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
telnetd.
-
-
- 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 telnetd.
-
-
- -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 (
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.