TELNETD(8) UNIX System V TELNETD(8)
NAME
telnetd - DARPA TELNET protocol server
SYNOPSIS
/usr/libexec/telnetd [-a authmode] [-B] [-D] [debugmode]
[-edebug] [-h] [-Iinitid] [-l] [-k] [-n] [-rlowpty-highpty]
[-s] [-S tos] [-U] [-X authtype] [-w
[ip|maxhostlen[,[no]striplocal]]] [-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.
valid
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.
user 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.
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TELNETD(8) UNIX System V TELNETD(8)
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.
-B Specifies bftp server mode. In this mode, telnetd
causes login to start a bftp(1) session rather than the
user's normal shell. In bftp daemon mode, normal
logins are not supported, and it must be used on a port
other than the normal TELNET port.
-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.
-h Disables the printing of host-specific information
before login has been completed.
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TELNETD(8) UNIX System V TELNETD(8)-I initid
This option is only applicable to UNICOS systems prior
to 7.0. It specifies the ID from /etc/inittab to use
when init starts login sessions. The default ID is fe.
-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.
-r lowpty-highpty
This option is only enabled when telnetd is compiled
for UNICOS. It specifies an inclusive range of
pseudo-terminal devices to use. If the system has
sysconf variable _SC_CRAY_NPTY configured, the default
pty search range is 0 to _SC_CRAY_NPTY; otherwise, the
default range is 0 to 128. Either lowpty or highpty
may be omitted to allow changing either end of the
search range. If lowpty is omitted, the - character is
still required so that telnetd can differentiate
highpty from lowpty.
-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, and is usually
useful for controlling remote logins from outside of a
firewall.
-S tos
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TELNETD(8) UNIX System V TELNETD(8)-U This option causes telnetd to refuse connections from
addresses that cannot be mapped back into a symbolic
name via the gethostbyaddr(3) routine.
-w [ip|maxhostlen[,[no]striplocal]]
Controls the form of the remote hostname passed to
login(1). Specifying ip results in the numeric IP
address always being passed to login(1). Specifying a
number, maxhostlen, sets the maximum length of the
hostname passed to login(1) before it will be passed as
a numeric IP address. If maxhostlen is 0, then the
system default, as determined by the utmp or utmpx
structures, is used. The nostriplocal and striplocal
options, which must be preceded by a comma, control
whether or not the local host domain is stripped from
the remote hostname. By default, the equivalent of
striplocal is in effect.
-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.
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
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TELNETD(8) UNIX System V TELNETD(8)
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
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TELNETD(8) UNIX System V TELNETD(8)
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,
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TELNETD(8) UNIX System V TELNETD(8)
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/inittab (UNICOS systems only)
/etc/iptos (if supported)
/usr/ucb/bftp (if supported)
SEE ALSO
telnet(1), login(1), bftp(1) (if supported)
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
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TELNETD(8) UNIX System V TELNETD(8)
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.
Page 8 (printed 4/3/05)
