GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1572

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.

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.
 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

 IAC WILL NEW-ENVIRON
    The sender of this command is willing to send environment
    variables.
 IAC WONT NEW-ENVIRON
    The sender of this command refuses to send environment variables.

Telnet Working Group [Page 1] RFC 1572 Telnet Environment Option January 1994

 IAC DO NEW-ENVIRON
    The sender of this command is willing to receive environment
    variables.
 IAC DONT NEW-ENVIRON
    The sender of this command refuses to accept environment
    variables.
 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.

Telnet Working Group [Page 2] RFC 1572 Telnet Environment Option January 1994

 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
    WONT NEW-ENVIRON
    DONT NEW-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 1572 Telnet Environment Option January 1994

 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:
                <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 [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

Telnet Working Group [Page 4] RFC 1572 Telnet Environment Option January 1994

 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 DO NEW-ENVIRON
                                      IAC WILL NEW-ENVIRON
     [ Host1 is now free to request environment information ]
     IAC SB NEW-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 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:
    IAC SB NEW-ENVIRON IS IAC SE
 is a valid response to any of the following:

Telnet Working Group [Page 5] RFC 1572 Telnet Environment Option January 1994

    IAC SB NEW-ENVIRON SEND IAC SE
    IAC SB NEW-ENVIRON SEND VAR IAC SE
    IAC SB NEW-ENVIRON SEND USERVAR IAC SE
    IAC SB NEW-ENVIRON SEND VAR USERVAR IAC SE
 (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.

Telnet Working Group [Page 6] RFC 1572 Telnet Environment Option January 1994

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

Acknowledgements

 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

Telnet Working Group [Page 7]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1572.txt · Last modified: 1994/01/12 23:15 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki