Virtual Box 6.1 (or greater) and Host-Only-Adapter

In VirtualBox there is the capability to setup a so called Host-Only Adapter, that is a network layer that machines can share. I usually use a NAT-onlyu interface to let the machine go out on the Intenet, and an Host-Only Adapter to let my host machine to connect into the virtual machines.

Host-only adapter are configured, in the GUI application, via the File, Host Network Manager:



After an upgrade to VirtualBox 6.1.32 I tried to configure the above window, but it resulted aslways in an error with access denied.
After digging a little, I found out that there is now the need for some policy option into a file named /etc/vbox/network.conf. In short, such file must contain the list of available address ranges, otherwise Virtual Box will allow you insert only addresses in the range 192.168.56.0/24.
The syntax of this file is awkward: every line needs an asterisk on its beginning, and this lead me to some errors in the beginning. So, for instance, the following is my network configuration file:



And that allows me to decide the range of subnetworks to use without any error. Clearly, you need to reload the Virtual Box application.