curl / libcurl / API / curl_easy_setopt / CURLOPT_HSTS

CURLOPT_HSTS explained


CURLOPT_HSTS - HSTS cache filename


#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTS, char *filename);


Make the filename point to a filename to load an existing HSTS cache from, and to store the cache in when the easy handle is closed. Setting a file name with this option also enables HSTS for this handle (the equivalent of setting CURLHSTS_ENABLE with CURLOPT_HSTS_CTRL).

If the given file does not exist or contains no HSTS entries at startup, the HSTS cache simply starts empty. Setting the filename to NULL or "" only enables HSTS without reading from or writing to any file.

If this option is set multiple times, libcurl loads cache entries from each given file but only stores the last used name for later writing.

File format

The HSTS cache is saved to and loaded from a text file with one entry per physical line. Each line in the file has the following format:

[host] [stamp]

[host] is the domain name for the entry and the name is dot-prefixed if it is an entry valid for all subdomains to the name as well or only for the exact name.

[stamp] is the time (in UTC) when the entry expires and it uses the format "YYYYMMDD HH:MM:SS".

Lines starting with "#" are treated as comments and are ignored. There is currently no length or size limit.


NULL, no filename


This functionality affects http only


int main(void)
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_HSTS, "/home/user/.hsts-cache");


Added in curl 7.74.0

Return value

Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.

See also


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