In this link at the last it mentions
http://comments.gmane.org/gmane.comp.web.curl.library/40895
libcurl uses its DNS cache by default as long as you re-use the handle. You can change the time it'll hold entries in the cache - it is only meant to aid "spikes" or rapid requests to the same host names as it doesn't get the "true" TTL values.
I have gone through all the documentation of curl http://curl.haxx.se/docs/manpage.html but I could not find any way to change the time (TTL) curl hold entries in the cache.
How to set the curl cache item holding-time & how to clear the curl cache?