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:

[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

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