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Plugins

Writing Plugins

The main method that gets called is ServeDNS. It has three parameters:

  • a context.Context;
  • dns.ResponseWriter that is, basically, the client's connection;
  • *dns.Msg the request from the client.

ServeDNS returns two values, a response code and an error. If the error is not nil, CoreDNS will return a SERVFAIL to the client. The response code tells CoreDNS if a reply has been written by the plugin chain or not. In the latter case CoreDNS will take care of that.

CoreDNS treats:

  • SERVFAIL (dns.RcodeServerFailure)
  • REFUSED (dns.RcodeRefused)
  • FORMERR (dns.RcodeFormatError)
  • NOTIMP (dns.RcodeNotImplemented)

as special and will then assume nothing has been written to the client. In all other cases it assumes something has been written to the client (by the plugin).

The example plugin shows a bare-bones implementation that can be used as a starting point for your plugin. This plugin has tests and extensive comments in the code.

Hooking It Up

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

Logging

If your plugin needs to output a log line you should use the plugin/pkg/log package. This package implements log levels. The standard way of outputting is: log.Info for info level messages. The levels available are log.Info, log.Warning, log.Error, log.Debug. Each of these also has a f variant. The plugin's name should be included, by using the log package like so:

import clog "github.com/coredns/coredns/plugin/pkg/log"

var log = clog.NewWithPlugin("whoami")

log.Info("message") // outputs: [INFO] plugin/whoami: message

In general, logging should be left to the higher layers by returning an error. However, if there is a reason to consume the error and notify the user, then logging in the plugin itself can be acceptable. The Debug* functions only output something when the debug plugin is loaded in the server.

Metrics

When exporting metrics the Namespace should be plugin.Namespace (="coredns"), and the Subsystem must be the name of the plugin. The README.md for the plugin should then also contain a Metrics section detailing the metrics.

Readiness

If the plugin supports signalling readiness it should have a Ready section detailing how it works, and implement the ready.Readiness interface.

Opening Sockets

See the plugin/pkg/reuseport for Listen and ListenPacket functions. Using these functions makes your plugin handle reload events better.

Context

Every request get a context.Context these are pre-filled with 2 values:

  • Key: holds a pointer to the current server, this can be useful for logging or metrics. It is infact used in the metrics plugin to tie a request to a specific (internal) server.
  • LoopKey: holds an integer to detect loops within the current context. The file plugin uses this to detect loops when resolving CNAMEs.

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: "Name" with PLUGIN - one line description.
  • Subsection titled: "Description" has a longer description.
  • Subsection titled: "Syntax", syntax and supported directives.
  • 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 and tests 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 and all subdomains.

. {
    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 may be reasons to "fallthrough" to the next plugin, meaning a plugin is only responsible for a subset of names within the zone.

The fallthrough directive should optionally accept a list of zones. Only queries for records in one of those zones should be allowed to fallthrough. See plugin/pkg/fallthrough for the implementation.

Mutating a Response

Using a custom ResponseWriter, a plugin can mutate a response when another plugin further down the chain writes the response to the client. If a plugin mutates a response it MUST make a copy of the entire response before doing so. A response is a pointer to a dns.Msg and as such you will be manipulating the original response, which could have been generated from a data store. E.g. the file plugin creates a response that the rewrite plugin then rewrites; not copying the data, means it's also mutating the data of the file's data store. A response can be copied by using the Copy() method.

General Guidelines

Some general guidelines:

  • logging time duration should be done in seconds (call the Seconds() method on any duration).
  • keep logging to a minimum.
  • call the main config parse function just parse.
  • try to minimize the number of knobs in the configuration.
  • use plugin.Error() to wrap errors returned from the setup function.

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.