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Versions: 03 RFC 1408

Network Working Group                    Internet Engineering Task Force
Internet-Draft                                      Telnet Working Group
                                                       D. Borman, Editor
                                                     Cray Research, Inc.
                                                               July 1992


                       Telnet Environment Option

Status of this Memo

   This document is an Internet Draft.  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.

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   Please check the 1id-abstracts.txt listing contained in the
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   nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the
   current status of any Internet Draft.

Abstract

   This document specifies a mechanism for passing environment informa-
   tion between a telnet client and server.  Use of this mechanism en-
   ables a telnet user to propagate configuration information to a re-
   mote host when connecting.  Distribution of this memo is unlimited.

1.  Command Names and Codes

      ENVIRON         36
          IS               0
          SEND             1
          INFO             2

          VAR              0
          VALUE            1
          ESC              2
          USERVAR          3

2.  Command Meanings



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   IAC WILL ENVIRON

      The sender of this command is willing to send environment vari-
      ables.

   IAC WONT ENVIRON

      The sender of this command refuses to send environment variables.

   IAC DO ENVIRON

      The sender of this command is willing to receive environment vari-
      ables.

   IAC DONT ENVIRON

      The sender of this command refuses to accept environment vari-
      ables.

   IAC SB ENVIRON SEND [ type ... [ type ... [ ... ] ] ] IAC SE

      The sender of this command requests that the remote side send its
      environment variables.  The "type" may be either VAR or USERVAR,
      to indicate either well known or user variable names.  Only the
      side that is DO ENVIRON may initiate a SEND command.  If a list of
      variables is specified, then only those variables should be sent.
      If no list is specified, then the default environment, of both
      well known and user defined variables, should be sent.  If one of
      the variables has no name, then all the variables of that type
      (well known or user defined)  in the default environment should be
      sent.

   IAC SB ENVIRON IS type ... [ VALUE ... ] [ type ... [ VALUE ... ] [
   ... ] ] IAC SE

      The sender of this command is sending environment variables.  This
      command is sent in response to a SEND request.  Only the side that
      is WILL ENVIRON may send an IS command.  The "type"/VALUE pairs
      must be returned in the same order as the SEND request specified
      them, and there must be a response for each "type ..." explicitly
      requested.  The "type" will be VAR or USERVAR.  Multiple environ-
      ment variables may be sent.  The characters following a "type" up
      to the next "type" or VALUE specify the variable name.  The char-
      acters following a VALUE up to the next "type" specify the value
      of the variable.  If a "type" is not followed by a VALUE (e.g., by
      another VAR, USERVAR, or IAC SE) then that variable is undefined.
      If a VALUE is immediately followed by a "type" or IAC, then the
      variable is defined, but has no value.  If an IAC is contained
      between the IS and the IAC SE, it must be sent as IAC IAC.  If a
      variable or a value contains a VAR, it must be sent as ESC VAR.



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      If a variable or a value contains a USERVAR, it must be sent as
      ESC USERVAR.  If a variable or a value contains a VALUE, it must
      be sent as ESC VALUE.  If a variable or a value contains an ESC,
      it must be sent as ESC ESC.

   IAC SB ENVIRON INFO type ... [ VALUE ... ] [ type ... [ VALUE ... ] [
   ... ] ] IAC SE

      The sender of this command is sending information about environ-
      ment variables that have changed.  It is identical to the IS com-
      mand, except that the command is INFO instead of IS.  Only the
      side that is WILL ENVIRON may send an INFO command.  The INFO com-
      mand is not to be used to send initial information; the SEND/IS
      sequence is to be used for that.  The INFO command is to be used
      to propagate changes in environment variables, and may be spon-
      taneously generated.


3.  Default Specification

   The default specification for this option is

      WONT ENVIRON
      DONT ENVIRON

   meaning there will not be any exchange of environment information.

4.  Motivation

   Many operating systems have startup information and environment vari-
   ables that contain information that should be propagated to remote
   machines when Telnet connections are established.  Rather than create
   a new Telnet option each time someone comes up with some new informa-
   tion that they need propagated through a Telnet session, but that the
   Telnet session itself doesn't really need to know about, this generic
   information option can be used.

5.  Well Known Variables

   USER        This variable is used to transmit the user or account
               name that the client wishes to log into on the remote
               system.  The format of the value the USER variable is
               system dependent, as determined by the remote system.

   JOB         This variable is used to transmit the job ID that the
               client wishes to use when logging into the remote system.
               The format of the value the JOB variable is system depen-
               dent, as determined by the remote system.

   ACCT        This variable is used to transmit the account ID that the



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               client wishes to use when logging into the remote system.
               The format of the value the ACCT variable is system
               dependent, as determined by the remote system.

   PRINTER     This variable is used to identify the default location
               for printer output.  Because there does not currently ex-
               ist a standard way of naming a printer on a network, the
               format of this variable is currently undefined.

   SYSTEMTYPE  This is used to transmit the type of operating system on
               the system that sends this variable.  It value is identi-
               cal to the value of the SYSTEM (SYST) command in FTP [2].
               The format of the value shall have as its first word one
               of the system names listed in the current version of the
               Assigned Numbers document [3].

   DISPLAY     This variable is used to transmit the X display location
               of the client.  The format for the value of the DISPLAY
               variable is:
                  <host>:<dispnum>[.<screennum>]
               This information is identical to the information passed
               using the Telnet X-DISPLAY-LOCATION option.  If both the
               DISPLAY environment variable, and the X-DISPLAY-LOCATION
               option[4] are received, and they contain conflicting in-
               formation, the most recently received information re-
               ceived should be used.


     Because it is impossible to anticipate all variables that users may
     wish to exchange, the USERVAR type is provided to allow users to
     transmit arbitrary variable/value pairs.  The use of an additional
     type allows implementations to distinguish between values derived
     by the remote host software and values supplied by the user.
     Paranoid implementations will most likely treat both types with an
     equal level of distrust.  The results of a name-space collision
     between a well-known and a user variable are implementation specif-
     ic.

  6.  Implementation Rules

     WILL and DO are used only at the beginning of the connection to ob-
     tain and grant permission for future negotiations.

     Once the two hosts have exchanged a WILL and a DO, the sender of
     the DO ENVIRON is free to request that environment variables be
     sent.  Only the sender of the DO may send requests (IAC SB ENVIRON
     SEND IAC SE) and only the sender of the WILL may transmit actual
     environment information (via the IAC SB ENVIRON IS ... IAC SE com-
     mand).  Though this option may be used at anytime throughout the
     life of the telnet connection, the exchange of environment informa-



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     tion will usually happen at the startup of the connection.  This is
     because many operating systems only have mechanisms for propagating
     environment information at process creation, so the information is
     needed before the user logs in.

     The receiving host is not required to put all variables that it re-
     ceives into the environment.  For example, if the client should
     send across USERVAR "TERM" VALUE "xterm" as an environment vari-
     able, and the TERMINAL-TYPE [1] option has already been used to
     determine the terminal type, the server may safely ignore the TERM
     variable.  Also, some startup information may be used in other
     ways; for example, the values for "USER", "ACCT" and "PROJ" values
     might be used to decide which account to log into, and might never
     be put into the users environment.  In general, if the server has
     already determined the value of an environment variable by some
     more accurate means, or if it does not understand a variable name,
     it may ignore the value sent in the ENVIRON option.  The server may
     also prefer to just put all unknown information into the users en-
     vironment.  This is the suggested method of implementation, because
     it allows the user the most flexibility.

     The following is an example of use of the option:

       Host1                            Host2
       IAC DO ENVIRON
                                        IAC WILL ENVIRON
       [ Host1 is now free to request environment information ]
       IAC SB ENVIRON SEND VAR "USER"
       VAR "ACCT" VAR USERVAR IAC SE
       [ The server has now explicitly asked for the USER and ACCT
         variables, the default set of well known environment variables,
         and the default set of user defined variables.  Note that the
         client includes the USER information twice; once because it was
         explicitly asked for, and once because it is part of the
         default environment.  ]
                                        IAC SB ENVIRON IS VAR "USER"
                                        VALUE "joe" VAR "ACCT" VALUE
                                        "kernel" VAR "USER" VALUE "joe"
                                        VAR "DISPLAY" VALUE "foo:0.0"
                                        USERVAR "SHELL" VALUE "/bin/csh"
                                        IAC SE

   It is legal for a client to respond with an empty environment (no
   data between the IAC SB and IAC SE) when no well-defined or user
   variables are currently defined.  For example:

      IAC SB ENVIRON IS IAC SE

   is a valid response to any of the following:




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      IAC SB ENVIRON SEND IAC SE
      IAC SB ENVIRON SEND VAR IAC SE
      IAC SB ENVIRON SEND USERVAR IAC SE
      IAC SB ENVIRON SEND VAR USERVAR IAC SE

   (The last example is equivalent to the first...)

   It is expected that any implementation that supports the Telnet EN-
   VIRON option will support all of this specification.

7.  Security Concerns

   It is important for an implementor of the ENVIRON option to under-
   stand the interaction of setting options and the login/authentication
   process. Specifically careful analysis should be done to determine
   which variables are "safe" to set prior to having the client login.
   An example of a bad choice would be permitting a variable to be
   changed that allows an intruder to circumvent or compromise the
   login/authentication program itself.

8.  References

   [1] VanBokkelen, J., "Telnet Terminal-Type Option", RFC 1091, FTP
       Software, Inc., February 1989.
   [2] Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", RFC
       959, ISI, October 1985
   [3] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1060, ISI,
       March 1990
   [4] Marcy, G., "Telnet X Display Location Option", RFC 1096, Carnegie
       Mellon University, March 1989.

Author's Address

   David A. Borman, Editor
   Cray Research, Inc.
   655F Lone Oak Drive
   Eagan, MN 55123

   Phone: (612) 452-6650

   Mailing List: telnet-ietf@CRAY.COM
   EMail: dab@CRAY.COM

Chair's Address

   The working group can be contacted via the current chair:

   Steve Alexander
   INTERACTIVE Systems Corporation
   1901 North Naper Boulevard



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   Naperville, IL 60563-8895

   Phone: (708) 505-9100 x256
   EMail: stevea@isc.com

















































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