#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <netdb.h>
#include <regex.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main(int argc, char *argv[]) {
  int i;
  struct hostent *host;
  struct in_addr **addr_list;
  struct sockaddr_in sin;
  char domain[512];
  regex_t er;

  if (argc != 2) {
    fprintf(stderr,"usage: %s {hostname|ip_address}\n",argv[0]);
    return 1;
  }

  regcomp(&er, "[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}", REG_EXTENDED|REG_NOSUB);
  if ((regexec(&er, argv[1],0,NULL,0))==0) {  // is a IP Address
    sin.sin_addr.s_addr = inet_addr(argv[1]);
    host = gethostbyaddr((char *)&sin.sin_addr.s_addr,sizeof(struct in_addr), AF_INET);
    if (host!=(struct hostent *)0) {
      strcpy(domain,host->h_name);
      printf("%s domain name pointer %s\n", argv[1], domain);
    } else {
      printf("gethostbyaddr failed\n");
    }

  } else { // is a hostname
    if ((host = gethostbyname(argv[1])) != NULL) {
      printf("IP Addresses:\n");
      addr_list = (struct in_addr **)host->h_addr_list;
      for(i = 0; addr_list[i] != NULL; i++)
        printf("%s has address %s\n", argv[1], inet_ntoa(*addr_list[i]));
    } else {
      printf("gethostbyname failed\n");
    }
  }
  return 0;
}