Chapter 4. Configuring kernel command-line parameters

With kernel command-line parameters, you can change the behavior of certain aspects of the Red Hat Enterprise Linux kernel at boot time. As a system administrator, you have full control over what options get set at boot. Certain kernel behaviors can only be set at boot time, so understanding how to make these changes is a key administration skill.

Important

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

4.1. What are kernel command-line parameters

With kernel command-line parameters, you can overwrite default values and set specific hardware settings. At boot time, you can configure the following features:

  • The Red Hat Enterprise Linux kernel
  • The initial RAM disk
  • The user space features

By default, the kernel command-line parameters for systems 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.

Additional resources

4.2. Understanding boot entries

A boot entry is a collection of options which are 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 and can look like this: 

d8712ab6d4f14683c5625e87b52b6b6e-5.14.0-1.el9.x86_64.conf

The file name above 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 (5.14.0-1.el9.x86_64) 9.0 (Plow)
version 5.14.0-1.el9.x86_64
linux /vmlinuz-5.14.0-1.el9.x86_64
initrd /initramfs-5.14.0-1.el9.x86_64.img
options root=/dev/mapper/rhel_kvm--02--guest08-root ro crashkernel=1G-4G:192M,4G-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

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

Important

When installing a newer version of the kernel in RHEL 9 systems, the grubby tool passes the kernel command-line arguments from the previous kernel version.

However, this does not apply to RHEL version 9.0 in which newly installed kernels lose previous command-line options. You must run the grub2-mkconfig command on the newly installed kernel to pass the parameters to your new kernel. For more information about this known issue, see Boot loader.

Prerequisites

  • Verify that the grubby utility is installed on your system.
  • Verify that the 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

Additional resources

4.4. Changing kernel command-line parameters for a single boot entry

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

Prerequisites

  • Verify that the 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: 

      # zipl

  • To remove a parameter: 

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

    • On IBM Z, update the boot menu: 

      # 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.

Additional resources

4.5. Changing kernel command-line parameters temporarily at boot time

Make temporary changes to a Kernel Menu Entry by changing the kernel parameters only during a single boot process.

Note

This procedure applies only for a single boot and does not persistently make the changes.

Procedure

  1. Boot into the GRUB 2 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. The kernel command line starts with linux on 64-Bit IBM Power Series and x86-64 BIOS-based systems, or linuxefi on UEFI systems.

  5. Move the cursor to the end of the line.

    Note

    Press Ctrl+a to jump to the start of the line and Ctrl+e to jump to the end of the line. On some systems, Home and End keys might also work.

  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-5.14.0-63.el9.x86_64 root=/dev/mapper/rhel-root ro crashkernel=1G-4G:192M,4G-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

The serial console is beneficial when you need to connect to a headless server or an embedded system and the network is down. Or when you need to avoid security rules and obtain login access on a different system.

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

Prerequisites

  • You have root permissions.

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 much higher baud rate, for example 115200, is preferable for tasks such as following log files.

  2. Update the GRUB configuration file.

    • On BIOS-based machines: 

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

    • On UEFI-based machines: 

      # grub2-mkconfig -o /boot/grub2/grub.cfg
  3. Reboot the system for the changes to take effect.

4.7. Changing boot entries with the GRUB configuration file

The /etc/default/grub GRUB configuration file contains the GRUB_CMDLINE_LINUX key, which lists kernel command-line arguments to add to boot entries for the Linux kernel. For example: 

GRUB_CMDLINE_LINUX="crashkernel=1G-4G:192M,4G-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 RHEL 9 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-5.14.0-362.8.1.el9_3.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), do get propagated to the new grub.cfg by default.

Verification

  1. Reboot your operating system.

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

    For example, /proc/cmdline contains the noapic kernel parameter: 

    BOOT_IMAGE=(hd0,gpt2)/vmlinuz-4.18.0-425.3.1.el8.x86_64 root=/dev/mapper/RHELCSB-Root ro vconsole.keymap=us crashkernel=auto 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