The host *zbx0.pyrocufflink.blue* (a Raspberry Pi) runs the Zabbix
server and web UI. It has a reserved IPv4 address to simplify reverse
DNS management for now, since Samba's dynamic DNS client does not
register PTR records.
Instead of listing the addresses for DNS and NTP servers again in the
DHCP server configuration, these are now taken from the canonical
definitions in the `dch_networks` variable.