Harden networking (mostly)

flake.nix

@@ -25,6 +25,7 @@
           common
           admin
           hardware-hetzner
+          hardened
           agenix.nixosModule
           forgejo
           caddy

modules/common.nix

@@ -2,14 +2,7 @@
 
 {
   time.timeZone = "UTC";
-  services.openssh = {
-    enable = true;
-    # require public key authentication for better security
-    settings = {
-      PasswordAuthentication = false;
-      KbdInteractiveAuthentication = false;
-    };
-  };
+  services.openssh.enable = true;
   services.journald.extraConfig = ''
 SystemMaxUse=1G
 SystemMaxFileSize=100M

modules/hardened.nix

@@ -0,0 +1,97 @@
+{ config, pkgs, inputs, ... }:
+
+# A bunch of this is lifted from
+# <https://mdleom.com/blog/2020/03/04/caddy-nixos-part-2>
+
+{
+  # require public key authentication for better security
+  services.openssh.settings = {
+    PasswordAuthentication = false;
+    KbdInteractiveAuthentication = false;
+  };
+  # Use when PasswordAuthentication/KbdInteractiveAuthentication is enabled
+  # services.fail2ban.enable = true;
+
+  # Disable `useradd` and `passwd`
+  users.mutableUsers = false;
+
+  # DNS over TLS
+  services.stubby = {
+    enable = true;
+    settings = {
+      # ::1 cause error, use 0::1 instead
+      listen_addresses = [ "127.0.0.1" "0::1" ];
+      # https://github.com/getdnsapi/stubby/blob/develop/stubby.yml.example
+      resolution_type = "GETDNS_RESOLUTION_STUB";
+      dns_transport_list = [ "GETDNS_TRANSPORT_TLS" ];
+      tls_authentication = "GETDNS_AUTHENTICATION_REQUIRED";
+      tls_query_padding_blocksize = 128;
+      idle_timeout = 10000;
+      round_robin_upstreams = 1;
+      tls_min_version = "GETDNS_TLS1_3";
+      dnssec = "GETDNS_EXTENSION_TRUE";
+      upstream_recursive_servers = [
+        {address_data = "1.1.1.2"; # .2 is the malware-blocked version
+         tls_auth_name = "cloudflare-dns.com";}
+        {address_data = "1.0.0.2";
+         tls_auth_name = "cloudflare-dns.com";}
+        {address_data = "2606:4700:4700::1112";
+         tls_auth_name = "cloudflare-dns.com";}
+        {address_data = "2606:4700:4700::1002";
+         tls_auth_name = "cloudflare-dns.com";}
+        {address_data = "9.9.9.9";
+         tls_auth_name = "dns.quad9.net";}
+        {address_data = "149.112.112.112";
+         tls_auth_name = "dns.quad9.net";}
+        {address_data = "2620:fe::fe";
+         tls_auth_name = "dns.quad9.net";}
+        {address_data = "2620:fe::9";
+         tls_auth_name = "dns.quad9.net";}
+      ];
+    };
+  };
+  # Fallback incase stubby/DNS-over-TLS is unresponsive
+  networking.nameservers = ["::1" "127.0.0.1"];
+  services.resolved = {
+    enable = true;
+    fallbackDns = ["2606:4700:4700::1112" "2606:4700:4700::1002"
+                   "1.1.1.2" "1.0.0.2"];
+  };
+
+  # Network stack hardening + perf
+  boot.kernelModules = [ "tcp_bbr" ];
+  boot.kernel.sysctl = {
+    # Disable magic SysRq key
+    "kernel.sysrq" = 0;
+    # Ignore ICMP broadcasts to avoid participating in Smurf attacks
+    "net.ipv4.icmp_echo_ignore_broadcasts" = 1;
+    # Ignore bad ICMP errors
+    "net.ipv4.icmp_ignore_bogus_error_responses" = 1;
+    # Reverse-path filter for spoof protection
+    "net.ipv4.conf.default.rp_filter" = 1;
+    "net.ipv4.conf.all.rp_filter" = 1;
+    # SYN flood protection
+    "net.ipv4.tcp_syncookies" = 1;
+    # Do not accept ICMP redirects (prevent MITM attacks)
+    "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;
+    # Do not send ICMP redirects (we are not a router)
+    "net.ipv4.conf.all.send_redirects" = 0;
+    # Do not accept IP source route packets (we are not a router)
+    "net.ipv4.conf.all.accept_source_route" = 0;
+    "net.ipv6.conf.all.accept_source_route" = 0;
+    # Protect against tcp time-wait assassination hazards
+    "net.ipv4.tcp_rfc1337" = 1;
+    # TCP Fast Open (TFO)
+    "net.ipv4.tcp_fastopen" = 3;
+    ## Bufferbloat mitigations
+    # Requires >= 4.9 & kernel module
+    "net.ipv4.tcp_congestion_control" = "bbr";
+    # Requires >= 4.19
+    "net.core.default_qdisc" = "cake";
+  };
+}

modules/hardware-hetzner.nix

@@ -12,7 +12,6 @@
       availableKernelModules = [ "ahci" "xhci_pci" "virtio_pci" "sd_mod" "sr_mod" ];
       kernelModules = [ ];
     };
-    kernelModules = [ ];
     kernelParams = [
       "console=tty1"
       "console=ttyS0,115200"