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Free SSL on Kubernetes with Cert-manager

SSL certificate is the single most crucial component enabling secure communication between server and client. When deploying a fleet of services on Kubernetes, securing both the internal and external network traffic can be tricky. To our rescue comes Cert-manager, a Kubernetes native certificate manager.


At the end of this tutorial, we'll have secure cluster ingress traffic with a free SSL certificate issued from Let's Encrypt using cert-manager.

What is cert-manager?

Cert-manager is a native Kubernetes certificate management controller. It can help with issuing certificates from a variety of sources, such as Let’s Encrypt, HashiCorp Vault, self-signed certs, etc.


It will ensure certificates are valid and up to date, and attempt to renew certificates at a configured time before expiry.


Well, cert-manager comprises a variety of components and extensions, but we'll discuss those that are required for this tutorial. Do check out related concepts here.


  • Issuer - component that talks to CA authorities and requests a certificate on demand.

  • Certificate - it defines a certificate request as a Kubernetes resource, and this request is pushed to Issuer, and the issued certificate is then stored as a k8s secret object.


Requirements

Kubernetes cluster installed with Nginx-ingress and accessible with kubectl is required. I am using Kind to deploy a test kubernetes cluster, but you can choose any distribution of kubernetes of your choice.

For using Kind, you may refer to its installation docs.

Installing cert-manager with regular manifests

Install the CustomResourceDefinitions and cert-manager:

kubectl create ns cert-manager

kubectl -n cert-manager apply -f https://github.com/jetstack/cert-manager/releases/download/v1.3.1/cert-manager.yaml

You can download any other stable release for cert-manager from its releases page.


It's a good practice to store the YAML files that you apply to the cluster, in case something goes wrong you can restore the state of the cluster by re-applying them. To store the cert-manager manifest we just applied, run the following scripts.

curl -LO https://github.com/jetstack/cert-manager/releases/download/v1.3.1/cert-manager.yaml

mv cert-manager.yaml cert-manager-1.3.1.yaml

To verify the installation

kubectl -n cert-manager get all
NAME                                           READY   STATUS    RESTARTS   AGE
pod/cert-manager-86548b886-s64ql               1/1     Running   0          2m42s
pod/cert-manager-cainjector-6d59c8d4f7-mg25z   1/1     Running   0          2m43s
pod/cert-manager-webhook-578954cdd-9xjj7       1/1     Running   0          2m42s

NAME                           TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
service/cert-manager           ClusterIP   10.105.244.77   <none>        9402/TCP   2m46s
service/cert-manager-webhook   ClusterIP   10.107.103.46   <none>        443/TCP    2m44s

NAME                                      READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/cert-manager              1/1     1            1           2m42s
deployment.apps/cert-manager-cainjector   1/1     1            1           2m43s
deployment.apps/cert-manager-webhook      1/1     1            1           2m42s

NAME                                                 DESIRED   CURRENT   READY   AGE
replicaset.apps/cert-manager-86548b886               1         1         1       2m42s
replicaset.apps/cert-manager-cainjector-6d59c8d4f7   1         1         1       2m43s
replicaset.apps/cert-manager-webhook-578954cdd       1         1         1       2m42s

Deploying sample app

For testing issued SSL certificate, let's deploy a simple REST API in the cluster as well. Once deployed we'll route the traffic to this app using Nginx-ingress.


The source code for the sample app is inside the app directory and infrastructure files are contained inside the infra folder.


kubectl apply -f ./deployment.yaml
kubectl apply -f ./service.yaml

Do make sure service and deployment components are up and running.

Setting up domain and ingress controller

Since we'll obtain SSL certificate from let's encrypt, in order to verify ownership of the domain, let's encrypt will send an ACME challenge. So before setting up Issuer and certificate we need to point our domain name to our cluster.


If you are using a cloud provider point domain to the load balancer, for kind cluster or any other bare metal setup pointing to public IP of host machine will work fine.


Also, make sure the ingress controller is running and accepting traffic for your domain. Ping your-domain.com and you should be able to see a 404 Not Found page. This indicates there are no routes to / and the ingress controller is running.

If you are not able to get the above result, make sure Nginx-ingress is installed and enabled inside the cluster. Refer to its installation guide here.

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    kubernetes.io/ingress.class: "nginx"
  name: certmanager-app-ingress

  rules:
    - host: yourdomain.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: certmanager-app-service
                port:
                  number: 80

Copy the above code to ingress.yaml and apply it in the default namespace.

kubectl apply -f ./ingress.yaml

To verify the routing, access any endpoint of your REST API.

Let's issue a free SSL!

Files used in this section are present in certmanager dir.

Step 1 Create a Let's Encrypt Issuer for our cluster

Creating a ClusterIssuer, allows us to issue and manage certificates in any namespace.

kubectl apply -f issuer.yaml

Verify the issuer is running and set to make the request.

kubectl describe clusterissuer letsencrypt-cluster-issuer
Name:         letsencrypt-cluster-issuer
Namespace:
Labels:       <none>
Annotations:  <none>
API Version:  cert-manager.io/v1
Kind:         ClusterIssuer

Status:
  Acme:
    Last Registered Email:  [email protected]
    Uri:                    https://acme-v02.api.letsencrypt.org/acme/acct/107894884
  Conditions:
    Last Transition Time:  2021-04-30T17:40:12Z
    Message:               The ACME account was registered with the ACME server
    Reason:                ACMEAccountRegistered
    Status:                True
    Type:                  Ready
Events:                    <none>

Step 2 Issue a certificate for our domain 

kubectl apply -f certificate.yaml

Verify if the certificate has been issued.

kubectl describe certificate certmanager-app
Events:
  Type    Reason     Age    From          Message
  ----    ------     ----   ----          -------
  Normal  Issuing    2m22s  cert-manager  Issuing certificate as Secret does not exist
  Normal  Generated  2m22s  cert-manager  Stored new private key in temporary Secret resource "certmanager-app-z88vv"
  Normal  Requested  2m22s  cert-manager  Created new CertificateRequest resource "certmanager-app-hnhn6"
  Normal  Issuing    104s   cert-manager  The certificate has been successfully issued

A tls secret would have been successfully created.

kubectl get secrets
NAME                       TYPE                                  DATA   AGE
cert-manager-app-tls       kubernetes.io/tls                     2      84m

To learn more about kubernetes secret and tls secret click here.

Step 3 Update the ingress to enforce issued SSL

Copy the ingress configuration from certmanager/ingress.yaml and paste it inside ingress.yaml

kubectl apply -f ./ingress.yaml

Open the link https://your-domain.com inside the browser and you'll be able to see the lock sign(left to the domain) ensuring that the SSL certificate is installed and the payload to-from server is encrypted.

Conclusion

While there are many ways to set up SSL certificates inside your cluster, I like to use cert-manager as it's

  • Easy to install.
  • Supports ACME (Let's Encrypt), HashiCorp Vault, Venafi, self-signed, and internal certificate authorities.
  • Provide tools to manage certificates.

If you want to use any other CA or configuration, checkout certmanager-configuration-docs

Author

Akshit Sadana [email protected]