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 indi-
cating 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
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 intelli-
gent flushing of input and output queues.
Telnetd never sends TELNET go ahead commands.
4.2 Berkeley Distribution November 16, 1996 TELNETD(8)