nixos-modules/host: add microvm.host.tapScript

Closes Github issue #251

Author: astro

nixos-modules/host/default.nix

@@ -160,7 +160,7 @@ in
             fi
 
             ${pkgs.iproute2}/bin/ip tuntap add name $id mode tap user ${user} $TAP_FLAGS
-            ${pkgs.iproute2}/bin/ip link set $id up
+            ${config.microvm.host.tapScript}
           done
         '';
       };
@@ -342,6 +342,10 @@ in
       wants = map (name: "microvm@${name}.service") config.microvm.autostart;
     };
 
+    microvm.host.tapScript = lib.mkDefault ''
+      ${pkgs.iproute2}/bin/ip link set $id up
+    '';
+
     # This helper creates tap interfaces and attaches them to a bridge
     # for qemu regardless if it is run as root or not.
     security.wrappers.qemu-bridge-helper = lib.mkIf (!config.virtualisation.libvirtd.enable) {

nixos-modules/host/options.nix

@@ -21,6 +21,23 @@
       '';
     };
 
+    host.tapScript = mkOption {
+      description = ''
+        Commands to run after creating a tap interface
+
+        Defaults to bring the interface up.
+
+        If you do not want the interface to be automatically created
+        at all, just set
+        `systemd.services."microvm-tap-interfaces@%i.service".enable = false`
+      '';
+      example = ''
+        # Attach tap interface to bridge br0, and set it up
+        ''${pkgs.iproute2}/bin/ip link set $id master br0 up
+      '';
+      type = types.lines;
+    };
+
     vms = mkOption {
       type = with types; attrsOf (submodule ({ config, name, ... }: {
         options = {