custom-coreos-disk-images

This repo contains files and instructions for building customized RHCOS (Red Hat CoreOS) disk images that are used for installation and bootstrapping of OpenShift Clusters.

Creating a custom RHCOS container Image

Some background context and some examples for creating layered RHCOS container imags can be found in the OpenShift Documentation. Some of that is reproduced here to provide a full example.

For this to work you will need a registry pull secret. If you have a cluster up and running already then you most likely have that set up. If not, then you should be log in and grab your pull secret from console.redhat.com.

In order to figure out what container image to base your layered container on you can get that from your cluster like:

oc adm release info --image-for rhel-coreos

or from quay using a pull secret like:

oc adm release info --registry-config /path/to/pull-secret --image-for=rhel-coreos quay.io/openshift-release-dev/ocp-release:4.15.1-x86_64

where you can replace 4.15.1 with the version of OpenShift you are currently targeting.

Now you can do a container build. Here is an example Containerfile that layers a single package from EPEL:

FROM scratch
#Enable EPEL (more info at https://docs.fedoraproject.org/en-US/epel/ ) and install htop
RUN rpm-ostree install https://dl.fedoraproject.org/pub/epel/epel-release-latest-9.noarch.rpm && \
    rpm-ostree install podman-tui && \
    ostree container commit

Note that in RHCOS 4.16 and newer, you can also use dnf install instead of rpm-ostree install.

And the command to build the container would look like:

RHCOS_CONTAINER='quay.io/openshift-release-dev/ocp-v4.0-art-dev@sha256:....'
PULL_SECRET=/path/to/pull-secret
podman build \
    --from $RHCOS_CONTAINER \
    --authfile $PULL_SECRET \
    --file Containerfile    \
    --tag quay.io/myorg/myrepo:mytag

Creating disk boot images from the container image

First, we need to convert the image to an OCI archive:

# to pull from local storage
skopeo copy containers-storage:quay.io/myorg/myrepo:mytag oci-archive:my-custom-rhcos.ociarchive
# OR to pull from a registry
skopeo copy --authfile /path/to/pull-secret docker://registry.com/org/repo:latest oci-archive:./my-custom-rhcos.ociarchive

You can now take that ociarchive and create a disk image for a platform (i.e. qemu, metal or gcp). First you need an environment to run OSBuild in. Right now this needs to be a fully up to date Fedora 41 machine with SELinux in permissive mode and some software installed:

sudo dnf update -y
sudo setenforce 0
sudo sed -i -e 's/SELINUX=enforcing/SELINUX=permissive/' /etc/selinux/config
sudo dnf install -y osbuild osbuild-tools osbuild-ostree podman jq xfsprogs e2fsprogs zip

Now you should be able to generate an image with something like:

ociarchive=/path/to/my-custom-rhcos.ociarchive
platform=qemu
sudo ./custom-coreos-disk-images.sh --ociarchive $ociarchive --platforms $platform

Which will create the file my-custom-rhcos.ociarchive.x86_64.qcow2 in the current working directory that can then be used.

Using the container image in the cluster

You will also want to push the custom container image to a registry and point OpenShift at it using a MachineConfig with the osImageURL field set to the image. Otherwise, upon booting, the node will immediately be switched to the default OS image for the target OpenShift version.

Create a MachineConfig like the following:

apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
metadata:
  labels:
    machineconfiguration.openshift.io/role: worker 
  name: custom-image
spec:
  osImageURL: example.com/my/custom-image@sha256... 

If scaling up, you can specify this MachineConfig as usual using oc apply -f.

If installing a cluster, you can specify the MachineConfig at that point so that it's part of the initial bootstrapping. For examples of this, see the documentation at:

https://docs.openshift.com/container-platform/4.17/installing/installing_bare_metal/installing-bare-metal.html#installation-user-infra-generate-k8s-manifest-ignition_installing-bare-metal