The
/usr/libexec/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
/usr/libexec/telnetd
manually, instead of through
inetd(8).
If started up this way,
port
may be specified to run
/usr/libexec/telnetd
on an alternate
TCP
port number.
The
/usr/libexec/telnetd
command accepts the following options:
-4
Forces
/usr/libexec/telnetd
to use IPv4 addresses only.
-6
Forces
/usr/libexec/telnetd
to use IPv6 addresses only.
-a authmode
This option may be used for specifying what mode should
be used for authentication.
Note that this option is only useful if
/usr/libexec/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
-avalid
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,
/usr/libexec/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.
-D debugmode
This option may be used for debugging purposes.
This allows
/usr/libexec/telnetd
to print out debugging information
to the connection, allowing the user to see what
/usr/libexec/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
.
ptydata
Display data written to the pty.
exercise
Has not been implemented yet.
-debug
Enable debugging on each socket created by
/usr/libexec/telnetd
(see
SO_DEBUG
in
socket(2)).
-edebug
If
/usr/libexec/telnetd
has been compiled with support for data encryption, then the
-edebug
option may be used to enable encryption debugging code.
-h
Disable the printing of host-specific information before
login has been completed.
-k
This option is only useful if
/usr/libexec/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
/usr/libexec/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 linemode.
-n
Disable
TCP
keep-alives.
Normally
/usr/libexec/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.
-p loginprog
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.
-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
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.
Specifying
-u0
indicates that only dotted decimal addresses
should be put into the
utmp
file.
-U
This option causes
/usr/libexec/telnetd
to refuse connections from addresses that
cannot be mapped back into a symbolic name
via the
gethostbyaddr(3)
routine.
-X authtype
This option is only valid if
/usr/libexec/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
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,
/usr/libexec/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
XTABS andCRMOD
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
/usr/libexec/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
/usr/libexec/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 Terminal.
"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
/usr/libexec/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
/usr/libexec/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
BSD 4.2
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 connection.
"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
/usr/libexec/telnetd
is compiled with support for linemode, and
requests that the client do line by line processing.
"DO TIMING-MARK"
Only sent if
/usr/libexec/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
/usr/libexec/telnetd
is compiled with support for authentication, and
indicates a willingness to receive authentication
information for automatic login.
"DO ENCRYPT"
Only sent if
/usr/libexec/telnetd
is compiled with support for data encryption, and
indicates a willingness to decrypt
the data stream.
NOTES
By default
/usr/libexec/telnetd
will read the
hehn
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.
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
HISTORY
IPv6 support was added by WIDE/KAME project.
BUGS
Some
TELNET
commands are only partially implemented.
Because of bugs in the original
BSD 4.2
telnet(1),
/usr/libexec/telnetd
performs some dubious protocol exchanges to try to discover if the remote
client is, in fact, a
BSD 4.2
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.
