K8s networking - where's my Flannel ?

Whilst setting up a new "vanilla" Kubernetes (K8s) cluster across two Ubuntu 20.04.5 VMs, I kept hitting a networking aka  - issue.

Having created the cluster using kubeadm init as per the following: -

export ip_address=$(ifconfig eth0 | grep inet | awk '{print $2}')

kubeadm init --apiserver-advertise-address=$ip_address --pod-network-cidr=172.20.0.0/16 --cri-socket unix:///run/containerd/containerd.sock

and having added Flannel as my Container Network Interface (CNI), as follows: -

curl -sL -o /tmp/kube-flannel.yml https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml

kubectl apply -f /tmp/kube-flannel.yml

I was having problems with the kube-flannel pods not starting.

Initially, I thought it was because I was specifying the --pod-network-cidr switch which, I'd read, was only required for Calico CNI.

Therefore, I reset the cluster using kubeadm reset and re-ran the init as follows: -

kubeadm init --apiserver-advertise-address=$ip_address --cri-socket unix:///run/containerd/containerd.sock

but, this time around, the Flannel pods failed with: -

pod cidr not assigned

I resorted to Google, and found this: -

pod cidr not assgned #728

in the Flannel repo on GitHub.

One response said: -

The node needs to have a podCidr. Can you check if it does - kubectl get nodes -o jsonpath='{.items[*].spec.podCIDR}'

When I checked: -

kubectl get nodes -o jsonpath='{.items[*].spec.podCIDR}'

nothing was returned, which was worrying.

I then read on ...

Did you see this note in the kubeadm docs

    There are pod network implementations where the master also plays a role in allocating a set of network address space for each node. When using flannel as the pod network (described in step 3), specify --pod-network-cidr=10.244.0.0/16. This is not required for any other networks besides Flannel.

So, third time lucky: -

kubeadm init --apiserver-advertise-address=$ip_address --pod-network-cidr=10.244.0.0/16 --cri-socket unix:///run/containerd/containerd.sock

and ... IT WORKED! 

kubectl get nodes

NAME                  STATUS   ROLES           AGE   VERSION

acarids1.foobar.com   Ready    control-plane   16m   v1.25.4

acarids2.foobar.com   Ready    <none>          14m   v1.25.4

kubectl get pods -A

NAMESPACE      NAME                                            READY   STATUS    RESTARTS   AGE

kube-flannel   kube-flannel-ds-p9rxb                           1/1     Running   0          13m

kube-flannel   kube-flannel-ds-ztbj7                           1/1     Running   0          13m

kube-system    coredns-565d847f94-r8fdp                        1/1     Running   0          15m

kube-system    coredns-565d847f94-x4qhk                        1/1     Running   0          15m

kube-system    etcd-acarids1.foobar.com.                       1/1     Running   2          16m

kube-system    kube-apiserver-acarids1.foobar.com.             1/1     Running   2          16m

kube-system    kube-controller-manager-acarids1.foobar.com.    1/1     Running   0          16m

kube-system    kube-proxy-2nzbd                                1/1     Running   0          14m

kube-system    kube-proxy-jcwzr                                1/1     Running   0          15m

kube-system    kube-scheduler-acarids1.foobar.com.             1/1     Running   2          16m