Finally

modules/metanix/networkd.nix

@@ -5,16 +5,24 @@ let
 
   metanix = config.metanix or { };
 
-  thisHostName = config.metanix.thisHost or config.networking.hostName;
+  # Prefer explicit metanix.thisHost, fall back to networking.hostName
+  thisHostName =
+    if metanix ? thisHost && metanix.thisHost != null then metanix.thisHost
+    else config.networking.hostName or null;
 
-  hosts = (metanix.network or { }).hosts or { };
-  host = hosts.${thisHostName} or null;
+  network = metanix.network or { };
+  hosts = network.hosts   or { };
+  subnets = network.subnets or { };
+
+  host =
+    if thisHostName == null || !builtins.hasAttr thisHostName hosts
+    then null
+    else hosts.${thisHostName};
 
-  subnets = (metanix.network or { }).subnets or { };
   subnet =
-    if host != null && host ? subnetKey
-    then subnets.${host.subnetKey} or null
-    else null;
+    if host == null || !(host ? subnetKey) || !builtins.hasAttr host.subnetKey subnets
+    then null
+    else subnets.${host.subnetKey};
 
   cidrPrefix =
     cidr:
@@ -27,8 +35,10 @@ let
     if subnet == null then 32 else cidrPrefix subnet.cidr;
 
   unitName =
-    if host == null then "metanix"
+    if host == null || !(host ? interface)
+    then null
     else "10-${host.location}-${host.subnetType}-${host.interface}";
+
 in
 {
   options.metanix.thisHost = mkOption {
@@ -40,7 +50,7 @@ in
     '';
   };
 
-  config = mkIf (host != null && host ? interface) {
+  config = mkIf (host != null && host ? interface && unitName != null) {
     systemd.network.enable = true;
 
     systemd.network.networks.${unitName} = {
@@ -53,6 +63,8 @@ in
       linkConfig = mkIf (host ? "hw-address") {
         MACAddress = host."hw-address";
       };
+
+      # Later: DNS, gateways, routes from policy
     };
   };
 }

tests/networkd-phobos.nix

@@ -2,17 +2,28 @@
 let
   system = "x86_64-linux";
 
-  # Grab nixpkgs lib for convenience
+  # Real nixpkgs for addressing lib
   pkgs = import <nixpkgs> { inherit system; };
-  lib = pkgs.lib;
+  realLib = pkgs.lib;
 
-  # Metanix world + addressing
+  # Minimal "testing lib" that makes mkIf actually behave like a real if
+  testLib = realLib // {
+    mkOption = x: x;
+    mkIf = cond: body: if cond then body else { };
+
+    types = {
+      str = "str";
+      nullOr = t: "nullOr " + builtins.toString t;
+    };
+  };
+
+  # Your world + addressing lib
   meta = import ../meta.nix;
-  addressing = import ../lib/addressing { inherit lib; };
+  addressing = import ../lib/addressing { lib = realLib; };
 
   network = addressing.mkNetworkFromSpec meta;
 
-  # Fake NixOS config that your module expects
+  # Fake NixOS config for this test
   config = {
     networking.hostName = "phobos";
     metanix = {
@@ -21,16 +32,18 @@ let
     };
   };
 
-  # Call the networkd module directly as a function
+  # Call the Metanix networkd module with the fake lib + config
   networkdModule = import ../modules/metanix/networkd.nix {
-    inherit lib config;
+    lib = testLib;
+    inherit config;
   };
+
 in
 {
-  # Raw Metanix view
+  # Raw Metanix view for sanity
   metanixHost = network.hosts.phobos;
   metanixSubnet = network.subnets."home-main";
 
-  # What the module actually emits
+  # What the module actually emits once mkIf is forced
   systemdNetwork = networkdModule.config.systemd.network;
 }