| <!-- | |
| Copyright (C) Daniel Stenberg, <[email protected]>, et al. | |
| SPDX-License-Identifier: curl | |
| --> | |
| # HSTS support | |
| HTTP Strict-Transport-Security. Added as experimental in curl | |
| 7.74.0. Supported "for real" since 7.77.0. | |
| ## Standard | |
| [HTTP Strict Transport Security](https://datatracker.ietf.org/doc/html/rfc6797) | |
| ## Behavior | |
| libcurl features an in-memory cache for HSTS hosts, so that subsequent | |
| HTTP-only requests to a hostname present in the cache gets internally | |
| "redirected" to the HTTPS version. | |
| ## `curl_easy_setopt()` options: | |
| - `CURLOPT_HSTS_CTRL` - enable HSTS for this easy handle | |
| - `CURLOPT_HSTS` - specify filename where to store the HSTS cache on close | |
| (and possibly read from at startup) | |
| ## curl command line options | |
| - `--hsts [filename]` - enable HSTS, use the file as HSTS cache. If filename | |
| is `""` (no length) then no file is used, only in-memory cache. | |
| ## HSTS cache file format | |
| Lines starting with `#` are ignored. | |
| For each hsts entry: | |
| [host name] "YYYYMMDD HH:MM:SS" | |
| The `[host name]` is dot-prefixed if it includes subdomains. | |
| The time stamp is when the entry expires. | |
| ## Possible future additions | |
| - `CURLOPT_HSTS_PRELOAD` - provide a set of HSTS hostnames to load first | |
| - ability to save to something else than a file | |