Kube API Server (Part 2)

# Manifest 1
apiVersion: v1
kind: Pod
metadata:
  annotations:
    kubeadm.kubernetes.io/kube-apiserver.advertise-address.endpoint: 192.168.102.134:6443
  creationTimestamp: null
  labels:
    component: kube-apiserver
    tier: control-plane
  name: kube-apiserver
  namespace: kube-system
spec:
  containers:
  - command:
    - kube-apiserver
    - --advertise-address=192.168.102.134
    - --allow-privileged=true
    - --authorization-mode=Node,RBAC
    - --client-ca-file=/etc/kubernetes/pki/ca.crt
    - --enable-admission-plugins=NodeRestriction
    - --enable-bootstrap-token-auth=true
    - --etcd-cafile=/etc/kubernetes/pki/etcd/ca.crt
    - --etcd-certfile=/etc/kubernetes/pki/apiserver-etcd-client.crt
    - --etcd-keyfile=/etc/kubernetes/pki/apiserver-etcd-client.key
    - --etcd-servers=https://127.0.0.1:2379
    - --kubelet-client-certificate=/etc/kubernetes/pki/apiserver-kubelet-client.crt
    - --kubelet-client-key=/etc/kubernetes/pki/apiserver-kubelet-client.key
    - --kubelet-preferred-address-types=InternalIP,ExternalIP,Hostname
    - --proxy-client-cert-file=/etc/kubernetes/pki/front-proxy-client.crt
    - --proxy-client-key-file=/etc/kubernetes/pki/front-proxy-client.key
    - --requestheader-allowed-names=front-proxy-client
    - --requestheader-client-ca-file=/etc/kubernetes/pki/front-proxy-ca.crt
    - --requestheader-extra-headers-prefix=X-Remote-Extra-
    - --requestheader-group-headers=X-Remote-Group
    - --requestheader-username-headers=X-Remote-User
    - --secure-port=6443
    - --service-account-issuer=https://kubernetes.default.svc.cluster.local
    - --service-account-key-file=/etc/kubernetes/pki/sa.pub
    - --service-account-signing-key-file=/etc/kubernetes/pki/sa.key
    - --service-cluster-ip-range=172.20.0.0/16
    - --tls-cert-file=/etc/kubernetes/pki/apiserver.crt
    - --tls-private-key-file=/etc/kubernetes/pki/apiserver.key
    image: registry.k8s.io/kube-apiserver:v1.33.0
    imagePullPolicy: IfNotPresent
    livenessProbe:
      failureThreshold: 8
      httpGet:
        host: 192.168.102.134
        path: /livez
        port: 6443
        scheme: HTTPS
      initialDelaySeconds: 10
      periodSeconds: 10
      timeoutSeconds: 15
    name: kube-apiserver
    readinessProbe:
      failureThreshold: 3
      httpGet:
        host: 192.168.102.134
        path: /readyz
        port: 6443
        scheme: HTTPS
      periodSeconds: 1
      timeoutSeconds: 15
    resources:
      requests:
        cpu: 250m
    startupProbe:
      failureThreshold: 24
      httpGet:
        host: 192.168.102.134
        path: /livez
        port: 6443
        scheme: HTTPS
      initialDelaySeconds: 10
      periodSeconds: 10
      timeoutSeconds: 15
    volumeMounts:
    - mountPath: /etc/ssl/certs
      name: ca-certs
      readOnly: true
    - mountPath: /etc/ca-certificates
      name: etc-ca-certificates
      readOnly: true
    - mountPath: /etc/kubernetes/pki
      name: k8s-certs
      readOnly: true
    - mountPath: /usr/local/share/ca-certificates
      name: usr-local-share-ca-certificates
      readOnly: true
    - mountPath: /usr/share/ca-certificates
      name: usr-share-ca-certificates
      readOnly: true
  hostNetwork: true
  priority: 2000001000
  priorityClassName: system-node-critical
  securityContext:
    seccompProfile:
      type: RuntimeDefault
  volumes:
  - hostPath:
      path: /etc/ssl/certs
      type: DirectoryOrCreate
    name: ca-certs
  - hostPath:
      path: /etc/ca-certificates
      type: DirectoryOrCreate
    name: etc-ca-certificates
  - hostPath:
      path: /etc/kubernetes/pki
      type: DirectoryOrCreate
    name: k8s-certs
  - hostPath:
      path: /usr/local/share/ca-certificates
      type: DirectoryOrCreate
    name: usr-local-share-ca-certificates
  - hostPath:
      path: /usr/share/ca-certificates
      type: DirectoryOrCreate
    name: usr-share-ca-certificates
status: {}

Great catch ❤️ — this is one of those things that often confuses people in CKA prep, because you expect to see containerPort: in the pod spec, but here you don’t. Let’s break it down:

🔑 Why no containerPort in kube-apiserver manifest?

  • containerPort in a Pod manifest is only metadata for Kubernetes (used for Service definition, network policy, etc).
  • It does not actually open a port inside the container. Ports are opened by the process running inside the container (in this case, the kube-apiserver binary).
  • Since this is a static pod (created directly by kubelet from /etc/kubernetes/manifests/kube-apiserver.yaml), there is no Service automatically referencing it. So Kubernetes doesn’t need to know containerPort.
  • The kube-apiserver itself listens on --secure-port=6443 (defined in the command args).
  • Additionally, hostNetwork: true is set → the pod shares the host’s network namespace. That means the API server is directly reachable on the host IP (192.168.102.134:6443) without needing port mapping.

📝 Rule of Thumb for CKA:

👉 If hostNetwork: true, you don’t need containerPort. The process inside the pod listens directly on the host network.

🧠 What to remember for the exam:

  • containerPort: = declarative metadata, not required if you’re not exposing ports via Service/Pod definitions.
  • In control-plane static pods (like kube-apiserver, kube-scheduler, kube-controller-manager, etcd) → you won’t see containerPort. Instead, ports are controlled by command-line flags (--secure-port, --etcd-servers, etc).
  • hostNetwork: true → Pod skips port mapping and uses host’s network stack.

✅ Exam tip: If you’re asked why kube-apiserver listens on 6443 even though no containerPort is defined, the answer is: “Because it uses hostNetwork: true, and the kube-apiserver binary itself opens port 6443 as defined by --secure-port=6443. containerPort is optional metadata, not an actual port binding.”

# Mnifest 2
controlplane ~ ➜  cat /etc/kubernetes/manifests/kube-apiserver.yaml 
apiVersion: v1
kind: Pod
metadata:
  annotations:
    kubeadm.kubernetes.io/kube-apiserver.advertise-address.endpoint: 192.168.102.164:6443
  labels:
    component: kube-apiserver
    tier: control-plane
  name: kube-apiserver
  namespace: kube-system
spec:
  containers:
  - command:
    - kube-apiserver
    - --advertise-address=192.168.102.164
    - --allow-privileged=true
    - --authorization-mode=Node,RBAC
    - --client-ca-file=/etc/kubernetes/pki/ca.crt
    - --enable-admission-plugins=NodeRestriction
    - --enable-bootstrap-token-auth=true
    - --etcd-cafile=/etc/kubernetes/pki/etcd/ca.crt
    - --etcd-certfile=/etc/kubernetes/pki/apiserver-etcd-client.crt
    - --etcd-keyfile=/etc/kubernetes/pki/apiserver-etcd-client.key
    - --etcd-servers=https://127.0.0.1:2379
    - --kubelet-client-certificate=/etc/kubernetes/pki/apiserver-kubelet-client.crt
    - --kubelet-client-key=/etc/kubernetes/pki/apiserver-kubelet-client.key
    - --kubelet-preferred-address-types=InternalIP,ExternalIP,Hostname
    - --proxy-client-cert-file=/etc/kubernetes/pki/front-proxy-client.crt
    - --proxy-client-key-file=/etc/kubernetes/pki/front-proxy-client.key
    - --requestheader-allowed-names=front-proxy-client
    - --requestheader-client-ca-file=/etc/kubernetes/pki/front-proxy-ca.crt
    - --requestheader-extra-headers-prefix=X-Remote-Extra-
    - --requestheader-group-headers=X-Remote-Group
    - --requestheader-username-headers=X-Remote-User
    - --secure-port=6443
    - --service-account-issuer=https://kubernetes.default.svc.cluster.local
    - --service-account-key-file=/etc/kubernetes/pki/sa.pub
    - --service-account-signing-key-file=/etc/kubernetes/pki/sa.key
    - --service-cluster-ip-range=172.20.0.0/16
    - --tls-cert-file=/etc/kubernetes/pki/apiserver.crt
    - --tls-private-key-file=/etc/kubernetes/pki/apiserver.key
    image: registry.k8s.io/kube-apiserver:v1.34.0
    imagePullPolicy: IfNotPresent
    livenessProbe:
      failureThreshold: 8
      httpGet:
        host: 192.168.102.164
        path: /livez
        port: probe-port
        scheme: HTTPS
      initialDelaySeconds: 10
      periodSeconds: 10
      timeoutSeconds: 15
    name: kube-apiserver
    ports:
    - containerPort: 6443
      name: probe-port
      protocol: TCP
    readinessProbe:
      failureThreshold: 3
      httpGet:
        host: 192.168.102.164
        path: /readyz
        port: probe-port
        scheme: HTTPS
      periodSeconds: 1
      timeoutSeconds: 15
    resources:
      requests:
        cpu: 250m
    startupProbe:
      failureThreshold: 24
      httpGet:
        host: 192.168.102.164
        path: /livez
        port: probe-port
        scheme: HTTPS
      initialDelaySeconds: 10
      periodSeconds: 10
      timeoutSeconds: 15
    volumeMounts:
    - mountPath: /etc/ssl/certs
      name: ca-certs
      readOnly: true
    - mountPath: /etc/ca-certificates
      name: etc-ca-certificates
      readOnly: true
    - mountPath: /etc/kubernetes/pki
      name: k8s-certs
      readOnly: true
    - mountPath: /usr/local/share/ca-certificates
      name: usr-local-share-ca-certificates
      readOnly: true
    - mountPath: /usr/share/ca-certificates
      name: usr-share-ca-certificates
      readOnly: true
  hostNetwork: true
  priority: 2000001000
  priorityClassName: system-node-critical
  securityContext:
    seccompProfile:
      type: RuntimeDefault
  volumes:
  - hostPath:
      path: /etc/ssl/certs
      type: DirectoryOrCreate
    name: ca-certs
  - hostPath:
      path: /etc/ca-certificates
      type: DirectoryOrCreate
    name: etc-ca-certificates
  - hostPath:
      path: /etc/kubernetes/pki
      type: DirectoryOrCreate
    name: k8s-certs
  - hostPath:
      path: /usr/local/share/ca-certificates
      type: DirectoryOrCreate
    name: usr-local-share-ca-certificates
  - hostPath:
      path: /usr/share/ca-certificates
      type: DirectoryOrCreate
    name: usr-share-ca-certificates
status: {}

Perfect observation again 👌 — this second manifest is a slightly different variant of kube-apiserver, and yes, here you actually see a ports: section with containerPort: 6443.

Now let’s clear the confusion:

🔎 Why is ports.containerPort present here, but absent in your earlier manifest?

  • First manifest (no containerPort): The kube-apiserver pod is running with --secure-port=6443 and hostNetwork: true. Since probes were defined with port: 6443 directly, Kubernetes didn’t need a named containerPort.

  • Second manifest (with containerPort): Here, the probes (livenessProbe, readinessProbe, startupProbe) don’t reference the port number 6443 directly. Instead, they reference a named port:

port: probe-port

And in order for probe-port to mean something, you need:

ports:
- containerPort: 6443
  name: probe-port

Otherwise Kubernetes wouldn’t know which port the name probe-port refers to.

🔑 So, the difference is:

  • Without containerPort: Probes (or Services) use the numeric port (6443).
  • With containerPort: Probes (or Services) use a named port (probe-port), which must be mapped to a real number via containerPort.

🧠 Exam takeaway:

  • containerPort is not required for a container to listen on a port. The process itself opens the port.
  • You must define containerPort only if you want to use named ports in probes or Services.
  • If you’re using just numbers (e.g., port: 6443), you can omit it.
  • With hostNetwork: true, port binding still happens on the host’s IP, but containerPort metadata can still be used for probes and Service discovery.

✅ One-liner rule for exam: 👉 Use containerPort only when you want to reference the port by name (e.g., in probes or Services). If you reference the port by number directly, containerPort is optional.