117 lines
3.1 KiB
Org Mode
117 lines
3.1 KiB
Org Mode
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#+title: Kernel Hardening
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#+setupfile: ../headers
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* Kernel Hardening
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Some of my machines are exposed to the Internet, and therefore get
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their kernel hardened. First, let me declare the Nix file’s skeleton,
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with the =nixos.hardened= module.
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#+begin_src nix :tangle yes
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{
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flake.modules.nixos.hardened = {
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boot = {
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<<kernel-modules>>
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<<kernel-options>>
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};
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};
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}
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#+end_src
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** Kernel Modules
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The very first thing to do is to load the =tcp_bbr= kernel module. It
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increases the connection speed of the system with a better congestion
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control. It is particularly interesting for my servers, as they may
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have to deal with high traffic if a crawler ever decides to explore
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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
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Nixcraft for more details.
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#+name: kernel-modules
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#+begin_src nix
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kernelModules = ["tcp_bbr"];
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#+end_src
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** Kernel Options
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Next are a series of kernel options.
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#+name: kernel-options
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#+begin_src nix
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kernel.sysctl = {
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<<sysrq-key>>
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<<icmp>>
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<<icmp-no-accept-redirects>>
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<<icmp-no-send-redirects>>
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<<ip-source-route-packets>>
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<<syn>>
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<<tcp-time-wait>>
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<<bufferfloat>>
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<<latency>>
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};
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#+end_src
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First, we’ll disable the magic SysRq key. Not that I expect anyone to
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have physical access to my servers, but it is a really powerful tool
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that I’d rather have off.
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#+name: sysrq-key
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#+begin_src nix
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"kernel.sysrq" = 0;
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#+end_src
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Next, we’ll ignore ICMP broadcasts to avoid participating in Smurf
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attacks, and we’ll also ignore ICMP errors.
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#+name: icmp
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#+begin_src nix
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"net.ipv4.icmp_echo_ignore_broadcasts" = 1;
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"net.ipv4.icmp_ignore_bogus_error_responses" = 1;
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#+end_src
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Speaking of ICMP, we won’t accept ICMP redirects to prevent some MITM
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attacks.
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#+name: icmp-no-accept-redirects
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#+begin_src nix
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"net.ipv4.conf.all.accept_redirects" = 0;
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"net.ipv4.conf.default_accept_redirects" = 0;
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"net.ipv4.conf.all.secure_redirects" = 0;
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"net.ipv4.conf.default.secure_redirects" = 0;
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"net.ipv6.conf.all.accept_redirects" = 0;
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"net.ipv6.conf.default.accept_redirects" = 0;
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#+end_src
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And we won’t send ICMP redirects (we’re not a router).
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#+name: icmp-no-send-redirects
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#+begin_src nix
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"net.ipv4.conf.all.send_redirects" = 0;
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#+end_src
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We’re stil not a router, so we’ll refuse IP source route packets, both
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on IPV4 and IPV6.
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#+name: ip-source-route-packets
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#+begin_src nix
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"net.ipv4.conf.all.accept_source_route" = 0;
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"net.ipv6.conf.all.accept_source_route" = 0;
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#+end_src
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Now, let’s get some SYN flood protection.
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#+name: syn
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#+begin_src nix
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"net.ipv4.tcp_syncookies" = 1;
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#+end_src
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And protection against TCP time-wait assassination hazards.
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#+name: tcp-time-wait
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#+begin_src nix
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"net.ipv4.tcp_rfc1337" = 1;
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#+end_src
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We will also mitigate bufferfloat, including with BBR (hey, we enabled that above!)
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#+name: bufferfloat
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#+begin_src nix
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"net.ipv4.tcp_congestion_control" = "bbr";
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"net.core.default_qdisc" = "cake";
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#+end_src
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And lastly, we’ll reduce latency on IPV4.
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#+name: latency
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#+begin_src nix
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"net.ipv4.tcp_fastopen" = 3;
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#+end_src
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And we should be good to go!
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