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README.md: Elaborate a bit more on node join #150

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README.md: Elaborate a bit more on node join #150

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Expand Up @@ -47,6 +47,19 @@ automatically approves all CSRs. This bootstrap service will end up approving
the CSRs for the control plane nodes, while `cluster-machine-approver` will
take over for future new CSRs from worker nodes.

### Understanding node join

The default OCP flow uses CoreOS (e.g. RHEL CoreOS), which is provisioned via
Ignition. All the initial node configuration is rendered into Ignition
by the [MCO](https://github.com/openshift/machine-config-operator/). Further,
before kubelet even starts, the OS is upgraded to the latest image. For
more information on this, see:
https://github.com/openshift/machine-config-operator/blob/master/docs/OSUpgrades.md

And specifically for the initial kubelet config, see [cluster_server.go](https://github.com/openshift/machine-config-operator/blob/0476b259ab6895e7aaca237581bc19b4d4610e12/pkg/server/cluster_server.go#L58)
which is part of the "Machine Config Server" that provides Ignition
when the node requests it on the first boot.

### Disabling Node Client CSR Approvals

It is possible to disable node client CSR approvals completely. This is done
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