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Standalone Kubelet on Fedora 31

What is this about?

This gist describes how to set up standalone kubelet + CRI-O + CNI on Fedora Core 31. The goal is to place a Kubernetes Pod manifest on an single node and access the application from the network. This guide has been tested on x86-64 and armv7 deployments.

Prepare the system

Make sure the system is up to date:

dnf -y update

On Fedora 31 by default cgroup v2 are used. However the kubelet does not seem to be compatible with this as of time of writing. Revert this by adding the systemd.unified_cgroup_hierarchy=0 switch to the kernel command line in

For x86-64 systems, modify /etc/default/grub like below:

 GRUB_DEFAULT=saved
 GRUB_DISABLE_SUBMENU=true
 GRUB_TERMINAL_OUTPUT="console"
-GRUB_CMDLINE_LINUX="resume=/dev/mapper/fedora-swap rd.lvm.lv=fedora/root rd.lvm.lv=fedora/swap rhgb quiet"
+GRUB_CMDLINE_LINUX="resume=/dev/mapper/fedora-swap rd.lvm.lv=fedora/root rd.lvm.lv=fedora/swap rhgb quiet systemd.unified_cgroup_hierarchy=0"
 GRUB_DISABLE_RECOVERY="true"
 GRUB_ENABLE_BLSCFG=true

Regenerate your bootloader configuration using Grub2...

on a BIOS-based system install grub2-mkconfig -o /boot/grub2/grub.cfg

or on UEFI-based installs with grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

For armv7 based systems which employ extlinux, modify /boot/extlinux/extlinux.conf:

default=Fedora-Minimal-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
 label Fedora (5.4.17-200.fc31.armv7hl) 31 (Thirty One)
 	kernel /vmlinuz-5.4.17-200.fc31.armv7hl
-	append ro root=UUID=b0abeeb4-7c58-4b29-9c60-3377437cde8a rhgb quiet LANG=en_US.UTF-8
+	append ro root=UUID=b0abeeb4-7c58-4b29-9c60-3377437cde8a rhgb quiet LANG=en_US.UTF-8 systemd.unified_cgroup_hierarchy=0
 	fdtdir /dtb-5.4.17-200.fc31.armv7hl/
 	initrd /initramfs-5.4.17-200.fc31.armv7hl.img

 label Fedora-Minimal-armhfp-31-1.9 (5.3.7-301.fc31.armv7hl)
 	kernel /vmlinuz-5.3.7-301.fc31.armv7hl
-	append ro root=UUID=b0abeeb4-7c58-4b29-9c60-3377437cde8a rhgb quiet LANG=en_US.UTF-8
+	append ro root=UUID=b0abeeb4-7c58-4b29-9c60-3377437cde8a rhgb quiet LANG=en_US.UTF-8 systemd.unified_cgroup_hierarchy=0
 	fdtdir /dtb-5.3.7-301.fc31.armv7hl/
 	initrd /initramfs-5.3.7-301.fc31.armv7hl.img

Reboot the system.

systemctl reboot

Install CRI-O

CRI-O is available as a DNF module stream. On FC31 be sure to install the latest version to circumvent this bug: https://bugzilla.redhat.com/show_bug.cgi?id=1754170

dnf -y module enable cri-o:1.16
dnf -y install crio

To interact with the CRI-O environment also install critctl

dnf -y install cri-tools

Configure CNI

CNI will be responsible for connection containers launched through CRI-O to the hosts network. Portmapping (to enable use of hostPort in pod manifest) and firewall (to configure firewalld to permit traffic) will be used.

Delete the default CNI config:

rm /etc/cni/net.d/*.conf

Place the following file into /etc/cni/net.d/100-crio-bridge.conflist

{
  "cniVersion": "0.4.0",
  "name": "bridge-firewalld",
  "plugins": [
    {
      "type": "bridge",
      "bridge": "cni0",
      "isDefaultGateway": true,
      "isGateway": true,
      "ipMasq": true,
      "ipam": {
        "type": "host-local",
        "subnet": "10.88.0.0/16",
        "routes": [
          {
            "dst": "0.0.0.0/0"
          }
        ]
      }
    },
    { 
      "type": "portmap",
      "capabilities": {
	"portMappings": true
     }
    },
    {
      "type": "firewall"
    }
  ]
}

The above configuration will cause CNI to create Linux bridge cni0 and attach veth-pairs between the host and the container. The containers will receive IPs in the 10.88.0.0/16 in the process. The bridge will act as gateway and IP masquerading will be configured to allow containers to networks external to the host (e.g. internet). Portmapping and firewalld rule manipulation will be conducted.

Install Standalone kubelet

dnf -y install kubernetes-node

Configure Standalone Kubelet

Disable Docker

You will notice that containerd and docker have been pulled in as a dependency. This is because by default the kubelet depends on Docker as the runtime. We will disable it and replace with CRI-O.

systemctl mask docker

We need to reconfigure the Kubelet systemd unit to not require docker but crio. Copy the unit file into the drop-in location:

cp /usr/lib/systemd/system/kubelet.service /etc/systemd/system/kubelet.service

Modify the unit file /etc/systemd/system/kubelet.service and replace the references to the docker unit with crio:

 [Unit]
 Description=Kubernetes Kubelet Server
 Documentation=https://github.com/GoogleCloudPlatform/kubernetes
-After=docker.service
-Requires=docker.service
+After=crio.service
+Requires=crio.service
 
 [Service]
 WorkingDirectory=/var/lib/kubelet

Notify systemd about these changes:

systemctl daemon-reload

Configure Kubelet

Remove the now obsolete flag --allow-privileged from /etc/kubernetes/config that would prevent the service from starting:

 KUBE_LOG_LEVEL="--v=0"
 
 # Should this cluster be allowed to run privileged docker containers
-KUBE_ALLOW_PRIV="--allow-privileged=true"
+KUBE_ALLOW_PRIV=""
 
 # How the controller-manager, scheduler, and proxy find the apiserver
 KUBE_MASTER="--master=http://127.0.0.1:8080"

Apply the below modifications to /etc/kubernetes/kubelet in order to:

  • enable static pod manifests stored on disk
  • enable the use of runc through CRIO as the container runtime (details)
 KUBELET_HOSTNAME="--hostname-override=127.0.0.1"
 
 # Add your own!
-KUBELET_ARGS="--cgroup-driver=systemd --fail-swap-on=false"
+KUBELET_ARGS="--cgroup-driver=systemd --fail-swap-on=false --pod-manifest-path=/etc/kubernetes/manifests --container-runtime=remote --container-runtime-endpoint=unix:///var/run/crio/crio.sock --runtime-request-timeout=10m"

Create the manifest directory:

mkdir /etc/kubernetes/manifests

Finally, enable and start the Kubelet.

systemctl enable kubelet --now

Verify the kubelet status

systemctl status kubelet

Verify the status of CRI-O

systemctl status crio

Verify that both the runtime and CNI are ready:

crictl info

You should see the following:

{
  "status": {
    "conditions": [
      {
        "type": "RuntimeReady",
        "status": true,
        "reason": "",
        "message": ""
      },
      {
        "type": "NetworkReady",
        "status": true,
        "reason": "",
        "message": ""
      }
    ]
  }
}

When succcessful, place the following example pod manifest in /etc/kubernetes/manifests/echoserver.yaml

apiVersion: v1
kind: Pod
metadata:
  name: echoserver
spec:
  containers:
    - name: echoserver
      image: gcr.io/google-containers/echoserver:1.10
      ports:
        - name: web
          containerPort: 8080
          hostPort: 9091
          protocol: TCP
      resources:
        limits:
          cpu: "100m"
          memory: "50Mi"

Warning Note that this container image is x86-64 only. Attempting to start it on a non-x86-64 platform will likely result in the container crashing (exec format error). If you are running on an ARM 32/64 bit platform, feel free to use docker.io/dmesser/echoserver:1.10.

Verify that the pod is running:

crictl ps -o table

You should the pod running:

CONTAINER           IMAGE                                                              CREATED             STATE               NAME                ATTEMPT             POD ID
99cb17ca96800       365ec60129c5426b4cf160257c06f6ad062c709e0576c8b3d9a5dcc488f5252d   11 minutes ago      Running             echoserver          2                   adfbfd4a31754

Verify that the firewall CNI plugin has created a rule to allow traffic to the container:

firewall-cmd --info-zone=trusted

You should see the IP address of the container/pod in the source list of the trusted zone:

trusted (active)
  target: ACCEPT
  icmp-block-inversion: no
  interfaces: 
  sources: 10.88.0.18/32
  services: 
  ports: 
  protocols: 
  masquerade: no
  forward-ports: 
  source-ports: 
  icmp-blocks: 
  rich rules: 

You should be able to curl the container on its container port:

curl http://10.88.0.18:8080

You should also be able to curl the container on its host port from another system:

curl http://<host-ip>:9091

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