#+title: Kernel Hardening
#+setupfile: ../headers

* Kernel Hardening
Some of my machines are exposed to the Internet, and therefore get
their kernel hardened. First, let me declare the Nix file’s skeleton,
with the =nixos.hardened= module.

#+begin_src nix :tangle yes
{
  flake.modules.nixos.hardened = {
    boot = {
      <<kernel-modules>>
      <<kernel-options>>
    };
  };
}
#+end_src

** Kernel Modules
The very first thing to do is to load the =tcp_bbr= kernel module. It
increases the connection speed of the system with a better congestion
control. It is particularly interesting for my servers, as they may
have to deal with high traffic if a crawler ever decides to explore
all webpages offered by some websites I host. See [[https://www.cyberciti.biz/cloud-computing/increase-your-linux-server-internet-speed-with-tcp-bbr-congestion-control/][this article]] by
Nixcraft for more details.
#+name: kernel-modules
#+begin_src nix
kernelModules = ["tcp_bbr"];
#+end_src

** Kernel Options
Next are a series of kernel options.
#+name: kernel-options
#+begin_src nix
kernel.sysctl = {
  <<sysrq-key>>
  <<icmp>>
  <<icmp-no-accept-redirects>>
  <<icmp-no-send-redirects>>
  <<ip-source-route-packets>>
  <<syn>>
  <<tcp-time-wait>>
  <<bufferfloat>>
  <<latency>>
};
#+end_src

First, we’ll disable the magic SysRq key. Not that I expect anyone to
have physical access to my servers, but it is a really powerful tool
that I’d rather have off.
#+name: sysrq-key
#+begin_src nix
"kernel.sysrq" = 0;
#+end_src

Next, we’ll ignore ICMP broadcasts to avoid participating in Smurf
attacks, and we’ll also ignore ICMP errors.
#+name: icmp
#+begin_src nix
"net.ipv4.icmp_echo_ignore_broadcasts" = 1;
"net.ipv4.icmp_ignore_bogus_error_responses" = 1;
#+end_src

Speaking of ICMP, we won’t accept ICMP redirects to prevent some MITM
attacks.
#+name: icmp-no-accept-redirects
#+begin_src nix
"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;
#+end_src

And we won’t send ICMP redirects (we’re not a router).
#+name: icmp-no-send-redirects
#+begin_src nix
"net.ipv4.conf.all.send_redirects" = 0;
#+end_src

We’re stil not a router, so we’ll refuse IP source route packets, both
on IPV4 and IPV6.
#+name: ip-source-route-packets
#+begin_src nix
"net.ipv4.conf.all.accept_source_route" = 0;
"net.ipv6.conf.all.accept_source_route" = 0;
#+end_src

Now, let’s get some SYN flood protection.
#+name: syn
#+begin_src nix
"net.ipv4.tcp_syncookies" = 1;
#+end_src

And protection against TCP time-wait assassination hazards.
#+name: tcp-time-wait
#+begin_src nix
"net.ipv4.tcp_rfc1337" = 1;
#+end_src

We will also mitigate bufferfloat, including with BBR (hey, we enabled that above!)
#+name: bufferfloat
#+begin_src nix
"net.ipv4.tcp_congestion_control" = "bbr";
"net.core.default_qdisc" = "cake";
#+end_src

And lastly, we’ll reduce latency on IPV4.
#+name: latency
#+begin_src nix
"net.ipv4.tcp_fastopen" = 3;
#+end_src

And we should be good to go!