The
telnetd
program is a server which supports the
DARPAtelnet
interactive communication 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
program 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 useful if
telnetd
has been compiled with support for authentication, which is not
available in the current version. The following values of
authmode
are understood:
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 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
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.
-D debugmode
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.
-edebug
If
telnetd
has been compiled with support for 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.
-L loginprg
This option may be used to specify a different login program.
By default,
/bin/login
is used.
-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
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. This is usually useful for controlling
remote logins from outside of a firewall.
-S tos
Sets the IP type-of-service (TOS) option for the telnet
connection to the value
tos
-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 recompile
telnetd
If the file
/etc/issue.net
is present,
telnetd
will display its contents before the login prompt of a telnet session (see
issue.net5).
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
stdinstdout
and
stderrTelnetd
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 AUTHENTICATION
WILL ENCRYPT
DO TERMINAL TYPE
DO TSPEED
DO XDISPLOC
DO NEW-ENVIRON
DO ENVIRON
WILL SUPPRESS GO AHEAD
DO ECHO
DO LINEMODE
DO NAWS
WILL STATUS
DO LFLOW
DO TIMING-MARK
The pseudo-terminal allocated to the client is configured
to operate in ``cooked'' mode, and with
XTABSCRMOD
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 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 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 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 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.
issue.net5).
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
BUGS
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
TELNETIAC GA
(go ahead) commands.
