How to Boot into a Specific Kernel Version
DigitalOcean Droplets are Linux-based virtual machines (VMs) that run on top of virtualized hardware. Each Droplet you create is a new server you can use, either standalone or as part of a larger, cloud-based infrastructure.
By default, your Droplet boots into the highest-versioned kernel. If you want to use a different kernel version, you’ll need to:
- Verify that your Droplet uses internal kernel management.
- Find and install the kernel version you want.
- Set which kernel the bootloader loads by default.
Verify the Droplet’s Kernel Management Method
Before you can modify your Droplet’s kernel, you need to verify that your Droplet is using internally-managed kernels.
All Droplets created after March 2017 use internal kernels by default, and older Droplets can be configured to support internal kernels with the DigitalOcean GrubLoader kernel.
If you’re not sure whether your Droplet manages its kernels internally, visit its detail page in the control panel and click Kernel in the navigation. If the kernel management page has the following message, your Droplet is set to use internal kernels natively:
The kernel for this Droplet is not managed within the control panel. Instead, you can upgrade the kernel from within the Droplet.
If you see a Select a Kernel menu with a Change button and the following description instead, your Droplet is using legacy external kernel management:
This will update your configuration. Then power off the server from the command line and boot it from the control panel and the new kernel will be active. To revert, select ‘Original Kernel’ and follow the same process.
If your Droplet is using legacy kernel management, you can switch to the DigitalOcean GrubLoader kernel to support internal kernels.
Find and Install the Kernel Package
Once your Droplet is using internal kernel management, you can find and install the kernel package you want to switch to.
Ubuntu and Debian
Search the apt repositories for available kernels.
sudo apt-get update
apt-cache search --names-only linux-image
This returns a list of available kernels similar to this:
linux-image-4.4.0-21-generic - Linux kernel image for version 4.4.0 on 64 bit x86 SMP
linux-image-4.4.0-21-lowlatency - Linux kernel image for version 4.4.0 on 64 bit x86 SMP
linux-image-extra-4.4.0-21-generic - Linux kernel extra modules for version 4.4.0 on 64 bit x86 SMP
. . .
The package name of the kernel is linux-image- followed by the version, like linux-image-4.4.0-21-generic. The associated headers file for a kernel has the same package name with image replaced by headers.
Install the kernel you want, and we recommend installing the headers as well. For example:
sudo apt-get install linux-image-4.4.0-21-generic linux-headers-4.4.0-21-generic
Once the kernel is installed, you need to modify your Droplet’s bootloader to boot into that kernel version by default.
CentOS and Fedora
Search the yum repositories for available kernels.
yum list --showduplicates kernel
This returns a list of the installed and available kernels similar to this:
Installed Packages
kernel.x86_64 3.10.0-327.22.2.el7 @updates
Available Packages
kernel.x86_64 3.10.0-327.el7 base
kernel.x86_64 3.10.0-327.3.1.el7 updates
kernel.x86_64 3.10.0-327.4.4.el7 updates
. . .
The package name of the kernel is kernel- with the version from the second column, like kernel-3.10.0-327-18.3.el7.
Install the kernel you want. For example:
sudo yum install kernel-3.10.0-327-18.3.el7
Once the kernel is installed, you need to modify your Droplet’s bootloader to boot into that kernel version by default.
Setting the Default Kernel in the Bootloader
To override the default kernel selection, you need to modify the kernel that the bootloader loads by default. The procedure to do so depends heavily which bootloader your Droplet uses.
Most distributions on DigitalOcean use the Grub 2 bootloader, including:
- Ubuntu 18.04, 16.04, 14.04, and 12.04
- Debian 9, 8, and 7
- Fedora 28, 27, 26, 25, and 24
- CentOS 7
Note
CentOS 6 and 5 use the legacy Grub 1 bootloader. These distributions only have internal kernel management available via the DigitalOcean GrubLoader kernel, which can only boot to the highest available kernel. As a result, to boot to a different kernel, you must remove all other installed kernels.
You can display a list of all installed kernel packages with rpm -q kernel. Because yum can’t uninstall kernels packages, you should use rpm -e to delete all but the kernel you want to use, and then reboot your Droplet. For example:
sudo rpm -e kernel-2.6.18-410.el5
sudo rpm -e kernel-2.6.18-409.el5
# repeat for all kernels you want to remove
sudo reboot
Your Droplet boots into the remaining kernel.
The appropriate procedure for Grub 2 depends on whether your Droplet uses internal kernels natively or via the DigitalOcean GrubLoader kernel.
For Grub 2 Droplets using Native Internal Kernel Management
In /etc/default/grub, find and modify the following settings that determine how Grub chooses the default kernel. If the settings are not present in your version of the file, add them.
. . .
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true
GRUB_DISABLE_SUBMENU=y
. . .
The GRUB_DEFAULT setting allows Grub to use whatever value we save as the default kernel instead of hard-coding a default. The GRUB_SAVEDEFAULT tells Grub to set the default kernel to anything we explicitly select in a menu. The GRUB_DISABLE_SUBMENU option flattens the menu structure to simplify parsing it. Grub 2 does not include a utility to display its menu on the command line, so you must manually parse the configuration.
There should only be one occurrence of each setting in the file. Save and close the file when you are finished.
Next, if you are running a Debian or Ubuntu Droplet, there is a DigitalOcean-specific Grub file in /etc/default/grub.d named 50-cloudimg-settings.cfg. This file is necessary for the DigitalOcean to boot the image properly, but it contains a default selection that overrides the settings in /etc/default/grub.
In /etc/default/grub.d/50-cloudimg-settings.cfg, comment out the GRUB_DEFAULT line if one is present by adding a # to the beginning of the line.
#GRUB_DEFAULT=0
The location of the grub.cfg file varies based on distribution, so create an environmental variable called GRUB_CONFIG that finds it. You’ll use this variable in commands later in the process.
export GRUB_CONFIG=`sudo find /boot -name "grub.cfg"`
Rebuild the grub.cfg file with the changes you made.
On Ubuntu and Debian, use update-grub:
sudo update-grub
On CentOS and Fedora, use grub2-mkconfig:
sudo grub2-mkconfig -o $GRUB_CONFIG
Now that the configuration is rebuilt and the menu is flattened, we can parse the file for the available entries. The following command displays the entry index number and title. You can use either of these to refer to a specific entry.
sudo grep 'menuentry ' $GRUB_CONFIG | cut -f 2 -d "'" | nl -v 0
This returns all of your available boot options. Take a note of the index number or title of the entry you wish to boot.
0 CentOS Linux (3.10.0-327.22.2.el7.x86_64) 7 (Core)
1 CentOS Linux (3.10.0-327.18.2.el7.x86_64) 7 (Core)
2 CentOS Linux (0-rescue-d4c27651599f460481bc723aafe36177) 7 (Core)
3 CentOS Linux (0-rescue-ce700ac27e6f4161b0730bad2182e0b1) 7 (Core)
Grub includes commands to set a new default kernel from the command line. You can use either the index number or the entry title to specify a boot option.
For Ubuntu and Debian users, use grub-set-default. For example, by index:
sudo grub-set-default 2
For CentOS and Fedora Droplets, use grub2-set-default. For example, by entry title:
sudo grub2-set-default 'CentOS Linux (3.10.0-327.18.2.el7.x86_64) 7 (Core)'
You can also choose a temporary boot option which applies to the next boot only.
For Ubuntu and Debian, use grub-reboot:
sudo grub-reboot 2
For CentOS and Fedora, use grub2-reboot:
sudo grub2-reboot 'CentOS Linux (3.10.0-327.18.2.el7.x86_64) 7 (Core)'
Once you’ve selected the kernel to use, reboot the Droplet.
sudo reboot
Your Droplet boots into the kernel you selected.
For Grub 2 Droplets Using the GrubLoader Kernel
If your Droplet uses the GrubLoader kernel in the control panel to implement internal kernels, you have to follow a different procedure.
In /etc/default/grub, find and modify the following settings that determine how Grub chooses the default kernel. If the settings are not present in your version of the file, add them.
. . .
GRUB_DISABLE_SUBMENU=y
. . .
The GRUB_DISABLE_SUBMENU option flattens the menu structure to simplify parsing it. Grub 2 does not include a utility to display its menu on the command line, so you must manually parse the configuration.
There should only be one occurrence of this setting. Save and close the file when you are finished.
The location of the grub.cfg file varies based on distribution, so create an environmental variable called GRUB_CONFIG that finds it. You’ll use this variable in commands later in the process.
export GRUB_CONFIG=`sudo find /boot -name "grub.cfg"`
Rebuild the grub.cfg file with the changes we’ve made.
On Ubuntu and Debian, use update-grub:
sudo update-grub
On CentOS and Fedora, use grub2-mkconfig:
sudo grub2-mkconfig -o $GRUB_CONFIG
Now that the configuration is rebuilt and the menu is flattened, we can parse the file for the available entries. The following command displays the entry index number and title. You can use either of these to refer to a specific entry.
sudo grep 'menuentry ' $GRUB_CONFIG | cut -f 2 -d "'" | nl -v 0
This returns all of your available boot options. Take a note of the index number or title of the entry you wish to boot.
0 CentOS Linux (3.10.0-327.22.2.el7.x86_64) 7 (Core)
1 CentOS Linux (3.10.0-327.18.2.el7.x86_64) 7 (Core)
2 CentOS Linux (0-rescue-d4c27651599f460481bc723aafe36177) 7 (Core)
3 CentOS Linux (0-rescue-ce700ac27e6f4161b0730bad2182e0b1) 7 (Core)
In /etc/default/grub, look for the GRUB_DEFAULT setting. Set this to either the numerical index of the entry you discovered, or the title of the entry:
. . .
GRUB_DEFAULT=1
. . .
Next, if you are running a Debian or Ubuntu Droplet, there is a DigitalOcean-specific Grub file in /etc/default/grub.d named 50-cloudimg-settings.cfg. This file is necessary for the DigitalOcean to boot the image properly, but it contains a default selection that overrides the settings in /etc/default/grub.
In /etc/default/grub.d/50-cloudimg-settings.cfg, comment out the GRUB_DEFAULT line if one is present by adding a # to the beginning of the line.
#GRUB_DEFAULT=0
Next, regenerate the grub.cfg file using the same procedure as before. This applies the new default selection.
On Ubuntu and Debian:
sudo update-grub
On CentOS and Fedora:
sudo grub2-mkconfig -o $GRUB_CONFIG
Finally, reboot the Droplet.
sudo reboot
Your Droplet should now boot into the kernel you selected.