HTTP Strict-Transport-Security. Added as experimental in curl 7.74.0. Supported "for real" since 7.77.0.
libcurl features an in-memory cache for HSTS hosts, so that subsequent HTTP-only requests to a host name present in the cache will get internally "redirected" to the HTTPS version.
CURLOPT_HSTS_CTRL- enable HSTS for this easy handle
CURLOPT_HSTS- specify file name where to store the HSTS cache on close (and possibly read from at startup)
curl cmdline options
--hsts [filename]- enable HSTS, use the file as HSTS cache. If filename is
""(no length) then no file will be used, only in-memory cache.
HSTS cache file format
Lines starting with
# are ignored.
For each hsts entry:
[host name] "YYYYMMDD HH:MM:SS"
[host name] is dot-prefixed if it is a includeSubDomain.
The time stamp is when the entry expires.
I considered using wget's file format for the HSTS cache. However, they store the time stamp as the epoch (number of seconds since 1970) and I strongly disagree with using that format. Instead I opted to use a format similar to the curl alt-svc cache file format.
Possible future additions
CURLOPT_HSTS_PRELOAD- provide a set of preloaded HSTS host names
- ability to save to something else than a file