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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 .\" .Dd September 19, 2006 .Dt TELNETD 8 .Os BSD 4.2 .Sh NAME .Nm telnetd .Nd DARPA .Tn TELNET protocol server .Sh SYNOPSIS .Nm telnetd .Op Fl BeUhkln .Op Fl D Ar debugmode .Op Fl S Ar tos .Op Fl X Ar authtype .Op Fl a Ar authmode .Op Fl r Ns Ar lowpty-highpty .Op Fl u Ar len .Op Fl debug .Op Fl L Ar /bin/login .Op Fl y .Op Ar port .Sh DESCRIPTION The .Nm telnetd 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 telnetd manually, instead of through .Xr inetd 8 . If started up this way, .Ar port may be specified to run .Nm telnetd on an alternate .Tn TCP port number. .Pp The .Nm telnetd 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 telnetd 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 otp Only allow authenticated connections (as with .Fl a .Cm user ) and also logins with one-time passwords (OTPs). This option will call login with an option so that only OTPs are accepted. The user can of course still type secret information at the prompt. .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 B Ignored. .It Fl D Ar debugmode This option may be used for debugging purposes. This allows .Nm telnetd to print out debugging information to the connection, allowing the user to see what .Nm telnetd 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 telnetd . .It Cm ptydata Displays data written to the pty. .It Cm exercise Has not been implemented yet. .El .It Fl e require encryption to be turned on (in both direction) by the client and disconnects if the client tries to turn the encryption off (in either direction). .It Fl h Disables the printing of host-specific information before login has been completed. .It Fl k .It Fl l Ignored. .It Fl n Disable .Dv TCP keep-alives. Normally .Nm telnetd 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 r Ar lowpty-highpty This option is only enabled when .Nm telnetd is compiled for .Dv UNICOS . It specifies an inclusive range of pseudo-terminal devices to use. If the system has sysconf variable .Dv _SC_CRAY_NPTY configured, the default pty search range is 0 to .Dv _SC_CRAY_NPTY ; otherwise, the default range is 0 to 128. Either .Ar lowpty or .Ar highpty may be omitted to allow changing either end of the search range. If .Ar lowpty is omitted, the - character is still required so that .Nm telnetd can differentiate .Ar highpty from .Ar lowpty . .It Fl S Ar tos .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 telnetd 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 telnetd 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 telnetd . .It Fl L Ar pathname Specify pathname to an alternative login program. .It Fl y Makes .Nm not warn when a user is trying to login with a cleartext password. .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 telnetd 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 .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 telnetd 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 .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 4.2BSD .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 telnetd is compiled with support for linemode, and requests that the client do line by line processing. .It "DO TIMING-MARK" Only sent if .Nm telnetd 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 telnetd is compiled with support for authentication, and indicates a willingness to receive authentication information for automatic login. .It "DO ENCRYPT" Only sent if .Nm telnetd is compiled with support for data encryption, and indicates a willingness to decrypt the data stream. .El .Sh FILES .Bl -tag -width /etc/services -compact .It Pa /etc/services .It Pa /etc/inittab (UNICOS systems only) .It Pa /etc/iptos (if supported) .El .Sh "SEE ALSO" .Xr telnet 1 , .Xr login 1 .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 BUGS Some .Tn TELNET commands are only partially implemented. .Pp Because of bugs in the original 4.2 BSD .Xr telnet 1 , .Nm telnetd performs some dubious protocol exchanges to try to discover if the remote client is, in fact, a 4.2 BSD .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.