net: add Resolver.PreferTCP test + GODEBUG=netdns=go,tcp doc

Author: jfbus

src/net/dnsclient_unix_test.go

@@ -1621,3 +1621,30 @@ func TestTXTRecordTwoStrings(t *testing.T) {
 		t.Errorf("txt[1], got %q, want %q", txt[1], want)
 	}
 }
+
+// Issue 29358. Add configuration knob to force TCP-only DNS requests in the pure Go resolver.
+func TestPreferTCP(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, PreferTCP: 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)
+	if err != nil {
+		t.Fatal("exchange failed:", err)
+	}
+}

src/net/net.go

@@ -66,6 +66,9 @@ GODEBUG environment variable (see package runtime) to go or cgo, as in:
 The decision can also be forced while building the Go source tree
 by setting the netgo or netcgo build tag.
 
+The pure Go resolver can be configured to use only TCP to communicate
+by using the tcp option, as in GODEBUG=netdns=go,tcp.
+
 A numeric netdns setting, as in GODEBUG=netdns=1, causes the resolver
 to print debugging information about its decisions.
 To force a particular resolver while also printing debugging information,