27.6. Setting Up the Serial Console

Contributed by Kazutaka YOKOTA.
Based on a document by Bill Paul.

FreeBSD has the ability to boot a system with a dumb terminal on a serial port as a console. This configuration is useful for system administrators who wish to install FreeBSD on machines that have no keyboard or monitor attached, and developers who want to debug the kernel or device drivers.

As described in Chapter 13, The FreeBSD Booting Process, FreeBSD employs a three stage bootstrap. The first two stages are in the boot block code which is stored at the beginning of the FreeBSD slice on the boot disk. The boot block then loads and runs the boot loader as the third stage code.

In order to set up booting from a serial console, the boot block code, the boot loader code, and the kernel need to be configured.

27.6.1. Quick Serial Console Configuration

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.

Procedure 27.1. Configuring a Serial Console on COM1
  1. Connect the serial cable to COM1 and the controlling terminal.

  2. To configure boot messages to display on the serial console, issue the following command as the superuser:

    # echo 'console="comconsole"' >> /boot/loader.conf
  3. Edit /etc/ttys and change off to on and dialup to vt100 for the ttyu0 entry. Otherwise, a password will not be required to connect via the serial console, resulting in a potential security hole.

  4. Reboot the system to see if the changes took effect.

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

27.6.2. In-Depth Serial Console Configuration

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

Procedure 27.2. Configuring a Serial Console
  1. Prepare a serial cable.

    Use either a null-modem cable or a standard serial cable and a null-modem adapter. See Section 27.2.1, “Serial Cables and Ports” for a discussion on serial cables.

  2. Unplug the keyboard.

    Many systems probe for the keyboard during the Power-On Self-Test (POST) and will generate an error if the keyboard is not detected. Some machines will refuse to boot until the keyboard is plugged in.

    If the computer complains about the error, but boots anyway, no further configuration is needed.

    If the computer refuses to boot without a keyboard attached, configure the BIOS so that it ignores this error. Consult the motherboard's manual for details on how to do this.


    Try setting the keyboard to Not installed in the BIOS. This setting tells the BIOS not to probe for a keyboard at power-on so it should not complain if the keyboard is absent. If that option is not present in the BIOS, look for an Halt on Error option instead. Setting this to All but Keyboard or to No Errors will have the same effect.

    If the system has a PS/2® mouse, unplug it as well. PS/2® mice share some hardware with the keyboard and leaving the mouse plugged in can fool the keyboard probe into thinking the keyboard is still there.


    While most systems will boot without a keyboard, quite a few will not boot without a graphics adapter. Some systems can be configured to boot with no graphics adapter by changing the graphics adapter setting in the BIOS configuration to Not installed. Other systems do not support this option and will refuse to boot if there is no display hardware in the system. With these machines, leave some kind of graphics card plugged in, even if it is just a junky mono board. A monitor does not need to be attached.

  3. Plug a dumb terminal, an old computer with a modem program, or the serial port on another UNIX® box into the serial port.

  4. Add the appropriate hint.sio.* entries to /boot/device.hints for the serial port. Some multi-port cards also require kernel configuration options. Refer to sio(4) for the required options and device hints for each supported serial port.

  5. Create boot.config in the root directory of the a partition on the boot drive.

    This file instructs the boot block code how to boot the system. In order to activate the serial console, one or more of the following options are needed. When using multiple options, include them all on the same line:


    Toggles between the internal and serial consoles. Use this to switch console devices. For instance, to boot from the internal (video) console, use -h to direct the boot loader and the kernel to use the serial port as its console device. Alternatively, to boot from the serial port, use -h to tell the boot loader and the kernel to use the video display as the console instead.


    Toggles between the single and dual console configurations. In the single configuration, the console will be either the internal console (video display) or the serial port, depending on the state of -h. In the dual console configuration, both the video display and the serial port will become the console at the same time, regardless of the state of -h. However, the dual console configuration takes effect only while the boot block is running. Once the boot loader gets control, the console specified by -h becomes the only console.


    Makes the boot block probe the keyboard. If no keyboard is found, the -D and -h options are automatically set.


    Due to space constraints in the current version of the boot blocks, -P is capable of detecting extended keyboards only. Keyboards with less than 101 keys and without F11 and F12 keys may not be detected. Keyboards on some laptops may not be properly found because of this limitation. If this is the case, do not use -P.

    Use either -P to select the console automatically or -h to activate the serial console. Refer to boot(8) and boot.config(5) for more details.

    The options, except for -P, are passed to the boot loader. The boot loader will determine whether the internal video or the serial port should become the console by examining the state of -h. This means that if -D is specified but -h is not specified in /boot.config, the serial port can be used as the console only during the boot block as the boot loader will use the internal video display as the console.

  6. Boot the machine.

    When FreeBSD starts, the boot blocks echo the contents of /boot.config to the console. For example:

    /boot.config: -P
    Keyboard: no

    The second line appears only if -P is in /boot.config and indicates the presence or absence of the keyboard. These messages go to either the serial or internal console, or both, depending on the option in /boot.config:

    OptionsMessage goes to
    noneinternal console
    -hserial console
    -Dserial and internal consoles
    -Dhserial and internal consoles
    -P, keyboard presentinternal console
    -P, keyboard absentserial console

    After the message, there will be a small pause before the boot blocks continue loading the boot loader and before any further messages are printed to the console. Under normal circumstances, there is no need to interrupt the boot blocks, but one can do so in order to make sure things are set up correctly.

    Press any key, other than Enter, at the console to interrupt the boot process. The boot blocks will then prompt for further action:

    >> FreeBSD/i386 BOOT
    Default: 0:ad(0,a)/boot/loader

    Verify that the above message appears on either the serial or internal console, or both, according to the options in /boot.config. If the message appears in the correct console, press Enter to continue the boot process.

    If there is no prompt on the serial terminal, something is wrong with the settings. Enter -h then Enter or Return to tell the boot block (and then the boot loader and the kernel) to choose the serial port for the console. Once the system is up, go back and check what went wrong.

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.


This line in /boot/loader.conf or /boot/loader.conf.local configures the boot loader and the kernel to send their boot messages to the serial console, regardless of the options in /boot.config:


That line should be the first line of /boot/loader.conf so that boot messages are displayed on the serial console as early as possible.

If that line does not exist, or if it is set to console="vidconsole", the boot loader and the kernel will use whichever console is indicated by -h in the boot block. See loader.conf(5) for more information.

At the moment, the boot loader has no option equivalent to -P in the boot block, and there is no provision to automatically select the internal console and the serial console based on the presence of the keyboard.


While it is not required, it is possible to provide a login prompt over the serial line. To configure this, edit the entry for the serial port in /etc/ttys using the instructions in Section 27.3.1, “Terminal Configuration”. If the speed of the serial port has been changed, change std.9600 to match the new setting.

27.6.3. Setting a Faster Serial Port Speed

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:

  • Edit /etc/make.conf and set BOOT_COMCONSOLE_SPEED to the new console speed. Then, recompile and install the boot blocks and the boot loader:

    # cd /sys/boot
    # make clean
    # make
    # make install

    If the serial console is configured in some other way than by booting with -h, or if the serial console used by the kernel is different from the one used by the boot blocks, add the following option, with the desired speed, to a custom kernel configuration file and compile a new kernel:

    options CONSPEED=19200
  • Add the -S19200 boot option to /boot.config, replacing 19200 with the speed to use.

  • Add the following options to /boot/loader.conf. Replace 115200 with the speed to use.


27.6.4. Entering the DDB Debugger from the Serial Line

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 Chapter 8, Configuring the FreeBSD Kernel. 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.

options DDB

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