telnetd(8)
NAME
telnetd — DARPA telnet protocol server
SYNOPSIS
/usr/sbin/in.telnetd [-hnNs] [-a authmode] [-D debugmode] [-L loginprg]
[-S tos] [-X authtype] [-edebug] [-debug port]
[-z sslopt]
DESCRIPTION
The telnetd program is a server which supports the DARPA telnet
interactive communication protocol. Telnetd is normally invoked by the
internet server (see inetd(8)) for requests to connect to the telnet port
as indicated 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 program accepts the following options:
-a authmode 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
authentication, which is not available in the current
version. The following values of authmode are understood:
debug Turns 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 without 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 valid.
none This is the default state. Authentication
information is not required. If no or insufficient
authentication information is provided, then the
login(1) program will provide the necessary user
verification.
off This disables the authentication code. All user
verification will happen through the login(1)
program.
-D debugmode
This option may be used for debugging purposes. It allows
telnetd to print out debugging information to the
connection, allowing the user to see what telnetd is doing.
Repeated use of the option arranges composite debug reports.
There are several possible values for debugmode:
options Prints information about the negotiation of telnet
options.
report Prints the options information, plus some
additional information about what processing is
going on.
netdata Displays the data stream received by telnetd.
ptydata Displays data written to the pty.
exercise Has not been implemented yet.
-edebug If telnetd has been compiled with support for encryption,
then the -edebug option may be used to enable encryption
debugging code.
-h Disables the printing of host-specific information before
login has been completed.
-L loginprg This option may be used to specify a different login
program. By default, /usr/lib/telnetlogin is used.
-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.
-N Disable reverse DNS lookups and use the numeric IP address
in logs and REMOTEHOST environment variable.
-s This option is only enabled if telnetd is compiled with
support for SecurID cards. It causes the -s option to be
passed on to login(1), and thus is only useful if login(1)
supports the -s flag to indicate that only SecurID validated
logins are allowed. This is usually useful for controlling
remote logins from outside of a firewall.
-S tos Sets the IP type-of-service (TOS) option for the telnet
connection to the value tos.
-X authtype 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.
-z SSL-parameter
This option is only valid if telnetd has been built with SSL
(Secure Socket Layer) support.
debug Enable SSL related debugging.
ssl Negotiate SSL at first, then use telnet
protocol. In this mode telnetd only accepts
connections from SSL enhanced telnet with option
-z ssl
nossl, !ssl
switch off SSL negotiation
certsok Look username up in /etc/ssl.users. The format
of this file is lines of this form:
user1,user2:/C=US/..... where user1 and user2
are usernames and /C=US/... is the subject name
of the certificate. Use openssl x509 -subject
-noout to extract the subject name. If client
certificate is valid, authenticate without
password.
certrequired
client certificate is mandatory
secure Don't switch back to unencrypted mode (no SSL)
if SSL is not available.
verify=int Set the SSL verify flags (SSL_VERIFY_* in
ssl/ssl.h ).
cert=cert_file
Use the certificate(s) in cert_file.
key=key_file
Use the key(s) in key_file.
cipher=ciph_list
Set the preferred ciphers to ciph_list. (See
ssl/ssl.h ).
If the file /etc/issue.net is present, telnetd will display its contents
before the login prompt of a telnet session (see issue.net(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 the following telnet options,
which are described in more detail below:
DO AUTHENTICATION
WILL ENCRYPT
DO TERMINAL TYPE
DO TSPEED
DO XDISPLOC
DO NEW-ENVIRON
WILL SUPPRESS GO AHEAD
DO ECHO
DO LINEMODE
DO NAWS
WILL STATUS
DO LFLOW
DO TIMING-MARK
The pseudo-terminal allocated to the client is configured to operate in
“cooked” mode, and with XTABS 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 Indicates that the client is willing to send a 8 bits
of data, rather than the normal 7 bits of the Network
Virtual Terminal.
WILL SGA Indicates that it will not be sending IAC GA, go
ahead, commands.
WILL STATUS Indicates 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
data stream.
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
characters remotely.
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 Indicates 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 Indicates 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 Indicates a desire to be able to request information
about the speed of the serial line to which the client
is attached.
DO XDISPLOC Indicates a desire to be able to request the name of
the X windows display that is associated with the
telnet client.
DO NEW-ENVIRON Indicates a desire to be able to request environment
variable information, as described in RFC 1572.
DO LINEMODE Only sent if telnetd is compiled with support for
linemode, and requests that the client do line by line
processing.
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
disable this.
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
data stream.
FILES
/etc/services, /etc/issue.net
SEE ALSO
telnet(1), login(1), issue.net(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
BUGS
Some TELNET commands are only partially implemented.
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.
Telnetd never sends TELNET IAC GA (go ahead) commands.
The source code is not comprehensible.
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