VirtualDocumentRoot handling non-existent folders - apache

Question

Using Apache, I have set up the VirtualDocumentRoot and it works this way:
<VirtualHost *:80>
UseCanonicalName Off
ServerName app.example.com
ServerAlias *.example.com
VirtualDocumentRoot "/home/domains/example.com/%1"
<Directory "/home/domains/example.com/*">
Options Indexes FollowSymLinks
AllowOverride All
Order Allow,Deny
Allow from all
</Directory>
</VirtualHost>
If I put the URL of http://mail.example.com, the DocRoot becomes /home/domains/example.com/mail, if it is http://www.example.com, the DocRoot becomes /home/domains/example.com/www, and so on.
Now I would like to check for the folder if it exists, like when the user requests a domain like http://myusername.example.com, the DocRoot checks /home/domains/example.com/myusername and it doesn't exist, so it should refer to /home/domains/example.com/404. Is this possible?
I have referred the following questions but no good:
VirtualDocumentRoot Domains and Subdomains
Apache VirtualDocumentRoot redirect non-existing domains

Answer 1

If you have virtual subdomains that are mapped to folders then couldn't you just check for the existance of the document root? If not exists redirect to the 404 subdomain which in turn should use /home/domains/example.com/404 perhaps like this.
<VirtualHost *:80>
UseCanonicalName Off
ServerName app.example.com
ServerAlias *.example.com
VirtualDocumentRoot "/home/domains/example.com/%1"
<Directory "/home/domains/example.com/*">
Options Indexes FollowSymLinks
AllowOverride All
Order Allow,Deny
Allow from all
RewriteEngine On
RewriteCond %{DOCUMENT_ROOT} !-d
RewriteRule ^(.*) http://404.example.com/ [R=301,L]
</Directory>
</VirtualHost>
Or if the full virtualdocumentroot path is not being picked up by %{DOCUMENT_ROOT} variable because of the dynamic assignment, maybe try it this way by assiging subdomain to a env variable then use it to check the path.
<VirtualHost *:80>
UseCanonicalName Off
ServerName app.example.com
ServerAlias *.example.com
VirtualDocumentRoot "/home/domains/example.com/%1"
<Directory "/home/domains/example.com/*">
Options Indexes FollowSymLinks
AllowOverride All
Order Allow,Deny
Allow from all
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(.+)\.example\.com [NC]
RewriteRule ^(.*)$ - [E=virt_path:%1,N,NS]
RewriteCond %{DOCUMENT_ROOT}/%{ENV:virt_path} !-d
RewriteRule ^(.*) http://404.example.com/ [R=301,L]
</Directory>
</VirtualHost>

Answer 2

Interesting problem indeed. You need to handle it by placing a custom 404 handler in <VirtualHost> section.
Add this line just below <VirtualHost *:80>:
ErrorDocument 404 http://www.example.com/404.html
This will try to open /home/domains/example.com/www/404.html when you visit:
http://myusername.example.com
And corresponding folder doesn't exist in your filesystem.