OpenNDS is a high performance, small footprint Captive Portal, offering by default a simple splash page restricted Internet connection yet incorporates an API that allows the creation of sophisticated authentication applications.
After installing the snap you can adjust the configuration file in /var/snap/opennds/common/opennds/opennds.conf following the documentation at https://openndsdocs.readthedocs.io/en/stable/
Then you need to allow the snap access to the firewall control, network information, mount information and some system and hardware resources with the commands
sudo snap connect opennds:firewall-control
sudo snap connect opennds:hardware-observe
sudo snap connect opennds:mount-observe
sudo snap connect opennds:network-observe
sudo snap connect opennds:system-observe
After connecting these snap interfaces, the opennds daemon will start
automatically. To interact with the daemon the snap package ships
the opennds.ndsctl command
You can just build this snap by calling snapcraft in the toplevel
of this source tree