nix/module/Kernel.nix

{ pkgs, config, lib, ... }: with lib; let
	cfg = config.module.kernel;
in {
	options = {
		module.kernel = {
			enable = mkEnableOption "Enable kernel tweaks.";
			hardening = mkOption {
				default = false;
				type    = types.bool;
			};
			hotspotTtlBypass = mkOption {
				default = false;
				type    = types.bool;
			};
			latest = mkOption {
				default = false;
				type    = types.bool;
			};
		};
	};

	config = mkIf cfg.enable (mkMerge [
		{
			boot.kernel.sysctl = {
				# Allow sysrq.
				"kernel.sysrq" = 1;

				# Increase file watchers.
				"fs.inotify.max_user_instances" = 9999999;
				"fs.inotify.max_user_watches"   = 9999999;
				"fs.inotify.max_user_event"     = 9999999;
				# "fs.file-max"                   = 999999;
			};
		}

		(mkIf cfg.hardening {
			boot.kernel.sysctl = {
				# Spoof protection.
				"net.ipv4.conf.all.rp_filter"     = 1;
				"net.ipv4.conf.default.rp_filter" = 1;

				# Packet forwarding.
				"net.ipv4.ip_forward"          = 0;
				"net.ipv6.conf.all.forwarding" = 1;

				# MITM protection.
				"net.ipv4.conf.all.accept_redirects" = 0;
				"net.ipv6.conf.all.accept_redirects" = 0;

				# Do not send ICMP redirects (we are not a router).
				"net.ipv4.conf.all.send_redirects" = 0;

				# Do not accept IP source route packets (we are not a router).
				"net.ipv4.conf.all.accept_source_route" = 0;
				"net.ipv6.conf.all.accept_source_route" = 0;

				# Protect filesystem links.
				"fs.protected_hardlinks" = 0;
				"fs.protected_symlinks"  = 0;

				# Lynis config.
				"kernel.core_uses_pid" = 1;
				"kernel.kptr_restrict" = 2;

				# IP hardening.
				"net.ipv4.conf.all.log_martians"            = 1;
				"net.ipv4.conf.default.accept_redirects"    = 0;
				"net.ipv4.conf.default.accept_source_route" = 0;
				"net.ipv4.conf.default.log_martians"        = 0;
				"net.ipv4.tcp_timestamps"                   = 0;
				"net.ipv6.conf.default.accept_redirects"    = 0;

				# Disable ipv6.
				"net.ipv6.conf.all.disable_ipv6"     = 1;
				"net.ipv6.conf.default.disable_ipv6" = 1;
				"net.ipv6.conf.lo.disable_ipv6"      = 1;
			};
		})

		(mkIf cfg.hotspotTtlBypass {
			boot.kernel.sysctl."net.ipv4.ip_default_ttl" = 65;
		})

		(mkIf cfg.latest {
			boot.kernelPackages = pkgs.linuxPackages_latest;
		})
	]);
}