Provided by: telnetd-ssl_0.17.40+0.2-1_amd64 bug


     telnetd — DARPA telnet protocol server


     /usr/sbin/in.telnetd [-hnNs] [-a authmode] [-D debugmode] [-L loginprg] [-S tos]
                          [-X authtype] [-edebug] [-debug port] [-z sslopt]


     The telnetd program is a server which supports the DARPA telnet interactive communication
     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 program 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 authentication, which is not available in the current
                  version.  The following values of authmode are understood:

                  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 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.

     -D debugmode
                  This option may be used for debugging purposes.  It allows telnetd to print out
                  debugging information to the connection, allowing the user to see what telnetd
                  is doing.  Repeated use of the option arranges composite debug reports.  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.

     -edebug      If telnetd has been compiled with support for 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

     -L loginprg  This option may be used to specify a different login program.  By default,
                  /usr/lib/telnetlogin is used.

     -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.

     -N           Disable reverse DNS lookups and use the numeric IP address in logs and
                  REMOTEHOST environment variable.

     -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. This is usually useful for controlling remote logins from
                  outside of a firewall.

     -S tos       Sets the IP type-of-service (TOS) option for the telnet connection to the value

     -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.

     -z SSL-parameter
                  This option is only valid if telnetd has been built with SSL (Secure Socket
                  Layer) support.

                  debug       Enable SSL related debugging.

                  ssl         Negotiate SSL at first, then use telnet protocol. In this mode
                              telnetd only accepts connections from SSL enhanced telnet with
                              option -z ssl

                  nossl, !ssl
                              switch off SSL negotiation

                  certsok     Look username up in /etc/ssl.users. The format of this file is
                              lines of this form: user1,user2:/C=US/..... where user1 and user2
                              are usernames and /C=US/... is the subject name of the certificate.
                              Use openssl x509 -subject -noout to extract the subject name.  If
                              client certificate is valid, authenticate without password.

                              client certificate is mandatory

                  secure      Don't switch back to unencrypted mode (no SSL) if SSL is not

                  verify=int  Set the SSL verify flags (SSL_VERIFY_* in ssl/ssl.h ).

                              Use the certificate(s) in cert_file.

                              Use the key(s) in key_file.

                              Set the preferred ciphers to ciph_list.  (See ssl/ssl.h ).

     If the file /etc/ is present, telnetd will display its contents before the login
     prompt of a telnet session (see

     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 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 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

     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 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.


     /etc/services, /etc/


     telnet(1), login(1),,


     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


     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.

     The source code is not comprehensible.