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CoreDNS - A Flexible and Powerful DNS Server

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Introduction

CoreDNS is a DNS server software that is often used to support service discovery in containerized environments, particularly those managed by Kubernetes. The original version of CoreDNS was written by Miek Gieben in 2016. Miek had previously written a DNS server called SkyDNS and a popular library of DNS functions in Go called Go DNS. Like its predecessor, CoreDNS’s primary purpose is to support service discovery. However, Miek was impressed by the architecture of a Go-based web server called Caddy, so he forked Caddy to create CoreDNS. As a result, CoreDNS inherited many of Caddy’s major advantages, such as its simple configuration syntax, its powerful plug-in-based architecture, and its foundation in Go.

Compared to the configuration file syntax of BIND, for example, CoreDNS’s Corefile is refreshingly straightforward. The Corefile for a basic CoreDNS-based DNS server is often just a few lines long and relatively easy to read.

CoreDNS uses plug-ins to provide DNS functionality. There are plug-ins for caching, forwarding, configuring a primary DNS server that reads zone data from a file, and configuring a secondary DNS server. Not only is configuring each plug-in straightforward (as mentioned in the previous paragraph), but if you need to create your own plug-in. That makes CoreDNS faster and more secure.

Configuring CoreDNS

CoreDNS is configured using a Corefile, which is a text file that defines the DNS zones and how they are resolved. The Corefile consists of a series of blocks, each of which is enclosed in curly braces. Each block defines a DNS zone and how it is resolved. Here is an example of a simple Corefile:

. {
    forward . 8.8.8.8
}

In this example, the . block defines the root DNS zone and the forward plugin is used to forward requests to the Google DNS server at 8.8.8.8.

Using CoreDNS Plugins

CoreDNS provides a wide variety of plugins that can be used to customize its behavior. Here are some examples of common plugins:

Hosts Plugin

The hosts plugin allows you to define DNS records using a hosts file format. Here’s an example Corefile that uses the hosts plugin:

. {
    hosts {
        10.0.0.1 example.com
    }
}

In this example, any request for example.com will be resolved to 10.0.0.1.

Proxy Plugin

The proxy plugin allows you to serve DNS requests by proxying them to another DNS server. Here’s an example Corefile that uses the proxy plugin:

. {
    proxy . 8.8.8.8
}

In this example, any request for a DNS record that is not defined in the Corefile will be forwarded to the Google DNS server at 8.8.8.8.

Rewrite Plugin

The rewrite plugin allows you to rewrite DNS requests based on regular expressions. Here’s an example Corefile that uses the rewrite plugin:

. {
    rewrite name example.com www.example.com
    forward . 8.8.8.8
}

In this example, any request for example.com will be rewritten to www.example.com before being forwarded to the Google DNS server at 8.8.8.8.

Conclusion

CoreDNS is a powerful and flexible DNS server that can be used for a wide variety of use cases. In this blog post, we have explored how to configure CoreDNS using a Corefile and provided some examples of common plugins that can be used to customize its behavior. With CoreDNS, you can easily create a DNS server that meets your specific needs.

Reference

  • Learning CoreDNS by John Belamaric and Cricket Liu
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