telnetd —
DARPA
TELNET protocol server
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
telnetd command accepts the following options:
-
-a,
--authmode
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:
- 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.
-
-S,
--server-principal
name
- Set the Kerberos principal name for this server instance,
with or without an explicit realm.
-
-X,
--disable-auth-type
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.
-
-D,
--debug
[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.
- auth
- Displays authentication information.
- encr
- Displays encryption information.
-
-E,
--exec-login
string
- Set the program to be executed instead of
/bin/login.
-
-h,
--no-hostinfo
- Disables the printing of host-specific information before
login has been completed.
-
-l,
--linemode
[mode]
- Specifies line mode. Tries to force clients to use line-
at-a-time mode. If mode is
nokludge, then automatic klugde linemode will
be disabled. If the
LINEMODE
option is
not supported, it will go into kludge linemode.
-
-n,
--no-keepalive
- 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.
-
-U,
--reverse-lookup
- This option causes telnetd to
refuse connections from addresses that cannot be mapped back into a
symbolic name via the gethostbyaddr(3)
routine.
-
-?,
--help
- Display a help list.
- --usage
- Display a short usage message.
-
-V,
--version
- Display program version.
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.
/etc/services
telnet(1),
login(1),
bftp(1) (if supported)
- 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 4.2 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 TELNET
IAC GA
(go ahead) commands.