.htaccess - Apache DirectorySlash Off - Site breaks

Question

If i set DirectorySlash Off in my .htaccess file and call the directory without the trailing slash i get an 403-Forbidden from my server. If i call it with slash everything works fine.

Could anyone explain why? Here are my fully anonymized .htaccess:

# GLOBAL CONFIG
Options +FollowSymlinks
DirectorySlash Off
AddDefaultCharset utf-8
php_value post_max_size 256M
php_value upload_max_filesize 256M

# BEGIN WordPress
RewriteEngine On
RewriteBase /folder/
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /folder/index.php [L]
# END WordPress

# REMOVE WWW
RewriteCond %{HTTP_HOST} ^([^.]+).domain.com$ [NC]
RewriteRule ^(.*)$ http://domain.com$1 [R=301,L]

Answer 1

As you know per the documentation, when DirectorySlash is set to Off, requests to /folder do not have DirectoryIndex evaluated. This means that the request will not be automatically mapped to /folder/index.php.

mod_dir performs this check in the "fixup" phase of the request processing. mod_rewrite, which is responsible for your RewriteRule definitions, also performs its processing in this phase when you specify the rules in a .htaccess file.

However, it was programmed with an awareness of modules like mod_dir, and includes a check to make sure that the current directory was requested with a trailing slash. If not, it declines to handle the request, since doing so might lead to undefined behaviour.

The request then moves on to the content-generation phase, which, since the request was not mapped to a real file, is handled by mod_autoindex. Given that Indexes are disabled on your host by default, mod_autoindex returns 403 Forbidden which is what you see.

Note that since DirectoryIndex is not evaluated, even if mod_rewrite were to process the request, it would still fail, because no auto-resolution to index.php would occur, and your rule

RewriteRule . /folder/index.php [L]

wouldn't match, because the . requires a match on something (but the request would be blank).

Enabling DirectorySlash prevents this scenario by correcting the prevented actions in all of the previously mentioned scenarios except the last note, which is taken care of by the fact that DirectoryIndex maps the request to index.php anyway.