Simple Golang DNS Server

main.go

package main

import (
	"fmt"
	"log"
	"strconv"

	"github.com/miekg/dns"
)

var records = map[string]string{
	"test.service.": "192.168.0.2",
}

func parseQuery(m *dns.Msg) {
	for _, q := range m.Question {
		switch q.Qtype {
		case dns.TypeA:
			log.Printf("Query for %s\n", q.Name)
			ip := records[q.Name]
			if ip != "" {
				rr, err := dns.NewRR(fmt.Sprintf("%s A %s", q.Name, ip))
				if err == nil {
					m.Answer = append(m.Answer, rr)
				}
			}
		}
	}
}

func handleDnsRequest(w dns.ResponseWriter, r *dns.Msg) {
	m := new(dns.Msg)
	m.SetReply(r)
	m.Compress = false

	switch r.Opcode {
	case dns.OpcodeQuery:
		parseQuery(m)
	}

	w.WriteMsg(m)
}

func main() {
	// attach request handler func
	dns.HandleFunc("service.", handleDnsRequest)

	// start server
	port := 5353
	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())
	}
}

test.sh

dig @localhost -p 5353 test.service

How do I get the client IP address?

It seems you can get it via
dns.Responsewriter.RemoteAddr()

The error is pretty obvious: you already have a dns service running on your machine.

…

On Tue, Mar 29, 2022 at 10:46 AM yolo_gopher ***@***.***> wrote: ***@***.**** commented on this gist. ------------------------------ I always get error: Failed to start server: listen udp :53: bind: address already in use whenever I run the program with go run. Do you know why ? — Reply to this email directly, view it on GitHub <https://gist.github.com/0d67b4fb2d5daf3edd4fad3e13b162cb#gistcomment-4114142>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AABY7EHMIF3BJWERFJW7BXLVCK7OLANCNFSM4JLKVJDQ> . You are receiving this because you commented.Message ID: <walm/main. ***@***.***>

nslookup test.service
;; Got recursion not available from 127.0.0.1, trying next server
Server:		8.8.8.8
Address:	8.8.8.8#53

** server can't find test.service: NXDOMAIN

I made a version that supports multiple A records (and also multiple SRV records): https://gist.github.com/NinoM4ster/edaac29339371c6dde7cdb48776d2854

@NinoM4ster Are you interested in communicating with dns servers?

@NinoM4ster Are you interested in communicating with dns servers?

I'm not sure what you mean.

127.0.0.1:53
curl nslookup

@Ran-Xing I used the dig command.

oh,no,dig work, but system not work,you can try set 127.0.0.1 on system, then run

get ipaddress

docker run -itd --name nginx --rm nginx
docker run -itd --name ubuntu --rm ubuntu bash
docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' nginx
docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' ubuntu

records add

	"nginx.service.": {"192.168.0.2"},
	"ubuntu.service.": {"192.168.0.2"},

at last

set word_system dns

127.0.0.1

nslookup nginx.service
curl nginx.service -v

@NinoM4ster I also wrote a dns server according to this writing method, which supports A records, but it does not work on macOS and Linux

dig works fine

oh, in my case I'm not using this as a system-wide DNS server, but using a service to query it directly.