From 2fea36c19eb904125e2db5ba230b28d72dc881db Mon Sep 17 00:00:00 2001
From: Franck Cuny <franck@fcuny.net>
Date: Fri, 12 Sep 2025 13:11:20 -0700
Subject: start to refactor nixos modules

---
 modules/nixos/base.nix    | 65 ++++++++++++++++++++++++++++++++++++++++
 modules/nixos/cgroups.nix | 75 +++++++++++++++++++++++++++++++++++++++++++++++
 modules/nixos/default.nix |  8 +++++
 modules/nixos/ssh.nix     | 21 +++++++++++++
 4 files changed, 169 insertions(+)
 create mode 100644 modules/nixos/base.nix
 create mode 100644 modules/nixos/cgroups.nix
 create mode 100644 modules/nixos/default.nix
 create mode 100644 modules/nixos/ssh.nix

(limited to 'modules/nixos')

diff --git a/modules/nixos/base.nix b/modules/nixos/base.nix
new file mode 100644
index 0000000..f3dece1
--- /dev/null
+++ b/modules/nixos/base.nix
@@ -0,0 +1,65 @@
+{
+  self,
+  config,
+  pkgs,
+  lib,
+  ...
+}:
+{
+  boot = {
+    kernelPackages = pkgs.linuxPackages_latest;
+    kernel.sysctl = {
+      "net.ipv4.tcp_congestion_control" = "bbr";
+      "net.ipv4.tcp_ecn" = 1;
+      "net.ipv4.tcp_fastopen" = 3;
+      "net.ipv4.tcp_tw_reuse" = 1;
+    };
+  };
+
+  i18n = {
+    defaultLocale = "en_US.UTF-8";
+    supportedLocales = [
+      "en_US.UTF-8/UTF-8"
+    ];
+  };
+
+  time.timeZone = "America/Los_Angeles";
+
+  users.motdFile = "/etc/motd";
+
+  environment.etc.motd.text = ''
+    Machine  ${config.networking.hostName}
+     NixOS   ${config.system.nixos.release}
+       @     ${self.shortRev or self.dirtyShortRev}
+  '';
+
+  ## disable that slow "building man-cache" step
+  documentation.man.generateCaches = lib.mkForce false;
+
+  users = {
+    mutableUsers = false;
+    users.root.openssh.authorizedKeys.keys = [
+      "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAINBkozy+X96u5ciX766bJ/AyQ3xm1tXZTIr5+4PVFZFi"
+    ];
+  };
+
+  security.sudo.wheelNeedsPassword = false;
+
+  environment.systemPackages = with pkgs; [
+    curl
+    dysk
+    fd
+    fish
+    git
+    htop
+    jq
+    mtr
+    pciutils
+    powertop
+    ripgrep
+    tcpdump
+    traceroute
+    vim
+    wireguard-tools
+  ];
+}
diff --git a/modules/nixos/cgroups.nix b/modules/nixos/cgroups.nix
new file mode 100644
index 0000000..07dc964
--- /dev/null
+++ b/modules/nixos/cgroups.nix
@@ -0,0 +1,75 @@
+# Stolen from https://git.lix.systems/the-distro/infra/src/branch/main/common/cgroups.nix
+# Relatively inspired by fbtax2:
+# https://facebookmicrosites.github.io/cgroup2/docs/fbtax-results.html
+{ ... }:
+let
+  systemCriticalSliceConfig = {
+    ManagedOOMMemoryPressure = "kill";
+
+    # guarantee availability of memory
+    MemoryMin = "192M";
+    # default 100
+    IOWeight = 1000;
+    # default 100
+    CPUWeight = 1000;
+  };
+in
+{
+  systemd.oomd = {
+    enable = true;
+    enableRootSlice = true;
+    enableSystemSlice = true;
+    enableUserSlices = true;
+  };
+
+  systemd.services.nix-daemon = {
+    serviceConfig = {
+      CPUWeight = 10;
+      IOWeight = 10;
+    };
+  };
+
+  systemd.slices.hostcritical = {
+    description = "Ensures that services to keep the system alive remain alive";
+
+    unitConfig = {
+      # required to avoid a dependency cycle on systemd-oomd. systemd will
+      # actually guess this right but we should fix it anyway.
+      DefaultDependencies = false;
+    };
+
+    sliceConfig = systemCriticalSliceConfig;
+  };
+
+  # make root logins higher priority for resources
+  systemd.slices."user-0" = {
+    sliceConfig = systemCriticalSliceConfig;
+  };
+
+  systemd.slices.system = {
+    sliceConfig = {
+      ManagedOOMMemoryPressure = "kill";
+      ManagedOOMMemoryPressureLimit = "50%";
+
+      IOWeight = 100;
+    };
+  };
+
+  systemd.services.sshd = {
+    serviceConfig = {
+      Slice = "hostcritical.slice";
+    };
+  };
+
+  systemd.services.systemd-oomd = {
+    serviceConfig = {
+      Slice = "hostcritical.slice";
+    };
+  };
+
+  systemd.services.systemd-journald = {
+    serviceConfig = {
+      Slice = "hostcritical.slice";
+    };
+  };
+}
diff --git a/modules/nixos/default.nix b/modules/nixos/default.nix
new file mode 100644
index 0000000..669cd75
--- /dev/null
+++ b/modules/nixos/default.nix
@@ -0,0 +1,8 @@
+{ ... }:
+{
+  imports = [
+    ./base.nix
+    ./cgroups.nix
+    ./ssh.nix
+  ];
+}
diff --git a/modules/nixos/ssh.nix b/modules/nixos/ssh.nix
new file mode 100644
index 0000000..b4c8772
--- /dev/null
+++ b/modules/nixos/ssh.nix
@@ -0,0 +1,21 @@
+{ lib, ... }:
+{
+  networking.firewall.allowedTCPPorts = [ 22 ];
+
+  services.openssh = {
+    enable = lib.mkDefault true;
+    settings = {
+      PasswordAuthentication = lib.mkForce false;
+      KbdInteractiveAuthentication = lib.mkForce false;
+
+      PermitRootLogin = lib.mkForce "prohibit-password";
+    };
+    openFirewall = lib.mkDefault true;
+    hostKeys = [
+      {
+        path = "/etc/ssh/ssh_host_ed25519_key";
+        type = "ed25519";
+      }
+    ];
+  };
+}
-- 
cgit v1.2.3