Sockopt: Fix Windows Multicast `interface` bind (#4568)

https://github.com/XTLS/Xray-core/pull/4568#issuecomment-2763492336

transport/internet/sockopt_windows.go

@@ -13,6 +13,8 @@ const (
 	TCP_FASTOPEN    = 15
 	IP_UNICAST_IF   = 31
 	IPV6_UNICAST_IF = 31
+	IP_MULTICAST_IF = 9
+	IPV6_MULTICAST_IF = 9
 )
 
 func setTFO(fd syscall.Handle, tfo int) error {
@@ -41,10 +43,16 @@ func applyOutboundSocketOptions(network string, address string, fd uintptr, conf
 			if err := syscall.SetsockoptInt(syscall.Handle(fd), syscall.IPPROTO_IP, IP_UNICAST_IF, int(idx)); err != nil {
 				return errors.New("failed to set IP_UNICAST_IF").Base(err)
 			}
+			if err := syscall.SetsockoptInt(syscall.Handle(fd), syscall.IPPROTO_IP, IP_MULTICAST_IF, int(idx)); err != nil {
+				return errors.New("failed to set IP_MULTICAST_IF").Base(err)
+			}
 		} else {
 			if err := syscall.SetsockoptInt(syscall.Handle(fd), syscall.IPPROTO_IPV6, IPV6_UNICAST_IF, inf.Index); err != nil {
 				return errors.New("failed to set IPV6_UNICAST_IF").Base(err)
 			}
+			if err := syscall.SetsockoptInt(syscall.Handle(fd), syscall.IPPROTO_IPV6, IPV6_MULTICAST_IF, inf.Index); err != nil {
+				return errors.New("failed to set IPV6_MULTICAST_IF").Base(err)
+			}
 		}
 	}