of MIT revised it to reflect the latest
   implementation experience.  Cliff Neuman and Prasad Upasani of USC's
   Information Sciences Institute developed the credential forwarding
   support.

   In addition, the contributions of the Telnet Working Group are also
   gratefully acknowledged.

8. References

   [1] Kohl, J. and B. Neuman, "The Kerberos Network Authentication
       System (V5)", RFC 1510, September 1993.

   [2] Ts'o, T. and J. Altman, "Telnet Authentication Option", RFC 2941,
       September 2000.

   [3] Ts'o, T., "Telnet Data Encryption Option", RFC 2946, September
       2000.

   [4] Postel, J. and J. Reynolds, "Telnet Option Specifications", STD
       8, RFC 855, May 1983.

9. Editor's Address

   Theodore Ts'o
   VA Linux Systems
   43 Pleasant St.
   Medford, MA 02155

   Phone: (781) 391-3464
   EMail: tytso@mit.edu

10. Full Copyright Statement

   Copyright (C) The Internet Society (2000).  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 assigns.

   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.

 

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