--- name: "ktelnetd" section: "8" platform: "linux" package: "kerberos4kth-servers" version: "1.2.2-11.2" release: "Debian 3.1 (Sarge)" platforms: ["linux"] url: "https://new.carta.tech/ktelnetd" --- # ktelnetd(8) DARPA TELNET protocol server ## NAME **telnetd** – DARPA TELNET protocol server ## SYNOPSIS **telnetd** \[**-BUhkln**\] \[**-D** *debugmode*\] \[**-S** *tos*\] \[**-X** *authtype*\] \[**-a** *authmode*\] \[**-r***lowpty-highpty*\] \[**-u** *len*\] \[**-debug**\] \[**-L** */bin/login*\] \[**-y**\] \[*port*\] ## 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)](/inetd/8.md)) for requests to connect to the TELNET port as indicated by the */etc/services* file (see [services(5)](/services/5.md)). The **-debug** option may be used to start up **telnetd** manually, instead of through [inetd(8)](/inetd/8.md). 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 Turns 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 providing a password. - valid Only allow connections when the remote user can provide valid authentication information to identify the remote user. The [login(1)](/login/1.md) command will provide any additional user verification 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 currently not supported by any of the existing authentication mechanisms, and is thus the same as specifying **-a** **valid**. - otp Only allow authenticated connections (as with **-a** **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. - none This is the default state. Authentication information is not required. If no or insufficient authentication information is provided, then the [login(1)](/login/1.md) program will provide the necessary user verification. - off This disables the authentication code. All user verification will happen through the [login(1)](/login/1.md) program. - **-B** Ignored. - **-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 several possible values for *debugmode*: - **options** Prints information about the negotiation of TELNET options. - **report** Prints the **options** information, plus some additional information about what processing is going on. - **netdata** Displays the data stream received by **telnetd**. - **ptydata** Displays data written to the pty. - **exercise** Has not been implemented yet. - **-h** Disables the printing of host-specific information before login has been completed. - **-k** - **-l** Ignored. - **-n** Disable TCP keep-alives. Normally **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 **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 **telnetd** can differentiate *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 **telnetd** to refuse connections from addresses that cannot be mapped back into a symbolic name via the [gethostbyaddr(3)](/gethostbyaddr/3.md) 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 specific authentication type without having to recompile **telnetd**. - **-L** *pathname* Specify pathname to an alternative login program. - **-y** Makes **telnetd** not warn when a user is trying to login with a cleartext password. **Telnetd** operates by allocating a pseudo-terminal device (see [pty(4)](/pty/4.md)) 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 characters 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 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)](/tty/4.md)). **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 terminal 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 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 **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)](/telnet/1.md) 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 telnet 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 **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. ## FILES - */etc/services* - */etc/inittab* (UNICOS systems only) - */etc/iptos* (if supported) ## SEE ALSO [telnet(1)](/telnet/1.md), [login(1)](/login/1.md) ## 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)](/telnet/1.md), **telnetd** performs some dubious protocol exchanges to try to discover if the remote client is, in fact, a 4.2 BSD [telnet(1)](/telnet/1.md). 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.