CoreDNS Fundamentals

This comprehensive hands-on course teaches you how to deploy, configure, and troubleshoot CoreDNS—the default DNS server for Kubernetes and a CNCF graduated project. You'll progress from basic concepts to advanced production configurations through practical exercises.

Course Topics

Topic 1: Introduction to CoreDNS

What you'll learn:

• CoreDNS History & Architecture: Understand the origin story (forked from Caddy), CNCF graduation, and adoption by Kubernetes
• Plugin-Based Design Philosophy: Learn why “everything is a plugin” makes CoreDNS flexible and modular
• Corefile Fundamentals: Master the main configuration file that defines domains, ports, and plugin chains
• Query Processing Model: Understand how DNS queries flow through the sequential plugin chain
• Installation & Setup: Download, install, and configure CoreDNS v1.12.3 on Linux
• First DNS Server: Build and test your first CoreDNS server using the whoami plugin

Hands-on exercises:

• Install CoreDNS binary and set up system PATH
• Run CoreDNS with the default configuration
• Test DNS queries using the dig command
• Observe query logs and understand server behavior

Topic 2: Corefile Syntax & Configuration

What you'll learn:

• Production-Ready DNS Architecture: Build DNS servers from scratch
• Caching & Forwarding: Set up a basic DNS resolver with caching and upstream forwarding to 8.8.8.8
• Local Zone Management: Create and serve custom DNS zones from zone files
• Multi-Port Configuration: Serve different zones on dedicated ports (e.g., port 5300)
• Intelligent Fallback: Use the fallthrough keyword for incomplete zone files
• Server Block Structure: Understand zone definitions, port bindings, and plugin ordering
• Zone File Format: Work with SOA records, NS records, and A records

Hands-on exercises:

• Configure a basic forwarder with caching on port 53
• Add a local zone example.local using the file plugin
• Serve the internal zone on a separate port 5300
• Implement fallthrough for incomplete internal.network zone
• Test configurations using dig queries

Topic 3: CoreDNS Plugins Deep Dive

What you'll learn:

• Plugin Architecture: Understand modularity, flexibility, and extensibility

Essential Plugins:

• hosts: Serve DNS from local hosts file (like /etc/hosts)
• forward: Route queries to upstream DNS servers
• etcd: Dynamic service discovery using etcd key-value store
• rewrite: Transform DNS queries (e.g., rewrite search.local → google.com
• loadbalance: Round-robin distribution across multiple IPs
• log: Enable query logging for debugging
• errors: Log DNS errors to stdout
• cache: Store responses to speed up lookups
• Plugin Chaining: Combine multiple plugins with proper ordering
• Fallthrough Mechanism: Pass queries to the next plugin when no match is found
• Docker Integration: Run etcd containers for a dynamic DNS backend

Hands-on exercises:

• Configure a hosts plugin to resolve service-a.lab.local
• Set up forwarding to Google DNS (8.8.8.8)
• Deploy an etcd container and configure the CoreDNS etcd plugin
• Implement DNS rewriting for development environments
• Configure load balancing for multi-backend services
• Chain plugins for a complete DNS resolution pipeline

Topic 4: Service Discovery in Kubernetes

What you'll learn:

• DNS in Kubernetes Clusters: Identify and understand the CoreDNS deployment in kube-system

Two Service Discovery Methods:

• Environment Variable-Based Discovery (automatic injection by kubelet)
• Cluster DNS-Based Discovery (preferred method)

DNS and Service Resolution Concepts:

• Pod DNS Configuration: Examine /etc/resolv.conf and understand nameserver and search domains
• DNS Naming Conventions: Master the structure ..svc.cluster.local
• Intra-Namespace Resolution: Resolve services within the same namespace using short names
• Cross-Namespace Resolution: Access services in different namespaces using qualified names
• Troubleshooting Connectivity: Debug and fix service connection issues between pods

Hands-on exercises:

• Inspect CoreDNS pods and services in a Kubernetes cluster
• Use nslookup to test DNS resolution from inside pods
• Resolve services using short names vs FQDNs
• Fix webapp-to-database connectivity across namespaces
• Redirect DNS query outputs to files for verification

Topic 5: Inspection & Debugging CoreDNS

What you'll learn:

• Diagnosing Complete DNS Failures: Identify when internal service resolution fails
• RBAC Permission Issues: Understand CoreDNS service account requirements (list/watch on services, endpoints, endpointslices)
• External Domain Resolution Problems: Fix forwarding configuration for internet domains
• Custom Cluster Domains: Change from cluster.local to custom domains like hello.kodekloud
• Log Analysis: Read and interpret CoreDNS error messages
• DNS Resolution Flow: Understand the complete path from pod query to API server
• Search Domain Behavior: Master how DNS search domains affect query resolution

Hands-on exercises:

• Fix RBAC permissions by editing ClusterRole for CoreDNS
• Uncomment and configure the forward plugin for external DNS
• Update kubelet and CoreDNS for the custom cluster domain
• Analyze CoreDNS logs to identify permission errors
• Validate DNS changes using new test pods

Course Learning Outcomes

After completing this course, you will be able to:

• Install and configure CoreDNS from scratch
• Understand the plugin-based architecture and create custom plugin chains
• Deploy and manage CoreDNS in Kubernetes environments
• Troubleshoot DNS resolution issues using logs and debugging tools
• Configure service discovery for microservices architectures
• Implement custom DNS zones and domain configurations
• Integrate CoreDNS with etcd for dynamic service registration
• Apply RBAC best practices for CoreDNS in Kubernetes
• Build production-ready, observable DNS infrastructure

Prerequisites

• Basic understanding of DNS concepts
• Familiarity with the Linux command line
• Basic Kubernetes knowledge (for Topics 4–5)
• Understanding of Docker containers (for Topic 3)

Course Format

• Hands-on labs with real configurations
• Multiple choice questions to test understanding
• Practical troubleshooting scenarios
• Progressive complexity from basics to advanced topics
• Production-ready examples you can use in real environments

This course takes you on a complete journey from CoreDNS fundamentals to advanced production configurations, with a strong emphasis on Kubernetes integration and real-world troubleshooting.