NetworkPluginNotReady message:Network plugin returns error: cni plugin not initialized

We have a cluster with 4 worker nodes and 1 master, and the flannel CNI installed. 1 kube-flannel-ds-xxxx pod running on every node.

They used to run fine, but 1 node suddenly entered NotReady state and does not come out of it anymore.

journalctl -u kubelet -f on the node constanly emits "cni plugin not initialized"

Jul 25 14:44:05 ubdock09 kubelet[13076]: E0725 14:44:05.916280   13076 kubelet.go:2349] "Container runtime network not ready" networkReady="NetworkReady=false reason:NetworkPluginNotReady message:Network plugin returns error: cni plugin not initialized"

Deleting the flannel pod makes a new one start up but the pluin keeps being uninitialized.
What can we do or check to fix this?

Found the cause in journalctl of containerd.

Jul 25 15:10:36 ubdock09 containerd[23164]: time="2022-07-25T15:10:36.480398235+02:00" level=error msg="failed to reload cni configuration after receiving fs change event(\"/etc/cni/net.d/.10-flannel.conf.swp\": REMOVE)" error="cni config load failed: failed to load CNI config file /etc/cni/net.d/10-flannel.conf: error parsing configuration: missing 'type': invalid cni config: failed to load cni config"

The Ready machines did not have /etc/cni/net.d/10-flannel.conf so I just removed the /etc/cni/net.d directory and the network device cni0 that was created by the container network interface.

id@machine:/# ip -4 addr show
6: cni0:  mtu 1500 qdisc noqueue state UP group default qlen 1000
    inet 10.244.11.1/24 brd 10.244.11.255 scope global cni0
       valid_lft forever preferred_lft forever

ip link delete cni0

Then restarted containerd and the flannel pod. Now the node is ready and cni0 is recreated.

StackExchange DevOps Q#16344, answer score: 6