---
name: "CURLOPT_HSTS_CTRL"
section: "3"
platform: "linux"
package: "curl"
version: "8.23.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/CURLOPT_HSTS_CTRL"
---

# CURLOPT\_HSTS\_CTRL(3)

control HSTS behavior

## NAME

CURLOPT\_HSTS\_CTRL - control HSTS behavior

## SYNOPSIS

```
#include <curl/curl.h>
#define CURLHSTS_ENABLE       (1L << 0)
#define CURLHSTS_READONLYFILE (1L << 1)
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTS_CTRL, long bitmask);
```

## DESCRIPTION

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 then automatically redirects 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.

## BITS

- CURLHSTS\_ENABLE

  Enable the in-memory HSTS cache for this handle.

- CURLHSTS\_READONLYFILE

  Make the HSTS file (if specified) read-only - makes libcurl not save the cache to the file when closing the handle.

## DEFAULT

0

## PROTOCOLS

This functionality affects http only

## EXAMPLE

```
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode result;
    /* enable HSTS */
    curl_easy_setopt(curl, CURLOPT_HSTS_CTRL, CURLHSTS_ENABLE);
    /* specify where to store the HSTS cache */
    curl_easy_setopt(curl, CURLOPT_HSTS, "/home/user/.hsts-cache");
    result = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
}
```

## AVAILABILITY

Added in curl 7.74.0

## RETURN VALUE

*[curl\_easy\_setopt(3)](/curl_easy_setopt/3.md)* returns a CURLcode indicating success or error.

CURLE\_OK (0) means everything was OK, non-zero means an error occurred, see *[libcurl-errors(3)](/libcurl-errors/3.md)*.

## SEE ALSO

[**CURLOPT\_ALTSVC**(3)](/CURLOPT_ALTSVC/3.md), [**CURLOPT\_CONNECT\_TO**(3)](/CURLOPT_CONNECT_TO/3.md), [**CURLOPT\_HSTS**(3)](/CURLOPT_HSTS/3.md), [**CURLOPT\_RESOLVE**(3)](/CURLOPT_RESOLVE/3.md)