tl;dr: /mnt/data/udapi-config/dnsmasq.lease
So I just went on an excursion trying to find all dhcp leases dhcpd on our new UDM had handed out.
Reason: anything behind unmanaged switches behind an AP serving as bridge will not show up in the clients overview in the controller, while being perfectly functional.
Following the official guide to validate static assignments doesn’t work for dream machines – and logging into the USG to run show dhcp leases will simply tell you that show isn’t a command found in its lingo. This is exacerbated by the fact that it’s actually impossible to log in to the UDM as a regular user (ie the user configured through the controller (after enabling advanced features and SSH authentication subsequently)), and you’ll have to login as root with your regular controller password as documented in the getting started guide.
On the brighter side, the controller will show static configurations in the clients dashboard, once you select the “All configured clients” filter.
cat /mnt/data/udapi-config/dnsmasq.leaseTo show current dhcp leases.