The challenge for the #KubeWeek

Santosh Nellagi

·Apr 25, 2023·

5 min read

TABLE OF CONTENTS

☸ What is Cluster Networking?

Cluster Networking provides communication between different pods and containers within a pod use networking to communicate via loopback

Networking is a central part of Kubernetes, but it can be challenging to understand exactly how it is expected to work. There are 4 distinct networking problems to address:

1. Highly-coupled container-to-container communications: this is solved by Pods and localhost communications.

2. Pod-to-Pod communications

3. Pod-to-Service communications

4. External-to-Service communications

☸Within pod containers communication

containers within a pod use networking to communicate via loopback. Highly-coupled container-to-container communications: this is solved by Pods and localhost communications

Example of Manifest file.

save this file as pod1.yml

kind: Pod
apiVersion: v1
metadata:
  name: testpod
spec:
  containers:
    - name: c00
      image: ubuntu
      command: ["/bin/bash", "-c", "while true; do echo Hello-Bhupinder; sleep 5 ; done"]
    - name: c01
      image: httpd
      ports:
       - containerPort: 80

To apply this file use the below command

kubectl apply -f pod1.yml

To get pods details.

kubectl get pods

To get inside the container

kubectl ecec testpod -it -c c00 -- /bin/bash

Update the container and install curl inside it

apt update && apt install curl

To check the communication of containers.

curl localhost:80

Output: <it works>

☸Services

• The service resources let's you expose an application running in pods to be reachable from outside your cluster.

• you can also use services to publish services only for consumption inside your cluster

  

Manifest file for the example. save as service.yml

kind: Service                             # Defines to create Service type Object
apiVersion: v1
metadata:
  name: demoservice
spec:
  ports:
    - port: 80                               # Containers port exposed
      targetPort: 80                     # Pods port
  selector:
    name: deployment                    # Apply this service to any pods which has the specific label
  type: ClusterIP                       # Specifies the service type i.e ClusterIP or NodePort

To apply this file use the below command

kubectl apply -f service.yml

To get services details.

kubectl get svc

To check the communication of containers.

curl ip address:80

☸Cluster Ip

• Exposes virtual IP only reachable from within the cluster.

• mainly used to communicate between components of microservices.

☸Node port

• makes a service accessible from the outside the cluster.

• exposes the service on the same port of each selected node in the cluster using NAT.

☸Ingress

• an Ingress is an object that allows access to Kubernetes services from outside the Kubernetes cluster.

    apiVersion: networking.k8s.io/v1
    kind: Ingress
    metadata:
      name: test-ingress
      annotations:
        kubernetes.io/ingress.class: "gce"
    spec:
      tls:
        - secretName: my-ssl-secret
      rules:
      - host: testhost.com
        http:
          paths:
          - path: /*
            backend:
              serviceName: service-test
              servicePort: 80
  

☸ Network Policies

NetworkPolicies are an application-centric construct which allow you to specify how a pod is allowed to communicate with various network "entities" (we use the word "entity" here to avoid overloading the more common terms such as "endpoints" and "services", which have specific Kubernetes connotations) over the network.

•   apiVersion: networking.k8s.io/v1
    kind: NetworkPolicy
    metadata:
      name: deny-all
      namespace: default
    spec:
      podSelector: {}
      policyTypes:
      - Ingress
      - Egress
  

DNS

1. Kubernetes creates DNS records for Services and Pods. You can contact Services with consistent DNS names instead of IP addresses..

2. Kubernetes publishes information about Pods and Services which is used to program DNS. Kubelet configures Pods' DNS so that running containers can lookup Services by name rather than IP.

CNI Plugins

Kubernetes Container Network Interface (CNI) plugins for cluster networking

• Different plugins are available (both open- and closed- source) in the wider Kubernetes ecosystem.

• A CNI plugin is required to implement the Kubernetes network model.