How to redirect only 404 pages with htaccess in Wordpress

Question

For a client I want to do the following thing with .htaccess:

domainname.com/path must redirect to sub.domainname.com/path, but only if the first page is a deleted page / 404 page.

So all the existing pages must stay intact, and the deleted pages must redirect to a subdomain with the same path.

I used the following code on a Wordpress website, to only redirect 404 error pages to a new domain:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain\.com$ [NC]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d 
RewriteRule ^ http://example.com%{REQUEST_URI} [L,R=301]

I have overwritten the following code in the existing htaccess file:

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

It is not working. The problem is that now ALL pages are redirected to the new domain.

Only the 404's must be redirected.

What do I wrong?

Thanks!

Answer 1

As far I understand, .htaccess rewrite rules are executed before the request is proccesed by WordPress, or even by PHP engine, so you can not know if a URL will trigger a 404 error in .htaccess, you can not check if a given path will trigger a 404 at that level. You need to do it in WordPress itself:

// See https://developer.wordpress.org/reference/hooks/template_redirect/
add_action( 'template_redirect', 'cyb_redirect_not_found_paths' );
function cyb_redirect_not_found_paths() {

    // See https://developer.wordpress.org/reference/functions/is_404/
    if( is_404() ) {

        if( $_SERVER['REQUEST_URI'] == '/your_path' ) {

            $url_to = 'redirecton_rul';

            // See https://developer.wordpress.org/reference/functions/wp_redirect/
            wp_redirect( $url_to );
            exit;

        }

    }

}

Answer 2

Use this direction:

ErrorDocument 404 http://example.com/404/