How to reuse CURL and set different doh url at every time in libcurl?

I set the hosts file on PC like this:

127.0.0.1 www.baidu.com

Then I wrote a simple test:

CURL *curl = curl_easy_init();
for (int i = 0; i < 3; ++i) {
    if (curl) {
        curl_easy_setopt(curl, CURLOPT_DNS_CACHE_TIMEOUT, 0L);
        curl_easy_setopt(curl, CURLOPT_URL, "www.baidu.com");
        if (i == 1)
            curl_easy_setopt(curl, CURLOPT_DOH_URL, "https://dns.alidns.com/dns-query");
        else
            curl_easy_setopt(curl, CURLOPT_DOH_URL, NULL);
            
        CURLcode res = curl_easy_perform(curl);
        std::cout << std::endl;
        std::cout << "==================================================" << std::endl;
        std::cout << "Time: " << i << std::endl;
        std::cout << "res = " << res << std::endl;
        char *url = NULL;
        curl_easy_getinfo(curl, CURLINFO_EFFECTIVE_URL, &url);
        if (url)
            printf("Redirect to: %s\n", url);
        std::cout << "==================================================" << std::endl;
        curl_easy_reset(curl);
    }
}

And the result is like:

==================================================
Time: 0
res = 7
Redirect to: http://www.baidu.com/
==================================================
==================================================
Time: 1
res = 0
Redirect to: http://www.baidu.com/
==================================================
==================================================
Time: 2
res = 0
Redirect to: http://www.baidu.com/
==================================================

As defined in CURLcode, r = 0 means CURLE_OK, and r = 7 means CURLE_COULDNT_CONNECT.

I don’t understand the result. When i == 0, I set DOH_URL to NULL, so it is normal that it couldn’t connect. But when i == 2, I also set DOH_URL to NULL, and I do reset operation, and set DNS_CACHE_TIMEOUT to 0, why it still could connect?

>Solution :

curl easy uses the existing connection of the 2nd iteration on the 3rd iteration. See Name Resolving

libcurl tries hard to re-use an existing connection rather than to create a new one. The function that checks for an existing connection to use is based purely on the name and is performed before any name resolving is attempted.

See curl_easy_reset()

It does not change the following information kept in the handle: live connections, the Session ID cache, the DNS cache, the cookies, the shares or the alt-svc cache.