NAME
telnetd
- DARPA
TELNET
protocol server
SYNOPSIS
/usr/libexec/telnetd
[-Uhlkns46]
[-D debugmode]
[-S tos]
[-X authtype]
[-a authmode]
[-edebug]
[-g gettyent]
[-u len]
[-debug[ port]]
DESCRIPTION
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 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.
- 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
-
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
.
- 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
socket(2)).
- -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 completed.
- -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 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 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.
Specifying
-u0
indicates that only dotted decimal addresses
should be put into the
utmp
file.
- -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 recompile
.
- -4
-
- -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
inetd(8).
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 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
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 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.
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.
FILES
/etc/services
/etc/iptos
(if supported)
SEE ALSO
login(1),
skey(1),
telnet(1),
ttyaction(3)
STANDARDS
- 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
BUGS
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
in this case
(UNIX.)
The terminal type name received from the remote client is converted to
lower case.
telnetd
never sends
TELNET
IAC
GA
(go ahead) commands.