说明：  综合应用libnet和libpcap的简单例程，其功能是在接收到一个来自特定主机的ARP请求报文之后，发出ARP回应报文，通知该主机请求的IP地址对应的MAC地址.该程序利用了libpcap在数据链路层抓包，利用了libnet向数据链路层发包
(Integrated application libnet and libpcap simple routines, its function is to receive a request from a particular host s ARP packets, the ARP response packet sent to notify the host requests IP address of the corresponding MAC address. The program utilizes Ethereal libpcap data link layer, using the data link layer to the contracting libnet)