telnet, tn, or tn3270 Command

Purpose

Connects the local host with a remote host, using the Telnet interface.

Syntax

{ telnet | tn | tn3270 } [ -d ] [ -p ] [ -n TraceFile ] [ -e TerminalType ] [ -f | -F ] [ -k realm ] [ -l user ] [ Host [ Port ] ]

Description

The telnet command, which is also referred to as the tn or tn3270 command, operates in two different modes: command mode and input mode.

System

The user is assigned a default login Sensitivity Label (SL) and Integrity Label (TL), which is SL and TL of the user's process after successful login.

If the user does not want to login using the default login SL, the user can choose to supply a different SL at the login time using the -e option. The SL that is supplied by the user must be dominated by the user's clearance and contained in the system accreditation range. The TL cannot be specified by the user at login time. The default login SL and TL are defined in the /etc/security/user file along with the username and clearance for each user. To use the -e option, the server side's kernel trusted network bit must be turned off.

Command Mode

When the telnet command is issued without arguments, it enters command mode, as indicated by the telnet>, tn>, or the tn3270> prompt. A user can also enter command mode from input mode by pressing Ctrl-] for the telnet command, Ctrl-T for the tn command, or Ctrl-C for the tn3270 command. In command mode, subcommands can be entered to manage the remote system. Some of these subcommands return you to the remote session upon completion. For those subcommands that do not, pressing the Enter key returns you to the remote session.

To enter telnet command mode while connected to a remote host, type the Telnet escape key sequence. When in command mode, the standard operating system editing conventions, such as backspace, are available.

Input Mode

When the telnet command is issued with arguments, it performs an open subcommand with those arguments and then enters input mode. The type of input mode is either character-at-a-time or line-by-line, depending on what the remote system supports. In character-at-a-time mode, most text that is typed is immediately sent to the remote host for processing. In line-by-line mode, all text is echoed locally and completed lines are sent to the remote host.

In either input mode, if the toggle localchars subcommand has a value of True, the user's QUIT, INTR, and FLUSH characters are trapped locally and sent as Telnet Protocol sequences to the remote host. The toggle autoflush and toggle autosynch subcommands cause this action to flush subsequent output to the terminal until the remote host acknowledges the Telnet sequence and to flush previous terminal input (in the case of QUIT and INTR characters).

Arabic/Hebrew Support

The telnet, tn, and tn3270 command supports the Arabic and Hebrew texts, allowing the user to type Arabic or Hebrew characters while in an emulation session. The Ar_AA locale displays the Arabic characters in their correct shapes. The following functions support the bidirectional Arabic and Hebrew texts:

Language Selection

This function allows you to toggle the language layer. Activate the Arabic/Hebrew language selection with the following key combinations:

Activate the Latin language layer with the following key combinations:

Screen Reverse

This function reverses the screen image and invokes the default language of the new screen orientation. Thus, if the screen is reversed to right-to-left, the language is changed to Arabic/Hebrew. If the screen is reversed to left-to-right, the language is changed to Latin.

If symmetric character swapping is enabled, reversing the screen causes bidirectional characters to be replaced by their counterparts. For example, if numeric character swapping is enabled, reversing the screen causes Hindi numerals to be replaced by their Arabic counterparts and the Arabic numerals to be replaced by their Hindi counterparts.

Activate screen reverse with the following key combinations:

Push/End Push

The Push function allows you to edit text whose direction is opposite the screen orientation. When you activate this function, the cursor orientation is reversed, the language layer is changed accordingly, and a Push segment is created.

The Push function has two secondary modes:

Activate this function with the following key combinations:

The End Push function terminates the Push function. The cursor jumps to the end of the Push segment and its direction changes to the original direction. You can activate End Push by pressing any field exit keys such as cursor up, cursor down, or any attention identifier (AID) key such as the Enter key. You can also activate this function with the following key combinations:

Field Reverse

This function toggles the field orientation to either the opposite of or the same as the screen orientation. This function does not invert the text in the field. The cursor orientation is set to the new field orientation and the language layer is selected accordingly.

For example, if the cursor is in the first logical position of a field or line when you activate the field reverse function, the cursor skips to the opposite side of that field or line. This position is now the first logical position. If the cursor is not in the first position of the field or line when you activate field reverse function, the cursor remains in its position and allows natural and correct editing of the existing text. Activate this function with the following key combinations:

Autopush

This function assists you in typing mixed left-to-right and right-to-left text. When enabled, reversed segments are automatically initiated and terminated according to the typed characters or the selected language layer. Thus, this mode automatically invokes the Push mode and relieves you of invoking the Push function.

When you type a digit or Latin character in a right-to-left field, the Autopush function automatically initiates the Push function without changing the language. If you type additional digits or Latin character, the Push function continues; otherwise, the Push function automatically terminates. Thus, you can type Arabic/Hebrew text with embedded digits or Latin characters without invoking the Push/End Push functions.

When you type an Arabic/Hebrew character in a left-to-right field, the Autopush function automatically initiates the Push function without a language change. If you then type a digit or Latin character, the Autopush function automatically terminates. Thus, you can type Latin text with embedded Arabic/Hebrew text using the Language Selection function rather than the Push/End Push functions.

Activate this function with the following key combinations:

Field Shape

This function shapes the Arabic characters in the current field or line. Activate this function with the following key combinations:

Field Deshape

This function deshapes Arabic text in the current field or line. Activate this function with the following key combinations:

Contextual Shape Determination

This function determines the shape of an Arabic character based on the surrounding text. Use the Contextual Shape Determination function only when typing or editing right-to-left text. This function is terminated when any of the specific shape selection keys is pressed. This is the default function. Activate this function with the following key combinations:

Initial Shape Determination

This function shapes Arabic characters in their initial shapes. Activate this function with the following key combinations:

Middle Shape Determination

This function shapes Arabic characters in their middle shapes. Activate this function with the following key combinations:

Isolated Shape Determination

This function shapes Arabic characters in their isolated shapes. Activate this function with the following key combinations:

Final Shape Determination

This function shapes Arabic characters in their final shapes. Activate this function with the following key combinations:

Miscellaneous Functions

To activate numeric swapping, type the following line at the command line:

export ARB_NUM_SWAP=1 

To activate symmetric swapping, that is, to swap bidirectional characters such as braces, brackets, and so on, type the following line at the command line:

export ARB_SYM_SWAP=1

To specify the code page that the host uses, type the following line at the command line:

export RM_HOST_LANG=IBM-420

Terminal Type Negotiation

The telnet command negotiates the terminal type, using the Telnet protocol, and it sets the TERM environment variable according to what has been negotiated.

To override the terminal negotiation from the console, use the EMULATE environment variable or the -e flag; or invoke the tn3270 command if you require 3270 emulation. To determine whether terminal-type negotiation is performed, the following list describes the order of the telnet command processing:

1. The -e command-line flag. (No negotiation.)
2. The EMULATE environment variable. (No negotiation.)
3. The tn3270 command. (No negotiation.)
4. If steps 1, 2, and 3 are not present, terminal-type negotiation occurs automatically.

If the client and the server negotiate to use a 3270 data stream, the keyboard mapping is determined by the following precedence:

Secure Attention Key (SAK) Option

In addition to terminal negotiation, the telnet command allows negotiation for the Secure Attention Key (SAK) option. This option, when supported, provides the local user with a secure communication path to the remote host for tasks such as changing user IDs or passwords. If the remote host supports the SAK function, a trusted shell is opened on the remote host when the telnet send sak subcommand is issued. The SAK function can also be assigned to a single key available in telnet input mode, using the set sak subcommand.

End-of-Line Convention

The Telnet protocol defines the carriage-return line-feed (CR-LF) sequence to mean "end-of-line." For terminal input, this corresponds to a command-completion or end-of-line key being pressed on a user terminal. On an ASCII terminal, this is the CR key, but it may also be labeled "Return" or "Enter."

When a Telnet server receives the Telnet end-of-line sequence, CR-LF, as input from a remote terminal, the effect is the same as if the user had pressed the end-of-line key on a local terminal.

On ASCII servers, receiving the Telnet sequence CR-LF causes the same effect as a local user pressing the CR key on a local terminal. CR-LF and CR-NUL have the same effect on an ASCII server when received as input over a Telnet connection.

A Telnet user on an ASCII host should have a user-controllable mode to send either CR-LF or CR-NULL when the user presses the end-of-line key. The CR-LF should be the default. The Telnet end-of-line sequence, CR-LF, must be used to send Telnet data that is not terminal-to-computer. This occurs, for example, when a Telnet server sends output or when the Telnet protocol incorporates another application protocol.

The telnet command "execs" (using the exec command) the /usr/sbin/login command to validate a user. This 1) allows all user and device attributes to take effect on telnet connections and 2) causes telnet connections to count against the maximum number of login sessions allowable at a time (determined by the maxlogins attribute). Attributes are defined in the /etc/security/user and /etc/security/login.cfg files.

Restrictions

• Earlier versions of the telnet command are not compatible with AIX Version 4 and later of the telnet command in sending escapes that emulate a high function terminal (HFT). AIX Version 4 and later of the telnet command sends only one escape when the escape key is hit, while prior versions send two escape characters.
• The telnet command must allow transmission of 8-bit characters that are not in binary mode to implement ISO 8859 Latin code page. This is necessary for internationalization of the TCP/IP commands.
• In order to support new character sets, the following was added to the hft-m, ibm5081, hft, hft-nam, hft-c, aixterm-m, and aixterm entries in the terminfo file:

  box1=\154\161\153\170\152\155\167\165\166\164\156,     batt1=f1,
  box2=\154\161\153\170\152\155\167\165\166\164\156,     batt2=f1md,
  font0=\E(B,     font1=\E(0,

• The rlogind and telnetd daemons use POSIX line discipline to change the line discipline on the local TTY. If POSIX line discipline is not used on the local TTY, echoing other line disciplines may result in improper behavior. AIX TCP/IP must have POSIX line discipline to function properly.
• The mouse cannot be used as an input device with the telnet command.
• The telnet command does not support the APL data stream.

Environment Variables

The following environment variables can be used with the telnet command:

Flags

Subcommands

Before entering each subcommand, press the escape key sequence. The escape sequence tells the program that non-text information follows. Otherwise, the program interprets subcommands as text.

For each of the subcommands in the following list, you only need to type enough letters to uniquely identify the subcommand. (For example, q is sufficient for the quit subcommand.) This is also true for the arguments to the display, emulate, mode, set, and toggle subcommands.

The telnet subcommands are:

Authentication

If the system is configured for Kerberos 5 authentication, the telnet client will attempt authentication negotiation. The authentication negotiation used by telnet and the definitions of the options and suboptions for this are defined in rfc 1416.

If the client and server agree on an authentication type, they will exchange authentication information including the account the client wants to access. This will be the local user unless the -l flag is set.

If they cannot agree on the authentication information or if it fails, the telnet connection will continue with the standard connection (provided Standard AIX is configured).

The remote host allows access only if all of the following conditions are satisfied:

• The local user has current DCE credentials.
• The remote system accepts the DCE credentials as sufficient for access to the remote account. See the kvalid_user function for additional information.

Security

Examples

In the following examples, if you enter the tn command instead of the telnet command, the command mode prompt is displayed as tn>.

1. To log in to the remote host host1 and perform terminal negotiation, enter:

   telnet host1

2. To log in to host1 as a vt100 terminal (no terminal type negotiation), choose one of the following methods:
   1. Use the following commands to set the EMULATE environment variable for this login session, then enter the telnet command:

      EMULATE=vt100; export EMULATE
      telnet host1

   2. Use the -e flag to set the terminal type for this telnet session only:

      telnet  -e vt100 host1

3. To log in to a remote host and then check the status of the telnet program, enter:

   telnet host3

   When the login prompt appears, enter your login ID and password. Press the Ctrl-T key sequence to receive the telnet> prompt. Enter the following at the telnet> prompt:

   status

   Information similar to the following is displayed on your screen:

   Connected to host3.
   Operating in character-at-a-time mode.
   Escape character is '^]'.

   Upon completion of the status subcommand, press the Enter key to return to the remote prompt.

   Once you have completed your login, you can issue commands. To log out of the system and close the connection, press the Ctrl-D key sequence, or exit.

4. To log in to a remote host using the tn3270 command, enter:

   tn3270 hostname

   The host login screen should be displayed. You can now enter your login ID and password. Once you have completed your login, you can issue commands. To log out of the system and close the connection, press Ctrl-D or exit.
5. To connect to the icehouse.austin.ibm.com remote host with the telnet command with a user name david of specific SLs sec a b, enter the following commands:
   1. In the command line, enter telnet icehouse.aoot.austin.ibm.com to connect to the icehouse.austin.ibm.com
   2. In the login field, enter david -e "sec a b"
   3. In the passwords field, enter david's passwords.

   To disconnect from the remote server, use the Ctrl-T key sequence.

Files