I'm trying to install a kubernetes cluster, one server and two slaves running Ubuntu 16.04, following the instructions in this article

when arriving at the steps to join slave nodes to the master, I perform (as suggested by the kubeadm init output step) the following command:

sudo kubeadm join <master ip>:6443 --token 6wz<...etc...>goj --discovery-token-ca-cert-hash sha256:ac3<...etc...>54a

Here, I get an error

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[discovery] Trying to connect to API Server "<master ip>:6443"
[discovery] Created cluster-info discovery client, requesting info from "https://<master ip>:6443"
[discovery] Failed to request cluster info, will try again: [Get https://<master ip>:6443/api/v1/namespaces/kube-public/configmaps/cluster-info: 
    dial tcp <master ip>:6443: getsockopt: connection refused]
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My questions are:

• how can I check that the master is correctly running? (kubeadm init printed a message Your Kubernetes master has initialized successfully!, but I don't know how to actually check this is true)
• what could cause the connection refused error?
• could the error depend on the docker version? (during kubeadm init I got a wanring docker version is greater than the most recently validated version. Docker version: 17.05.0-ce. Max validated version: 17.03, don't know if this could be related to my problem)