Programming

void got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet)
{
       printf("got packet\n);
}

int main()
{
 int ret = 0;
 char *dev = NULL;   /* capture device name */
 char errbuf[PCAP_ERRBUF_SIZE];  /* error buffer */
 pcap_t *handle;    /* packet capture handle */

 char filter_exp[] = "udp dst port 1500";  /* filter expression */
 struct bpf_program fp;   /* compiled filter program (expression) */
 bpf_u_int32 mask;   /* subnet mask */
 bpf_u_int32 net;   /* ip */
 int num_packets = 10;   /* number of packets to capture */


 /* get network number and mask associated with capture device */
 if (pcap_lookupnet(dev, &net, &mask, errbuf) == -1) {
  fprintf(stderr, "Couldn't get netmask for device %s: %s\n",
      dev, errbuf);
  net = 0;
  mask = 0;
 }


 /* print capture info */
 printf("Device: %s\n", dev);
 printf("Number of packets: %d\n", num_packets);
 printf("Filter expression: %s\n", filter_exp);




 /* open capture device */
 handle = pcap_open_live(dev, SNAP_LEN, 1, 1000, errbuf);
 if (handle == NULL) {
  fprintf(stderr, "Couldn't open device %s: %s\n", dev, errbuf);
  exit(EXIT_FAILURE);
 }



 /* compile the filter expression */
 if (pcap_compile(handle, &fp, filter_exp, 0, net) == -1) {
  fprintf(stderr, "Couldn't parse filter %s: %s\n",
      filter_exp, pcap_geterr(handle));
  exit(EXIT_FAILURE);
 }

 /* apply the compiled filter */
 if (pcap_setfilter(handle, &fp) == -1) {
  fprintf(stderr, "Couldn't install filter %s: %s\n",
      filter_exp, pcap_geterr(handle));
  exit(EXIT_FAILURE);
 }

 /* now we can set our callback function */
 pcap_loop(handle, num_packets, got_packet, NULL);

 /* cleanup */
 pcap_freecode(&fp);
 pcap_close(handle);
}