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 :

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.

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.

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:

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.

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 :

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.”

As with terminals, init spawns a getty process for each configured serial port used for dial-in connections. When a user dials the modem’s line and the modems connect, the “Carrier Detect” signal is reported by the modem. The kernel notices that the carrier has been detected and instructs getty to open the port and display a prompt at the specified initial line speed. In a typical configuration, if garbage characters are received, usually due to the modem’s connection speed being different than the configured speed, getty tries adjusting the line speeds until it receives reasonable characters. After the user enters their login name, getty executes login, which completes the login process by asking for the user’s password and then starting the user’s shell.

There are two schools of thought regarding dial-up modems. One configuration method is to set the modems and systems so that no matter at what speed a remote user dials in, the dial-in RS-232 interface runs at a locked speed. The benefit of this configuration is that the remote user always sees a system login prompt immediately. The downside is that the system does not know what a user’s true data rate is, so full-screen programs like Emacs will not adjust their screen-painting methods to make their response better for slower connections.

The second method is to configure the RS-232 interface to vary its speed based on the remote user’s connection speed. Because getty does not understand any particular modem’s connection speed reporting, it gives a message at an initial speed and watches the characters that come back in response. If the user sees junk, they should press Enter until they see a recognizable prompt. If the data rates do not match, getty sees anything the user types as junk, tries the next speed, and gives the prompt again. This procedure normally only takes a keystroke or two before the user sees a good prompt. This login sequence does not look as clean as the locked-speed method, but a user on a low-speed connection should receive better interactive response from full-screen programs.

When locking a modem’s data communications rate at a particular speed, no changes to /etc/gettytab should be needed. However, for a matching-speed configuration, additional entries may be required in order to define the speeds to use for the modem. This example configures a 14.4 Kbps modem with a top interface speed of 19.2 Kbps using 8-bit, no parity connections. It configures getty to start the communications rate for a V.32bis connection at 19.2 Kbps, then cycles through 9600 bps, 2400 bps, 1200 bps, 300 bps, and back to 19.2 Kbps. Communications rate cycling is implemented with the nx= (next table) capability. Each line uses a tc= (table continuation) entry to pick up the rest of the settings for a particular data rate.

For a 28.8 Kbps modem, or to take advantage of compression on a 14.4 Kbps modem, use a higher communications rate, as seen in this example:

For a slow CPU or a heavily loaded system without 16550A-based serial ports, this configuration may produce sio“silo” errors at 57.6 Kbps.

The configuration of /etc/ttys is similar to Configuring Terminal Entries, but a different argument is passed to getty and dialup is used for the terminal type. Replace xxx with the process init will run on the device:

The dialup terminal type can be changed. For example, setting vt102 as the default terminal type allows users to use VT102 emulation on their remote systems.

For a locked-speed configuration, specify the speed with a valid type listed in /etc/gettytab . This example is for a modem whose port speed is locked at 19.2 Kbps:

In a matching-speed configuration, the entry needs to reference the appropriate beginning “auto-baud” entry in /etc/gettytab . To continue the example for a matching-speed modem that starts at 19.2 Kbps, use this entry:

After editing /etc/ttys , wait until the modem is properly configured and connected before signaling init:

High-speed modems, like V.32, V.32bis, and V.34 modems, use hardware (RTS/CTS) flow control. Use stty to set the hardware flow control flag for the modem port. This example sets the crtscts flag on COM2 's dial-in and dial-out initialization devices:

This section provides a few tips for troubleshooting a dial-up modem that will not connect to a FreeBSD system.

Hook up the modem to the FreeBSD system and boot the system. If the modem has status indication lights, watch to see whether the modem’s DTR indicator lights when the prompt appears on the system’s console. If it lights up, that should mean that FreeBSD has started a getty process on the appropriate communications port and is waiting for the modem to accept a call.

If the DTR indicator does not light, login to the FreeBSD system through the console and type ps ax to see if FreeBSD is running a getty process on the correct port:

If the second column contains a d0 instead of a ?? and the modem has not accepted a call yet, this means that getty has completed its open on the communications port. This could indicate a problem with the cabling or a misconfigured modem because getty should not be able to open the communications port until the carrier detect signal has been asserted by the modem.

If no getty processes are waiting to open the port, double-check that the entry for the port is correct in /etc/ttys . Also, check /var/log/messages to see if there are any log messages from init or getty.

Next, try dialing into the system. Be sure to use 8 bits, no parity, and 1 stop bit on the remote system. If a prompt does not appear right away, or the prompt shows garbage, try pressing Enter about once per second. If there is still no prompt, try sending a BREAK. When using a high-speed modem, try dialing again after locking the dialing modem’s interface speed.

If there is still no prompt, check /etc/gettytab again and double-check that:

If the modem on the FreeBSD system will not answer, make sure that the modem is configured to answer the phone when DTR is asserted. If the modem seems to be configured correctly, verify that the DTR line is asserted by checking the modem’s indicator lights.

If it still does not work, try sending an email to the link:FreeBSD general questions mailing list describing the modem and the problem.

This section provides a fast overview of setting up the serial console. This procedure can be used when the dumb terminal is connected to COM1 .

If a different configuration is required, see the next section for a more in-depth configuration explanation.

This section provides a more detailed explanation of the steps needed to setup a serial console in FreeBSD.

During the third stage of the boot process, one can still switch between the internal console and the serial console by setting appropriate environment variables in the boot loader. See loader(8) for more information.

By default, the serial port settings are 9600 baud, 8 bits, no parity, and 1 stop bit. To change the default console speed, use one of the following options:

To configure the ability to drop into the kernel debugger from the serial console, add the following options to a custom kernel configuration file and compile the kernel using the instructions in [_kernelconfig]. Note that while this is useful for remote diagnostics, it is also dangerous if a spurious BREAK is generated on the serial port. Refer to ddb(4) and ddb(8) for more information about the kernel debugger.