TELNETD(8) BSD System Manager's Manual TELNETD(8)
NAME
telnetd -- DARPA TELNET protocol server
SYNOPSIS
/usr/libexec/telnetd [-Uhlkns] [-D debugmode] [-Iinitid] [-S tos] [-X authtype] [-a authmode] [-edebug]
[-p loginprog] [-rlowpty-highpty] [-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 Turn 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 pro-viding providing
viding 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 ver-ification verification
ification 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 cur-rently currently
rently 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 insuf-ficient insufficient
ficient authentication information is provided, then the login(1) program will provide
the necessary user verification.
off Disable 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 sev-eral several
eral possible values for debugmode:
options Print information about the negotiation of TELNET options.
report Print the options information, plus some additional information about what processing
is going on.
netdata Display the data stream received by telnetd.
ptydata Display data written to the pty.
exercise Has not been implemented yet.
-debug Enable 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.
-p loginprog
Specify an alternate login(1) command to run to complete the login. The alternate command must
understand the same command arguments as the standard login.
-h Disable the printing of host-specific information before login has been completed.
-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 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 heuris-tic heuristic
tic (if they respond with WILL TIMING-MARK in response to a DO TIMING-MARK) for kludge linemode
support.
-l Specify line mode. Try 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 con-nections connections
nections 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 con-figured, configured,
figured, 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 differenti-ate differentiate
ate 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 con-trolling controlling
trolling remote logins from outside of a firewall.
-S tos
-u len This option is used to specify the size of the field in the utmpx 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 utmpx file.
-U This option causes telnetd to refuse connections from addresses that cannot be mapped back into
a symbolic name via the gethostbyaddr(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 spe-cific specific
cific 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 char-acters characters
acters 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 will-ingness willingness
ingness 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 ter-minal terminal
minal 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 Indicate 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 Indicate that it will not be sending IAC GA, go ahead, commands.
WILL STATUS Indicate 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 ses-sion session
sion 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 Indicate 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 Indicate 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 Indicate a desire to be able to request information about the speed of the serial
line to which the client is attached.
DO XDISPLOC Indicate a desire to be able to request the name of the X Window System display that
is associated with the telnet client.
DO NEW-ENVIRON Indicate a desire to be able to request environment variable information, as
described in RFC 1572.
DO ENVIRON Indicate 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.
NOTES
By default telnetd will read the he, hn, and im capabilities from /etc/gettytab and use that informa-tion information
tion (if present) to determine what to display before the login: prompt. You can also use a System V
style /etc/issue file by using the if capability, which will override im. The information specified in
either im or if will be displayed to both console and remote logins.
FILES
/etc/services
/etc/gettytab
/etc/inittab (UNICOS systems only)
/etc/iptos (if supported)
/usr/ucb/bftp (if supported)
SEE ALSO
bftp(1), login(1), gettytab(5), telnet(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
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.
HISTORY
IPv6 support was added by WIDE/KAME project.
BSD March 1, 1994 BSD
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