I think there's an error here. Let's see what a DNS is first.

https://en.wikipedia.org/wiki/Domain_Name_System

Now while a private network can have a DNS server this doesn't mean it will work from say the internet since your entries won't sync with the internet DNS servers.

For most home DVR CCTV systems I can skip port forwarding and place the DVR's IP into the DMZ entry.

Sorry for the TLA.