protocol server

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() 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() program will provide the necessary user verification.

off

This disables the authentication code. All user verification will happen through the login() 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 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() 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 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 characters remotely.

DO ECHO

This is not really supported, but is sent to identify a 4.2BSD telnet() 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.

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

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.