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draft-ietf-telnet-envmnt-option-02.txt
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Network Working Group S. Alexander, Editor
Internet-Draft Lachman Technology, Inc.
<draft-ietf-telnet-envmnt-option-02.txt> October 1993
Telnet Environment Option
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a "working
draft" or "work in progress."
Please check the 1id-abstracts.txt listing contained in the
internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net,
nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the
current status of any Internet Draft.
This memo is found in file "draft-ietf-telnet-envmnt-option-02.txt".
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
ENVIRON 39
IS 0
SEND 1
INFO 2
VAR 0
VALUE 1
ESC 2
USERVAR 3
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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.
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 ... ] [
... ] ] 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 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,
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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 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 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
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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:
<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 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
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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 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
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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...)
The earlier version of this specification [1] incorrectly reversed
the values for VAR and VALUE, which this 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 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
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] Borman, D., "Telnet Environment Option", RFC 1408, Cray Research,
Inc., January 1993.
[2] Borman, D., "Telnet Environment Option Interoperability Issues",
Internet Draft, Cray Research, Inc., April 1993.
[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
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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 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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