I yesterday I noticed the following situation with Kubernetes. I had to install Kubernetes on Redhat 7.3 OS with kubeadm. As network plugin I used flannel. Everything went ok until I found that my DNS pod couldn't start with the following error:

Normal   SandboxChanged          13m (x42200 over 13h)  kubelet, k8s-master  Pod sandbox changed, it will be killed and re-created.
Warning  FailedCreatePodSandBox  3m (x42687 over 13h)   kubelet, k8s-master  Failed create pod sandbox.

I checked my cni interface and found the following:

cni0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
    ether 00:00:00:00:00:00  txqueuelen 1000  (Ethernet)
    RX packets 0  bytes 0 (0.0 B)
    RX errors 0  dropped 0  overruns 0  frame 0
    TX packets 0  bytes 0 (0.0 B)
    TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

There was no IP address and mac. Then I checked directory /var/lib/cni/networks/cbr0/. There were files which correspond to the network 10.244.0.0/24 that is allocated to this host by flannel.

Then I checked logs:

journalctl -xe | grep cni
Jan 24 20:35:02 k8s-master kubelet[3353]: E0124 20:35:02.979434    3353     cni.go:259] Error adding network: open /proc/sys/net/ipv6/conf/eth0/accept_dad: no such file or directory
Jan 24 20:35:02 k8s-master kubelet[3353]: E0124 20:35:02.979452    3353 cni.go:227] Error while adding to cni network: open /proc/sys/net/ipv6/conf/eth0/accept_dad: no such file or directory
Jan 24 20:35:03 k8s-master kubelet[3353]: E0124 20:35:03.260145    3353 remote_runtime.go:92] RunPodSandbox from runtime service failed: rpc error: code = Unknown desc = NetworkPlugin cni failed to set up pod "kube-dns-6f4fd4bdf-mcztl_kube-system" network: open /proc/sys/net/ipv6/conf/eth0/accept_dad: no such file or directory

So something connected to ipv6 but I don't know why because we don't use IPv6 protocol. Then I checked IPv6 module on kernel level and found that it's disabled:

GRUB_CMDLINE_LINUX="ipv6.disable=1"

Just for a test, I tried to enable it, and it helped pod got IP everything became ok.

Could somebody clarify this situation, maybe you faced the same issue? Why I can't disable IPv6 if I don't use it?

Thank you in advance.