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

with lib;

let
  cfg = config.services.tailscale;
  firewallOn = config.networking.firewall.enable;
  rpfMode = config.networking.firewall.checkReversePath;
  isNetworkd = config.networking.useNetworkd;
  rpfIsStrict = rpfMode == true || rpfMode == "strict";
in {
  meta.maintainers = with maintainers; [ danderson mbaillie twitchyliquid64 ];

  options.services.tailscale = {
    enable = mkEnableOption (lib.mdDoc "Tailscale client daemon");

    port = mkOption {
      type = types.port;
      default = 41641;
      description = lib.mdDoc "The port to listen on for tunnel traffic (0=autoselect).";
    };

    interfaceName = mkOption {
      type = types.str;
      default = "tailscale0";
      description = lib.mdDoc ''The interface name for tunnel traffic. Use "userspace-networking" (beta) to not use TUN.'';
    };

    permitCertUid = mkOption {
      type = types.nullOr types.nonEmptyStr;
      default = null;
      description = lib.mdDoc "Username or user ID of the user allowed to to fetch Tailscale TLS certificates for the node.";
    };

    package = mkOption {
      type = types.package;
      default = pkgs.tailscale;
      defaultText = literalExpression "pkgs.tailscale";
      description = lib.mdDoc "The package to use for tailscale";
    };
  };

  config = mkIf cfg.enable {
    warnings = optional (firewallOn && rpfIsStrict) "Strict reverse path filtering breaks Tailscale exit node use and some subnet routing setups. Consider setting `networking.firewall.checkReversePath` = 'loose'";
    environment.systemPackages = [ cfg.package ]; # for the CLI
    systemd.packages = [ cfg.package ];
    systemd.services.tailscaled = {
      wantedBy = [ "multi-user.target" ];
      path = [
        config.networking.resolvconf.package # for configuring DNS in some configs
        pkgs.procps     # for collecting running services (opt-in feature)
        pkgs.glibc      # for `getent` to look up user shells
      ];
      serviceConfig.Environment = [
        "PORT=${toString cfg.port}"
        ''"FLAGS=--tun ${lib.escapeShellArg cfg.interfaceName}"''
      ] ++ (lib.optionals (cfg.permitCertUid != null) [
        "TS_PERMIT_CERT_UID=${cfg.permitCertUid}"
      ]);
      # Restart tailscaled with a single `systemctl restart` at the
      # end of activation, rather than a `stop` followed by a later
      # `start`. Activation over Tailscale can hang for tens of
      # seconds in the stop+start setup, if the activation script has
      # a significant delay between the stop and start phases
      # (e.g. script blocked on another unit with a slow shutdown).
      #
      # Tailscale is aware of the correctness tradeoff involved, and
      # already makes its upstream systemd unit robust against unit
      # version mismatches on restart for compatibility with other
      # linux distros.
      stopIfChanged = false;
    };

    networking.dhcpcd.denyInterfaces = [ cfg.interfaceName ];

    systemd.network.networks."50-tailscale" = mkIf isNetworkd {
      matchConfig = {
        Name = cfg.interfaceName;
      };
      linkConfig = {
        Unmanaged = true;
        ActivationPolicy = "manual";
      };
    };
  };
}