rfc2941.txt

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Network Working Group                                     T. Ts'o, Editor
Request for Comments: 2941                               VA Linux Systems
Obsoletes: 1416                                                 J. Altman
Category: Standards Track                             Columbia University
                                                           September 2000


                      Telnet Authentication Option

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 the authentication option to the telnet [1]
   protocol as a generic method for negotiating an authentication type
   and mode including whether encryption should be used and if
   credentials should be forwarded.  While this document summarizes
   currently utilized commands and types it does not define a specific
   authentication type.  Separate documents are to be published defining
   each authentication type.

   This document updates a previous specification of the telnet
   authentication option, RFC 1416 [2], so that it can be used to
   securely enable the telnet encryption option [3].

1.  Command Names and Codes

      AUTHENTICATION          37

          Authentication Commands
          IS                       0
          SEND                     1
          REPLY                    2
          NAME                     3

          Authentication Types
          NULL                     0
          KERBEROS_V4              1



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          KERBEROS_V5              2
          SPX*                     3
          MINK*                    4
          SRP                      5
          RSA*[also used by SRA*]  6
          SSL*                     7
          [unassigned]             8
          [unassigned]             9
          LOKI*                   10
          SSA*                    11
          KEA_SJ                  12
          KEA_SJ_INTEG            13
          DSS                     14
          NTLM*                   15

       Authentication types followed by (*) were never submitted to the
       IETF for consideration as an Internet standard.

       Following historical practice, future authentication type numbers
       and authentication modifiers will be assigned by the IANA under a
       First Come First Served policy as outlined by RFC 2434 [4].
       Despite the fact that authentication type numbers are allocated
       out of an 8-bit number space (as are most values in the telnet
       specification) it is not anticipated that the number space is or
       will become in danger of being exhausted.  However, if this
       should become an issue, when over 50% of the number space becomes
       allocated, the IANA shall refer allocation requests to either the
       IESG or a designated expert for approval.  IANA is instructed not
       to issue new suboption values without submission of documentation
       of their use.

          Modifiers
          AUTH_WHO_MASK        1
          AUTH_CLIENT_TO_SERVER    0
          AUTH_SERVER_TO_CLIENT    1

          AUTH_HOW_MASK        2
          AUTH_HOW_ONE_WAY         0
          AUTH_HOW_MUTUAL          2

          ENCRYPT_MASK        20
          ENCRYPT_OFF              0
          ENCRYPT_USING_TELOPT     4
          ENCRYPT_AFTER_EXCHANGE  16
          ENCRYPT_RESERVED        20

          INI_CRED_FWD_MASK    8
          INI_CRED_FWD_OFF         0



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          INI_CRED_FWD_ON          8

2.  Command Meanings

   This document makes reference to a "server" and a "client".  For the
   purposes of this document, the "server" is the side of the connection
   that performed the passive TCP open (TCP LISTEN state), and the
   "client" is the side of the connection that did the active open.

   IAC WILL AUTHENTICATION

      The client side of the connection sends this command to indicate
      that it is willing to send and receive authentication information.

   IAC DO AUTHENTICATION

      The servers side of the connection sends this command to indicate
      that it is willing to send and receive authentication information.

   IAC WONT AUTHENTICATION

      The client side of the connection sends this command to indicate
      that it refuses to send or receive authentication information; the
      server side must send this command if it receives a DO
      AUTHENTICATION command.

   IAC DONT AUTHENTICATION

      The server side of the connection sends this command to indicate
      that it refuses to send or receive authentication information; the
      client side must send this command if it receives a WILL
      AUTHENTICATION command.

   IAC SB AUTHENTICATION SEND authentication-type-pair-list IAC SE

      The sender of this command (the server) requests that the remote
      side send authentication information for one of the authentication
      types listed in "authentication-type-pair-list".  The
      "authentication-type-pair-list" is an ordered list of
      "authentication-type" pairs.  Only the server side (DO
      AUTHENTICATION) is allowed to send this.

   IAC SB AUTHENTICATION IS authentication-type-pair <auth data> IAC SE

      The sender of this command (the client) is sending the
      authentication information for authentication type
      "authentication-type-pair".  Only the client side (WILL
      AUTHENTICATION) is allowed to send this.



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   IAC SB AUTHENTICATION REPLY authentication-type-pair <auth data> IAC
   SE

      The sender of this command (the server) is sending a reply to the
      the authentication information received in a previous IS command.
      Only the server side (DO AUTHENTICATION) is allowed to send this.

   IAC SB AUTHENTICATION NAME remote-user IAC SE

      This optional command is sent to specify the account name on the
      remote host that the user wishes to be authorized to use.  Note
      that authentication may succeed, and the authorization to use a
      particular account may still fail.  Some authentication mechanisms
      may ignore this command.

   The "authentication-type-pair" is two octets, the first is the
   authentication type, and the second is a modifier to the type.  The
   authentication type may or may not include built-in encryption.  For
   instance, when the Kerberos 4 authentication type is negotiated
   encryption must be negotiated with the telnet ENCRYPT option.
   However, the SSL and KEA_SJ authentication types provide an encrypted
   channel as part of a successful telnet AUTH option negotiation.

   There are currently five one bit fields defined in the modifier.  The
   first two of these bits are processed as a pair, the AUTH_WHO_MASK
   bit and the AUTH_HOW_MASK bit.  There are four possible combinations
   of these two bits:

      AUTH_CLIENT_TO_SERVER
      AUTH_HOW_ONE_WAY

         The client will send authentication information about the local
         user to the server.  If the negotiation is successful, the
         server will have authenticated the user on the client side of
         the connection.

      AUTH_SERVER_TO_CLIENT
      AUTH_HOW_ONE_WAY

         The server will authenticate itself to the client.  If the
         negotiation is successful, the client will know that it is
         connected to the server that it wants to be connected to.

      AUTH_CLIENT_TO_SERVER
      AUTH_HOW_MUTUAL

         The client will send authentication information about the local
         user to the server, and then the server will authenticate



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         itself to the client.  If the negotiation is successful, the
         server will have authenticated the user on the client side of
         the connection, and the client will know that it is connected
         to the server that it wants to be connected to.

      AUTH_SERVER_TO_CLIENT
      AUTH_HOW_MUTUAL

         The server will authenticate itself to the client, and then the
         client will authenticate itself to the server.  If the
         negotiation is successful, the client will know that it is
         connected to the server that it wants to be connected to, and
         the server will know that the client is who it claims to be.

      The third and fifth bits in the modifier are the ENCRYPT_MASK
      bits.  These bits are used to determine if and how encryption
      should be enabled.  Of the four possible combinations only three
      are currently defined:

         ENCRYPT_OFF

            Encryption will not be used for this session.  TELOPT
            ENCRYPT SHOULD NOT be negotiated.  This mode MUST be used
            with all AUTH types that do not provide a shared secret to
            be used as a session key.

         ENCRYPT_USING_TELOPT

            Encryption will be negotiated via the use of TELOPT ENCRYPT.
            Immediately after authentication has completed TELOPT
            ENCRYPT MUST be negotiated in both directions.  This is
            required to occur before credentials forwarding; other
            telnet options are negotiated; or any user data is
            transmitted.  A failure to successfully negotiate TELOPT
            ENCRYPT in either direction MUST result in immediate session
            termination.

         ENCRYPT_AFTER_EXCHANGE

            Encryption will be activated in both directions immediately
            after the successful exchange of the shared secret to be
            used as the session key.  The encryption algorithm to be
            used MUST be implied by the AUTH type.

      The fourth bit field in the modifier is the INI_CRED_FWD_MASK bit.
      This bit is either set to INI_CRED_FWD_ON or INI_CRED_FWD_OFF.
      This bit is set by the client to advise the server to expect
      forwarded credentials from the client.



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