How to connect to kubernetes internal network using WireGuard

When you are testing your deployments in a kubernetes cluster on the cloud you have a few options to expose your services outside world, for example you can use a NodePort service, but also you need to configure the firewall rules for each NodePort service, the other type of service that you can use is LoadBalancer however each of them is billed by cloud provider. To solve this problem you can use a vpn running within your k8s cluster, this vpn can be exposed outside the cluster with a NodePort or LoadBalancer service. As client you can access to you kubernetes internal network using service FQDN in your local machine.

In this tutorial we gonna setup a pod that run wireguard server, this wireguard will be configured with the kube-dns service and generate cliente credentials automatically the diagram will be like this:

Assuming that you are in a testing k8s cluster in the cloud with multiple namespaces and services (for example a web app).

First we need to know the kube-dns IP address with the following command

$ kubectl -n kube-system get svc | grep kube-dns | awk '{print $3}'
# output example: 10.124.0.10

In order to isolate wireguard server from another apps, we need to create a wireguard namespace named wireguard

apiVersion: v1
kind: Namespace
metadata:
  name: wireguard
  labels:
    name: wireguard

To store wireguard config files, we need a persistent volume, in my case i’m using a gke managed service that provides me a storage class, so i’m gonna create a persistent volume claim to that storage class.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pv-claim-wireguard 
  namespace: wireguard
spec:
  storageClassName: "standard"
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 10M

The next thing to configure is the environment variables of wireguard server, this will be do with a config map. The kube-dns IP from steps earlier will be set in PEERDNS field.

apiVersion: v1
kind: ConfigMap
metadata:
  name: wireguard-configmap
  namespace: wireguard
data:
  PUID: "1000"
  PGID: "1000"
  TZ: "America/Mexico_City"
  SERVERPORT: "31820"
  PEERS: "2"
  PEERDNS: "10.124.0.10"
  ALLOWEDIPS: "0.0.0.0/0, ::/0"
  INTERNAL_SUBNET: "10.13.13.0"

Now we can create the wireguard server pod, this pod needs to be privileged with NET_ADMIN and SYS_MODULE capabilities and needs to mount /lib/modules directory from the host. The image used is ghcr.io/linuxserver/wireguard from linuxserver.io.

apiVersion: v1
kind: Pod
metadata:
  name: wireguard
  namespace: wireguard
  labels:
    app: wireguard
spec:
  containers:
  - name: wireguard
    image: ghcr.io/linuxserver/wireguard
    envFrom:
    - configMapRef:
        name: wireguard-configmap 
    securityContext:
      capabilities:
        add:
          - NET_ADMIN
          - SYS_MODULE
      privileged: true
    volumeMounts:
      - name: wg-config
        mountPath: /config
      - name: host-volumes
        mountPath: /lib/modules
    ports:
    - containerPort: 51820
      protocol: UDP
    resources:
      requests:
        memory: "64Mi"
        cpu: "100m"
      limits:
        memory: "128Mi"
        cpu: "200m"
  volumes:
    - name: wg-config
      persistentVolumeClaim:
        claimName: pv-claim-wireguard 
    - name: host-volumes
      hostPath:
        path: /lib/modules
        type: Directory

Finally to access to wireguard server, we need to create a service, this service could be a NodePort or LoadBalancer, in my case i used a NodePort service on port 31820, take in mind that you probably need to configure a firewall rule to access at this service.

kind: Service
apiVersion: v1
metadata:
  labels:
    k8s-app: wireguard
  name: wireguard-service
  namespace: wireguard
spec:
  type: NodePort
  ports:
  - port: 51820
    nodePort: 31820
    protocol: UDP
    targetPort: 51820
  selector:
    app: wireguard

This configurations are in a single file wireguard-pod.yaml to execute just apply the file with kubectl command

$ kubectl apply -f wireguard-pod.yaml

The container generate a QR code for each peer, these QR appears in the logs of the pod, to see just type the following command

$ kubectl -n wireguard logs wireguard

The output will be like this

In order to connect to wireguard server download mobile app of install in your local machine. See wireguard.com.

You can scan the code with the mobile app or copy the config file in your computer at ~/peer1.conf

$ kubectl -n wireguard exec wireguard -- cat /config/peer1/peer1.conf > ~/peer1.conf

Now you can utilize the config file to activate the vpn. With NetworManager you can import the config file

$ nmcli connection import type wireguard file ~/peer1.conf

And activate or deactivate the connection

$ nmcli connection up peer1 
$ nmcli connection down peer1

Finally to access a ClusterIP service within k8s cluster just use the IP of ClusterIP service or use the FQDN of the service using the following rule

<clusterip-service>.<namespace>.svc.cluster.local

Check the output of dig in a FQDN inside a remote k8s cluster, note that the query is answered by kube-dns IP inside the k8s cluster.

For example to access a ClusterIP service named thingsboard-service in the namespace thingsboard at 9090 port from our local machine through wireguard vpn:

http://thingsboard-service.thingsboard.svc.cluster.local:9090

And the output in our local environment

Conclusion

This method is very useful for a managed kubernetes service in the cloud in a development environment because we can test our services without configure a nodePort for each service and his respectively firewall rule.

WARNING: Only use this method in a development environment, don’t use in a production environment

Source Code