Chapter 6. Red Hat Enterprise Linux CoreOS (RHCOS)

Red Hat Enterprise Linux CoreOS (RHCOS) represents the next generation of single-purpose container operating system technology by providing the quality standards of Red Hat Enterprise Linux (RHEL) with automated, remote upgrade features.

RHCOS is supported only as a component of OpenShift Container Platform 4.8 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. There are two general ways RHCOS is deployed in OpenShift Container Platform 4.8:

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": {
    "version": "3.2.0"
  },
  "passwd": {
    "users": [
      {
        "name": "core",
        "sshAuthorizedKeys": [
          "ssh-rsa AAAAB3NzaC1yc...."
        ]
      }
    ]
  },
  "storage": {
    "files": [
      {
        "overwrite": false,
        "path": "/etc/motd",
        "user": {
          "name": "root"
        },
        "append": [
          {
            "source": "data:text/plain;charset=utf-8;base64,VGhpcyBpcyB0aGUgYm9vdHN0cmFwIG5vZGU7IGl0IHdpbGwgYmUgZGVzdHJveWVkIHdoZW4gdGhlIG1hc3RlciBpcyBmdWxseSB1cC4KClRoZSBwcmltYXJ5IHNlcnZpY2VzIGFyZSByZWxlYXNlLWltYWdlLnNlcnZpY2UgZm9sbG93ZWQgYnkgYm9vdGt1YmUuc2VydmljZS4gVG8gd2F0Y2ggdGhlaXIgc3RhdHVzLCBydW4gZS5nLgoKICBqb3VybmFsY3RsIC1iIC1mIC11IHJlbGVhc2UtaW1hZ2Uuc2VydmljZSAtdSBib290a3ViZS5zZXJ2aWNlCg=="
          }
        ],
        "mode": 420
      },
...

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 VGhpcyBpcyB0aGUgYm9vdHN0cmFwIG5vZGU7IGl0IHdpbGwgYmUgZGVzdHJveWVkIHdoZW4gdGhlIG1hc3RlciBpcyBmdWxseSB1cC4KClRoZSBwcmltYXJ5IHNlcnZpY2VzIGFyZSByZWxlYXNlLWltYWdlLnNlcnZpY2UgZm9sbG93ZWQgYnkgYm9vdGt1YmUuc2VydmljZS4gVG8gd2F0Y2ggdGhlaXIgc3RhdHVzLCBydW4gZS5nLgoKICBqb3VybmFsY3RsIC1iIC1mIC11IHJlbGVhc2UtaW1hZ2Uuc2VydmljZSAtdSBib290a3ViZS5zZXJ2aWNlCg== | base64 --decode

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

The primary services are release-image.service followed by bootkube.service. To watch their status, run e.g.

  journalctl -b -f -u release-image.service -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:

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

NAME                                      GENERATEDBYCONTROLLER   IGNITIONVERSION   CREATED   OSIMAGEURL

00-master                                 4.0.0-0.150.0.0-dirty   3.2.0             16m
00-master-ssh                             4.0.0-0.150.0.0-dirty                     16m
00-worker                                 4.0.0-0.150.0.0-dirty   3.2.0             16m
00-worker-ssh                             4.0.0-0.150.0.0-dirty                     16m
01-master-kubelet                         4.0.0-0.150.0.0-dirty   3.2.0             16m
01-worker-kubelet                         4.0.0-0.150.0.0-dirty   3.2.0             16m
master-1638c1aea398413bb918e76632f20799   4.0.0-0.150.0.0-dirty   3.2.0             16m
worker-2feef4f8288936489a5a832ca8efe953   4.0.0-0.150.0.0-dirty   3.2.0             16m

The Machine Config Operator acts somewhat differently than Ignition when it comes to applying these machine configs. The machine configs are read in order (from 00* to 99*). Labels inside the machine configs identify the type of node each is for (master or worker). If the same file appears in multiple machine config 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 machine config pool.

To see what files are being managed from a machine config, look for "Path:" inside a particular MachineConfig object. 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

Be sure to give the machine config file 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 machine config to the cluster.