#include <stdio.h>      // printf
#include <stdlib.h>     // exit
#include <arpa/inet.h>  // inet_ntop
#include <netdb.h>      // gethostbyname

/*

It is a two-step process to connect to an Internet server when we are using a
domain name, since domain names, though understood by humans, are not understood
by the IP protocol through which Internet systems communicate:

  - First we must resolve the domain name to an IP address.
  - Then we may use the IP address to correctly state the destination of our
    data.

Here we focus on the domain name look-up, using gethostbyname().

*/


int main(int argc, char *argv[]) {

  if (argc != 2) {
    printf("\n\nUSAGE: %s <hostname>\n\n", argv[0]);
    exit(1);
  }

  char* hostname = argv[1];
  printf("Looking up the IP address of '%s'\n", hostname);

  // The hostent struct is used to describe domain name information returned
  // from a look-up, which can include multiple addresses for the same domain
  // name.
  struct hostent *hostinfo;

  // Resolve the hostname to an IP address, ultimately via a DNS look-up.  For
  // convenience, this will also accept addresses as strings (e.g.
  // '72.21.210.250') and convert them to proper address structures, allowing
  // users to specify either domain names or addresses in their applications.
  hostinfo = gethostbyname(hostname);

  // Check if the look-up failed.
  if (hostinfo == NULL) {
    fprintf(stderr, "Unknown host %s.\n", hostname);
    exit(1);
  }

  // We are interested inthe h_addr attribute of the returned hostinfo, since
  // this contains the first address.  Note that the OS networking services are
  // generic, so gethostbyname could be used with many other protocols other
  // than the now de facto IPv4, but here we are expecting a 4-byte IPv4 address
  // so cast it appropriately to the in_addr (IPv4 address) structure.
  struct in_addr *ip_address = (struct in_addr *) hostinfo->h_addr;

  // The IP address returned is simply 4 bytes, but often addresses are printed
  // out in the dot notation (e.g. 72.21.210.250), so here we use the function
  // inet_ntop to format our obtained address for display.
  char ip_address_string[256];
  // Flag AF_INET means: format the bytes in the Internet address family style.
  inet_ntop(AF_INET, ip_address, (char *) &ip_address_string, sizeof(ip_address_string));
  printf("'%s' -> %s\n", hostname, ip_address_string);

  return 0;
}