Rewrite subdomain to existing folders - .htaccess

Question

I have a folder structure like this:
public_html
...images
...v2020
...v2021
With mod_rewrite, I redirect to the folder v2021 and force to https://www.example.com. Now I struggle with adding subdomains. When I go to images.example.com I want the redirection to the folder images, with images.example.com being unchanged in the address field.
I tested the code in two parts separately, first is the forwarding towards https://www including subdomain. It works without subdomains, but if I add any subdomain it leads to nowhere.
RewriteEngine on
# "example.com" to "www.example.com" (and HTTPS)
RewriteCond %{HTTP_HOST} ^(example\.com) [NC]
RewriteRule ^ https://www.%1%{REQUEST_URI} [NE,R=301,L]
# HTTP to HTTPS (same host) - preserve subdomain
RewriteCond %{HTTPS} off
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [NE,R=301,L]
The second part as a minimal working example is the forwording of the standard domain (no subdomain) and subdomain forwarding to a debug script.
RewriteCond %{HTTP_HOST} ^www\.example\.com
RewriteRule ^((?!v2021/).*)$ v2021/$1 [L]
RewriteCond %{HTTP_HOST} ^(.+?)\.example\.com
RewriteRule (.*) echo.php\?a=%1\&b=$1 [L]
The script works fine and outputs a="www" when using it with the upper condition, but it is not execute with the lower one. I get forwarded to nowhere.
Thanks for your help!

Answer 1

There seem to be several issues here...
RewriteCond %{HTTP_HOST} !^www\. [OR,NC]
RewriteCond %{HTTPS} off
RewriteRule ^ https://www.example.com%{REQUEST_URI} [NE,R=301,L]
You first rule redirects all your subdomains (every hostname that does not start www.) to www.example.com, so the subdomain is lost. You probably need to change the first condition so that it only redirects requests for example.com (the domain apex) to www.example.com. But you also need to split the HTTP to HTTPS redirect in order to preserve the subdomain (images etc.)
For example:
# "example.com" to "www.example.com" (and HTTPS)
RewriteCond %{HTTP_HOST} ^(example\.com) [NC]
RewriteRule ^ https://www.%1%{REQUEST_URI} [NE,R=301,L]
# HTTP to HTTPS (same host) - preserve subdomain
RewriteCond %{HTTPS} off
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [NE,R=301,L]
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com
RewriteRule ^(.*)$ %1/$1 [L,NC,QSA]
This will result in an internal rewrite loop (500 error) since it repeatedly rewrites to subdomain/url-path resulting in subdomain/subdomain/subdomain/url-path etc. etc.
You need a condition to either only rewrite the direct/initial request (not rewritten requests), or check that the subdomain has not already been rewritten to a subdirectory of the same name.
However, this also rewrites the www subdomain, which I assume is not the intention, so another condition is required to exclude this.
For example, to only rewrite the initial request and exclude the www subdomain:
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{HTTP_HOST} !^www\.
RewriteCond %{HTTP_HOST} ^(.*)\.example\.com
RewriteRule (.*) %1/$1 [L]
By always rewriting the initial request and not explicitly checking that the subdirectory does not match the subdomain means that you can have a sub-subdirectory of the same name as the subdomain (if required). eg. /images/images/foo.jpg is not a problem.
The NC and QSA flags on the RewriteRule are not required. (Since it's already case-insensitive and you have no query string to append to.)
The REDIRECT_STATUS environment variable is empty on the initial request and set to 200 (as in 200 OK status) after the first successful rewrite, so by checking that it is empty it avoids a rewrite loop.
If you've configured a wildcard subdomain then you might want to check that the subdirectory exists before rewriting. (Although filesystem checks are relatively expensive, so this is probably best avoided if possible. Particularly when serving assets.)
RewriteRule ^((?!v2021/).*)$ /v2021/$1 [L]
This would seem to conflict with the above rule. Shouldn't this only apply to the www subdomain (the opposite of the above rewrite)? For example:
RewriteCond %{HTTP_HOST} ^www\.
RewriteRule ^((?!v2021/).*)$ v2021/$1 [L]