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

     telnetd - DARPA TELNET protocol server

     /usr/libexec/telnetd [-Uhlkns46] [-D debugmode] [-S tos] [-X authtype]
                          [-a authmode] [-edebug] [-g gettyent] [-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:

     -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

                  debug  Turns 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 providing 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 verification 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 currently
                         not supported by any of the existing authentication
                         mechanisms, and is thus the same as specifying -a

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

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

     -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 several possible values for debugmode:

                  options   Prints information about the negotiation of TELNET

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

                  netdata   Displays the data stream received by telnetd.

                  ptydata   Displays data written to the pty.

                  exercise  Has not been implemented yet.

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

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

     -g gettyent  Specifies which entry from /etc/gettytab should be used to
                  get banner strings, login program and other information.
                  The default entry is default.

     -h           Disables 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 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 heuristic (if they respond
                  with WILL TIMING-MARK in response to a DO TIMING-MARK) for
                  kludge linemode support.

     -l           Specifies line mode.  Tries 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 connections 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           This option is only enabled if telnetd is compiled with
                  support for secure logins.  It causes the -s option to be
                  passed on to login(1), and thus is only useful if login(1)
                  supports the -s flag to indicate that only Kerberos or S/Key
                  validated logins are allowed, and is usually useful for
                  controlling remote logins from outside of a firewall.

     -S tos       This option sets the IP Type-of Service (TOS) option on the
                  connection to the value tos, which may be a numeric TOS
                  value or a symbolic TOS name found in the /etc/iptos file.
                  This option has no effect on NetBSD.

     -u len       This option is used to specify the size of the field in the
                  utmp 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 utmp file.

     -U           This option causes telnetd to refuse connections from
                  addresses that cannot be mapped back into a symbolic name
                  via the getnameinfo(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 specific authentication type without having to
                  recompile telnetd.


     -6           Specifies address family to be used on -debug mode.  During
                  normal operation (called from inetd(8)) telnetd will use the
                  file descriptor passed from inetd(8).

     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 characters 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 willingness 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 terminal 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 needed.

     WILL BINARY        Indicates 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           Indicates that it will not be sending IAC GA, go
                        ahead, commands.

     WILL STATUS        Indicates 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 TIMING-MARK

     WILL LOGOUT        When a DO LOGOUT is received, a WILL LOGOUT is sent in
                        response, and the TELNET session 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   Indicates 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             Indicates 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  Indicates a desire to be able to request information
                        about the speed of the serial line to which the client
                        is attached.

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

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

     DO ENVIRON         Indicates 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

     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.

     At the end of a login session, telnetd invokes the ttyaction(3) facility
     with an action of "telnetd" and user "root" to execute site-specific

     /etc/iptos (if supported)

     login(1), skey(1), telnet(1), ttyaction(3)

     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 1571    Telnet Environment Option Interoperability Issues
     RFC 1572    Telnet Environment Option

     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.

NetBSD 10.99                   December 16, 2018                  NetBSD 10.99