The plugin.md in the CoreDNS source tree has some pointers on what a plugin for CoreDNS should have as minimum requirements. It basically boils down to: “it should add something unique and useful to CoreDNS”. Further more documentation, tests and functionality should all be excellent.

It is easier to list when a plugin can be included in CoreDNS than to say it should stay external, so we will do that:

• First, the plugin should be useful for other people. “Useful” is a subjective term, but the plugin needs to fill a niche that appeals to more than one person.
• It should be sufficiently different from other plugins 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.
• Care must be taken to make it efficient in both memory and CPU.

Plugins for CoreDNS can easily live out-of-tree, plugin.cfg defaults to CoreDNS’ repo but other repos work just as well.