Minimal DNS proxy in golang. Reply NXDOMAIN for AAAA queries.

dns.go

package main

import (
	"log"
	"net"
	"os"
	"strconv"

	"github.com/miekg/dns"
)

func getDNSRequestHandler(client *dns.Client, config *dns.ClientConfig) func(dns.ResponseWriter, *dns.Msg) {
	return func(w dns.ResponseWriter, r *dns.Msg) {

		// Reply NXDOMAIN for AAAA queries
		questionWithoutAAAA := []dns.Question{}
		for _, q := range r.Question {
			if q.Qtype == 28 {
				emptyRR, _ := dns.NewRR(q.Name + "\tIN\tAAAA")
				r.Answer = append(r.Answer, emptyRR)
				r.Rcode = dns.RcodeNameError
				w.WriteMsg(r)
				continue
			}
			questionWithoutAAAA = append(questionWithoutAAAA, q)
		}

		r.Question = questionWithoutAAAA
		r, _, err := client.Exchange(r, net.JoinHostPort(config.Servers[0], config.Port))

		if err != nil {
			log.Printf("ERROR: %s\n", err.Error())
			return
		}

		w.WriteMsg(r)
	}
}

func main() {
	resolv := "/etc/resolv.conf"
	if resolvEnv := os.Getenv("RESOLV"); resolvEnv != "" {
		resolv = resolvEnv
	}

	port := 53
	if portEnv := os.Getenv("PORT"); portEnv != "" {
		var err error
		port, err = strconv.Atoi(portEnv)
		if err != nil {
			log.Fatalf("Invalid port: %s", portEnv)
		}
	}

	config, _ := dns.ClientConfigFromFile(resolv)
	client := new(dns.Client)
	dns.HandleFunc(".", getDNSRequestHandler(client, config))

	server := &dns.Server{Addr: ":" + strconv.Itoa(port), Net: "udp"}
	log.Printf("Starting at %d\n", port)
	err := server.ListenAndServe()
	defer server.Shutdown()
	if err != nil {
		log.Fatalf("Failed to start server: %s\n ", err.Error())
	}
}