Red Hat Enterprise Linux CoreOS (RHCOS) represents the next generation of single-purpose container operating system technology. Created by the same development teams that created Red Hat Enterprise Linux Atomic Host and CoreOS Container Linux, RHCOS combines the quality standards of Red Hat Enterprise Linux (RHEL) with the automated, remote upgrade features from Container Linux.

RHCOS is supported only as a component of OpenShift Container Platform 4.1 for all OpenShift Container Platform machines. RHCOS is the only supported operating system for OpenShift Container Platform control plane, or master, machines. While RHCOS is the default operating system for all cluster machines, you can create compute machines, which are also known as worker machines, that use RHEL as their operating system.

If you install your cluster on infrastructure that the cluster provisions, RHCOS images are downloaded to the target platform during installation, and suitable Ignition config files, which control the RHCOS configuration, are used to deploy the machines. If you install your cluster on infrastructure that you manage, you must follow the installation documentation to obtain the RHCOS images, generate Ignition config files, and use the Ignition config files to provision your machines.

Key RHCOS features

The following list describes key features of the RHCOS operating system:

The underlying operating system consists primarily of RHEL components. The same quality, security, and control measures that support RHEL also support RHCOS. For example, RHCOS software is in RPM packages, and each RHCOS system starts up with a RHEL kernel and a set of services that are managed by the systemd init system.

Although it contains RHEL components, RHCOS is designed to be managed more tightly than a default RHEL installation. Management is performed remotely from the OpenShift Container Platform cluster. When you set up your RHCOS machines, you can modify only a few system settings. This controlled immutability allows OpenShift Container Platform to store the latest state of RHCOS systems in the cluster so it is always able to create additional machines and perform updates based on the latest RHCOS configurations.

Although RHCOS contains features for running the OCI- and libcontainer-formatted containers that Docker requires, it incorporates the CRI-O container engine instead of the Docker container engine. By focusing on features needed by Kubernetes platforms, such as OpenShift Container Platform, CRI-O can offer specific compatibility with different Kubernetes versions. CRI-O also offers a smaller footprint and reduced attack surface than is possible with container engines that offer a larger feature set. At the moment, CRI-O is only available as a container engine within OpenShift Container Platform clusters.

For tasks such as building, copying, and otherwise managing containers, RHCOS replaces the Docker CLI tool with a compatible set of container tools. The podman CLI tool supports many container runtime features, such as running, starting, stopping, listing, and removing containers and container images. The skopeo CLI tool can copy, authenticate, and sign images. You can use the crictl CLI tool to work with containers and pods from the CRI-O container engine. While direct use of these tools in RHCOS is discouraged, you can use them for debugging purposes.

RHCOS features transactional upgrades using the rpm-ostree system. Updates are delivered via container images and are part of the OpenShift update process. When deployed, the container image is pulled, extracted, and written to disk, then the bootloader is modified to boot into the new version. The machine will reboot into the update in a rolling manner to ensure cluster capacity is minimally impacted.

For RHCOS systems, the layout of the rpm-ostree file system has the following characteristics:

  • /usr is where the operating system binaries and libraries are stored and is read-only. We do not support altering this.

  • /etc, /boot, /var are writable on the system but only intended to be altered by the Machine Config Operator.

  • /var/lib/containers is the graph storage location for storing container images.

In OpenShift Container Platform, the Machine Config Operator handles operating system upgrades. Instead of upgrading individual packages, as is done with yum upgrades, rpm-ostree delivers upgrades of the OS as an atomic unit. The new OS deployment is staged during upgrades and goes into effect on the next reboot. If something goes wrong with the upgrade, a single rollback and reboot returns the system to the previous state. RHCOS upgrades in OpenShift Container Platform are performed during cluster updates.

Configuring RHCOS in OpenShift Container Platform

For OpenShift Container Platform, RHCOS images are set up initially with a feature called Ignition, which runs only on the system’s first boot. After first boot, RHCOS systems are managed by the Machine Config Operator (MCO) that runs in the OpenShift Container Platform cluster.

Because RHCOS systems in OpenShift Container Platform are designed to be fully managed from the OpenShift Container Platform cluster, directly logging in to a RHCOS machine is discouraged. Limited direct access to RHCOS machines in an OpenShift Container Platform cluster can be completed for debugging purposes.

About Ignition

Ignition is the utility that is used by RHCOS to manipulate disks during initial configuration. It completes common disk tasks, including partitioning disks, formatting partitions, writing files, and configuring users. On first boot, Ignition reads its configuration from the installation media or the location that you specify and applies the configuration to the machines.

Whether you are installing your cluster or adding machines to it, Ignition always performs the initial configuration of the OpenShift Container Platform cluster machines. Most of the actual system setup happens on each machine itself. For each machine, Ignition takes the RHCOS image and boots the RHCOS kernel. Options on the kernel command line, identify the type of deployment and the location of the Ignition-enabled initial Ram Disk (initramfs).

OpenShift Container Platform uses Ignition version 2 and Ignition config version 2.3

How Ignition works

To create machines by using Ignition, you need Ignition config files. The OpenShift Container Platform installation program creates the Ignition config files that you need to deploy your cluster. These files are based on the information that you provide to the installation program directly or through an install-config.yaml file.

The way that Ignition configures machines is similar to how tools like cloud-init or Linux Anaconda kickstart configure systems, but with some important differences:

  • Ignition runs from an initial RAM disk that is separate from the system you are installing to. Because of that, Ignition can repartition disks, set up file systems, and perform other changes to the machine’s permanent file system. In contrast, cloud-init runs as part of a machine’s init system when the system boots, so making foundational changes to things like disk partitions cannot be done as easily. With cloud-init, it is also difficult to reconfigure the boot process while you are in the middle of the node’s boot process.

  • Ignition is meant to initialize systems, not change existing systems. After a machine initializes and the kernel is running from the installed system, the Machine Config Operator from the OpenShift Container Platform cluster completes all future machine configuration.

  • Instead of completing a defined set of actions, Ignition implements a declarative configuration. It checks that all partitions, files, services, and other items are in place before the new machine starts. It then makes the changes, like copying files to disk that are necessary for the new machine to meet the specified configuration.

  • After Ignition finishes configuring a machine, the kernel keeps running but discards the initial RAM disk and pivots to the installed system on disk. All of the new system services and other features start without requiring a system reboot.

  • Because Ignition confirms that all new machines meet the declared configuration, you cannot have a partially-configured machine. If a machine’s setup fails, the initialization process does not finish, and Ignition does not start the new machine. Your cluster will never contain partially-configured machines. If Ignition cannot complete, the machine is not added to the cluster. You must add a new machine instead. This behavior prevents the difficult case of debugging a machine when the results of a failed configuration task are not known until something that depended on it fails at a later date.

  • If there is a problem with an Ignition config that causes the setup of a machine to fail, Ignition will not try to use the same config to set up another machine. For example, a failure could result from an Ignition config made up of a parent and child config that both want to create the same file. A failure in such a case would prevent that Ignition config from being used again to set up an other machines, until the problem is resolved.

  • If you have multiple Ignition config files, you get a union of that set of configs. Because Ignition is declarative, conflicts between the configs could cause Ignition to fail to set up the machine. The order of information in those files doesn’t matter. Ignition will sort and implement each setting in ways that make the most sense. For example, if a file needs a directory several levels deep, if another file needs a directory along that path, the later file is created first. Ignition sorts and creates all files, directories, and links by depth.

  • Because Ignition can start with a completely empty hard disk, it can do something cloud-init can’t do: set up systems on bare metal from scratch (using features such as PXE boot). In the bare metal case, the Ignition config is injected into the boot partition so Ignition can find it and configure the system correctly.

The Ignition sequence

The Ignition process for an RHCOS machine in an OpenShift Container Platform cluster involves the following steps:

  • The machine gets its Ignition config file. Master machines get their Ignition config files from the bootstrap machine, and worker machines get Ignition config files from a master.

  • Ignition creates disk partitions, file systems, directories, and links on the machine. It supports RAID arrays but does not support LVM volumes

  • Ignition mounts the root of the permanent file system to the /sysroot directory in the initramfs and starts working in that /sysroot directory.

  • Ignition configures all defined file systems and sets them up to mount appropriately at runtime.

  • Ignition runs systemd temporary files to populate required files in the /var directory.

  • Ignition runs the Ignition config files to set up users, systemd unit files, and other configuration files.

  • Ignition unmounts all components in the permanent system that were mounted in the initramfs.

  • Ignition starts up new machine’s init process which, in turn, starts up all other services on the machine that run during system boot.

The machine is then ready to join the cluster and does not require a reboot.

Viewing Ignition configuration files

To see the Ignition config file used to deploy the bootstrap machine, run the following command:

$ openshift-install create ignition-configs --dir $HOME/testconfig

After you answer a few questions, the bootstrap.ign, master.ign, and worker.ign files appear in the directory you entered.

To see the contents of the bootstrap.ign file, pipe it through the jq filter. Here’s a snippet from that file:

$ cat $HOME/testconfig/bootstrap.ign | jq


  "ignition": \\{

        "config": \\{},


  "storage": \\{

        "files": [


            "filesystem": "root",

            "path": "/etc/motd",

            "user": \\{

              "name": "root"


            "append": true,

            "contents": \\{

              "source": "data:text/plain;charset=utf-8;base64,VGhpcyBpcyB0aGUgYm9vdHN0cmFwIG5vZGU7IGl0IHdpbGwgYmUgZGVzdHJveWVkIHdoZW4gdGhlIG1hc3RlciBpcyBmdWxseSB1cC4KClRoZSBwcmltYXJ5IHNlcnZpY2UgaXMgImJvb3RrdWJlLnNlcnZpY2UiLiBUbyB3YXRjaCBpdHMgc3RhdHVzLCBydW4gZS5nLgoKICBqb3VybmFsY3RsIC1iIC1mIC11IGJvb3RrdWJlLnNlcnZpY2UK",

To decode the contents of a file listed in the bootstrap.ign file, pipe the base64-encoded data string representing the contents of that file to the base64 -d command. Here’s an example using the contents of the /etc/motd file added to the bootstrap machine from the output shown above:

$ echo VGhpcyBpcyB0aGUgYm9vdHN0cmFwIG5vZGU7IGl0IHdpbGwgYmUgZGVzdHJveWVkIHdoZW4gdGhlIG1hc3RlciBpcyBmdWxseSB1cC4KClRoZSBwcmltYXJ5IHNlcnZpY2UgaXMgImJvb3RrdWJlLnNlcnZpY2UiLiBUbyB3YXRjaCBpdHMgc3RhdHVzLCBydW4gZS5nLgoKICBqb3VybmFsY3RsIC1iIC1mIC11IGJvb3RrdWJlLnNlcnZpY2UK | base64 -d

This is the bootstrap machine; it will be destroyed when the master is fully up.

The primary service is "bootkube.service". To watch its status, run, e.g.:

journalctl -b -f -u bootkube.service

Repeat those commands on the master.ign and worker.ign files to see the source of Ignition config files for each of those machine types.  You should see a line like the following for the worker.ign, identifying how it gets its Ignition config from the bootstrap machine:

"source": "https://api.myign.develcluster.example.com:22623/config/worker",

Here are a few things you can learn from the bootstrap.ign file:

  • Format: The format of the file is defined in the Ignition config spec. Files of the same format are used later by the MCO to merge changes into a machine’s configuration.

  • Contents: Because the bootstrap machine serves the Ignition configs for other machines, both master and worker machine Ignition config information is stored in the bootstrap.ign, along with the bootstrap machine’s configuration.

  • Size: The file is more than 1300 lines long, with path to various types of resources.

  • The content of each file that will be copied to the machine is actually encoded into data URLs, which tends to make the content a bit clumsy to read. (Use the jq and base64 commands shown previously to make the content more readable.)

  • Configuration: The different sections of the Ignition config file are generally meant to contain files that are just dropped into a machine’s file system, rather than commands to modify existing files. For example, instead of having a section on NFS that configures that service, you would just add an NFS configuration file, which would then be started by the init process when the system comes up.

  • users: A user named core is created, with your ssh key assigned to that user. This allows you to log in to the cluster with that user name and your credentials.

  • storage: The storage section identifies files that are added to each machine. A few notable files include /root/.docker/config.json (which provides credentials your cluster needs to pull from container image registries) and a bunch of manifest files in /opt/openshift/manifests that are used to configure your cluster.

  • systemd: The systemd section holds content used to create systemd unit files. Those files are used to start up services at boot time, as well as manage those services on running systems.

  • Primitives: Ignition also exposes low-level primitives that other tools can build on.

Changing Ignition Configs after installation

Machine Config Pools manage a cluster of nodes and their corresponding Machine Configs. Machine Configs contain configuration information for a cluster. To list all Machine Config Pools that are known:

$ oc get machineconfigpools

NAME   CONFIG                                  UPDATED UPDATING DEGRADED
master master-1638c1aea398413bb918e76632f20799 False   False    False
worker worker-2feef4f8288936489a5a832ca8efe953 False   False    False

To list all Machine Configs:

$ oc get machineconfig


00-master                                 4.0.0-   2.2.0             16m
00-master-ssh                             4.0.0-                     16m
00-worker                                 4.0.0-   2.2.0             16m
00-worker-ssh                             4.0.0-                     16m
01-master-kubelet                         4.0.0-   2.2.0             16m
01-worker-kubelet                         4.0.0-   2.2.0             16m
master-1638c1aea398413bb918e76632f20799   4.0.0-   2.2.0             16m
worker-2feef4f8288936489a5a832ca8efe953   4.0.0-   2.2.0             16m

The Machine Config Operator acts somewhat differently than Ignition when it comes to applying these machineconfigs. The machineconfigs are read in order (from 00* to 99*). Labels inside the machineconfigs identify the type of node each is for (master or worker). If the same file appears in multiple machineconfig files, the last one wins. So, for example, any file that appears in a 99* file would replace the same file that appeared in a 00* file. The input machineconfig objects are unioned into a "rendered" machineconfig object, which will be used as a target by the operator and is the value you can see in the machineconfigpool.

To see what files are being managed from a machineconfig, look for “Path:” inside a particular machineconfig. For example:

$ oc describe machineconfigs 01-worker-container-runtime | grep Path:
            Path:            /etc/containers/registries.conf
            Path:            /etc/containers/storage.conf
            Path:            /etc/crio/crio.conf

If you wanted to change a setting in one of those files, for example to change pids_limit to 1500 (pids_limit = 1500) inside the crio.conf file, you could create a new machineconfig containing only the file you want to change.

Be sure to give the machineconfig a later name (such as 10-worker-container-runtime). Keep in mind that the content of each file is in URL-style data. Then apply the new machineconfig to the cluster.