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{ config, lib, ... }:
{
# Security-related system tweaks
# Prevent replacing the running kernel without reboot.
security.protectKernelImage = true;
# mount /tmp in ram. This makes temp file management faster
# on ssd systems, and volatile! Because it's wiped on reboot.
boot.tmp.useTmpfs = false;
boot.tmp.tmpfsSize = "80%";
# Purge /tmp on boot. (fallback option)
boot.tmp.cleanOnBoot = lib.mkDefault (!config.boot.tmp.useTmpfs);
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" ];
# Accept CA's ToS.
security.acme.acceptTerms = true;
}
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