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TELNETD(8) System Manager's Manual TELNETD(8)
telnetd - DARPA TELNET protocol server
/usr/sbin/telnetd [-BUhlkns] [-D debugmode] [-Iinitid] [-S tos]
[-X authtype] [-a authmode] [-edebug]
[-rlowpty-highpty] [-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 indi-
cated 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 use-
ful 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 pro-
vide 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 pro-
vide 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 authentica-
tion information. This option is currently not sup-
ported by any of the existing authentication mecha-
nisms, and is thus the same as specifying -a valid.
none This is the default state. Authentication informa-
tion is not required. If no or insufficient authen-
tication information is provided, then the login(1)
program will provide the necessary user verification.
off This disables the authentication code. All user ver-
ification 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 nor-
mal shell. In bftp daemon mode normal logins are not sup-
ported, and it must be used on a port other than the normal
This option may be used for debugging purposes. This allows
telnetd to print out debugging information to the connec-
tion, 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 addi-
tional information about what processing is going
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 encryp-
tion, then the -edebug option may be used to enable encryp-
tion 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.
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 differenti-
ate highpty from lowpty.
-s This option is only enabled if telnetd is compiled with sup-
port 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.
-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
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 TERMINAL TYPE
WILL SUPPRESS GO AHEAD
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
WILL SGA Indicates that it will not be sending IAC GA, go
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
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 charac-
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
DO XDISPLOC Indicates a desire to be able to request the name of
the X windows display that is associated with the tel-
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
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
/etc/inittab (UNICOS systems only)
/etc/iptos (if supported)
/usr/ucb/bftp (if supported)
login(1), rlogin(1,) rlogind(8,) ssh(1,) sshd(8,) telnet(1), (if sup-
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
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
Telnetd never sends TELNET IAC GA (go ahead) commands.
Interix March 17, 2006 Interix