TELNETD(8) BSD System Manager's Manual TELNETD(8)NAMEtelnetd — DARPA TELNET protocol server
/usr/libexec/telnetd [-46BUhlkn] [-D debugmode] [-S tos] [-X authtype]
[-a authmode] [-edebug] [-p loginprog] [-u len]
The telnetd command is a server which supports the DARPA standard TELNET
virtual terminal protocol. Telnetd is normally invoked by the internet
server (see inetd(8)) for requests to connect to the TELNET port as indi‐
cated by the /etc/services file (see services(5)). The -debug option may
be used to start up telnetd manually, instead of through inetd(8). If
started up this way, port may be specified to run telnetd on an alternate
TCP port number.
The telnetd command accepts the following options:
-4 Forces telnetd to use IPv4 addresses only.
-6 Forces telnetd to use IPv6 addresses only.
This option may be used for specifying what mode should be used
for authentication. Note that this option is only useful if
telnetd has been compiled with support for the AUTHENTICATION
option. There are several valid values for authmode:
debug Turn on authentication debugging code.
user Only allow connections when the remote user can provide
valid authentication information to identify the remote
user, and is allowed access to the specified account with‐
out providing a password.
valid Only allow connections when the remote user can provide
valid authentication information to identify the remote
user. The login(1) command will provide any additional
user verification needed if the remote user is not allowed
automatic access to the specified account.
other Only allow connections that supply some authentication
information. This option is currently not supported by
any of the existing authentication mechanisms, and is thus
the same as specifying -a valid.
none This is the default state. Authentication information is
not required. If no or insufficient authentication infor‐
mation is provided, then the login(1) program will provide
the necessary user verification.
off Disable the authentication code. All user verification
will happen through the login(1) program.
-B Specify bftp server mode. In this mode, telnetd causes login to
start a bftp(1) session rather than the user's normal shell. In
bftp daemon mode normal logins are not supported, and it must be
used on a port other than the normal TELNET port.
This option may be used for debugging purposes. This allows
telnetd to print out debugging information to the connection,
allowing the user to see what telnetd is doing. There are sev‐
eral possible values for debugmode:
options Print information about the negotiation of TELNET
report Print the options information, plus some additional
information about what processing is going on.
netdata Display the data stream received by telnetd.
ptydata Display data written to the pty.
exercise Has not been implemented yet.
-debug Enable debugging on each socket created by telnetd (see SO_DEBUG
If telnetd has been compiled with support for data encryption,
then the -edebug option may be used to enable encryption debug‐
-h Disable the printing of host-specific information before login
has been completed.
-k This option is only useful if telnetd has been compiled with both
linemode and kludge linemode support. If the -k option is speci‐
fied, then if the remote client does not support the LINEMODE
option, then telnetd will operate in character at a time mode.
It will still support kludge linemode, but will only go into
kludge linemode if the remote client requests it. (This is done
by the client sending DONT SUPPRESS-GO-AHEAD and DONT ECHO.) The
-k option is most useful when there are remote clients that do
not support kludge linemode, but pass the heuristic (if they
respond with WILL TIMING-MARK in response to a DO TIMING-MARK)
for kludge linemode support.
-l Specify line mode. Try to force clients to use line-at-a-time
mode. If the LINEMODE option is not supported, it will go into
-n Disable TCP keep-alives. Normally telnetd enables the TCP keep-
alive mechanism to probe connections that have been idle for some
period of time to determine if the client is still there, so that
idle connections from machines that have crashed or can no longer
be reached may be cleaned up.
Specify an alternate login(1) command to run to complete the
login. The alternate command must understand the same command
arguments as the standard login.
-S tos Sets the IP type-of-service (TOS) option for the telnet connec‐
tion to the value tos, which can be a numeric TOS value or, on
systems that support it, a symbolic TOS name found in the
-u len This option is used to specify the size of the field in the utmp
structure that holds the remote host name. If the resolved host
name is longer than len, the dotted decimal value will be used
instead. This allows hosts with very long host names that over‐
flow this field to still be uniquely identified. Specifying -u0
indicates that only dotted decimal addresses should be put into
the utmp file.
-U This option causes telnetd to refuse connections from addresses
that cannot be mapped back into a symbolic name via the
This option is only valid if telnetd has been built with support
for the authentication option. It disables the use of authtype
authentication, and can be used to temporarily disable a specific
authentication type without having to recompile telnetd. Avail‐
able authtype values include KERBEROS_V4, KERBEROS_V5, SPX, MINK,
and SRA. These options are completely independent of the -a
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 the following TELNET options,
which are described in more detail below:
DO TERMINAL TYPE
WILL SUPPRESS GO AHEAD
The pseudo-terminal allocated to the client is configured to operate in
“cooked” mode, and with XTABS and CRMOD enabled (see tty(4)).
Telnetd has support for enabling locally the following TELNET options:
WILL ECHO When the LINEMODE option is enabled, a WILL ECHO or
WONT ECHO will be sent to the client to indicate the
current state of terminal echoing. When terminal echo
is not desired, a WILL ECHO is sent to indicate that
telnetd will take care of echoing any data that needs
to be echoed to the terminal, and then nothing is
echoed. When terminal echo is desired, a WONT ECHO is
sent to indicate that telnetd will not be doing any
terminal echoing, so the client should do any terminal
echoing that is needed.
WILL BINARY Indicate that the client is willing to send a 8 bits
of data, rather than the normal 7 bits of the Network
WILL SGA Indicate that it will not be sending IAC GA, go ahead,
WILL STATUS Indicate a willingness to send the client, upon
request, of the current status of all TELNET options.
WILL TIMING-MARK Whenever a DO TIMING-MARK command is received, it is
always responded to with a WILL TIMING-MARK.
WILL LOGOUT When a DO LOGOUT is received, a WILL LOGOUT is sent in
response, and the TELNET session is shut down.
WILL ENCRYPT Only sent if telnetd is compiled with support for data
encryption, and indicates a willingness to decrypt the
Telnetd has support for enabling remotely the following TELNET options:
DO BINARY Sent to indicate that telnetd is willing to receive an
8 bit data stream.
DO LFLOW Requests that the client handle flow control charac‐
DO ECHO This is not really supported, but is sent to identify
a 4.2BSD telnet(1) client, which will improperly
respond with WILL ECHO. If a WILL ECHO is received, a
DONT ECHO will be sent in response.
DO TERMINAL-TYPE Indicate a desire to be able to request the name of
the type of terminal that is attached to the client
side of the connection.
DO SGA Indicate that it does not need to receive IAC GA, the
go ahead command.
DO NAWS Requests that the client inform the server when the
window (display) size changes.
DO TERMINAL-SPEED Indicate a desire to be able to request information
about the speed of the serial line to which the client
DO XDISPLOC Indicate a desire to be able to request the name of
the X Window System display that is associated with
the telnet client.
DO NEW-ENVIRON Indicate a desire to be able to request environment
variable information, as described in RFC 1572.
DO ENVIRON Indicate a desire to be able to request environment
variable information, as described in RFC 1408.
DO LINEMODE Only sent if telnetd is compiled with support for
linemode, and requests that the client do line by line
DO TIMING-MARK Only sent if telnetd is compiled with support for both
linemode and kludge linemode, and the client responded
with WONT LINEMODE. If the client responds with WILL
TM, the it is assumed that the client supports kludge
linemode. Note that the [-k] option can be used to
DO AUTHENTICATION Only sent if telnetd is compiled with support for
authentication, and indicates a willingness to receive
authentication information for automatic login.
DO ENCRYPT Only sent if telnetd is compiled with support for data
encryption, and indicates a willingness to decrypt the
By default telnetd will read the he, hn, and im capabilities from
/etc/gettytab and use that information (if present) to determine what to
display before the login: prompt. You can also use a System V style
/etc/issue file by using the if capability, which will override im. The
information specified in either im or if will be displayed to both con‐
sole and remote logins.
/etc/iptos (if supported)
/usr/ucb/bftp (if supported)
SEE ALSObftp(1), login(1), telnet(1) (if supported), gettytab(5)STANDARDS
RFC-854 TELNET PROTOCOL SPECIFICATION
RFC-855 TELNET OPTION SPECIFICATIONS
RFC-856 TELNET BINARY TRANSMISSION
RFC-857 TELNET ECHO OPTION
RFC-858 TELNET SUPPRESS GO AHEAD OPTION
RFC-859 TELNET STATUS OPTION
RFC-860 TELNET TIMING MARK OPTION
RFC-861 TELNET EXTENDED OPTIONS - LIST OPTION
RFC-885 TELNET END OF RECORD OPTION
RFC-1073 Telnet Window Size Option
RFC-1079 Telnet Terminal Speed Option
RFC-1091 Telnet Terminal-Type Option
RFC-1096 Telnet X Display Location Option
RFC-1123 Requirements for Internet Hosts -- Application and Support
RFC-1184 Telnet Linemode Option
RFC-1372 Telnet Remote Flow Control Option
RFC-1416 Telnet Authentication Option
RFC-1411 Telnet Authentication: Kerberos Version 4
RFC-1412 Telnet Authentication: SPX
RFC-1571 Telnet Environment Option Interoperability Issues
RFC-1572 Telnet Environment Option
IPv6 support was added by WIDE/KAME project.
Some TELNET commands are only partially implemented.
Because of bugs in the original 4.2BSD telnet(1), telnetd performs some
dubious protocol exchanges to try to discover if the remote client is, in
fact, a 4.2BSD 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
Telnetd never sends TELNET IAC GA (go ahead) commands.
BSD August 28, 2008 BSD