NAME¶
telnetd
—
DARPA telnet protocol server
SYNOPSIS¶
/usr/sbin/in.telnetd |
[ -hnNs ]
[-a
authmode ]
[-D
debugmode ]
[-L
loginprg ]
[-S
tos ]
[-X
authtype ]
[-edebug ]
[-debug
port ]
[-z
sslopt ] |
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.
-N
- Disable reverse DNS lookups and use the numeric IP address in logs and
REMOTEHOST environment variable.
-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
.
-z
SSL-parameter
- This option is only valid if
telnetd
has been built with SSL (Secure Socket Layer) support.
debug
- Enable SSL related debugging.
ssl
- Negotiate SSL at first, then use telnet protocol. In this mode telnetd
only accepts connections from SSL enhanced telnet with option
-z ssl
nossl,
!ssl
- switch off SSL negotiation
certsok
- Look username up in /etc/ssl.users. The format of this file is lines
of this form: user1,user2:/C=US/.....
where user1 and user2 are usernames and /C=US/... is the subject name
of the certificate. Use openssl x509
-subject -noout to extract the subject name. If client
certificate is valid, authenticate without password.
certrequired
- client certificate is mandatory
secure
- Don't switch back to unencrypted mode (no SSL) if SSL is not
available.
verify=int
- Set the SSL verify flags (SSL_VERIFY_* in
ssl/ssl.h ).
cert=cert_file
- Use the certificate(s) in
cert_file.
key=key_file
- Use the key(s) in key_file.
cipher=ciph_list
- Set the preferred ciphers to
ciph_list. (See
ssl/ssl.h ).
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.
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.