TELNETD(8) BSD System Manager's Manual TELNETD(8)NAMEin.telnetd — DARPA TELNET protocol server
SYNOPSISin.telnetd [-Uhlkn46H] [-D debugmode] [-Iinitid] [-S tos] [-X authtype]
[-rlowpty-highpty] [-u len] [-L /bin/login] [-debug [port]]
DESCRIPTION
The in.telnetd command is a server which supports the DARPA standard
TELNET virtual terminal protocol. in.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 in.telnetd manually, instead of through
inetd(8). If started up this way, port may be specified to run
in.telnetd on an alternate TCP port number.
The in.telnetd command accepts the following options:
-D debugmode This option may be used for debugging purposes. This
allows in.telnetd to print out debugging information to the
connection, allowing the user to see what in.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 addi‐
tional information about what processing is going
on.
netdata Displays the data stream received by in.telnetd.
ptydata Displays data written to the pty.
exercise Has not been implemented yet.
-debug Enables debugging on each socket created by in.telnetd (see
SO_DEBUG in socket(2)).
-h Disables the printing of host-specific information before
login has been completed.
-H Instruct the login program to suppress printing the host‐
name before the prompt.
-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 in.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 in.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 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 in.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 in.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 in.telnetd can dif‐
ferentiate highpty from lowpty.
-S tos
-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 in.telnetd to refuse connections from
addresses that cannot be mapped back into a symbolic name
via the via the gethostbyaddr(3) routine.
-X authtype This option is only valid if in.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 in.telnetd.
-L pathname Specify pathname to an alternative login program.
-4
-6 Specifies address family to be used on -debug mode. During
normal operation (called from inetd(8)) in.telnetd will use
the file descriptor passed from inetd(8).
in.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. in.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, in.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)).
in.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
in.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 in.telnetd will not be doing
any terminal echoing, so the client should do any ter‐
minal 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 in.telnetd is compiled with support for
data encryption, and indicates a willingness to
decrypt the data stream.
in.telnetd has support for enabling remotely the following TELNET
options:
DO BINARY Sent to indicate that in.telnetd is willing to receive
an 8 bit data stream.
DO LFLOW Requests that the client handle flow control charac‐
ters 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 tel‐
net 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 in.telnetd is compiled with support for
linemode, and requests that the client do line by line
processing.
DO TIMING-MARK Only sent if in.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 sup‐
ports kludge linemode. Note that the [-k] option can
be used to disable this.
DO AUTHENTICATION Only sent if in.telnetd is compiled with support for
authentication, and indicates a willingness to receive
authentication information for automatic login.
DO ENCRYPT Only sent if in.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)
SEE ALSOlogin(1), telnet(1)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), in.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.
in.telnetd never sends TELNET IAC GA (go ahead) commands.
WONT CRYPT. in.telnetd talks over insecure, unencrypted communications
channels. Please use sshd(8) instead.
BSD March 1, 1994 BSD
