diff --git a/lib/qemu-flags.nix b/lib/qemu-flags.nix
new file mode 100644
index 000000000000..05edc1d2cc6c
--- /dev/null
+++ b/lib/qemu-flags.nix
@@ -0,0 +1,15 @@
+# QEMU flags shared between various Nix expressions.
+
+{
+
+  qemuNICFlags = nic: net:
+    "-net nic,vlan=${toString nic},model=virtio " +
+    # Use 232.0.1.<vlan> as the multicast address to connect VMs on
+    # the same vlan, but allow it to be overriden using the
+    # $QEMU_MCAST_ADDR_<vlan> environment variable.  The test driver
+    # sets this variable to prevent collisions between parallel
+    # builds.
+    "-net socket,vlan=${toString nic},mcast=" +
+    "\${QEMU_MCAST_ADDR_${toString net}:-232.0.1.${toString net}:1234} ";
+    
+}
diff --git a/lib/test-driver/Machine.pm b/lib/test-driver/Machine.pm
index ed43d8d255a4..5ff7bf6cea19 100644
--- a/lib/test-driver/Machine.pm
+++ b/lib/test-driver/Machine.pm
@@ -34,6 +34,7 @@ sub new {
             if defined $args->{hda};
         $startCommand .= "-cdrom $args->{cdrom} "
             if defined $args->{cdrom};
+        $startCommand .= $args->{qemuFlags} || "";
     }
 
     my $name = $args->{name};
diff --git a/tests/installer.nix b/tests/installer.nix
index 26423cd9d061..883138f17608 100644
--- a/tests/installer.nix
+++ b/tests/installer.nix
@@ -1,6 +1,7 @@
 { pkgs, nixpkgs, system, ... }:
 
 with pkgs.lib;
+with import ../lib/qemu-flags.nix;
 
 let
 
@@ -104,7 +105,7 @@ let
     ''
       createDisk("harddisk", 4 * 1024);
 
-      my $machine = Machine->new({ hda => "harddisk", cdrom => glob("${iso}/iso/*.iso") });
+      my $machine = Machine->new({ hda => "harddisk", cdrom => glob("${iso}/iso/*.iso"), qemuFlags => '${qemuNICFlags 1 1}' });
       $machine->start;
 
       ${optionalString testChannel ''