Dnsmasq is a lightweight, easy to configure, DNS forwarder, which can be used to answer to dns queries from your network.
Similar functionality is also provided by “Unbound DNS”, our standard enabled forward/resolver service.
In some cases people prefer to use dnsmasq or combine it with our default enabled resolver (Unbound).
Since OPNsense 17.7 Unbound has been our standard DNS service, the main reason for Dnsmasq being shipped in our product is for compatibility. Although there are some use-cases that require Dnsmasq specifically, most users better opt for Unbound.
Most settings are pretty straightforward here, enable the service, listen on a port (53 when empty) and assign interfaces to listen to. You can register dhcp static mappings as well, so the forwarder will know internally defined hosts.
Here you define static hostnames, which allow you to reply a specific address when being asked.
If a specific domain should be answered by a different DNS server, you can configure it here.