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Network Working Group                                  D. Borman, EditorRequest for Comments: 1408                           Cray Research, Inc.                                                            January 1993Telnet Environment OptionStatus 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.Abstract   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          32.  Command Meanings   IAC WILL ENVIRON      The sender of this command is willing to send environment      variables.   IAC WONT ENVIRON      The sender of this command refuses to send environment variables.Telnet Working Group                                            [Page 1]

RFC 1408               Telnet Environment Option            January 1993   IAC DO ENVIRON      The sender of this command is willing to receive environment      variables.   IAC DONT ENVIRON      The sender of this command refuses to accept environment      variables.   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 ... ] [   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.Telnet Working Group                                            [Page 2]

RFC 1408               Telnet Environment Option            January 1993   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      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   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.Telnet Working Group                                            [Page 3]

RFC 1408               Telnet Environment Option            January 1993   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:                  <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 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 isTelnet Working Group                                            [Page 4]

RFC 1408               Telnet Environment Option            January 1993   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 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 SETelnet Working Group                                            [Page 5]

RFC 1408               Telnet Environment Option            January 1993   is a valid response to any of the following:         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   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 inSection 7.Telnet Working Group                                            [Page 6]

RFC 1408               Telnet Environment Option            January 1993Author'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.COMChair'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   EMail: stevea@isc.comTelnet Working Group                                            [Page 7]