net: set AD bit in DNS queries when trust-ad in resolv.conf

src/net/dnsclient_unix.go

@@ -51,9 +51,9 @@ var (
 	errServerTemporarilyMisbehaving = errors.New("server misbehaving")
 )
 
-func newRequest(q dnsmessage.Question) (id uint16, udpReq, tcpReq []byte, err error) {
+func newRequest(q dnsmessage.Question, ad bool) (id uint16, udpReq, tcpReq []byte, err error) {
 	id = uint16(randInt())
-	b := dnsmessage.NewBuilder(make([]byte, 2, 514), dnsmessage.Header{ID: id, RecursionDesired: true})
+	b := dnsmessage.NewBuilder(make([]byte, 2, 514), dnsmessage.Header{ID: id, RecursionDesired: true, AuthenticData: ad})
 	if err := b.StartQuestions(); err != nil {
 		return 0, nil, nil, err
 	}
@@ -157,9 +157,9 @@ func dnsStreamRoundTrip(c Conn, id uint16, query dnsmessage.Question, b []byte)
 }
 
 // exchange sends a query on the connection and hopes for a response.
-func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Question, timeout time.Duration, useTCP bool) (dnsmessage.Parser, dnsmessage.Header, error) {
+func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Question, timeout time.Duration, useTCP, ad bool) (dnsmessage.Parser, dnsmessage.Header, error) {
 	q.Class = dnsmessage.ClassINET
-	id, udpReq, tcpReq, err := newRequest(q)
+	id, udpReq, tcpReq, err := newRequest(q, ad)
 	if err != nil {
 		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotMarshalDNSMessage
 	}
@@ -273,7 +273,7 @@ func (r *Resolver) tryOneName(ctx context.Context, cfg *dnsConfig, name string,
 		for j := uint32(0); j < sLen; j++ {
 			server := cfg.servers[(serverOffset+j)%sLen]
 
-			p, h, err := r.exchange(ctx, server, q, cfg.timeout, cfg.useTCP)
+			p, h, err := r.exchange(ctx, server, q, cfg.timeout, cfg.useTCP, cfg.trustAD)
 			if err != nil {
 				dnsErr := &DNSError{
 					Err:    err.Error(),

src/net/dnsclient_unix_test.go

@@ -79,7 +79,7 @@ func TestDNSTransportFallback(t *testing.T) {
 	for _, tt := range dnsTransportFallbackTests {
 		ctx, cancel := context.WithCancel(context.Background())
 		defer cancel()
-		_, h, err := r.exchange(ctx, tt.server, tt.question, time.Second, useUDPOrTCP)
+		_, h, err := r.exchange(ctx, tt.server, tt.question, time.Second, useUDPOrTCP, false)
 		if err != nil {
 			t.Error(err)
 			continue
@@ -135,7 +135,7 @@ func TestSpecialDomainName(t *testing.T) {
 	for _, tt := range specialDomainNameTests {
 		ctx, cancel := context.WithCancel(context.Background())
 		defer cancel()
-		_, h, err := r.exchange(ctx, server, tt.question, 3*time.Second, useUDPOrTCP)
+		_, h, err := r.exchange(ctx, server, tt.question, 3*time.Second, useUDPOrTCP, false)
 		if err != nil {
 			t.Error(err)
 			continue
@@ -1593,7 +1593,7 @@ func TestDNSDialTCP(t *testing.T) {
 	}
 	r := Resolver{PreferGo: true, Dial: fake.DialContext}
 	ctx := context.Background()
-	_, _, err := r.exchange(ctx, "0.0.0.0", mustQuestion("com.", dnsmessage.TypeALL, dnsmessage.ClassINET), time.Second, useUDPOrTCP)
+	_, _, err := r.exchange(ctx, "0.0.0.0", mustQuestion("com.", dnsmessage.TypeALL, dnsmessage.ClassINET), time.Second, useUDPOrTCP, false)
 	if err != nil {
 		t.Fatal("exhange failed:", err)
 	}
@@ -1746,7 +1746,7 @@ func TestDNSUseTCP(t *testing.T) {
 	r := Resolver{PreferGo: true, Dial: fake.DialContext}
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-	_, _, err := r.exchange(ctx, "0.0.0.0", mustQuestion("com.", dnsmessage.TypeALL, dnsmessage.ClassINET), time.Second, useTCPOnly)
+	_, _, err := r.exchange(ctx, "0.0.0.0", mustQuestion("com.", dnsmessage.TypeALL, dnsmessage.ClassINET), time.Second, useTCPOnly, false)
 	if err != nil {
 		t.Fatal("exchange failed:", err)
 	}
@@ -2344,3 +2344,68 @@ func TestLongDNSNames(t *testing.T) {
 		}
 	}
 }
+
+func TestDNSTrustAD(t *testing.T) {
+	fake := fakeDNSServer{
+		rh: func(_, _ string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {
+			if q.Questions[0].Name.String() == "notrustad.go.dev." && q.Header.AuthenticData {
+				t.Error("unexpected AD bit")
+			}
+
+			if q.Questions[0].Name.String() == "trustad.go.dev." && !q.Header.AuthenticData {
+				t.Error("expected AD bit")
+			}
+
+			r := dnsmessage.Message{
+				Header: dnsmessage.Header{
+					ID:       q.Header.ID,
+					Response: true,
+					RCode:    dnsmessage.RCodeSuccess,
+				},
+				Questions: q.Questions,
+			}
+			if q.Questions[0].Type == dnsmessage.TypeA {
+				r.Answers = []dnsmessage.Resource{
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:   q.Questions[0].Name,
+							Type:   dnsmessage.TypeA,
+							Class:  dnsmessage.ClassINET,
+							Length: 4,
+						},
+						Body: &dnsmessage.AResource{
+							A: TestAddr,
+						},
+					},
+				}
+			}
+
+			return r, nil
+		}}
+
+	r := &Resolver{PreferGo: true, Dial: fake.DialContext}
+
+	conf, err := newResolvConfTest()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conf.teardown()
+
+	err = conf.writeAndUpdate([]string{"nameserver 127.0.0.1"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := r.LookupIPAddr(context.Background(), "notrustad.go.dev"); err != nil {
+		t.Errorf("lookup failed: %v", err)
+	}
+
+	err = conf.writeAndUpdate([]string{"nameserver 127.0.0.1", "options trust-ad"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := r.LookupIPAddr(context.Background(), "trustad.go.dev"); err != nil {
+		t.Errorf("lookup failed: %v", err)
+	}
+}

src/net/dnsconfig.go

@@ -29,6 +29,7 @@ type dnsConfig struct {
 	soffset       uint32        // used by serverOffset
 	singleRequest bool          // use sequential A and AAAA queries instead of parallel queries
 	useTCP        bool          // force usage of TCP for DNS resolutions
+	trustAD       bool          // add AD flag to queries
 }
 
 // serverOffset returns an offset that can be used to determine

src/net/dnsconfig_unix.go

@@ -113,6 +113,8 @@ func dnsReadConfig(filename string) *dnsConfig {
 					// https://www.freebsd.org/cgi/man.cgi?query=resolv.conf&sektion=5&manpath=freebsd-release-ports
 					// https://man.openbsd.org/resolv.conf.5
 					conf.useTCP = true
+				case s == "trust-ad":
+					conf.trustAD = true
 				case s == "edns0":
 					// We use EDNS by default.
 					// Ignore this option.