Harden & optimize networking settings

default.nix

@@ -45,12 +45,13 @@ with lib.my;
   fileSystems."/".device = mkDefault "/dev/disk/by-label/nixos";
 
   # Use the latest kernel
-  boot.kernelPackages = mkDefault pkgs.linuxPackages_5_10;
-
-  boot.loader = {
-    efi.canTouchEfiVariables = mkDefault true;
-    systemd-boot.configurationLimit = 10;
-    systemd-boot.enable = mkDefault true;
+  boot = {
+    kernelPackages = mkDefault pkgs.linuxPackages_5_10;
+    loader = {
+      efi.canTouchEfiVariables = mkDefault true;
+      systemd-boot.configurationLimit = 10;
+      systemd-boot.enable = mkDefault true;
+    };
   };
 
   # Just the bear necessities...

hosts/ao/default.nix

@@ -5,6 +5,7 @@
 with lib.my;
 {
   imports = [
+    ../server.nix
     ../personal.nix
     ./hardware-configuration.nix
 

hosts/linode.nix

@@ -53,6 +53,7 @@ with lib;
 {
   imports = [
     "${modulesPath}/profiles/qemu-guest.nix"
+    ./server.nix
   ];
 
   environment.systemPackages =

hosts/personal.nix

@@ -26,7 +26,12 @@ with lib;
     longitude = 12.5;
   } else {});
 
-  ##
+
+  # Refuse ICMP echo requests on my desktop/laptop; nobody has any business
+  # pinging them, unlike my servers.
+  boot.kernel.sysctl."net.ipv4.icmp_echo_ignore_broadcasts" = 1;
+
+  # So thw bitwarden CLI knows where to find my server.
   modules.shell.bitwarden.config.server = "p.v0.io";
 
   services.syncthing.declarative = {

hosts/server.nix

@@ -0,0 +1,20 @@
+{ config, lib, pkgs, ... }:
+
+{
+  boot.kernelPackages = pkgs.linuxPackages_latest_hardened;
+
+  systemd = {
+    services.clear-log = {
+      description = "Clear >1 month-old logs every week";
+      serviceConfig = {
+        Type = "oneshot";
+        ExecStart = "${pkgs.systemd}/bin/journalctl --vacuum-time=30d";
+      };
+    };
+    timers.clear-log = {
+      wantedBy = [ "timers.target" ];
+      partOf = [ "clear-log.service" ];
+      timerConfig.OnCalendar = "weekly UTC";
+    };
+  };
+}

modules/security.nix

@@ -1,7 +1,12 @@
-{ lib, config, ... }:
+{ config, lib, ... }:
 
 {
   ## System security tweaks
+  # sets hidepid=2 on /proc (make process info visible only to owning user)
+  security.hideProcessInformation = true;
+  # Prevent replacing the running kernel w/o reboot
+  security.protectKernelImage = true;
+
   # tmpfs = /tmp is mounted in ram. Doing so makes temp file management speedy
   # on ssd systems, and volatile! Because it's wiped on reboot.
   boot.tmpOnTmpfs = lib.mkDefault true;
@@ -9,20 +14,59 @@
     what = "tmpfs";
     where = "/tmp";
     type = "tmpfs";
-    # Added noexec (for added security), and reduced size from 50% to 25%
+    # Added noexec (for added security) and reduce size from 50% to 25%
     options = "mode=1777,strictatime,rw,nosuid,nodev,noexec,size=25%";
   }];
   # If not using tmpfs, which is naturally purged on reboot, we must clean it
   # /tmp ourselves. /tmp should be volatile storage!
   boot.cleanTmpDir = lib.mkDefault (!config.boot.tmpOnTmpfs);
 
-  security.hideProcessInformation = true;
-  security.protectKernelImage = true;
-
   # Fix a security hole in place for backwards compatibility. See desc in
   # nixpkgs/nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix
   boot.loader.systemd-boot.editor = false;
 
+  boot.kernel.sysctl = {
+    # The Magic SysRq key is a key combo that allows users connected to the
+    # system console of a Linux kernel to perform some low-level commands.
+    # Disable it, since we don't need it, and is a potential security concern.
+    "kernel.sysrq" = 0;
+
+    ## TCP hardening
+    # Prevent bogus ICMP errors from filling up logs.
+    "net.ipv4.icmp_ignore_bogus_error_responses" = 1;
+    # Reverse path filtering causes the kernel to do source validation of
+    # packets received from all interfaces. This can mitigate IP spoofing.
+    "net.ipv4.conf.default.rp_filter" = 1;
+    "net.ipv4.conf.all.rp_filter" = 1;
+    # Do not accept IP source route packets (we're not a router)
+    "net.ipv4.conf.all.accept_source_route" = 0;
+    "net.ipv6.conf.all.accept_source_route" = 0;
+    # Don't send ICMP redirects (again, we're on a router)
+    "net.ipv4.conf.all.send_redirects" = 0;
+    "net.ipv4.conf.default.send_redirects" = 0;
+    # Refuse ICMP redirects (MITM mitigations)
+    "net.ipv4.conf.all.accept_redirects" = 0;
+    "net.ipv4.conf.default.accept_redirects" = 0;
+    "net.ipv4.conf.all.secure_redirects" = 0;
+    "net.ipv4.conf.default.secure_redirects" = 0;
+    "net.ipv6.conf.all.accept_redirects" = 0;
+    "net.ipv6.conf.default.accept_redirects" = 0;
+    # Protects against SYN flood attacks
+    "net.ipv4.tcp_syncookies" = 1;
+    # Incomplete protection again TIME-WAIT assassination
+    "net.ipv4.tcp_rfc1337" = 1;
+
+    ## TCP optimization
+    # TCP Fast Open is a TCP extension that reduces network latency by packing
+    # data in the sender’s initial TCP SYN. Setting 3 = enable TCP Fast Open for
+    # both incoming and outgoing connections:
+    "net.ipv4.tcp_fastopen" = 3;
+    # Bufferbloat mitigations + slight improvement in throughput & latency
+    "net.ipv4.tcp_congestion_control" = "bbr";
+    "net.core.default_qdisc" = "cake";
+  };
+  boot.kernelModules = [ "tcp_bbr" ];
+
   # Change me later!
   user.initialPassword = "nixos";
   users.users.root.initialPassword = "nixos";