我有一个 Kubernetes 集群，有 1 个主节点和 5 个工作节点，当我尝试加入第 6 个工作节点时，出现以下错误：
$ sudo kubeadm join 172.22.20.20:6443 --token xxxxx.yyyyyyyy --discovery-token-ca-cert-hash sha256:zzzzzzz
[preflight] Running pre-flight checks.
[WARNING FileExisting-crictl]: crictl not found in system path
Suggestion: go get github.com/kubernetes-incubator/cri-tools/cmd/crictl
[discovery] Trying to connect to API Server "172.22.20.20:6443"
[discovery] Created cluster-info discovery client, requesting info from "https://172.22.20.20:6443"
[discovery] Failed to request cluster info, will try again: [Get https://172.22.20.20:6443/api/v1/namespaces/kube-public/configmaps/cluster-info: dial tcp 172.22.20.20:6443: getsockopt: connection refused]
所有工作节点在 Ubuntu Server 16.04 上都以完全相同的方式（脚本）构建。
加入有限制吗？什么样的日志会有用？