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Viktor Szakats ac6264366f tidy-up: miscellaneous

- tool_bname: scope an include.
- `endif` comments.
- Markdown fixes.
- comment tidy-ups.
- whitespace, newlines, indent.

Closes #20309

2026-01-15 13:06:13 +01:00

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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

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:

[hostname] "YYYYMMDD HH:MM:SS"

The [hostname] 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

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