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 namespaces that cert-manager will use to store its webhook-related resources. For simplicity, we suggest using the defaule namespace linkerd uses:

# control plane core
kubectl create namespace linkerd

# viz (ignore if not using the viz extension)
kubectl create namespace linkerd-viz

# viz (ignore if not using the jaeger extension)
kubectl create namespace linkerd-jaeger

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

# ignore if not using the viz extension
kubectl create secret tls webhook-issuer-tls --cert=ca.crt --key=ca.key --namespace=linkerd-viz

# ignore if not using the jaeger extension
kubectl create secret tls webhook-issuer-tls --cert=ca.crt --key=ca.key --namespace=linkerd-jaeger

Create Issuers referencing the secrets

With the Secrets in place, we can create cert-manager “Issuer” resources that reference them:

cat <<EOF | kubectl apply -f -
apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: webhook-issuer
  namespace: linkerd
spec:
  ca:
    secretName: webhook-issuer-tls
---
# ignore if not using the viz extension
apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: webhook-issuer
  namespace: linkerd-viz
spec:
  ca:
    secretName: webhook-issuer-tls
---
# ignore if not using the jaeger extension
apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: webhook-issuer
  namespace: linkerd-jaeger
spec:
  ca:
    secretName: webhook-issuer-tls
EOF

Issuing certificates and writing them to secrets

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

cat <<EOF | kubectl apply -f -
apiVersion: cert-manager.io/v1
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
  dnsNames:
  - linkerd-proxy-injector.linkerd.svc
  isCA: false
  privateKey:
    algorithm: ECDSA
  usages:
  - server auth
---
apiVersion: cert-manager.io/v1
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
  dnsNames:
  - linkerd-sp-validator.linkerd.svc
  isCA: false
  privateKey:
    algorithm: ECDSA
  usages:
  - server auth
---
# ignore if not using the viz extension
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: tap
  namespace: linkerd-viz
spec:
  secretName: tap-k8s-tls
  duration: 24h
  renewBefore: 1h
  issuerRef:
    name: webhook-issuer
    kind: Issuer
  commonName: tap.linkerd-viz.svc
  dnsNames:
  - tap.linkerd-viz.svc
  isCA: false
  privateKey:
    algorithm: ECDSA
  usages:
  - server auth
---
# ignore if not using the viz extension
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: linkerd-tap-injector
  namespace: linkerd-viz
spec:
  secretName: tap-injector-k8s-tls
  duration: 24h
  renewBefore: 1h
  issuerRef:
    name: webhook-issuer
    kind: Issuer
  commonName: tap-injector.linkerd-viz.svc
  dnsNames:
  - tap-injector.linkerd-viz.svc
  isCA: false
  privateKey:
    algorithm: ECDSA
  usages:
  - server auth
---
# ignore if not using the jaeger extension
apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: jaeger-injector
  namespace: linkerd-jaeger
spec:
  secretName: jaeger-injector-k8s-tls
  duration: 24h
  renewBefore: 1h
  issuerRef:
    name: webhook-issuer
    kind: Issuer
  commonName: jaeger-injector.linkerd.svc
  dnsNames:
  - jaeger-injector.linkerd.svc
  isCA: false
  privateKey:
    algorithm: 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, tap-k8s-tls, tap-injector-k8s-tls and jaeger-injector-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
EOF

# ignore if not using the viz extension
cat > config-viz.yml <<EOF
tap:
  externalSecret: true
  caBundle: |
$CA
tapInjector:
  externalSecret: true
  caBundle: |
$CA
EOF

# ignore if not using the jaeger extension
cat > config-jaeger.yml <<EOF
webhook:
  externalSecret: true
  caBundle: |
$CA
EOF

Now we can install Linkerd using these config files:

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

# ignore if not using the viz extension
linkerd viz install --config=config-viz.yml | kubectl apply -f -

# ignore if not using the jaeger extension
linkerd jaeger install --config=config-jaeger.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 \
  linkerd/linkerd2 \
  -n linkerd

# ignore if not using the viz extension
helm install linkerd-viz \
  --set installNamespace=false \
  --set tap.externalSecret=true \
  --set-file tap.caBundle=ca.crt \
  --set tapInjector.externalSecret=true \
  --set-file tapInjector.caBundle=ca.crt \
  linkerd/linkerd-viz \
  -n linkerd-viz

# ignore if not using the jaeger extension
helm install linkerd-jaeger \
  --set installNamespace=false \
  --set webhook.externalSecret=true \
  --set-file webhook.caBundle=ca.crt \
  linkerd/linkerd-jaeger \
  -n linkerd-jaeger

See Automatically Rotating Control Plane TLS Credentials for details on how to do something similar for control plane credentials.