Integrate Kubernetes cert-manager with an internal ACME CA

About this tutorial

In this example, we'll configure Kubernetes cert-manager to get a certificate from an internal ACME server, using cert-manager's ACME issuer.

Estimated effort: Reading time ~4 mins, Lab time ~20 to 60 mins.

If you run into any issues please let us know in GitHub Discussions.

Requirements

Open source - You have initialized and started up a step-ca ACME instance using the steps in our ACME server tutorial.
Smallstep Certificate Manager - this tutorial assumes you have created a hosted or linked authority and created an ACME provisioner with External Account Binding enabled.
You'll need the root certificate PEM file for your CA.

0. Before you begin

This example uses the ACME dns-01 challenge type, with Google Cloud DNS. We'll create a service account on Google Cloud that cert-manager will use to solve DNS challenges. For other DNS providers, or other ACME challenge types, you'll need to change the challenge solver settings below.

1. Create a Kubernetes cluster

For this tutorial, I created a Google Compute Engine VM running a kind cluster. I'm using kind for testing, but pretty much any Kubernetes cluster will do.

$ kind create cluster

2. Install cert-manager

Let's install Kubernetes cert-manager

First, install cert-manager:

$ kubectl apply --validate=false -f https://github.com/jetstack/cert-manager/releases/download/v1.12.0/cert-manager.yaml

3. Configure a challenge solver

Not using Google Cloud Platform? You can skip this step and configure the cert-manager Issuer in step 4 to use a different challenge solver. See cert-manager's documentation for http-01 and dns-01 solvers.

We're going to have cert-manager solve dns-01 ACME challenges against a public Google Cloud Platform DNS zone. For this, we're going to create a Google Cloud Platform service account and import its credentials. The service account will need permission to manage DNS entries.

Let's create a Google Cloud Platform service account with the roles/dns.admin role. Replace the PROJECT_ID here with your own:

$ export PROJECT_ID=step-lan
$ gcloud iam service-accounts create dns01-solver \
   --project $PROJECT_ID --display-name "dns01-solver"
$ gcloud projects add-iam-policy-binding $PROJECT_ID \
   --member serviceAccount:dns01-solver@$PROJECT_ID.iam.gserviceaccount.com \
   --role roles/dns.admin

Now import the service account's credentials as a Kubernetes secret:

$ gcloud iam service-accounts keys create key.json \
   --iam-account dns01-solver@$PROJECT_ID.iam.gserviceaccount.com
$ kubectl create secret generic clouddns-dns01-solver-svc-acct \
   --from-file=key.json

4. Create the cert-manager Issuer

Finally, let's create an cert-manager Issuer to perform dns-01 ACME challenges. For this, we'll need a base64-encoded PEM file containing ACME server's CA certificate:

ROOT_CA=$(step ca root | base64)

Make a new file called acme-issuer.yaml:

apiVersion: cert-manager.io/v1
kind: Issuer
metadata:
  name: acme-issuer
spec:
  acme:
    email: carl@smallstep.com
    server: https://example.ca.smallstep.com/acme/acme/directory
    caBundle: LS0tLS1DRUdJTiBDRVJUSUZJEXAMPLE2UE11OWN4ckRNYWpQTlRTbkxCcEkxd1K4VnQ3SHBSK3A5b1JUTEzKcXxBPqotLSVOEXAMPLEUSUZJQ0FURS0tLS0tCg==
    privateKeySecretRef:
      name: acme-issuer-account-key
    solvers:
    - dns01:
        cloudDNS:
          # Your Google Cloud Platform project ID:
          project: step-gcp-test
          # Your Google CloudDNS zone name we will use for DNS01 challenges:
          hostedZoneName: step-public-zone
          serviceAccountSecretRef:
            name: clouddns-dns01-solver-svc-acct
            key: key.json

Replace the values for email, server URL, caBundle, project and hostedZoneName with your own. Your Smallstep ACME endpoint typically takes the form of https://[your CA hostname]/acme/acme/directory.

Optional: Enabling ACME External Account Binding (EAB)

Smallstep Certificate Manager uses ACME External Account Binding (EAB). When you get an EAB key from Smallstep, you'll need to convert it to base64URL before creating a Kubernetes secret for it:

echo 'yEZNEXAMPLEnu43wV/LNZYjL3ezwnd+GOd01TaID0EE=' | sed -e 's/+/-/g' -e 's/\//_/g' -e 's/=//g'

Output:

yEZNEXAMPLEnu43wV_LNZYjL3ezwnd-GOd01TaID0EE

Add this secret to Kubernetes:

kubectl create secret generic eab-secret --from-literal \
  secret=yEZNEXAMPLEnu43wV_LNZYjL3ezwnd-GOd01TaID0EE

Next, see cert-manager's documentation for details on configure your EAB key and secret in your Issuer.

5. Apply your Issuer

Finally, apply your Issuer configuration:

$ kubectl apply -f acme-issuer.yaml

You now have an automated ACME certificate manager running inside your Kubernetes cluster.

6. Issue a test certificate

Let's get a test certificate from our ACME CA, using a Certificate object. Create a file called tls-certificate.yaml:

apiVersion: cert-manager.io/v1
kind: Certificate
metadata:
  name: k8s-internal
  namespace: default
spec:
  secretName: k8s-internal-tls
  issuerRef:
    name: acme-issuer
  dnsNames:
  - k8s.smallstep.internal

Replace the dnsNames value with a DNS name that's inside your zone.

Apply it:

$ kubectl apply -f tls-certificate.yaml

You can check the status with kubectl get certificaterequest or kubectl describe certificate:

$ kubectl get certificaterequest
NAME                    READY   AGE
k8s-internal-nzbnm      True    7s
$ kubectl describe certificate k8s-internal
Name:         k8s-internal
Namespace:    default
...
Kind:         Certificate
Metadata:
  Creation Timestamp:  2020-11-03T23:06:46Z
...
Spec:
  Dns Names:
    k8s.smallstep.internal
  Issuer Ref:
    Name:       acme-issuer
  Secret Name:  k8s-internal-tls
Status:
  Conditions:
    Last Transition Time:  2020-11-03T23:11:01Z
    Message:               Certificate is up to date and has not expired
    Reason:                Ready
    Status:                True
    Type:                  Ready
  Not After:               2020-11-04T23:11:01Z
  Not Before:              2020-11-03T23:11:01Z
  Renewal Time:            2020-11-04T15:11:01Z
  Revision:                1
Events:
  Type    Reason     Age    From          Message
  ----    ------     ----   ----          -------
  Normal  Issuing    10m    cert-manager  Issuing certificate as Secret does not exist
  Normal  Generated  10m    cert-manager  Stored new private key in temporary Secret resource "k8s-internal-g79jq"
  Normal  Requested  10m    cert-manager  Created new CertificateRequest resource "k8s-internal-nzbnm"
  Normal  Issuing    9m33s  cert-manager  The certificate has been successfully issued

As you can see, cert-manager will automatically renew the certificate when approximately 2/3 of its lifetime has elapsed.

That's it! You now have automated, short-lived certificates for your Kubernetes cluster. There are many use cases for X.509 certificates issued through cert-manager.