How to make the pod CIDR range larger in kubernetes cluster deployed with kubeadm?

I deployed my cluster with the --pod-network-cidr added, and have created the new ip pool using calicoctl to change the pods to this range. The problem I am having is exactly what I need to change on the kubernetes side to make the pod cidr range changes? Do I make changes in the API server, Controller manager, and scheduler or is there only specific parts I need to change. I have attempted only changing the controller manager, and those control plane pods go into a crash loop after changing the --cluster-cidr in the yaml.

The output in the controller-manager logs are below?

controllermanager.go:235] error starting controllers: failed to mark cidr[192.168.0.0/24] at idx [0] as occupied for node: : cidr 192.168.0.0/24 is out the range of cluster cidr 10.0.0.0/16

Solution

Changing a cluster CIDR isn't a simple task. I managed to reproduce your scenario and I managed to change it using the following steps.

Changing an IP pool

The process is as follows :

Install calicoctl as a Kubernetes pod (Source)
Add a new IP pool (Source).
Disable the old IP pool. This prevents new IPAM allocations from the old IP pool without affecting the networking of existing workloads.
Change nodes podCIDR parameter (Source)
Change --cluster-cidr on kube-controller-manager.yaml on master node. (Credits to OP on that)
Recreate all existing workloads that were assigned an address from the old IP pool.
Remove the old IP pool.

Let’s get started.

In this example, we are going to replace 192.168.0.0/16 to 10.0.0.0/8.

Installing calicoctl as a Kubernetes pod

$ kubectl apply -f https://docs.projectcalico.org/manifests/calicoctl.yaml

Setting an alias:

$ alias calicoctl="kubectl exec -i -n kube-system calicoctl -- /calicoctl "

Add a new IP pool:

calicoctl create -f -<<EOF
apiVersion: projectcalico.org/v3
kind: IPPool
metadata:
  name: new-pool
spec:
  cidr: 10.0.0.0/8
  ipipMode: Always
  natOutgoing: true
EOF

We should now have two enabled IP pools, which we can see when running calicoctl get ippool -o wide:

NAME                  CIDR             NAT    IPIPMODE   DISABLED
default-ipv4-ippool   192.168.0.0/16   true   Always     false
new-pool              10.0.0.0/8       true   Always     false

Disable the old IP pool.

First save the IP pool definition to disk:

calicoctl get ippool -o yaml > pool.yaml

pool.yaml should look like this:

apiVersion: projectcalico.org/v3
items:
- apiVersion: projectcalico.org/v3
  kind: IPPool
  metadata:
    name: default-ipv4-ippool
  spec:
    cidr: 192.168.0.0/16
    ipipMode: Always
    natOutgoing: true
- apiVersion: projectcalico.org/v3
  kind: IPPool
  metadata:
    name: new-pool
  spec:
    cidr: 10.0.0.0/8
    ipipMode: Always
    natOutgoing: true

Note: Some extra cluster-specific information has been redacted to improve readibility.

Edit the file, adding disabled: true to the default-ipv4-ippool IP pool:

apiVersion: projectcalico.org/v3
kind: IPPool
metadata:5
  name: default-ipv4-ippool
spec:
  cidr: 192.168.0.0/16
  ipipMode: Always
  natOutgoing: true
  disabled: true

Apply the changes:

calicoctl apply -f pool.yaml

We should see the change reflected in the output of calicoctl get ippool -o wide:

NAME                  CIDR             NAT    IPIPMODE   DISABLED
default-ipv4-ippool   192.168.0.0/16   true   Always     true
new-pool              10.0.0.0/8       true   Always     false

Change nodes podCIDR parameter:

Override podCIDR parameter on the particular k8s Node resource with a new IP source range, desirable way with the following commands:

$ kubectl get no kubeadm-0 -o yaml > file.yaml; sed -i "s~192.168.0.0/24~10.0.0.0/16~" file.yaml; kubectl delete no kubeadm-0 && kubectl create -f file.yaml
$ kubectl get no kubeadm-1 -o yaml > file.yaml; sed -i "s~192.168.1.0/24~10.1.0.0/16~" file.yaml; kubectl delete no kubeadm-1 && kubectl create -f file.yaml
$ kubectl get no kubeadm-2 -o yaml > file.yaml; sed -i "s~192.168.2.0/24~10.2.0.0/16~" file.yaml; kubectl delete no kubeadm-2 && kubectl create -f file.yaml

We had to perform this action for every node we have. Pay attention to the IP Ranges, they are different from one node to the other.

Change CIDR on kubeadm-config ConfigMap and kube-controller-manager.yaml

Edit kubeadm-config ConfigMap and change podSubnet to the new IP Range:

kubectl -n kube-system edit cm kubeadm-config

Also, change the --cluster-cidr on /etc/kubernetes/manifests/kube-controller-manager.yaml located in the master node.

$ sudo cat /etc/kubernetes/manifests/kube-controller-manager.yaml
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=10.0.0.0/8
    - --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
    - --node-cidr-mask-size=24
    - --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=10.96.0.0/12
    - --use-service-account-credentials=true

Recreate all existing workloads using IPs from the disabled pool. In this example, kube-dns is the only workload networked by Calico:
```
kubectl delete pod -n kube-system kube-dns-6f4fd4bdf-8q7zp
```
Check that the new workload now has an address in the new IP pool by running calicoctl get wep --all-namespaces:
```
NAMESPACE     WORKLOAD                   NODE      NETWORKS            INTERFACE
kube-system   kube-dns-6f4fd4bdf-8q7zp   vagrant   10.0.24.8/32   cali800a63073ed
```

Delete the old IP pool:

calicoctl delete pool default-ipv4-ippool

Creating it correctly from scratch

To deploy a cluster under a specific IP range using Kubeadm and Calico you need to init the cluster with --pod-network-cidr=192.168.0.0/24 (where 192.168.0.0/24 is your desired range) and than you need to tune the Calico manifest before applying it in your fresh cluster.

To tune Calico before applying, you have to download it's yaml file and change the network range.

Download the Calico networking manifest for the Kubernetes.

$ curl https://docs.projectcalico.org/manifests/calico.yaml -O

If you are using pod CIDR 192.168.0.0/24, skip to the next step. If you are using a different pod CIDR, use the following commands to set an environment variable called POD_CIDR containing your pod CIDR and replace 192.168.0.0/24 in the manifest with your pod CIDR.
```
$ POD_CIDR="<your-pod-cidr>" \
sed -i -e "s?192.168.0.0/16?$POD_CIDR?g" calico.yaml
```
Apply the manifest using the following command.
```
$ kubectl apply -f calico.yaml
```