DNS server example

dns-server.go

// dns server

package dnser

import (
	"net"

	"golang.org/x/net/dns/dnsmessage"
)

// Context ...
type Context struct {
	host string
	ln   *net.UDPConn
}

// Server ...
func Server(host string) (*Context, error) {
	conn, err := net.ListenUDP("udp", &net.UDPAddr{Port: 53})
	if err != nil {
		return nil, err
	}
	defer conn.Close()

	return &Context{
		host: host,
		ln:   conn,
	}, nil
}

// Do ...
func (ctx *Context) Do() {
	for {
		buf := make([]byte, 512)
		_, addr, _ := ctx.ln.ReadFromUDP(buf)

		go ctx.handle(addr, buf)
	}
}

func (ctx *Context) handle(addr *net.UDPAddr, data []byte) {
	var m dnsmessage.Message
	_ = m.Unpack(data)

	m.Answers = []dnsmessage.Resource{
		{
			Header: dnsmessage.ResourceHeader{
				Name:  m.Questions[0].Name,
				Type:  dnsmessage.TypeCNAME,
				Class: dnsmessage.ClassINET,
				TTL:   300,
			},
			Body: &dnsmessage.CNAMEResource{CNAME: dnsmessage.MustNewName(ctx.host)},
		},
	}

	data, _ = m.Pack()

	ctx.ln.WriteToUDP(data, addr)
}