easy:SetOpt_LocalPortRange(range)
range
argument is the number of attempts libcurl will
make to find a working local port number. It starts with the given
#CURLOPT_LOCALPORT
and adds one to the number for each retry. Setting
this option to 1 or below will make libcurl do only one try for the exact port
number. Port numbers by nature are scarce resources that will be busy at times
so setting this value to something too low might cause unnecessary connection
setup failures.