{pkgs, config, ...}:

###### interface
let
  inherit (pkgs.lib) mkOption mkIf;

  options = {
    services = {
      dhcpd = {

        enable = mkOption {
          default = false;
          description = "
            Whether to enable the DHCP server.
          ";
        };

        extraConfig = mkOption {
          default = "";
          example = "
            option subnet-mask 255.255.255.0;
            option broadcast-address 192.168.1.255;
            option routers 192.168.1.5;
            option domain-name-servers 130.161.158.4, 130.161.33.17, 130.161.180.1;
            option domain-name \"example.org\";
            subnet 192.168.1.0 netmask 255.255.255.0 {
              range 192.168.1.100 192.168.1.200;
            }
          ";
          description = "
            Extra text to be appended to the DHCP server configuration
            file.  Currently, you almost certainly need to specify
            something here, such as the options specifying the subnet
            mask, DNS servers, etc.
          ";
        };

        configFile = mkOption {
          default = null;
          description = "
            The path of the DHCP server configuration file.  If no file
            is specified, a file is generated using the other options.
          ";
        };

        interfaces = mkOption {
          default = ["eth0"];
          description = "
            The interfaces on which the DHCP server should listen.
          ";
        };

        machines = mkOption {
          default = [];
          example = [
            { hostName = "foo";
              ethernetAddress = "00:16:76:9a:32:1d";
              ipAddress = "192.168.1.10";
            }
            { hostName = "bar";
              ethernetAddress = "00:19:d1:1d:c4:9a";
              ipAddress = "192.168.1.11";
            }
          ];
          description = "
            A list mapping ethernet addresses to IP addresses for the
            DHCP server.
          ";
        };

      };
    };
  };
in

###### implementation

let

  cfg = config.services.dhcpd;

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

  machines = pkgs.lib.concatMapStrings (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;
    log-facility local1; # see dhcpd.nix
    ${cfg.extraConfig}
    ${machines}
  '';

in
  

mkIf config.services.dhcpd.enable {
  require = [
    options
  ];

  services = {
    extraJobs = [{
      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
      '';
    }];
  };
}