Wildcard Certificate

Now that we have an Issuer and the necessary credentials, we can create a wildcard certificate, which we can then feed to our Ingresses.

Certificates are Kubernetes secrets, and so are subject to the same limitations / RBAC controls as other secrets. Importantly, they are namespaced, so it's not possible to refer to a secret in one namespace, from a pod in another namespace. This restriction also applies to Ingress resources (although there are workarounds) - An Ingress can only refer to TLS secrets in its own namespace.

This behaviour can be prohibitive, because (a) we don't want to have to request/renew certificates for every single FQDN served by our cluster, and (b) we don't want more than one wildcard certificate if possible, to avoid being rate-limited at request/renewal time.

To take advantage of the various workarounds available, I find it best to put the certificates into a dedicated namespace, which I name.. letsencrypt-wildcard-cert.

Preparation

DNS01 Validation Secret

The simplest way to validate ownership of a domain to LetsEncrypt is to use DNS-01 validation. In this mode, we "prove" our ownership of a domain name by creating a special TXT record, which LetsEncrypt will check and confirm for validity, before issuing us any certificates for that domain name.

The ClusterIssuers we created earlier included a field solvers.dns01.cloudflare.apiTokenSecretRef.name. This value points to a secret (in the same namespace as cert-manager) containing credentials necessary to create DNS records automatically. (again, my examples are for cloudflare, but the other supported providers will have similar secret requirements)

Thanks to Sealed Secrets, we have a safe way of committing secrets into our repository, so to create necessary secret, you'd run something like this:

  kubectl create secret generic cloudflare-api-token-secret \
  --namespace cert-manager \
  --dry-run=client \
  --from-literal=api-token=gobbledegook -o json \
  | kubeseal --cert <path to public cert> \
  > <path to repo>/cert-manager/sealedsecret-cloudflare-api-token-secret.yaml

Staging Certificate

Finally, we create our certificates! Here's an example certificate resource which uses the letsencrypt-staging issuer (to avoid being rate-limited while learning!). I save this in my flux repo as /letsencrypt-wildcard-cert/certificate-wildcard-cert-letsencrypt-staging.yaml

apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: letsencrypt-wildcard-cert-example.com-staging
  namespace: letsencrypt-wildcard-cert
spec:
  # secretName doesn't have to match the certificate name, but it may as well, for simplicity!
  secretName: letsencrypt-wildcard-cert-example.com-staging 
  issuerRef:
    name: letsencrypt-staging
    kind: ClusterIssuer
  dnsNames:
    - "example.com"
    - "*.example.com"

Serving

Did it work?

After committing the above to the repo, provided the YAML syntax is correct, you should end up with a "Certificate" resource in the letsencrypt-wildcard-cert namespace. This doesn't mean that the certificate has been issued by LetsEncrypt yet though - describe the certificate for more details, using kubectl describe certificate -n letsencrypt-wildcard-cert letsencrypt-wildcard-cert-staging. The status field will show you whether the certificate is issued or not:

Status:
  Conditions:
    Last Transition Time:  2021-11-19T01:09:32Z
    Message:               Certificate is up to date and has not expired
    Observed Generation:   1
    Reason:                Ready
    Status:                True
    Type:                  Ready
  Not After:               2022-02-17T00:09:26Z
  Not Before:              2021-11-19T00:09:27Z
  Renewal Time:            2022-01-18T00:09:26Z
  Revision:                1

Troubleshooting

If your certificate does not become Ready within a few minutes ¹, try watching the logs of cert-manager to identify the issue, using kubectl logs -f -n cert-manager -l app.kubernetes.io/name=cert-manager.

Production Certificate

Once you know you can happily deploy a staging certificate, it's safe enough to attempt your "prod" certificate. I save this in my flux repo as /letsencrypt-wildcard-cert/certificate-wildcard-cert-letsencrypt-prod.yaml

apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: letsencrypt-wildcard-cert-example.com
  namespace: letsencrypt-wildcard-cert
spec:
# secretName doesn't have to match the certificate name, but it may as well, for simplicity!
  secretName: letsencrypt-wildcard-cert-example.com 
  issuerRef:
    name: letsencrypt-prod
    kind: ClusterIssuer
  dnsNames:
  - "example.com"
  - "*.example.com"

Commit the certificate and follow the steps above to confirm that your prod certificate has been issued.

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