nixos/qemu-vm: add option "restrictNetwork"

nixos/modules/virtualisation/qemu-vm.nix

@@ -528,6 +528,20 @@ in
         '';
     };
 
+    virtualisation.restrictNetwork =
+      mkOption {
+        type = types.bool;
+        default = false;
+        example = true;
+        description =
+          lib.mdDoc ''
+            If this option is enabled, the guest will be isolated, i.e. it will
+            not be able to contact the host and no guest IP packets will be
+            routed over the host to the outside. This option does not affect
+            any explicitly set forwarding rules.
+          '';
+      };
+
     virtualisation.vlans =
       mkOption {
         type = types.listOf types.ints.unsigned;
@@ -934,10 +948,11 @@ in
               else "'guestfwd=${proto}:${guest.address}:${toString guest.port}-" +
                    "cmd:${pkgs.netcat}/bin/nc ${host.address} ${toString host.port}',"
           );
+        restrictNetworkOption = lib.optionalString cfg.restrictNetwork "restrict=on,";
       in
       [
         "-net nic,netdev=user.0,model=virtio"
-        "-netdev user,id=user.0,${forwardingOptions}\"$QEMU_NET_OPTS\""
+        "-netdev user,id=user.0,${forwardingOptions}${restrictNetworkOption}\"$QEMU_NET_OPTS\""
       ];
 
     # FIXME: Consolidate this one day.

nixos/tests/all-tests.nix

@@ -533,6 +533,7 @@ in {
   public-inbox = handleTest ./public-inbox.nix {};
   pulseaudio = discoverTests (import ./pulseaudio.nix);
   qboot = handleTestOn ["x86_64-linux" "i686-linux"] ./qboot.nix {};
+  qemu-vm-restrictnetwork = handleTest ./qemu-vm-restrictnetwork.nix {};
   quorum = handleTest ./quorum.nix {};
   quake3 = handleTest ./quake3.nix {};
   rabbitmq = handleTest ./rabbitmq.nix {};

nixos/tests/qemu-vm-restrictnetwork.nix

@@ -0,0 +1,36 @@
+import ./make-test-python.nix ({
+  name = "qemu-vm-restrictnetwork";
+
+  nodes = {
+    unrestricted = { config, pkgs, ... }: {
+      virtualisation.restrictNetwork = false;
+    };
+
+    restricted = { config, pkgs, ... }: {
+      virtualisation.restrictNetwork = true;
+    };
+  };
+
+  testScript = ''
+    import os
+
+    if os.fork() == 0:
+      # Start some HTTP server on the qemu host to test guest isolation.
+      from http.server import HTTPServer, BaseHTTPRequestHandler
+      HTTPServer(("", 8000), BaseHTTPRequestHandler).serve_forever()
+
+    else:
+      start_all()
+      unrestricted.wait_for_unit("network-online.target")
+      restricted.wait_for_unit("network-online.target")
+
+      # Guests should be able to reach each other on the same VLAN.
+      unrestricted.succeed("ping -c1 restricted")
+      restricted.succeed("ping -c1 unrestricted")
+
+      # Only the unrestricted guest should be able to reach host services.
+      # 10.0.2.2 is the gateway mapping to the host's loopback interface.
+      unrestricted.succeed("curl -s http://10.0.2.2:8000")
+      restricted.fail("curl -s http://10.0.2.2:8000")
+    '';
+})