TELNETD(8) System Manager's Manual TELNETD(8)
telnetd - DARPA TELNET protocol server
telnetd [-a authmode] [-B] [-D] [debugmode] [-e] [-h] [-Iinitid] [-l] [-k] [-n] [-rlowpty-highpty] [-s] [-S tos] [-U] [-X authtype] [-w
[ip|maxhostlen[,[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 number.
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.
valid 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.
user 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.
Enables encryption debugging code.
Has not been implemented yet.
-debug Enables debugging on each socket created by telnetd (see SO_DEBUG in socket(2)).
-e This option causes telnetd to refuse unencrypted connections.
-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 speci-
fied, 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 sup-
port kludge linemode, but pass the heuristic (if they respond with WILL TIMING-MARK in response to a DO TIMING-MARK) for kludge
-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
-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 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.
-U This option causes telnetd to refuse connections from addresses that cannot be mapped back into a symbolic name via the gethost-
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 nos-
triplocal 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 equivalent of striplocal is in effect.
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 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 Virtual
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
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
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
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 authentica-
tion 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
/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 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.