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OpenShift 4 is an operator-focused platform, and the Machine Config operator extends that to the operating system itself, managing updates and configuration changes to essentially everything between the kernel and kubelet.

To repeat for emphasis, this operator manages updates to systemd, cri-o/kubelet, kernel, NetworkManager, etc. It also offers a new MachineConfig CRD that can write configuration files onto the host.

The approach here is a "fusion" of code from the original CoreOS Tectonic as well as some components of Red Hat Enterprise Linux Atomic Host, as well as some fundamentally new design.

The MCO (for short) interacts closely with both the installer as well as Red Hat CoreOS. See also the machine-api-operator which handles provisioning of new machines - once the machine-api-operator provisions a machine (with a "pristine" base Red Hat CoreOS), the MCO will take care of configuring it.

One way to view the MCO is to treat the operating system itself as "just another Kubernetes component" that you can inspect and manage with oc.

The MCO uses CoreOS Ignition as a configuration format. Operating system updates use rpm-ostree, with ostree updates encapsulated inside a container image. More information in

As of release 4.12, you can try out OCP CoreOS Layering which lets you use more familiar "Containerfile" (Dockerfile) syntax to apply configuration to your pools.

Sub-components and design

This one git repository generates 4 components in a cluster; the machine-config-operator pod manages the remaining 3 sub-components. Here are links to design docs:

Interacting with the MCO

Because the MCO is a cluster-level operator, you can inspect its status just like any other operator that is part of the release image. If it's reporting success, then that means that the operating system is up to date and configured.

oc describe clusteroperator/machine-config

One level down from the operator CRD, the machineconfigpool objects track updates to a group of nodes. You will often want to run a command like this:

oc describe machineconfigpool

Particularly note the Updated and Updating columns.

Applying configuration changes to the cluster

The MCO has "high level" knobs for some components of the cluster state; for example, SSH keys and kubelet configuration. However, there are obviously a quite large number of things one may want to configure on a system. For example, offline environments may want to specify an internal NTP pool. Another example is static network configuration. By providing a MachineConfig object containing Ignition configuration, systemd units can be provided, arbitrary files can be laid down into writable locations (i.e. /etc and /var).

See the OCP product documentation for more information.

What to look at after creating a MachineConfig

Once you create a MachineConfig fragment like the above, the controller will generate a new "rendered" version that will be used as a target. For more information, see MachineConfig.

In particular, you should look at oc describe machineconfigpool and oc describe clusteroperator/machine-config as noted above.

More information about OS updates

The model implemented by the MCO is that the cluster controls the operating system. OS updates are just another entry in the release image. For more information, see

Developing the MCO


Frequently Asked Questions


Security Response

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