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Exposing Services in Kubernetes

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There are different ways to expose services in Kubernetes so both internal endpoints and external endpoints can access them. This Kubernetes configuration is pretty critical as the administrator could give access to attackers to services they shouldn't be able to access.

Automatic Enumeration

Before starting enumerating the ways K8s offers to expose services to the public, know that if you can list namespaces, services and ingresses, you can find everything exposed to the public with:

kubectl get namespace -o custom-columns='NAME:.metadata.name' | grep -v NAME | while IFS='' read -r ns; do
    echo "Namespace: $ns"
    kubectl get service -n "$ns"
    kubectl get ingress -n "$ns"
    echo "=============================================="
    echo ""
    echo ""
done | grep -v "ClusterIP"
# Remove the last '| grep -v "ClusterIP"' to see also type ClusterIP

ClusterIP

A ClusterIP service is the default Kubernetes service. It gives you a service inside your cluster that other apps inside your cluster can access. There is no external access.

However, this can be accessed using the Kubernetes Proxy:

kubectl proxy --port=8080

Now, you can navigate through the Kubernetes API to access services using this scheme:

http://localhost:8080/api/v1/proxy/namespaces/<NAMESPACE>/services/<SERVICE-NAME>:<PORT-NAME>/

For example you could use the following URL:

http://localhost:8080/api/v1/proxy/namespaces/default/services/my-internal-service:http/

to access this service:

apiVersion: v1
kind: Service
metadata:  
  name: my-internal-service
spec:
  selector:
    app: my-app
  type: ClusterIP
  ports:  
  - name: http
    port: 80
    targetPort: 80
    protocol: TCP

This method requires you to run kubectl as an authenticated user.

List all ClusterIPs:

kubectl get services --all-namespaces -o=custom-columns='NAMESPACE:.metadata.namespace,NAME:.metadata.name,TYPE:.spec.type,CLUSTER-IP:.spec.clusterIP,PORT(S):.spec.ports[*].port,TARGETPORT(S):.spec.ports[*].targetPort,SELECTOR:.spec.selector' | grep ClusterIP

NodePort

When NodePort is utilised, a designated port is made available on all Nodes (representing the Virtual Machines). Traffic directed to this specific port is then systematically routed to the service. Typically, this method is not recommended due to its drawbacks.

List all NodePorts:

kubectl get services --all-namespaces -o=custom-columns='NAMESPACE:.metadata.namespace,NAME:.metadata.name,TYPE:.spec.type,CLUSTER-IP:.spec.clusterIP,PORT(S):.spec.ports[*].port,NODEPORT(S):.spec.ports[*].nodePort,TARGETPORT(S):.spec.ports[*].targetPort,SELECTOR:.spec.selector' | grep NodePort

An example of NodePort specification:

apiVersion: v1
kind: Service
metadata:  
  name: my-nodeport-service
spec:
  selector:    
    app: my-app
  type: NodePort
  ports:  
  - name: http
    port: 80
    targetPort: 80
    nodePort: 30036
    protocol: TCP

If you don't specify the nodePort in the yaml (it's the port that will be opened) a port in the range 30000–32767 will be used.

LoadBalancer

Exposes the Service externally using a cloud provider's load balancer. On GKE, this will spin up a Network Load Balancer that will give you a single IP address that will forward all traffic to your service. In AWS it will launch a Load Balancer.

You have to pay for a LoadBalancer per exposed service, which can be expensive.

List all LoadBalancers:

kubectl get services --all-namespaces -o=custom-columns='NAMESPACE:.metadata.namespace,NAME:.metadata.name,TYPE:.spec.type,CLUSTER-IP:.spec.clusterIP,EXTERNAL-IP:.status.loadBalancer.ingress[*],PORT(S):.spec.ports[*].port,NODEPORT(S):.spec.ports[*].nodePort,TARGETPORT(S):.spec.ports[*].targetPort,SELECTOR:.spec.selector' | grep LoadBalancer

External IPs

External IPs are exposed by services of type Load Balancers and they are generally used when an external Cloud Provider Load Balancer is being used.

For finding them, check for load balancers with values in the EXTERNAL-IP field.

Traffic that ingresses into the cluster with the external IP (as destination IP), on the Service port, will be routed to one of the Service endpoints. externalIPs are not managed by Kubernetes and are the responsibility of the cluster administrator.

In the Service spec, externalIPs can be specified along with any of the ServiceTypes. In the example below, "my-service" can be accessed by clients on "80.11.12.10:80" (externalIP:port)

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  selector:
    app: MyApp
  ports:
    - name: http
      protocol: TCP
      port: 80
      targetPort: 9376
  externalIPs:
    - 80.11.12.10

ExternalName

From the docs: Services of type ExternalName map a Service to a DNS name, not to a typical selector such as my-service or cassandra. You specify these Services with the spec.externalName parameter.

This Service definition, for example, maps the my-service Service in the prod namespace to my.database.example.com:

apiVersion: v1
kind: Service
metadata:
  name: my-service
  namespace: prod
spec:
  type: ExternalName
  externalName: my.database.example.com

When looking up the host my-service.prod.svc.cluster.local, the cluster DNS Service returns a CNAME record with the value my.database.example.com. Accessing my-service works in the same way as other Services but with the crucial difference that redirection happens at the DNS level rather than via proxying or forwarding.

List all ExternalNames:

kubectl get services --all-namespaces | grep ExternalName

Ingress

Unlike all the above examples, Ingress is NOT a type of service. Instead, it sits in front of multiple services and act as a “smart router” or entrypoint into your cluster.

You can do a lot of different things with an Ingress, and there are many types of Ingress controllers that have different capabilities.

The default GKE ingress controller will spin up a HTTP(S) Load Balancer for you. This will let you do both path based and subdomain based routing to backend services. For example, you can send everything on foo.yourdomain.com to the foo service, and everything under the yourdomain.com/bar/ path to the bar service.

The YAML for a Ingress object on GKE with a L7 HTTP Load Balancer might look like this:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: my-ingress
spec:
  backend:
    serviceName: other
    servicePort: 8080
  rules:
  - host: foo.mydomain.com
    http:
      paths:
      - backend:
          serviceName: foo
          servicePort: 8080
  - host: mydomain.com
    http:
      paths:
      - path: /bar/*
        backend:
          serviceName: bar
          servicePort: 8080

List all the ingresses:

kubectl get ingresses --all-namespaces -o=custom-columns='NAMESPACE:.metadata.namespace,NAME:.metadata.name,RULES:spec.rules[*],STATUS:status'

Although in this case it's better to get the info of each one by one to read it better:

kubectl get ingresses --all-namespaces -o=yaml

References

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