Until recently, eksctl provided its own bootstrapping/userdata logic on disk, over-writing or ignore those which came with the AMIs. This caused numerous headaches after changes upstream and we got tired of maintaining these extra pieces.

Implementation

For Unmanaged nodes there is an interface:

type Bootstrapper interface {
  UserData() (string, error)
}

AMI families Ubuntu, AmazonLinux2, Bottlerocket and Windows all fulfil this.

As of eksctl version 0.45.0 unmanaged nodes of these families, as well as managed nodes (different interface), will defer to the native bootstrap script which comes built into the image.

This script is found on disk at /etc/eks/bootstrap.sh. UserData will provide a wrapper script which will set custom values and delegate to the official bootstrap script.

Ubuntu & AmazonLinux2

The bootstrapping "prep" for these 2 are fairly similar. Common setup lives in userdata.go. Individual scripts are prepped in ubuntu.go and al2.go.

Non-dynamic assets live in assets/.

Both bootstrappers add assets/bootstrap.helper.sh to the node along with either assets/bootstrap.ubuntu.sh or assets/bootstrap.al2.sh.

The call to UserData will also dynamically add the following:

kubelet-extra.json - user configuration for kubelet
docker-extra.json - extra config for docker daemon
kubelet.env - env vars for kubelet

The bootstrap wrapper scripts will use jq and sed to get user and our config into various files, and then call /etc/eks/bootstrap.sh.

AmazonLinux2023

While AL2023 implements the Bootstrapper interface, the underlying userdata will be entirely different from other AMI families. Specifically, AL2023 introduces a new node initialization process nodeadm that uses a YAML configuration schema, dropping the use of /etc/eks/bootstrap.sh script. For self-managed nodes, and for EKS-managed nodes based on custom AMIs, eksctl will populate userdata in the fashion below:

MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=//

--//
Content-Type: application/node.eks.aws

apiVersion: node.eks.aws/v1alpha1
kind: NodeConfig
spec:
  cluster:
    apiServerEndpoint: https://XXXX.us-west-2.eks.amazonaws.com
    certificateAuthority: XXXX
    cidr: 10.100.0.0/16
    name: my-cluster
  kubelet:
    config:
      clusterDNS:
      - 10.100.0.10
    flags:
    - --node-labels=alpha.eksctl.io/cluster-name=my-cluster,alpha.eksctl.io/nodegroup-name=my-nodegroup
    - --register-with-taints=special=true:NoSchedule (only for EKS-managed nodes)

--//--

Troubleshooting

Ubuntu

sudo snap logs kubelet-eks [-n=all/20]
systemctl status docker.service

Files:

/etc/eks/bootstrap.sh
/var/lib/cloud/scripts/eksctl/bootstrap.ubuntu.sh
/etc/kubernetes/kubelet/kubelet-config.json
/etc/docker/daemon.json

AmazonLinux2

Status:

systemctl status kubelet
systemctl status docker

Logs:

journalctl -u kubelet.service