add declarative port forwarding

Author: Mic92

Makefile

@@ -10,7 +10,7 @@ test-resources:
 	$(NIXOS_SHELL) examples/vm-resources.nix
 
 test-forward:
-	QEMU_NET_OPTS="hostfwd=tcp::2222-:22" $(NIXOS_SHELL) examples/vm.nix
+	$(NIXOS_SHELL) examples/vm-forward.nix
 
 test-graphics:
 	$(NIXOS_SHELL) examples/vm-graphics.nix

README.md

@@ -52,9 +52,12 @@ Or switch to qemu console with `Ctrl-a c` and type:
 ## Port forwarding
 
 To forward ports from the virtual machine to the host, override the
-`QEMU_NET_OPTS` environment variable. 
-In this example the tcp port 2222 on the host is forwarded to port 22 in the virtual
-machine:
+`virtualisation.qemu.networkingOptions` NixOS option.
+See `examples/vm-forward.nix` where the ssh server running on port 22 in the
+virtual machine is made accessible through port 2222 on the host.
+
+If `virtualisation.qemu.networkingOptions` is not overridden the same can be
+also achieved by using the `QEMU_NET_OPTS` environment variable.
 
 ```console
 $ QEMU_NET_OPTS="hostfwd=tcp::2222-:22" nixos-shell

examples/vm-forward.nix

@@ -0,0 +1,10 @@
+{ pkgs, ... }: {
+  services.openssh.enable = true;
+  virtualisation.qemu.networkingOptions = [
+    # We need to re-define our usermode network driver
+    # since we are overriding the default value.
+    "-net nic,netdev=user.0,model=virtio"
+    # Than we can use qemu's hostfwd option to forward ports.
+    "-netdev user,id=user.0,hostfwd=tcp::2222-:22"
+  ];
+}