How to restrict the direct access to wp-admin?

Question

How to restrict the direct access to wp-admin?

Answer

Create a rewrite rule in .htaccess as the following:

RewriteEngine On

RewriteBase /

RewriteRule ^index\.php$ - [L]

# uploaded files

RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin

RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

# Fix using the previous answer. 

RewriteRule ^admin/(.*)$ wp-admin/$1 [QSA,L] 

RewriteCond %{REQUEST_FILENAME} -f [OR]

RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]

RewriteRule  ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]

#Fix to load css js image files for multi site admin urls. 

#old code RewriteRule  ^[_0-9a-zA-Z-]+/(.*\.php)$ $1 [L]

RewriteRule  ^[_0-9a-zA-Z-]+/(.*\.(php|css|js|png|jpg|gif))$ $1 [L]

RewriteRule . index.php [L]