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TELNETD(8)                BSD System Manager's Manual               TELNETD(8)

     telnetd -- DARPA TELNET protocol server

     /usr/libexec/telnetd [-46BUhlkns] [-D debugmode] [-S tos] [-X authtype] [-a authmode] [-edebug]
                          [-p loginprog] [-u len] [-debug [port]]

     The telnetd command is a server which supports the DARPA standard TELNET virtual terminal protocol.
     Telnetd is normally invoked by the internet server (see inetd(8)) for requests to connect to the TELNET
     port as indicated by the /etc/services file (see services(5)).  The -debug option may be used to start
     up telnetd manually, instead of through inetd(8).  If started up this way, port may be specified to run
     telnetd on an alternate TCP port number.

     The telnetd command accepts the following options:

     -4      Forces telnetd to use IPv4 addresses only.

     -6      Forces telnetd to use IPv6 addresses only.

     -a authmode
             This option may be used for specifying what mode should be used for authentication.  Note that
             this option is only useful if telnetd has been compiled with support for the AUTHENTICATION
             option.  There are several valid values for authmode:

             debug  Turn on authentication debugging code.

             user   Only allow connections when the remote user can provide valid authentication information
                    to identify the remote user, and is allowed access to the specified account without pro-viding providing
                    viding a password.

             valid  Only allow connections when the remote user can provide valid authentication information
                    to identify the remote user.  The login(1) command will provide any additional user ver-ification verification
                    ification needed if the remote user is not allowed automatic access to the specified

             other  Only allow connections that supply some authentication information.  This option is cur-rently currently
                    rently not supported by any of the existing authentication mechanisms, and is thus the
                    same as specifying -a valid.

             none   This is the default state.  Authentication information is not required.  If no or insuf-ficient insufficient
                    ficient authentication information is provided, then the login(1) program will provide
                    the necessary user verification.

             off    Disable the authentication code.  All user verification will happen through the login(1)

     -B      Specify bftp server mode.  In this mode, telnetd causes login to start a bftp(1) session rather
             than the user's normal shell.  In bftp daemon mode normal logins are not supported, and it must
             be used on a port other than the normal TELNET port.

     -D debugmode
             This option may be used for debugging purposes.  This allows telnetd to print out debugging
             information to the connection, allowing the user to see what telnetd is doing.  There are sev-eral several
             eral possible values for debugmode:

             options   Print information about the negotiation of TELNET options.

             report    Print the options information, plus some additional information about what processing
                       is going on.

             netdata   Display the data stream received by telnetd.

             ptydata   Display data written to the pty.

             exercise  Has not been implemented yet.

     -debug  Enable debugging on each socket created by telnetd (see SO_DEBUG in socket(2)).

             If telnetd has been compiled with support for data encryption, then the -edebug option may be
             used to enable encryption debugging code.

     -p loginprog
             Specify an alternate login(1) command to run to complete the login.  The alternate command must
             understand the same command arguments as the standard login.

     -h      Disable the printing of host-specific information before login has been completed.

     -k      This option is only useful if telnetd has been compiled with both linemode and kludge linemode
             support.  If the -k option is specified, then if the remote client does not support the
             LINEMODE option, then telnetd will operate in character at a time mode.  It will still support
             kludge linemode, but will only go into kludge linemode if the remote client requests it.  (This
             is done by the client sending DONT SUPPRESS-GO-AHEAD and DONT ECHO.)  The -k option is most
             useful when there are remote clients that do not support kludge linemode, but pass the heuris-tic heuristic
             tic (if they respond with WILL TIMING-MARK in response to a DO TIMING-MARK) for kludge linemode

     -l      Specify line mode.  Try to force clients to use line-at-a-time mode.  If the LINEMODE option is
             not supported, it will go into kludge linemode.

     -n      Disable TCP keep-alives.  Normally telnetd enables the TCP keep-alive mechanism to probe con-nections connections
             nections that have been idle for some period of time to determine if the client is still there,
             so that idle connections from machines that have crashed or can no longer be reached may be
             cleaned up.

     -S tos  Sets the IP type-of-service (TOS) option for the telnet connection to the value tos, which can
             be a numeric TOS value or, on systems that support it, a symbolic TOS name found in the
             /etc/iptos file.

     -u len  This option is used to specify the size of the field in the utmpx structure that holds the
             remote host name.  If the resolved host name is longer than len, the dotted decimal value will
             be used instead.  This allows hosts with very long host names that overflow this field to still
             be uniquely identified.  Specifying -u0 indicates that only dotted decimal addresses should be
             put into the utmpx file.

     -U      This option causes telnetd to refuse connections from addresses that cannot be mapped back into
             a symbolic name via the gethostbyaddr(3) routine.

     -X authtype
             This option is only valid if telnetd has been built with support for the authentication option.
             It disables the use of authtype authentication, and can be used to temporarily disable a spe-cific specific
             cific authentication type without having to recompile telnetd.

     Telnetd operates by allocating a pseudo-terminal device (see pty(4)) for a client, then creating a
     login process which has the slave side of the pseudo-terminal as stdin, stdout and stderr.  Telnetd
     manipulates the master side of the pseudo-terminal, implementing the TELNET protocol and passing char-acters characters
     acters between the remote client and the login process.

     When a TELNET session is started up, telnetd sends TELNET options to the client side indicating a will-ingness willingness
     ingness to do the following TELNET options, which are described in more detail below:

           WILL ENCRYPT
           DO TSPEED
           DO XDISPLOC
           DO NEW-ENVIRON
           DO ENVIRON
           DO ECHO
           DO LINEMODE
           DO NAWS
           WILL STATUS
           DO LFLOW
           DO TIMING-MARK

     The pseudo-terminal allocated to the client is configured to operate in ``cooked'' mode, and with XTABS
     and CRMOD enabled (see tty(4)).

     Telnetd has support for enabling locally the following TELNET options:

     WILL ECHO          When the LINEMODE option is enabled, a WILL ECHO or WONT ECHO will be sent to the
                        client to indicate the current state of terminal echoing.  When terminal echo is not
                        desired, a WILL ECHO is sent to indicate that telnetd will take care of echoing any
                        data that needs to be echoed to the terminal, and then nothing is echoed.  When ter-minal terminal
                        minal echo is desired, a WONT ECHO is sent to indicate that telnetd will not be
                        doing any terminal echoing, so the client should do any terminal echoing that is

     WILL BINARY        Indicate that the client is willing to send a 8 bits of data, rather than the normal
                        7 bits of the Network Virtual Terminal.

     WILL SGA           Indicate that it will not be sending IAC GA, go ahead, commands.

     WILL STATUS        Indicate a willingness to send the client, upon request, of the current status of
                        all TELNET options.

     WILL TIMING-MARK   Whenever a DO TIMING-MARK command is received, it is always responded to with a WILL

     WILL LOGOUT        When a DO LOGOUT is received, a WILL LOGOUT is sent in response, and the TELNET ses-sion session
                        sion is shut down.

     WILL ENCRYPT       Only sent if telnetd is compiled with support for data encryption, and indicates a
                        willingness to decrypt the data stream.

     Telnetd has support for enabling remotely the following TELNET options:

     DO BINARY          Sent to indicate that telnetd is willing to receive an 8 bit data stream.

     DO LFLOW           Requests that the client handle flow control characters remotely.

     DO ECHO            This is not really supported, but is sent to identify a 4.2BSD telnet(1) client,
                        which will improperly respond with WILL ECHO.  If a WILL ECHO is received, a DONT
                        ECHO will be sent in response.

     DO TERMINAL-TYPE   Indicate a desire to be able to request the name of the type of terminal that is
                        attached to the client side of the connection.

     DO SGA             Indicate that it does not need to receive IAC GA, the go ahead command.

     DO NAWS            Requests that the client inform the server when the window (display) size changes.

     DO TERMINAL-SPEED  Indicate a desire to be able to request information about the speed of the serial
                        line to which the client is attached.

     DO XDISPLOC        Indicate a desire to be able to request the name of the X Window System display that
                        is associated with the telnet client.

     DO NEW-ENVIRON     Indicate a desire to be able to request environment variable information, as
                        described in RFC 1572.

     DO ENVIRON         Indicate a desire to be able to request environment variable information, as
                        described in RFC 1408.

     DO LINEMODE        Only sent if telnetd is compiled with support for linemode, and requests that the
                        client do line by line processing.

     DO TIMING-MARK     Only sent if telnetd is compiled with support for both linemode and kludge linemode,
                        and the client responded with WONT LINEMODE.  If the client responds with WILL TM,
                        the it is assumed that the client supports kludge linemode.  Note that the [-k]
                        option can be used to disable this.

     DO AUTHENTICATION  Only sent if telnetd is compiled with support for authentication, and indicates a
                        willingness to receive authentication information for automatic login.

     DO ENCRYPT         Only sent if telnetd is compiled with support for data encryption, and indicates a
                        willingness to decrypt the data stream.

     By default telnetd will read the he, hn, and im capabilities from /etc/gettytab and use that informa-tion information
     tion (if present) to determine what to display before the login: prompt.  You can also use a System V
     style /etc/issue file by using the if capability, which will override im.  The information specified in
     either im or if will be displayed to both console and remote logins.

     /etc/iptos     (if supported)
     /usr/ucb/bftp  (if supported)

     bftp(1), login(1), telnet(1) (if supported), gettytab(5)

     RFC-1073  Telnet Window Size Option
     RFC-1079  Telnet Terminal Speed Option
     RFC-1091  Telnet Terminal-Type Option
     RFC-1096  Telnet X Display Location Option
     RFC-1123  Requirements for Internet Hosts -- Application and Support
     RFC-1184  Telnet Linemode Option
     RFC-1372  Telnet Remote Flow Control Option
     RFC-1416  Telnet Authentication Option
     RFC-1411  Telnet Authentication: Kerberos Version 4
     RFC-1412  Telnet Authentication: SPX
     RFC-1571  Telnet Environment Option Interoperability Issues
     RFC-1572  Telnet Environment Option

     IPv6 support was added by WIDE/KAME project.

     Some TELNET commands are only partially implemented.

     Because of bugs in the original 4.2BSD telnet(1), telnetd performs some dubious protocol exchanges to
     try to discover if the remote client is, in fact, a 4.2BSD telnet(1).

     Binary mode has no common interpretation except between similar operating systems (Unix in this case).

     The terminal type name received from the remote client is converted to lower case.

     Telnetd never sends TELNET IAC GA (go ahead) commands.

BSD                             January 9, 2005                            BSD

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