When I first set up my home network using my OPNsense router and was learning firewall rules, I took the approach of allowing only the Unbound DNS service on OPNsense to be accessed and blocking access to all other DNS servers. This simplistic approach works well enough since any rogue access to external DNS servers are simply blocked. Only the DNS resolver on the local network is allowed (unless the DNS requests are encrypted, of course – see note below).
If you have software/services running on your local network that you want to remotely access, you may have encountered a situation in which your IP address changes periodically. Many ISPs do not assign static IP addresses to non-business accounts especially for IPv4 addresses because they are extremely limited and have technically been exhausted for some time now. When your modem stays connected for a long period of time with an ISP, it is possible your IP address will not change often.
Historically, DNS is a service that was designed to be unencrypted. Whenever a device from your network is trying to go to a web address, it needs to determine the IP address of the website in order to access it. With the increasing levels of tracking and data sharing/selling, a growing awareness that having DNS traffic unencrypted is not a good idea from a privacy and security standpoint. ISPs and other entities are able to know which sites you visit even if all of your web traffic is encrypted.
On my home network, I host a few public facing services that my family and I make use of when away from home such as Plex Media Server. On Plex I have limited the bandwidth remote users may use to be slightly less than the maximum of my upload speed so my home network is still usable. I am using my own registered domain name which I use to refer to devices on my network (both internally and externally, which you can read more about with another article I wrote).
One nice thing about utilizing more advanced router features is that you can do neat things such as using your own domain name in your home network. This allows you to refer to devices on your network by using the device name/hostname followed by the domain name. For instance, server.homenetworkguy.com could refer to a host named “server” that exists on the network using the domain name “homenetworkguy.com”. Of course, depending on your configuration, you could just use the hostname “server” to access various services on your device(s) such as a fileshare, which will be shorter than including the domain name.