telnetd - DARPA TELNET protocol server
/usr/kerberos/sbin/telnetd [-a authmode] [-B] [-D] [debugmode] [-edebug] [-h] [-Iinitid]
[-l] [-k] [-n] [-rlowpty-highpty] [-s] [-S tos] [-U] [-X authtype] [-w [ip|max-
hostlen[,[no]striplocal]]] [-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 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 num-
The telnetd command accepts the following options:
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 speci-
fied 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.
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:
Prints information about the negotiation of TELNET options.
report Prints the options information, plus some additional information about what
processing is going on.
Displays the data stream received by telnetd.
Displays data written to the pty.
Has not been implemented yet.
-debug Enables debugging on each socket created by telnetd (see SO_DEBUG in socket(2)).
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.
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
-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 vari-
able _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
-U This option causes telnetd to refuse connections from addresses that cannot be
mapped back into a symbolic name via the gethostbyaddr(3) routine.
Controls the form of the remote hostname passed to login(1). Specifying ip results
in the numeric IP address always being passed to login(1). Specifying a number,
maxhostlen, sets the maximum length of the hostname passed to login(1) before it
will be passed as a numeric IP address. If maxhostlen is 0, then the system
default, as determined by the utmp or utmpx structures, is used. The nostriplocal
and striplocal options, which must be preceded by a comma, control whether or not
the local host domain is stripped from the remote hostname. By default, the equiv-
alent of striplocal is in effect.
This option is only valid if telnetd has been built with support for the authenti-
cation option. It disables the use of authtype authentication, and can be used to
temporarily disable a specific authentication type without having to recompile tel-
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 indi-
cating a willingness to do the following TELNET options, which are described in more
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 termi-
nal 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 cur-
rent 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
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 tel-
net(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 com-
DO NAWS Requests that the client inform the server when the window (display)
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 infor-
mation, as described in RFC 1572.
DO ENVIRON Indicates a desire to be able to request environment variable infor-
mation, 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
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/inittab (UNICOS systems only)
/etc/iptos (if supported)
/usr/ucb/bftp (if supported)
telnet(1), login(1), bftp(1) (if supported)
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
The terminal type name received from the remote client is converted to lower case.
Telnetd never sends TELNET IAC GA (go ahead) commands.