telnetd -- DARPA TELNET protocol service

The telnetd command is a 10/2016/2019/11/2022/2025 service that supports the DARPA standard TELNET virtual terminal protocol. The telnetd service is normally started by 10/2016/2019/11/2022/2025 service control manager when the system boots and is normally configured through a control panel applet.

telnetd operates by allocating a pseudo-terminal device for a client, then creating a login process which has the slave side of the pseudo-terminal as stdin, 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.

telnetd has support for enabling locally the following TELNET options:

WILL ECHO: When the LINEMODE option is enabled, a WILL ECHO or WONT ECHO is 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: indicate 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: is 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: is 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: is not really supported, but is sent to identify a 4.2BSD telnet client, which improperly responds with WILL ECHO. If a WILL ECHO is received, a DONT ECHO is 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: Indicate 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 Window System 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: is sent only if telnetd is compiled with support for linemode, and requests that the client do line by line processing.
DO TIMING-MARK: is 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: is only sent if telnetd is compiled with support for authentication, and indicates a willingness to receive authentication information for automatic login.
DO ENCRYPT: is only sent if telnetd is compiled with support for data encryption, and indicates a willingness to decrypt the data stream.

FILES

None.

PORTABILITY

Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022. Windows Server 2025.

The following Internet RFCs are supported:

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

NOTES

Normally, the PTC MKS Toolkit version of telnetd comes with a default license that allows only one person connect to telnetd at a time. Licenses for multiple users are available from MKS.

When connecting to telnetd, you may find that neither the backspace or delete key work correctly to erase characters. You can correct this by using either:

stty erase ^H

stty erase ^?

to set the erase character to either backspace or delete, respectively.

AVAILABILITY

PTC MKS Toolkit for System Administrators
PTC MKS Toolkit for Developers
PTC MKS Toolkit for Interoperability
PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Professional Developers 64-Bit Edition
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition

BUGS

Some TELNET commands are only partially implemented.

Because of bugs in the original 4.2 BSD telnet, telnetd performs some dubious protocol exchanges to try to discover if the remote client is, in fact, a 4.2 BSD telnet.

Binary mode has no common interpretation except between similar operating systems.

The terminal type name received from the remote client is converted to lower case.

telnetd never sends TELNET IAC GA

HISTORY

IPv6 support was added by WIDE/KAME project.

telnetd
DARPA TELNET protocol service

Service

SYNOPSIS

DESCRIPTION