Add ControlD public DNS to OS resolver

Since the OS resolver only returns response with NOERROR first, it's
safe to use ControlD public DNS in parallel with system DNS. Local
domains would resolve only though local resolvers, because public ones
will return NXDOMAIN response.

resolver_test.go

@@ -2,6 +2,8 @@ package ctrld
 
 import (
 	"context"
+	"net"
+	"sync"
 	"testing"
 	"time"
 
@@ -28,6 +30,57 @@ func Test_osResolver_Resolve(t *testing.T) {
 	}
 }
 
+func Test_osResolver_ResolveWithNonSuccessAnswer(t *testing.T) {
+	ns := make([]string, 0, 2)
+	servers := make([]*dns.Server, 0, 2)
+	successHandler := dns.HandlerFunc(func(w dns.ResponseWriter, msg *dns.Msg) {
+		m := new(dns.Msg)
+		m.SetRcode(msg, dns.RcodeSuccess)
+		w.WriteMsg(m)
+	})
+	nonSuccessHandlerWithRcode := func(rcode int) dns.HandlerFunc {
+		return dns.HandlerFunc(func(w dns.ResponseWriter, msg *dns.Msg) {
+			m := new(dns.Msg)
+			m.SetRcode(msg, rcode)
+			w.WriteMsg(m)
+		})
+	}
+
+	handlers := []dns.Handler{
+		nonSuccessHandlerWithRcode(dns.RcodeRefused),
+		nonSuccessHandlerWithRcode(dns.RcodeNameError),
+		successHandler,
+	}
+	for i := range handlers {
+		pc, err := net.ListenPacket("udp", ":0")
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+
+		s, addr, err := runLocalPacketConnTestServer(t, pc, handlers[i])
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		ns = append(ns, addr)
+		servers = append(servers, s)
+	}
+	defer func() {
+		for _, server := range servers {
+			server.Shutdown()
+		}
+	}()
+	resolver := &osResolver{nameservers: ns}
+	msg := new(dns.Msg)
+	msg.SetQuestion(".", dns.TypeNS)
+	answer, err := resolver.Resolve(context.Background(), msg)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if answer.Rcode != dns.RcodeSuccess {
+		t.Errorf("unexpected return code: %s", dns.RcodeToString[answer.Rcode])
+	}
+}
+
 func Test_upstreamTypeFromEndpoint(t *testing.T) {
 	tests := []struct {
 		name         string
@@ -51,3 +104,33 @@ func Test_upstreamTypeFromEndpoint(t *testing.T) {
 		})
 	}
 }
+
+func runLocalPacketConnTestServer(t *testing.T, pc net.PacketConn, handler dns.Handler, opts ...func(*dns.Server)) (*dns.Server, string, error) {
+	t.Helper()
+
+	server := &dns.Server{
+		PacketConn:   pc,
+		ReadTimeout:  time.Hour,
+		WriteTimeout: time.Hour,
+		Handler:      handler,
+	}
+
+	waitLock := sync.Mutex{}
+	waitLock.Lock()
+	server.NotifyStartedFunc = waitLock.Unlock
+
+	for _, opt := range opts {
+		opt(server)
+	}
+
+	addr, closer := pc.LocalAddr().String(), pc
+	go func() {
+		if err := server.ActivateAndServe(); err != nil {
+			t.Error(err)
+		}
+		closer.Close()
+	}()
+
+	waitLock.Lock()
+	return server, addr, nil
+}