Default handling for unmatched domains/subdomains in Apache

Apache uses the first virtualhost if no name matches. Just configure a new virtualhost as the first one with a random name, displaying whatever you like - or returning a 404 page.

Wildcard include your site configuration files:

Include path/to/site/confs/*httpd.conf

Organize your site conf files so they are loaded in an expected order. Example...

01-httpd.conf

02-site1-httpd.conf

03-site2-httpd.conf

etc...

Apache will read these in order. Then create one that will always load last to catch any unmatched virtual hosts and return a 404 instead of loading a default site.

99-catchall-httpd.conf

<VirtualHost *:8080>
 ServerName null
 ServerAlias *
 Redirect 404 /
</VirtualHost>

<VirtualHost *:8443>
 ServerName null
 ServerAlias *
 Redirect 404 /
</VirtualHost>

Be sure to replace the ports with whatever ports your httpd listens on. Or if you have httpd listening on specific interfaces, you'll need to add a catchall for each interface instead, like so:

<VirtualHost 192.168.1.101:8080>
 ServerName null
 ServerAlias *
 Redirect 404 /
</VirtualHost>
<VirtualHost 192.168.1.101:8443>
 ServerName null
 ServerAlias *
 Redirect 404 /
</VirtualHost>

<VirtualHost 192.168.1.102:8080>
 ServerName null
 ServerAlias *
 Redirect 404 /
</VirtualHost>

<VirtualHost 192.168.1.102:8443>
 ServerName null
 ServerAlias *
 Redirect 404 /
</VirtualHost>

Hope this helps. I use this method to load sites in the order I specify and prevent unmatched virtual hosts from loading an unexpected site unintentionally.