cmd/sniproxy: reimplement DNS server

Switch to golang.org/x/net/dns/dnsmessage, due to
https://github.com/miekg/dns/issues/1427

Updates https://github.com/tailscale/tailscale/issues/1748

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

cmd/sniproxy/snipproxy.go

@@ -9,14 +9,13 @@ package main
 import (
 	"context"
 	"flag"
-	"fmt"
 	"log"
 	"net"
 	"net/netip"
 	"strings"
 	"time"
 
-	"github.com/miekg/dns"
+	"golang.org/x/net/dns/dnsmessage"
 	"inet.af/tcpproxy"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/net/netutil"
@@ -26,6 +25,7 @@ import (
 var (
 	ports   = flag.String("ports", "443", "comma-separated list of ports to proxy")
 	dnsserv = flag.Bool("dns", true, "run a small DNS server to reply to any query with its own address")
+	tsMBox  = dnsmessage.MustNewName("support.tailscale.com.")
 )
 
 func main() {
@@ -113,46 +113,100 @@ func (s *server) getAddresses() (ip4, ip6 netip.Addr) {
 }
 
 func (s *server) serveDns() {
-	dns.HandleFunc(".", func(w dns.ResponseWriter, r *dns.Msg) {
-		switch r.Opcode {
-		case dns.OpcodeQuery:
-			m := s.dnsResponse(r)
-			m.SetReply(r)
-			w.WriteMsg(m)
-		}
-	})
-
+	buf := make([]byte, 1024)
 	pconn, err := s.ts.ListenPacket("udp", ":53")
 	if err != nil {
-		log.Printf("Failed to start DNS listener: %s\n ", err.Error())
+		log.Fatal(err)
+	}
+
+	for {
+		_, addr, err := pconn.ReadFrom(buf)
+		if err != nil {
+			log.Printf("pconn.ReadFrom failed: %v\n ", err)
+			continue
+		}
+
+		var msg dnsmessage.Message
+		err = msg.Unpack(buf)
+		if err != nil {
+			log.Printf("dnsmessage.Message unpack failed: %v\n ", err)
+			continue
+		}
+
+		buf, err := s.dnsResponse(&msg)
+		if err != nil {
+			log.Printf("s.dnsResponse failed: %v\n", err)
+			continue
+		}
+
+		_, err = pconn.WriteTo(buf, addr)
+		if err != nil {
+			log.Printf("pconn.WriteTo failed: %v\n", err)
+			continue
+		}
+	}
+}
+
+func (s *server) dnsResponse(req *dnsmessage.Message) (buf []byte, err error) {
+	resp := dnsmessage.NewBuilder(buf,
+		dnsmessage.Header{
+			ID:            req.Header.ID,
+			Response:      true,
+			Authoritative: true,
+		})
+	resp.EnableCompression()
+
+	if len(req.Questions) == 0 {
+		buf, _ = resp.Finish()
 		return
 	}
 
-	dnsServer := &dns.Server{PacketConn: pconn}
-	err = dnsServer.ActivateAndServe()
+	q := req.Questions[0]
+	err = resp.StartQuestions()
 	if err != nil {
-		log.Printf("Failed to start DNS server: %s\n ", err.Error())
+		return
 	}
-}
+	resp.Question(q)
 
-func (s *server) dnsResponse(requestMsg *dns.Msg) *dns.Msg {
-	responseMsg := new(dns.Msg)
-	if len(requestMsg.Question) == 0 {
-		return responseMsg
-	}
-
-	q := requestMsg.Question[0]
-	var rr dns.RR
 	ip4, ip6 := s.getAddresses()
-
-	switch q.Qtype {
-	case dns.TypeAAAA:
-		rr, _ = dns.NewRR(fmt.Sprintf("%s 120 IN AAAA %s", q.Name, ip6.String()))
-
-	case dns.TypeA:
-		rr, _ = dns.NewRR(fmt.Sprintf("%s 120 IN A %s", q.Name, ip4.String()))
+	err = resp.StartAnswers()
+	if err != nil {
+		return
 	}
 
-	responseMsg.Answer = append(responseMsg.Answer, rr)
-	return responseMsg
+	switch q.Type {
+	case dnsmessage.TypeAAAA:
+		err = resp.AAAAResource(
+			dnsmessage.ResourceHeader{Name: q.Name, Class: q.Class, TTL: 120},
+			dnsmessage.AAAAResource{AAAA: ip6.As16()},
+		)
+
+	case dnsmessage.TypeA:
+		err = resp.AResource(
+			dnsmessage.ResourceHeader{Name: q.Name, Class: q.Class, TTL: 120},
+			dnsmessage.AResource{A: ip4.As4()},
+		)
+	case dnsmessage.TypeSOA:
+		err = resp.SOAResource(
+			dnsmessage.ResourceHeader{Name: q.Name, Class: q.Class, TTL: 120},
+			dnsmessage.SOAResource{NS: q.Name, MBox: tsMBox, Serial: 2023030600,
+				Refresh: 120, Retry: 120, Expire: 120, MinTTL: 60},
+		)
+	case dnsmessage.TypeNS:
+		err = resp.NSResource(
+			dnsmessage.ResourceHeader{Name: q.Name, Class: q.Class, TTL: 120},
+			dnsmessage.NSResource{NS: tsMBox},
+		)
+	}
+
+	if err != nil {
+		return
+	}
+
+	buf, err = resp.Finish()
+	if err != nil {
+		return
+	}
+
+	return
 }