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 pro
vide 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 pro
vide 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 authentica
tion 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 informa
tion 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 ver
ification will happen through the login(1) program.
-D debugmode: 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 several possible values for debugmode:; options Prints information about the negotiation of telnet
options.
report Prints the options information, plus some addi: tional 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 pro: gram. 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 sup: port 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 con: nection 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 proto
col. 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
DO 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 charac: ters 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 ENVIRON Indicates 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 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

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.

docs.sk

comprehensive documentation repository

See also