CURLOPT_HEADEROPT man page
CURLOPT_HEADEROPT — set how to send HTTP headers
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADEROPT, long bitmask);
Pass a long that is a bitmask of options of how to deal with headers. The two mutually exclusive options are:
CURLHEADER_UNIFIED - the headers specified in CURLOPT_HTTPHEADER(3) will be used in requests both to servers and proxies. With this option enabled, CURLOPT_PROXYHEADER(3) will not have any effect.
CURLHEADER_SEPARATE - makes CURLOPT_HTTPHEADER(3) headers only get sent to a server and not to a proxy. Proxy headers must be set with CURLOPT_PROXYHEADER(3) to get used. Note that if a non-CONNECT request is sent to a proxy, libcurl will send both server headers and proxy headers. When doing CONNECT, libcurl will send CURLOPT_PROXYHEADER(3) headers only to the proxy and then CURLOPT_HTTPHEADER(3) headers only to the server.
CURLHEADER_SEPARATE (changed in 7.42.1, ased CURLHEADER_UNIFIED before then)
Added in 7.37.0
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
curl_easy_setopt(3), CURLOPT_HTTPHEADER(3), CURLOPT_PROXYHEADER(3).