telnet(1) telnet(1)
NAME [Toc] [Back]
telnet - user interface to the TELNET protocol
SYNOPSIS [Toc] [Back]
telnet [[options] host [port]]
DESCRIPTION [Toc] [Back]
telnet is used to communicate with another host using the TELNET
protocol. If telnet is invoked without arguments, it enters command
mode, indicated by its prompt (telnet>). In this mode, it accepts and
executes the commands listed below. If telnet is invoked with
arguments, it performs an open command (see below) with those
arguments.
Once a connection has been opened, telnet enters an input mode. The
input mode will be either "character at a time" or "line by line" ,
depending on what the remote system supports.
In "character at a time" mode, most text typed is immediately sent to
the remote host for processing.
In "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) can be used to turn-off and turn-on the
local echo (this would mostly be used to enter passwords without the
password being echoed).
In either mode, if the localchars toggle is TRUE (the default in line
mode; see below), the user's quit and intr characters are trapped
locally, and sent as TELNET protocol sequences to the remote side.
There are options (see toggle autoflush and toggle autosynch below)
that 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 can be entered
by typing the telnet "escape character" (initially ^]). When in
command mode, the normal terminal editing conventions are available.
telnet supports 8-bit characters when communicating with the server on
the remote host. To use eight-bit characters you may need to
reconfigure your terminal or the remote host appropriately (see
stty(1)). Furthermore, you may have to use the binary toggle to
enable an 8-bit data stream between telnet and the remote host.
Note: Some remote hosts may not provide the necessary support for
eight-bit characters.
If, at any time, telnet is unable to read from or write to the server
over the connection, the message Connection closed by foreign host. is
printed on standard error. telnet then exits with a value of 1.
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telnet supports the TAC User ID (also known as the TAC Access Control
System, or TACACS User ID) option. Enabling the option on a host
server allows the user to telnet into that host without being prompted
for a second login sequence. The TAC User ID option uses the same
security mechanism as rlogin for authorizing access by remote hosts
and users. The system administrator must enable the (telnetd) option
only on systems, which are designated as participating hosts. The
system administrator must also assign to each user of TAC User ID the
very same UID on every system for which he is allowed to use the
feature. See the telnetd(1M) manpage and the Managing Systems and
Workgroups manual.
Options [Toc] [Back]
The following telnet options are available:
-8 Enable cs8 (8 bit transfer) on local tty.
-e c Set the telnet command mode escape character to be ^c
instead of its default value of ^].
-l Disable the TAC User ID option if enabled on the client, to
cause the user to be prompted for login username and
password. Omitting the -l option executes the default
setting.
Kerberos-Specific Options [Toc] [Back]
By default, or by use of the -a or the -l option, the Kerberos version
of telnet behaves as a client which supports authentication based on
Kerberos V5. As a Kerberos client, telnet will authenticate and
authorize the user to access the remote system. See the sis(5)
manpage for details on Kerberos authentication and authorization.
However, it will not support integrity-checked or encrypted sessions.
The default Kerberos options for the applications are set in the
krb5.conf configuration file. Refer to the appdefaults Section in the
krb5.conf(4) manpage for more information. The options -a, -f, and -F
described in the subsequent paragraphs, can be set in the krb5.conf
file with the tag names autologin, forward, and forwardable
respectively. Refer to the appdefaults Section of the krb5.conf(4)
manpage for more information.
The fallback option can be set in the krb5.conf file within
appdefaults Section. If fallback is set to true and the kerberos
authentication fails, telnet will use the non-secure mode of
authentication.
Note: Command line options override configuration file options.
The following options are Kerboros-specific:
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-a This option is applicable only in a secure environment based
on Kerberos V5. Attempt automatic login into the Kerberos
realm and disable the TAC User ID option.
Note: This is the default login mode.
Sends the user name via the NAME subnegotiation of the
Authentication option. The name used is that of the current
user as returned by the USER environment variable. If this
variable is not defined, the name used is that returned by
getpwnam() (see getpwent(3C)) if it agrees with the current
user ID. Otherwise, it is the name associated with the user
ID.
-P This option is applicable only in a secure environment based
on Kerberos V5. Disable use of Kerberos authentication and
authorization. When this option is specified, a password is
required that is sent across the network in a readable form.
See the sis(5) manpage.
-f This option is applicable only in a secure environment based
on Kerberos V5. Allows local credentials to be forwarded to
the remote system. Only one -f or -F option is allowed.
They cannot be used together.
-F This option is applicable only in a secure environment based
on Kerberos V5. Allows local credentials to be forwarded to
the remote system including any credentials that have
already been forwarded into the local environment. Only one
-f or -F option is allowed. They cannot be used together.
-l user This option is applicable only in a secure environment based
on Kerberos V5. Attempt automatic login into the Kerberos
realm as the specified user and disable the TAC User ID
option. The user name specified is sent via the NAME
subnegotiation of the Authentication option. Omitting the
-l option executes the default setting. Only one -l option
is allowed.
Commands [Toc] [Back]
The following commands are available in command mode. You need to
type only sufficient prefix of each command to uniquely identify it
(this is also true for arguments to the mode, set, toggle, and display
commands).
open host [port]
Open a connection to the named host at the indicated
port. If no port is specified, telnet attempts to
contact a TELNET server at the standard TELNET port.
In the IPv4 environment, hostname can be either the
official name or an alias as understood by
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gethostbyname() (see gethostent(3N)) or an Internet
address specified in the dot notation as described in
hosts(4). In the IPv6 environment, hostname can be
either the official name or an alias as understood by
getnameinfo() (see getnameinfo(3N)) or an Internet
address specified in the colon notation as described in
hosts(4). If no hostname is given, telnet prompts for
one.
close Close a TELNET session. If the session was started
from command mode, telnet returns to command mode;
otherwise telnet exits.
quit Close any open TELNET session and exit telnet. An end
of file (in command mode) will also close a session and
exit.
z Suspend telnet. If telnet is run from a shell that
supports job control, (such as cs
), the z
command suspends the TELNET session and returns the
user to the shell that invoked telnet. Then the job
can be resumed by using the fg command (see csh(1) or
ksh(1)).
mode mode Change telnet's user input mode to mode, which can be
character (for "character at a time" mode) or line (for
"line by line" mode). 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 is entered. In character mode, telnet
sends each character to the remote host as it is typed.
In line mode, telnet gathers user input into lines and
transmits each line to the remote host when the user
types carriage return, linefeed, or EOF (normally ^D;
see stty(1)).
Note: Setting line-mode also sets local echo.
Applications that expect to interpret user input
character by character (such as more, csh, ksh, and vi)
do not work properly in line mode.
status Show current status of telnet. telnet reports the
current escape character. If telnet is connected, it
reports the host to which it is connected and the
current mode. If telnet is not connected to a remote
host, it reports No connection. Once telnet has been
connected, it reports the local flow control toggle
value.
display [argument ...]
Displays all or some of the set and toggle values (see
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below).
? [command] Get help. With no arguments, telnet prints a help
summary. If a command is specified, telnet prints the
help information available about that command only.
Help information is limited to a one-line description
of the command.
! [shell_command]
Shell escape. The SHELL environment variable is
checked for the name of a shell to use to execute the
command. If no shell_command is specified, a shell is
started and connected to the user's terminal. If SHELL
is undefined, /usr/bin/sh is used.
send arguments Sends one or more special character sequences to the
remote host. Each argument can have any of the
following values (multiple arguments can be specified
with each send command):
escape Sends the current telnet escape character
(initially ^]).
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 to some systems -- if
it doesn't work, a lower case ``r'' may be
echoed on the terminal).
brk Sends the TELNET BRK (Break) sequence, which
may have significance to the remote system.
ip Sends the TELNET IP (Interrupt Process)
sequence, which should cause the remote
system to abort the currently running
process.
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.
ec Sends the TELNET EC (Erase Character)
sequence, which should cause the remote
system to erase the last character entered.
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el Sends the TELNET EL (Erase Line) sequence,
which should cause the remote system to erase
the line currently being entered.
ga Sends the TELNET GA (Go Ahead) sequence,
which is likely to have no significance to
the remote system.
nop Sends the TELNET NOP (No OPeration) sequence.
? Prints out help information for the send
command.
set variable_name value
Set any of the telnet variables to a specific value.
The special value off turns off the function associated
with the variable. The values of variables can be
shown by using the display command. The following
variable_names can be specified:
echo This is the value (initially ^E) which,
toggles between doing local echoing of
entered characters (for normal processing),
and suppressing echoing of entered characters
(for entering, for example, a password) in
line-by-line mode.
escape This is the telnet escape character
(initially ^]) that causes entry into telnet
command mode (when connected to a remote
system).
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.
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.
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.
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The initial value for the flush character is
^O.
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.
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.
eof If telnet is operating in line-by-line mode,
entering this character as the first
character on a line causes 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.
toggle arguments ...
Toggle (between TRUE and FALSE ) various flags that
control how telnet responds to events. More than one
argument can be specified. The state of these flags
can be shown by using the display command. Valid
arguments are:
localchars
If TRUE, the flush, interrupt, quit, erase,
and kill characters (see set above) are
recognized locally, and transformed into
appropriate TELNET control sequences
(respectively ao, ip, brk, ec, and el; see
send above). The initial value for this
toggle is TRUE in line-by-line mode, and
FALSE in character-at-a-time mode.
autoflush If autoflush and localchars are both TRUE,
whenever the ao, intr, 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
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sequences. The initial value for this toggle
is TRUE.
autologin This flag is available only in a secure
environment based on Kerberos V5. Enable or
disable automatic login into the Kerberos
realm. Using this option yields the same
results as using the -a option. The initial
value for this toggle is TRUE.
autosynch If autosynch and localchars are both TRUE,
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 causes
the remote system to begin discarding 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. This option should be
enabled in order to send and receive 8-bit
characters to and from the TELNET server.
crlf If TRUE, end-of-line sequences are sent as an
ASCII carriage-return and line-feed pair. If
FALSE, end-of-line sequences are sent as an
ASCII carriage-return and NUL character pair.
The initial value for this toggle is FALSE.
crmod Toggle carriage return mode. When this mode
is enabled, any carriage return characters
received from the remote host are mapped into
a carriage return and a line feed. This mode
does not affect those characters typed by the
user; only those received. This mode is only
required for some hosts that require the
client to do local echoing, but output
``naked'' carriage returns. The initial
value for this toggle is FALSE.
echo Toggle local echo mode or remote echo mode.
In local echo mode, user input is echoed to
the terminal by the local telnet before being
transmitted to the remote host. In remote
echo, any echoing of user input is done by
the remote host. Applications that handle
echoing of user input themselves, such as C
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shell, Korn shell, and vi (see csh(1),
ksh(1), and vi(1)), do not work correctly
with local echo.
options Toggle viewing of TELNET options processing.
When options viewing is enabled, all TELNET
option negotiations are displayed. Options
sent by telnet are displayed as ``SENT'',
while options received from the TELNET server
are displayed as ``RCVD''. The initial value
for this toggle is FALSE.
netdata Toggles the display of all network data (in
hexadecimal format). The initial value for
this toggle is FALSE.
? Displays the legal toggle commands.
RETURN VALUE [Toc] [Back]
In the event of an error, or if the TELNET connection is closed by the
remote host, telnet returns a value of 1. Otherwise, it returns 0.
DIAGNOSTICS [Toc] [Back]
The following diagnostic messages are displayed by telnet:
Error! Could not retrieve authentication type.
There are two authentication mechanisms used by TELNET. One
authentication mechanism is based on Kerberos and the other
is not. The type of authentication mechanism is obtained
from a system file, which is updated by inetsvcs_sec. If
the system file on either the local host or the remote host
does not contain known authentication types, the above error
is displayed.
telnet/tcp: Unknown service
telnet was unable to find the TELNET service entry in the
services(4) database.
hostname: Unknown host
telnet was unable to map the host name to an Internet
address. Your next step should be to contact the system
administrator to check whether there is an entry for the
remote host in the hosts database (see hosts(4)).
?Invalid command
An invalid command was typed in telnet command mode.
system call>: ...
An error occurred in the specified system call. See the
appropriate manual entry for a description of the error.
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AUTHOR [Toc] [Back]
telnet was developed by the University of California, Berkeley.
SEE ALSO [Toc] [Back]
csh(1), ksh(1), login(1), rlogin(1), stty(1), telnetd(1M),
inetsvcs_sec(1M), hosts(4), krb5.conf(4), services(4), sis(5),
termio(7).
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