{ lib, ... }:
let
  inherit (lib) mkOption mkEnableOption types;
  rlib = import ./lib.nix { inherit lib; };
in
{
  options.router = {
    enable = mkEnableOption "the router role";

    wan = {
      interface = mkOption {
        type = types.str;
        example = "eth0";
        description = "Name of the uplink interface.";
      };
    };

    lans = mkOption {
      default = { };
      description = "LAN segments served by this router, keyed by name.";
      type = types.attrsOf (
        types.submodule (
          { config, ... }:
          {
            options = {
              interface = mkOption {
                type = types.str;
                example = "eth1";
                description = "Interface (or bridge/VLAN device) carrying this LAN.";
              };

              ipv4 = {
                address = mkOption {
                  type = types.str;
                  example = "192.168.10.1";
                  description = "Router address on this LAN.";
                };
                prefixLength = mkOption {
                  type = types.ints.between 8 30;
                  default = 24;
                };
                subnet = mkOption {
                  type = types.str;
                  readOnly = true;
                  description = "Network in CIDR notation, derived from `address` and `prefixLength`.";
                };
              };

              dhcp = {
                enable = mkOption {
                  type = types.bool;
                  default = true;
                };
                pool = {
                  start = mkOption {
                    type = types.str;
                    example = "192.168.10.100";
                  };
                  end = mkOption {
                    type = types.str;
                    example = "192.168.10.199";
                  };
                };
              };

              allowManagement = mkOption {
                type = types.bool;
                default = true;
                description = "Whether clients on this LAN may reach SSH on the router.";
              };
            };

            # "invalid" is only ever seen when assertions.nix is already failing.
            config.ipv4.subnet =
              if rlib.ipv4.parse config.ipv4.address == null then
                "invalid"
              else
                rlib.ipv4.cidr config.ipv4.address config.ipv4.prefixLength;
          }
        )
      );
    };

    dns.forwarders = mkOption {
      type = types.listOf types.str;
      default = [ ];
      description = "Upstream resolvers. When empty, Unbound resolves recursively.";
    };
  };
}