View Source: telnet(1) - Waikato Linux Users Group

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__NAME__


telnet - user interface to the TELNET protocol


__SYNOPSIS__


telnet [[-468ELadr] [[-S tos] [[-e escapechar] [[-l user] [[-n
tracefile] [[ host [[port] ]


__DESCRIPTION__


The telnet command is used for interactive communication
with another host using the TELNET protocol.
It begins in command mode, where it prints a telnet prompt
(


Options:
       -4   Force [IPv4] address resolution.


-6Force [IPv6] address resolution.-8Request 8-bit operation. This causes  an  attempt  tonegotiate the TELNET BINARY option for both input and output. By default telnet is not 8-bit clean.


-E
Disables the escape character functionality; that is, sets
the escape character to ``no character''.


-L
Specifies an 8-bit data path on output. This causes the
TELNET BINARY option to be negotiated on just
output.


-a
Attempt automatic login. Currently, this sends the user name
via the USER variable of the ENVIRON
option if supported by the remote system. The username is
retrieved via getlogin(3).


-d
Sets the initial value of the debug toggle to
TRUE.


-r
Emulate rlogin(1). In this mode, the default escape
character is a tilde. Also, the interpretation of the escape
character is changed: an escape character fol- lowed by a
dot causes telnet to disconnect from the remote host. A ^Z
instead of a dot suspends telnet, and a ^] (the default
telnet escape character) gener- ates a normal telnet prompt.
These codes are accepted only at the beginning of a
line.


-S tos
Sets the [IP] type-of-service (TOS) option for the tel- net
connection to the value tos.


-e escapechar
Sets the escape character to escapechar. If no char- acter
is supplied, no escape character will be used. Entering the
escape character while connected causes telnet to drop to
command mode.


-l user
Specify user as the user to log in as on the remote system.
This is accomplished by sending the specified name as the
USER environment variable, so it requires that the
remote system support the TELNET ENVIRON option.
This option implies the -a option, and may also be used with
the open command.


-n tracefile
Opens tracefile for recording trace information. See the set
tracefile command below.


host
Specifies a host to contact over the network.


port
Specifies a port number or service name to contact. If not
specified, the telnet port (23) is used.


Protocol:


Once a connection has been opened, telnet will attempt to
enable the TELNET LINEMODE option. If this fails,
then telnet will revert to one of two input modes: either
``char- acter at a time'' or ``old line by line'' depending
on what the remote system supports.


When LINEMODE is enabled, character processing is
done on the local system, under the control of the remote
system. When input editing or character echoing is to be
disabled, the remote system will relay that information. The
remote system will also relay changes to any special
characters that happen on the remote system, so that they
can take effect on the local system.


In ``character at a time'' mode, most text typed is immedi-
ately sent to the remote host for processing.


In ``old line by line'' mode, all text is echoed locally,
and (normally) only completed lines are sent to the remote
host. The ``local echo character'' (initially ``^E'') may be
used to turn off and on the local echo (this would mostly be
used to enter passwords without the password being
echoed).


If the LINEMODE option is enabled, or if the
localchars tog- gle is TRUE (the default for ``old
line by line``; see below), the user's quit, intr, and flush
characters are trapped locally, and sent as
TELNET protocol sequences to the remote side.
If LINEMODE has ever been enabled, then the user's
susp and eof are also sent as TELNET protocol
sequences, and quit is sent as a TELNET ABORT
instead of BREAK There are options (see toggle
autoflush and toggle autosynch below) which cause this
action to flush subsequent output to the terminal (until the
remote host acknowledges the TELNET sequence)
and flush previous terminal input (in the case of quit and
intr).


Commands:


The following telnet commands are available. Unique prefixes
are understood as abbreviations.


auth argument ...
The auth command controls the TELNET AUTHENTICATE
protocol option. If telnet was compiled without
authentication, the auth command will not be sup- ported.
Valid arguments are as follows:


disable type
Disable the specified type of authen- tication. To obtain a
list of avail- able types, use the auth disable ?
command.


enable type
Enable the specified type of authen- tication. To obtain a
list of avail- able types, use the auth enable ?
command.


status
List the current status of the vari- ous types of
authentication.


Note that the current version of telnet does not support
authentication.


close
Close the connection to the remote host, if any, and return
to command mode.


display argument ...
Display all, or some, of the set and toggle values (see
below).


encrypt argument ...
The encrypt command controls the TELNET ENCRYPT
protocol option. If telnet was compiled without encryption,
the encrypt command will not be sup- ported.


Valid arguments are as follows:


disable type [[input|output]
Disable the specified type of encryp- tion. If you do not
specify input or output, encryption of both is dis- abled.
To obtain a list of available types, use ``encrypt disable
?''.


enable type [[input|output]
Enable the specified type of encryp- tion. If you do not
specify input or output, encryption of both is enabled. To
obtain a list of avail- able types, use ``encrypt enable
?''.


input
This is the same as ``encrypt start input''.


-input
This is the same as ``encrypt stop input''.


output
This is the same as ``encrypt start output''.


-output
This is the same as ``encrypt stop output''.


start [[input|output]
Attempt to begin encrypting. If you do not specify input or
output, encryption of both input and output is
started.


status
Display the current status of the encryption
module.


stop [[input|output]
Stop encrypting. If you do not spec- ify input or output,
encryption of both is stopped.


type type
Sets the default type of encryption to be used with later
``encrypt start'' or ``encrypt stop'' commands.


Note that the current version of telnet does not support
encryption.


environ arguments...
The environ command is used to propagate environ- ment
variables across the telnet link using the TELNET
ENVIRON protocol option. All variables exported from
the shell are defined, but only the DISPLAY and
PRINTER variables are marked to be sent by default.
The USER variable is marked to be sent if the -a or
-l command-line options were used.


Valid arguments for the environ command are:


define variable value
Define the variable variable to have a value of value. Any
variables defined by this command are automatically marked
for propagation (``exported''). The value may be enclosed in
single or double quotes so that tabs and spaces may be
included.


undefine variable
Remove any existing definition of variable.


export variable
Mark the specified variable for propa- gation to the remote
host.


unexport variable
Do not mark the specified variable for propagation to the
remote host. The remote host may still ask explicitly for
variables that are not exported.


list
List the current set of environment variables. Those marked
with a will be propagated to the remote host. The remote
host may still ask explicitly for the rest.


?
Prints out help information for the environ
command.


logout
Send the TELNET LOGOUT protocol option to the
remote host. This command is similar to a close command. If
the remote host does not support the LOGOUT option,
nothing happens. But if it does, this command should cause
it to close the connec- tion. If the remote side also
supports the con- cept of suspending a user's session for
later reattachment, the logout command indicates that the
session should be terminated immediately.


mode type
Type is one of several options, depending on the state of
the session. Telnet asks the remote host to
go into the requested mode. If the remote host says it can,
that mode takes effect.


character
Disable the TELNET LINEMODE option, or, if the
remote side does not understand the LINEMODE
option, then enter ``character at a time``
mode.


line
Enable the TELNET LINEMODE option, or, if the
remote side does not understand the LINEMODE
option, then attempt to enter ``old-line-by-line``
mode.


isig (-isig)
Attempt to enable (disable) the TRAPSIG mode of the
LINEMODE option. This requires that the
LINEMODE option be enabled.


edit (-edit)
Attempt to enable (disable) the EDIT mode of the
LINEMODE option. This requires that the
LINEMODE option be enabled.


softtabs (-softtabs)
Attempt to enable (disable) the SOFT_TAB mode of
the LINEMODE option. This requires that the
LINEMODE option be enabled.


litecho (-litecho)
Attempt to enable (disable) the LIT_ECHO mode of
the LINEMODE option. This requires that the
LINEMODE option be enabled.


?
Prints out help information for the mode
command.


open host
[[
[[-l] user
][[
- port]
Open a connection to the named host. If no port number is
specified, telnet will attempt to con- tact a
telnet daemon at the standard port (23). The
host specification may be a host name or IP address. The -l
option may be used to specify a user name to be passed to
the remote system, like the -l command-line
option.


When connecting to ports other than the telnet port, telnet
does not attempt telnet protocol
negotiations. This makes it possible to connect to services
that do not support the telnet protocol
without making a mess. Protocol negotiation can be forced by
placing a dash before the port number.


After establishing a connection, any commands associated
with the remote host in /etc/telnetrc and the
user's .telnetrc file are executed, in that
order.


The format of the telnetrc files is as follows: Lines
beginning with a #, and blank lines, are ignored. The rest
of the file should consist of hostnames and sequences of
telnet commands to use with that host. Commands should be
one per line, indented by whitespace; lines beginning
without whitespace are interpreted as hostnames. Lines
beginning with the special hostname DEFAULT will
apply to all hosts. Upon connecting to a particu- lar host,
the commands associated with that host are
executed.


quit
Close any open session and exit telnet. An end of file
condition on input, when in command mode, will trigger this
operation as well.


send arguments
Send one or more special telnet protocol
character sequences to the remote host. The following are
the codes which may be specified (more than one may be used
in one command):


abort
Sends the TELNET ABORT (Abort Processes)
sequence.


ao
Sends the TELNET AO (Abort Output) sequence, which
should cause the remote system to flush all output
''from'' the remote system ''to'' the user's
terminal.


ayt
Sends the TELNET AYT (Are You There?) sequence, to
which the remote system may or may not choose to
respond.


brk
Sends the TELNET BRK (Break) sequence, which may
have significance to the remote system.


ec
Sends the TELNET EC (Erase Character) sequence,
which should cause the remote system to erase the last
character entered.


el
Sends the TELNET EL (Erase Line) sequence, which
should cause the remote system to erase the line currently
being entered.


eof
Sends the TELNET EOF (End Of File)
sequence.


eor
Sends the TELNET EOR (End of Record)
sequence.


escape
Sends the current telnet escape character.


ga
Sends the TELNET GA (Go Ahead) sequence, which
likely has no significance to the remote
system.


getstatus
If the remote side supports the TELNET STATUS
command, getstatus will send the subnegotiation to request
that the server send its current option status.


ip
Sends the TELNET IP (Interrupt Process) sequence,
which should cause the remote system to abort the currently
running pro- cess.


nop
Sends the TELNET NOP (No Operation)
sequence.


susp
Sends the TELNET SUSP (Suspend Process)
sequence.


synch
Sends the TELNET SYNCH sequence. This sequence
causes the remote system to dis- card all previously typed
(but not yet read) input. This sequence is sent as
TCP urgent data (and may not work if the
remote system is a 4.2 BSD system -- if it
doesn't work, a lower case ``r'' may be echoed on the
terminal).


do cmd


dont cmd


will cmd


wont cmd
Sends the TELNET DO cmd sequence. cmd can be either
a decimal number between 0 and 255, or a symbolic name for a
specific TELNET command. cmd can also be either
help or ? to print out help information, including a list of
known symbolic names.


?
Prints out help information for the send
command.


set argument value


unset argument value
The set command will set any one of a number of telnet
variables to a specific value or to TRUE. The
special value off turns off the function asso- ciated with
the variable. This is equivalent to using the unset command.
The unset command will disable or set to FALSE any
of the specified vari- ables. The values of variables may be
interro- gated with the display command. The variables which
may be set or unset, but not toggled, are listed here. In
addition, any of the variables for the toggle command may be
explicitly set or unset.


ayt
If telnet is in localchars mode, or
LINEMODE is enabled, and the status charac- ter is
typed, a TELNET AYT sequence is sent to the remote
host. The initial value for the


echo
This is the value (initially ``^E'') which, when in ``line
by line'' mode, toggles between doing local echoing of
entered characters (for normal processing), and suppressing
echoing of entered characters (for entering, say, a
password).


eof
If telnet is operating in LINEMODE or ``old line by
line'' mode, entering this charac- ter as the first
character on a line will cause this character to be sent to
the remote system. The initial value of the eof character is
taken to be the terminal's eof character.


erase
If telnet is in localchars mode (see toggle localchars
below), __and__ if telnet is operat- ing in ``character
at a time'' mode, then when this character is typed, a
TELNET EC sequence (see send ec above) is sent to
the remote system. The initial value for the erase character
is taken to be the termi- nal's erase
character.


escape
This is the telnet escape character (ini- tially ``^[['')
which causes entry into telnet command mode (when connected
to a remote system).


flushoutput
If telnet is in localchars mode (see toggle localchars
below) and the flushoutput char- acter is typed, a
TELNET AO sequence (see send ao above) is sent to
the remote host. The initial value for the flush character
is taken to be the terminal's flush charac-
ter.


forw1


forw2
If TELNET is operating in LINEMODE,
these are the characters that, when typed, cause partial
lines to be forwarded to the remote system. The initial
value for the forward- ing characters are taken from the
termi- nal's eol and eol2 characters.


interrupt
If telnet is in localchars mode (see toggle localchars
below) and the interrupt charac- ter is typed, a TELNET
IP sequence (see send ip above) is sent to the remote
host. The initial value for the interrupt charac- ter is
taken to be the terminal's intr character.


kill
If telnet is in localchars mode (see toggle localchars
below), and if telnet is operat- ing in ``character at a
time'' mode, then when this character is typed, a TELNET
EL sequence (see send el above) is sent to the remote
system. The initial value for the kill character is taken to
be the termi- nal's kill character.


lnext
If telnet is operating in LINEMODE or ``old line by
line`` mode, then this character is taken to be the
terminal's lnext character. The initial value for the lnext
character is taken to be the terminal's lnext charac-
ter.


quit
If telnet is in localchars mode (see toggle localchars
below) and the quit character is typed, a TELNET
BRK sequence (see send brk above) is sent to the remote
host. The initial value for the quit character is taken to
be the terminal's quit character.


reprint
If telnet is operating in LINEMODE or ``old line by
line`` mode, then this character is taken to be the
terminal's reprint charac- ter. The initial value for the
reprint character is taken to be the terminal's reprint
character.


rlogin
This is the rlogin mode escape character. Setting it enables
rlogin mode, as with the r command-line option
(q.v.)


start
If the TELNET TOGGLE-FLOW-CONTROL option has been
enabled, then this character is taken to be the terminal's
start character. The initial value for the kill character is
taken to be the terminal's start character.


stop
If the TELNET TOGGLE-FLOW-CONTROL option has been
enabled, then this character is taken to be the terminal's
stop character. The initial value for the kill character is
taken to be the terminal's stop character.


susp
If telnet is in localchars mode, or LINEMODE is
enabled, and the suspend char- acter is typed, a TELNET
SUSP sequence (see send susp above) is sent to the
remote host. The initial value for the suspend character is
taken to be the terminal's suspend character.


tracefile
This is the file to which the output, caused by netdata or
option tracing being TRUE, will be written. If it
is set to ``-'', then tracing information will be written to
standard output (the default).


worderase
If telnet is operating in LINEMODE or ``old line by
line`` mode, then this character is taken to be the
terminal's worderase char- acter. The initial value for the
worderase character is taken to be the terminal's worderase
character.


?
Displays the legal set (unset) commands.


slc state
The slc command (Set Local Characters) is used to set or
change the state of the the special charac- ters when the
TELNET LINEMODE option has been enabled. Special
characters are characters that get mapped to
TELNET commands sequences (like ip or quit)
or line editing characters (like erase and kill). By
default, the local special charac- ters are
exported.


check
Verify the current settings for the current special
characters. The remote side is requested to send all the
cur- rent special character settings, and if there are any
discrepancies with the local side, the local side will
switch to the remote value.


export
Switch to the local defaults for the special characters. The
local default characters are those of the local ter- minal
at the time when telnet was started.


import
Switch to the remote defaults for the special characters.
The remote default characters are those of the remote sys-
tem at the time when the TELNET connec- tion
was established.


?
Prints out help information for the slc
command.


status
Show the current status of telnet. This includes the name of
the remote host, if any, as well as the current
mode.


toggle arguments ...
Toggle (between TRUE and FALSE) various
flags that control how telnet responds to events. These
flags may be set explicitly to TRUE or
FALSE using the set and unset commands. More than
one flag may be toggled at once. The state of these flags
may be examined with the display command. Valid flags
are:


authdebug
Turns on debugging for the authenti- cation code. This flag
only exists if authentication support is
enabled.


autoflush
If autoflush and localchars are both TRUE, then
when the ao, or quit char- acters are recognized (and trans-
formed into TELNET sequences; see set above
for details), telnet refuses to display any data on the
user's termi- nal until the remote system acknowl- edges
(via a TELNET TIMING MARK option) that it has
processed those TELNET sequences. The initial
value for this toggle is TRUE if the termi- nal
user had not done an
FALSE (see stty(1)).


autodecrypt
When the TELNET ENCRYPT option is negotiated, by
default the actual encryption (decryption) of the data
stream does not start automatically. The autoencrypt
(autodecrypt) command states that encryption of the output
(input) stream should be enabled as soon as
possible.


Note that this flag exists only if encryption support is
enabled.


autologin
If the remote side supports the TELNET
AUTHENTICATION option, telnet attempts
to use it to perform auto- matic authentication. If the
TELNET AUTHENTICATION option is not sup- ported,
the user's login name is propagated using the TELNET
ENVIRON option. Setting this flag is the same as
specifying the a option to the open command or on the
command line.


autosynch
If autosynch and localchars are both TRUE, then
when either the intr or quit characters is typed (see set
above for descriptions of the intr and quit characters), the
resulting telnet sequence sent is followed by
the TELNET SYNCH sequence. This pro- cedure should
cause the remote system to begin throwing away all
previously typed input until both of the
telnet sequences have been read and acted
upon. The initial value of this tog- gle is
FALSE.


binary
Enable or disable the TELNET BINARY option on both
input and output.


inbinary
Enable or disable the TELNET BINARY option on
input.


outbinary
Enable or disable the TELNET BINARY option on
output.


crlf
If this is TRUE, then carriage returns will be sent
as . If this is FALSE,
then carriage returns will be send as
. The ini- tial value for this
toggle is FALSE.


crmod
Toggle carriage return mode. When this mode is enabled, most
carriage return characters received from the remote host
will be mapped into a carriage return followed by a line
feed. This mode does not affect those characters typed by
the user, only those received from the remote host. This
mode is not very useful unless the remote host only sends
carriage return, but never line feed. The initial value for
this toggle is FALSE.


debug
Toggles socket level debugging (use- ful only to the super
user). The initial value for this toggle is
FALSE.


encdebug
Turns on debugging information for the encryption code. Note
that this flag only exists if encryption sup- port is
available.


localchars
If this is TRUE, then the flush, interrupt, quit,
erase, and kill characters (see set above) are recog- nized
locally, and transformed into (hopefully) appropriate
TELNET con- trol sequences (respectively ao,
ip, brk, ec, and el; see send above). The initial value for
this toggle is TRUE in ``old line by line'' mode,
and FALSE in ``character at a time'' mode. When the
LINEMODE option is enabled, the value of localchars
is ignored, and assumed to always be TRUE. If
LINEMODE has ever been enabled, then quit is sent
as abort, and eof and are sent as eof and susp, see send
above).


netdata
Toggles the display of all network data (in hexadecimal
format). The initial value for this toggle is
FALSE.


options
Toggles the display of some internal telnet protocol
processing (having to do with telnet
options). The initial value for this toggle is
FALSE.


prettydump
When the netdata toggle is enabled, if prettydump is enabled
the output from the netdata command will be for- matted in a
more user-readable for- mat. Spaces are put between each
character in the output, and the beginning of
telnet escape sequences are preceded by a ''
to aid in locat- ing them.


skiprc
When the skiprc toggle is TRUE,
telnet does not read the telnetrc files. The
initial value for this toggle is
FALSE.


termdata
Toggles the display of all terminal data (in hexadecimal
format). The initial value for this toggle is
FALSE.


verbose_encrypt
When the verbose_encrypt toggle is TRUE,
TELNET prints out a message each time
encryption is enabled or disabled. The initial value for
this toggle is FALSE. This flag only exists if
encryption support is available.


?
Displays the legal toggle commands.


z
Suspend telnet. This command only works when the user is
using the csh(1).


! [[command]
Execute a single command in a subshell on the local system.
If command is omitted, then an interactive subshell is
invoked.


? [[command]
Get help. With no arguments, telnet prints a help summary.
If a command is specified, telnet will print the help
information for just that command.


__ENVIRONMENT__


Telnet uses at least the HOME, SHELL,
DISPLAY, and TERM environment variables.
Other environment variables may be propagated to the other
side via the TELNET ENVIRON option.
__FILES__
       /etc/telnetrc


global telnet startup values
~/.telnetrc
user customized telnet startup values


__HISTORY__


The Telnet command appeared in 4.2 BSD
.
__NOTES__


On some remote systems, echo has to be turned off manually when in ``old line by line'' mode.


In ``old line by line'' mode or LINEMODE the
terminal's eof character is only recognized (and sent to the
remote system) when it is the first character on a
line.
__BUGS__


The source code is not comprehensible.


Linux !NetKit (0.17) August 15, 1999 1
----
CategoryNetworking
14 pages link to telnet(1):
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Last edited on Monday, January 5, 2004 11:59:57 pm by StuartYeates
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