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Set up a High-Availability Etcd Cluster With Kubeadm

Kubeadm defaults to running a single member etcd cluster in a static pod managed by the kubelet on the control plane node. This is not a highly-available setup as the the etcd cluster contains only one member and cannot sustain any members becoming unavailable. This task walks through the process of creating a highly available etcd cluster of three members that can be used as an external etcd when using kubeadm to set up a kubernetes cluster.

Before you begin

The general approach is to generate all certs on one node and only distribute the necessary files to the other nodes.

Create configuration files for kubeadm

Using the template provided below, create one kubeadm configuration file for each host that will have an etcd member running on it. Update the value of CURRENT_HOST and NAME before running the cat command.

export HOST0=10.0.0.1 # Update HOST0, HOST1, and HOST2 with the IPs or resolvable names of your hosts
export HOST1=10.0.0.2
export HOST2=10.0.0.3

# Create temp directories to store files that will end up on other hosts.
mkdir -p /tmp/${HOST0}/certs /tmp/${HOST1}/certs /tmp/${HOST2}/certs

export CURRENT_HOST="${HOST0}" # Update on each ranging through HOST0, HOST1 and HOST2
export NAME=infra0 # Update to use infra0 for HOST0, infra1 for HOST1 and infra2 for HOST2

cat << EOF > /tmp/${CURRENT_HOST}/kubeadmcfg.yaml
apiVersion: "kubeadm.k8s.io/v1alpha1"
kind: MasterConfiguration
etcd:
    serverCertSANs:
    - "${CURRENT_HOST}"
    peerCertSANs:
    - "${CURRENT_HOST}"
    extraArgs:
        initial-cluster: infra0=https://${HOST0}:2380,infra1=https://${HOST1}:2380,infra2=https://${HOST2}:2380
        initial-cluster-state: new
        name: ${NAME}
        listen-peer-urls: https://${CURRENT_HOST}:2380
        listen-client-urls: https://${CURRENT_HOST}:2379
        advertise-client-urls: https://${CURRENT_HOST}:2379
        initial-advertise-peer-urls: https://${CURRENT_HOST}:2380
EOF

Generate certificates needed for the etcd cluster

Certificate Authority

If you already have a CA then the only action that is copying the CA’s crt and key file to /etc/kubernetes/pki/etcd/ca.crt and /etc/kubernetes/pki/etcd/ca.key. After those files have been copied, please skip to the Certificate Swizzling section below.

If you do not already have a CA then run this command on $HOST0 (where you generated the configuration files for kubeadm).

kubeadm alpha phase certs etcd-ca

This creates two files

  1. /etc/kubernetes/pki/etcd/ca.crt
  2. /etc/kubernetes/pki/etcd/ca.key

Create certificates for each member

In this step we create all the certs for each host in our cluster.

kubeadm alpha phase certs etcd-server --config=/tmp/${HOST2}/kubeadmcfg.yaml
kubeadm alpha phase certs etcd-peer --config=/tmp/${HOST2}/kubeadmcfg.yaml
kubeadm alpha phase certs etcd-healthcheck-client --config=/tmp/${HOST2}/kubeadmcfg.yaml
# Move the generated certs out of the generated directory
find /etc/kubernetes/pki/etcd -not -name ca.crt -not -name ca.key -type f -exec mv {} /tmp/${HOST2}/certs \;
cp /etc/kubernetes/pki/etcd/ca.crt /tmp/${HOST2}/certs

kubeadm alpha phase certs etcd-server --config=/tmp/${HOST1}/kubeadmcfg.yaml
kubeadm alpha phase certs etcd-peer --config=/tmp/${HOST1}/kubeadmcfg.yaml
kubeadm alpha phase certs etcd-healthcheck-client --config=/tmp/${HOST1}/kubeadmcfg.yaml
# Move the generated certs out of the generated directory
find /etc/kubernetes/pki/etcd -not -name ca.crt -not -name ca.key -type f -exec mv {} /tmp/${HOST1}/certs \;
cp /etc/kubernetes/pki/etcd/ca.crt /tmp/${HOST1}/certs

kubeadm alpha phase certs etcd-server --config=/tmp/${HOST0}/kubeadmcfg.yaml
kubeadm alpha phase certs etcd-peer --config=/tmp/${HOST0}/kubeadmcfg.yaml
kubeadm alpha phase certs etcd-healthcheck-client --config=/tmp/${HOST0}/kubeadmcfg.yaml
# No need to move the certs because they are for HOST0

Copy certs and configs to other hosts

Copy the certs and configs in each tmp directory to the respective hosts and put the certs owned by root:root in /etc/kubernetes/pki/etcd/.

The steps to get these files on $HOST1 might look like this if you can ssh between hosts:

root@HOST0 $ scp -i /home/ubuntu/.ssh/id_rsa -r /tmp/${HOST1}/* ubuntu@${HOST1}:/home/ubuntu
root@HOST0 $ ssh -i /home/ubuntu/.ssh/id_rsa ubuntu@${HOST1}
ubuntu@HOST1 $ sudo -s
root@HOST1 $ chown -R root:root certs
root@HOST1 $ mv certs/* /etc/kubernetes/pki/etcd/
# Repeat for HOST2

List of all generated certs

This is a list of all the files you have generated and where on which host they should live.

Host 0

  1. /etc/kubernetes/pki/etcd/ca.crt
  2. /etc/kubernetes/pki/etcd/ca.key
  3. /etc/kubernetes/pki/etcd/server.crt
  4. /etc/kubernetes/pki/etcd/server.key
  5. /etc/kubernetes/pki/etcd/peer.crt
  6. /etc/kubernetes/pki/etcd/peer.key
  7. /etc/kubernetes/pki/etcd/healthcheck-client.crt
  8. /etc/kubernetes/pki/etcd/healthcheck-client.key
  9. /tmp/${HOST0}/kubeadmcfg.yaml

Host 1

  1. /etc/kubernetes/pki/etcd/ca.crt
  2. /etc/kubernetes/pki/etcd/server.crt
  3. /etc/kubernetes/pki/etcd/server.key
  4. /etc/kubernetes/pki/etcd/peer.crt
  5. /etc/kubernetes/pki/etcd/peer.key
  6. /etc/kubernetes/pki/etcd/healthcheck-client.crt
  7. /etc/kubernetes/pki/etcd/healthcheck-client.key
  8. /home/ubuntu/kubeadmcfg.yaml

Host 2

  1. /etc/kubernetes/pki/etcd/ca.crt
  2. /etc/kubernetes/pki/etcd/server.crt
  3. /etc/kubernetes/pki/etcd/server.key
  4. /etc/kubernetes/pki/etcd/peer.crt
  5. /etc/kubernetes/pki/etcd/peer.key
  6. /etc/kubernetes/pki/etcd/healthcheck-client.crt
  7. /etc/kubernetes/pki/etcd/healthcheck-client.key
  8. /home/ubuntu/kubeadmcfg.yaml

Manifests

Now that the certs and configs are in place we can create the manifest. On each host run the kubeadm command to generate a static manifest for etcd.

root@HOST0 $ kubeadm alpha phase etcd local --config=/tmp/${HOST0}/kubeadmcfg.yaml
root@HOST1 $ kubeadm alpha phase etcd local --config=/home/ubuntu/kubeadmcfg.yaml
root@HOST2 $ kubeadm alpha phase etcd local --config=/home/ubuntu/kubeadmcfg.yaml

Optional: Check the cluster health

docker run --rm -it --net host -v /etc/kubernetes:/etc/kubernetes quay.io/coreos/etcd:v3.2.14 etcdctl --cert-file /etc/kubernetes/pki/etcd/peer.crt --key-file /etc/kubernetes/pki/etcd/peer.key --ca-file /etc/kubernetes/pki/etcd/ca.crt --endpoints https://${HOST0}:2379 cluster-health
...
cluster is healthy

What's next

Once your have a working 3 member etcd cluster, you can continue setting up an HA control plane using kubeadm.

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