{ config, lib, pkgs, ...}:

with lib;

let
  cfg = config.services.stubby;

  fallbacks = concatMapStringsSep "\n  " (x: "- ${x}") cfg.fallbackProtocols;
  listeners = concatMapStringsSep "\n  " (x: "- ${x}") cfg.listenAddresses;

  # By default, the recursive resolvers maintained by the getdns
  # project itself are enabled. More information about both getdns's servers,
  # as well as third party options for upstream resolvers, can be found here:
  # https://dnsprivacy.org/wiki/display/DP/DNS+Privacy+Test+Servers
  #
  # You can override these values by supplying a yaml-formatted array of your
  # preferred upstream resolvers in the following format:
  #
  # 106 # - address_data: IPv4 or IPv6 address of the upstream
  #   port: Port for UDP/TCP (default is 53)
  #   tls_auth_name: Authentication domain name checked against the server
  #                  certificate
  #   tls_pubkey_pinset: An SPKI pinset verified against the keys in the server
  #                      certificate
  #     - digest: Only "sha256" is currently supported
  #       value: Base64 encoded value of the sha256 fingerprint of the public
  #              key
  #   tls_port: Port for TLS (default is 853)

  defaultUpstream = ''
    - address_data: 145.100.185.15
      tls_auth_name: "dnsovertls.sinodun.com"
      tls_pubkey_pinset:
        - digest: "sha256"
          value: 62lKu9HsDVbyiPenApnc4sfmSYTHOVfFgL3pyB+cBL4=
    - address_data: 145.100.185.16
      tls_auth_name: "dnsovertls1.sinodun.com"
      tls_pubkey_pinset:
        - digest: "sha256"
          value: cE2ecALeE5B+urJhDrJlVFmf38cJLAvqekONvjvpqUA=
    - address_data: 185.49.141.37
      tls_auth_name: "getdnsapi.net"
      tls_pubkey_pinset:
        - digest: "sha256"
          value: foxZRnIh9gZpWnl+zEiKa0EJ2rdCGroMWm02gaxSc9Q=
    - address_data: 2001:610:1:40ba:145:100:185:15
      tls_auth_name: "dnsovertls.sinodun.com"
      tls_pubkey_pinset:
        - digest: "sha256"
          value: 62lKu9HsDVbyiPenApnc4sfmSYTHOVfFgL3pyB+cBL4=
    - address_data: 2001:610:1:40ba:145:100:185:16
      tls_auth_name: "dnsovertls1.sinodun.com"
      tls_pubkey_pinset:
        - digest: "sha256"
          value: cE2ecALeE5B+urJhDrJlVFmf38cJLAvqekONvjvpqUA=
    - address_data: 2a04:b900:0:100::38
      tls_auth_name: "getdnsapi.net"
      tls_pubkey_pinset:
        - digest: "sha256"
          value: foxZRnIh9gZpWnl+zEiKa0EJ2rdCGroMWm02gaxSc9Q=
  '';

  # Resolution type is not changeable here because it is required per the
  # stubby documentation:
  #
  # "resolution_type: Work in stub mode only (not recursive mode) - required for Stubby
  # operation."
  #
  # https://dnsprivacy.org/wiki/display/DP/Configuring+Stubby

  confFile = pkgs.writeText "stubby.yml" ''
    resolution_type: GETDNS_RESOLUTION_STUB
    dns_transport_list:
      ${fallbacks}
    appdata_dir: "/var/cache/stubby"
    tls_authentication: ${cfg.authenticationMode}
    tls_query_padding_blocksize: ${toString cfg.queryPaddingBlocksize}
    edns_client_subnet_private: ${if cfg.subnetPrivate then "1" else "0"}
    idle_timeout: ${toString cfg.idleTimeout}
    listen_addresses:
      ${listeners}
    round_robin_upstreams: ${if cfg.roundRobinUpstreams then "1" else "0"}
    ${cfg.extraConfig}
    upstream_recursive_servers:
    ${cfg.upstreamServers}
  '';
in

{
  options = {
    services.stubby = {

      enable = mkEnableOption "Stubby DNS resolver";

      fallbackProtocols = mkOption {
        default = [ "GETDNS_TRANSPORT_TLS" ];
        type = with types; listOf (enum [
          "GETDNS_TRANSPORT_TLS"
          "GETDNS_TRANSPORT_TCP"
          "GETDNS_TRANSPORT_UDP"
        ]);
        description = ''
          Ordered list composed of one or more transport protocols.
          Strict mode should only use <literal>GETDNS_TRANSPORT_TLS</literal>.
          Other options are <literal>GETDNS_TRANSPORT_UDP</literal> and
          <literal>GETDNS_TRANSPORT_TCP</literal>.
        '';
      };

      authenticationMode = mkOption {
        default = "GETDNS_AUTHENTICATION_REQUIRED";
        type = types.enum [
          "GETDNS_AUTHENTICATION_REQUIRED"
          "GETDNS_AUTHENTICATION_NONE"
        ];
        description = ''
          Selects the Strict or Opportunistic usage profile.
          For strict, set to <literal>GETDNS_AUTHENTICATION_REQUIRED</literal>.
          for opportunistic, use <literal>GETDNS_AUTHENTICATION_NONE</literal>.
        '';
      };

      queryPaddingBlocksize = mkOption {
        default = 128;
        type = types.int;
        description = ''
          EDNS0 option to pad the size of the DNS query to the given blocksize.
        '';
      };

      subnetPrivate = mkOption {
        default = true;
        type = types.bool;
        description = ''
          EDNS0 option for ECS client privacy. Default is
          <literal>true</literal>. If set, this option prevents the client
          subnet from being sent to authoritative nameservers.
        '';
      };

      idleTimeout = mkOption {
        default = 10000;
        type = types.int;
        description = "EDNS0 option for keepalive idle timeout expressed in
        milliseconds.";
      };

      listenAddresses = mkOption {
        default = [ "127.0.0.1" "0::1" ];
        type = with types; listOf str;
        description = ''
          Sets the listen address for the stubby daemon.
          Uses port 53 by default.
          Ise IP@port to specify a different port.
        '';
      };

      roundRobinUpstreams = mkOption {
        default = true;
        type = types.bool;
        description = ''
          Instructs stubby to distribute queries across all available name
          servers. Default is <literal>true</literal>. Set to
          <literal>false</literal> in order to use the first available.
        '';
      };

      upstreamServers = mkOption {
        default = defaultUpstream;
        type = types.lines;
        description = ''
          Replace default upstreams. See <citerefentry><refentrytitle>stubby
          </refentrytitle><manvolnum>1</manvolnum></citerefentry> for an
          example of the entry formatting. In Strict mode, at least one of the
          following settings must be supplied for each nameserver:
          <literal>tls_auth_name</literal> or
          <literal>tls_pubkey_pinset</literal>.
        '';
      };

      debugLogging = mkOption {
        default = false;
        type = types.bool;
        description = "Enable or disable debug level logging.";
      };

      extraConfig = mkOption {
        default = "";
        type = types.lines;
        description = ''
          Add additional configuration options. see <citerefentry>
          <refentrytitle>stubby</refentrytitle><manvolnum>1</manvolnum>
          </citerefentry>for more options.
        '';
      };
    };
  };

  config = mkIf cfg.enable {
    environment.systemPackages = [ pkgs.stubby ];
    systemd.services.stubby = {
      description = "Stubby local DNS resolver";
      after = [ "network.target" ];
      before = [ "nss-lookup.target" ];
      wantedBy = [ "multi-user.target" ];

      serviceConfig = {
        AmbientCapabilities = "CAP_NET_BIND_SERVICE";
        CapabilityBoundingSet = "CAP_NET_BIND_SERVICE";
        ExecStart = "${pkgs.stubby}/bin/stubby -C ${confFile} ${optionalString cfg.debugLogging "-l"}";
        DynamicUser = true;
        CacheDirectory = "stubby";
      };
    };
  };
}