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machine-config-operator

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 OSUpgrades.md.

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).

One known ergonomic issue right now for supplying files is that you must encode file contents via data: URIs. This is part of the current Ignition specification. The easiest way to encode file contents using this scheme is via base64. See the example MachineConfig below on how to provide base64 encoded file contents

In the example below, the mode is in octal (notice the leading 0); however, decimal is the canonical representation for mode when inspecting MachineConfigs (in the example, it's 420 below).

This example MachineConfig object replaces /etc/chrony.conf with some custom NTP time servers; see the chrony docs.

# This example MachineConfig replaces /etc/chrony.conf
apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
metadata:
  labels:
    machineconfiguration.openshift.io/role: worker
  name: 50-examplecorp-chrony
spec:
  config:
    ignition:
      version: 2.2.0
    storage:
      files:
      - contents:
          source: data:text/plain;charset=utf-8;base64,c2VydmVyIGZvby5leGFtcGxlLm5ldCBtYXhkZWxheSAwLjQgb2ZmbGluZQpzZXJ2ZXIgYmFyLmV4YW1wbGUubmV0IG1heGRlbGF5IDAuNCBvZmZsaW5lCnNlcnZlciBiYXouZXhhbXBsZS5uZXQgbWF4ZGVsYXkgMC40IG9mZmxpbmUKZHJpZnRmaWxlIC92YXIvbGliL2Nocm9ueS9kcmlmdAptYWtlc3RlcCAxLjAgMwpydGNzeW5jCmxvZ2RpciAvdmFyL2xvZy9jaHJvbnkK
        filesystem: root
        mode: 0644
        path: /etc/chrony.conf
# oc get machineconfigs -o yaml 50-examplecorp-chrony
apiVersion: machineconfiguration.openshift.io/v1
kind: MachineConfig
metadata:
  creationTimestamp: 2019-03-25T18:25:39Z
  generation: 1
  labels:
    machineconfiguration.openshift.io/role: worker
  name: 50-examplecorp-chrony
  resourceVersion: "186713"
  selfLink: /apis/machineconfiguration.openshift.io/v1/machineconfigs/50-examplecorp-chrony
  uid: 6445154f-4f2b-11e9-91e1-021aaf2ce4c0
spec:
  config:
    ignition:
      version: 2.2.0
    storage:
      files:
      - contents:
          source: data:text/plain;charset=utf-8;base64,c2VydmVyIGZvby5leGFtcGxlLm5ldCBtYXhkZWxheSAwLjQgb2ZmbGluZQpzZXJ2ZXIgYmFyLmV4YW1wbGUubmV0IG1heGRlbGF5IDAuNCBvZmZsaW5lCnNlcnZlciBiYXouZXhhbXBsZS5uZXQgbWF4ZGVsYXkgMC40IG9mZmxpbmUKZHJpZnRmaWxlIC92YXIvbGliL2Nocm9ueS9kcmlmdAptYWtlc3RlcCAxLjAgMwpydGNzeW5jCmxvZ2RpciAvdmFyL2xvZy9jaHJvbnkK
        filesystem: root
        mode: 420
        path: /etc/chrony.conf

The controller will notice the new MachineConfig and generate a new "rendered" version that looks like worker-<hash>. Use oc describe machineconfigpool/worker to monitor the status of the rollout of the new rendered config to each node.

Note this configuration only applies to workers (see the role: worker label); currently if you want to apply to both master and workers, you must create two separate MachineConfig objects.

Practically speaking, one may find it useful to generate your custom MachineConfig objects from a higher level tool. Although in the future ergonomic improvements are planned such as having a single MC apply to multiple labels, inline file encoding, etc.

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 MachineConfiguration.

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 OSUpgrades.md.

Developing the MCO

See HACKING.md.

Frequently Asked Questions

See FAQ.md.

Security Response

If you've found a security issue that you'd like to disclose confidentially please contact Red Hat's Product Security team. Details at https://access.redhat.com/security/team/contact