NAME

telnetd − DARPA TELNET protocol server

SYNOPSIS

/etc/telnetd [ −Xdfsw ]

DESCRIPTION

The telnetd server supports the DARPA standard TELNET virtual terminal protocol.  The telnetd server is invoked by the internet server (see inetd(1M)), normally for requests to connect to the TELNET port as indicated by the /etc/services file (see services(5)).

The telnetd server operates by allocating a pseudo-terminal device (see pty(7)) 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 that it is ready to do remote echo of characters, to suppress go-ahead, and to receive terminal type information from the remote client.  If the remote client is willing, the remote terminal type is propagated in the environment of the created login process.  The pseudo-terminal allocated to the client is configured to operate in “canonical” mode and with TAB3, CS7, OCRNL, and ONLCR enabled (see termio(7)).

telnetd is willing to do echo, binary, suppress go-ahead, negotiate flow control, and timing mark.  telnetd is willing to have the remote client do binary, terminal type, X display location, speed, window size, and suppress go-ahead. 

If any of the options negotiate flow control, X display location, and speed cause problems with clients, the server can be told to not volunteer the options with the −X option followed by any of the letters f, d, s, or w, which represent the options flow control, display location, terminal speed, and window size, respectively.

SEE ALSO

inetd(1M), telnet(1C), services(5), pty(7). 
termio(7) in the INTERACTIVE UNIX System User’s/System Administrator’s Reference Manual.

BUGS

Some TELNET commands are only partially implemented. 

The TELNET protocol allows for the exchange of the number of lines and columns on the user’s terminal, but telnetd does not make use of them. 

Because of bugs in the original 4.2BSD telnet(1C), telnetd performs some dubious protocol exchanges to try to discover if the remote client is, in fact, a 4.2BSD telnet(1C).

Binary mode has no common interpretation except between similar operating systems (the UNIX System in this case). 
The terminal type name received from the remote client is converted to lowercase letters.

The telnetd server never sends TELNET go-ahead commands. 

\*U  —  Version 1.0