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Using Kubeadm to Add New Control Plane Nodes with AWS Integration

In my recent post on using kubeadm to set up a Kubernetes 1.13 cluster with AWS integration, I mentioned that I was still working out the details on enabling AWS integration (via the AWS cloud provider) while also using new functionality in kubeadm (specifically, the --experimental-control-plane flag) to make it easier to join new control plane nodes to the cluster. In this post, I’ll share with you what I’ve found to make this work.

The challenge here, by the way, is that you can’t use the --config <filename>.yaml flag and the --experimental-control-plane flag at the same time. I did try this, and the results of my testing led me to believe that although kubeadm doesn’t report an error, it does ignore the --experimental-control-plane flag. (Kubernetes experts/contributors, feel free to let me know if I’ve missed something here.)

After some trial-and-error—mostly my own fault because I didn’t take the time to review the v1beta1 kubeadm API docs ahead of time—I finally arrived at a working configuration that allows you to use kubeadm join --config <filename>.yaml to join a control plane node to an existing AWS-integrated Kubernetes cluster.

Credit for finding the solution goes to Rafael Fernández López, a developer based in Madrid who works for SUSE. Rafael and I had been chatting for a bit on the Kubernetes Slack team, and he pointed me to a particular value he’d been using with kubeadm that “flipped the bit,” so to speak, in telling kubeadm that the node you’re joining to the cluster is a control plane node and not a worker node. As proof, he pointed me to this snippet of code that was working for him.

So, based on Rafael’s input and my own subsequent testing, I arrived at this YAML configuration file:

kind: JoinConfiguration
    token: hjnu4i.ebza7otvg9axuqv7
    apiServerEndpoint: ""
    unsafeSkipCAVerification: true
    cloud-provider: aws

Naturally, you can’t just copy-and-paste this and use it; you’ll need to change a few values that are specific to your particular environment/situation:

  • The token field needs to specify a valid token. Bootstrap tokens generated by kubeadm init have a TTL of 24 hours, so you may need to use kubeadm token create to create a new token and specify that value here. Try to keep your tokens as short-lived as possible (use the --ttl flag to specify a short lifetime), as these are powerful authentication secrets.
  • The apiServerEndpoint will need to point to the DNS name of the load balancer that sits in front of your control plane.
  • You’ll have to specify unsafeSkipCAVerification: true unless you know the SHA256 hash of the CA certificate. If you do know the SHA256 hash, then you’d replace this line with caCertHashes: [<hash value>]. (The brackets are required.)
  • The value is one I’ve discussed multiple times; it need to match the EC2 Private DNS entry (and this is also what the OS should have configured for hostname).
  • Finally, the controlPlane.localAPIEndpoint.advertiseAddress should be the IP address of the instance you’re joining to the cluster as a new control plane node. This is the “magic sauce,” so to speak, that tells kubeadm you’re joining a control plane node to the cluster.

You’d use this configuration file by running kubeadm join --config <filename>.yaml. The node should join the cluster as a control plane node, and since the configuration file specifies the AWS cloud provider, you should also get AWS integration.

There you have it—a kubeadm configuration file that allows you to use kubeadm join to join new control plane nodes to a cluster while enabling the AWS cloud provider.

Additional Notes

I tested this using Kubernetes 1.14.0, but it should work the same way with Kubernetes 1.13.x as well. I used an external etcd cluster, but it should also work with stacked masters (etcd co-located with the control plane components). Note that I did not test the --experimental-upload-certs functionality in Kubernetes 1.14 (it’s on my list of things to do).

If you have any questions, comments, suggestions, or corrections, please contact me on Twitter. I’d love to hear from you.

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