**************************************************************************
* _ _ ____ _
* Project ___| | | | _ \| |
* / __| | | | |_) | |
* | (__| |_| | _ <| |___
* \___|\___/|_| \_\_____|
*
* Copyright (C) 2020, 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
*
* This software is licensed as described in the file COPYING, which
* you should have received as part of this distribution. The terms
* are also available at https://curl.se/docs/copyright.html.
*
* You may opt to use, copy, modify, merge, publish, distribute and/or sell
* copies of the Software, and permit persons to whom the Software is
* furnished to do so, under the terms of the COPYING file.
*
* This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
* KIND, either express or implied.
*
**************************************************************************

CURLOPT_HSTS_CTRL - control HSTS behavior

#include <curl/curl.h>

#define CURLHSTS_ENABLE (1<<0)
#define CURLHSTS_READONLYFILE (1<<1)

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTS_CTRL, long bitmask);

HSTS (HTTP Strict Transport Security) means that an HTTPS server can instruct the client to not contact it again over clear-text HTTP for a certain period into the future. libcurl will then automatically redirect HTTP attempts to such hosts to instead use HTTPS. This is done by libcurl retaining this knowledge in an in-memory cache. Populate the long bitmask with the correct set of features to instruct libcurl how to handle HSTS for the transfers using this handle. Enable the in-memory HSTS cache for this handle. Make the HSTS file (if specified) read-only - makes libcurl not save the cache to the file when closing the handle. 0. HSTS is disabled by default. HTTPS and HTTP

CURL *curl = curl_easy_init();
if(curl) {
 curl_easy_setopt(curl, CURLOPT_HSTS_CTRL, CURLHSTS_ENABLE);
 curl_easy_perform(curl);
}

Added in 7.74.0 Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not. CURLOPT_HSTS "(3), " CURLOPT_CONNECT_TO "(3), " CURLOPT_RESOLVE "(3), " CURLOPT_ALTSVC "(3), "