Ubuntu Manpage: telnetd — DARPA TELNET protocol server

Provided by: inetutils-telnetd_1.9.2-1_amd64

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)

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.