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RFC 1572 - Telnet Environment Option

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Network Working Group                               S. Alexander, Editor
Request for Comments: 1572                      Lachman Technology, Inc.
Category: Standards Track                                   January 1994

                       Telnet Environment 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.


   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.

   This document corrects some errors in [1].

1.  Command Names and Codes

      NEW-ENVIRON     39
          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.


      The sender of this command is willing to receive environment


      The sender of this command refuses to accept environment

   IAC SB NEW-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 NEW-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 NEW-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 NEW-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.

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

      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 NEW-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 generated.

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.

   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 [4].  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 [5].

   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 [6] 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 NEW-ENVIRON is free to request that environment variables be sent.
   Only the sender of the DO may send requests (IAC SB NEW-ENVIRON SEND
   IAC SE) and only the sender of the WILL may transmit actual
   environment information (via the IAC SB NEW-ENVIRON IS ... IAC SE
   command).  Though this option may be used at any time 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.

   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 [3] 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 NEW-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 NEW-ENVIRON
       [ Host1 is now free to request environment information ]
       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 NEW-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:


   is a valid response to any of the following:


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

   The earlier version of this specification [1] incorrectly reversed
   the values for VAR and VALUE,  which put the specification at odds
   with existing implementations.  In order to resolve that problem, as
   well as other minor problems, a new option number has been assigned
   to the NEW-ENVIRON option.  This allows implementations of this memo
   to interoperate with no ambiguity.

   For a discussion on how to implement to interoperate with the various
   implementations that pre-date this memo, see [2].

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

7.  Security Concerns

   It is important for an implementor of the NEW-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] Borman, D., Editor, "Telnet Environment Option", RFC 1408, Cray
       Research, Inc., January 1993.

   [2] Borman, D., "Telnet Environment Option Interoperability Issues",
       RFC 1571, Cray Research, Inc., January 1994.

   [3] VanBokkelen, J., "Telnet Terminal-Type Option", RFC 1091, FTP
       Software, Inc., February 1989.

   [4] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD
       9, RFC 959, USC/Information Sciences Institute, October 1985.

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

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


   The original version of this document was written by Dave Borman of
   Cray Research, Inc.  In addition, the comments of the Telnet Working
   Group of the IETF are gratefully acknowledged.

Security Considerations

   Security issues are discussed in Section 7.

Editor's Address

   Steve Alexander
   Lachman Technology, Inc.
   1901 North Naper Boulevard
   Naperville, IL 60563-8895

   Phone: (708) 505-9555 x256
   EMail: stevea@lachman.com


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