IPv6 and Let's Encrypt TLS on Google Kubernetes Engine

In a previous article I described how I deployed my blog on kubernetes and served it over HTTP. Today I’d like to add three more pieces:

  • Automate Let’s Encrypt certificate retrieval (and renewal)
  • Add a TLS-capable load balancer
  • Add IPv6 support (because it’s 2017)

Automating certificate management

Thanks to Let’s Encrypt web servers can request trusted and signed certificate for free in a fully automated manner. A web traffic load balancer is basically a proxy server, acting like a web server on the frontend and like a HTTP client towards the backend. So why not let the load balancer’s fronted (the web server part) take care of fetching a certificate from Let’s Encrypt? We have seen other web servers, such as Caddy, taking care of certificate management.

Unfortunately, this is not a feature that is available on Google Cloud Platform (GCP). Furthermore, I can imagine this working fine with a single load balancer, but failing at scale in a multi-balancer setup. The reason is, that Let’s Encrypt has an API limit. One can request only so many certificates in a week. But even if we had access to an unlimited API, it would still be a non-trivial task to make sure the right load balancer is responding to the HTTP challenge request from Let’s Encrypt.

What we need to address the problem is a software that retrieves and renews certificates and deploys them to our load balancer(s) whenever a relevant change occurs. A relevant change in this sense could be a modified hostname, a new subdomain, or the nearing expiration date of a currently deployed certificate. Fortunately, there is a tool for that already. There are actually multiple tools, and they run on kubernetes, making deployment really straightforward:

In this article we will use kube-lego, but I can highly recommend cert-manager, too. Of course, for non-production use cases only. 😉

Note: If your kubernetes cluster has Role Based Access Control (RBAC) enabled then apply a profile to kube-lego that grants the required privileges before you proceed!

Deploying kube-lego

Like every other workload, we like to cage kube-lego into a dedicated namespace. We define the namespace in k8s/kube-lego.ns.yaml:

apiVersion: v1
kind: Namespace
metadata:
  name: kube-lego

And create it via the command line tool kubectl:

$ kubectl create -f k8s/kube-lego.ns.yaml

The next step is to define and configure the kube-lego deployment in k8s/kube-lego.deployment.yaml. For the initial deployment of kube-lego, I recommend setting LEGO_LOG_LEVEL to debug:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: kube-lego
  namespace: kube-lego
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: kube-lego
    spec:
      containers:
      - name: kube-lego
        image: jetstack/kube-lego:0.1.5
        imagePullPolicy: Always
        ports:
        - containerPort: 8080
        env:
        - name: LEGO_LOG_LEVEL
          value: info  # more verbose: debug
        - name: LEGO_EMAIL
          value: mail@example.com  # change this!
        - name: LEGO_URL
          value: https://acme-v01.api.letsencrypt.org/directory
        - name: LEGO_NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace
        - name: LEGO_POD_IP
          valueFrom:
            fieldRef:
              fieldPath: status.podIP
        resources:
          limits:
            cpu: 100m
            memory: 50Mi
          requests:
            cpu: 50m
            memory: 50Mi
        readinessProbe:
          httpGet:
            path: /healthz
            port: 8080
          initialDelaySeconds: 5
          timeoutSeconds: 1

Once the namespace is ready we can deploy and check if the deployment succeeded:

$ kubectl create -f k8s/kube-lego.deployment.yaml
✂ī¸
$ kubectl -n kube-lego get deployments
NAME        DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
kube-lego   1         1         1            1           10m

Tip: Consider using a configmap as alternative to hard-coding configuration parameters into a deployment.

Addding a TLS-enabled load balancer

With kube-lego there are two different ways of defining a load balancer. The easier (but more expensive) one is to use a load balancer provided by GCP. The alternative is deploying an nginx ingress pod and using that as the load balancer. I got good results from both in my experiments. For the sake of brevity, we will use the quicker GCP way in this article.

First, we need to create a kubernetes ingress object to balance and proxy incoming web traffic. The important part here is, that we can influence the behavior of the ingress object by providing annotations.

  • kubernetes.io/ingress.class: "gce" This annotation let’s kubernetes know that we want to use a GCP load balancer for ingress traffic. Obviously, this annotation does not make sense on kubernetes installations which do not run on GCP.
  • kubernetes.io/tls-acme: "true"` This annotation allows kube-lego to manage the domains and certificates referenced in this ingress object for us. If we leave out this annotation, kube-lego will refrain from touching it or its associated kubernetes secrets.
apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  annotations:
    kubernetes.io/tls-acme: "true"
    kubernetes.io/ingress.class: "gce"
  name: website
  namespace: website
spec:
  rules:
  - host: test.danrl.com
    http:
      paths:
      - backend:
          serviceName: website
          servicePort: 80
        path: /
  tls:
  - hosts:
    - test.danrl.com
    secretName: test-danrl-com-certificate
$ kubectl create -f k8s/website.ingress.yaml

It may take a while for the ingress object to become fully visible. GCP is not the fastest fellow to spin up new load balancers in my experience. ⏱

$ kubectl -n website get ingress
NAME      HOSTS            ADDRESS       PORTS     AGE
website   test.danrl.com   35.196.54.8   80, 443   3m

Very soon after the load balancer is up and running, kube-lego should jump in and notice the lack of a certificate. It will fetch one and deploy it automatically. Awesome! We can watch this process in the logs. I use Stackdriver for collecting logs from kubernetes workloads, but there are many other options as well. Wherever your logs are, lookout for a line similar to this one:

level=info msg="requesting certificate for test.danrl.com" context="ingress_tls" name=website namespace=website

Once the requested certificate has been received, kube-lego will create or update the secret for it. We can verify the existence of the secret:

$ kubectl -n website get secrets
NAME                          TYPE                                  DATA      AGE
✂ī¸
test-danrl-com-certificate    kubernetes.io/tls                     2         22m

From now on, kube-lego will monitor the certificate and renew and replace it as necessary. The certificate should also show up in the load balancer configuration on the GCP console at Network Services → Load balancing → Certificates (you may have to enable the advanced menu at the bottom):

initial certificate,small

To test the automation further we could trigger a certificate renewal by tweaking the LEGO_MINIMUM_VALIDITY environment variable (optional). For reference, here is the automatically retrieved follow-up certificate I got:

followup certificate,small

Adding IPv6 to the load balancer

In the standard configuration GCP load balancers are started without an IPv6 address assigned. Technically, they can handle IPv6 traffic and we are free to assign IPv6 addresses to a GCP load balancer. To do this, we first have to reserve a static IPv6 address. This is done at VPC network → External IP addresses.

vpc external addresses,small

Reserving an address means, that this address can not be used by anyone else on the platform. If we reverse addresses but don’t use them charges will apply.

reserve static address,small

Once the address is reserved, we can assign it to the load balancer. To do that, we have to add an additional frontend for every address and every protocol (HTTP, HTTPS). That is, two frontends for each additional address.

add ipv6 to load balancer

We have to do the same for HTTPS, too, of course. When setting the IPv6 HTTPS frontend, we select the current certificate from the dropdown menu.

Almost automated… 😤

And now I have some bad news for you. ☚ī¸ IPv6 load balancer frontends, certificate renewal via kube-lego, and GCP load balancers do not go very well together (as of time of writing). When kube-lego renews the certificates it ignores manually added frontends. This means, the certificate for the IPv6 address will not be replaced automatically. Very frustrating!

certificates differ

In the screenshot we can see the new certificate k8s-ssl-1-website2-website2–a02b6ae745a706f8 alongside the old one k8s-ssl-website2-website2–a02b6ae745a706f8. Only for the IPv4 frontend was the certificate replaced.