net: use DNS over TCP when use-vc is set in resolv.conf

Fixes #29358

src/net/dnsclient_unix.go

@@ -131,13 +131,19 @@ 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) (dnsmessage.Parser, dnsmessage.Header, error) {
+func (r *Resolver) exchange(ctx context.Context, server string, q dnsmessage.Question, timeout time.Duration, usetcp bool) (dnsmessage.Parser, dnsmessage.Header, error) {
 	q.Class = dnsmessage.ClassINET
 	id, udpReq, tcpReq, err := newRequest(q)
 	if err != nil {
 		return dnsmessage.Parser{}, dnsmessage.Header{}, errCannotMarshalDNSMessage
 	}
-	for _, network := range []string{"udp", "tcp"} {
+	var networks []string
+	if usetcp {
+		networks = []string{"tcp"}
+	} else {
+		networks = []string{"udp", "tcp"}
+	}
+	for _, network := range networks {
 		ctx, cancel := context.WithDeadline(ctx, time.Now().Add(timeout))
 		defer cancel()
 
@@ -241,7 +247,7 @@ 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)
+			p, h, err := r.exchange(ctx, server, q, cfg.timeout, cfg.usetcp)
 			if err != nil {
 				dnsErr := &DNSError{
 					Err:    err.Error(),

src/net/dnsclient_unix_test.go

@@ -81,7 +81,7 @@ 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)
+		_, h, err := r.exchange(ctx, tt.server, tt.question, time.Second, false)
 		if err != nil {
 			t.Error(err)
 			continue
@@ -137,7 +137,7 @@ 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)
+		_, h, err := r.exchange(ctx, server, tt.question, 3*time.Second, false)
 		if err != nil {
 			t.Error(err)
 			continue
@@ -1564,7 +1564,7 @@ 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)
+	_, _, err := r.exchange(ctx, "0.0.0.0", mustQuestion("com.", dnsmessage.TypeALL, dnsmessage.ClassINET), time.Second, false)
 	if err != nil {
 		t.Fatal("exhange failed:", err)
 	}
@@ -1695,3 +1695,30 @@ func TestSingleRequestLookup(t *testing.T) {
 		}
 	}
 }
+
+// Issue 29358. Add configuration knob to force TCP-only DNS requests in the pure Go resolver.
+func TestDNSUseTCP(t *testing.T) {
+	fake := fakeDNSServer{
+		rh: func(n, _ string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {
+			r := dnsmessage.Message{
+				Header: dnsmessage.Header{
+					ID:       q.Header.ID,
+					Response: true,
+					RCode:    dnsmessage.RCodeSuccess,
+				},
+				Questions: q.Questions,
+			}
+			if n == "udp" {
+				t.Fatal("udp protocol was used instead of tcp")
+			}
+			return r, nil
+		},
+	}
+	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, true)
+	if err != nil {
+		t.Fatal("exchange failed:", err)
+	}
+}

src/net/dnsconfig_unix.go

@@ -33,6 +33,7 @@ type dnsConfig struct {
 	mtime         time.Time     // time of resolv.conf modification
 	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
 }
 
 // See resolv.conf(5) on a Linux machine.
@@ -123,6 +124,8 @@ func dnsReadConfig(filename string) *dnsConfig {
 					//  This option disables the behavior and makes glibc
 					//  perform the IPv6 and IPv4 requests sequentially."
 					conf.singleRequest = true
+				case s == "use-vc":
+					conf.usetcp = true
 				default:
 					conf.unknownOpt = true
 				}

src/net/dnsconfig_unix_test.go

@@ -124,6 +124,17 @@ var dnsReadConfigTests = []struct {
 			search:        []string{"domain.local."},
 		},
 	},
+	{
+		name: "testdata/use-vc-resolv.conf",
+		want: &dnsConfig{
+			servers:  defaultNS,
+			ndots:    1,
+			usetcp:   true,
+			timeout:  5 * time.Second,
+			attempts: 2,
+			search:   []string{"domain.local."},
+		},
+	},
 }
 
 func TestDNSReadConfig(t *testing.T) {

src/net/testdata/use-vc-resolv.conf

@@ -0,0 +1 @@
+options use-vc