nix-config/modules/boot/hardened.nix

{
  flake.modules.nixos.hardened = {
    boot = {
      kernelModules = ["tcp_bbr"];
      kernel.sysctl = {
        "kernel.sysrq" = 0;
        "net.ipv4.icmp_echo_ignore_broadcasts" = 1;
        "net.ipv4.icmp_ignore_bogus_error_responses" = 1;
        "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;
        "net.ipv4.conf.all.send_redirects" = 0;
        "net.ipv4.conf.all.accept_source_route" = 0;
        "net.ipv6.conf.all.accept_source_route" = 0;
        "net.ipv4.tcp_syncookies" = 1;
        "net.ipv4.tcp_rfc1337" = 1;
        "net.ipv4.tcp_congestion_control" = "bbr";
        "net.core.default_qdisc" = "cake";
        "net.ipv4.tcp_fastopen" = 3;
      };
    };
  };
}