{ config, lib, ... }:
let
  cfg = config.router;
  dhcpLans = lib.filter (l: l.dhcp.enable) (lib.attrValues cfg.lans);
  ipv4 = (import ./lib.nix { inherit lib; }).ipv4;
in
{
  config = lib.mkIf (cfg.enable && dhcpLans != [ ]) {
    services.kea.dhcp4 = {
      enable = true;
      settings = {
        interfaces-config = {
          interfaces = map (l: l.device) dhcpLans;
          # Interfaces are configured asynchronously by networkd.
          re-detect = true;
        };
        lease-database = {
          type = "memfile";
          persist = true;
          name = "/var/lib/kea/dhcp4.leases";
        };
        valid-lifetime = 3600;
        # The ID keys persistent leases, so derive it from the network address:
        # stable when LANs are added or removed (an index would shift).
        subnet4 = map (l: {
          id = ipv4.networkOf (ipv4.parse l.ipv4.address) l.ipv4.prefixLength;
          inherit (l.ipv4) subnet;
          interface = l.device;
          pools = [ { pool = "${l.dhcp.pool.start} - ${l.dhcp.pool.end}"; } ];
          option-data = [
            {
              name = "routers";
              data = l.ipv4.address;
            }
            {
              name = "domain-name-servers";
              data = l.ipv4.address;
            }
          ];
        }) dhcpLans;
      };
    };
  };
}