CURLOPT_CONNECT_ONLY (3) - Linux Man Pages
CURLOPT_CONNECT_ONLY: stop when connected to target server
NAMECURLOPT_CONNECT_ONLY - stop when connected to target server
DESCRIPTIONPass a long. If the parameter equals 1, it tells the library to perform all the required proxy authentication and connection setup, but no data transfer, and then return.
The option can be used to simply test a connection to a server, but is more useful when used with the CURLINFO_LASTSOCKET option to curl_easy_getinfo(3) as the library can set up the connection and then the application can obtain the most recently used socket for special data transfers.