Plugins

Writing Plugins

See a couple of blog posts on how to write and add plugin to CoreDNS:

https://blog.coredns.io/2017/03/01/how-to-add-plugins-to-coredns/
https://blog.coredns.io/2016/12/19/writing-plugin-for-coredns/, slightly older, but useful.

Metrics

When exporting metrics the Namespace should be plugin.Namespace (="coredns"), and the Subsystem should be the name of the plugin. The README.md for the plugin should then also contain a Metrics section detailing the metrics. If the plugin supports dynamic health reporting it should also have Health section detailing on some of its inner workings.

Documentation

Each plugin should have a README.md explaining what the plugin does and how it is configured. The file should have the following layout:

Title: use the plugin's name
Subsection titled: "Syntax"
Subsection titled: "Examples"

More sections are of course possible.

Style

We use the Unix manual page style:

The name of plugin in the running text should be italic: plugin.
all CAPITAL: user supplied argument, in the running text references this use strong text: **: EXAMPLE.
Optional text: in block quotes: [optional].
Use three dots to indicate multiple options are allowed: arg....
Item used literal: literal.

Example Domain Names

Please be sure to use example.org or example.net in any examples you provide. These are the standard domain names created for this purpose.

Fallthrough

In a perfect world the following would be true for plugin: "Either you are responsible for a zone or not". If the answer is "not", the plugin should call the next plugin in the chain. If "yes" it should handle all names that fall in this zone and the names below - i.e. it should handle the entire domain.

. {
    file example.org db.example
}

In this example the file plugin is handling all names below (and including) example.org. If a query comes in that is not a subdomain (or equal to) example.org the next plugin is called.

Now, the world isn't perfect, and there are good reasons to "fallthrough" to the next middlware, meaning a plugin is only responsible for a subset of names within the zone. The first of these to appear was the reverse plugin that synthesis PTR and A/AAAA responses (useful with IPv6).

The nature of the reverse plugin is such that it only deals with A,AAAA and PTR and then only for a subset of the names. Ideally you would want to layer reverse in front off another plugin such as file or auto (or even proxy). This means reverse handles some special reverse cases and all other request are handled by the backing plugin. This is exactly what "fallthrough" does. To keep things explicit we've opted that plugins implement such behavior should implement a fallthrough keyword.

Qualifying for main repo

Plugins for CoreDNS can live out-of-tree, plugin.cfg defaults to CoreDNS' repo but other repos work just as well. So when do we consider the inclusion of a new plugin in the main repo?

First, the plugin should be useful for other people. "Useful" is a subjective term. We will probably need to further refine this.
It should be sufficiently different from other plugin to warrant inclusion.
Current internet standards need be supported: IPv4 and IPv6, so A and AAAA records should be handled (if your plugin is in the business of dealing with address records that is).
It must have tests.
It must have a README.md for documentation.

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