and forums.domain.com are the same machine, then this isn't a DNS question it is an Apache question. You need to create a virtual web server on the machine to support forums.domain.com.
In your /etc/httpd/conf/http.conf file (you don't tell us which distro so I'm assuming RHEL or a derivative), add something like this:-
Google "Apache virtual host" if you need to understand these settings.
If this is all too complicated, install WebMin
and use it to configure your Virtual Host settings for you.