CURLOPT_HSTS(3) Library Functions Manual CURLOPT_HSTS(3)
NAME
CURLOPT_HSTS - HSTS cache filename
SYNOPSIS
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTS, char *filename);
DESCRIPTION
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(3)).
If the given file does not exist or contains no HSTS entries at
startup, the HSTS cache starts empty. Setting the filename to NULL
allows HSTS without reading from or writing to any file. NULL also
makes libcurl clear the list of files to read HSTS data from, if any
such were previously set.
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.
DEFAULT
NULL, no filename
SECURITY CONCERNS
libcurl cannot fully protect against attacks where an attacker has
write access to the same directory where it is directed to save files.
This is particularly sensitive if you save files using elevated
privileges.
libcurl creates the file to store HSTS data in using default file
permissions, meaning that on *nix systems you may need to restrict your
umask to prevent other users on the same system to access the file.
PROTOCOLS
This functionality affects http only
EXAMPLE
int main(void)
{
CURL *curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_HSTS, "/home/user/.hsts-cache");
curl_easy_perform(curl);
}
}
AVAILABILITY
Added in curl 7.74.0
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_ALTSVC(3), CURLOPT_HSTS_CTRL(3), CURLOPT_RESOLVE(3)
libcurl 2026-03-23 CURLOPT_HSTS(3)
curl 8.19.0 - Generated Fri Mar 27 14:45:47 CDT 2026