man telnet – Man page for telnet

June 1, 2007 – 1:21 am

TELNET

Section: User Commands (1)
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---

BSD mandoc

 

NAME

telnet

– user interface to the

TELNET

protocol
 

SYNOPSIS

telnet

[–8EFKLacdfrx

]

[–X authtype

]

[–b hostalias

]

[–e escapechar

]

[–k realm

]

[–l user

]

[–n tracefile

]

[host

[port

]

]
 

DESCRIPTION

The
telnet

command
is used to communicate with another host using the
TELNET

protocol.
If
telnet

is invoked without the
host

argument, it enters command mode,
indicated by its prompt
(telnet>

)

In this mode, it accepts and executes the commands listed below.
If it is invoked with arguments, it performs an
open

command with those arguments.

The options are as follows:

–7

Strip 8th bit on input and output. Telnet is 8–bit clean by default but doesn’t send the TELNET BINARY option unless forced.

–8

Specifies an 8–bit data path.
This causes an attempt to negotiate the
TELNET BINARY

option on both input and output.

–E

Stops any character from being recognized as an escape character.

–F

If Kerberos V5 authentication is being used, the
–F

option allows the local credentials to be forwarded
to the remote system, including any credentials that
have already been forwarded into the local environment.

–K

Specifies no automatic login to the remote system.

–L

Specifies an 8–bit data path on output.
This causes the BINARY option to be negotiated on output.

–X atype

Disables the
atype

type of authentication.

–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 name used is that of the current user as returned by
getlogin(2)

if it agrees with the current user ID,
otherwise it is the name associated with the user ID.

–b hostalias

Uses
bind(2)

on the local socket to bind it to an aliased address (see
ifconfig(8)

and the “alias” specifier) or to the address of
another interface than the one naturally chosen by
connect(2).

This can be useful when connecting to services which use IP addresses
for authentication and reconfiguration of the server is undesirable (or
impossible).

–c

Disables the reading of the user’s
.telnetrc

file.
(See the
toggle skiprc

command on this man page.)

–d

Sets the initial value of the
debug

toggle to
TRUE

–e escapechar

Sets the initial
telnet

escape character to
escapechar

If
escapechar

is omitted, then
there will be no escape character.

–f

If Kerberos V5 authentication is being used, the
–f

option allows the local credentials to be forwarded to the remote system.

–k realm

If Kerberos authentication is being used, the
–k

option requests that
telnet

obtain tickets for the remote host in
realm
realm

instead of the remote host’s realm, as determined
by
krb_realmofhost3.

–l user

When connecting to the remote system, if the remote system
understands the
ENVIRON

option, then
user

will be sent to the remote system as the value for the variable USER.
This option implies the
–a

option.
This option may also be used with the
open

command.

–n tracefile

Opens
tracefile

for recording trace information.
See the
set tracefile

command below.

–r

Specifies a user interface similar to
rlogin(1).

In this
mode, the escape character is set to the tilde (~) character,
unless modified by the
–e

option.

–x

Turns on encryption of the data stream if possible.

host

Indicates the official name, an alias, or the Internet address
of a remote host.

port

Indicates a port number (address of an application).
If a number is not specified, the default
telnet

port is used.

When in rlogin mode, a line of the form ~.
disconnects from the
remote host; ~ is the telnet escape character.
Similarly, the line ~^Z suspends the telnet session.
The line ~^] escapes to the normal telnet escape prompt.

Once a connection has been opened,
telnet

will attempt to enable the
TELNET LINEMODE

option.
If this fails,
telnet

will revert to one of two input modes:
either “character 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 immediately 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

toggle 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 )

While connected to a remote host,
telnet

command mode may be entered by typing the
telnet

“escape character” (initially “^]”).
When in command mode, the normal terminal editing conventions are available.
Note that the escape character will return to the command mode of the initial
invocation of
telnet

that has the controlling terminal.
Use the
send escape

command to switch to command mode in subsequent
telnet

processes on remote hosts.

The following
telnet

commands are available.
Only enough of each command to uniquely identify it need be typed
(this is also true for arguments to the
mode

set

toggle

unset

slc

environ

and
display

commands).

auth argument [...

]

The
auth

command manipulates the information sent through the
TELNET AUTHENTICATE

option.
Valid arguments for the
auth

command are as follows:

disable type

Disables the specified
type

of authentication.
To obtain a list of available types, use the
auth disable ?

command.

enable type

Enables the specified
type

of authentication.
To obtain a list of available types, use the
auth enable ?

command.

status

Lists the current status of the various types of
authentication.

close

Close a
TELNET

session and return to command mode.

display argument [...

]

Displays all, or some, of the
set

and
toggle

values (see below).

encrypt argument [...

]

The
encrypt

command manipulates the information sent through the
TELNET ENCRYPT

option.

Valid arguments for the encrypt command are as follows:

disable type [input|output]

Disables the specified
type

of encryption.
If you omit
input

and
output

both input and output
are disabled.
To obtain a list of available types, use the
encrypt disable ?

command.

enable type [input|output]

Enables the specified
type

of encryption.
If you omit
input

and
output

both input and output are
enabled.
To obtain a list of available types, use the
encrypt enable ?

command.

input

This is the same as the
encrypt start input

command.

–input

This is the same as the
encrypt stop input

command.

output

This is the same as the
encrypt start output

command.

–output

This is the same as the
encrypt stop output

command.

start [input|output]

Attempts to start encryption.
If you omit
input

and
output

both input and output are enabled.
To obtain a list of available types, use the
encrypt enable ?

command.

status

Lists the current status of encryption.

stop [input|output]

Stops encryption.
If you omit
input

and
output

encryption is on both input and output.

type type

Sets the default type of encryption to be used
with later
encrypt start

or
encrypt stop

commands.

environ arguments [...

]

The
environ

command is used to manipulate the
variables that may be sent through the
TELNET ENVIRON

option.
The initial set of variables is taken from the users
environment, with only the
DISPLAY

and
PRINTER

variables being exported by default.
The
USER

variable is also exported if the
–a

or
–l

options are 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 exported.
The
value

may be enclosed in single or double quotes so
that tabs and spaces may be included.

undefine variable

Remove
variable

from the list of environment variables.

export variable

Mark the variable
variable

to be exported to the remote side.

unexport variable

Mark the variable
variable

to not be exported unless
explicitly asked for by the remote side.

list

List the current set of environment variables.
Those marked with a
*

will be sent automatically,
other variables will only be sent if explicitly requested.

?

Prints out help information for the
environ

command.

logout

Sends the
TELNET LOGOUT

option to the remote side.
This command is similar to a
close

command; however, if the remote side does not support the
LOGOUT

option, nothing happens.
If, however, the remote side does support the
LOGOUT

option, this command should cause the remote side to close the
TELNET

connection.
If the remote side also supports the concept of
suspending a user’s session for later reattachment,
the logout argument indicates that you
should terminate the session immediately.

mode type

type

is one of several options, depending on the state of the
TELNET

session.
The remote host is asked for permission to go into the requested mode.
If the remote host is capable of entering that mode, the requested
mode will be entered.

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 contact a
TELNET

server at the default port.
The host specification may be either a host name (see
hosts(5))

or an Internet address specified in the “dot notation” (see
inet(3)).

The
–l

option may be used to specify the user name
to be passed to the remote system via the
ENVIRON

option.
When connecting to a non–standard port,
telnet

omits any automatic initiation of
TELNET

options.
When the port number is preceded by a minus sign,
the initial option negotiation is done.
After establishing a connection, the file
.telnetrc

in the
user’s home directory is opened.
Lines beginning with a “#” are
comment lines.
Blank lines are ignored.
Lines that begin
without whitespace are the start of a machine entry.
The first thing on the line is the name of the machine that is
being connected to.
The rest of the line, and successive
lines that begin with whitespace are assumed to be
telnet

commands and are processed as if they had been typed
in manually to the
telnet

command prompt.

quit

Close any open
TELNET

session and exit
telnet

An end–of–file (in command mode) will also close a session and exit.

send arguments

Sends one or more special character sequences to the remote host.
The following are the arguments which may be specified
(more than one argument may be specified at a time):

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 (initially “^]”).

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 process.

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 discard 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
BSD 4.2
system –– if
it doesn’t work, a lower case “r” may be echoed on the terminal).

do 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.

dont cmd

Sends the
TELNET DONT

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.

will cmd

Sends the
TELNET WILL

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.

wont cmd

Sends the
TELNET WONT

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 associated with
the variable; this is equivalent to using the
unset

command.
The
unset

command will disable or set to
FALSE

any of the specified functions.
The values of variables may be interrogated 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 using
the
set

and
unset

commands.

ayt

If
TELNET

is in
localchars

mode, or
LINEMODE

is enabled, and the status character is typed, a
TELNET AYT

sequence (see
send ayt

preceding) is sent to the
remote host.
The initial value for the "Are You There"
character is the terminal’s status character.

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 character
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 operating 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 terminal’s
erase

character.

escape

This is the
telnet

escape character (initially “^['') 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

character 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

character.

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 forwarding characters are taken from the terminal's
eol and eol2 characters.

interrupt

If
telnet

is in
localchars

mode (see
toggle

localchars

below)
and the
interrupt

character is typed, a
TELNET IP

sequence (see
send

ip

above)
is sent to the remote host.
The initial value for the
interrupt

character is taken to be
the terminal's
intr

character.

kill

If
telnet

is in
localchars

mode (see
toggle

localchars

below),
and if
telnet

is operating 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 terminal'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

character.

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

character.
The initial value for the
reprint

character is taken to be
the terminal's
reprint

character.

rlogin

This is the rlogin escape character.
If set, the normal
TELNET

escape character is ignored unless it is
preceded by this character at the beginning of a line.
This character, at the beginning of a line, followed by
a "." closes the connection; when followed by a ^Z it
suspends the
telnet

command.
The initial state is to
disable the
rlogin

escape character.

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
start

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
stop

character is taken to be
the terminal's
stop

character.

susp

If
telnet

is in
localchars

mode, or
LINEMODE

is enabled, and the
suspend

character 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

character.
The initial value for the
worderase

character is taken to be
the terminal's
worderase

character.

?

Displays the legal
set

(unset

)

commands.

skey sequence challenge

The
skey

command computes a response to the S/Key challenge.
See
skey(1)

for more information on the S/Key system.

slc state

The
slc

command (Set Local Characters) is used to set
or change the state of the special
characters 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 characters are exported.

check

Verify the current settings for the current special characters.
The remote side is requested to send all the current 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 terminal 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 system
at the time when the
TELNET

connection was established.

?

Prints out help information for the
slc

command.

status

Show the current status of
telnet

This includes the peer one is connected to, 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 listed above.
More than one argument may be specified.
The state of these flags may be interrogated with the
display

command.
Valid arguments are:

authdebug

Turns on debugging information for the authentication code.

autoflush

If
autoflush

and
localchars

are both
TRUE

then when the
ao

or
quit

characters are recognized (and transformed into
TELNET

sequences; see
set

above for details),
telnet

refuses to display any data on the user’s terminal
until the remote system acknowledges (via a
TELNET TIMING MARK

option)
that it has processed those
TELNET

sequences.
The initial value for this toggle is
TRUE

if the terminal user had not
done an "stty noflsh", otherwise
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.

autologin

If the remote side supports the
TELNET AUTHENTICATION

option
TELNET

attempts to use it to perform automatic authentication.
If the
AUTHENTICATION

option is not supported, the user’s login
name are propagated through the
TELNET ENVIRON

option.
This command is the same as specifying
a

option on the
open

command.

autosynch

If
autosynch

and
localchars

are both
TRUE

then when either the
intr

or
quit

character 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 procedure
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 toggle 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
<CR><LF>

If this is
FALSE

then carriage returns will be send as
<CR><NUL>

The initial 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 feeds.
The initial value for this toggle is
FALSE

debug

Toggles socket level debugging (useful only to the superuser).
The initial value for this toggle is
FALSE

encdebug

Turns on debugging information for the encryption code.

localchars

If this is
TRUE

then the
flush

interrupt

quit

erase

and
kill

characters (see
set

above) are recognized locally, and transformed into (hopefully) appropriate
TELNET

control 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
suspend

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 formatted in a more user readable format.
Spaces are put between each character in the output, and the
beginning of any
TELNET

escape sequence is preceded by a ‘*’ to aid in locating them.

skiprc

When the skiprc toggle is
TRUE

TELNET

skips the reading of the
.telnetrc

file in the user’s home
directory when connections are opened.
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

?

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

~/.telnetrc

user customized telnet startup values

 

HISTORY

The
telnet

command appeared in
BSD 4.2

 

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.

Source routing is not supported yet for IPv6.

---

 

Index

NAME

SYNOPSIS

DESCRIPTION

ENVIRONMENT

FILES

HISTORY

NOTES

---

Related posts:

1. man fence_bladecenter – Man page for fence_bladecenter
2. man talk – Man page for talk
3. man stdio – Man page for stdio
4. man ed – Man page for ed
5. man slattach – Man page for slattach
6. man mvget_wch – Man page for mvget_wch
7. man gimp-remote-2.2 – Man page for gimp-remote-2.2
8. man termname – Man page for termname
9. man rsautl – Man page for rsautl
10. man has_key – Man page for has_key