TELNETD(8) BSD System Manager's Manual TELNETD(8)
telnetd -- DARPA TELNET protocol server
/usr/libexec/telnetd [-Uhlkns46] [-D debugmode] [-S tos] [-X authtype] [-a authmode]
[-edebug] [-g gettyent] [-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:
-a authmode This option may be used for specifying what mode should be used for authentica-
tion. Note that this option is only useful 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 provide valid authenti-
cation 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 authenti-
cation 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 authenti-
cation 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 hap-
pen through the login(1) program.
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
-edebug If telnetd has been compiled with support for data encryption, then the -edebug
option may be used to enable encryption debugging code.
-g gettyent Specifies which entry from /etc/gettytab should be used to get banner strings,
login program and other information. The default entry is default.
-h Disables the printing of host-specific information before login has been com-
-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 char-
acter 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.
-s This option is only enabled if telnetd is compiled with support for secure
logins. 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 Kerberos or S/Key
validated logins are allowed, and is usually useful for controlling remote
logins from outside of a firewall.
-S tos This option sets the IP Type-of Service (TOS) option on the connection to the
value tos, which may be a numeric TOS value or a symbolic TOS name found in the
/etc/iptos file. This option has no effect on NetBSD.
-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. Specify-
ing -u0 indicates that only dotted decimal addresses should be put into the
-U This option causes telnetd to refuse connections from addresses that cannot be
mapped back into a symbolic name via the getnameinfo(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
-6 Specifies address family to be used on -debug mode. During normal operation
(called from inetd(8)) telnetd will use the file descriptor passed from
telnetd operates by allocating a pseudo-terminal device (see pty(4)) for a client, then cre-
ating 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 detail
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 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
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 dis-
play that is associated with the telnet client.
DO NEW-ENVIRON Indicates a desire to be able to request environment variable informa-
tion, as described in RFC 1572.
DO ENVIRON Indicates a desire to be able to request environment variable informa-
tion, 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 auto-
DO ENCRYPT Only sent if telnetd is compiled with support for data encryption, and
indicates a willingness to decrypt the data stream.
At the end of a login session, telnetd invokes the ttyaction(3) facility with an action of
"telnetd" and user "root" to execute site-specific commands.
/etc/iptos (if supported)
login(1), skey(1), telnet(1), ttyaction(3)
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 4.2BSD 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
The terminal type name received from the remote client is converted to lower case.
telnetd never sends TELNET IAC GA (go ahead) commands.
BSD July 17, 2004 BSD