bionic (8) in.telnetd.8.gz

Provided by: inetutils-telnetd_1.9.4-3ubuntu0.1_amd64 bug

NAME

     telnetd — DARPA TELNET protocol server

SYNOPSIS

     /usr/libexec/telnetd [-BUhlkns] [-D debugmode] [-Iinitid] [-S tos] [-X authtype] [-a authmode] [-edebug]
                          [-rlowpty-highpty] [-u len] [-debug [port]]

DESCRIPTION

     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 authmode:

                  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 account.

                  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 valid.

                  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 verification.

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

     -B           Specifies 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
                  several possible values for debugmode:

                  options   Prints information about the negotiation of TELNET options.

                  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.

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

     -I initid    This option is only applicable to UNICOS systems prior to 7.0.  It specifies the ID from
                  /etc/inittab to use when init starts login sessions.  The default ID is fe.

     -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.

     -r lowpty-highpty
                  This option is only enabled when telnetd is compiled for UNICOS. It specifies an inclusive
                  range of pseudo-terminal devices to use.  If the system has sysconf variable _SC_CRAY_NPTY
                  configured, the default pty search range is 0 to _SC_CRAY_NPTY; otherwise, the default range
                  is 0 to 128.  Either lowpty or highpty may be omitted to allow changing either end of the
                  search range.  If lowpty is omitted, the - character is still required so that telnetd can
                  differentiate highpty from lowpty.

     -s           This option is only enabled if telnetd is compiled with support for SecurID cards.  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 SecurID validated logins are allowed, and is usually useful for
                  controlling remote logins from outside of a firewall.

     -S tos

     -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 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 specific 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 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:

           DO AUTHENTICATION
           WILL ENCRYPT
           DO TERMINAL TYPE
           DO TSPEED
           DO XDISPLOC
           DO NEW-ENVIRON
           DO ENVIRON
           WILL SUPPRESS GO AHEAD
           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 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.

ENVIRONMENT

FILES

     /etc/services
     /etc/inittab (UNICOS systems only)
     /etc/iptos (if supported)
     /usr/ucb/bftp (if supported)

SEE ALSO

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

STANDARDS

     RFC-854   TELNET PROTOCOL SPECIFICATION
     RFC-855   TELNET OPTION SPECIFICATIONS
     RFC-856   TELNET BINARY TRANSMISSION
     RFC-857   TELNET ECHO OPTION
     RFC-858   TELNET SUPPRESS GO AHEAD OPTION
     RFC-859   TELNET STATUS OPTION
     RFC-860   TELNET TIMING MARK OPTION
     RFC-861   TELNET EXTENDED OPTIONS - LIST OPTION
     RFC-885   TELNET END OF RECORD OPTION
     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

BUGS

     Some TELNET commands are only partially implemented.

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