26.3. Terminals

Contributed by Sean Kelly.

Terminals provide a convenient and low-cost way to access a FreeBSD system when not at the computer's console or on a connected network. This section describes how to use terminals with FreeBSD.

The original UNIX® systems did not have consoles. Instead, users logged in and ran programs through terminals that were connected to the computer's serial ports.

The ability to establish a login session on a serial port still exists in nearly every UNIX®-like operating system today, including FreeBSD. By using a terminal attached to an unused serial port, a user can log in and run any text program that can normally be run on the console or in an xterm window.

Many terminals can be attached to a FreeBSD system. An older spare computer can be used as a terminal wired into a more powerful computer running FreeBSD. This can turn what might otherwise be a single-user computer into a powerful multiple-user system.

FreeBSD supports three types of terminals:

Dumb terminals

Dumb terminals are specialized hardware that connect to computers over serial lines. They are called dumb because they have only enough computational power to display, send, and receive text. No programs can be run on these devices. Instead, dumb terminals connect to a computer that runs the needed programs.

There are hundreds of kinds of dumb terminals made by many manufacturers, and just about any kind will work with FreeBSD. Some high-end terminals can even display graphics, but only certain software packages can take advantage of these advanced features.

Dumb terminals are popular in work environments where workers do not need access to graphical applications.

Computers Acting as Terminals

Since a dumb terminal has just enough ability to display, send, and receive text, any spare computer can be a dumb terminal. All that is needed is the proper cable and some terminal emulation software to run on the computer.

This configuration can be useful. For example, if one user is busy working at the FreeBSD system's console, another user can do some text-only work at the same time from a less powerful personal computer hooked up as a terminal to the FreeBSD system.

There are at least two utilities in the base-system of FreeBSD that can be used to work through a serial connection: cu(1) and tip(1).

For example, to connect from a client system that runs FreeBSD to the serial connection of another system:

# cu -l serial-port-device

Replace serial-port-device with the device name of the connected serial port. These device files are called /dev/cuauN on FreeBSD versions 10.x and higher and /dev/cuadN on FreeBSD versions 9.x and lower. In either case, N is the serial port number, starting from zero. This means that COM1 is /dev/cuau0 or /dev/cuad0 in FreeBSD.

Additional programs are available through the Ports Collection, such as comms/minicom.

X Terminals

X terminals are the most sophisticated kind of terminal available. Instead of connecting to a serial port, they usually connect to a network like Ethernet. Instead of being relegated to text-only applications, they can display any Xorg application.

This chapter does not cover the setup, configuration, or use of X terminals.

26.3.1. Terminal Configuration

This section describes how to configure a FreeBSD system to enable a login session on a serial terminal. It assumes that the system recognizes the serial port to which the terminal is connected and that the terminal is connected with the correct cable.

In FreeBSD, init reads /etc/ttys and starts a getty process on the available terminals. The getty process is responsible for reading a login name and starting the login program. The ports on the FreeBSD system which allow logins are listed in /etc/ttys. For example, the first virtual console, ttyv0, has an entry in this file, allowing logins on the console. This file also contains entries for the other virtual consoles, serial ports, and pseudo-ttys. For a hardwired terminal, the serial port's /dev entry is listed without the /dev part. For example, /dev/ttyv0 is listed as ttyv0.

The default /etc/ttys configures support for the first four serial ports, ttyu0 through ttyu3:

ttyu0   "/usr/libexec/getty std.9600"   dialup  off secure
ttyu1   "/usr/libexec/getty std.9600"   dialup  off secure
ttyu2   "/usr/libexec/getty std.9600"   dialup  off secure
ttyu3   "/usr/libexec/getty std.9600"   dialup  off secure

When attaching a terminal to one of those ports, modify the default entry to set the required speed and terminal type, to turn the device on and, if needed, to change the port's secure setting. If the terminal is connected to another port, add an entry for the port.

Example 26.1, “Configuring Terminal Entries” configures two terminals in /etc/ttys. The first entry configures a Wyse-50 connected to COM2. The second entry configures an old computer running Procomm terminal software emulating a VT-100 terminal. The computer is connected to the sixth serial port on a multi-port serial card.

Example 26.1. Configuring Terminal Entries
ttyu11  "/usr/libexec/getty std.38400"2  wy503  on4  insecure5
ttyu5   "/usr/libexec/getty std.19200"  vt100  on insecure

1

The first field specifies the device name of the serial terminal.

2

The second field tells getty to initialize and open the line, set the line speed, prompt for a user name, and then execute the login program. The optional getty type configures characteristics on the terminal line, like bps rate and parity. The available getty types are listed in /etc/gettytab. In almost all cases, the getty types that start with std will work for hardwired terminals as these entries ignore parity. There is a std entry for each bps rate from 110 to 115200. Refer to gettytab(5) for more information.

When setting the getty type, make sure to match the communications settings used by the terminal. For this example, the Wyse-50 uses no parity and connects at 38400 bps. The computer uses no parity and connects at 19200 bps.

3

The third field is the type of terminal. For dial-up ports, unknown or dialup is typically used since users may dial up with practically any type of terminal or software. Since the terminal type does not change for hardwired terminals, a real terminal type from /etc/termcap can be specified. For this example, the Wyse-50 uses the real terminal type while the computer running Procomm is set to emulate a VT-100.

4

The fourth field specifies if the port should be enabled. To enable logins on this port, this field must be set to on.

5

The final field is used to specify whether the port is secure. Marking a port as secure means that it is trusted enough to allow root to login from that port. Insecure ports do not allow root logins. On an insecure port, users must login from unprivileged accounts and then use su or a similar mechanism to gain superuser privileges, as described in Section 4.3.1.3, “The Superuser Account”. For security reasons, it is recommended to change this setting to insecure.


After making any changes to /etc/ttys, send a SIGHUP (hangup) signal to the init process to force it to re-read its configuration file:

# kill -HUP 1

Since init is always the first process run on a system, it always has a process ID of 1.

If everything is set up correctly, all cables are in place, and the terminals are powered up, a getty process should now be running on each terminal and login prompts should be available on each terminal.

26.3.2. Troubleshooting the Connection

Even with the most meticulous attention to detail, something could still go wrong while setting up a terminal. Here is a list of common symptoms and some suggested fixes.

If no login prompt appears, make sure the terminal is plugged in and powered up. If it is a personal computer acting as a terminal, make sure it is running terminal emulation software on the correct serial port.

Make sure the cable is connected firmly to both the terminal and the FreeBSD computer. Make sure it is the right kind of cable.

Make sure the terminal and FreeBSD agree on the bps rate and parity settings. For a video display terminal, make sure the contrast and brightness controls are turned up. If it is a printing terminal, make sure paper and ink are in good supply.

Use ps to make sure that a getty process is running and serving the terminal. For example, the following listing shows that a getty is running on the second serial port, ttyu1, and is using the std.38400 entry in /etc/gettytab:

# ps -axww|grep ttyu
22189  d1  Is+    0:00.03 /usr/libexec/getty std.38400 ttyu1

If no getty process is running, make sure the port is enabled in /etc/ttys. Remember to run kill -HUP 1 after modifying /etc/ttys.

If the getty process is running but the terminal still does not display a login prompt, or if it displays a prompt but will not accept typed input, the terminal or cable may not support hardware handshaking. Try changing the entry in /etc/ttys from std.38400 to 3wire.38400, then run kill -HUP 1 after modifying /etc/ttys. The 3wire entry is similar to std, but ignores hardware handshaking. The baud rate may need to be reduced or software flow control enabled when using 3wire to prevent buffer overflows.

If garbage appears instead of a login prompt, make sure the terminal and FreeBSD agree on the bps rate and parity settings. Check the getty processes to make sure the correct getty type is in use. If not, edit /etc/ttys and run kill -HUP 1.

If characters appear doubled and the password appears when typed, switch the terminal, or the terminal emulation software, from half duplex or local echo to full duplex.

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