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