DNS data not refreshed after CURLOPT_DNS_CACHE_TIMEOUT

Hi,

I am running a test application to periodically download a file from the
server.
Here I am creating an curl easy handle at once and using the same for all
the future downloads.
If I set CURLOPT_DNS_CACHE_TIMEOUT to 2 sec, I expect the application to
contact the DNS
server and resolve the name every 2 seconds. But I see that DNS requests
are sent only once for the
first time and never thereafter.

Curl version info:
curl-7.72.0.
c-ares is not used for test. (Even when the c-ares is used, result is the
same)
operating system: CentOS - 7

I check the DNS requests using tcpdump:
tcpdump -i interface port 53

Could someone explain if I am missing any thing?

Below is my code:
#include "curl.h"
#include <unistd.h>
#include <stdio.h>

static size_t cb(void *data, size_t size, size_t nmemb, void *userp)
{
  printf("Got data :%u\n",size);
  size_t realsize = size * nmemb;
  return realsize;
}

static void download(CURL* curl,int i)
{
  if(curl) {
    CURLcode res;
    curl_easy_setopt(curl, CURLOPT_DNS_CACHE_TIMEOUT,2);
    curl_easy_setopt(curl, CURLOPT_URL, "google.com");

    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, cb);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *)0);

    // curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);

    res = curl_easy_perform(curl);
    if (res== CURLE_OK)
    {
      printf("(%u) Curl get success\n",i);
    }
    else
    {
      printf("(%u) Curl get failed\n",i);
    }
  }
  printf("_______________________\n");
}

static void get(CURL* curl, int loop, int pause)
{
  int i;
  for(i=0; i<loop; i++)
  {
    download(curl,i);
    sleep(pause);
  }
}

int main()
{
  curl_global_init(CURL_GLOBAL_DEFAULT);
  CURL *curl = curl_easy_init();
  get(curl, 100, 1);
  curl_easy_cleanup(curl);
}

Thanks,
Narasimha

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Received on 2020-08-29