Of course we are discussing here about DNS to be used by Zentyal server, not DNS to be set as DHCP option for clients.
Very obviously, it is not mandatory, for DNS server, to use itself as DNS server. One may imagine that DNS server is using another DNS and never itself.
However, for what concerns Zentyal, when configured as Internet gateway, it has to rely on external (public) server but may also have to resolve internal names.
In such case, the right place to reach is localhost (127.0.0.1) because it avoid to go through physical interface.
This obviously assumes that DNS binds (if I can say so
) on all interfaces.
When trying to implement, e.g. reverse proxy using fqdn instead of IP address in the redirect directive, resolving internal names is mandatory.
Another reason why one would like to have Zentyal resolving internal names: say you are using Zentyal as cache proxy: all browsers will access this proxy, even for internal web servers and fqdn has to be resolved by proxy, meaning Zentyal itself.
I also suppose there is something strange with virtual hosts if internal DNS is missing...
To make it the other way around: if all machines in pone consistent domain are using same DNS (here Zentyal), why would the machine providing this DNS service be an exception. Thus what would it bring not to resolve internal names for Zentyal having one leg inside