CoreDNS

Kubernetes has become the de facto standard for container orchestration, and one of its critical components is CoreDNS. CoreDNS is a flexible, extensible DNS server that serves as the default DNS service in Kubernetes clusters. In this blog, we'll explore what CoreDNS is, how it works, its architecture, and how it integrates with Kubernetes.

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What is CoreDNS?

CoreDNS is a DNS server that is designed to be fast, flexible, and modular. It was introduced as the default DNS service in Kubernetes starting with version 1.13, replacing kube-dns. CoreDNS is written in Go and is highly extensible through plugins, making it a versatile tool for service discovery and name resolution in Kubernetes clusters.

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Why CoreDNS in Kubernetes?

In Kubernetes, services and pods are dynamically created and destroyed. To enable communication between these ephemeral components, Kubernetes relies on DNS for service discovery. CoreDNS plays a crucial role in this process by:

1. Resolving Service Names: CoreDNS maps Kubernetes service names to their corresponding IP addresses.

2. Pod DNS Resolution: It can also resolve pod hostnames to their IP addresses.

3. Custom DNS Configuration: CoreDNS allows administrators to customize DNS behavior using plugins.

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CoreDNS Architecture

CoreDNS operates as a single binary and uses a modular architecture. Its functionality is driven by plugins, which can be chained together to create custom DNS workflows. Here's a high-level overview of CoreDNS architecture:

1. CoreDNS Server: The main server process that listens for DNS queries.

2. Plugins: CoreDNS uses plugins to handle different aspects of DNS resolution. Some plugins are enabled by default, while others can be added as needed.

3. Kubernetes Integration: CoreDNS interacts with the Kubernetes API to dynamically update DNS records based on the state of the cluster.

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How CoreDNS Works in Kubernetes

When CoreDNS is deployed in a Kubernetes cluster, it runs as a pod (or multiple pods for high availability) in the kube-system namespace. Here's how it works:

1. Service Discovery:

   • When a service is created in Kubernetes, CoreDNS automatically creates a DNS record for it.

   • For example, a service named my-service in the default namespace can be resolved as my-service.default.svc.cluster.local.

2. Pod DNS:

   • CoreDNS can also resolve pod hostnames if the pods are configured with hostnames and subdomains.

3. DNS Queries:

   • When a pod in the cluster makes a DNS query, the query is sent to the CoreDNS service.

   • CoreDNS processes the query using its configured plugins and returns the appropriate response.

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CoreDNS Configuration in Kubernetes

CoreDNS is configured using a Corefile, which is a configuration file written in a simple syntax. The Corefile specifies which plugins to use and how they should behave. Here's an example Corefile:

corefile

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.:53 {
    errors
    health
    kubernetes cluster.local in-addr.arpa ip6.arpa {
        pods insecure
        fallthrough in-addr.arpa ip6.arpa
    }
    prometheus :9153
    forward . /etc/resolv.conf
    cache 30
    loop
    reload
    loadbalance
}

Key Plugins in the Corefile:

• kubernetes: Enables CoreDNS to interact with the Kubernetes API and resolve services and pods.

• forward: Forwards DNS queries to upstream DNS servers for external name resolution.

• cache: Caches DNS responses to improve performance.

• prometheus: Exposes metrics for monitoring CoreDNS performance.

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Deploying CoreDNS in Kubernetes

CoreDNS is typically deployed as a Kubernetes Deployment. Here's an example manifest:

yaml

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apiVersion: apps/v1
kind: Deployment
metadata:
  name: coredns
  namespace: kube-system
  labels:
    k8s-app: kube-dns
spec:
  replicas: 2
  selector:
    matchLabels:
      k8s-app: kube-dns
  template:
    metadata:
      labels:
        k8s-app: kube-dns
    spec:
      containers:
      - name: coredns
        image: coredns/coredns:1.10.1
        args:
        - "-conf"
        - "/etc/coredns/Corefile"
        volumeMounts:
        - name: config-volume
          mountPath: /etc/coredns
      volumes:
      - name: config-volume
        configMap:
          name: coredns-config

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CoreDNS vs kube-dns

Before CoreDNS, Kubernetes used kube-dns for DNS resolution. Here's a comparison:

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Customizing CoreDNS

CoreDNS can be customized to meet specific requirements. For example:

• Add custom DNS records using the hosts plugin.

• Enable logging and monitoring using the log and prometheus plugins.

• Configure caching and load balancing for better performance.

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Troubleshooting CoreDNS

If you encounter DNS issues in your Kubernetes cluster, here are some steps to troubleshoot:

1. Check the CoreDNS pods: Ensure they are running and healthy.

2. Verify the Corefile: Ensure the configuration is correct.

3. Check logs: Use kubectl logs to inspect CoreDNS logs for errors.

4. Test DNS resolution: Use tools like nslookup or dig from within a pod to test DNS queries.

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Conclusion

CoreDNS is a powerful and flexible DNS server that plays a critical role in Kubernetes service discovery. Its modular architecture, performance, and ease of configuration make it an excellent choice for modern Kubernetes clusters. By understanding how CoreDNS works and how to configure it, you can ensure reliable and efficient DNS resolution in your Kubernetes environment.

If you're running Kubernetes, CoreDNS is likely already working behind the scenes to keep your services connected. Take some time to explore its configuration and capabilities—it's a tool worth mastering!