nslookup.c

UNIX/Linux环境下的使用gethostbyname()实现域名解析的代码。

#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>
#include <arpa/inet.h>

extern int h_errno;

int main(int argc, char **argv)
{
  if (argc < 2)
  {
    fprintf(stdout, "Usage: %s <domain name/ip>\n", argv[0]);
    exit(1);
  }

  struct hostent *h;

  //struct hostent *gethostbyname(const char *name);
  h = gethostbyname(argv[1]);

  /*
   * The gethostbyname() and gethostbyaddr() functions return the hostent structure or a NULL pointer  if  an
   * error  occurs.   On  error, the h_errno variable holds an error number.  When non-NULL, the return value
   * may point at static data, see the Notes below.
   */

  if (h == NULL)
  {
    //fprintf(stderr, "gethostbyname() failed: %s\n", strerror(h_errno));
    switch (h_errno)
    {
      case HOST_NOT_FOUND:
	fprintf(stderr, "The specified host is unknown.\n");
	break;
      case NO_ADDRESS:		//NO_DATA:
	fprintf(stderr, "The requested name is valid but does not have an IP address.\n");
	break;
      case NO_RECOVERY:
	fprintf(stderr, "A non - recoverable name server error occurred.\n");
	break;
      case TRY_AGAIN:
	fprintf(stderr, "A temporary error occurred on an authoritative name server.Try again later.\n");
	break;
      default:
	break;
    }
  }

  fprintf(stdout, "gethostbyname() successed.\n");

#if 0
  struct hostent
  {
    char *h_name;		/* official name of host */
    char **h_aliases;		/* alias list */
    int h_addrtype;		/* host address type */
    int h_length;		/* length of address */
    char **h_addr_list;		/* list of addresses */
  }

#  define h_addr  h_addr_list[0]	/* for backward compatibility */
#endif

  fprintf(stdout, "official name: %s\n", h->h_name);

  // XXX: output h_aliases
  char **p;

  p = h->h_aliases;

  while (p && *p)
  {
    fprintf(stdout, "alias: %s\n", *p);
    p++;
  }

  fprintf(stdout, "host address type: %d(AF_INET: %d, AF_INET6: %d)\n", h->h_addrtype, AF_INET, AF_INET6);
  fprintf(stdout, "address length: %d\n", h->h_length);

  // XXX: output h_addr_list
  p = h->h_addr_list;

  while (p && *p)
  {
    fprintf(stdout, "Address: %s\n", inet_ntoa(*(struct in_addr *) *p));

    p++;
  }

  return 0;
}