CURLcode curl_easy_setopt(CURL *handle, CURLOPT_DNS_CACHE_TIMEOUT, long age);
The name resolve functions of various libc implementations don't re-read name server information unless explicitly told so (for example, by calling res_init(3)). This may cause libcurl to keep using the older server even if DHCP has updated the server info, and this may look like a DNS cache issue to the casual libcurl-app user.
Note that DNS entries have a "TTL" property but libcurl doesn't use that. This DNS cache timeout is entirely speculative that a name will resolve to the same address for a certain small amount of time into the future.
CURL *curl = curl_easy_init(); if(curl) { curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/foo.bin"); /* only reuse addresses for a very short time */ curl_easy_setopt(curl, CURLOPT_DNS_CACHE_TIMEOUT, 2L); ret = curl_easy_perform(curl); /* in this second request, the cache will not be used if more than two seconds have passed since the previous name resolve */ ret = curl_easy_perform(curl); curl_easy_cleanup(curl); }