In the previous blog on What Is Kubernetes, you must have got an understanding on Kubernetes. In this blog on Kubernetes networking, I will primarily focus on the networking concepts involved in Kubernetes.
In this blog on Kubernetes Networking, you will understand the following topics:
You can define Kubernetes as an open-source container orchestration tool that provides a portable platform for automating the deployment of containerized applications.
Now, anybody working with Kubernetes must have a clear understanding of Kubernetes Cluster as that will help you in understanding Kubernetes Networking.
The Kubernetes platform offers desired state management, which enables the cluster services to run, the fed configuration in the infrastructure. Let me explain with an example.
Consider a YAML file which has all the configuration information which needs to be fed into the cluster services. So, this file is fed to the API of cluster services, and then it will be up to the cluster services to figure out how to schedule pods in the environment. So, suppose there are two container images for pod 1 with three replicas, and one container image for pod 2 with two replicas, it will be up to the cluster services to allocate these pod-replica pairs to the workers.
Refer to the above diagram. Now, as you can see that the cluster services have allotted the first worker with two pod replica pairs, the second worker with a single pod-replica pair, and the third worker with two pod replica pairs. Now, it is the Kubelet process which is responsible for communicating the cluster services with workers.
So, this whole setup of cluster services and the workers themselves makes up this Kubernetes cluster!!
How, do you think these individually allocated pods communicate with each other?
The answer lies in Kubernetes Networking!
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There are mainly 4 problems to solve with the networking concepts.
- Container to container communication
- Pod to pod Communication
- Pod to service communication
- External to service Communication
The communication between pods, services and external services to the ones in a cluster brings in the concept of Kubernetes networking.
So, for your better understanding let me divide the concepts into the following.
- Pods & Container Communication
- Connecting External to Services via Ingress Network
Pods & Container Communication
Before I tell you how do pods communicate, let me introduce you what are pods?
Pods are basic units of Kubernetes applications, that consists of one or more containers allocated on the same host to share a network stack and other resources. So, this implies that all containers in a pod can reach other on a local host.
Now, let me brief you on how do these pods communicate?
There are 2 types of communication. The inter-node communication and the intra-node communication.
So, let’s start with intra-node communication, but before that let me introduce to you the components of the pod network.
Intra-node Pod Network
Intra-node pod network is basically the communication between two different nodes on the same pod. Let me explain you with an example.
Assume a packet is going from pod1 to pod2.
- The packet leaves Pod 1’s network at eth0 and enters the root network at veth0
- Then, the packet passes onto the Linux bridge(cbr0) which discovers the destination using an ARP request
- So, if veth1 has the IP, the bridge now knows where to forward the packet.
Now, similarly let me tell you about the inter-node pod communication.
Inter-node pod network
Consider two nodes having various network namespaces, network interfaces, and a Linux bridge.
Now, assume a packet travels from pod1 to a pod4 which is on a different node.
- The packet leaves the pod 1 network and enters the root network at veth0
- Then the packet passes on to the Linux bridge (cbr0) whose responsibility is to make an ARP request to find the destination.
- After the bridge realizes that this pod doesn’t have the destination address, the packet comes back to the main network interface eth0.
- The packet now leaves the node 1 to find it’s destination on the other node and enters the route table who routes the packet to the node whose CIDR block contains the pod4.
- So, now the packet reaches node2 and then the bridge takes the packet which makes an ARP request to find out that the IP belonging to veth0.
- Finally, the packet crosses the pipe-pair and reaches pod4.
So, that’s how pods communicate with each other. Now, lets’ move on and see how services help in the communication of pods.
So, what do you think the services are?
Basically, services are a type of resource that configures a proxy to forward the requests to a set of pods, which will receive traffic & is determined by the selector. Once the service is created it has an assigned IP address which will accept requests on the port.
Now, there are various service types that give you the option for exposing a service outside of your cluster IP address.
Types of Services
There are mainly 4 types of services.
ClusterIP: This is the default service type which exposes the service on a cluster-internal IP by making the service only reachable within the cluster.
NodePort: This exposes the service on each Node’s IP at a static port. Since, a ClusterIP service, to which the NodePort service will route, is automatically created. We can contact the NodePort service outside the cluster.
LoadBalancer: This is the service type which exposes the service externally using a cloud provider’s load balancer. So, the NodePort and ClusterIP services, to which the external load balancer will route, are automatically created.
ExternalName: This service type maps the service to the contents of the externalName field by returning a CNAME record with its value.
So, guys that was all about services. Now, you might be wondering how do external services connect to these networks right?
Well, that’s by none other than Ingress Network.
Well, Ingress network is the most powerful way of exposing services as it is a collection of rules that allow inbound connections, that can be configured to give services externally through reachable URLs. So, it basically acts as an entry point to the Kubernetes cluster that manages external access to the services in a cluster.
Now, let me explain to you the working of Ingress Network with an example.
We have 2 nodes, having the pod and root network namespaces with a Linux bridge. In addition to this, we also have a new virtual ethernet device called flannel0(network plugin) added to the root network.
Now, we want the packet to flow from pod1 to pod 4.
- So, the packet leaves pod1’s network at eth0 and enters the root network at veth0.
- Then it is passed on to cbr0, which makes the ARP request to find the destination and it thereafter finds out that nobody on this node has the destination IP address.
- So, the bridge sends the packet to flannel0 as the node’s route table is configured with flannel0.
- Now, the flannel daemon talks to the API server of Kubernetes to know all the pod IPs and their respective nodes to create mappings for pods IPs to node IPs.
- The network plugin wraps this packet in a UDP packet with extra headers changing the source and destination IP’s to their respective nodes and sends this packet out via eth0.
- Now, since the route table already knows how to route traffic between nodes, it sends the packet to the destination node2.
- The packet arrives at eth0 of node2 and goes back to flannel0 to de-capsulate and emits it back in the root network namespace.
- Again, the packet is forwarded to the Linux bridge to make an ARP request to find out the IP that belongs to veth1.
- The packet finally crosses the root network and reaches the destination Pod4.
So, that’s how external services are connected with the help of an ingress network. Now, as I was talking about Network plugins, let me introduce you to the list of popular network plugins available.
Case Study: Wealth Wizard Using Kubernetes Networking
Wealth Wizards is an online financial planning platform that combines financial planning, and smart software technology to deliver expert advice at an affordable cost.
Now, it was extremely important for the company to quickly discover and eliminate code vulnerabilities with full visibility of their cloud environment but wanted to control traffic through access restrictions.
So, they used Kubernetes infrastructure to manage the provisioning and rollout of the clusters with the help of tools to manage the deployment and configuration of microservices across the Kube clusters.
They also used a network policy feature of Kubernetes to allow them to control traffic through access restrictions.
Now, the problem was, these policies are application-oriented and can only evolve with the applications, but, there was no component to enforce these policies.
So, the only solution the company could find for this was to use a network plugin, and hence they started using Weave Net.
This network plugin creates a virtual network that has a network policy controller to manage and enforce the rules in Kubernetes. Not only this, but it also connects Docker containers across multiple hosts and enables their automatic discovery.
So, suppose you have a workload in the cluster and you want to stop any other workload in the cluster talking to it. You can achieve this by creating a network policy that restricts access and only allows ingress to it via the ingress controller on a specific port.
Now, with his deployment on each Kubernetes node, the plugin manages inter-pod routing and has access to manipulate the IPtables rules. In simple terms, each policy is converted to a collection of IPtables rules, coordinated and configured across each machine to translate the Kubernetes tags.
So, with an assumption that all of you have installed Kubernetes on your systems, I have a scenario to showcase.
Suppose you want to store product name and product ID, for that you will need a web application. Basically, you need one container for web application and you need one more container as MySQL for the backend, and that MySQL container should be linked to the web application container.
How about I execute the above-stated example practically.
Let’s get started!
Step 1: Create a folder in your desired directory and change the working directory path to that folder.
mkdir HandsOn cd HandsOn/
Step2: Now create deployment YAML files, for the web application and MySQL database.
Step3: Once you create the deployment files, deploy both the applications.
kubectl apply -f webapp.yml kubectl apply -f mysql.yml
Step 3.1: Check both the deployments.
kubectl get deployment
Step 4: Now, you have to create services for both the applications.
kubectl apply -f webservice.yml kubectl apply -f sqlservice.yml
Step 4.1: Once the services are created, deploy the services.
Step 4.2: Check whether the services have been created or not.
kubectl get service
Step 5: Now, check the configuration of running pods.
kubectl get pods
Step 6: Go into the container inside the webapp pod.
kubectl exec -it container_id bash nano var/www/html/index.php
Step 6.1: Now, change the $servername from localhost to the SQL service name which is “webapp-sql1” in this case, and $password from “” to “edureka”. Also, fill all the database details required and save your index.php file by using the keyboard shortcut Ctrl+x and after that press y to save and press enter.
Step 7: Now, go into the MySQL container present in the pod.
kubectl exec it container_id bash
Step 7.1: Get the access to use the MySQL container.
mysql -u root -p edureka
Where -u represents the user and -p is the password of your machine.
Step 7.2: Create a database in MySQL which will be used to get data from webapp.
CREATE DATABASE ProductDetails;
Step 7.3: Use the created database.
Step 7.4: Create a table in this database in MySQL which will be used to get data from webapp.
CREATE TABLE products(product_name VARCHAR(10), product_id VARCHAR(11));
Step 7.5: Now, exit MySQL container as well using the command exit.
Step 8: Check the port number on which your web application is working.
kubectl get services
Step 8.1: Now, open the web application on it’s allocated port number.
Step 9: Once you click on Submit Query, go to the node in which your MySQL service is running and then go inside the container.
This will show you the output of all the list products, of which you have filled in the details.
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