If you already have a vpn set up and established, you can use the following to setup DNS forwarding for your domain from your remote site to you main site. This is done in CLI which you can access from the web UI if you want.
configure
set service dns forwarding options server=/privdomain.local/192.168.1.50
set service dns forwarding options server=/privdomain.com/192.168.1.50
commit
save
exit
