coredns无法启动

core dns无法启动:
[root@master ~]# kubectl get pod -n kube-system
NAME                             READY   STATUS    RESTARTS   AGE
coredns-6d56c8448f-5vmfg         0/1     Pending   0          28m
coredns-6d56c8448f-gjf4h         0/1     Pending   0          28m
etcd-master                      1/1     Running   1          28m
kube-apiserver-master            1/1     Running   1          28m
kube-controller-manager-master   1/1     Running   1          28m

kubectl apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml

[root@master ~]# kubectl describe pod  coredns-6d56c8448f-5vmfg -n kube-system
Name:                 coredns-6d56c8448f-5vmfg
Namespace:            kube-system
Priority:             2000000000
Priority Class Name:  system-cluster-critical
Node:                
Labels:               k8s-app=kube-dns