RFC1408 - Telnet Environment Option

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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 RFCspecifies 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.

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.

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.

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

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

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", RFC1091, FTP
Software, Inc., February 1989.

[2] Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", STD
9, RFC959, USC/Information Sciences Institute, October 1985.

[3] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC1340,
USC/Information Sciences Institute, July 1992.

[4] Marcy, G., "Telnet X Display Location Option", RFC1096, Carnegie
Mellon University, March 1989.

Security Considerations

Security issues are discussed in Section 7.

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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