telnetd, in.telnetd - TCP/IP TELNET protocol server
This program is available with the Networking software installation
option. Refer to for information on how to install optional software.
telnetd is a server which supports the TCP/IP standard TELNET virtual
terminal protocol. telnetd is invoked by the internet server (see
inetd(8C)), 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 its standard input, output, and error. 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 charac-
ters, 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, ICRNL, and ONLCR enabled (see
telnetd is willing to do: echo, binary, suppress go ahead, and timing
mark. telnetd is willing to have the remote client do: binary, termi-
nal type, and suppress go ahead.
Postel, Jon, and Joyce Reynolds, ``Telnet Protocol Specification,'' RFC
854, Network Information Center, SRI International, Menlo Park, Calif.,
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 operat-
The terminal type name received from the remote client is converted to
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.
telnetd can only support 64 pseudo-terminals.
18 December 1989 TELNETD(8C)