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The booting keyword
The booting flag is used to tell dhcpd whether or not to respond to queries from a particular client. This keyword only has meaning when it appears in a host declaration. By default, booting is allowed, but if it is disabled for a particular client, then that client will not be able to get an address from the DHCP server.
It looks like an indirect way to add a reservation without address. The only action should be to check Kea supports this case?