Automatically Rotating Webhook TLS Credentials

The Linkerd control plane contains several components, called webhooks, which are called directly by Kubernetes itself. The traffic from Kubernetes to the Linkerd webhooks is secured with TLS and therefore each of the webhooks requires a secret containing TLS credentials. These certificates are different from the ones that the Linkerd proxies use to secure pod-to-pod communication and use a completely separate trust chain. For more information on rotating the TLS credentials used by the Linkerd proxies, see Automatically Rotating Control Plane TLS Credentials.

By default, when Linkerd is installed with the Linkerd CLI or with the Linkerd Helm chart, TLS credentials are automatically generated for all of the webhooks. If these certificates expire or need to be regenerated for any reason, performing a Linkerd upgrade (using the Linkerd CLI or using Helm) will regenerate them.

This workflow is suitable for most users. However, if you need these webhook certificates to be rotated automatically on a regular basis, it is possible to use cert-manager to automatically manage them.

Install Cert manager

As a first step, install cert-manager on your cluster and create the namespace that cert-manager will use to store its webhook-related resources. For simplicity, we suggest the default Linkerd control plane namespace:

kubectl create namespace linkerd

Save the signing key pair as a Secret

Next, we will use the step tool, to create a signing key pair which will be used to sign each of the webhook certificates:

step certificate create webhook.linkerd.cluster.local ca.crt ca.key \
  --profile root-ca --no-password --insecure --san webhook.linkerd.cluster.local &&
  kubectl create secret tls \
   webhook-issuer-tls \
   --cert=ca.crt \
   --key=ca.key \
   --namespace=linkerd

Create an Issuer referencing the secret

With the Secret in place, we can create a cert-manager “Issuer” resource that references it:

cat <<EOF | kubectl apply -f -
apiVersion: cert-manager.io/v1alpha3
kind: Issuer
metadata:
  name: webhook-issuer
  namespace: linkerd
spec:
  ca:
    secretName: webhook-issuer-tls
EOF

Issuing certificates and writing them to a secret

Finally, we can create cert-manager “Certificate” resources which use the Issuer to generate the desired certificates:

cat <<EOF | kubectl apply -f -
apiVersion: cert-manager.io/v1alpha3
kind: Certificate
metadata:
  name: linkerd-proxy-injector
  namespace: linkerd
spec:
  secretName: linkerd-proxy-injector-k8s-tls
  duration: 24h
  renewBefore: 1h
  issuerRef:
    name: webhook-issuer
    kind: Issuer
  commonName: linkerd-proxy-injector.linkerd.svc
  isCA: false
  keyAlgorithm: ecdsa
  usages:
  - server auth
---
apiVersion: cert-manager.io/v1alpha3
kind: Certificate
metadata:
  name: linkerd-sp-validator
  namespace: linkerd
spec:
  secretName: linkerd-sp-validator-k8s-tls
  duration: 24h
  renewBefore: 1h
  issuerRef:
    name: webhook-issuer
    kind: Issuer
  commonName: linkerd-sp-validator.linkerd.svc
  isCA: false
  keyAlgorithm: ecdsa
  usages:
  - server auth
---
apiVersion: cert-manager.io/v1alpha3
kind: Certificate
metadata:
  name: linkerd-tap
  namespace: linkerd
spec:
  secretName: linkerd-tap-k8s-tls
  duration: 24h
  renewBefore: 1h
  issuerRef:
    name: webhook-issuer
    kind: Issuer
  commonName: linkerd-tap.linkerd.svc
  isCA: false
  keyAlgorithm: ecdsa
  usages:
  - server auth
EOF

At this point, cert-manager can now use these Certificate resources to obtain TLS credentials, which are stored in the linkerd-proxy-injector-k8s-tls, linkerd-sp-validator-k8s-tls, and linkerd-tap-k8s-tls secrets respectively.

Now we just need to inform Linkerd to consume these credentials.

Using these credentials with CLI installation

To configure Linkerd to use the credentials from cert-manager rather than generating its own, we generate a supplemental config file:

CA=$(awk '{ print "    " $0 }' ca.crt); cat > config.yml <<EOF
proxyInjector:
  externalSecret: true
  caBundle: |
$CA
profileValidator:
  externalSecret: true
  caBundle: |
$CA
tap:
  externalSecret: true
  caBundle: |
$CA
EOF

Now we can install Linkerd using this config file:

linkerd install --config=config.yml | kubectl apply -f -

Installing with Helm

For Helm installation, we can configure the Helm values directly:

helm install linkerd2 \
  --set installNamespace=false \
  --set proxyInjector.externalSecret=true \
  --set-file proxyInjector.caBundle=ca.crt \
  --set profileValidator.externalSecret=true \
  --set-file profileValidator.caBundle=ca.crt \
  --set tap.externalSecret=true \
  --set-file tap.caBundle=ca.crt \
  linkerd/linkerd2 \
  -n linkerd