NAME¶
telnetd —
DARPA telnet protocol
server
SYNOPSIS¶
/usr/sbin/in.telnetd |
[-hns]
[-a
authmode]
[-D
debugmode]
[-L
loginprg]
[-S tos]
[-X
authtype]
[-edebug]
[-debug
port] |
DESCRIPTION¶
The
telnetd program is a server which supports the DARPA
telnet 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, /usr/lib/telnetlogin 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.net(5)).
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
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. issue.net(5)).
FILES¶
/etc/services,
/etc/issue.net
SEE ALSO¶
telnet(1),
login(1),
issue.net(5),
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 4.2 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.
The source code is not comprehensible.