Chapter 16. RHCOS image layering

PDF

Red Hat Enterprise Linux CoreOS (RHCOS) image layering allows you to easily extend the functionality of your base RHCOS image by layering additional images onto the base image. This layering does not modify the base RHCOS image. Instead, it creates a custom layered image that includes all RHCOS functionality and adds additional functionality to specific nodes in the cluster.

You create a custom layered image by using a Containerfile and applying it to nodes by using a MachineConfig object. The Machine Config Operator overrides the base RHCOS image, as specified by the osImageURL value in the associated machine config, and boots the new image. You can remove the custom layered image by deleting the machine config, The MCO reboots the nodes back to the base RHCOS image.

With RHCOS image layering, you can install RPMs into your base image, and your custom content will be booted alongside RHCOS. The Machine Config Operator (MCO) can roll out these custom layered images and monitor these custom containers in the same way it does for the default RHCOS image. RHCOS image layering gives you greater flexibility in how you manage your RHCOS nodes.

Important

Installing realtime kernel and extensions RPMs as custom layered content is not recommended. This is because these RPMs can conflict with RPMs installed by using a machine config. If there is a conflict, the MCO enters a degraded state when it tries to install the machine config RPM. You need to remove the conflicting extension from your machine config before proceeding.

As soon as you apply the custom layered image to your cluster, you effectively take ownership of your custom layered images and those nodes. While Red Hat remains responsible for maintaining and updating the base RHCOS image on standard nodes, you are responsible for maintaining and updating images on nodes that use a custom layered image. You assume the responsibility for the package you applied with the custom layered image and any issues that might arise with the package.

Important

Image layering is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information about the support scope of Red Hat Technology Preview features, see Technology Preview Features Support Scope.

Currently, RHCOS image layering allows you to work with Customer Experience and Engagement (CEE) to obtain and apply Hotfix packages on top of your RHCOS image. In some instances, you might want a bug fix or enhancement before it is included in an official OpenShift Container Platform release. RHCOS image layering allows you to easily add the Hotfix before it is officially released and remove the Hotfix when the underlying RHCOS image incorporates the fix.

Important

Some Hotfixes require a Red Hat Support Exception and are outside of the normal scope of OpenShift Container Platform support coverage or life cycle policies.

In the event you want a Hotfix, it will be provided to you based on Red Hat Hotfix policy. Apply it on top of the base image and test that new custom layered image in a non-production environment. When you are satisfied that the custom layered image is safe to use in production, you can roll it out on your own schedule to specific node pools. For any reason, you can easily roll back the custom layered image and return to using the default RHCOS.

Note

It is planned for future releases that you can use RHCOS image layering to incorporate third-party software packages such as libreswan or numactl.

To apply a custom layered image, you create a Containerfile that references an OpenShift Container Platform image and the Hotfix that you want to apply. For example:

Example Containerfile to apply a Hotfix

# Using a 4.12.0 image
FROM quay.io/openshift-release-dev/ocp-release@sha256...
#Install hotfix rpm
RUN rpm-ostree override replace https://example.com/myrepo/haproxy-1.0.16-5.el8.src.rpm && \
    rpm-ostree cleanup -m && \
    ostree container commit

Note

Use the same base RHCOS image installed on the rest of your cluster. Use the oc adm release info --image-for rhel-coreos-8 command to obtain the base image used in your cluster.

Push the resulting custom layered image to an image registry. In a non-production OpenShift Container Platform cluster, create a MachineConfig object for the targeted node pool that points to the new image.

The Machine Config Operator (MCO) updates the operating system with content provided in the machine config. This creates a custom layered image that overrides the base RHCOS image on those nodes.

After you create the machine config, the MCO:

Renders a new machine config for the specified pool or pools.
Performs cordon and drain operations on the nodes in the pool or pools.
Writes the rest of the machine config parameters onto the nodes.
Applies the custom layered image to the node.
Reboots the node using the new image.

Important

It is strongly recommended that you test your images outside of your production environment before rolling out to your cluster.

16.1. Applying a RHCOS custom layered image

You can easily configure Red Hat Enterprise Linux CoreOS (RHCOS) image layering on the nodes in specific machine config pools. The Machine Config Operator (MCO) reboots those nodes with the new custom layered image, overriding the base Red Hat Enterprise Linux CoreOS (RHCOS) image.

To apply a custom layered image to your cluster, you must have the custom layered image in a repository that your cluster can access. Then, create a MachineConfig object that points to the custom layered image. You need a separate MachineConfig object for each machine config pool that you want to configure.

Important

When you configure a custom layered image, OpenShift Container Platform no longer automatically updates any node that uses the custom layered image. You become responsible for manually updating your nodes as appropriate. If you roll back the custom layer, OpenShift Container Platform will again automatically update the node. See the Additional resources section that follows for important information about updating nodes that use a custom layered image.

Prerequisites

You must create a custom layered image that is based on an OpenShift Container Platform image digest, not a tag.
Note
You should use the same base RHCOS image that is installed on the rest of your cluster. Use the oc adm release info --image-for rhel-coreos-8 command to obtain the base image being used in your cluster.
For example, the following Containerfile creates a custom layered image from an OpenShift Container Platform 4.12 image and overrides the kernel package with one from CentOS 8 Stream:
Example Containerfile for a custom layer image
```
# Using a 4.12.0 image
FROM quay.io/openshift-release/ocp-release@sha256... 1
#Install hotfix rpm
RUN rpm-ostree cliwrap install-to-root / \ 2
    rpm-ostree override replace http://mirror.centos.org/centos/8-stream/BaseOS/x86_64/os/Packages/kernel-{,core-,modules-,modules-extra-}4.18.0-483.el8.x86_64.rpm && \ 3
    rpm-ostree cleanup -m && \
    ostree container commit
```
1
Specifies the RHCOS base image of your cluster.
2
Enables cliwrap. This is currently required to intercept some command invocations made from kernel scripts.
3
Replaces the kernel packages.
Note
Instructions on how to create a Containerfile are beyond the scope of this documentation.
Because the process for building a custom layered image is performed outside of the cluster, you must use the --authfile /path/to/pull-secret option with Podman or Buildah. Alternatively, to have the pull secret read by these tools automatically, you can add it to one of the default file locations: ~/.docker/config.json, $XDG_RUNTIME_DIR/containers/auth.json, ~/.docker/config.json, or ~/.dockercfg. Refer to the containers-auth.json man page for more information.
You must push the custom layered image to a repository that your cluster can access.

Procedure

Create a machine config file.
1. Create a YAML file similar to the following:
```
apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
metadata:
  labels:
    machineconfiguration.openshift.io/role: worker 1
  name: os-layer-hotfix
spec:
  osImageURL: quay.io/my-registry/custom-image@sha256... 2
```
  1
  Specifies the machine config pool to apply the custom layered image.
  2
  Specifies the path to the custom layered image in the repository.
2. Create the MachineConfig object:
```
$ oc create -f <file_name>.yaml
```
  Important
  It is strongly recommended that you test your images outside of your production environment before rolling out to your cluster.

Verification

You can verify that the custom layered image is applied by performing any of the following checks:

Check that the worker machine config pool has rolled out with the new machine config:

Check that the new machine config is created:

$ oc get mc

Sample output

NAME                                               GENERATEDBYCONTROLLER                      IGNITIONVERSION   AGE
00-master                                          5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
00-worker                                          5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
01-master-container-runtime                        5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
01-master-kubelet                                  5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
01-worker-container-runtime                        5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
01-worker-kubelet                                  5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
99-master-generated-registries                     5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
99-master-ssh                                                                                 3.2.0             98m
99-worker-generated-registries                     5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
99-worker-ssh                                                                                 3.2.0             98m
os-layer-hotfix                                                                                                 10s 1
rendered-master-15961f1da260f7be141006404d17d39b   5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
rendered-worker-5aff604cb1381a4fe07feaf1595a797e   5bdb57489b720096ef912f738b46330a8f577803   3.2.0             95m
rendered-worker-5de4837625b1cbc237de6b22bc0bc873   5bdb57489b720096ef912f738b46330a8f577803   3.2.0             4s  2

1: New machine config
2: New rendered machine config

Check that the osImageURL value in the new machine config points to the expected image:

$ oc describe mc rendered-master-4e8be63aef68b843b546827b6ebe0913

Example output

Name:         rendered-master-4e8be63aef68b843b546827b6ebe0913
Namespace:
Labels:       <none>
Annotations:  machineconfiguration.openshift.io/generated-by-controller-version: 8276d9c1f574481043d3661a1ace1f36cd8c3b62
              machineconfiguration.openshift.io/release-image-version: 4.12.0-ec.3
API Version:  machineconfiguration.openshift.io/v1
Kind:         MachineConfig
...
  Os Image URL: quay.io/my-registry/custom-image@sha256...

Check that the associated machine config pool is updating with the new machine config:

$ oc get mcp

Sample output

NAME     CONFIG                                             UPDATED   UPDATING   DEGRADED   MACHINECOUNT   READYMACHINECOUNT   UPDATEDMACHINECOUNT   DEGRADEDMACHINECOUNT   AGE
master   rendered-master-6faecdfa1b25c114a58cf178fbaa45e2   True      False      False      3              3                   3                     0                      39m
worker   rendered-worker-6b000dbc31aaee63c6a2d56d04cd4c1b   False     True       False      3              0                   0                     0                      39m 1

1: When the UPDATING field is True, the machine config pool is updating with the new machine config. When the field becomes False, the worker machine config pool has rolled out to the new machine config.

Check the nodes to see that scheduling on the nodes is disabled. This indicates that the change is being applied:

$ oc get nodes

Example output

NAME                                         STATUS                     ROLES                  AGE   VERSION
ip-10-0-148-79.us-west-1.compute.internal    Ready                      worker                 32m   v1.25.0+3ef6ef3
ip-10-0-155-125.us-west-1.compute.internal   Ready,SchedulingDisabled   worker                 35m   v1.25.0+3ef6ef3
ip-10-0-170-47.us-west-1.compute.internal    Ready                      control-plane,master   42m   v1.25.0+3ef6ef3
ip-10-0-174-77.us-west-1.compute.internal    Ready                      control-plane,master   42m   v1.25.0+3ef6ef3
ip-10-0-211-49.us-west-1.compute.internal    Ready                      control-plane,master   42m   v1.25.0+3ef6ef3
ip-10-0-218-151.us-west-1.compute.internal   Ready                      worker                 31m   v1.25.0+3ef6ef3

When the node is back in the Ready state, check that the node is using the custom layered image:
1. Open an oc debug session to the node. For example:
```
$ oc debug node/ip-10-0-155-125.us-west-1.compute.internal
```
2. Set /host as the root directory within the debug shell:
```
sh-4.4# chroot /host
```
3. Run the rpm-ostree status command to view that the custom layered image is in use:
```
sh-4.4# sudo rpm-ostree status
```
  Example output
```
State: idle
Deployments:
* ostree-unverified-registry:quay.io/my-registry/...
                   Digest: sha256:...
```

Additional resources

Updating with a RHCOS custom layered image

16.2. Removing a RHCOS custom layered image

You can easily revert Red Hat Enterprise Linux CoreOS (RHCOS) image layering from the nodes in specific machine config pools. The Machine Config Operator (MCO) reboots those nodes with the cluster base Red Hat Enterprise Linux CoreOS (RHCOS) image, overriding the custom layered image.

To remove a Red Hat Enterprise Linux CoreOS (RHCOS) custom layered image from your cluster, you need to delete the machine config that applied the image.

Procedure

Delete the machine config that applied the custom layered image.
```
$ oc delete mc os-layer-hotfix
```
After deleting the machine config, the nodes reboot.

Verification

You can verify that the custom layered image is removed by performing any of the following checks:

Check that the worker machine config pool is updating with the previous machine config:

$ oc get mcp

Sample output

NAME     CONFIG                                             UPDATED   UPDATING   DEGRADED   MACHINECOUNT   READYMACHINECOUNT   UPDATEDMACHINECOUNT   DEGRADEDMACHINECOUNT   AGE
master   rendered-master-6faecdfa1b25c114a58cf178fbaa45e2   True      False      False      3              3                   3                     0                      39m
worker   rendered-worker-6b000dbc31aaee63c6a2d56d04cd4c1b   False     True       False      3              0                   0                     0                      39m 1

1: When the UPDATING field is True, the machine config pool is updating with the previous machine config. When the field becomes False, the worker machine config pool has rolled out to the previous machine config.

Check the nodes to see that scheduling on the nodes is disabled. This indicates that the change is being applied:

$ oc get nodes

Example output

NAME                                         STATUS                     ROLES                  AGE   VERSION
ip-10-0-148-79.us-west-1.compute.internal    Ready                      worker                 32m   v1.25.0+3ef6ef3
ip-10-0-155-125.us-west-1.compute.internal   Ready,SchedulingDisabled   worker                 35m   v1.25.0+3ef6ef3
ip-10-0-170-47.us-west-1.compute.internal    Ready                      control-plane,master   42m   v1.25.0+3ef6ef3
ip-10-0-174-77.us-west-1.compute.internal    Ready                      control-plane,master   42m   v1.25.0+3ef6ef3
ip-10-0-211-49.us-west-1.compute.internal    Ready                      control-plane,master   42m   v1.25.0+3ef6ef3
ip-10-0-218-151.us-west-1.compute.internal   Ready                      worker                 31m   v1.25.0+3ef6ef3

When the node is back in the Ready state, check that the node is using the base image:

Open an oc debug session to the node. For example:

$ oc debug node/ip-10-0-155-125.us-west-1.compute.internal

Set /host as the root directory within the debug shell:
```
sh-4.4# chroot /host
```

Run the rpm-ostree status command to view that the custom layered image is in use:

sh-4.4# sudo rpm-ostree status

Example output

State: idle
Deployments:
* ostree-unverified-registry:podman pull quay.io/openshift-release-dev/ocp-release@sha256:e2044c3cfebe0ff3a99fc207ac5efe6e07878ad59fd4ad5e41f88cb016dacd73
                   Digest: sha256:e2044c3cfebe0ff3a99fc207ac5efe6e07878ad59fd4ad5e41f88cb016dacd73

16.3. Updating with a RHCOS custom layered image

When you configure Red Hat Enterprise Linux CoreOS (RHCOS) image layering, OpenShift Container Platform no longer automatically updates the node pool that uses the custom layered image. You become responsible to manually update your nodes as appropriate.

To update a node that uses a custom layered image, follow these general steps:

The cluster automatically upgrades to version x.y.z+1, except for the nodes that use the custom layered image.
You could then create a new Containerfile that references the updated OpenShift Container Platform image and the RPM that you had previously applied.
Create a new machine config that points to the updated custom layered image.

Updating a node with a custom layered image is not required. However, if that node gets too far behind the current OpenShift Container Platform version, you could experience unexpected results.

Chapter 16. RHCOS image layering

16.1. Applying a RHCOS custom layered image

16.2. Removing a RHCOS custom layered image

16.3. Updating with a RHCOS custom layered image

Learn

Try, buy, & sell

Communities

About Red Hat Documentation

Making open source more inclusive

About Red Hat

Red Hat legal and privacy links

Red Hat legal and privacy links