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11-tls-bootstrapping-kubernetes-workers.md

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TLS Bootstrapping Worker Nodes

In the previous step we configured a worker node by

  • Creating a set of key pairs for the worker node by ourself
  • Getting them signed by the CA by ourself
  • Creating a kube-config file using this certificate by ourself
  • Everytime the certificate expires we must follow the same process of updating the certificate by ourself

This is not a practical approach when you have 1000s of nodes in the cluster, and nodes dynamically being added and removed from the cluster. With TLS boostrapping:

  • The Nodes can generate certificate key pairs by themselves
  • The Nodes can generate certificate signing request by themselves
  • The Nodes can submit the certificate signing request to the Kubernetes CA (Using the Certificates API)
  • The Nodes can retrieve the signed certificate from the Kubernetes CA
  • The Nodes can generate a kube-config file using this certificate by themselves
  • The Nodes can start and join the cluster by themselves
  • The Nodes can request new certificates via a CSR, but the CSR must be manually approved by a cluster administrator

In Kubernetes 1.11 a patch was merged to require administrator or Controller approval of node serving CSRs for security reasons.

Reference: https://kubernetes.io/docs/reference/access-authn-authz/kubelet-tls-bootstrapping/#certificate-rotation

So let's get started!

What is required for TLS Bootstrapping

Certificates API: The Certificate API (as discussed in the lecture) provides a set of APIs on Kubernetes that can help us manage certificates (Create CSR, Get them signed by CA, Retrieve signed certificate etc). The worker nodes (kubelets) have the ability to use this API to get certificates signed by the Kubernetes CA.

Pre-Requisite

kube-apiserver - Ensure bootstrap token based authentication is enabled on the kube-apiserver.

--enable-bootstrap-token-auth=true

kube-controller-manager - The certificate requests are signed by the kube-controller-manager ultimately. The kube-controller-manager requires the CA Certificate and Key to perform these operations.

  --cluster-signing-cert-file=/var/lib/kubernetes/ca.crt \\
  --cluster-signing-key-file=/var/lib/kubernetes/ca.key

Note: We have already configured these in lab 8 in this course

Step 1 Create the Boostrap Token to be used by Nodes(Kubelets) to invoke Certificate API

Run the following steps on master-1

For the workers(kubelet) to access the Certificates API, they need to authenticate to the kubernetes api-server first. For this we create a Bootstrap Token to be used by the kubelet

Bootstrap Tokens take the form of a 6 character token id followed by 16 character token secret separated by a dot. Eg: abcdef.0123456789abcdef. More formally, they must match the regular expression [a-z0-9]{6}.[a-z0-9]{16}

Set an expiration date for the bootstrap token of 7 days from now (you can adjust this)

EXPIRATION=$(date -u --date "+7 days" +"%Y-%m-%dT%H:%M:%SZ")
cat > bootstrap-token-07401b.yaml <<EOF
apiVersion: v1
kind: Secret
metadata:
  # Name MUST be of form "bootstrap-token-<token id>"
  name: bootstrap-token-07401b
  namespace: kube-system

# Type MUST be 'bootstrap.kubernetes.io/token'
type: bootstrap.kubernetes.io/token
stringData:
  # Human readable description. Optional.
  description: "The default bootstrap token generated by 'kubeadm init'."

  # Token ID and secret. Required.
  token-id: 07401b
  token-secret: f395accd246ae52d

  # Expiration. Optional.
  expiration: ${EXPIRATION}

  # Allowed usages.
  usage-bootstrap-authentication: "true"
  usage-bootstrap-signing: "true"

  # Extra groups to authenticate the token as. Must start with "system:bootstrappers:"
  auth-extra-groups: system:bootstrappers:worker
EOF


kubectl create -f bootstrap-token-07401b.yaml --kubeconfig admin.kubeconfig

Things to note:

  • expiration - make sure its set to a date in the future. The computed shell variable EXPIRATION ensures this.
  • auth-extra-groups - this is the group the worker nodes are part of. It must start with "system:bootstrappers:" This group does not exist already. This group is associated with this token.

Once this is created the token to be used for authentication is 07401b.f395accd246ae52d

Reference: https://kubernetes.io/docs/reference/access-authn-authz/bootstrap-tokens/#bootstrap-token-secret-format

Step 2 Authorize workers(kubelets) to create CSR

Next we associate the group we created before to the system:node-bootstrapper ClusterRole. This ClusterRole gives the group enough permissions to bootstrap the kubelet

kubectl create clusterrolebinding create-csrs-for-bootstrapping \
  --clusterrole=system:node-bootstrapper \
  --group=system:bootstrappers \
  --kubeconfig admin.kubeconfig

--------------- OR ---------------

cat > csrs-for-bootstrapping.yaml <<EOF
# enable bootstrapping nodes to create CSR
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: create-csrs-for-bootstrapping
subjects:
- kind: Group
  name: system:bootstrappers
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: system:node-bootstrapper
  apiGroup: rbac.authorization.k8s.io
EOF


kubectl create -f csrs-for-bootstrapping.yaml --kubeconfig admin.kubeconfig

Reference: https://kubernetes.io/docs/reference/access-authn-authz/kubelet-tls-bootstrapping/#authorize-kubelet-to-create-csr

Step 3 Authorize workers(kubelets) to approve CSRs

kubectl create clusterrolebinding auto-approve-csrs-for-group \
  --clusterrole=system:certificates.k8s.io:certificatesigningrequests:nodeclient \
  --group=system:bootstrappers \
  --kubeconfig admin.kubeconfig

--------------- OR ---------------

cat > auto-approve-csrs-for-group.yaml <<EOF
# Approve all CSRs for the group "system:bootstrappers"
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: auto-approve-csrs-for-group
subjects:
- kind: Group
  name: system:bootstrappers
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: system:certificates.k8s.io:certificatesigningrequests:nodeclient
  apiGroup: rbac.authorization.k8s.io
EOF

kubectl create -f auto-approve-csrs-for-group.yaml --kubeconfig admin.kubeconfig

Reference: https://kubernetes.io/docs/reference/access-authn-authz/kubelet-tls-bootstrapping/#approval

Step 4 Authorize workers(kubelets) to Auto Renew Certificates on expiration

We now create the Cluster Role Binding required for the nodes to automatically renew the certificates on expiry. Note that we are NOT using the system:bootstrappers group here any more. Since by the renewal period, we believe the node would be bootstrapped and part of the cluster already. All nodes are part of the system:nodes group.

kubectl create clusterrolebinding auto-approve-renewals-for-nodes \
  --clusterrole=system:certificates.k8s.io:certificatesigningrequests:selfnodeclient \
  --group=system:nodes \
  --kubeconfig admin.kubeconfig

--------------- OR ---------------

cat > auto-approve-renewals-for-nodes.yaml <<EOF
# Approve renewal CSRs for the group "system:nodes"
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: auto-approve-renewals-for-nodes
subjects:
- kind: Group
  name: system:nodes
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: system:certificates.k8s.io:certificatesigningrequests:selfnodeclient
  apiGroup: rbac.authorization.k8s.io
EOF


kubectl create -f auto-approve-renewals-for-nodes.yaml --kubeconfig admin.kubeconfig

Reference: https://kubernetes.io/docs/reference/access-authn-authz/kubelet-tls-bootstrapping/#approval

Step 5 Configure the Binaries on the Worker node

Going forward all activities are to be done on the worker-2 node until step 11.

Download and Install Worker Binaries

Note that kubectl is required here to assist with creating the boostrap kubeconfigs for kubelet and kube-proxy

KUBE_VERSION=$(curl -L -s https://dl.k8s.io/release/stable.txt)

wget -q --show-progress --https-only --timestamping \
  https://dl.k8s.io/release/${KUBE_VERSION}/bin/linux/amd64/kube-proxy \
  https://dl.k8s.io/release/${KUBE_VERSION}/bin/linux/amd64/kubelet

Reference: https://kubernetes.io/releases/download/#binaries

Create the installation directories:

sudo mkdir -p \
  /var/lib/kubelet/pki \
  /var/lib/kube-proxy \
  /var/lib/kubernetes/pki \
  /var/run/kubernetes

Install the worker binaries:

{
  chmod +x kube-proxy kubelet
  sudo mv kube-proxy kubelet /usr/local/bin/
}

Move the certificates and secure them.

{
  sudo mv ca.crt kube-proxy.crt kube-proxy.key /var/lib/kubernetes/pki
  sudo chown root:root /var/lib/kubernetes/pki/*
  sudo chmod 600 /var/lib/kubernetes/pki/*
}

Step 6 Configure Kubelet to TLS Bootstrap

It is now time to configure the second worker to TLS bootstrap using the token we generated

For worker-1 we started by creating a kubeconfig file with the TLS certificates that we manually generated. Here, we don't have the certificates yet. So we cannot create a kubeconfig file. Instead we create a bootstrap-kubeconfig file with information about the token we created.

This is to be done on the worker-2 node. Note that now we have set up the load balancer to provide high availibilty across the API servers, we point kubelet to the load balancer.

Set up some shell variables for nodes and services we will require in the following configurations:

LOADBALANCER=$(dig +short loadbalancer)
POD_CIDR=10.244.0.0/16
SERVICE_CIDR=10.96.0.0/16
CLUSTER_DNS=$(echo $SERVICE_CIDR | awk 'BEGIN {FS="."} ; { printf("%s.%s.%s.10", $1, $2, $3) }')

Set up the bootstrap kubeconfig.

{
  sudo kubectl config --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig \
    set-cluster bootstrap --server="https://${LOADBALANCER}:6443" --certificate-authority=/var/lib/kubernetes/pki/ca.crt

  sudo kubectl config --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig \
    set-credentials kubelet-bootstrap --token=07401b.f395accd246ae52d

  sudo kubectl config --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig \
    set-context bootstrap --user=kubelet-bootstrap --cluster=bootstrap

  sudo kubectl config --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig \
    use-context bootstrap
}

--------------- OR ---------------

cat <<EOF | sudo tee /var/lib/kubelet/bootstrap-kubeconfig
apiVersion: v1
clusters:
- cluster:
    certificate-authority: /var/lib/kubernetes/pki/ca.crt
    server: https://${LOADBALANCER}:6443
  name: bootstrap
contexts:
- context:
    cluster: bootstrap
    user: kubelet-bootstrap
  name: bootstrap
current-context: bootstrap
kind: Config
preferences: {}
users:
- name: kubelet-bootstrap
  user:
    token: 07401b.f395accd246ae52d
EOF

Reference: https://kubernetes.io/docs/reference/access-authn-authz/kubelet-tls-bootstrapping/#kubelet-configuration

Step 7 Create Kubelet Config File

Create the kubelet-config.yaml configuration file:

Reference: https://kubernetes.io/docs/reference/config-api/kubelet-config.v1beta1/

cat <<EOF | sudo tee /var/lib/kubelet/kubelet-config.yaml
kind: KubeletConfiguration
apiVersion: kubelet.config.k8s.io/v1beta1
authentication:
  anonymous:
    enabled: false
  webhook:
    enabled: true
  x509:
    clientCAFile: /var/lib/kubernetes/pki/ca.crt
authorization:
  mode: Webhook
containerRuntimeEndpoint: unix:///var/run/containerd/containerd.sock
cgroupDriver: systemd
clusterDomain: "cluster.local"
clusterDNS:
  - ${CLUSTER_DNS}
registerNode: true
resolvConf: /run/systemd/resolve/resolv.conf
rotateCertificates: true
runtimeRequestTimeout: "15m"
serverTLSBootstrap: true
EOF

Note: We are not specifying the certificate details - tlsCertFile and tlsPrivateKeyFile - in this file

Step 8 Configure Kubelet Service

Create the kubelet.service systemd unit file:

cat <<EOF | sudo tee /etc/systemd/system/kubelet.service
[Unit]
Description=Kubernetes Kubelet
Documentation=https://github.com/kubernetes/kubernetes
After=containerd.service
Requires=containerd.service

[Service]
ExecStart=/usr/local/bin/kubelet \\
  --bootstrap-kubeconfig="/var/lib/kubelet/bootstrap-kubeconfig" \\
  --config=/var/lib/kubelet/kubelet-config.yaml \\
  --kubeconfig=/var/lib/kubelet/kubeconfig \\
  --cert-dir=/var/lib/kubelet/pki/ \\
  --v=2
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target
EOF

Things to note here:

  • bootstrap-kubeconfig: Location of the bootstrap-kubeconfig file.
  • cert-dir: The directory where the generated certificates are stored.
  • kubeconfig: We specify a location for this but we have not yet created it. Kubelet will create one itself upon successful bootstrap.

Step 9 Configure the Kubernetes Proxy

In one of the previous steps we created the kube-proxy.kubeconfig file. Check here if you missed it.

{
  sudo mv kube-proxy.kubeconfig /var/lib/kube-proxy/
  sudo chown root:root /var/lib/kube-proxy/kube-proxy.kubeconfig
  sudo chmod 600 /var/lib/kube-proxy/kube-proxy.kubeconfig
}

Create the kube-proxy-config.yaml configuration file:

Reference: https://kubernetes.io/docs/reference/config-api/kube-proxy-config.v1alpha1/

cat <<EOF | sudo tee /var/lib/kube-proxy/kube-proxy-config.yaml
kind: KubeProxyConfiguration
apiVersion: kubeproxy.config.k8s.io/v1alpha1
clientConnection:
  kubeconfig: /var/lib/kube-proxy/kube-proxy.kubeconfig
mode: ipvs
clusterCIDR: ${POD_CIDR}
EOF

Create the kube-proxy.service systemd unit file:

cat <<EOF | sudo tee /etc/systemd/system/kube-proxy.service
[Unit]
Description=Kubernetes Kube Proxy
Documentation=https://github.com/kubernetes/kubernetes

[Service]
ExecStart=/usr/local/bin/kube-proxy \\
  --config=/var/lib/kube-proxy/kube-proxy-config.yaml
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target
EOF

Step 10 Start the Worker Services

On worker-2:

{
  sudo systemctl daemon-reload
  sudo systemctl enable kubelet kube-proxy
  sudo systemctl start kubelet kube-proxy
}

Remember to run the above commands on worker node: worker-2

Optional - Check Certificates and kubeconfigs

At worker-2 node, run the following, selecting option 5

./cert_verify.sh

Step 11 Approve Server CSR

Now, go back to master-1 and approve the pending kubelet-serving certificate

//: # (command:kubectl certificate approve --kubeconfig admin.kubeconfig $(kubectl get csr --kubeconfig admin.kubeconfig -o json | jq -r '.items | .[] | select(.spec.username == "system:node:worker-2") | .metadata.name'))

kubectl get csr --kubeconfig admin.kubeconfig

Output - Note the name will be different, but it will begin with csr-

NAME        AGE   SIGNERNAME                                    REQUESTOR                 REQUESTEDDURATION   CONDITION
csr-7k8nh   85s   kubernetes.io/kubelet-serving                 system:node:worker-2      <none>              Pending
csr-n7z8p   98s   kubernetes.io/kube-apiserver-client-kubelet   system:bootstrap:07401b   <none>              Approved,Issued

Approve the pending certificate. Note that the certificate name csr-7k8nh will be different for you, and each time you run through.

kubectl certificate approve --kubeconfig admin.kubeconfig csr-7k8nh

Note: In the event your cluster persists for longer than 365 days, you will need to manually approve the replacement CSR.

Reference: https://kubernetes.io/docs/reference/access-authn-authz/kubelet-tls-bootstrapping/#kubectl-approval

Verification

List the registered Kubernetes nodes from the master node:

kubectl get nodes --kubeconfig admin.kubeconfig

output

NAME       STATUS      ROLES    AGE   VERSION
worker-1   NotReady    <none>   93s   v1.28.4
worker-2   NotReady    <none>   93s   v1.28.4

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