unsigned short hard_header_len;unsigned short type; hard_header_len是硬件头的长度,在以太网设备的初始化函数中,该成员被赋为ETH_HLEN,即以太网头的长度,该值为14,下面是以太网头的定义:struct ethhdr { unsigned char h_dest[ETH_ALEN]; /* destination eth addr */ unsigned char h_source[ETH_ALEN]; /* source ether addr */ unsigned short h_proto; /* packet type ID field */} __attribute__((packed)); ETH_ALEN被定义为6,即以太网MAC地址的长度。h_proto保存type的值。type是接口的硬件类型,以太网设备的初始化函数中将其赋值为ARPHRD_ETHER,即10Mb以太网。