Niklas Tech Blog

mTLS is a widely used for securing sensitive data the world over. It is dependent on that both sides of a communication trust each other before a transaction can happen. The trust is based on certificates and can be devided into two parts, one part where the clients needs to trust the servers presented certificate and another part where the server needs to trust the clients presented certificate. We are now going to see an example of how to set this up in Kubernetes

All certificates are going to be self-signed in this example, regular certificates from trusted sources like Thwate, GlobalSign and many others will also work.

For Kubernetes I will use Minikube with the Ingress addon:

minikube addons enable ingress

1. First we need a server certificate to present to any client wanting to connect

openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout server.key -out server.crt -subj "/CN=test.localdev.me/O=test.localdev.me"

This should give you two files, a server.key and a server.crt file with the private key and the certificate to present to clients trying to connect.
NOTE: the “domain” test.localdev.me will normally return 127.0.0.1 automatically which makes it perfect to use in cases like this

2. Lets add the certificate to the cluster via a Secret and the special type tls

kubectl create secret tls server-certificate --key server.key --cert server.crt

3. Now we need the client key and certificate. We start by creating our own “CA Authority”

openssl req -x509 -sha256 -newkey rsa:4096 -keyout ca.key -out ca.crt -days 356 -nodes -subj "/CN=My CA"

4. Add the CA to the cluster as a Secret with the type ca-secret

kubectl create secret generic ca-secret --from-file=ca.crt=ca.crt

5. A CSR for our client cert

openssl req -new -newkey rsa:4096 -keyout client.key -out client.csr -nodes -subj "/CN=My Client"

6. Sign the CSR with our CA (same we put into the cluster)

openssl x509 -req -sha256 -days 365 -in client.csr -CA ca.crt -CAkey ca.key -set_serial 02 -out client.crt

We should now have a client.key and a client.crt ready to use

7. Now we need an application to call. We create one with the Deployment below:

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: mywebserver
  name: mywebserver
spec:
  replicas: 1
  selector:
    matchLabels:
      app: mywebserver
  template:
    metadata:
      labels:
        app: mywebserver
    spec:
      containers:
      - image: httpd
        name: httpd
        ports:
        - containerPort: 80

8. A Service to expose the application to the cluster

apiVersion: v1
kind: Service
metadata:
  labels:
    app: my-service
  name: my-service
spec:
  ports:
  - port: 80
    protocol: TCP
    targetPort: 80
  selector:
    app: mywebserver

9. A Ingress to handle the authentication

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    nginx.ingress.kubernetes.io/auth-tls-pass-certificate-to-upstream: "true"
    nginx.ingress.kubernetes.io/auth-tls-secret: default/ca-secret
    nginx.ingress.kubernetes.io/auth-tls-verify-client: "on"
    nginx.ingress.kubernetes.io/auth-tls-verify-depth: "1"
  name: mtls-ingress
  namespace: default
spec:
  ingressClassName: nginx
  rules:
  - host: test.localdev.me
    http:
      paths:
      - backend:
          service:
            name: my-service
            port:
              number: 80
        path: /
        pathType: Prefix
  tls:
  - hosts:
    - test.localdev.me
    secretName: server-certificate

10. Time to test our mTLS setup
First we need to setup a port binding to port 443 on our local machine

sudo kubectl port-forward -n ingress-nginx service/ingress-nginx-controller 443:443

and now we can test with a call with our client certificate and key

curl -k -v https://test.localdev.me/ --key client.key --cert client.crt

If everything is working we should get “It works!” from the Web Server

Tested in Minikube 1.26.0 and with OpenSSL 1.1.1f on Ubuntu 20.08