Comment # 1 on bug 1218694 from Priyanka Saggu

Thanks for the ticket, @Egbert Eich!

For the crashing flannel pods:

flannel, by default, expects the podCIDR to be set to "10.244.0.0/16"[2] at the
time of "kubeadm init":

> sudo kubeadm init --pod-network-cidr 10.244.0.0/16

(Or in case of custom podCIDR, same needs to be updated in the
kube-flannel.yaml[1] file as well:

```
  net-conf.json: |
    {
      "Network": "<customCIDR>",
      "Backend": {
        "Type": "vxlan"
      }
    }
```
)

[1]
https://github.com/flannel-io/flannel/tree/master?tab=readme-ov-file#deploying-flannel-with-kubectl
[2]
https://github.com/flannel-io/flannel/releases/latest/download/kube-flannel.yml


-----

For the coreDNS pods, they get stuck in "pending/containerCreating" state due
to missing CNI plugins in /opt/cni/bin path.

```
> k describe pod coredns-86ccd44ff8-hz2lj -n kube-system

Events:
  Type     Reason                  Age   From               Message
  ----     ------                  ----  ----               -------
  Normal   Scheduled               14s   default-scheduler  Successfully
assigned kube-system/coredns-86ccd44ff8-hz2lj to 5
  Warning  FailedCreatePodSandBox  13s   kubelet            Failed to create
pod sandbox: rpc error: code = Unknown desc = failed to setup network for
sandbox "de201303e9a5802dbae988d668bed2640315da55b4654c2cdda3e5cffeec8ab4":
plugin type="loopback" failed (add): failed to find plugin "loopback" in path
[/opt/cni/bin]
  Normal   SandboxChanged          13s   kubelet            Pod sandbox
changed, it will be killed and re-created.
```

Flannel doesn't add these CNI binaries into the /opt/cni/bin path[3]

Adding the binaries manually in the required path, fixes it:

> mkdir -p /opt/cni/bin
> curl -O -L https://github.com/containernetworking/plugins/releases/download/v1.4.0/cni-plugins-linux-amd64-v1.4.0.tgz
> sudo tar -C /opt/cni/bin -xzf cni-plugins-linux-amd64-v1.4.0.tgz

[3]https://github.com/flannel-io/flannel/issues/1715

I'll send an update to kube-flannel package for above.