UDP Socket 소켓 프로그래밍 질문~!! (포트 열기)

#include <pcap.h>
#include <net/ethernet.h>
#include <netinet/in.h>
#include <netinet/ip.h>
#include <netinet/udp.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
 
#define PROMISCUOUS 1
#define NONPROMISCUOUS 0
 
void callback(u_char *useless, const struct pcap_pkthdr *pkthdr,
    const u_char *packet)
{
    struct ether_header *ep;
    struct iphdr *iph;
    struct udphdr *udph;
    unsigned short ether_type;
    int chcnt = 0;
    int len;
    int i;
 
    // Get Ethernet header.
    ep = (struct ether_header *)packet;
    // Get network layer protocol type.
    ether_type = ntohs(ep->ether_type);
 
    if (ether_type == ETHERTYPE_IP) {
        // Move packet pointer for upper protocol header.
        packet += sizeof(struct ether_header);        
        // Get IP header.
        iph = (struct iphdr *)packet;
 
        // Get transport layer protocol type.
        if (iph->protocol == 17) {
            packet += sizeof(struct iphdr);
            udph = (struct udphdr *)packet;
            len = ntohs(udph->len)-sizeof(struct udphdr);
 
            // Print UDP header infomation.
            printf("UDP packet, source port = %d, dest port = %d, len = %d\n",
                    ntohs(udph->source), ntohs(udph->dest), len);
            // Print Data.
            printf("< Data >\n");
            packet += sizeof(struct udphdr);
            for (i=0; i<len; ++i) {
                printf("%.2X ", packet[i]);
                if ((i % 16) == 15) 
                    printf("\n");
            }
            printf("\n");
        }
     }
}
 
int main(int argc, char **argv)
{
  char *dev;
  char *net;
  char *mask;
  char errbuf[PCAP_ERRBUF_SIZE];
  struct bpf_program fp;
  pcap_t *pcd; // packet caputre descriptor.
  bpf_u_int32 netp;
  bpf_u_int32 maskp;
  struct in_addr net_addr, mask_addr;
 
  dev = pcap_lookupdev(errbuf);
  if (dev == NULL)
  {
      printf("%s\n", errbuf);
      exit(1);
  }
  printf("DEV : %s\n", dev);
 
  // Get netmask
  if (pcap_lookupnet(dev, &netp, &maskp, errbuf) == -1) {
    fprintf(stderr, "%s\n", errbuf);
    return 1;
  }
 
  net_addr.s_addr = netp;
  net = inet_ntoa(net_addr);
  printf("NET : %s\n", net);
  mask_addr.s_addr = maskp;
  mask = inet_ntoa(mask_addr);
  printf("MASK : %s\n", mask);
 
  // Get packet capture descriptor.
  pcd = pcap_open_live(dev, BUFSIZ, NONPROMISCUOUS, -1, errbuf);
  if (pcd == NULL) {
    fprintf(stderr, "%s\n", errbuf);
    return 1;
  }
 
  // Set compile option.
  if (pcap_compile(pcd, &fp, "udp", 0, netp) == -1) {
    fprintf(stderr, "compile error\n");
    return 1; }
 
  // Set packet filter role by compile option.
  if (pcap_setfilter(pcd, &fp) == -1) {
    fprintf(stderr, "set filter error\n");
    return 1;
  }
 
  // Capture packet. When packet captured, call callback function.
  pcap_loop(pcd, 0, callback, NULL);
  return 0;
}