I have provisioned a Kubernetes cluster on RHEL 7 and deployed Calico to it. The calic-node pods running on the master and worker nodes do not become ready until I SSH into the master and temporarily disable iptables with systemctl stop iptables.
I have configured iptables on the master to allow incoming access over TCP to 6443, 2379:2380, and 10250:10252. So it should be configured correctly as per the documented required ports. After all, the nodes are able to join the cluster via kubeadm join which requires the ability to network with the master over 6443.
The solution was also including the ports defined here.