CURLOPT_DNS_CACHE_TIMEOUT (3) - Linux Man Pages
CURLOPT_DNS_CACHE_TIMEOUT: set life-time for DNS cache entries
NAMECURLOPT_DNS_CACHE_TIMEOUT - set life-time for DNS cache entries
DESCRIPTIONPass a long, this sets the timeout in seconds. Name resolves will be kept in memory and used for this number of seconds. Set to zero to completely disable caching, or set to -1 to make the cached entries remain forever. By default, libcurl caches this info for 60 seconds.
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.