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

let
  cfg = config.services.prometheus.exporters.dnstt;
  inherit (lib)
    escapeShellArgs
    mkDefault
    mkEnableOption
    mkIf
    mkOption
    optionals
    types
    ;

  listenAddress = "${cfg.listenAddress}:${toString cfg.port}";
  args = [
    "-web.listen-address"
    listenAddress
    "-web.telemetry-path"
    cfg.telemetryPath
    "-dnstt.port"
    (toString cfg.dnsttPort)
  ]
  ++ lib.concatMap (domain: [
    "-dnstt.domain"
    domain
  ]) cfg.domains
  ++ optionals (cfg.geoip.countryDatabase != null) [
    "-geoip.country-database"
    cfg.geoip.countryDatabase
  ]
  ++ optionals (cfg.geoip.asnDatabase != null) [
    "-geoip.asn-database"
    cfg.geoip.asnDatabase
  ]
  ++ cfg.extraFlags;
in
{
  options.services.prometheus.exporters.dnstt = {
    enable = mkEnableOption "the Prometheus DNSTT exporter";

    package = mkOption {
      type = types.package;
      description = ''
        Package providing the `dnstt_exporter` binary.
      '';
    };

    domains = mkOption {
      type = types.nonEmptyListOf types.str;
      example = [ "tunnel.example.com" ];
      description = ''
        DNSTT tunnel domains to observe. The exporter matches DNS query names
        below these domains and decodes DNSTT session IDs from the query prefix.
      '';
    };

    dnsttPort = mkOption {
      type = types.port;
      default = 53;
      description = ''
        UDP port where DNSTT DNS traffic is observed.
      '';
    };

    port = mkOption {
      type = types.port;
      default = 9713;
      description = ''
        Port to listen on for Prometheus metrics.
      '';
    };

    listenAddress = mkOption {
      type = types.str;
      default = "0.0.0.0";
      description = ''
        Address to listen on for Prometheus metrics.
      '';
    };

    telemetryPath = mkOption {
      type = types.str;
      default = "/metrics";
      description = ''
        HTTP path under which to expose Prometheus metrics.
      '';
    };

    geoip = {
      countryDatabase = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = "/var/lib/GeoIP/GeoLite2-Country.mmdb";
        description = ''
          Optional MaxMind GeoIP2 or GeoLite2 Country database path. When set,
          exported DNSTT metrics include a `country` label for the resolver
          address seen by the server.
        '';
      };

      asnDatabase = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = "/var/lib/GeoIP/GeoLite2-ASN.mmdb";
        description = ''
          Optional MaxMind GeoIP2 or GeoLite2 ASN database path. When set,
          exported DNSTT metrics include an `asn` label for the resolver address
          seen by the server.
        '';
      };
    };

    extraFlags = mkOption {
      type = types.listOf types.str;
      default = [ ];
      description = ''
        Extra command line flags to pass to `dnstt_exporter`.
      '';
    };

    openFirewall = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Open the exporter metrics port in the firewall.
      '';
    };
  };

  config = mkIf cfg.enable {
    assertions = [
      {
        assertion = cfg.domains != [ ];
        message = "services.prometheus.exporters.dnstt.domains must contain at least one DNSTT domain.";
      }
    ];

    networking.firewall.allowedTCPPorts = mkIf cfg.openFirewall [ cfg.port ];

    systemd.services.prometheus-dnstt-exporter = {
      description = "Prometheus DNSTT exporter";
      wantedBy = [ "multi-user.target" ];
      after = [ "network.target" ];

      serviceConfig = {
        ExecStart = "${cfg.package}/bin/dnstt_exporter ${escapeShellArgs args}";
        Restart = "always";
        DynamicUser = true;

        AmbientCapabilities = [ "CAP_NET_RAW" ];
        CapabilityBoundingSet = [ "CAP_NET_RAW" ];
        NoNewPrivileges = true;

        LockPersonality = true;
        MemoryDenyWriteExecute = true;
        PrivateDevices = mkDefault false;
        PrivateTmp = true;
        ProtectClock = true;
        ProtectControlGroups = true;
        ProtectHome = true;
        ProtectHostname = true;
        ProtectKernelLogs = true;
        ProtectKernelModules = true;
        ProtectKernelTunables = true;
        ProtectSystem = "strict";
        RemoveIPC = true;
        RestrictAddressFamilies = [
          "AF_INET"
          "AF_INET6"
          "AF_PACKET"
        ];
        RestrictNamespaces = true;
        RestrictRealtime = true;
        RestrictSUIDSGID = true;
        SystemCallArchitectures = "native";
        UMask = "0077";
      };
    };
  };

  meta.maintainers = [ ];
  meta.doc = false;
  meta.buildDocsInSandbox = false;
}