RKE and ssh - How to

I have one issue with RKE when installing my first cluster with 3 node roles (etcd, control, worker) each one being on a specific VM.
I am left with this error:
Error waiting for container [rke-port-checker] on host []: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

EDIT: googling a little helped me to find it is the TCP protocol.
I suspect I have this issue due to either iptables or SELinux. These two services are usually a good culprit for this kind of permission error. Reading the RKE github REDAME, I saw a few settings I need to do, like allow some ports in my iptables. I need to know which protocol: TCP or UDP, or both?

Then, I read RKE works by connecting to each server via SSH. My sshd config is to allow specific user with no password but a key. Shall I add any user, maybe docker, in my sshd config?

Thank you

So, setting aside Rancher and RKE for a moment, can you connect to the docker daemon directly using the API just using the docker client (I’m assuming you have configured the daemon to enable remote connection) ?

The error is confusing and I have created this issue: https://github.com/rancher/rke/issues/507

But the root cause is still that the required ports are not opened between the hosts.