数据链路层sniffer（二）sniffer实例

实例

下面的代码创建一个直接读取链路层数据包的原始套接字,并从中分析出源MAC地址和目的MAC地址,源IP和目的IP,以及对应的传输层协议,如果是TCP/UDP协议的话,打印其目的和源端口.为了方便阅读,程序中避免了使用任何与协议有关的数据结构,如
struct ether_header ,struct iphdr 等

其中需要掌握数据链路层、网络层、传输层数据包头格式，格式如下：

• MAC帧格式
  
• ip头格式

• TCP头格式

• UDP头格式
  
• 代码如下

/***************SimpelSniffer.c*************/
#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <linux/if_ether.h>
#include <linux/in.h>
#define BUFFER_MAX 2048
int main(int argc, char *argv[])
{

        int sock, n_read, proto;
        char buffer[BUFFER_MAX];
        char  *ethhead, *iphead, *tcphead,
                         *udphead, *icmphead, *p;

if((sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_IP))) < 0)
    {
        fprintf(stdout, "create socket error\n");
        exit(0);
    }

while(1)
{
     n_read = recvfrom(sock, buffer, 2048, 0, NULL, NULL);
        /*
        14   6(dest)+6(source)+2(type or length)
        +
        20   ip header
        +
        8   icmp,tcp or udp header
        = 42
        */
if(n_read < 42)
   {
      fprintf(stdout, "Incomplete header, packet corrupt\n");
      continue;
   }

        ethhead = buffer;
        p = ethhead;
        int n = 0XFF;
                printf("MAC: %.2X:%02X:%02X:%02X:%02X:%02X==>"
                           "%.2X:%.2X:%.2X:%.2X:%.2X:%.2X,",
        p[6]&n, p[7]&n, p[8]&n, p[9]&n, p[10]&n, p[11]&n,
        p[0]&n, p[1]&n, p[2]&n,p[3]&n, p[4]&n, p[5]&n);

                   iphead = ethhead + 14;
                   p = iphead + 12;

           printf("IP: %d.%d.%d.%d => %d.%d.%d.%d,",
           p[0]&0XFF, p[1]&0XFF, p[2]&0XFF, p[3]&0XFF,
           p[4]&0XFF, p[5]&0XFF, p[6]&0XFF, p[7]&0XFF);
            proto = (iphead + 9)[0];
            p = iphead + 20;
             printf("Protocol: ");
            switch(proto)
              {
                case IPPROTO_ICMP: printf("ICMP\n");break;
                case IPPROTO_IGMP: printf("IGMP\n");break;
                case IPPROTO_IPIP: printf("IPIP\n");break;
                case IPPROTO_TCP :
                case IPPROTO_UDP :
    printf("%s,", proto == IPPROTO_TCP ? "TCP": "UDP");
    printf("source port: %u,",(p[0]<<8)&0XFF00 |  p[1]&0XFF);
    printf("dest port: %u\n", (p[2]<<8)&0XFF00 | p[3]&0XFF);
         break;
    case IPPROTO_RAW : printf("RAW\n");break;
    default:printf("Unkown, please query in include/linux/in.h/n");
        }
   }
}