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Chapter 4. Configuring kernel command-line parameters

Use kernel command-line parameters to change the behavior of certain aspects of the Red Hat Enterprise Linux kernel at boot time. System administrators control which options are set at boot. Note that certain kernel behaviors can only be set at boot time.

Important

Changing the behavior of the system by modifying kernel command-line parameters can have negative effects on your system. Always test changes before deploying them in production. For further guidance, contact Red Hat Support.

4.1. What are kernel command-line parameters
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With kernel command-line parameters, you can overwrite default values and set specific hardware settings. At boot time, you can configure the Red Hat Enterprise Linux kernel, the initial RAM disk, and user space features.

By default, the kernel command-line parameters for systems by using the GRUB boot loader are defined in the boot entry configuration file for each kernel boot entry.

You can manipulate boot loader configuration files by using the grubby utility. With grubby, you can perform these actions:

  • Change the default boot entry.
  • Add or remove arguments from a GRUB menu entry.

4.2. Understanding boot entries
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Understand boot entries to manage your system’s kernel configurations.

A boot entry is a collection of options stored in a configuration file and tied to a particular kernel version. In practice, you have at least as many boot entries as your system has installed kernels. The boot entry configuration file is located in the /boot/loader/entries/ directory: 

d8712ab6d4f14683c5625e87b52b6b6e-6.12.0.el10_0.x86_64.conf

The file name consists of a machine ID stored in the /etc/machine-id file, and a kernel version.

The boot entry configuration file contains information about the kernel version, the initial ramdisk image, and the kernel command-line parameters. The example contents of a boot entry config can be seen below: 

title Red Hat Enterprise Linux (6.12.0-0.el10_0.x86_64) 10.0
version 6.12.0-0.el10_0.x86_64
linux /vmlinuz-6.12.0-0.el10_0.x86_64
initrd /initramfs-6.12.0-0.el10_0.x86_64.img
options root=/dev/mapper/rhel_kvm--02--guest08-root ro crashkernel=2G-64G:256M,64G-:512M resume=/dev/mapper/rhel_kvm--02--guest08-swap rd.lvm.lv=rhel_kvm-02-guest08/root rd.lvm.lv=rhel_kvm-02-guest08/swap console=ttyS0,115200
grub_users $grub_users
grub_arg --unrestricted
grub_class kernel

4.3. Changing kernel command-line parameters for all boot entries
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Change kernel command-line parameters for all boot entries on your system.

Important

When installing a newer version of the kernel in Red Hat Enterprise Linux 10 systems, the grubby tool passes the kernel command-line arguments from the previous kernel version.

Prerequisites

  • grubby utility is installed on your system.
  • zipl utility is installed on your IBM Z system.

Procedure

  • To add a parameter: 

    # grubby --update-kernel=ALL --args="<NEW_PARAMETER>"

    For systems that use the GRUB boot loader and, on IBM Z that use the zIPL boot loader, the command adds a new kernel parameter to each /boot/loader/entries/<ENTRY>.conf file.

    • On IBM Z, update the boot menu: 

      # zipl

  • To remove a parameter: 

    # grubby --update-kernel=ALL --remove-args="<PARAMETER_TO_REMOVE>"

    • On IBM Z, update the boot menu: 

      # zipl
      Note

      There is no need to update boot menu for the systems using the GRUB boot loader.

4.4. Changing kernel command-line parameters for a single boot entry
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Make changes in kernel command-line parameters for a single boot entry on your system.

Prerequisites

  • grubby and zipl utilities are installed on your system.

Procedure

  • To add a parameter: 

    # grubby --update-kernel=/boot/vmlinuz-$(uname -r) --args="<NEW_PARAMETER>"

    • On IBM Z, update the boot menu: 

      # grubby --args="<NEW_PARAMETER> --update-kernel=ALL --zipl

  • To remove a parameter: 

    # grubby --update-kernel=/boot/vmlinuz-$(uname -r) --remove-args="<PARAMETER_TO_REMOVE>"

    • On IBM Z, update the boot menu: 

      # grubby --args="<NEW_PARAMETER> --update-kernel=ALL --zipl
      Important

      grubby modifies and stores the kernel command-line parameters of an individual kernel boot entry in the /boot/loader/entries/<ENTRY>.conf file.

4.5. Changing kernel command-line parameters temporarily at boot time
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Make temporary changes to a Kernel Menu Entry by changing the kernel parameters only during a single boot process. This procedure applies only for a single boot and does not persist after system reboot.

Procedure

  1. Boot into the GRUB boot menu.
  2. Select the kernel you want to start.
  3. Press the e key to edit the kernel parameters.
  4. Find the kernel command line by moving the cursor down.
  5. Move the cursor to the end of the line.

  6. Edit the kernel parameters as required. For example, to run the system in emergency mode, add the emergency parameter at the end of the linux line: 

    linux   ($root)/vmlinuz-6.12.0-0.el10_0.x86_64 root=/dev/mapper/rhel-root ro crashkernel=2G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet emergency

    To enable the system messages, remove the rhgb and quiet parameters.

  7. Press Ctrl+x to boot with the selected kernel and the modified command line parameters.

    Important

    If you press the Esc key to leave command line editing, it will drop all the user made changes.

4.6. Configuring GRUB settings to enable serial console connection
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To connect to a headless server or embedded system when the network is down, you can configure GRUB 2 to enable a serial console connection. This also provides login access on a different system to avoid security rules.

You need to configure some default GRUB settings to use the serial console connection.

Prerequisites

  • You have root permissions on the system.

Procedure

  1. Add the following two lines to the /etc/default/grub file: 

    GRUB_TERMINAL="serial"
GRUB_SERIAL_COMMAND="serial --speed=9600 --unit=0 --word=8 --parity=no --stop=1"

    The first line disables the graphical terminal. The GRUB_TERMINAL key overrides values of GRUB_TERMINAL_INPUT and GRUB_TERMINAL_OUTPUT keys.

    The second line adjusts the baud rate (--speed), parity and other values to fit your environment and hardware. Note that a higher baud rate, for example 115200, is preferable for tasks such as following log files.

  2. Update the GRUB configuration file: 

    # grub2-mkconfig -o /boot/grub2/grub.cfg

    This applies to both, BIOS and UEFI based machines.

  3. Reboot the system for the changes to take effect.

4.7. Changing boot entries with the GRUB configuration file
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To change boot entries for the Linux kernel, you can modify the /etc/default/grub configuration file. This file contains the GRUB_CMDLINE_LINUX key, which lists the kernel command-line arguments.

For example: 

GRUB_CMDLINE_LINUX="crashkernel=2G-64G:256M,64G-:512M resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap"

To change the boot entries, overwrite Boot Loader Specification (BLS) snippets with the contents of the GRUB_CMDLINE_LINUX values.

Prerequisites

  • A fresh Red Hat Enterprise Linux 10 installation.

Procedure

  1. Add or remove a kernel parameter for individual kernels in a post installation script with grubby: 

    # grubby --update-kernel <PATH_TO_KERNEL> --args "<NEW_ARGUMENTS>"

    For example, add the noapic parameter to the chosen kernel: 

    # grubby --update-kernel /boot/vmlinuz-6.12.0-0.el10_0.x86_64 --args "noapic"

    The parameter is propagated into the BLS snippets, but not into the /etc/default/grub file.

  2. Overwrite BLS snippets with the contents of the GRUB_CMDLINE_LINUX values present in the /etc/default/grub file: 

    # grub2-mkconfig -o /boot/grub2/grub.cfg --update-bls-cmdline
     
    Generating grub configuration file ...
Adding boot menu entry for UEFI Firmware Settings ...
done
    Note

    Other changes, such as changes made to GRUB_TIMEOUT key (also included in the /etc/default/grub GRUB configuration file) are propagated to the new grub.cfg file by executing grub2-mkconfig command.

Verification

  1. Reboot your system.

  2. Verify that the parameters are included in the /proc/cmdline file.

    For example, if you added the noapic: 

    BOOT_IMAGE=(hd0,gpt2)/vmlinuz-6.12.0-0.el10_0.x86_64 root=/dev/mapper/RHELCSB-Root ro vconsole.keymap=us crashkernel=2G-64G:256M,64G-:512M rd.lvm.lv=RHELCSB/Root rd.luks.uuid=luks-d8a28c4c-96aa-4319-be26-96896272151d rhgb quiet noapic rd.luks.key=d8a28c4c-96aa-4319-be26-96896272151d=/keyfile:UUID=c47d962e-4be8-41d6-8216-8cf7a0d3b911 ipv6.disable=1
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