telnet

telnet(1) User's Reference Manual telnet(1)

NAME

   telnet  user interface to the TELNET protocol

SYNOPSIS

   telnet [options ...] [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.

OPTIONS

   -4, --ipv4
           Use IPv4 to connect to hosts.

   -6, --ipv6
           Use IPv6 to connect to hosts.

   -8, --binary
           Specifies an 8-bit data path.  This causes an attempt to negotiate the TELNET BINARY option on both input and output.

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

   -e, --escape escape-char
           Sets the initial telnet escape character to escape-char. If escape-char is omitted, then there will be no escape character.

   -E, --no-escape
           Stops any character from being recognized as an escape character.

   -a, --login
           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 cur
           rent user as returned by getlogin(2) if it agrees with the current user ID, otherwise it is the name associated with the user ID.

   -K, --no-login
           Specifies no automatic login to the remote system.

   -l, --user 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, --trace tracefile
           Opens tracefile for recording trace information.  See the set tracefile command below.

   -r, --rlogin
           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.

   -c, --no-rc
           Disables the reading of the user's .telnetrc file.  (See the toggle skiprc command on this man page.)

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

   -x, --encrypt
           Turns on encryption of the data stream if possible.

   -X, --disable-auth atype
           Disables the atype type of authentication.

   -k, --realm 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  deter
           mined by krb_realmofhost(3).

   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.

OPERATION

   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 ~^] es
   capes to the normal telnet escape prompt.

   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 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 re
   mote 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 lo
   cal 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 lo‐
   cally, 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 (un
   til 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  con
   ventions are available.

   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.

              Note:  Because of export controls, the TELNET ENCRYPT option is not supported outside of the United States and Canada.

              Valid arguments for the encrypt command are as follows:

              disable type [input|output]
                            Disables the specified type of encryption.  If you omit the 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 the variables that my be sent through the TELNET ENVIRON option.  The initial set of variables is taken from the  users  envi
              ronment, 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 hap
              pens.   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 sup
              ports 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 ei‐
              ther 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 pre‐
              ceded by a minus sign, the initial option negotiation is done.  After establishing a connection, the file .telnetrc in the users home directory is opened.  Lines  beginning
              with  a  #  are comment lines.  Blank lines are ignored.  Lines that begin without white space 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 white space 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 ur
                      gent data (and may not work if the remote system is a 4.2BSD 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 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  ex‐
              plicitly 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  sup
                      pressing 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 kill char
                      acter 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 charac
                      ter 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 writ
                      ten 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.

   slc state  The  slc  command (Set Local Characters) is used to set or change the state of the the special characters when the TELNET LINEMODE option has been enabled.  Special charac
              ters 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 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 com
              mands 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  de
                            tails), 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.

                            Note:  Because of export controls, the TELNET ENCRYPT option is not supported outside the United States and Canada.

              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 characters is typed (see set above for descriptions of the intr and quit charac
                            ters), 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 use
                            ful 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 (useful only to the super user).  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)  appro
                            priate 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 users 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.
                            Note:  Because of export controls, data encryption is not supported outside of the United States and Canada.

              ?             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 4.2BSD.

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.

GNU Network Utilities February 9, 2019 telnet(1)