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OKD uses a KubeletConfig custom resource (CR) to manage the configuration of nodes. By creating an instance of a KubeletConfig object, a managed machine config is created to override setting on the node.

Logging in to remote machines for the purpose of changing their configuration is not supported.

Modifying nodes

To make configuration changes to a cluster, or machine pool, you must create a custom resource definition (CRD), or kubeletConfig object. OKD uses the Machine Config Controller to watch for changes introduced through the CRD to apply the changes to the cluster.

Because the fields in a kubeletConfig object are passed directly to the kubelet from upstream Kubernetes, the validation of those fields is handled directly by the kubelet itself. Please refer to the relevant Kubernetes documentation for the valid values for these fields. Invalid values in the kubeletConfig object can render cluster nodes unusable.

Procedure
  1. Obtain the label associated with the static CRD, Machine Config Pool, for the type of node you want to configure. Perform one of the following steps:

    1. Check current labels of the desired machine config pool.

      For example:

      $  oc get machineconfigpool  --show-labels
      Example output
      NAME      CONFIG                                             UPDATED   UPDATING   DEGRADED   LABELS
      master    rendered-master-e05b81f5ca4db1d249a1bf32f9ec24fd   True      False      False      operator.machineconfiguration.openshift.io/required-for-upgrade=
      worker    rendered-worker-f50e78e1bc06d8e82327763145bfcf62   True      False      False
    2. Add a custom label to the desired machine config pool.

      For example:

      $ oc label machineconfigpool worker custom-kubelet=enabled
  2. Create a kubeletconfig custom resource (CR) for your configuration change.

    For example:

    Sample configuration for a custom-config CR
    apiVersion: machineconfiguration.openshift.io/v1
    kind: KubeletConfig
    metadata:
      name: custom-config (1)
    spec:
      machineConfigPoolSelector:
        matchLabels:
          custom-kubelet: enabled (2)
      kubeletConfig: (3)
        podsPerCore: 10
        maxPods: 250
        systemReserved:
          cpu: 2000m
          memory: 1Gi
    1 Assign a name to CR.
    2 Specify the label to apply the configuration change, this is the label you added to the machine config pool.
    3 Specify the new value(s) you want to change.
  3. Create the CR object.

    $ oc create -f <file-name>

    For example:

    $ oc create -f master-kube-config.yaml

Most Kubelet Configuration options can be set by the user. The following options are not allowed to be overwritten:

  • CgroupDriver

  • ClusterDNS

  • ClusterDomain

  • RuntimeRequestTimeout

  • StaticPodPath

If a single node contains more than 50 images, pod scheduling might be imbalanced across nodes. This is because the list of images on a node is shortened to 50 by default. You can disable the image limit by editing the KubeletConfig object and setting the value of nodeStatusMaxImages to -1.

Configuring control plane nodes as schedulable

You can configure control plane nodes to be schedulable, meaning that new pods are allowed for placement on the master nodes. By default, control plane nodes are not schedulable.

You can set the masters to be schedulable, but must retain the worker nodes.

You can deploy OKD with no worker nodes on a bare metal cluster. In this case, the control plane nodes are marked schedulable by default.

You can allow or disallow control plane nodes to be schedulable by configuring the mastersSchedulable field.

When you configure control plane nodes from the default unschedulable to schedulable, additional subscriptions are required. This is because control plane nodes then become worker nodes.

Procedure
  1. Edit the schedulers.config.openshift.io resource.

    $ oc edit schedulers.config.openshift.io cluster
  2. Configure the mastersSchedulable field.

    apiVersion: config.openshift.io/v1
    kind: Scheduler
    metadata:
      creationTimestamp: "2019-09-10T03:04:05Z"
      generation: 1
      name: cluster
      resourceVersion: "433"
      selfLink: /apis/config.openshift.io/v1/schedulers/cluster
      uid: a636d30a-d377-11e9-88d4-0a60097bee62
    spec:
      mastersSchedulable: false (1)
      policy:
        name: ""
    status: {}
    1 Set to true to allow control plane nodes to be schedulable, or false to disallow control plane nodes to be schedulable.
  3. Save the file to apply the changes.

Setting SELinux booleans

OKD allows you to enable and disable an SELinux boolean on a Fedora CoreOS (FCOS) node. The following procedure explains how to modify SELinux booleans on nodes using the Machine Config Operator (MCO). This procedure uses container_manage_cgroup as the example boolean. You can modify this value to whichever boolean you need.

Prerequisites
  • You have installed the OpenShift CLI (oc).

Procedure
  1. Create a new YAML file with a MachineConfig object, displayed in the following example:

    apiVersion: machineconfiguration.openshift.io/v1
    kind: MachineConfig
    metadata:
      labels:
        machineconfiguration.openshift.io/role: worker
      name: 99-worker-setsebool
    spec:
      config:
        ignition:
          version: 2.2.0
        systemd:
          units:
          - contents: |
              [Unit]
              Description=Set SELinux booleans
              Before=kubelet.service
    
              [Service]
              Type=oneshot
              ExecStart=/sbin/setsebool container_manage_cgroup=on
              RemainAfterExit=true
    
              [Install]
              WantedBy=multi-user.target graphical.target
            enabled: true
            name: setsebool.service
  2. Create the new MachineConfig object by running the following command:

    $ oc create -f 99-worker-setsebool.yaml

Applying any changes to the MachineConfig object causes all affected nodes to gracefully reboot after the change is applied.

Adding kernel arguments to nodes

In some special cases, you might want to add kernel arguments to a set of nodes in your cluster. This should only be done with caution and clear understanding of the implications of the arguments you set.

Improper use of kernel arguments can result in your systems becoming unbootable.

Examples of kernel arguments you could set include:

  • enforcing=0: Configures Security Enhanced Linux (SELinux) to run in permissive mode. In permissive mode, the system acts as if SELinux is enforcing the loaded security policy, including labeling objects and emitting access denial entries in the logs, but it does not actually deny any operations. While not supported for production systems, permissive mode can be helpful for debugging.

  • nosmt: Disables symmetric multithreading (SMT) in the kernel. Multithreading allows multiple logical threads for each CPU. You could consider nosmt in multi-tenant environments to reduce risks from potential cross-thread attacks. By disabling SMT, you essentially choose security over performance.

See Kernel.org kernel parameters for a list and descriptions of kernel arguments.

In the following procedure, you create a MachineConfig object that identifies:

  • A set of machines to which you want to add the kernel argument. In this case, machines with a worker role.

  • Kernel arguments that are appended to the end of the existing kernel arguments.

  • A label that indicates where in the list of machine configs the change is applied.

Prerequisites
  • Have administrative privilege to a working OKD cluster.

Procedure
  1. List existing MachineConfig objects for your OKD cluster to determine how to label your machine config:

    $ oc get MachineConfig
    Example output
    NAME                                               GENERATEDBYCONTROLLER                      IGNITIONVERSION   AGE
    00-master                                          52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    00-worker                                          52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    01-master-container-runtime                        52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    01-master-kubelet                                  52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    01-worker-container-runtime                        52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    01-worker-kubelet                                  52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    99-master-generated-registries                     52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    99-master-ssh                                                                                 3.2.0             40m
    99-worker-generated-registries                     52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    99-worker-ssh                                                                                 3.2.0             40m
    rendered-master-23e785de7587df95a4b517e0647e5ab7   52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    rendered-worker-5d596d9293ca3ea80c896a1191735bb1   52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
  2. Create a MachineConfig object file that identifies the kernel argument (for example, 05-worker-kernelarg-selinuxpermissive.yaml)

    apiVersion: machineconfiguration.openshift.io/v1
    kind: MachineConfig
    metadata:
      labels:
        machineconfiguration.openshift.io/role: worker(1)
      name: 05-worker-kernelarg-selinuxpermissive(2)
    spec:
      config:
        ignition:
          version: 3.2.0
      kernelArguments:
        - enforcing=0(3)
    1 Applies the new kernel argument only to worker nodes.
    2 Named to identify where it fits among the machine configs (05) and what it does (adds a kernel argument to configure SELinux permissive mode).
    3 Identifies the exact kernel argument as enforcing=0.
  3. Create the new machine config:

    $ oc create -f 05-worker-kernelarg-selinuxpermissive.yaml
  4. Check the machine configs to see that the new one was added:

    $ oc get MachineConfig
    Example output
    NAME                                               GENERATEDBYCONTROLLER                      IGNITIONVERSION   AGE
    00-master                                          52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    00-worker                                          52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    01-master-container-runtime                        52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    01-master-kubelet                                  52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    01-worker-container-runtime                        52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    01-worker-kubelet                                  52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    05-worker-kernelarg-selinuxpermissive                                                         3.2.0             105s
    99-master-generated-registries                     52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    99-master-ssh                                                                                 3.2.0             40m
    99-worker-generated-registries                     52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    99-worker-ssh                                                                                 3.2.0             40m
    rendered-master-23e785de7587df95a4b517e0647e5ab7   52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
    rendered-worker-5d596d9293ca3ea80c896a1191735bb1   52dd3ba6a9a527fc3ab42afac8d12b693534c8c9   3.2.0             33m
  5. Check the nodes:

    $ oc get nodes
    Example output
    NAME                           STATUS                     ROLES    AGE   VERSION
    ip-10-0-136-161.ec2.internal   Ready                      worker   28m   v1.22.1
    ip-10-0-136-243.ec2.internal   Ready                      master   34m   v1.22.1
    ip-10-0-141-105.ec2.internal   Ready,SchedulingDisabled   worker   28m   v1.22.1
    ip-10-0-142-249.ec2.internal   Ready                      master   34m   v1.22.1
    ip-10-0-153-11.ec2.internal    Ready                      worker   28m   v1.22.1
    ip-10-0-153-150.ec2.internal   Ready                      master   34m   v1.22.1

    You can see that scheduling on each worker node is disabled as the change is being applied.

  6. Check that the kernel argument worked by going to one of the worker nodes and listing the kernel command line arguments (in /proc/cmdline on the host):

    $ oc debug node/ip-10-0-141-105.ec2.internal
    Example output
    Starting pod/ip-10-0-141-105ec2internal-debug ...
    To use host binaries, run `chroot /host`
    
    sh-4.2# cat /host/proc/cmdline
    BOOT_IMAGE=/ostree/rhcos-... console=tty0 console=ttyS0,115200n8
    rootflags=defaults,prjquota rw root=UUID=fd0... ostree=/ostree/boot.0/rhcos/16...
    coreos.oem.id=qemu coreos.oem.id=ec2 ignition.platform.id=ec2 enforcing=0
    
    sh-4.2# exit

    You should see the enforcing=0 argument added to the other kernel arguments.