NixOS/system/modules/openssh.nix

{ config, lib, ... }:
let
  cfg = config.profile.openssh;
in
{
  config = lib.mkIf cfg.enable {
    networking.firewall = {
      allowedTCPPorts = lib.mkAfter [ 22 ];
    };
    services.openssh = {
      enable = true;
      settings = {
        PasswordAuthentication = false;
        UseDns = true;
        X11Forwarding = false;
        PermitRootLogin = "no";
      };
    };
    services.fail2ban = {
      enable = true;
      # Ban IP after 5 failures
      maxretry = 5;
      ignoreIP = [
        # Whitelist some subnets
        "10.0.0.0/8"
        "172.16.0.0/12"
        "192.168.0.0/16"
      ];
      bantime = "24h"; # Ban IPs for one day on the first ban
      bantime-increment = {
        enable = true; # Enable increment of bantime after each violation
        formula = "ban.Time * math.exp(float(ban.Count+1)*banFactor)/math.exp(1*banFactor)";
        # multipliers = "1 2 4 8 16 32 64";
        maxtime = "168h"; # Do not ban for more than 1 week
        overalljails = true; # Calculate the bantime based on all the violations
      };
    };
  };
}