dns_server.go

package main

import (
	"fmt"
	"github.com/miekg/dns"
	"strings"
)

const DNSDefaultTTL = 1

type dnsRequestHandler struct{}

func newDNSReplyMsg() *dns.Msg {
	msg := dns.Msg{}

	msg.Compress = true

	// this is an authoritative DNS server
	msg.Authoritative = true
	msg.RecursionAvailable = false

	// DNSSEC disabled for now
	// TODO: fix DNSSEC
	msg.AuthenticatedData = false
	msg.CheckingDisabled = true

	return &msg
}

// send out the generated answer, and if the answer is not correct, send out a SERVFAIL
func finishAnswer(w *dns.ResponseWriter, r *dns.Msg) {
	err := (*w).WriteMsg(r)
	if err != nil {
		softFailIf(err)

		// if answer sanity check (miekg/dns automatically does this) fails, reply with SERVFAIL
		msg := newDNSReplyMsg()
		msg.SetReply(r)
		msg.Rcode = dns.RcodeServerFailure
		err = (*w).WriteMsg(msg)
		softFailIf(err)
	}

	// access log to stdout
	fmt.Printf("[DNS] C=%d Q=%d R=%d DOMAIN=%s %s => %s\n", r.Question[0].Qclass, r.Question[0].Qtype, r.Rcode, r.Question[0].Name, (*w).RemoteAddr(), (*w).LocalAddr())
}

// TODO: force TCP for 1) clients which requests too fast; 2) non-existent answers
// See: https://labs.apnic.net/?p=382
func (this *dnsRequestHandler) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
	msg := newDNSReplyMsg()
	msg.SetReply(r)

	// on function return, we send out the current answer
	defer finishAnswer(&w, msg)

	// sanity check
	if len(r.Question) != 1 {
		msg.Rcode = dns.RcodeRefused
		return
	}

	switch r.Question[0].Qclass {
	case dns.ClassINET:
		switch r.Question[0].Qtype {
		case dns.TypeA:
			handleA(this, r, msg)
			return

		case dns.TypeAAAA:
			handleAAAA(this, r, msg)
			return

		case dns.TypeSOA:
			handleSOA(this, r, msg)
			return

		default:
			handleDefault(this, r, msg)
			return
		}
	case dns.ClassCHAOS:
		switch r.Question[0].Qtype {
		case dns.TypeTXT:
			if strings.EqualFold(r.Question[0].Name, "version.bind.") {
				// we need to reply our software version
				// https://serverfault.com/questions/517087/dns-how-to-find-out-which-software-a-remote-dns-server-is-running
				handleTXTVersionRequest(this, r, msg)
			} else {
				handleDefault(this, r, msg)
			}
			return

		default:
			handleDefault(this, r, msg)
			return
		}
	}
}