Explanation
The unknown-clients statement in the ISC DHCPD configuration is used to specify whether or not an address pool can be used by nodes which have a corresponding host section in the configuration. A host section is a declaration that defines a static IP address for a specific client based on its MAC address or other identifier.
An unknown client is a client that does not have a host section. The unknown-clients statement can be either allow, deny, or ignore, and it can be applied to a pool, a subnet, or a shared-network. For example, the following configuration allows unknown clients to use the pool of addresses from 192.168.1.100 to
192.168.1.200, but denies them from using the pool of addresses from 192.168.1.201 to 192.168.1.254:
subnet 192.168.1.0 netmask 255.255.255.0 { pool { range 192.168.1.100 192.168.1.200; allow unknown-clients; } pool { range 192.168.1.201 192.168.1.254; deny unknown-clients; } } References:
ISC DHCP 4.4 Manual Pages - dhcpd.conf: The official documentation of ISC DHCPD on how to configure the dhcpd.conf file, which includes the description of the unknown-clients statement and examples.
How To Configure a DHCP Server on Ubuntu 20.04 | DigitalOcean: A tutorial from DigitalOcean on how to configure a DHCP server on Ubuntu 20.04, which includes the use of the unknown-clients statement and host sections.