How to set HSTS header from .htaccess only on HTTPS

My web application runs on a different number of hosts that I control. To prevent the need to change the Apache config of each vhost, I add most of the config using .htaccess files in my repo so the basic setup of each host is just a couple of lines. This also makes it possible to change the config upon deploying a new version. Currently the .htaccess (un)sets headers, does some rewrite magic and controls the caching of the UA.

I want to enable HSTS in the application using .htaccess. Just setting the header is easy:

Header always set Strict-Transport-Security "max-age=31536000"

But the spec clearly states: "An HSTS Host MUST NOT include the STS header field in HTTP responses conveyed over non-secure transport.". So I don't want to send the header when sending it over HTTP connections. See https://datatracker.ietf.org/doc/html/draft-ietf-websec-strict-transport-sec-14 .

I tried to set the header using environment vars, but I got stuck there. Anyone that knows how to do that?

Apparently there is a HTTPS environment variable available that can be used easily. For people with the same question:

Header set Strict-Transport-Security "max-age=31536000" env=HTTPS

To build on nielsr's answer, I used the following in the .htaccess to meet the secure deployment recommendations at https://hstspreload.org which will hardcode the domain into the Chrome browser. Be aware this will enforce HSTS across your subdomains, and that inclusion in the preload list cannot easily be undone, so rtfm.

<IfModule mod_headers.c>
Header set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload" env=HTTPS
</IfModule>

You can use this and put it on your htaccess file to comply with https://hstspreload.org. put this in your .htaccess file.

RewriteCond %{HTTPS} !=on
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R=301,L]

RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}/$1 [R=301,L,E=HTTPS:1]
Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload" env=HTTPS

first, it will do redirection for non https to https. and redirect non www https to www https with HSTS header.

(http://example.com -> https://example.com -> https://www.example.com - with HSTS header)

Tested and comply with https://hstspreload.org

For httpd.conf (if you have access to edit this) you can use

<VirtualHost 65.81.122.43:443>
    Header always set Strict-Transport-Security "max-age=31536000; includeSubdomains;"
</VirtualHost>

NOTE : You need to set it on the HTTPS vhost only and cannot be on http vhost.

When should I, and should I not use .htaccess files? >Allowing .htaccess files will make Apache look for them upon every access to your server. Since parent directories are searched as well, this will take some (small) amount of time, and can impact your server's performance. Source

Yet another alternative is to always set the header and conditionally remove it for non-ssl connections:

Header always set   Strict-Transport-Security "max-age=31536000" early
Header        unset Strict-Transport-Security env=!HTTPS

This has the advantage, that the Header directive can be used with both the env condition as well as the early flag. With a single Header directive, env and early cannot be used together, they are mutually exclusive (see official documentation: https://httpd.apache.org/docs/current/mod/mod_headers.html#header).