example/hsts-preload: show one way to HSTS preload

Closes #11337

docs/examples/Makefile.inc

@@ -46,6 +46,7 @@ check_PROGRAMS = \
   getredirect \
   getreferrer \
   headerapi \
+  hsts-preload \
   http-options \
   http-post \
   http2-download \

docs/examples/hsts-preload.c

@@ -0,0 +1,118 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 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.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+/* <DESC>
+ * Preload domains to HSTS
+ * </DESC>
+ */
+#include <stdio.h>
+#include <string.h>
+#include <curl/curl.h>
+
+struct entry {
+  const char *name;
+  const char *exp;
+};
+
+static const struct entry preload_hosts[] = {
+  { "example.com", "20370320 01:02:03" },
+  { "curl.se",     "20370320 03:02:01" },
+  { NULL, NULL } /* end of list marker */
+};
+
+struct state {
+  int index;
+};
+
+/* "read" is from the point of the library, it wants data from us. One domain
+   entry per invoke. */
+static CURLSTScode hstsread(CURL *easy, struct curl_hstsentry *e,
+                            void *userp)
+{
+  const char *host;
+  const char *expire;
+  struct state *s = (struct state *)userp;
+  (void)easy;
+  host = preload_hosts[s->index].name;
+  expire = preload_hosts[s->index++].exp;
+
+  if(host && (strlen(host) < e->namelen)) {
+    strcpy(e->name, host);
+    e->includeSubDomains = 0;
+    strcpy(e->expire, expire);
+    fprintf(stderr, "HSTS preload '%s' until '%s'\n", host, expire);
+  }
+  else
+    return CURLSTS_DONE;
+  return CURLSTS_OK;
+}
+
+static CURLSTScode hstswrite(CURL *easy, struct curl_hstsentry *e,
+                             struct curl_index *i, void *userp)
+{
+  (void)easy;
+  (void)userp; /* we have no custom input */
+  printf("[%u/%u] %s %s\n", (unsigned int)i->index, (unsigned int)i->total,
+         e->name, e->expire);
+  return CURLSTS_OK;
+}
+
+int main(void)
+{
+  CURL *curl;
+  CURLcode res;
+
+  curl = curl_easy_init();
+  if(curl) {
+    struct state st = {0};
+
+    /* enable HSTS for this handle */
+    curl_easy_setopt(curl, CURLOPT_HSTS_CTRL, (long)CURLHSTS_ENABLE);
+
+    /* function to call at first to populate the cache before the transfer */
+    curl_easy_setopt(curl, CURLOPT_HSTSREADFUNCTION, hstsread);
+    curl_easy_setopt(curl, CURLOPT_HSTSREADDATA, &st);
+
+    /* function to call after transfer to store the new state of the HSTS
+       cache */
+    curl_easy_setopt(curl, CURLOPT_HSTSWRITEFUNCTION, hstswrite);
+    curl_easy_setopt(curl, CURLOPT_HSTSWRITEDATA, NULL);
+
+    /* use the domain with HTTP but due to the preload, it should do the
+       transfer using HTTPS */
+    curl_easy_setopt(curl, CURLOPT_URL, "http://curl.se");
+
+    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
+
+    /* Perform the request, res will get the return code */
+    res = curl_easy_perform(curl);
+    /* Check for errors */
+    if(res != CURLE_OK)
+      fprintf(stderr, "curl_easy_perform() failed: %s\n",
+              curl_easy_strerror(res));
+
+    /* always cleanup */
+    curl_easy_cleanup(curl);
+  }
+  return 0;
+}