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os_boot_serial_console.md

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Enabling OS boot serial console and setting connection parameters

Intro

  • Instructions is written for Voyage Linux, but steps shown here should work for another Linux distributions
  • Used serial connection parameters: baudrate: 115200, data bits: 8, parity: none, stop bits: 1
  • Used text editor is nano, but it can be any another text editor e.g. vi
  • Root user is recommended

Enabling sending OS boot logs via serial console

Often sending OS boot logs via serial console is disabled as default setting. To enable that function menu.lst file has to be edited. To do that follow the steps below.

  1. Login to Linux Voyage. Default login/password: root/voyage.

  2. After logging to Voyage Linux filesystem is mounted as read-only. To change that type:

    remountrw

  3. Open menu.lst file with any text editor. E.g:

    nano /boot/grub/menu.lst

  4. Find the settings of configuration from grub menu which you choose. The structure should look like:

    title           Debian GNU/Linux, kernel 3.10.11
root            (hd0,0)
kernel          /boot/vmlinuz-3.10.11 root=UUID=f45cf8c7-311e-47d6-88d0-a3a8861f37be ro
initrd          /boot/initrd.img-3.10.11

  5. Add to the kernel line following options:

    vga=normal console=tty0 console=ttyS0,115200n8

    After that step settings of selected configuration should look like:

    title           Debian GNU/Linux, kernel 3.10.11
root            (hd0,0)
kernel          /boot/vmlinuz-3.10.11 root=UUID=f45cf8c7-311e-47d6-88d0-a3a8861f37be ro console=ttyS0,115200n8
initrd          /boot/initrd.img-3.10.11

  6. Save the changes and close the file. For nano it's Ctrl+O to save the file and Ctrl+X to exit from file.

  7. Now you can reboot the system and check that OS boot logs appear in serial console:

reboot

Changing serial console connection parameters

  1. Login to Linux Voyage. Default login/password: root/voyage.

  2. After logging to Voyage Linux filesystem is mounted as read-only. To change that type:

    remountrw

  3. Turn on serial redirection in the GRUB. To do that open grub.conf file with any text editor. There can be no grub.conf file in your OS. Opening text editor with correctly chosen directory and file name will create it automatically:

    nano /boot/grub/grub.conf

    Then add lines shown below to the file:

    serial --unit=1 --speed=19200
terminal --timeout=8 console serial

    Save the changes and exit from the file (for nano: Ctrl+O next Ctrl+X).

  4. Enable serial output from the Linux kernel. That step is descibed above.

  5. Turn on logging in via the serial console. Edit file named inittab:

    nano /etc/inittab

    Find uncommented line which looks similar to the shown below:

    T0:23:respawn:/sbin/getty -L ttyS0 115200

    The last parameter is responsible for the set baudrate. In the example that value is set to 115200. You can change it to the wanted baudrate.

    There can be more uncommented lines as indicated.

    Save the changes and exit from the file.

  6. Now you can reboot system and check that serial console baudrate was correctly changed:

    reboot

    If you are using serial console to perform steps from this instruction take a note that you should change connection parameters in used serial terminal (e.g. minicom) too. Because characters won't be shown correctly after reboot.