Ubuntu Manpage: telnetd — DARPA TELNET protocol server

Provided by: inetutils-telnetd_2.5-6ubuntu1_amd64

NAME

       telnetd — DARPA TELNET protocol server

SYNOPSIS

       telnetd [options ...]

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)).

OPTIONS

The telnetd command accepts the following options:

-a, --authmode 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:

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.

-S, --server-principal name
Set the Kerberos principal name for this server instance, with or without an explicit realm.

-X, --disable-auth-type 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.

-D, --debug [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.

auth Displays authentication information.

encr Displays encryption information.

-E, --exec-login string
Set the program to be executed instead of /bin/login.

-h, --no-hostinfo
Disables the printing of host-specific information before login has been completed.

-l, --linemode [mode]
Specifies line mode. Tries to force clients to use line- at-a-time mode. If mode is
nokludge, then automatic klugde linemode will be disabled. If the LINEMODE option is not
supported, it will go into kludge linemode.

-n, --no-keepalive
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.

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

-?, --help Display a help list.

--usage Display a short usage message.

-V, --version
Display program version.

OPERATION

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.

FILES

       /etc/services

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.

GNU Network Utilities                           February 9, 2019                                      telnetd(8)