27.6 Setting Up the Serial Console

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

Warning: As of FreeBSD 8.0, device nodes for serial ports have been renamed from /dev/ttydN to /dev/ttyuN. FreeBSD 7.X users will have to adapt the following documentation according to these changes.

27.6.1 Introduction

FreeBSD has the ability to boot on a system with only a dumb terminal on a serial port as a console. Such a configuration should be useful for two classes of people: 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, 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 will then load and run the boot loader (/boot/loader) as the third stage code.

In order to set up the serial console you must configure the boot block code, the boot loader code and the kernel.

27.6.2 Serial Console Configuration, Terse Version

This section assumes that you are using the default setup and just want a fast overview of setting up the serial console.

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

  2. To see all boot messages on the serial console, issue the following command while logged in 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, a more in depth configuration explanation exists in Section 27.6.3.

27.6.3 Serial Console Configuration

  1. Prepare a serial cable.

    You will need either a null-modem cable or a standard serial cable and a null-modem adapter. See Section 27.2.2 for a discussion on serial cables.

  2. Unplug your keyboard.

    Most 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 complain loudly about the lack of a keyboard and will not continue to boot until it is plugged in.

    If your computer complains about the error, but boots anyway, then you do not have to do anything special. (Some machines with Phoenix BIOS installed merely say “Keyboard failed” and continue to boot normally.)

    If your computer refuses to boot without a keyboard attached then you will have to configure the BIOS so that it ignores this error (if it can). Consult your motherboard's manual for details on how to do this.

    Tip: Set the keyboard to “Not installed” in the BIOS setup. You will still be able to use your keyboard. All this does is tell the BIOS not to probe for a keyboard at power-on. Your 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.

    Note: If your system has a PS/2® mouse, chances are very good that you may have to unplug your mouse as well as your keyboard. 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. It is said that a Gateway 2000 Pentium 90 MHz system with an AMI BIOS that behaves this way. In general, this is not a problem since the mouse is not much good without the keyboard anyway.

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

    If you do not have a dumb terminal, you can use an old PC/XT with a modem program, or the serial port on another UNIX® box. If you do not have a 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 you are already using COM1 for another device, you will have to temporarily remove that device and install a new boot block and kernel once you get FreeBSD up and running. (It is assumed that COM1 will be available on a file/compute/terminal server anyway; if you really need COM1 for something else (and you cannot switch that something else to COM2 (sio1)), then you probably should not even be bothering with all this in the first place.)

  4. Make sure the configuration file of your 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 the -h option described below, together with this flag.


    Forces this unit to be the console (unless there is another higher priority console), regardless of the -h option 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. You should not set this flag to the serial port unit which you want 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.


    device sio0 flags 0x10

    See the sio(4) manual page for more details.

    If the flags were not set, you need to 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 will instruct the boot block code how you would like to boot the system. In order to activate the serial console, you need one or more of the following options—if you want multiple options, include them all on the same line:


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


    Toggles 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 the -h option above. 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 the -h option. However, note that the dual console configuration takes effect only during the boot block is running. Once the boot loader gets control, the console specified by the -h option becomes the only console.


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

    Note: Due to space constraints in the current version of the boot blocks, the -P option 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 laptop computers may not be properly found because of this limitation. If this is the case with your system, you have to abandon using the -P option. Unfortunately there is no workaround for this problem.

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

    You may include other options described in boot(8) as well.

    The options, except for -P, will be passed to the boot loader (/boot/loader). The boot loader will determine which of the internal video or the serial port should become the console by examining the state of the -h option alone. This means that if you specify the -D option but not the -h option in /boot.config, you can use the serial port as the console only during the boot block; the boot loader will use the internal video display as the console.

  6. Boot the machine.

    When you start your FreeBSD box, the boot blocks will echo the contents of /boot.config to the console. For example:

    /boot.config: -P
    Keyboard: no

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

    Options Message goes to
    none internal console
    -h serial console
    -D serial and internal consoles
    -Dh serial and internal consoles
    -P, keyboard present internal console
    -P, keyboard absent serial console

    After the above messages, there will be a small pause before the boot blocks continue loading the boot loader and before any further messages printed to the console. Under normal circumstances, you do not need to interrupt the boot blocks, but you may want to do so in order to make sure things are set up correctly.

    Hit any key, other than Enter, at the console to interrupt the boot process. The boot blocks will then prompt you for further action. You should now see something like:

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

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

    If you want the serial console but you do not see the prompt on the serial terminal, something is wrong with your settings. In the meantime, you enter -h and hit Enter or Return (if possible) 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.

After the boot loader is loaded and you are in the third stage of the boot process you can still switch between the internal console and the serial console by setting appropriate environment variables in the boot loader. See Section 27.6.6.

27.6.4 Summary

Here is the summary of various settings discussed in this section and the console eventually selected. Case 1: You Set the Flags to 0x10 for sio0

device sio0 flags 0x10
Options in /boot.config Console during boot blocks Console during boot loader Console in kernel
nothing internal internal internal
-h serial serial serial
-D serial and internal internal internal
-Dh serial and internal serial serial
-P, keyboard present internal internal internal
-P, keyboard absent serial and internal serial serial Case 2: You Set the Flags to 0x30 for sio0

device sio0 flags 0x30
Options in /boot.config Console during boot blocks Console during boot loader Console in kernel
nothing internal internal serial
-h serial serial serial
-D serial and internal internal serial
-Dh serial and internal serial serial
-P, keyboard present internal internal serial
-P, keyboard absent serial and internal serial serial

27.6.5 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. If you wish to change the default console speed, you have the following options:

  • Recompile the boot blocks with BOOT_COMCONSOLE_SPEED set to the new console speed. See Section 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, then you must also add the following option to the kernel configuration file and compile a new kernel:

    options CONSPEED=19200
  • Use the -S boot option of the kernel. The -S command line option can be added to /boot.config. See the boot(8) manual page for a description of how to add options to /boot.config and a list of the supported options.

  • Enable the comconsole_speed option in your /boot/loader.conf file.

    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:

    console="comconsole,vidconsole" Using Serial Port Other Than sio0 for the Console

Using a port other than sio0 as the console requires some recompiling. If you want to use another serial port for whatever reasons, recompile the boot blocks, the boot loader and the kernel as follows.

  1. Get the kernel source. (See Chapter 25)

  2. Edit /etc/make.conf and set BOOT_COMCONSOLE_PORT to the address of the port you want to use (0x3F8, 0x2F8, 0x3E8 or 0x2E8). Only sio0 through sio3 (COM1 through COM4) can be used; 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 you want to use. For example, if you want 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

If you wish to drop into the kernel debugger from the serial console (useful for remote diagnostics, but also dangerous if you generate a spurious BREAK on the serial port!) then you should compile your kernel with the following options:

options DDB Getting a Login Prompt on the Serial Console

While this is not required, you may wish to get a login prompt over the serial line, now that you can see boot messages and can enter the kernel debugging session through the serial console. Here is how to do it.

Open the file /etc/ttys with an 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 corresponds to COM1 through COM4. Change off to on for the desired port. If you have changed the speed of the serial port, you need to change std.9600 to match the current setting, e.g., std.19200.

You may also want to change the terminal type from unknown to the actual type of your serial terminal.

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

27.6.6 Changing Console from the Boot Loader

Previous sections described how to set up the serial console by tweaking the boot block. This section shows that you can 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, after the boot block, the settings in the boot loader will override the settings in the boot block. Setting Up the Serial Console

You can easily specify the boot loader and the kernel to use the serial console by writing just one line in /boot/loader.conf:


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

You had better put the above line as the first line of /boot/loader.conf so as to see boot messages on the serial console as early as possible.

Likewise, you can specify the internal console as:


If you do not set the boot loader environment variable console, the boot loader, and subsequently the kernel, will use whichever console indicated by the -h option 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.

Note: At the moment, the boot loader has no option quivalent to the -P option 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

You need to recompile the boot loader to use a serial port other than sio0 for the serial console. Follow the procedure described in Section

27.6.7 Caveats

The idea here is to allow people to set up dedicated servers that require no graphics hardware or attached keyboards. Unfortunately, while most systems will let you boot without a keyboard, there are quite a few that will not let you 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 you have no display hardware in the system. With these machines, you will have to leave some kind of graphics card plugged in, (even if it is just a junky mono board) although you will not have to attach a monitor. You might also try installing an AMI BIOS.