网络通信编程笔记（三）：libnet库使用

非常有价值的参考资料：

http://blog.youkuaiyun.com/lianghe_work/article/details/45190379

http://www.cnblogs.com/hnrainll/archive/2011/09/21/2183940.html

接收IP包，封装成udp包，打上MAC帧头从网卡发出去的程序

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <libnet.h>
char * sock_ntop(const struct sockaddr* sa,socklen_t salen) {
	char portstr[8];
	static char str[128];
	struct sockaddr_in *sin=(struct sockaddr_in*)sa;
	if(inet_ntop(AF_INET,&sin->sin_addr,str,sizeof(str))==NULL) return NULL;
	if(ntohs(sin->sin_port)!=0) {
		snprintf(portstr,sizeof(portstr),":%d",ntohs(sin->sin_port));
		strcat(str,portstr);
	}
	return str;
}
/*以太网头*/
struct sniff_ethernet
{
        u_char ether_dhost[ETHER_ADDR_LEN];
        u_char ether_shost[ETHER_ADDR_LEN];
        u_short ether_type;
};
/*IP头*/
struct sniff_ip
{
        u_char ip_vhl;
        u_char ip_tos;
        u_short ip_len;