Automatically Rotating Control Plane TLS Credentials

    (Note that Linkerd’s trust anchor must still be manually rotated on long-lived clusters.)

    is a popular project for making TLS credentials from external sources available to Kubernetes clusters.

    As a first step, install cert-manager on your cluster.

    Note

    If you are installing cert-manager , you will need to have kubernetes >= 1.16. Legacy custom resource definitions in cert-manager for kubernetes <= 1.15 do not have a keyAlgorithm option, so the certificates will be generated using RSA and be incompatible with linkerd.

    See for more details on version requirements.

    In this case, rather than pulling credentials from an external source, we’ll configure it to act as an on-cluster CA and have it re-issue Linkerd’s issuer certificate and private key on a periodic basis.

    First, create the namespace that cert-manager will use to store its Linkerd-related resources. For simplicity, we suggest the default Linkerd control plane namespace:

    Save the signing key pair as a Secret

    1. step certificate create identity.linkerd.cluster.local ca.crt ca.key \
    2.   --profile root-ca --no-password --insecure --san identity.linkerd.cluster.local &&
    3.   kubectl create secret tls \
    4.    linkerd-trust-anchor \
    5.    --cert=ca.crt \
    6.    --key=ca.key \

    For a longer-lived trust anchor certificate, pass the --not-after argument to the step command with the desired value (e.g. --not-after=87600h).

    Create an Issuer referencing the secret

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

    Issuing certificates and writing them to a secret

    Finally, we can create a cert-manager “Certificate” resource which uses this Issuer to generate the desired certificate:

    1. cat <<EOF | kubectl apply -f -
    2. apiVersion: cert-manager.io/v1alpha3
    3. kind: Certificate
    4. metadata:
    5.   name: linkerd-identity-issuer
    6.   namespace: linkerd
    7. spec:
    9.   duration: 24h
    10.   renewBefore: 1h
    11.   issuerRef:
    12.     name: linkerd-trust-anchor
    13.     kind: Issuer
    14.   commonName: identity.linkerd.cluster.local
    15.   isCA: true
    16.   usages:
    17.   - cert sign
    18.   - crl sign
    19.   - server auth
    20.   - client auth
    21. EOF

    (In the YAML manifest above, the duration key instructs cert-manager to consider certificates as valid for 24 hours and the renewBefore key indicates that cert-manager will attempt to issue a new certificate one hour before expiration of the current one. These values can be customized to your liking.)

    At this point, cert-manager can now use this Certificate resource to obtain TLS credentials, which will be stored in a secret named linkerd-identity-issuer. To validate your newly-issued certificate, you can run:

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

    Note

    Due to a bug in cert-manager, if you are using cert-manager version 0.15 with experimental controllers, the certificate it issues are not compatible with with Linkerd versions <= stable-2.8.1.

    1. "Failed to initialize identity service: failed to read CA from disk:
    2. unsupported block type: 'PRIVATE KEY'"

    Some possible ways to resolve this issue are:

    • Upgrade Linkerd to the edge versions >= edge-20.6.4 which contains a .
    • Upgrade cert-manager to versions >= 0.16. (how to upgrade)
    • Turn off cert-manager experimental controllers.

    Alternative CA providers

    Instead of using Cert Manager as CA, you can configure it to rely on a number of other solutions such as . More detail on how to setup the existing Cert Manager to use different type of issuers can be found here.

    It is important to note that the mechanism that Linkerd provides is also usable outside of cert-manager. Linkerd will read the linkerd-identity-issuer Secret, and if it’s of type kubernetes.io/tls, will use the contents as its TLS credentials. This means that any solution that is able to rotate TLS certificates by writing them to this secret can be used to provide dynamic TLS certificate management.

    For CLI installation, the Linkerd control plane should be installed with the --identity-external-issuer flag, which instructs Linkerd to read certificates from the linkerd-identity-issuer secret. Whenever certificate and key stored in the secret are updated, the identity service will automatically detect this change and reload the new credentials.

    Voila! We have set up automatic rotation of Linkerd’s control plane TLS credentials. And if you want to monitor the update process, you can check the IssuerUpdated events emitted by the service:

    For Helm installation, rather than running linkerd install, set the global.identityTrustAnchorsPEM to the value of ca.crt in the linkerd-identity-issuer Secret:

    1. helm install linkerd2 \
    2.   --set-file global.identityTrustAnchorsPEM=ca.crt \
    3.   --set identity.issuer.scheme=kubernetes.io/tls \
    4.   --set installNamespace=false \

    Note

    For Helm versions < v3, --name flag has to specifically be passed. In Helm v3, It has been deprecated, and is the first argument as specified above.