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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.
+
+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 3
+
+2. 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 is
+
+
+
+Telnet 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 SE
+
+
+
+
+
+
+Telnet 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 in Section 7.
+
+
+
+
+
+
+
+
+
+
+
+
+Telnet Working Group [Page 6]
+
+RFC 1408 Telnet Environment Option January 1993
+
+
+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
+ EMail: stevea@isc.com
+
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+Telnet Working Group [Page 7]
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