add fail2ban, so far only banning ssh connections

fail2ban.nix

@@ -0,0 +1,19 @@
+{ ... }: {
+  services.fail2ban = {
+    enable = true;
+    maxretry = 5;
+    ignoreIP = [
+      # Whitelist some subnets
+      "10.0.0.0/8" "172.16.0.0/12" "192.168.0.0/16"
+      "matrix.org"
+      "app.element.io" # don't ratelimit matrix users
+    ];
+    bantime = "1h"; # Ban IPs for 1h at first.
+    bantime-increment = {
+      enable = true; # Enable increment of bantime after each violation
+      multipliers = "1 2 4 8 16 32 64 128 256";
+      maxtime = "168h"; # Do not ban for more than 1 week
+      overalljails = true; # Calculate the bantime based on all the violations
+    };
+  };
+}

flake.nix

@@ -29,6 +29,7 @@
           ./puffer.nix
           ./gitea.nix
           ./letsencrypt.nix
+          ./fail2ban.nix
           ./email.nix
           agenix.nixosModules.default
           nixos-mailserver.nixosModules.default