CURLOPT_LOCALPORT − set local port number to use for socket
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_LOCALPORT, long port);
Pass a long. This sets the local port number of the socket used for the connection. This can be used in combination with CURLOPT_INTERFACE(3) and you are recommended to use CURLOPT_LOCALPORTRANGE(3) as well when this option is set. Valid port numbers are 1 - 65535.
0, disabled - use whatever the system thinks is fine
Added in 7.15.2