home-assistant: add a stub network interface in c3d2 for mdns

hosts/home-assistant/default.nix

@@ -1,13 +1,37 @@
 { config, pkgs, ... }:
 
+let
+  c3d2MacAddress = "00:0b:ad:00:1d:ea";
+
+in
 {
   c3d2.deployment.server = "server10";
   microvm = {
     mem = 1 * 1024;
     vcpu = 2;
+    # add a network interface in c3d2 for mDNS
+    interfaces = [ {
+      type = "tap";
+      id = builtins.substring 0 15 "c3d2-${config.networking.hostName}";
+      mac = c3d2MacAddress;
+    } ];
   };
 
   networking.hostName = "home-assistant";
+  systemd.network = {
+    links."40-c3d2" = {
+      matchConfig.MACAddress = c3d2MacAddress;
+      # rename interface to net name
+      linkConfig.Name = "c3d2";
+    };
+    networks."40-c3d2" = {
+      matchConfig.MACAddress = c3d2MacAddress;
+      networkConfig = {
+        LinkLocalAddressing = "yes";
+        IPv6AcceptRA = "no";
+      };
+    };
+  };
 
   services = {
     avahi.enable = true;