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简介

k8sDNS也经历一些变化，之前的架构是 kube-dns，现在换成了CoreDNS。本文先简单描述下kube-dns之后详细介绍CoreDNS。

kube-dns

宏观上讲， kube-dns分两块，一块是与k8s强耦合的后端DNS服务用于解析Service地址等集群DNS的服务发现。一块是dnsmasq作为前端cache。总体上功能紧凑，但在定制开发方面很不友好。

CoreDNS

基于caddy开发的DNS插件框架，并且内置了很多已有的DNS相关插件。具体的插件位置在 ./plugin 文件夹下。

编译过程

通过配置文件来实现plugin的静态编译。可以通过Makefile来看到

core/plugin/zplugin.go core/dnsserver/zdirectives.go: plugin.cfg
  GO111MODULE=on go generate coredns.go

plugin.cfg文件中放的即是CoreDNS支持的插件名称以及对应的package库。因为后面要去引用这个package从而调用他们的init函数。

coredns.go中调用的gen指令的地方

///home/ry/go/src/github.com/coredns/coredns/coredns.go:3
//go:generate go run directives_generate.go
//go:generate go run owners_generate.go

查看对应源码，会生成这两个文件： core/plugin/zplugin.go core/dnsserver/zdirectives.go 一个是引用全部的plugin，一个是生成一个string slice并赋值给一个变量： Directives。 import某个插件为的是调用其init函数。而init函数中的作用是让CoreDNS依赖的caddy系统认识这个插件。可以找一个插件比如log来看一下注册细节。

genImports("core/plugin/zplugin.go", "plugin", mi)
genDirectives("core/dnsserver/zdirectives.go", "dnsserver", md)

==> 调用 init方法，以log为例

func init() {
  caddy.RegisterPlugin("log", caddy.Plugin{
    ServerType: "dns",
    Action:     setup,
  })
}

caddy是一个http2框架而CoreDNS是一个DNS服务，这里要让http框架“切换”7层协议。这是怎么做到的？ 答案是caddy的框架很框架。怎么监听怎么处理请求都是有个interface来定义的。具体的定义如下：

type Server interface {
  TCPServer
  UDPServer
}

type TCPServer interface {
  // Listen starts listening by creating a new listener
  // and returning it. It does not start accepting
  // connections. For UDP-only servers, this method
  // can be a no-op that returns (nil, nil).
  Listen() (net.Listener, error)

  // Serve starts serving using the provided listener.
  // Serve must start the server loop nearly immediately,
  // or at least not return any errors before the server
  // loop begins. Serve blocks indefinitely, or in other
  // words, until the server is stopped. For UDP-only
  // servers, this method can be a no-op that returns nil.
  Serve(net.Listener) error
}

caddy定义的Server接口是交给具体的插件来实现的。翻译成人话大概就是： 告诉caddy怎么监听，怎么处理请求，caddy会帮我们监听，请求来了会自动调用我们给的处理函数。而这一切又通过caddy所谓的context来统一封装。具体如下：

type Context interface {
  // Called after the Caddyfile is parsed into server
  // blocks but before the directives are executed,
  // this method gives you an opportunity to inspect
  // the server blocks and prepare for the execution
  // of directives. Return the server blocks (which
  // you may modify, if desired) and an error, if any.
  // The first argument is the name or path to the
  // configuration file (Caddyfile).
  //
  // This function can be a no-op and simply return its
  // input if there is nothing to do here.
  InspectServerBlocks(string, []caddyfile.ServerBlock) ([]caddyfile.ServerBlock, error)

  // This is what Caddy calls to make server instances.
  // By this time, all directives have been executed and,
  // presumably, the context has enough state to produce
  // server instances for Caddy to start.
  MakeServers() ([]Server, error)
}

给一个context，这个context用来创建Server，而这个Server要会怎么监听，怎么处理请求。就这样，caddy完美的把自己变成了一个框架，而不是gin这样的http框架。context怎么给过来？则要注册到caddy框架中。

///home/ry/go/src/github.com/coredns/coredns/core/dnsserver/register.go:23
func init() {
  flag.StringVar(&Port, serverType+".port", DefaultPort, "Default port")

  caddy.RegisterServerType(serverType, caddy.ServerType{
    Directives: func() []string { return Directives },
    DefaultInput: func() caddy.Input {
      return caddy.CaddyfileInput{
        Filepath:       "Corefile",
        Contents:       []byte(".:" + Port + " {\nwhoami\n}\n"),
        ServerTypeName: serverType,
      }
    },
    NewContext: newContext,
  })
}

这样就实现了CoreDNS到caddy的打通。caddy中的这行代码标记着caddy到CoreDNS的完美切换：

// /home/ry/go/src/github.com/coredns/coredns/vendor/github.com/mholt/caddy/caddy.go:604
  inst.context = stype.NewContext(inst)

抓住这些核心则看细节就很顺了。细节不再展开。

notes:

  1. 优先用proxy: the proxy plugin supports more protocols than forward
  2. https://github.com/coredns/example https://coredns.io/2017/03/01/how-to-add-plugins-to-coredns/ plugin example