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