TELNETD(8) BSD System Manager's Manual TELNETD(8)
telnetd -- DARPA TELNET protocol server
/usr/libexec/telnetd [-46BUhlkns] [-D debugmode] [-S tos] [-X authtype] [-a authmode] [-edebug] [-p loginprog] [-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 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:
-4 Forces telnetd to use IPv4 addresses only.
-6 Forces telnetd to use IPv6 addresses only.
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 Turn 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 Disable the authentication code. All user verification will happen through the login(1) program.
-B Specify 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:
options Print information about the negotiation of TELNET options.
report Print the options information, plus some additional information about what processing is going on.
netdata Display the data stream received by telnetd.
ptydata Display data written to the pty.
exercise Has not been implemented yet.
-debug Enable 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
Specify an alternate login(1) command to run to complete the login. The alternate command must understand the same command arguments
as the standard login.
-h Disable the printing of host-specific information before login has been completed.
-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 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 Specify line mode. Try to force clients to use line-at-a-time mode. If the LINEMODE option is not supported, it will go into kludge
-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.
-S tos Sets the IP type-of-service (TOS) option for the telnet connection to the value tos, which can be a numeric TOS value or, on systems
that support it, a symbolic TOS name found in the /etc/iptos file.
-u len This option is used to specify the size of the field in the utmpx 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 utmpx
-U This option causes telnetd to refuse connections from addresses that cannot be mapped back into a symbolic name via the
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 proto-
col 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 echo-
ing 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 Indicate that the client is willing to send a 8 bits of data, rather than the normal 7 bits of the Network Virtual Termi-
WILL SGA Indicate that it will not be sending IAC GA, go ahead, commands.
WILL STATUS Indicate 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 Indicate a desire to be able to request the name of the type of terminal that is attached to the client side of the con-
DO SGA Indicate 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 Indicate a desire to be able to request information about the speed of the serial line to which the client is attached.
DO XDISPLOC Indicate a desire to be able to request the name of the X Window System display that is associated with the telnet client.
DO NEW-ENVIRON Indicate a desire to be able to request environment variable information, as described in RFC 1572.
DO ENVIRON Indicate 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.
By default telnetd will read the he, hn, and im capabilities from /etc/gettytab and use that information (if present) to determine what to
display before the login: prompt. You can also use a System V style /etc/issue file by using the if capability, which will override im. The
information specified in either im or if will be displayed to both console and remote logins.
/etc/iptos (if supported)
/usr/ucb/bftp (if supported)
bftp(1), login(1), telnet(1) (if supported), gettytab(5)
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
IPv6 support was added by WIDE/KAME project.
Some TELNET commands are only partially implemented.
Because of bugs in the original 4.2BSD telnet(1), telnetd performs some dubious protocol exchanges to try to discover if the remote client
is, in fact, a 4.2BSD 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.
January 9, 2005 BSD