#include <stdio.h>

 #include <stdlib.h>

 #include <pcap.h>

 #include <errno.h>

 #include <sys/socket.h>

 #include <netinet/in.h>

 #include <arpa/inet.h>

 int main(int argc, char **argv)

 {

   char *dev;

   char *net;

   char *mask;

   int ret;

   char errbuf[PCAP_ERRBUF_SIZE];

   bpf_u_int32 netp;

   bpf_u_int32 maskp;

   struct in_addr addr;

   dev = pcap_lookupdev(errbuf);

   if(dev == NULL)

   {

    printf("%s\n",errbuf);

    exit();

   }

   printf("DEV: %s\n",dev);

   ret = pcap_lookupnet(dev,&netp,&maskp,errbuf);

   if(ret == -)

   {

    printf("%s\n",errbuf);

    exit();

   }

   addr.s_addr = netp;

   net = inet_ntoa(addr);

   if(net == NULL)

   {

     perror("inet_ntoa");

     exit();

   }

   printf("NET: %s\n",net);

   addr.s_addr = maskp;

   mask = inet_ntoa(addr);

   if(mask == NULL)

   {

     perror("inet_ntoa");

     exit();

   }

   printf("MASK: %s\n",mask);

   return ;

 }

 然后gcc -o pcap_1 pcap_1.c -lpcap（一定要-lpcap参数）

 编译ok~，执行./pcap_1，可以看到：

 DEV: eth0

 NET: 192.168.12.0

 MASK: 255.255.255.0

 好了，第一个pcap程序出炉了。。。。。

 但是（当然有但是了，要不然我后面写啥），上面那个程序除了向我们展现pcap_lookupdev和pcap_lookupnet之外什么都没有干，好，我们接着来，动手编写我们的第一个抓包程序。

 #include <stdio.h>

 #include <stdlib.h>

 #include <pcap.h>

 #include <errno.h>

 #include <sys/socket.h>

 #include <netinet/in.h>

 #include <arpa/inet.h>

 #include <netinet/if_ether.h>

 int main(int argc, char **argv)

 {

     int i;

     char *dev;

     char errbuf[PCAP_ERRBUF_SIZE];

     pcap_t* descr;

     const u_char *packet;

     struct pcap_pkthdr hdr;

     struct ether_header *eptr; 

     u_char *ptr;

     dev = pcap_lookupdev(errbuf);

     if(dev == NULL)

     {

         printf("%s\n",errbuf);

         exit();

     }

     printf("DEV: %s\n",dev);

     descr = pcap_open_live(dev,BUFSIZ,1,0,errbuf);

     if(descr == NULL)

     {

         printf("pcap_open_live(): %s\n",errbuf);

         exit();

     }

     packet = pcap_next(descr,&hdr);

     if(packet == NULL)

     {

         printf("Didn't grab packet\n");

         exit();

     }

     printf("Grabbed packet of length %d\n",hdr.len);

     printf("Recieved at ..... %s\n",ctime((const time_t*)&hdr.ts.tv_sec));

     printf("Ethernet address length is %d\n",ETHER_HDR_LEN);

     eptr = (struct ether_header *) packet;

     if (ntohs (eptr->ether_type) == ETHERTYPE_IP)

     {

         printf("Ethernet type hex:%x dec:%d is an IP packet\n",

                 ntohs(eptr->ether_type),

                 ntohs(eptr->ether_type));

     }else  if (ntohs (eptr->ether_type) == ETHERTYPE_ARP)

     {

         printf("Ethernet type hex:%x dec:%d is an ARP packet\n",

                 ntohs(eptr->ether_type),

                 ntohs(eptr->ether_type));

     }else {

         printf("Ethernet type %x not IP", ntohs(eptr->ether_type));

         exit();

     }

     ptr = eptr->ether_dhost;

     i = ETHER_ADDR_LEN;

     printf(" Destination Address:  ");

     do{

         printf("%s%x",(i == ETHER_ADDR_LEN) ? " " : ":",*ptr++);

     }while(--i>);

     printf("\n");

     ptr = eptr->ether_shost;

     i = ETHER_ADDR_LEN;

     printf(" Source Address:  ");

     do{

         printf("%s%x",(i == ETHER_ADDR_LEN) ? " " : ":",*ptr++);

     }while(--i>);

     printf("\n");

     return ;

 }

 好了,编译运行!

 [root@norman libpcap]# ./pcap_2

 DEV: eth0

 Grabbed packet of length

 Recieved at time..... Mon Mar  :: 

 Ethernet address length is

 Ethernet type hex: dec: is an IP packet

 Destination Address:   :::d1:e8:

 Source Address:   :a0:cc::c2:

 [root@pepe libpcap]#

 可能有人等了半天都没有一个包过来，有个好办法，再开一个控制台，ping一下某个网站，比如google~~，呵呵

 马上就有反应了~~

 这个程序是一个老外写的，大家看看注释应该没有问题吧~

 但是大家也发现了一个问题，就是上面的程序只能捕捉一个包，要不停的捕捉包怎么办，用循环？？libpcap提供了一个更好的方法：

 int pcap_loop(pcap_t *p, int cnt, pcap_handler callback, u_char *user);

 这个函数能够不停的捕捉以太网的包，cnt就是捕捉的次数，callback是处理函数，这个处理函数怎么写，看看pcap_3.c就知道了。user参数是干什么的？不要问我，我也不知道。

 #include <pcap.h>

 #include <stdio.h>

 #include <stdlib.h>

 #include <errno.h>

 #include <sys/socket.h>

 #include <netinet/in.h>

 #include <arpa/inet.h>

 #include <netinet/if_ether.h>

 void my_callback(u_char *useless,const struct pcap_pkthdr* pkthdr,const u_char*

         packet)

 {

     static int count = ;

     fprintf(stdout,"%d, ",count);

     if(count == )

         fprintf(stdout,"Come on baby sayyy you love me!!! ");

     if(count == )

         fprintf(stdout,"Tiiimmmeesss!! ");

     fflush(stdout);

     count++;

 }

 int main(int argc,char **argv)

 {

     int i;

     char *dev;

     char errbuf[PCAP_ERRBUF_SIZE];

     pcap_t* descr;

     const u_char *packet;

     struct pcap_pkthdr hdr;

     struct ether_header *eptr; 

     if(argc != ){ fprintf(stdout,"Usage: %s numpackets\n",argv[]);return ;}

     dev = pcap_lookupdev(errbuf);

     if(dev == NULL)

     { printf("%s\n",errbuf); exit(); }

     descr = pcap_open_live(dev,BUFSIZ,,-,errbuf);

     if(descr == NULL)

     { printf("pcap_open_live(): %s\n",errbuf); exit(); }

     pcap_loop(descr,atoi(argv[]),my_callback,NULL);

     fprintf(stdout,"\nDone processing packets... wheew!\n");

     return ;

 }

 运行./pcap_3

 , , , , Come on baby sayyy you love me!!! , , , Tiiimmmeesss!!

 Done processing packets... wheew!

 pcap_loop确实很好用,但是如果没有包包过来,只有干等在那里,pcap_dispatch就含有一个超时的功能，下面是man里面的一段话：

 pcap_dispatch() is used to collect and process packets. cnt specifies the maximum number of packets to process before returning. A cnt of - processes all the packets received in one buffer. A cnt of  processes all packets until an error occurs, EOF is reached, or the read times out (when doing live reads and a non-zero read timeout is specified). callback specifies a routine to be called with three arguments: a u_char pointer which is passed in from pcap_dispatch(), a pointer to the pcap_pkthdr struct (which precede the actual network headers and data), and a u_char pointer to the packet data. The number of packets read is returned. Zero is returned when EOF is reached in a ``savefile.'' A return of - indicates an error in which case pcap_perror() or pcap_geterr() may be used to display the error text. 

 另外的问题是，我们可能对抓取的包包太多而很头痛，可能很多都不是我们感兴趣的包，别急，pcap_compile和pcap_setfilter能帮我们解决问题。

 #include <pcap.h>

 #include <stdio.h>

 #include <stdlib.h>

 #include <errno.h>

 #include <sys/socket.h>

 #include <netinet/in.h>

 #include <arpa/inet.h>

 #include <netinet/if_ether.h>

 void my_callback(u_char *useless,const struct pcap_pkthdr* pkthdr,const u_char*

         packet)

 {

     static int count = ;

     fprintf(stdout,"%d, ",count);

     fflush(stdout);

     count++;

 }

 int main(int argc,char **argv)

 {

     int i;

     char *dev;

     char errbuf[PCAP_ERRBUF_SIZE];

     pcap_t* descr;

     const u_char *packet;

     struct pcap_pkthdr hdr;

     struct ether_header *eptr;

     struct bpf_program fp;

     bpf_u_int32 maskp;

     bpf_u_int32 netp;          

     if(argc != ){ fprintf(stdout,"Usage: %s \"filter program\"\n"

             ,argv[]);return ;}

     dev = pcap_lookupdev(errbuf);

     if(dev == NULL)

     { fprintf(stderr,"%s\n",errbuf); exit(); }

     pcap_lookupnet(dev,&netp,&maskp,errbuf);

     descr = pcap_open_live(dev,BUFSIZ,,-,errbuf);

     if(descr == NULL)

     { printf("pcap_open_live(): %s\n",errbuf); exit(); }

     if(pcap_compile(descr,&fp,argv[],,netp) == -)

     { fprintf(stderr,"Error calling pcap_compile\n"); exit(); }

     if(pcap_setfilter(descr,&fp) == -)

     { fprintf(stderr,"Error setting filter\n"); exit(); }

     pcap_loop(descr,-,my_callback,NULL);

     return ;

 }

 运行./pcap_4.c "host www.google.com"

 然后在另外一个控制台下面ping www.baidu.com

 哈哈

 没有反应吧

 接着再ping www.google.com

 就看到1, , , , , ,

 ok

 you got it!!