CURLOPT_DNS_LOCAL_IP6 man page
CURLOPT_DNS_LOCAL_IP6 — IPv6 address to bind DNS resolves to
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_DNS_LOCAL_IP6, char *address);
Set the local IPv6 address that the resolver should bind to. The argument should be of type char * and contain a single IPv6 address as a string. Set this option to NULL to use the default setting (don't bind to a specific IP address).
This option requires that libcurl was built with a resolver backend that supports this operation. The c-ares backend is the only such one.
Added in 7.33.0
Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, CURLE_NOT_BUILT_IN if support was disabled at compile-time, or CURLE_BAD_FUNCTION_ARGUMENT when given a bad address.