nixos-config/configs/common-linux.nix

{
  config,
  pkgs,
  lib,
  inputs,
  ...
}: {
  imports = [
    ./configs/common.nix
  ];

  boot = {
    loader = {
      efi.canTouchEfiVariables = true;
      grub = {
        enable = true;
        device = "nodev";
        efiSupport = true;
        configurationLimit = 10;
        enableCryptodisk = true;
        useOSProber = true;
      };
    };

    tmp = {
      useTmpfs = lib.mkDefault true;
      cleanOnBoot = true;
    };
    consoleLogLevel = 0;
    kernel.sysctl = {"vm.max_map_count" = 262144;};
    supportedFilesystems = ["ntfs" "btrfs"];

    initrd = {
      postDeviceCommands = pkgs.lib.mkBefore ''
        mkdir -p /mnt

        # We first mount the btrfs root to /mnt
        # so we can manipulate btrfs subvolumes.
        mount -o subvol=/ /dev/mapper/lvm-root /mnt

        # While we're tempted to just delete /root and create
        # a new snapshot from /root-blank, /root is already
        # populated at this point with a number of subvolumes,
        # which makes `btrfs subvolume delete` fail.
        # So, we remove them first.
        #
        # /root contains subvolumes:
        # - /root/var/lib/portables
        # - /root/var/lib/machines
        #
        # I suspect these are related to systemd-nspawn, but
        # since I don't use it I'm not 100% sure.
        # Anyhow, deleting these subvolumes hasn't resulted
        # in any issues so far, except for fairly
        # benign-looking errors from systemd-tmpfiles.
        btrfs subvolume list -o /mnt/root |
        cut -f9 -d' ' |
        while read subvolume; do
          echo "deleting /$subvolume subvolume..."
          btrfs subvolume delete "/mnt/$subvolume"
        done &&
        echo "deleting /root subvolume..." &&
        btrfs subvolume delete /mnt/root

        echo "restoring blank /root subvolume..."
        btrfs subvolume snapshot /mnt/root-blank /mnt/root

        # Once we're done rolling back to a blank snapshot,
        # we can unmount /mnt and continue on the boot process.
        umount /mnt
      '';
    };
  };

  sops = {
    defaultSopsFile = lib.mkDefault ../secrets/secrets.yaml;
    validateSopsFiles = true;
    age = {
      sshKeyPaths = ["/persist/etc/ssh/ssh_host_ed25519_key"];
      keyFile = "/persist/var/lib/sops-nix/key.txt";
      generateKey = true;
    };

    secrets = {
      hashedPassword = {
        neededForUsers = true;
        sopsFile = ../secrets/secrets.yaml;
      };
    };
  };

  environment = {
    # Don't install the /lib/ld-linux.so.2 stub. This saves one instance of nixpkgs.
    ldso32 = null;

    shells = with pkgs; [bashInteractive zsh];

    systemPackages = with pkgs; [
      btrfs-progs # utils for btrfs

      nethogs
      iotop
      nmon

      lm_sensors

      hdparm
    ];

    persistence."/persist" = {
      directories = [
        "/var/lib/nixos"
        "/var/lib/tailscale"
        "/var/lib/tuptime"
        "/var/lib/vnstat"
      ];
      files = [
        "/etc/machine-id"
        "/etc/NIXOS"
        "/etc/ssh/ssh_host_ed25519_key"
        "/etc/ssh/ssh_host_ed25519_key.pub"
        "/etc/ssh/ssh_host_rsa_key"
        "/etc/ssh/ssh_host_rsa_key.pub"
      ];
    };
  };

  i18n = {
    defaultLocale = "en_US.UTF-8";
    extraLocaleSettings = {
      LANGUAGE = "en_US.UTF-8";
      LC_ALL = "en_US.UTF-8";
    };
    supportedLocales = [
      "en_US.UTF-8/UTF-8"
      "de_DE.UTF-8/UTF-8"
      "de_DE/ISO-8859-1"
    ];
  };

  networking = {
    nameservers = ["127.0.0.1"];
    # If using dhcpcd:
    dhcpcd.extraConfig = "nohook resolv.conf";
    # If using NetworkManager:
    networkmanager.dns = "none";

    firewall = {
      # Allow PMTU / DHCP
      allowPing = true;

      # Keep dmesg/journalctl -k output readable by NOT logging
      # each refused connection on the open internet.
      logRefusedConnections = false;
    };

    # useNetworkd = true;
  };

  nix = {
    nixPath = ["nixpkgs=${inputs.nixpkgs-unstable}"];
    channel.enable = false;
    settings = {
      auto-optimise-store = true;
      experimental-features = ["nix-command" "flakes"];
      connect-timeout = 5;
      log-lines = 25;
      max-free = 3000 * 1024 * 1024;
      min-free = 512 * 1024 * 1024;
      builders-use-substitutes = true;
    };

    daemonCPUSchedPolicy = "batch";
    daemonIOSchedClass = "idle";
    daemonIOSchedPriority = 7;
  };

  programs = {
    nh = {
      enable = true;
      clean = {
        enable = true;
        extraArgs = "--keep-since 14d";
      };
      flake = "/home/alex/nixos-config";
    };
  };

  services = {
    vnstat.enable = true;
    tuptime.enable = true;
    locate.enable = true;
    tailscale.enable = true;

    openssh = {
      enable = true;
      settings = {
        PermitRootLogin = "no";
        PasswordAuthentication = false;
        X11Forwarding = false;
        KbdInteractiveAuthentication = false;
        UseDns = false;
        # unbind gnupg sockets if they exists
        StreamLocalBindUnlink = true;

        # Use key exchange algorithms recommended by `nixpkgs#ssh-audit`
        KexAlgorithms = [
          "curve25519-sha256"
          "curve25519-sha256@libssh.org"
          "diffie-hellman-group16-sha512"
          "diffie-hellman-group18-sha512"
          "sntrup761x25519-sha512@openssh.com"
        ];
      };
      openFirewall = true;
    };

    dnscrypt-proxy2 = {
      enable = true;
      settings = {
        ipv6_servers = true;
        require_dnssec = true;

        sources.public-resolvers = {
          urls = [
            "https://raw.githubusercontent.com/DNSCrypt/dnscrypt-resolvers/master/v3/public-resolvers.md"
            "https://download.dnscrypt.info/resolvers-list/v3/public-resolvers.md"
          ];
          cache_file = "/var/lib/dnscrypt-proxy2/public-resolvers.md";
          minisign_key = "RWQf6LRCGA9i53mlYecO4IzT51TGPpvWucNSCh1CBM0QTaLn73Y7GFO3";
        };

        # You can choose a specific set of servers from https://github.com/DNSCrypt/dnscrypt-resolvers/blob/master/v3/public-resolvers.md
        # server_names = [ ... ];
      };
    };

    journald = {extraConfig = "SystemMaxUse=500M";};
  };

  # The notion of "online" is a broken concept
  # https://github.com/systemd/systemd/blob/e1b45a756f71deac8c1aa9a008bd0dab47f64777/NEWS#L13
  systemd = {
    services.NetworkManager-wait-online.enable = false;
    network.wait-online.enable = false;

    # FIXME: Maybe upstream?
    # Do not take down the network for too long when upgrading,
    # This also prevents failures of services that are restarted instead of stopped.
    # It will use `systemctl restart` rather than stopping it with `systemctl stop`
    # followed by a delayed `systemctl start`.
    services.systemd-networkd.stopIfChanged = false;
    # Services that are only restarted might be not able to resolve when resolved is stopped before
    # services.systemd-resolved.stopIfChanged = false;

    services.nix-gc.serviceConfig = {
      CPUSchedulingPolicy = "batch";
      IOSchedulingClass = "idle";
      IOSchedulingPriority = 7;
    };

    services.nix-daemon.serviceConfig.OOMScoreAdjust = 250;

    # default is something like vt220... however we want to get alt least some colors...
    # services."serial-getty@".environment.TERM = "xterm-256color";
  };

  system.activationScripts.update-diff = {
    supportsDryActivation = true;
    text = ''
      if [[ -e /run/current-system ]]; then
        echo "--- diff to current-system"
        ${pkgs.nvd}/bin/nvd --nix-bin-dir=${config.nix.package}/bin diff /run/current-system "$systemConfig"
        echo "---"
      fi
    '';
  };

  # Turn off sudo lecture
  security = {
    sudo.extraConfig = ''
      # rollback results in sudo lectures after each reboot
      Defaults lecture = never
    '';
  };
}