26.6. Setting Up the Serial Console

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

26.6.1. Introduction

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.

26.6.2. Quick Serial Console Configuration

This section assumes the default setup and provides a fast overview of setting up the serial console.

  1. Connect the serial cable to COM1 and the controlling terminal.

  2. To see all the boot messages 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.

26.6.3. In-Depth Serial Console Configuration

  1. Prepare a serial cable.

    Use either a null-modem cable or a standard serial cable and a null-modem adapter. See Section 26.2.2, “Cables and Ports” for a discussion on serial cables.

  2. Unplug the keyboard.

    Many PC 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, the BIOS needs to be configured so that it ignores this error (if it can). Consult the motherboard's manual for details on how to do this.


    Try setting the keyboard to Not installed in the BIOS. The keyboard can still be used as this setting just tells the BIOS not to probe for a keyboard at power-on. The BIOS should not complain if the keyboard is absent. You can leave the keyboard plugged in even with this flag set to Not installed and the keyboard will still work. If the above option is not present in the BIOS, look for an Halt on Error option instead. Setting this to All but Keyboard or even to No Errors, will have the same effect.


    If the system has a PS/2® mouse, chances are good that both the mouse and keyboard need to be unplugged. This is because 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.

  3. Plug a dumb terminal into COM1 (sio0).

    If a dumb terminal is not available, use an old computer with a modem program, or the serial port on another UNIX® box. If there is no COM1 (sio0), get one. At this time, there is no way to select a port other than COM1 for the boot blocks without recompiling the boot blocks. If COM1 is being used by another device, temporarily remove that device and install a new boot block and kernel once FreeBSD is up and running.

  4. Make sure the configuration file of the custom kernel has appropriate flags set for COM1 (sio0).

    Relevant flags are:


    Enables console support for this unit. The other console flags are ignored unless this is set. Currently, at most one unit can have console support. The first one, in config file order, with this flag set is preferred. This option alone will not make the serial port the console. Set the following flag or use -h as described below, together with this flag.


    Forces this unit to be the console, unless there is another higher priority console, regardless of -h as discussed below. The flag 0x20 must be used together with the 0x10 flag.


    Reserves this unit (in conjunction with 0x10) and makes the unit unavailable for normal access. This flag should not be set to the serial port to use as the serial console. The only use of this flag is to designate the unit for kernel remote debugging. See The Developer's Handbook for more information on remote debugging.

    Here is an example setting:

    device sio0 flags 0x10

    Refer to sio(4) for more details.

    If the flags were not set, run UserConfig on a different console or recompile the kernel.

  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. Unfortunately there is no workaround for this problem.

    Use either -P to select the console automatically, or -h to activate the serial console.

    Other options are described in boot(8).

    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 boot:

    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 Section 26.6.6, “Changing Console from the Boot Loader” for more information.

26.6.4. Summary

Here is the summary of the various settings discussed in this section: Case 1: Set the Flags to 0x10 for sio0

device sio0 flags 0x10
Options in /boot.configConsole during boot blocksConsole during boot loaderConsole in kernel
-Dserial and internalinternalinternal
-Dhserial and internalserialserial
-P, keyboard presentinternalinternalinternal
-P, keyboard absentserial and internalserialserial Case 2: Set the Flags to 0x30 for sio0

device sio0 flags 0x30
Options in /boot.configConsole during boot blocksConsole during boot loaderConsole in kernel
-Dserial and internalinternalserial
-Dhserial and internalserialserial
-P, keyboard presentinternalinternalserial
-P, keyboard absentserial and internalserialserial

26.6.5. Tips for the Serial Console 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, the following options are available:

  • Recompile the boot blocks with BOOT_COMCONSOLE_SPEED set to the new console speed. See Section, “Using a Serial Port Other Than sio0 for the Console” for detailed instructions about building and installing new boot blocks.

    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 to a custom kernel configuration file and compile a new kernel:

    options CONSPEED=19200
  • Add the -S boot option to /boot.config. See boot(8) for a description of how to add options to /boot.config and a list of the supported options.

  • Enable comconsole_speed in /boot/loader.conf. This option depends on console, boot_serial, and boot_multicons being set in /boot/loader.conf too. An example of using comconsole_speed to change the serial console speed is:

    boot_multicons="YES" boot_serial="YES" comconsole_speed="115200" console="comconsole,vidconsole" Using a Serial Port Other Than sio0 for the Console

Using a port other than sio0 as the console requires the boot blocks, the boot loader, and the kernel to be recompiled as follows.

  1. Get the kernel source as described in Chapter 24, Updating and Upgrading FreeBSD.

  2. Edit /etc/make.conf and set BOOT_COMCONSOLE_PORT to the address of the port to use: 0x3F8, 0x2F8, 0x3E8 or 0x2E8. Only sio0 through sio3 (COM1 through COM4) can be used as multiport serial cards will not work. No interrupt setting is needed.

  3. Create a custom kernel configuration file and add appropriate flags for the serial port to use. For example, to make sio1 (COM2) the console:

    device sio1 flags 0x10


    device sio1 flags 0x30

    The console flags for the other serial ports should not be set.

  4. Recompile and install the boot blocks and the boot loader:

    # cd /sys/boot # make clean # make # make install
  5. Rebuild and install the kernel.

  6. Write the boot blocks to the boot disk with bsdlabel(8) and boot from the new kernel. Entering the DDB Debugger from the Serial Line

To drop into the kernel debugger from the serial console, compile a custom kernel with the following options. Note that while this is useful for remote diagnostics, it is also dangerous if a spurious BREAK is generated on the serial port.

options BREAK_TO_DEBUGGER options DDB Getting a Login Prompt on the Serial Console

While this is not required, it is possible to get a login prompt over the serial line. First, make sure that the boot messages are displayed and it is possible to enter the kernel debugging session through the serial console.

Open /etc/ttys with a text editor and locate the lines:

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

ttyu0 through ttyu3 correspond to COM1 through COM4. Change off to on for the desired port. If the speed of the serial port has been changed, change std.9600 to match the new setting.

The terminal type can also be changed from unknown to the actual type of the serial terminal.

After editing the file, type kill -HUP 1 to make this change take effect.

26.6.6. Changing Console from the Boot Loader

Previous sections described how to set up the serial console by tweaking the boot block. This section shows how to specify the console by entering some commands and environment variables in the boot loader. As the boot loader is invoked at the third stage of the boot process, the settings in the boot loader will override the settings in the boot block. Setting Up the Serial Console

The boot loader and the kernel to use the serial console can be specified by writing one line in /boot/loader.conf:


This will take effect regardless of the settings in the boot block discussed in the previous section.

This line should be the first line of /boot/loader.conf so as to see boot messages on the serial console as early as possible.

Likewise, to specify the internal console:


If the boot loader environment variable console is not set, the boot loader, and subsequently the kernel, will use whichever console is indicated by -h in the boot block.

The console can be specified in /boot/loader.conf.local or in /boot/loader.conf.

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. Using a Serial Port Other Than sio0 for the Console

The boot loader needs to be compiled in order to use a serial port other than sio0 for the serial console. Follow the procedure described in Section, “Using a Serial Port Other Than sio0 for the Console”.

26.6.7. Caveats

While most systems will boot without a keyboard, quite a few will not boot without a graphics adapter. Machines with AMI BIOSes can be configured to boot with no graphics adapter installed by changing the graphics adapter setting in the CMOS configuration to Not installed.

However, many machines 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. One might also try installing an AMI BIOS.

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