CoreDNS: A Comprehensive Overview

Table of Contents

Introduction

CoreDNS is a flexible, extensible DNS server written in Go. Unlike traditional DNS servers, CoreDNS relies heavily on plugins to provide functionality. This modular approach allows for great customization and extensibility.

Key features of CoreDNS:

• Written in Go
• Plugin-based architecture
• Supports various protocols: DNS, DNS over TLS (DoT), DNS over HTTPS (DoH), and DNS over gRPC
• Highly configurable

Installation

There are several ways to install CoreDNS:

1. Pre-compiled binaries: Available for various operating systems and architectures.
2. Docker: Images are available on the public Docker hub.
3. From source: Requires a working Go setup and uses Go modules for dependency management.

To test a basic CoreDNS installation:

$ ./coredns -dns.port=1053
$ dig @localhost -p 1053 a whoami.example.org

Configuration

CoreDNS uses a file called Corefile for configuration. The Corefile consists of one or more Server Blocks, each containing plugin configurations.

Environment Variables

CoreDNS supports environment variable substitution in its configuration using the syntax {$ENV_VAR} or {%ENV_VAR%}.

Importing Other Files

The import plugin allows including other configuration files or snippets.

Reusable Snippets

Snippets are defined using parentheses and can be imported into other parts of the configuration:

(snip) {
    prometheus
    log
    errors
}

. {
    whoami
    import snip
}

Plugins

Plugins are the core of CoreDNS functionality. They can:

• Process queries
• Forward queries
• Modify responses
• Implement non-DNS functionality (e.g., metrics, health checks)

The order of plugins in the Corefile does not determine execution order. The execution order is defined in plugin.cfg.

Example plugin configuration:

. {
    chaos CoreDNS-001 info@coredns.io
}

Server Blocks

Server Blocks define the zones a server is responsible for and the port it listens on. The basic syntax is:

zone:port {
    plugin1
    plugin2
}

Multiple zones can be specified in a single Server Block.

Common Setups

Authoritative Serving from Files

Use the file plugin to serve zone data from a file:

example.org {
    file db.example.org
    log
}

Forwarding

Forward queries to other DNS servers:

. {
    forward . 8.8.8.8 9.9.9.9
    log
}

Recursive Resolver

Use the unbound plugin (requires recompilation) for recursive resolution:

. {
    unbound
    cache
    log
}

Writing Plugins

To write a custom plugin:

1. Create a setup.go file for Corefile parsing
2. Implement the plugin logic in a separate file (e.g., example.go)
3. Write tests
4. Create a README.md documenting the plugin
5. Include a LICENSE file

Plugins should implement the plugin.Handler interface, which includes the ServeDNS method.

Best Practices

• Use example.org or example.net in examples and tests
• Implement fallthrough functionality when appropriate
• Follow the style guide for documentation
• Include metrics and readiness reporting
• Use proper logging practices

Remember to check the CoreDNS website and GitHub repository for the most up-to-date information and best practices.