The following lines in a site's .htaccess file will cause an Apache web server to perform reverse DNS lookups:
<Limit GET POST>
deny from none
allow from all
The reason for the hostname lookup is that there is no "none" directive. This will cause the Apache web server to view "deny from none" as "deny from none.vpsdefaulthostname.com". (Replace vpsdefaulthostname.com with the actual VPS default hostname.) This will then force a double reverse DNS lookup on every access to this site on the VPS.
Configuring a site this way will cause delays in each attempt to access the Apache web server if the primary DNS resolver is unavailable.