Kube Controller Manager

🧠 What is kube-controller-manager?¶

It is the automation engine of Kubernetes.

It runs dozens of controllers, such as:

• Node controller (detects node failure)
• Replication controller (ensures right number of pod replicas)
• Service account controller
• Token controller
• Persistent volume binder
• Garbage collector
• Job controller
• Many more...

In short: When something deviates from the desired state, controllers act to bring it back.

apiVersion: v1
kind: Pod
metadata:
  creationTimestamp: null
  labels:
    component: kube-controller-manager
    tier: control-plane
  name: kube-controller-manager
  namespace: kube-system
spec:
  containers:
  - command:
    - kube-controller-manager
    - --allocate-node-cidrs=true
    - --authentication-kubeconfig=/etc/kubernetes/controller-manager.conf
    - --authorization-kubeconfig=/etc/kubernetes/controller-manager.conf
    - --bind-address=127.0.0.1
    - --client-ca-file=/etc/kubernetes/pki/ca.crt
    - --cluster-cidr=172.17.0.0/16
    - --cluster-name=kubernetes
    - --cluster-signing-cert-file=/etc/kubernetes/pki/ca.crt
    - --cluster-signing-key-file=/etc/kubernetes/pki/ca.key
    - --controllers=*,bootstrapsigner,tokencleaner
    - --kubeconfig=/etc/kubernetes/controller-manager.conf
    - --leader-elect=true
    - --requestheader-client-ca-file=/etc/kubernetes/pki/front-proxy-ca.crt
    - --root-ca-file=/etc/kubernetes/pki/ca.crt
    - --service-account-private-key-file=/etc/kubernetes/pki/sa.key
    - --service-cluster-ip-range=172.20.0.0/16
    - --use-service-account-credentials=true
    image: registry.k8s.io/kube-controller-manager:v1.33.0
    imagePullPolicy: IfNotPresent
    livenessProbe:
      failureThreshold: 8
      httpGet:
        host: 127.0.0.1
        path: /healthz
        port: 10257
        scheme: HTTPS
      initialDelaySeconds: 10
      periodSeconds: 10
      timeoutSeconds: 15
    name: kube-controller-manager
    resources:
      requests:
        cpu: 200m
    startupProbe:
      failureThreshold: 24
      httpGet:
        host: 127.0.0.1
        path: /healthz
        port: 10257
        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: /usr/libexec/kubernetes/kubelet-plugins/volume/exec
      name: flexvolume-dir
    - mountPath: /etc/kubernetes/pki
      name: k8s-certs
      readOnly: true
    - mountPath: /etc/kubernetes/controller-manager.conf
      name: kubeconfig
      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: /usr/libexec/kubernetes/kubelet-plugins/volume/exec
      type: DirectoryOrCreate
    name: flexvolume-dir
  - hostPath:
      path: /etc/kubernetes/pki
      type: DirectoryOrCreate
    name: k8s-certs
  - hostPath:
      path: /etc/kubernetes/controller-manager.conf
      type: FileOrCreate
    name: kubeconfig
  - 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: {}

🔹 KEY FLAGS — Deep Explanation¶

✅ --allocate-node-cidrs=true¶

📌 Purpose: Tells the controller-manager to assign CIDR ranges (IP ranges) to nodes for pod IPs.

🧠 Required for --cluster-cidr (next flag). This is mostly used in cloud-native CNI like Calico, Weave, etc.

✅ --authentication-kubeconfig=/etc/kubernetes/controller-manager.conf¶

✅ --authorization-kubeconfig=/etc/kubernetes/controller-manager.conf¶

✅ --kubeconfig=/etc/kubernetes/controller-manager.conf¶

🔐 All of these deal with API communication:

All use the same file (controller-manager.conf).

✅ --bind-address=127.0.0.1¶

Locks down the API server listener to localhost only (security best practice). You’ll see this for all control plane components.

✅ --client-ca-file=/etc/kubernetes/pki/ca.crt¶

📜 Verifies client certs presented by other components. Only trusted clients are allowed to connect.

✅ --cluster-cidr=172.17.0.0/16¶

🌐 Defines the IP address range allocated for pods across the cluster.

Example:

• If Node A gets 172.17.1.0/24
• Node B gets 172.17.2.0/24
• All pods get IPs from this pool

Required for:

• IP assignment logic
• Network policies

✅ --cluster-name=kubernetes¶

A label for multi-cluster federation. Mostly defaults to kubernetes. Rarely changed.

✅ --cluster-signing-cert-file=/etc/kubernetes/pki/ca.crt¶

✅ --cluster-signing-key-file=/etc/kubernetes/pki/ca.key¶

🔐 Used to sign certificates automatically during:

• Node bootstrapping
• CSR approvals (if automated)
• Service accounts

This is how Kubernetes offers internal certificate management.

✅ --controllers=*,bootstrapsigner,tokencleaner¶

Controls which controllers run.

• * means: Run all default controllers
• bootstrapsigner: Signs bootstrap tokens used by new nodes
• tokencleaner: Periodically removes expired tokens

You can disable controllers if needed in custom setups, like:

--controllers=*,-garbagecollector

✅ --requestheader-client-ca-file=/etc/kubernetes/pki/front-proxy-ca.crt¶

📨 Used when the API server is fronted by a proxy (e.g., kube-aggregator). This CA validates incoming requests via proxy headers.

✅ --root-ca-file=/etc/kubernetes/pki/ca.crt¶

📎 Injected into service account secrets as ca.crt.

• Lets pods verify the identity of the kube-apiserver.
• Ensures secure communication.

✅ --service-account-private-key-file=/etc/kubernetes/pki/sa.key¶

🔑 Used to sign service account tokens, so the kube-apiserver can validate them.

• Every pod with a service account gets a token.
• This token is signed by this key.

✅ --service-cluster-ip-range=172.20.0.0/16¶

🎯 Defines the IP range for ClusterIP Services.

• Every Kubernetes Service gets a virtual IP from this pool.
• Must not overlap with pod CIDRs.

✅ --use-service-account-credentials=true¶

🔐 Ensures that individual controllers use service account tokens (rather than the main kubeconfig).

Security Best Practice: → Each controller acts under its own identity with limited permissions.

🔹 HEALTH CHECKS¶

✅ startupProbe and livenessProbe¶

• Both check: https://127.0.0.1:10257/healthz
• Periodically monitored by kubelet.
• Failure triggers restart.

⚠️ These endpoints are only accessible locally.

🔹 VOLUME MOUNTS¶

All these allow controller-manager to:

• Talk securely to the API server
• Access CA certificates
• Sign tokens and CSRs

🔹 OTHER IMPORTANT FIELDS¶

🧠 Summary Table (Plain English)¶