{pkgs, config}:

let

  cfg = config.services.dhcpd;

  stateDir = "/var/lib/dhcp"; # Don't use /var/state/dhcp; not FHS-compliant.

  machines = pkgs.lib.concatStrings (map (machine: "
    host ${machine.hostName} {
      hardware ethernet ${machine.ethernetAddress};
      fixed-address ${machine.ipAddress};
    }
  ") cfg.machines);

  configFile = if cfg.configFile != null then cfg.configFile else pkgs.writeText "dhcpd.conf" "
    default-lease-time 600;
    max-lease-time 7200;
    authoritative;
    ddns-update-style ad-hoc;
    ${cfg.extraConfig}
    ${machines}
  ";

in
  
{
  name = "dhcpd";
  
  job = "
description \"DHCP server\"

start on network-interfaces/started
stop on network-interfaces/stop

script

    mkdir -m 755 -p ${stateDir}

    touch ${stateDir}/dhcpd.leases

    exec ${pkgs.dhcp}/sbin/dhcpd -f -cf ${configFile} \\
        -lf ${stateDir}/dhcpd.leases \\
        ${toString cfg.interfaces}

end script
  ";
  
}