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:

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$ ./coredns -dns.port=1053
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$ 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.

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(snip) {
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    prometheus
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    log
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    errors
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}
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. {
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    whoami
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    import snip
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}

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:

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

Server Blocks#

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

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zone:port {
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    plugin1
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    plugin2
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}

Multiple zones can be specified in a single Server Block.

Common Setups#

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

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. {
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    forward . 8.8.8.8 9.9.9.9
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    log
4
}

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. {
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    unbound
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    cache
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    log
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}

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.