问题：
coredns始终处于containercreating状态
coredns镜像拉取不下来，只能手动拉去之后修改tag进行解决这个问题，具体步骤如下：

[root@master ~]# kubectl get pod -A
NAMESPACE     NAME                             READY   STATUS              RESTARTS   AGE
kube-system   coredns-bccdc95cf-86nc6          0/1     ContainerCreating   0          99s
kube-system   coredns-bccdc95cf-wsm6r          0/1     ContainerCreating   0          93s
kube-system   etcd-master                      1/1     Running             0          13h
kube-system   kube-apiserver-master            1/1     Running             0          13h
kube-system   kube-controller-manager-master   1/1     Running             1          13h
kube-system   kube-flannel-ds-amd64-dwc7j      1/1     Running             0          29m
kube-system   kube-flannel-ds-amd64-gxvpp      1/1     Running             0          29m
kube-system   kube-flannel-ds-amd64-mszpw      1/1     Running             0          29m
kube-system   kube-proxy-4twck                 1/1     Running             0          13h
kube-system   kube-proxy-stzxb                 1/1     Running             0          13h
kube-system   kube-proxy-w7v2b                 1/1     Running             0          13h
kube-system   kube-scheduler-master            1/1     Running             1          13h

Events:
  Type     Reason                  Age               From               Message
  ----     ------                  ----              ----               -------
  Normal   Scheduled               46s               default-scheduler  Successfully assigned kube-system/coredns-bccdc95cf-86nc6 to node1
  Warning  FailedCreatePodSandBox  45s               kubelet, node1     Failed create pod sandbox: rpc error: code = Unknown desc = [failed to set up sandbox container "bfadab3147650499aee00f528807c811bc9c072acc3630fd0ce47e3dc1687de6" network for pod "coredns-bccdc95cf-86nc6": NetworkPlugin cni failed to set up pod "coredns-bccdc95cf-86nc6_kube-system" network: failed to find plugin "flannel" in path [/opt/cni/bin], failed to clean up sandbox container "bfadab3147650499aee00f528807c811bc9c072acc3630fd0ce47e3dc1687de6" network for pod "coredns-bccdc95cf-86nc6": NetworkPlugin cni failed to teardown pod "coredns-bccdc95cf-86nc6_kube-system" network: failed to find plugin "flannel" in path [/opt/cni/bin]]
  Normal   SandboxChanged          8s (x4 over 45s)  kubelet, node1     Pod sandbox changed, it will be killed and re-created.

解决：
首先，确定coredns这两个pod处于哪个node上

kubectl get pods coredns-59d64cd4d4-cvgv5 -n kube-system -o wide

之后查看coredns所需镜像

[root@master ~]# kubectl get pods coredns-545d6fc579-nhqvj -n kube-system -o yaml | grep image:
    image: registry.aliyuncs.com/google_containers/coredns/coredns:v1.8.0

登录coredns所在的node机器，进行手动拉去镜像

docker pull coredns/coredns:1.8.0

拉取成功后，进行修改tag，修改为与所需要的一致即可

docker tag coredns/coredns registry.aliyuncs.com/google_containers/coredns:v1.8.0

修改完成之后，回到master查看，可以发现现在coredns就已经处于running状态了