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NetworkPolicy Tutorial

Welcome! 👋 This tutorial helps you get started with Kubernetes NetworkPolicy.

What is Kubernetes NetworkPolicy?

NetworkPolicy is a standardized Kubernetes object to control the allowed network traffic patterns between Kubernetes pods and namespaces as well as any traffic entering or leaving the cluster. However, Kubernetes itself does not provide an implementation of NetworkPolicy, it is typically provided by the CNI plugin. If no NetworkPolicy is loaded, all communication is allowed which is clearly violating least-privilege and zero-trust security privileges. For more information on the concept, see Network Policies in the Kubernetes documentation.

What will you learn in this Tutorial?

In this tutorial, we will cover the following topics:

  • Fundamentals:
    • Kubernetes 101
    • Default Allow & Default Deny
    • Ingress & Egress
  • Ingress Controls
    • Putting a pod into ingress default deny
    • Allow pod to pod connectivty (ingress)
    • Allow everything within a namespace (ingress)
    • Allow across namespaces
  • Egress Controls
    • Putting a pod into egress default deny
    • Allow pod to pod connectivity
    • Allow Kubernetes DNS (across namespaces)

Getting Started: Create a Kubernetes Cluster with a CNI

In order to enforce Kubernetes NetworkPolicies, your cluster must be running with a CNI that is capable of enforcing NetworkPolicies. Multiple options exist, for this tutorial we will be using Cilium as it provides nice visibility tooling to help us illustrate things as we go along.

If you already have a Kubernets cluster with an appropriate CNI plugin installed, then skip the following steps.

Create a Cluster

GKE

export CLUSTER_NAME=test-$(whoami)-$RANDOM
export CLUSTER_ZONE=us-west2-a
gcloud container clusters create $CLUSTER_NAME --image-type COS --num-nodes 2 --machine-type n1-standard-4 --zone $CLUSTER_ZONE
gcloud container clusters get-credentials $CLUSTER_NAME --zone $CLUSTER_ZONE

minikube

minikube start --network-plugin=cni

Install Cilium

Note: You can install and use any CNI that supports NetworkPolicy to complete this tutorial. Skip this section if already have a CNI installed with NetworkPolicy.

Darwin

curl -LO https://github.com/cilium/cilium-cli/releases/download/v0.4/cilium-darwin-amd64.tar.gz
tar xzvf cilium-darwin-amd64.tar.gz
sudo mv cilium /usr/local/bin
cilium install
cilium status

Linux

curl -LO https://github.com/cilium/cilium-cli/releases/download/v0.4/cilium-linux-amd64.tar.gz
tar xzvf cilium-linux-amd64.tar.gz
sudo mv cilium /usr/local/bin
cilium install
cilium status

Enable Hubble (Optional)

If you want, you can enable Hubble to get a fully distributed network monitor that will show you every forwarding and drop decision in your cluster.

  1. Install Hubble

  2. Enable Hubble in Cilium:

     cilium hubble enable
    
  3. Add a port forward with kubectl to expose Hubble. You can obviously also expose a LoadBalancer service if you want but for the simplicity of this tutorial, a port-forward is the simplest solution:

     kubectl port-forward -n kube-system deployment/hubble-relay 4245:4245&
    
  4. Run hubble observe to start observing all network traffic in the default namespace:

     hubble observe --server localhost:4245 -f -n default
    

Fundamentals

NetworkPolicy is a standardized Kubernetes object to control the allowed network traffic patterns between Kubernetes pods and namespaces as well as any traffic in and out of a cluster. However, Kubernetes itself does not provide an implementation of NetworkPolicy, it is typically provided by the CNI plugin. If no NetworkPolicy is loaded, all communication is allowed which is clearly violating least-privilege and zero-trust security privileges. It is thus in the best interest of any Kubernetes cluster operator to understand and use the concept of NetworkPolicy.

When referring to NetworkPolicy more broadly, users may refer to both the standard Kubernetes NetworkPolicy as well as the extended version with additional capabilities specific to the selected CNI plugin, e.g. CiliumNetworkPolicy. These extended versions are typically implemented as Custom Resources. In the initial chapters of this tutorial, we will focus on Kubernetes NetworkPolicy and then dive into some of the extensions to understand benefits and tradeoffs.

101 - Namespaces, Pods, & Services

In order to understand NetworkPolicy, you need to understand the basics of Kubernetes objects. This is a quick 101 of the Kubernetes objects that are relevant for NetworkPolicy. If you are already familiar with Kubernetes objects, feel free to skip this section. If are looking for a more detailed introduced, refer to the Kubernetes documentation Working with Kubernetes Objects.

A namespace groups Kubernetes objects into a specific context. If you want, you can consider it a virtual cluster. Namespaces allow to share a cluster among multiple tenants. In the context of NetworkPolicy, it is most common to define the scope of a NetworkPolicy to only apply within a particular namespace and specifically handle any cross-namespace communication. More on this later.

Pods are the smallest deployable unit of an application that you can create and manage. Pods are typically created via a higher-level object such as a Deployments, DaemonSets, Jobs, StatefulSets, etc. and are deployed directly into a particular Namespace. A pod consist of one or more application containers which all share the same storage and network resources and always run on the same host. If your application is scalable and deployed with multiple replicas then each replica is deployed as an individual pod. Pods can be assigned a set of Labels. These labels allow to select any number of pods and treat them in artbirary groups using Selectors. In the context of NetworkPolicy, a policy is applied to one or more pods by selecting pod labels in the NetworkPolicy. The policy is then applied to all application containers of any pod that matches the label selector.

Applications running as pods are made available to other pods and to external clients using services. A service makes a set of pods available using an internal or external IP address and assigns a stable DNS name to this IP to allow for the discovery of the service by other pods. In the context of NetworkPolicy, services typically play a minor role as policies are applied to pods directly but use of services can influence how NetworkPolicy has to be defined.

If you want to deepen your understanding of Kubernets objects before continuing, feel free to dive into the Kubernetes Concepts chapters of the Kubernetes documentation.

Default Allow & Default Deny

In the absence of any NetworkPolicy, the default behavior of a Kubernetes cluster is to allow everything on the network. It is your responsibility as an operator of a Kubernetes cluster to create NetworkPolicy objects to change the default allow setting to default deny and the explicitely allow all required traffic paterns.

If your Kubernetes cluster is already running many applications, this may appear difficult as changing from a default allow to a default deny security model in a single step may appear challenging. Fortunately, NetworkPolicy allows to change the default deny setting for individual groups of pods or individual namespaces so you can start locking down the most important applications first and improve your security posture step by step.

Before we now create our first NetworkPolicy, we need to understand one more fundamental concept:

Ingress & Egress

Ingress and egress are terms used in networking to describe traffic direction. Ingress refers to network traffic that is transmitted into a Kubernetes pod and egress is refers to network traffic that is emmited by a pod.

The default deny setting of a pod can be control for each traffic direction separately, i.e., you can initially deny all traffic into a pod and start allowing individual traffic patterns while still allowing all network traffic leaving a pod.

Ingress Controls

We are now ready to create our first NetworkPolicy to put a set of pods into default deny and then allow a set of network traffic.

Create Demo App

Create the following demo-app in your Kubernetes cluster. The examples will assume that you deploy the demo app into the default namespace but you can of course use any namespace and adjust the examples accordingly.

kubectl create -f tutorial/demo-app.yaml

Validate that you have frontend and backend pods running:

kubectl get pods,svc
NAME                            READY   STATUS    RESTARTS   AGE
pod/backend-69d87c4548-kx4sz    1/1     Running   0          8m18s
pod/frontend-6864ff68cc-7r2fc   1/1     Running   0          8m18s

NAME                 TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)    AGE
service/backend      ClusterIP   10.100.105.40   <none>        8080/TCP   8m19s

In order to simplify execution of commands in the pods, store the pod names in shell variables:

FRONTEND=$(kubectl get pods -l app=frontend -o json | jq -r '.items[0].metadata.name')
BACKEND=$(kubectl get pods -l app=backend -o json | jq -r '.items[0].metadata.name')

Finally, validate that the frontend pod can talk to the backend pod:

kubectl exec -ti $FRONTEND -- curl -I backend:8080 | head -1
HTTP/1.1 200 OK

Putting a Pod into ingress default deny mode

The following policy (01-backend-ingress-deny.yaml) will put the backend pod into default deny mode for the ingress direction. The pod will no longer be able to receive any traffic until we allow it but it can still egress all traffic.

kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: backend-ingress-deny
spec:
  podSelector:
    matchLabels:
      app: backend
  policyTypes:
  - Ingress

Create the above policy in your cluster:

kubectl create -f tutorial/01-backend-ingress-deny.yaml

A connection from frontend to backend will now timeout:

kubectl exec -ti $FRONTEND -- curl -I --connect-timeout 5 backend:8080
curl: (28) Connection timed out after 5001 milliseconds
command terminated with exit code 28

However, the backend can still reach kubernetes.io:

kubectl exec -ti $BACKEND -- curl -I kubernetes.io | head -1
HTTP/1.1 301 Moved Permanently

Looking at the specific policy, there is no mention of deny, why did it put the pod into default deny? Let's inspect the policy in steps to understand each part:

  1. The first part of the policy will be common among all policies, it describes the kind of Kubernetes object and the name of the policy:

    kind: NetworkPolicy
    apiVersion: networking.k8s.io/v1
    metadata:
      name: backend-ingress-deny
    
  2. The next part describes the podSelector which is the Selector to describe which pods this policy should apply to.

    spec:
      podSelector:
        matchLabels:
          app: backend
    

    As NetworkPolicy objects are namespaced a policy will only have effect in the namespace the policy has been created in. The above podSelector selects all pods in the namespace with the label app: backend. If our backend demo-app would be running as multiple replicas, then all replicas would be selected.

  3. The final part describes rules that specify what is allowed. As soon as a pod is selected by a NetworkPolicy it will change its default security posture from default allow to default deny in any direction for which rules are specified. For such pods, only traffic matching a rule will be allowed going forward:

    spec:
      policyTypes:
      - Ingress
    

    The above example is YAML syntax to specify an empty list and thus says that nothing is allowed in the ingress direction.

What is Stateful Policy Enforcement?

In the previous chapter, we put the ingress direction of thebackend pod into default deny. How was it possible for kubernetes.io to still be reachable from within that pod. Shouldn't the reply packets of the network connection to kubernetes.io have been dropped?

The reason why the example worked is because NetworkPolicy is typically enforced in a stateful manner. This is not required by the NetworkPolicy specification and will depend on the selected CNI plugin but stateful enforcement is the norm and typically expected by users.

Stateful enforcement enables allowing a network connection in the forward direction only and let the stateful enforcement automatically allow reply network traffic for known network connections. It is called stateful is because the CNI plugin will maintain state to keep track of all active network connections and associate all network packets to a connection in either the forward or reply direction.

Allowing Pod to Pod connectivity

Next, with the backend pod in ingress default deny, we will specifically allow the frontend pod to reach the backend (02-backend-ingress-allow-frontend.yaml):

kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: backend-ingress-allow-frontend
spec:
  podSelector:
    matchLabels:
      app: backend
  ingress:
    - from:
      - podSelector:
          matchLabels:
            app: frontend

Most aspects look similar to the previous policy, look at what is new:

spec:
  ingress:
    - from:
      - podSelector:
          matchLabels:
            app: frontend

Instead of an empty list, the above policy contains a rule that allows from a set of pods described as a podSelector. All pods which match the podSelector will be allowed. The podSelector uses the standard selector syntax that you might be familiar from other Kubernetes objects so you can also use sets and expressions to specify what pods should be allowed.

Import the above policy in your cluster and validate that the connectivity works again:

kubectl create -f tutorial/02-backend-ingress-allow-frontend.yaml
kubectl exec -ti $FRONTEND -- curl -I backend:8080 | head -1
HTTP/1.1 200 OK

You will now have two policies created in your namespace:

kubectl get networkpolicies
NAME                             POD-SELECTOR   AGE
backend-ingress-allow-frontend   app=backend    1h
backend-ingress-deny             app=backend    1h

This highlights another key aspect of NetworkPolicy: Policies are additive. If you create multiple policies that select the same pod then all rules specified in all policies are added together and will apply to the pod. In our example, the backend-ingress-deny policy puts the pod into ingress default deny mode and the backend-ingress-allow-frontend allows network communication from frontend. Technically, the backend-ingress-deny policy is no longer needed as the backend-ingress-allow-frontend policy also puts the pod into default deny. However, you will find that using a separate policy for default deny is preferred as it avoids accidental exposure of a pod on the removal of the allow policy. It also enables you to put all pods of a namespace into default deny and then describe allow rules for each deployment in a separte policy.

Allowing Everything in a Namespace

While it is clearly desirable to define all allowed network communication on a per pod level to achieve least-privilege security, doing so might be quite involved in your environment. Allowing everything within namespaces while denying cross namespace and external access can be a simple first step while already improving your security posture overall (03-all-pods-allow-within-namespace.yaml):

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: all-pods-ingress-allow-within-namespace
spec:
  podSelector: {}
  ingress:
    - from:
      - podSelector: {}

The above policy will apply to all pods in the namespace and allow from any other pod in the same namespace:

  1. The {} is YAML syntax for an empty object. An empty podSelector will select all pods in the namespace.:

    spec:
      podSelector: {}
  2. Similarly, the {} in the ingress podSelector will allow from all pods within the namespace

    spec:
      ingress:
        - from:
          - podSelector: {}

Create the above policy and validate that connectivity still works:

kubectl create -f tutorial/03-all-pods-allow-within-namespace.yaml
kubectl exec -ti $FRONTEND -- curl -I backend:8080 | head -1
HTTP/1.1 200 OK

This is not surprising as we also still have the policy backend-ingress-allow-frontend loaded. Let's remove that policy and rely on all-pods-ingress-allow-within-namespace to allow from the frontend pod to the backend pod:

  1. Delete the policy backend-ingress-allow-frontend:

    kubectl delete networkpolicy backend-ingress-allow-frontend
    networkpolicy.networking.k8s.io "backend-ingress-allow-frontend" deleted
  2. Validate that the connectivity still works:

    kubectl exec -ti $FRONTEND -- curl -I backend:8080 | head -1
    HTTP/1.1 200 OK

Allowing Across Namespaces

If an empty podSelector matches everything in a namespace, how can we allow network communication across namespaces? This is where the namespaceSelector comes in (04-backend-ingress-allow-monitoring-namespace.yaml):

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: backend-ingress-allow-other-namespace
spec:
  podSelector:
    matchLabels:
      app: backend
  ingress:
    - from:
        - namespaceSelector:
            matchLabels:
              type: monitoring

Allowing across namespaces is required when an otherwise isolated application running in its own namespace requires to access another application. A typical example is Prometheus running in a separate namespace accessing other pods to scrape metrics over the network.

Let's try this out. Create a new namespace monitoring, assign the label type: monitoring to it, and deploy the monitoring demo-app into the new namespace:

kubectl create namespace monitoring
kubectl label namespace monitoring type=monitoring
kubectl -n monitoring create -f tutorial/monitoring-app.yaml

Also store the pod name of the new monitoring deployment so we can exec from it:

MONITORING=$(kubectl -n monitoring get pods -l app=monitoring -o json | jq -r '.items[0].metadata.name')

Remember that the security posture of all pods in the default namespace is to only allow ingress from within the same namespace. Verify that connectivity is indeed denied from the new monitoring namespace:

kubectl -n monitoring exec -ti $MONITORING -- curl -I --connect-timeout 5 backend.default.svc.cluster.local:8080 | head -1
curl: (28) Connection timed out after 5001 milliseconds
command terminated with exit code 28

Now, create the policy backend-ingress-allow-monitoring-namespac to allow namespaces with the label type: monitoring to access the backend pod:

kubectl create -f tutorial/04-backend-ingress-allow-monitoring-namespace.yaml

And validate that connectivity is now allowed:

kubectl -n monitoring exec -ti $MONITORING -- curl -I backend.default.svc.cluster.local:8080 | head -1
HTTP/1.1 200 OK

Combining podSelector and namespaceSelector

To improve the security posture further, the namespaceSelector can be combined with a podSelector to only allow certain pods from the namespace (05-backend-ingress-allow-monitoring-app.yaml):

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: backend-ingress-allow-monitoring-namespace
spec:
  podSelector:
    matchLabels:
      app: backend
  ingress:
    - from:
        - namespaceSelector:
            matchLabels:
              type: monitoring
          podSelector:
            matchLabels:
              app: monitoring

Validate that this works by creating this new policy and then removing the previous policy we have created:

kubectl create -f tutorial/05-backend-ingress-allow-monitoring-app.yaml
kubectl delete networkpolicy backend-ingress-allow-monitoring-namespace
kubectl -n monitoring exec -ti $MONITORING -- curl -I backend.default.svc.cluster.local:8080 | head -1
HTTP/1.1 200 OK

Egress - Controlling what network traffic a pod can send

Putting a Pod into egress default deny mode

Following the example on how to put ingress into default deny, the same technique can be applied to egress as well but this time, we select the frontend pod (06-frontend-egress-deny.yaml):

kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: frontend-egress-deny
spec:
  podSelector:
    matchLabels:
      app: frontend
  policyTypes:
  - Egress

Create the egress default deny policy and validate that the frontend pod can't reach the backend pod anymore:

kubectl create -f tutorial/06-frontend-egress-deny.yaml
kubectl exec -ti $FRONTEND -- curl -I --connect-timeout 5 backend:8080 | head -1
curl: (28) Resolving timed out after 5000 milliseconds
command terminated with exit code 28

This is despite having an ingress policy loaded which allows the backend pod to receive network traffic from a frontend pod.

If both ingress and egress policies are in play, both have to allow the network traffic in order for the connection to be allowed.

Allowing Pod to Pod connectivity

Allowing pod to pod in the egress direction works exactly the same as for ingress (07-frontend-egress-allow-to-backend.yaml):

kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: frontend-egress-allow-to-backend
spec:
  podSelector:
    matchLabels:
      app: frontend
  policyTypes:
  - Egress
  egress:
    - to:
      - podSelector:
          matchLabels:
            app: backend

Import the policy:

kubectl create -f tutorial/07-frontend-egress-allow-to-backend.yaml

Test your newly allowed connection:

kubectl exec -ti $FRONTEND -- curl -I --connect-timeout 5 backend:8080 | head -1
curl: (28) Resolving timed out after 5000 milliseconds
command terminated with exit code 28

Why did it not work? DNS. It's always DNS. We will fix it in the next section. For now, confirm that you can reach the backend pod if you address it directly via the pod IP:

  1. Extract the pod IP of the backend pod:

    BACKEND_IP=$(kubectl get pods -l app=backend -o json | jq -r '.items[0].status.podIP')
  2. Confirm connectivity to backend without using DNS:

    kubectl exec -ti $FRONTEND -- curl -I -- $BACKEND_IP:8080 | head -1
    HTTP/1.1 200 OK

Allowing Kubernetes DNS (Across Namespaces)

Allowing DNS is something that you typically want to do for all pods running in a namespace. In future parts of this tutorial, we will introduce cluster-wide network policies that will even allow you do this at cluster scale. For now, we look at how to allow DNS for all pods in a single namespace: (08-all-pods-egress-allow-dns.yaml):

kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: all-pods-egress-allow-dns
spec:
  podSelector: {}
  policyTypes:
  - Egress
  egress:
    - to:
      - namespaceSelector:
          matchLabels:
            contains: coredns
        podSelector:
          matchLabels:
            k8s-app: kube-dns

As CoreDNS is running in a different namespace, the policy has to include both a namespaceSelector and a podSelector. In order to remain flexible, the namespace is selected with a labels contains: coredns which you will have to add to the namespace in which CoreDNS is running in. If CoreDNS is ever moved to a different namespace, setting the label on the namespace will be sufficient and the policy won't break.

Deploy it and test it out:

  1. Add the contains: coredns label to the kube-system namespace:

     kubectl label namespace kube-system contains=coredns
  2. Create the policy all-pods-egress-allow-dns:

    kubectl create -f tutorial/08-all-pods-egress-allow-dns.yaml
  3. Validate that the frontend pod can now perform DNS lookups and reach the backend pod:

     kubectl exec -ti $FRONTEND -- curl -I backend:8080 | head -1
     HTTP/1.1 200 OK

Bonus Question: Did you notice the side-effect of the above policy?

By creating a policy that selects all pods in the namespace, all pods in the namespace automatically went into default deny for the egress direction. This also includes the backend pod which was not restricted in its ability to perform egress network connections.

Let us validate this:

kubectl exec -ti $BACKEND -- curl -I --connect-timeout 5 kubernetes.io | head -1
curl: (28) Connection timed out after 5001 milliseconds
command terminated with exit code 28

Note that the DNS resolution succeeded because it is allowed but the connection to kubernetes.io then timed out because it is now allowed. We can tell based on the error message, Connection timed out after indicates that the connection to the desired destination timed out. If DNS failed, the error message would have been Resolving timed out.

Coming Soon

In the next parts of this tutorial we will start exploring how to secure access in and out of a cluster and start looking into special cases such as host-networking pods, network-based liveness and readiness probes, interaction with external LoadBalancers as well as how to troubleshoot and monitoring NetworkPolicy usage.

  • Part 2: Securing access in and out of your cluster
  • Part 3; Special cases: Host-networking, Loadbalancers, Health-checking
  • Part 4: How to troubleshoot NetworkPolicy and monitor Compliance