使用kubeadm部署k8s集群后，Master节点上coredns频繁restart，是何原因？

环境：centos7.3、kubeadm-1.12.2、Docker-1.13.1

问题：使用kubeadm部署Kubernetes后，发现Master节点上coredns pod频繁的restart？

# kubectl get pods --all-namespaces -o wide
NAMESPACE     NAME                              READY   STATUS    RESTARTS   AGE    IP           NODE      NOMINATED NODE
kube-system   coredns-576cbf47c7-5nnc8          1/1     Running   0          49m    10.36.0.0    kube-node1   
kube-system   coredns-576cbf47c7-lp5mh          1/1     Running   14         49m    10.32.0.3    kube-Master   

# kubectl describe pod coredns-576cbf47c7-lp5mh -n kube-system
Name:               coredns-576cbf47c7-lp5mh
Namespace:          kube-system
Priority:           0
PriorityClassName:  
Node:               kube-01/172.16.0.7
Start Time:         Mon, 05 Nov 2018 14:13:51 +0800
Labels:             k8s-app=kube-dns
                    pod-template-hash=576cbf47c7
Annotations:        ............

Events:
  Type     Reason     Age                   From               Message
  ----     ------     ----                  ----               -------
  Normal   Scheduled  50m                   default-scheduler  Successfully assigned kube-system/coredns-576cbf47c7-lp5mh to kube-01
  Normal   Pulled     46m (x3 over 50m)     kubelet, kube-Master   Container image "k8s.gcr.io/coredns:1.2.2" already present on machine
  Normal   Created    46m (x3 over 50m)     kubelet, kube-Master   Created container
  Normal   Started    46m (x3 over 50m)     kubelet, kube-Master   Started container
  Normal   Killing    30m (x8 over 48m)     kubelet, kube-Master   Killing container with id docker://coredns:Container failed liveness probe.. Container will be killed and recreated.
  Warning  BackOff    5m28s (x81 over 35m)  kubelet, kube-Master   Back-off restarting failed container
  Warning  Unhealthy  36s (x73 over 49m)    kubelet, kube-Master   Liveness probe failed: HTTP probe failed with statuscode: 503

# docker logs 343792b25758

E1105 07:04:07.695608       1 reflector.go:205] github.com/coredns/coredns/plugin/kubernetes/controller.go:348: Failed to list *v1.Service: Get https://10.96.0.1:443/api/v1/services?limit=500&resourceVersion=0: dial tcp 10.96.0.1:443: i/o timeout
E1105 07:04:09.698395       1 reflector.go:205] github.com/coredns/coredns/plugin/kubernetes/controller.go:355: Failed to list *v1.Namespace: Get https://10.96.0.1:443/api/v1/namespaces?limit=500&resourceVersion=0: dial tcp 10.96.0.1:443: connect: no route to host
E1105 07:04:11.702414       1 reflector.go:205] github.com/coredns/coredns/plugin/kubernetes/controller.go:348: Failed to list *v1.Service: Get https://10.96.0.1:443/api/v1/services?limit=500&resourceVersion=0: dial tcp 10.96.0.1:443: connect: no route to host
2018/11/05 07:04:12 [INFO] SIGTERM: Shutting down servers then terminating