xref: /netbsd-src/libexec/telnetd/telnetd.8 (revision 79d3071f0c04b87185c113a6fb64f03ec88d0331)

.\"	$NetBSD: telnetd.8,v 1.32 2018/12/15 23:23:51 maya Exp $
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.\"	from: @(#)telnetd.8	8.3 (Berkeley) 3/1/94
.\"
.Dd December 16, 2018
.Dt TELNETD 8
.Os
.Sh NAME
.Nm telnetd
.Nd DARPA
.Tn TELNET
protocol server
.Sh SYNOPSIS
.Nm /usr/libexec/telnetd
.Op Fl Uhlkns46
.Op Fl D Ar debugmode
.Op Fl S Ar tos
.Op Fl X Ar authtype
.Op Fl a Ar authmode
.Op Fl edebug
.Op Fl g Ar gettyent
.Op Fl u Ar len
.Op Fl debug Op Ar port
.Sh DESCRIPTION
The
.Nm
command is a server which supports the
.Tn DARPA
standard
.Tn TELNET
virtual terminal protocol.
.Nm
is normally invoked by the internet server (see
.Xr inetd 8 )
for requests to connect to the
.Tn TELNET
port as indicated by the
.Pa /etc/services
file (see
.Xr services 5 ) .
The
.Fl debug
option may be used to start up
.Nm
manually, instead of through
.Xr inetd 8 .
If started up this way,
.Ar port
may be specified to run
.Nm
on an alternate
.Tn TCP
port number.
.Pp
The
.Nm
command accepts the following options:
.Bl -tag -width "-a authmode"
.It Fl a Ar authmode
This option may be used for specifying what mode should
be used for authentication.
Note that this option is only useful if
.Nm
has been compiled with support for the
.Dv AUTHENTICATION
option.
There are several valid values for
.Ar authmode :
.Bl -tag -width debug
.It debug
Turns on authentication debugging code.
.It 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.
.It valid
Only allow connections when the remote user
can provide valid authentication information
to identify the remote user.
The
.Xr login 1
command will provide any additional user verification
needed if the remote user is not allowed automatic
access to the specified account.
.It 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
.Fl a
.Cm valid .
.It none
This is the default state.
Authentication information is not required.
If no or insufficient authentication information
is provided, then the
.Xr login 1
program will provide the necessary user
verification.
.It off
This disables the authentication code.
All user verification will happen through the
.Xr login 1
program.
.El
.It Fl D Ar debugmode
This option may be used for debugging purposes.
This allows
.Nm
to print out debugging information
to the connection, allowing the user to see what
.Nm
is doing.
There are several possible values for
.Ar debugmode :
.Bl -tag -width exercise
.It Cm options
Prints information about the negotiation of
.Tn TELNET
options.
.It Cm report
Prints the
.Cm options
information, plus some additional information
about what processing is going on.
.It Cm netdata
Displays the data stream received by
.Nm .
.It Cm ptydata
Displays data written to the pty.
.It Cm exercise
Has not been implemented yet.
.El
.It Fl debug
Enables debugging on each socket created by
.Nm
(see
.Dv SO_DEBUG
in
.Xr socket 2 ) .
.It Fl edebug
If
.Nm
has been compiled with support for data encryption, then the
.Fl edebug
option may be used to enable encryption debugging code.
.It Fl g Ar gettyent
Specifies which entry from
.Pa /etc/gettytab
should be used to get banner strings, login program and
other information.
The default entry is
.Dv default .
.It Fl h
Disables the printing of host-specific information before
login has been completed.
.It Fl k
This option is only useful if
.Nm
has been compiled with both linemode and kludge linemode support.
If the
.Fl k
option is specified,
then if the remote client does not support the
.Dv LINEMODE
option, then
.Nm
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
.Dv DONT SUPPRESS-GO-AHEAD
and
.Dv DONT ECHO . )
The
.Fl k
option is most useful when there are remote clients
that do not support kludge linemode, but pass the heuristic
(if they respond with
.Dv WILL TIMING-MARK
in response to a
.Dv DO TIMING-MARK )
for kludge linemode support.
.It Fl l
Specifies line mode.
Tries to force clients to use line-at-a-time mode.
If the
.Dv LINEMODE
option is not supported, it will go
into kludge linemode.
.It Fl n
Disable
.Dv TCP
keep-alives.
Normally
.Nm
enables the
.Tn 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.
.It Fl s
This option is only enabled if
.Nm
is compiled with support for secure logins.
It causes the
.Fl s
option to be passed on to
.Xr login 1 ,
and thus is only useful if
.Xr login 1
supports the
.Fl 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.
.It Fl S Ar 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
.Pa /etc/iptos
file.
This option has no effect on
.Nx .
.\"The option has no effect on systems that do not
.\"support
.\".Xr parsetos 3
.\"routine and the
.\".Pa /etc/iptos
.\"file.
.It Fl u Ar len
This option is used to specify the size of the field
in the
.Dv utmp
structure that holds the remote host name.
If the resolved host name is longer than
.Ar 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
.Fl u0
indicates that only dotted decimal addresses
should be put into the
.Pa utmp
file.
.It Fl U
This option causes
.Nm
to refuse connections from addresses that
cannot be mapped back into a symbolic name via the
.Xr getnameinfo 3
routine.
.It Fl X Ar authtype
This option is only valid if
.Nm
has been built with support for the authentication option.
It disables the use of
.Ar authtype
authentication, and can be used to temporarily disable
a specific authentication type without having to recompile
.Nm .
.It Fl 4
.It Fl 6
Specifies address family to be used on
.Fl debug
mode.
During normal operation
.Po
called from
.Xr inetd 8
.Pc
.Nm
will use the file descriptor passed from
.Xr inetd 8 .
.El
.Pp
.Nm
operates by allocating a pseudo-terminal device (see
.Xr pty 4 )
for a client, then creating a login process which has
the slave side of the pseudo-terminal as
.Dv stdin ,
.Dv stdout
and
.Dv stderr .
.Nm
manipulates the master side of the pseudo-terminal,
implementing the
.Tn TELNET
protocol and passing characters
between the remote client and the login process.
.Pp
When a
.Tn TELNET
session is started up,
.Nm
sends
.Tn TELNET
options to the client side indicating
a willingness to do the following
.Tn TELNET
options, which are described in more detail below:
.Bd -literal -offset indent
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
.Ed
.Pp
The pseudo-terminal allocated to the client is configured
to operate in \*(lqcooked\*(rq mode, and with
.Dv XTABS and
.Dv CRMOD
enabled (see
.Xr tty 4 ) .
.Pp
.Nm
has support for enabling locally the following
.Tn TELNET
options:
.Bl -tag -width "DO AUTHENTICATION"
.It "WILL ECHO"
When the
.Dv LINEMODE
option is enabled, a
.Dv WILL ECHO
or
.Dv WONT ECHO
will be sent to the client to indicate the
current state of terminal echoing.
When terminal echo is not desired, a
.Dv WILL ECHO
is sent to indicate that
.Tn 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
.Dv WONT ECHO
is sent to indicate that
.Tn telnetd
will not be doing any terminal echoing, so the
client should do any terminal echoing that is needed.
.It "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.
.It "WILL SGA"
Indicates that it will not be sending
.Dv IAC GA ,
go ahead, commands.
.It "WILL STATUS"
Indicates a willingness to send the client, upon
request, of the current status of all
.Tn TELNET
options.
.It "WILL TIMING-MARK"
Whenever a
.Dv DO TIMING-MARK
command is received, it is always responded
to with a
.Dv WILL TIMING-MARK
.It "WILL LOGOUT"
When a
.Dv DO LOGOUT
is received, a
.Dv WILL LOGOUT
is sent in response, and the
.Tn TELNET
session is shut down.
.It "WILL ENCRYPT"
Only sent if
.Nm
is compiled with support for data encryption, and
indicates a willingness to decrypt the data stream.
.El
.Pp
.Nm
has support for enabling remotely the following
.Tn TELNET
options:
.Bl -tag -width "DO AUTHENTICATION"
.It "DO BINARY"
Sent to indicate that
.Tn telnetd
is willing to receive an 8 bit data stream.
.It "DO LFLOW"
Requests that the client handle flow control
characters remotely.
.It "DO ECHO"
This is not really supported, but is sent to identify a
.Bx 4.2
.Xr telnet 1
client, which will improperly respond with
.Dv WILL ECHO .
If a
.Dv WILL ECHO
is received, a
.Dv DONT ECHO
will be sent in response.
.It "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.
.It "DO SGA"
Indicates that it does not need to receive
.Dv IAC GA ,
the go ahead command.
.It "DO NAWS"
Requests that the client inform the server when
the window (display) size changes.
.It "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.
.It "DO XDISPLOC"
Indicates a desire to be able to request the name
of the X windows display that is associated with
the telnet client.
.It "DO NEW-ENVIRON"
Indicates a desire to be able to request environment
variable information, as described in RFC 1572.
.It "DO ENVIRON"
Indicates a desire to be able to request environment
variable information, as described in RFC 1408.
.It "DO LINEMODE"
Only sent if
.Nm
is compiled with support for linemode, and
requests that the client do line by line processing.
.It "DO TIMING-MARK"
Only sent if
.Nm
is compiled with support for both linemode and
kludge linemode, and the client responded with
.Dv WONT LINEMODE .
If the client responds with
.Dv WILL TM ,
the it is assumed that the client supports kludge linemode.
Note that the
.Op Fl k
option can be used to disable this.
.It "DO AUTHENTICATION"
Only sent if
.Nm
is compiled with support for authentication, and
indicates a willingness to receive authentication
information for automatic login.
.It "DO ENCRYPT"
Only sent if
.Nm
is compiled with support for data encryption, and
indicates a willingness to decrypt the data stream.
.El
.Pp
At the end of a login session,
.Nm
invokes the
.Xr ttyaction 3
facility with an action of "telnetd" and user "root"
to execute site-specific commands.
.Sh FILES
.Bl -item -compact
.It
.Pa /etc/services
.It
.Pa /etc/iptos
(if supported)
.El
.Sh SEE ALSO
.Xr login 1 ,
.Xr skey 1 ,
.Xr telnet 1 ,
.Xr ttyaction 3
.Sh STANDARDS
.Bl -tag -compact -width "RFC 1572  "
.It RFC 854
.Tn TELNET
PROTOCOL SPECIFICATION
.It RFC 855
TELNET OPTION SPECIFICATIONS
.It RFC 856
TELNET BINARY TRANSMISSION
.It RFC 857
TELNET ECHO OPTION
.It RFC 858
TELNET SUPPRESS GO AHEAD OPTION
.It RFC 859
TELNET STATUS OPTION
.It RFC 860
TELNET TIMING MARK OPTION
.It RFC 861
TELNET EXTENDED OPTIONS - LIST OPTION
.It RFC 885
TELNET END OF RECORD OPTION
.It RFC 1073
Telnet Window Size Option
.It RFC 1079
Telnet Terminal Speed Option
.It RFC 1091
Telnet Terminal-Type Option
.It RFC 1096
Telnet X Display Location Option
.It RFC 1123
Requirements for Internet Hosts -- Application and Support
.It RFC 1184
Telnet Linemode Option
.It RFC 1372
Telnet Remote Flow Control Option
.It RFC 1416
Telnet Authentication Option
.It RFC 1411
Telnet Authentication: Kerberos Version 4
.It RFC 1571
Telnet Environment Option Interoperability Issues
.It RFC 1572
Telnet Environment Option
.El
.Sh BUGS
Some
.Tn TELNET
commands are only partially implemented.
.Pp
Because of bugs in the original
.Bx 4.2
.Xr telnet 1 ,
.Nm
performs some dubious protocol exchanges to try to discover if the remote
client is, in fact, a
.Bx 4.2
.Xr telnet 1 .
.Pp
Binary mode
has no common interpretation except between similar operating systems
.Po
.Ux
in this case
.Pc .
.Pp
The terminal type name received from the remote client is converted to
lower case.
.Pp
.Nm
never sends
.Tn TELNET
.Dv IAC GA
(go ahead) commands.