This example reproduces an issue with curl 7.32 or older: when adding an easy handle which sets CURLOPT_RESOLVE to a multi handle curl will sometimes consider the entry expired and check the default DNS servers instead.

curl_resolve_test.c

#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <unistd.h>
#include <curl/curl.h>

size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata)
{
  return size * nmemb;
}

int debug_callback(CURL *curl, curl_infotype info, char *msg, size_t len, void *ptr)
{
  if (info == CURLINFO_TEXT)
    fprintf(stderr, "debug: %.*s", (int) len, msg);
  
  return 0;
}

int do_request(CURLM *multi_handle, char const *url, char const *resolve)
{
  CURL *handle;

  int still_running;
  int i;
  int result = 0;
  
  struct curl_slist *resolve_list = NULL;

  CURLMsg *msg;
  int msgs_left;

  handle = curl_easy_init();
    
  resolve_list = curl_slist_append(resolve_list, resolve);

  curl_easy_setopt(handle, CURLOPT_URL, url);
  curl_easy_setopt(handle, CURLOPT_WRITEFUNCTION, write_callback);
  curl_easy_setopt(handle, CURLOPT_RESOLVE, resolve_list);
  curl_easy_setopt(handle, CURLOPT_DEBUGFUNCTION, debug_callback);
  curl_easy_setopt(handle, CURLOPT_VERBOSE, 1);
  curl_easy_setopt(handle, CURLOPT_DNS_CACHE_TIMEOUT, 2);

  curl_multi_add_handle(multi_handle, handle);
  curl_multi_perform(multi_handle, &still_running);

  do {
    struct timeval timeout;
    int rc; /* select() return code */

    fd_set fdread;
    fd_set fdwrite;
    fd_set fdexcep;
    int maxfd = -1;

    long curl_timeo = -1;

    FD_ZERO(&fdread);
    FD_ZERO(&fdwrite);
    FD_ZERO(&fdexcep);

    /* set a suitable timeout to play around with */
    timeout.tv_sec = 1;
    timeout.tv_usec = 0;

    curl_multi_timeout(multi_handle, &curl_timeo);
    if(curl_timeo >= 0) {
      timeout.tv_sec = curl_timeo / 1000;
      if(timeout.tv_sec > 1)
        timeout.tv_sec = 1;
      else
        timeout.tv_usec = (curl_timeo % 1000) * 1000;
    }

    /* get file descriptors from the transfers */
    curl_multi_fdset(multi_handle, &fdread, &fdwrite, &fdexcep, &maxfd);

    /* In a real-world program you OF COURSE check the return code of the
       function calls.  On success, the value of maxfd is guaranteed to be
       greater or equal than -1.  We call select(maxfd + 1, ...), specially in
       case of (maxfd == -1), we call select(0, ...), which is basically equal
       to sleep. */

    rc = select(maxfd+1, &fdread, &fdwrite, &fdexcep, &timeout);

    switch(rc) {
    case -1:
      /* select error */
      break;
    case 0: /* timeout */
    default: /* action */
      curl_multi_perform(multi_handle, &still_running);
      break;
    }
  } while(still_running);

  while ((msg = curl_multi_info_read(multi_handle, &msgs_left))) {
    if (msg->msg == CURLMSG_DONE && msg->easy_handle == handle) {
      result = msg->data.result;
      printf("HTTP transfer completed with status %d\n", result);
    }
  }

  curl_multi_remove_handle(multi_handle, handle);
  curl_easy_cleanup(handle);
  
  curl_slist_free_all(resolve_list);

  return result;
}

int main(void)
{
  int i;
  CURLM *multi;
  char const *url = "http://fake.host/";
  char const *resolve = "fake.host:80:137.56.161.173";

  multi = curl_multi_init();

  for (i = 0; i < 30; i++)
  {
    if (do_request(multi, url, resolve) == CURLE_COULDNT_RESOLVE_HOST)
      break;
    /* if you remove the following call the problem apparently goes away */
    sleep(2);
  }
  
  curl_multi_cleanup(multi);

  return 0;
}