.\" Copyright (c) 1983, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)telnetd.8 8.4 (Berkeley) 6/1/94 .\" $FreeBSD: src/contrib/telnet/telnetd/telnetd.8,v 1.21 2005/01/21 21:57:05 ru Exp $ .\" .Dd January 9, 2005 .Dt TELNETD 8 .Os .Sh NAME .Nm telnetd .Nd DARPA .Tn TELNET protocol server .Sh SYNOPSIS .Nm /usr/libexec/telnetd .Op Fl 46BUhlkns .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 p Ar loginprog .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 Telnetd 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 indent .It Fl 4 Forces .Nm to use IPv4 addresses only. .It Fl 6 Forces .Nm to use IPv6 addresses only. .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 Cm debug Turn on authentication debugging code. .It Cm 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 Cm 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 Cm 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 Cm 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 Cm off Disable the authentication code. All user verification will happen through the .Xr login 1 program. .El .It Fl B Specify bftp server mode. In this mode, .Nm causes login to start a .Xr 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 .Tn TELNET port. .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 Print information about the negotiation of .Tn TELNET options. .It Cm report Print the .Cm options information, plus some additional information about what processing is going on. .It Cm netdata Display the data stream received by .Nm . .It Cm ptydata Display data written to the pty. .It Cm exercise Has not been implemented yet. .El .It Fl debug Enable 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 p Ar loginprog Specify an alternate .Xr login 1 command to run to complete the login. The alternate command must understand the same command arguments as the standard login. .It Fl h Disable 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 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 Specify line mode. Try 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 Ar tos Sets the IP type-of-service (TOS) option for the telnet connection to the value .Ar tos , which can be a numeric TOS value or, on systems that support it, a symbolic TOS name found in the .Pa /etc/iptos file. .It Fl u Ar len This option is used to specify the size of the field in the .Dv utmpx 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 utmpx 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 gethostbyaddr 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 . .El .Pp .Nm Telnetd 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 Telnetd 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 .Dq cooked mode, and with .Dv XTABS and .Dv CRMOD enabled (see .Xr tty 4 ) . .Pp .Nm Telnetd 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 .Nm 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 .Nm will not be doing any terminal echoing, so the client should do any terminal echoing that is needed. .It "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. .It "WILL SGA" Indicate that it will not be sending .Dv IAC GA , go ahead, commands. .It "WILL STATUS" Indicate 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 Telnetd has support for enabling remotely the following .Tn TELNET options: .Bl -tag -width "DO AUTHENTICATION" .It "DO BINARY" Sent to indicate that .Nm 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" 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. .It "DO SGA" Indicate 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" Indicate a desire to be able to request information about the speed of the serial line to which the client is attached. .It "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. .It "DO NEW-ENVIRON" Indicate a desire to be able to request environment variable information, as described in RFC 1572. .It "DO ENVIRON" Indicate 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 .Sh NOTES By default .Nm will read the .Em \&he , .Em \&hn , and .Em \&im capabilities from .Pa /etc/gettytab and use that information (if present) to determine what to display before the login: prompt. You can also use a System V style .Pa /etc/issue file by using the .Em \&if capability, which will override .Em \&im . The information specified in either .Em \&im or .Em \&if will be displayed to both console and remote logins. .\" .Sh ENVIRONMENT .Sh FILES .Bl -tag -width /usr/ucb/bftp -compact .It Pa /etc/services .It Pa /etc/gettytab .It Pa /etc/iptos (if supported) .It Pa /usr/ucb/bftp (if supported) .El .Sh "SEE ALSO" .Xr bftp 1 , .Xr login 1 , .Xr telnet 1 (if supported), .Xr gettytab 5 .Sh STANDARDS .Bl -tag -compact -width RFC-1572 .It Cm RFC-854 .Tn TELNET PROTOCOL SPECIFICATION .It Cm RFC-855 TELNET OPTION SPECIFICATIONS .It Cm RFC-856 TELNET BINARY TRANSMISSION .It Cm RFC-857 TELNET ECHO OPTION .It Cm RFC-858 TELNET SUPPRESS GO AHEAD OPTION .It Cm RFC-859 TELNET STATUS OPTION .It Cm RFC-860 TELNET TIMING MARK OPTION .It Cm RFC-861 TELNET EXTENDED OPTIONS - LIST OPTION .It Cm RFC-885 TELNET END OF RECORD OPTION .It Cm RFC-1073 Telnet Window Size Option .It Cm RFC-1079 Telnet Terminal Speed Option .It Cm RFC-1091 Telnet Terminal-Type Option .It Cm RFC-1096 Telnet X Display Location Option .It Cm RFC-1123 Requirements for Internet Hosts -- Application and Support .It Cm RFC-1184 Telnet Linemode Option .It Cm RFC-1372 Telnet Remote Flow Control Option .It Cm RFC-1416 Telnet Authentication Option .It Cm RFC-1411 Telnet Authentication: Kerberos Version 4 .It Cm RFC-1412 Telnet Authentication: SPX .It Cm RFC-1571 Telnet Environment Option Interoperability Issues .It Cm RFC-1572 Telnet Environment Option .El .Sh HISTORY IPv6 support was added by WIDE/KAME project. .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 (Unix in this case). .Pp The terminal type name received from the remote client is converted to lower case. .Pp .Nm Telnetd never sends .Tn TELNET .Dv IAC GA (go ahead) commands.