DHCP server on Linux with ISC Kea: kea-dhcp4.conf, subnets, reservations and config testing

Goal

Configure ISC Kea's DHCPv4 server with a subnet, an address pool and a fixed host reservation, test the configuration before it is loaded, and know where leases are recorded.

Prerequisites

The kea-dhcp4 package installed; a network interface on the subnet to be served; no other DHCP server active on that segment.

Steps

1. Write /etc/kea/kea-dhcp4.conf as JSON under the top-level Dhcp4 object. List the serving interfaces in interfaces-config ("interfaces": ["eth0"]); without an entry there Kea opens no DHCP socket. Define at least one subnet in the subnet4 array, each with a unique, stable id (current Kea requires one; older releases auto-numbered subnets), a subnet in CIDR form, and a pools array such as {"pool": "10.0.0.100 - 10.0.0.200"}, following Kea's DHCPv4 server reference.
2. Add a fixed assignment where a device needs the same address every time: inside a subnet's reservations array, an entry keyed by hw-address (or client-id, duid) with a fixed ip-address gives that client the same address. Prefer reserved addresses outside the pools range, so no other client can already hold them as a dynamic lease.
3. Test the file before the server ever loads it for real: kea-dhcp4 -t /etc/kea/kea-dhcp4.conf (exit code 0 means it looks valid). The -t option checks the configuration file and reports the first error, if any; the check is not comprehensive. Recent releases also offer -T, which additionally loads hook libraries and connects to configured databases, still without opening sockets.
4. Start or restart the daemon (systemctl restart kea-dhcp4-server on Debian and Ubuntu, kea-dhcp4 on RHEL and Fedora, or config-reload through its control socket) and check the log for a successful load message.
5. Note where leases land: the lease database's name parameter specifies the lease file in which new leases and lease updates are recorded, by default kea-leases4.csv under the installation's var/lib/kea directory (/var/lib/kea/ for packages). Since Kea 2.7.9 the lease file must stay in that data directory, or the server refuses to start. Back this file up before any planned migration.
6. If migrating from ISC's own dhcpd, plan for a replacement rather than a repair: ISC's final maintenance releases (4.4.3-P1 and 4.1-ESV-R16-P2) shipped on 5 October 2022, and its end-of-life announcement states that administrators deploying DHCP in new environments should look beyond ISC DHCP for a solution, since it would be irresponsible to invest in new deployments of this software which is now end-of-life, and points toward Kea as the maintained alternative.

Expected result

kea-dhcp4 -t exits cleanly on a valid file; the daemon logs the subnets it loaded; a test client on the segment receives an offer from the configured pool, and a reserved client always receives its fixed address.

Limits and test basis

Neither -t nor -T opens sockets, and -t does not load hook libraries, so a port already in use, or a broken hook path, can still fail at real startup — test a real client lease after any config change, not only the syntax check. Running two DHCP servers on the same segment causes clients to receive conflicting offers; confirm any previous server is stopped before Kea goes live. Keep the previous configuration file and lease file backed up before a config change.