snowflake/modules/security.nix

# security tweaks borrowed from @hlissner
{
  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"];

  security.acme = {
    acceptTerms = true;
    defaults.email = "github@notohh.dev";
  };

  services = {
    openssh.settings.LogLevel = "VERBOSE";
    fail2ban = {
      enable = true;
      bantime = "1h";
      maxretry = 1;
      ignoreIP = [
        "192.168.0.0/16"
        "172.16.0.0/12"
        "10.0.0.0/8"
        "5.161.102.107/32"
        "100.71.49.65/10"
        "100.82.146.40/10"
      ];
      jails = {
        DEFAULT = {
          settings = {
            findtime = 100000;
            mode = "aggressive";
          };
        };
      };
    };
  };
}