Install CoreDNS On Kubernetes: A Simple Guide

Let's dive into how to get CoreDNS up and running in your Kubernetes cluster! CoreDNS is often the default cluster DNS for Kubernetes, and for good reason. It's flexible, powerful, and integrates seamlessly. So, if you're looking to understand how to install or even replace an existing DNS solution with CoreDNS, you've come to the right place. In this guide, we'll walk through the basics, the installation process, and some common configurations. So, buckle up, and let's get started!

What is CoreDNS?

Before we jump into the installation, let's quickly cover what CoreDNS actually is. Think of CoreDNS as a flexible, chainable DNS server. Unlike traditional DNS servers, CoreDNS uses a plugin architecture. This means its functionality is defined by a series of plugins that are chained together. Each plugin performs a specific DNS function, like resolving names from a file, forwarding queries to another server, or serving zone data directly. This plugin-based system makes CoreDNS incredibly adaptable.

Why is CoreDNS so cool for Kubernetes? Well, Kubernetes needs a way to resolve service names to IP addresses, right? CoreDNS can directly read Kubernetes' service and pod information and use it to answer DNS queries within the cluster. This allows pods to easily communicate with each other using simple service names instead of IP addresses. Plus, because it's so flexible, CoreDNS can be customized to handle all sorts of DNS scenarios.

One of the biggest advantages of CoreDNS is its simplicity. Configuration is done via a Corefile, which is a straightforward text file. You define the plugins you want to use and how they should be chained together. This makes it easier to understand and manage than many traditional DNS configurations. For instance, you can easily configure CoreDNS to forward external DNS queries to a public DNS server like Google's 8.8.8.8 while resolving internal Kubernetes services locally. This blend of internal and external resolution is crucial for many Kubernetes deployments.

Moreover, CoreDNS integrates well with Kubernetes' health checks and lifecycle management. Kubernetes can monitor the health of CoreDNS pods and automatically restart them if they fail, ensuring high availability of DNS services. This is especially important in production environments where DNS downtime can have significant impact.

In short, CoreDNS is a modern, adaptable, and Kubernetes-native DNS server that provides a robust and flexible solution for name resolution within your cluster. Its plugin architecture, ease of configuration, and seamless integration with Kubernetes make it a top choice for many deployments.

Prerequisites

Before we proceed with the installation, let's make sure you have everything you need. You'll need:

• A Running Kubernetes Cluster: This guide assumes you already have a Kubernetes cluster up and running. It could be a local cluster created with Minikube, Kind, or a cloud-based cluster like those offered by Google Kubernetes Engine (GKE), Amazon Elastic Kubernetes Service (EKS), or Azure Kubernetes Service (AKS).
• kubectl Configured: kubectl is the command-line tool for interacting with your Kubernetes cluster. Make sure it's installed and configured to communicate with your cluster. You should be able to run kubectl get nodes and see your cluster's nodes.
• Basic Kubernetes Knowledge: A basic understanding of Kubernetes concepts like Pods, Services, and Deployments will be helpful.
• (Optional) Helm: While we won't be using Helm in the basic installation, it's a popular package manager for Kubernetes. If you want a more automated installation, having Helm installed is a plus.

Let's elaborate on these prerequisites to ensure you're fully prepared. Firstly, having a functioning Kubernetes cluster is non-negotiable. The steps we'll cover are designed to work within a Kubernetes environment, so you need a cluster ready to go. This means your nodes should be healthy and reporting as Ready when you run kubectl get nodes. If you're using a managed Kubernetes service like GKE, EKS, or AKS, ensure that your cluster is properly provisioned and that you have the necessary permissions to deploy resources.

Next up, kubectl configuration is crucial. kubectl is your primary means of interacting with the Kubernetes API, and without it properly configured, you won't be able to deploy or manage anything. This typically involves setting up a kubeconfig file that contains the necessary credentials and cluster information. You can usually obtain this file from your Kubernetes provider or generate it if you're using a tool like Minikube. Verify that kubectl is working by running commands like kubectl get pods or kubectl get services. If you encounter errors, double-check your kubeconfig file and your cluster's connectivity.

Having a basic grasp of Kubernetes concepts is also essential. You should understand what Pods, Services, and Deployments are and how they relate to each other. Pods are the smallest deployable units in Kubernetes, Services provide a stable IP address and DNS name for accessing Pods, and Deployments manage the desired state of your application. Knowing these concepts will help you understand how CoreDNS fits into the overall Kubernetes architecture and how it interacts with other components.

Lastly, while Helm is optional, it can simplify the installation process, especially for more complex configurations. Helm uses charts to package and deploy applications on Kubernetes, and there are Helm charts available for CoreDNS. If you're comfortable with Helm, using a chart can save you time and effort. However, we'll focus on the manual installation method in this guide to provide a deeper understanding of the underlying components.

Installation Steps

Alright, with the prerequisites out of the way, let's get into the nitty-gritty of installing CoreDNS. There are a few ways to do this, but we'll focus on the most common method: deploying CoreDNS using YAML manifests.

1. Download the YAML Manifests: Kubernetes deployments are often defined using YAML files. We'll start by downloading the necessary YAML files for CoreDNS. You can usually find these on the CoreDNS website or in the Kubernetes documentation. Look for files that define the CoreDNS Deployment, Service, and ConfigMap.

2. Create the CoreDNS ConfigMap: The ConfigMap holds the CoreDNS configuration, which is defined in a file called Corefile. This file tells CoreDNS how to handle DNS queries. A basic Corefile might look like this:

   .:53 {
       forward . /etc/resolv.conf
       cache 30
       loop
       reload
       health
   }
   

   This configuration tells CoreDNS to forward all queries to the DNS servers defined in /etc/resolv.conf, cache the results for 30 seconds, prevent forwarding loops, reload the configuration when it changes, and provide a health endpoint. You'll create a ConfigMap from this file using kubectl. You must apply the configmap before applying the deployment otherwise you will have errors and the pods will not start correctly.

3. Deploy CoreDNS: Now that you have the ConfigMap, you can deploy CoreDNS using the Deployment YAML file. This file defines the number of CoreDNS replicas, the container image to use, and the resources allocated to each pod. Apply the Deployment using kubectl. The number of replicas is very important to ensure the HA of the DNS system of the cluster. The official documentation recommends using at least 2 replicas in a production environment.

4. Create the CoreDNS Service: The Service provides a stable IP address and DNS name for accessing the CoreDNS pods. Create a Service of type ClusterIP that targets the CoreDNS pods. This allows other pods in the cluster to resolve DNS queries through CoreDNS. Ensure the service is configured to use the DNS port 53.

Let's break down these steps in more detail to provide a comprehensive guide. First, downloading the YAML manifests is a critical step. These manifests define the desired state of your CoreDNS deployment, including the number of replicas, the container image, and the configuration. You can typically find these manifests in the official CoreDNS documentation or in example deployments on GitHub. Make sure to download the manifests that are compatible with your Kubernetes version. Once you have the manifests, review them carefully to understand what they're doing. Look for sections that define the Deployment, Service, and ConfigMap.

Next, creating the CoreDNS ConfigMap involves defining the Corefile, which is the heart of CoreDNS's configuration. The Corefile is a text file that specifies the plugins CoreDNS should use and how they should be chained together. A basic Corefile typically includes plugins like forward for forwarding queries to external DNS servers, cache for caching DNS responses, and loop for preventing forwarding loops. You can customize the Corefile to suit your specific needs, such as adding plugins for DNSSEC or zone transfers. Once you've defined the Corefile, you can create a ConfigMap using kubectl create configmap. Ensure that the ConfigMap is created in the kube-system namespace, as this is where CoreDNS is typically deployed.

Deploying CoreDNS involves creating a Deployment that manages the CoreDNS pods. The Deployment specifies the number of replicas, the container image to use, and the resources allocated to each pod. It also defines how the pods should be updated and rolled back. When creating the Deployment, make sure to specify the correct container image for CoreDNS. You can find the latest image tag on the CoreDNS website or in the Kubernetes documentation. Also, ensure that the Deployment is configured to use the ConfigMap you created earlier. This is typically done by mounting the ConfigMap as a volume in the CoreDNS pods. Once the Deployment is created, Kubernetes will automatically create and manage the CoreDNS pods.

Finally, creating the CoreDNS Service involves defining a Service that provides a stable IP address and DNS name for accessing the CoreDNS pods. The Service is typically of type ClusterIP, which means it's only accessible from within the cluster. The Service targets the CoreDNS pods using a selector that matches the labels on the pods. When creating the Service, make sure to specify the correct port (typically 53) and protocol (typically UDP and TCP). Once the Service is created, Kubernetes will assign it a stable IP address and DNS name, which can be used by other pods in the cluster to resolve DNS queries through CoreDNS.

Configuring Kubernetes to Use CoreDNS

Now that CoreDNS is installed, you need to tell Kubernetes to actually use it for DNS resolution. This usually involves modifying the kubelet configuration on each node. However, in most modern Kubernetes distributions, this is done automatically. Kubernetes typically configures itself to use the CoreDNS Service for cluster DNS. You can verify this by checking the /etc/resolv.conf file inside a pod. It should point to the CoreDNS Service's IP address.

To ensure that Kubernetes is properly configured to use CoreDNS, you can perform a few checks. First, verify the /etc/resolv.conf file inside a pod. This file tells the pod which DNS servers to use for resolving domain names. If CoreDNS is properly configured, the /etc/resolv.conf file should contain an entry that points to the CoreDNS Service's IP address. You can check this by running kubectl exec -it <pod-name> -- cat /etc/resolv.conf in a pod. Look for a line that starts with nameserver followed by the CoreDNS Service's IP address.

If the /etc/resolv.conf file is not configured correctly, you may need to modify the kubelet configuration on each node. The kubelet is the agent that runs on each node and manages the pods. The kubelet configuration specifies the DNS servers that the pods should use. You can modify the kubelet configuration by editing the /var/lib/kubelet/config.yaml file on each node. Look for the clusterDNS and clusterDomain settings. The clusterDNS setting should be set to the CoreDNS Service's IP address, and the clusterDomain setting should be set to the cluster's domain name (typically cluster.local). After modifying the kubelet configuration, you need to restart the kubelet service for the changes to take effect.

In most modern Kubernetes distributions, the kubelet configuration is managed automatically by the control plane. This means that you typically don't need to manually modify the kubelet configuration. However, it's still a good idea to verify that the /etc/resolv.conf file inside a pod is configured correctly. If it's not, you may need to investigate the control plane configuration to see why it's not being propagated to the nodes.

Another way to verify that CoreDNS is working correctly is to run a DNS query from inside a pod. You can use the nslookup or dig commands to perform a DNS query. For example, you can run kubectl exec -it <pod-name> -- nslookup kubernetes.default to resolve the DNS name of the Kubernetes API server. If CoreDNS is working correctly, the command should return the IP address of the Kubernetes API server.

Common CoreDNS Configurations

CoreDNS is incredibly flexible, and you can configure it in many different ways. Here are a few common configurations:

• Forwarding External Queries: You can configure CoreDNS to forward queries for external domains to a public DNS server like Google's 8.8.8.8 or Cloudflare's 1.1.1.1. This allows pods to resolve external domain names.
• Custom DNS Records: You can define custom DNS records in the Corefile to map specific domain names to specific IP addresses. This is useful for creating internal aliases or overriding external DNS records.
• Stub Domains: You can configure CoreDNS to forward queries for specific domains to other DNS servers. This is useful for integrating with existing DNS infrastructure.

Let's dive a bit deeper into these common configurations to give you a better understanding of how to implement them. Firstly, forwarding external queries is a fundamental configuration for most Kubernetes deployments. By default, CoreDNS is configured to resolve only internal DNS names within the cluster. To allow pods to resolve external domain names, you need to configure CoreDNS to forward queries for external domains to a public DNS server. This is typically done using the forward plugin in the Corefile. You can specify one or more public DNS servers as arguments to the forward plugin. For example, to forward queries to Google's public DNS servers, you would add the following line to your Corefile: forward . 8.8.8.8 8.8.4.4. This tells CoreDNS to forward all queries to the specified DNS servers. The . argument indicates that this rule applies to all domains.

Next, defining custom DNS records is a powerful way to customize DNS resolution within your cluster. You can define custom DNS records in the Corefile to map specific domain names to specific IP addresses. This is useful for creating internal aliases or overriding external DNS records. You can define custom DNS records using the hosts plugin in the Corefile. The hosts plugin allows you to specify a list of domain names and their corresponding IP addresses. For example, to map the domain name my-app.local to the IP address 10.0.0.1, you would add the following lines to your Corefile:

hosts {
  10.0.0.1 my-app.local
  fallthrough
}

This tells CoreDNS to resolve my-app.local to 10.0.0.1. The fallthrough directive tells CoreDNS to continue processing the query if the domain name is not found in the hosts file.

Finally, configuring stub domains is useful for integrating with existing DNS infrastructure. A stub domain is a domain for which CoreDNS is not authoritative but knows which DNS servers are authoritative. You can configure CoreDNS to forward queries for specific domains to other DNS servers using the forward plugin. For example, to forward queries for the example.com domain to the DNS servers at 192.168.1.1 and 192.168.1.2, you would add the following line to your Corefile: forward example.com 192.168.1.1 192.168.1.2. This tells CoreDNS to forward all queries for the example.com domain to the specified DNS servers.

Troubleshooting

Sometimes things don't go as planned. If you're having trouble with CoreDNS, here are a few things to check:

• CoreDNS Pod Status: Make sure the CoreDNS pods are running and healthy. Use kubectl get pods -n kube-system to check their status.
• CoreDNS Logs: Check the CoreDNS pod logs for errors. Use kubectl logs <pod-name> -n kube-system to view the logs.
• Corefile Syntax: Make sure your Corefile has no syntax errors. CoreDNS will fail to start if the Corefile is invalid.
• DNS Resolution: Test DNS resolution from inside a pod. Use nslookup or dig to query a domain name.

Let's expand on these troubleshooting steps to provide more detailed guidance. Firstly, checking the CoreDNS pod status is a crucial first step. If the CoreDNS pods are not running or are in a CrashLoopBackOff state, there's likely a problem with the deployment or configuration. Use the command kubectl get pods -n kube-system to check the status of the CoreDNS pods. Look for pods that are in the Running state and have a status of Ready. If any pods are in an error state, investigate further.

Next, examining the CoreDNS logs can provide valuable insights into what's going wrong. Use the command kubectl logs <pod-name> -n kube-system to view the logs of a specific CoreDNS pod. Look for error messages or warnings that might indicate a problem with the configuration or deployment. Pay attention to messages related to the Corefile, as syntax errors in the Corefile are a common cause of CoreDNS failures.

Ensuring the Corefile syntax is correct is essential. The Corefile is the configuration file for CoreDNS, and any syntax errors in this file will prevent CoreDNS from starting. Use a text editor or linter to check the Corefile for syntax errors. Pay attention to indentation, spacing, and the correct use of plugins and directives. You can also use the coredns -test command to test the Corefile for syntax errors. This command will parse the Corefile and report any errors it finds.

Finally, testing DNS resolution from inside a pod is a good way to verify that CoreDNS is working correctly. Use the nslookup or dig commands to query a domain name from inside a pod. For example, you can run kubectl exec -it <pod-name> -- nslookup kubernetes.default to resolve the DNS name of the Kubernetes API server. If CoreDNS is working correctly, the command should return the IP address of the Kubernetes API server. If the command fails or returns an unexpected result, there may be a problem with the CoreDNS configuration or deployment.

By following these troubleshooting steps, you can quickly identify and resolve common issues with CoreDNS.

Conclusion

And there you have it! Installing CoreDNS on Kubernetes isn't too scary, right? With a bit of YAML, a well-configured Corefile, and a few kubectl commands, you can have a robust and flexible DNS solution up and running in your cluster. CoreDNS is a cornerstone of modern Kubernetes deployments, providing the essential name resolution services that enable pods to communicate and applications to function correctly. Its flexibility, plugin-based architecture, and seamless integration with Kubernetes make it an ideal choice for managing DNS within your cluster.

Remember to explore the various CoreDNS plugins and configurations to tailor it to your specific needs. Whether you're forwarding external queries, defining custom DNS records, or integrating with existing DNS infrastructure, CoreDNS has you covered. And if you run into any issues, don't hesitate to consult the CoreDNS documentation, which is a wealth of information on all things CoreDNS. With a little patience and persistence, you can master CoreDNS and ensure that your Kubernetes cluster has a reliable and efficient DNS solution.

Happy clustering, folks! 🎉