Ubuntu Manpage: telnetd — DARPA telnet protocol server

Provided by: telnetd-ssl_0.17.41+0.2-3build1_amd64

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.

debug=log_file
Select in addition a specific location log_file for collecting
debug output, thus overriding the default file
/var/tmp/telnetd.log.

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 the 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. (See SSL_VERIFY_* in openssl/ssl.h ).

cacert=CA_file
A collection of trusted authority certificates for verification of
whatever the clients care to send as identifiers.

cert=cert_file
Present the certificate(s) in cert_file to any client. They are in
PEM-format, and the first certificate identifies the server itself.

key=key_file
Use the key(s) in key_file in case a key is not stored together
with the certificate.

cipher=ciph_list
Set the preferred ciphers to ciph_list. The environment variable
SSL_CIPHER serves the same purpose. (See openssl/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, /etc/ssl.users

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