With recent releases of OpenShift like 4.13 you can use CoreOS Layering to apply custom images to the nodes.

The feature allows to build, via a Dockerfile a custom image that can later be applied to our nodes.

Let’s review the steps:

  1. First we need to find the base image being used in our environment with oc adm release info quay.io/openshift-release-dev/ocp-release:4.13.5-aarch64 --image-for=rhel-coreos
  2. Then we use the returned value in the FROM line in our Dockerfile
  3. If we want to add custom packages, we should have a server which is reachable and run createrepo on the folder containing the rpm’s so that rpm-ostree can download them for installation.

Example dockerfile:

# Filled from the value obtained at 1st step
FROM quay.io/openshift-release-dev/ocp-v4.0-art-dev@sha256:394b83e0d67ea49314ba250e6e32710c5a7b807a19746c19f4f16d350b8636dd

RUN echo -e '[rt]\nbaseurl=\nenabled=1\ngpgcheck=0\n' > /etc/yum.repos.d/rtkernel.repo

# Note the following command is documented with a different approach, but this one is working, a bug was raised for fixing official docs.
RUN rpm-ostree cliwrap install-to-root /

# Perform the package installation step, in this case, removing the original kernel and installing another kernel instead
RUN rpm-ostree override remove kernel{,-core,-modules,-modules-extra,-modules-core} --install=gobject-introspection --install=hdparm --install=kernel-rt --install=kernel-rt-core --install=kernel-rt-modules-core --install=libperf --install=libtraceevent --install=python3-dbus --install=python3-linux-procfs --install=python3-perf --install=python3-six --install=realtime-setup --install=tuna --install=tuned --install=tuned-profiles-realtime --install=virt-what --install=python3-pyudev --install=python3-gobject-base --install=kernel-rt-modules --install=python3-gobject-base-noarch
RUN     rpm-ostree cleanup -m
RUN    ostree container commit

Later, to build the container we will execute podman build . --authfile /var/lib/kubelet/config.json so that it has access to download the required images (FROM field).

And we’re ready to push our newly created image to the registry, for example, if our image id is: b2d0af2a733

So as last step we can perform podman push b2d0af2a733 quay.io/youruser/yourimage:tag to upload the image for later consumption.

Then, we prepare a manifest for applying the change by MCO:

apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
    machineconfiguration.openshift.io/role: master
  name: rhcos-kernelrt
  osImageURL: myregistry/user/kcoreos-9:kernelrt

And later apply with oc apply -f mycustomkernel.yaml… MCO will take over and will start rolling out the change as we can monitor with oc describe mc rendered-master-XXXXXX.

If you want to rollback the change, we can of course, perform oc delete -f mycustomerkernel.yaml and MCO will revert the change.

We can check current booted OS image with rpm-ostree status:

sh-5.1# rpm-ostree  status
State: idle
* ostree-unverified-registry:myregistry/user/kcoreos-9:kernelrt
                   Digest: sha256:802d2f6bf8a6aef02c90ee1212de03f1888772dec6b34ab0dae97272cd6a917c
                  Version: 413.92.202307140015-0 (2023-11-07T17:16:18Z)

Enjoy! (and if you do, you can Buy Me a Coffee )