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.csdn.net/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;
u_short ip_id;
u_short ip_off;
#define IP_RF 0x8000
#define IP_DF 0x4000
#define IP_MF 0x2000
#define IP_OFFMASK 0x1fff
u_char ip_ttl;
u_char ip_p;
u_short ip_sum;
struct in_addr ip_src,ip_dst;
};
/*UDP报头*/
struct sniff_udp
{
u_short udp_sport;
u_short udp_dport;
u_short udp_len;
u_short udp_sum;
};
int main(int argc, char *argv[])
{
/*建立本地5010端口的udp接收端*/
int sockfd2=0;
struct sockaddr_in servaddr2;
sockfd2=socket(AF_INET,SOCK_DGRAM,0);
bzero(&servaddr2,sizeof(servaddr2));
servaddr2.sin_family=AF_INET;
servaddr2.sin_port=htons(5010);
servaddr2.sin_addr.s_addr=htonl(INADDR_ANY) ;
bind(sockfd2, (struct sockaddr *)&servaddr2, sizeof(servaddr2));

char errBuf[100];
libnet_t *lib_net = NULL;
libnet_ptag_t lib_t1 = 0;
libnet_ptag_t lib_t2 = 0;
libnet_ptag_t lib_t3 = 0;
unsigned char src_mac[6] = {0xb8,0xae,0xed,0x22,0x6c,0xa4};//发送者网卡地址00:0c:29:97:c7:c1
unsigned char dst_mac[6] = {0xb8,0xae,0xed,0x23,0x3a,0xf0};//接收者网卡地址‎b8:ae:ed:23:3a:f0
lib_net = libnet_init(LIBNET_LINK_ADV, argv[1], errBuf); //初始化，选择基于链路层LIBNET_LINK_ADV的发送方式
if(NULL == lib_net)
{
perror("libnet_init");
exit(-1);
}

unsigned char buffer[1500]; //设定缓冲区大小，考虑MTU1500
struct sockaddr_in addr;
socklen_t addr_len =sizeof(struct sockaddr_in);
int len=0;
while(1){
bzero(buffer,sizeof(buffer));
len = recvfrom(sockfd2,buffer,sizeof(buffer), 0 , (struct sockaddr *)&addr ,&addr_len);//收到数据存放在buffer里（IP包）
printf("receive %s: len:%d\n",sock_ntop((struct sockaddr *)&addr,addr_len),len);

struct sniff_ip *ip;//ip包头
struct sniff_udp *udp;//udp包头
u_char *payload;//数据包负载的数据
int payload_size;//数据包负载的数据大小
ip=(struct sniff_ip*)(buffer);//指向收到的数据，开始按照IP头处理数据
payload=(u_char *)(buffer+sizeof(struct sniff_ip)+sizeof(struct sniff_udp));//取出负载
payload_size=ntohs(udp->udp_len)-sizeof(struct sniff_udp);
printf("len:%d,srcport:%d,dstport:%d\n\n",payload_size,ntohs(udp->udp_sport),ntohs(udp->udp_dport));
lib_t1 = libnet_build_udp( //构造udp数据包
ntohs(udp->udp_sport),
ntohs(udp->udp_dport),
8+payload_size,
0,
payload,
payload_size,
lib_net,
lib_t1
);

lib_t2 = libnet_build_ipv4( //构造ip数据包
20+8+payload_size,
ip->ip_tos,
ntohs(ip->ip_id),
ntohs(ip->ip_off),
ip->ip_ttl,
ip->ip_p,
0,
*(u_int32_t *)&ip->ip_src,
*(u_int32_t *)&ip->ip_dst,
NULL,
0,
lib_net,
lib_t2
);

lib_t3 = libnet_build_ethernet( //构造以太网数据包
(u_int8_t *)dst_mac,
(u_int8_t *)src_mac,
0x800, // 或者，ETHERTYPE_IP
NULL,
0,
lib_net,
lib_t3
);
int res = 0;
res = libnet_write(lib_net); //发送数据包
if(-1 == res)
{
perror("libnet_write");
exit(-1);
}
}

libnet_destroy(lib_net); //销毁资源

printf("----ok-----\n");
return 0;
}