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RFC 1408

Telnet Environment Option

Pages: 7
Updated by:  1571

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Network Working Group                                  D. Borman, Editor
Request for Comments: 1408                           Cray Research, Inc.
                                                            January 1993

                       Telnet Environment Option

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.


   This document specifies a mechanism for passing environment
   information between a telnet client and server.  Use of this
   mechanism enables a telnet user to propagate configuration
   information to a remote host when connecting.

1.  Command Names and Codes

      ENVIRON         36
          IS               0
          SEND             1
          INFO             2

          VAR              0
          VALUE            1
          ESC              2
          USERVAR          3

2.  Command Meanings


      The sender of this command is willing to send environment


      The sender of this command refuses to send environment variables.
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      The sender of this command is willing to receive environment


      The sender of this command refuses to accept environment

   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

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

   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
      environment variables may be sent.  The characters following a
      "type" up to the next "type" or VALUE specify the variable name.
      The characters 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.

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

   The sender of this command is sending information about environment
      variables that have changed.  It is identical to the IS command,
      except that the command is INFO instead of IS.  Only the side that
      is WILL ENVIRON may send an INFO command.  The INFO command 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 spontaneously

3.  Default Specification

   The default specification for this option is


   meaning there will not be any exchange of environment information.

4.  Motivation

   Many operating systems have startup information and environment
   variables 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
   information 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
               dependent, as determined by the remote system.

   ACCT        This variable is used to transmit the account ID that the
               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.
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   PRINTER     This variable is used to identify the default location
               for printer output.  Because there does not currently
               exist 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
               identical 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:
               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 information, the most recently
               received information received 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 specific.

6.  Implementation Rules

   WILL and DO are used only at the beginning of the connection to
   obtain 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 command).  Though
   this option may be used at anytime throughout the life of the telnet
   connection, the exchange of environment information 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.  In this section, anything that is in quotes is
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   shorthand for a string of ASCII values.  For example, "joe" means the
   three octet sequence (in decimal) 106 111 101.

   The receiving host is not required to put all variables that it
   receives into the environment.  For example, if the client should
   send across USERVAR "TERM" VALUE "xterm" as an environment variable,
   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 environment.  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 WILL ENVIRON
       [ Host1 is now free to request environment information ]
       [ 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:

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   is a valid response to any of the following:


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

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

7.  Security Concerns

   It is important for an implementor of the ENVIRON option to
   understand 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)", STD
       9, RFC 959, USC/Information Sciences Institute, October 1985.

   [3] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
       USC/Information Sciences Institute, July 1992.

   [4] Marcy, G., "Telnet X Display Location Option", RFC 1096, Carnegie
       Mellon University, March 1989.

Security Considerations

   Security issues are discussed in Section 7.
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Author's Address

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

   Phone: (612) 452-6650
   EMail: dab@CRAY.COM

   Mailing List: telnet-ietf@CRAY.COM

Chair's Address

   The working group can be contacted via the current chair:

   Steve Alexander
   INTERACTIVE Systems Corporation
   1901 North Naper Boulevard
   Naperville, IL 60563-8895

   Phone: (708) 505-9100 x256