Sockopt: Handle single-family interface binding on Windows, including TUN inbound's autoOutboundsInterface (#7214)

And https://github.com/XTLS/Xray-core/pull/7214#issuecomment-6329118637

https://github.com/XTLS/Xray-core/pull/7201#issuecomment-6314453444

Fixes https://github.com/XTLS/Xray-core/issues/7160#issuecomment-6246944012

---------

Co-authored-by: patterniha <71074308+patterniha@users.noreply.github.com>

transport/internet/system_dialer.go

@@ -2,6 +2,8 @@ package internet
 
 import (
 	"context"
+	go_errors "errors"
+	"runtime"
 	"sync"
 	"syscall"
 	"time"
@@ -92,6 +94,16 @@ func (d *DefaultSystemDialer) Dial(ctx context.Context, src net.Address, dest ne
 			}
 		}
 		packetConn, err := lc.ListenPacket(ctx, srcAddr.Network(), srcAddr.String())
+		var bindErr *BindError
+		if err != nil && runtime.GOOS == "windows" && go_errors.As(err, &bindErr) {
+			// Keep dual-stack UDP when possible. If the interface cannot bind both
+			// families, retry a socket that cannot send on the unbound family.
+			network := "udp6"
+			if destAddr.IP.To4() != nil {
+				network = "udp4"
+			}
+			packetConn, err = lc.ListenPacket(ctx, network, srcAddr.String())
+		}
 		if err != nil {
 			return nil, err
 		}

transport/internet/system_dialer_windows_test.go

@@ -0,0 +1,63 @@
+package internet_test
+
+import (
+	"context"
+	"errors"
+	"slices"
+	"syscall"
+	"testing"
+
+	"github.com/xtls/xray-core/common/net"
+	"github.com/xtls/xray-core/transport/internet"
+	"golang.org/x/sys/windows"
+)
+
+// TestDialUDPBindFallback checks that a UDP dial whose dual-stack socket
+// cannot be bound to both IP versions, as on an interface without one of
+// them, moves to a socket of the destination's version only, and still
+// fails when that one cannot be bound either.
+func TestDialUDPBindFallback(t *testing.T) {
+	for _, c := range []struct {
+		dest   string
+		reject []string // the kinds of socket that cannot be bound
+		tried  []string // the kinds of socket tried, in order
+		ok     bool
+	}{
+		{"127.0.0.1", nil, []string{"dual"}, true},
+		{"127.0.0.1", []string{"dual"}, []string{"dual", "udp4"}, true},
+		{"::1", []string{"dual"}, []string{"dual", "udp6"}, true},
+		{"127.0.0.1", []string{"dual", "udp4"}, []string{"dual", "udp4"}, false},
+	} {
+		var tried []string
+		internet.ControllersLock.Lock()
+		saved := internet.Controllers
+		internet.Controllers = []func(string, string, syscall.RawConn) error{func(network, _ string, rc syscall.RawConn) error {
+			kind := network
+			if network == "udp6" {
+				rc.Control(func(fd uintptr) {
+					if v6only, _ := windows.GetsockoptInt(windows.Handle(fd), windows.IPPROTO_IPV6, windows.IPV6_V6ONLY); v6only == 0 {
+						kind = "dual"
+					}
+				})
+			}
+			tried = append(tried, kind)
+			if slices.Contains(c.reject, kind) {
+				return internet.NewBindError(errors.New("cannot bind " + kind))
+			}
+			return nil
+		}}
+		internet.ControllersLock.Unlock()
+
+		conn, err := internet.DialSystem(context.Background(), net.UDPDestination(net.ParseAddress(c.dest), 53), nil)
+		if conn != nil {
+			conn.Close()
+		}
+		internet.ControllersLock.Lock()
+		internet.Controllers = saved
+		internet.ControllersLock.Unlock()
+
+		if (err == nil) != c.ok || !slices.Equal(tried, c.tried) {
+			t.Errorf("%s rejecting %v: tried %v, err %v", c.dest, c.reject, tried, err)
+		}
+	}
+}