dotfiles/modules/security.nix

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{ config, lib, ... }:
{
## System security tweaks
# sets hidepid=2 on /proc (make process info visible only to owning user)
# NOTE Was removed on nixpkgs-unstable because it doesn't do anything
# security.hideProcessInformation = true;
# Prevent replacing the running kernel w/o reboot
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security.protectKernelImage = lib.mkDefault true;
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# tmpfs = /tmp is mounted in ram. Doing so makes temp file management speedy
# on ssd systems, and volatile! Because it's wiped on reboot.
boot.tmpOnTmpfs = lib.mkDefault true;
# If not using tmpfs, which is naturally purged on reboot, we must clean it
# /tmp ourselves. /tmp should be volatile storage!
boot.cleanTmpDir = lib.mkDefault (!config.boot.tmpOnTmpfs);
# Fix a security hole in place for backwards compatibility. See desc in
# nixpkgs/nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix
boot.loader.systemd-boot.editor = false;
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;
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# Log martian packages
"net.ipv4.conf.all.log_martians" = 1;
"net.ipv4.conf.default.log_martians" = 1;
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## TCP optimization
# TCP Fast Open is a TCP extension that reduces network latency by packing
# data in the senders 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" ];
# So we don't have to do this later...
security.acme.acceptTerms = true;
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# Enable doas as an alternative to sudo
security.doas = {
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enable = lib.mkDefault true;
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extraRules = [
# Do not ask for a password again for some time after the user successfully authenticates.
{
groups = [ "wheel" "doas" ];
persist = true;
}
];
};
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security.sudo.enable = !config.security.doas.enable;
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# SSH
services.openssh = {
# Disable ssh password login
passwordAuthentication = lib.mkDefault false;
logLevel = "VERBOSE";
extraConfig = ''
AllowAgentForwarding no
AllowTcpForwarding no
ClientAliveCountMax 2
Compression no
MaxAuthTries 3
MaxSessions 2
TCPKeepAlive no
'';
};
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}