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BSD 2.11 - man page for telnetd (bsd section 8)

TELNETD(8)				       System Manager's Manual					   TELNETD(8)

telnetd - DARPA TELNET protocol server
Telnetd is a server which supports the DARPA standard TELNET virtual terminal protocol. Telnetd is invoked by the internet server (see inetd(8)), normally for requests to connect to the TELNET port as indicated by the /etc/services file (see services(5)). Telnetd operates by allocating a pseudo-terminal device (see pty(4)) 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 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 "cooked" mode, and with XTABS and CRMOD enabled (see tty(4)). Telnetd is willing to do: echo, binary, suppress go ahead, and timing mark. Telnetd is willing to have the remote client do: binary, terminal type, and suppress go ahead.
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 doesn't make use of them. Because of bugs in the original 4.2 BSD telnet(1C), telnetd performs some dubious protocol exchanges to try to discover if the remote client is, in fact, a 4.2 BSD telnet(1C). 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. The packet interface to the pseudo-terminal (see pty(4)) should be used for more intelligent flushing of input and output queues. Telnetd never sends TELNET go ahead commands. 4.2 Berkeley Distribution November 16, 1996 TELNETD(8)

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