NAME
telnetd - DARPA TELNET protocol server

SYNOPSIS
/etc/telnetd

DESCRIPTION
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.

SEE ALSO
telnet(1)

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 doesn’t make use of
them.

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

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.