chore: refactor system modules

system/network/default.nix

@@ -0,0 +1,5 @@
+{
+  imports = [
+    ./networking.nix
+  ];
+}

system/network/networking.nix

@@ -0,0 +1,70 @@
+{
+  lib,
+  config,
+  ...
+}:
+with lib; let
+  cfg = config.system.networking;
+in {
+  options.system.networking = with types; {
+    hostname = mkOption {
+      type = str;
+      example = "gampo";
+    };
+    id = mkOption {
+      type = str;
+      example = "deadb33f";
+    };
+    domain = mkOption {
+      type = nullOr str;
+      example = "phundrak.com";
+      default = null;
+    };
+    hostFiles = mkOption {
+      type = listOf path;
+      example = [/path/to/hostFile];
+      default = [];
+    };
+    firewall = {
+      openPorts = mkOption {
+        type = listOf int;
+        example = [22 80 443];
+        default = [];
+      };
+      openPortRanges = mkOption {
+        type = listOf (attrsOf port);
+        default = [];
+        example = [
+          {
+            from = 8080;
+            to = 8082;
+          }
+        ];
+        description = ''
+          A range of TCP and UDP ports on which incoming connections are
+          accepted.
+        '';
+      };
+      extraCommands = mkOption {
+        type = nullOr lines;
+        example = "iptables -A INPUTS -p icmp -j ACCEPT";
+        default = null;
+      };
+    };
+  };
+
+  config.networking = {
+    hostName = cfg.hostname; # Define your hostname.
+    hostId = cfg.id;
+    networkmanager.enable = true;
+    inherit (cfg) hostFiles domain;
+    firewall = {
+      enable = true;
+      allowedTCPPorts = cfg.firewall.openPorts;
+      allowedUDPPorts = cfg.firewall.openPorts;
+      allowedTCPPortRanges = cfg.firewall.openPortRanges;
+      allowedUDPPortRanges = cfg.firewall.openPortRanges;
+      extraCommands = (mkIf (cfg.firewall.extraCommands != null)) cfg.firewall.extraCommands;
+    };
+  };
+}