CURLOPT\_HSTS\_CTRL(3) Library Functions Manual CURLOPT\_HSTS\_CTRL(3)

# [NAME](#NAME)

CURLOPT\_HSTS\_CTRL - control HSTS behavior

# [SYNOPSIS](#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](#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](#BITS)

[CURLHSTS\_ENABLE](#CURLHSTS_ENABLE)

Enable the in-memory HSTS cache for this handle.

[CURLHSTS\_READONLYFILE](#CURLHSTS_READONLYFILE)

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

# [DEFAULT](#DEFAULT)

0

# [PROTOCOLS](#PROTOCOLS)

This functionality affects http only

# [EXAMPLE](#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](#AVAILABILITY)

Added in curl 7.74.0

# [RETURN VALUE](#RETURN_VALUE)

[*curl\_easy\_setopt(3)*](/man/os/curl/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)*](/man/os/curl/libcurl-errors.3).

# [SEE ALSO](#SEE_ALSO)

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

2026-10-22 libcurl

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Document: CURLOPT_HSTS_CTRL(3) from curl 8.23.0-r2 (Chainguard OS)
Source: https://github.com/curl/curl.git