windows平台下选择外网网卡程序，使用了winpcap，特别在一个主机多个网卡，上网方式，或者存在虚拟网卡得情况下判断真实外网网卡特别有用。-windows platform choice of network card, the use of winpcap. especially in a number of mainframe card, Internet, Virtual LAN or the existence of a real judge outside network car