Some work to do with systemd-networkd. Still nowhere near enough to create an actual config.

flake.nix

@@ -45,7 +45,8 @@
 
               modules =
                 (sysCfg.modules or [ ]) ++ [
-                  ./modules/metanix-core.nix
+                  ./modules/metanix/core.nix
+                  ./modules/metanix/networkd.nix
 
                   # Disko wiring if present
                   (if sysCfg ? diskoConfig then

lib/addressing/default.nix

@@ -331,6 +331,11 @@ let
             {
               ip-address = ip;
               inherit hostname fqdn;
+
+              # New context fields:
+              location = locationName;
+              subnetType = typeName;
+              subnetKey = "${locationName}-${typeName}";
             }
             // optionalAttrs (hostCfg ? hw-address) {
               inherit (hostCfg) hw-address;
@@ -341,7 +346,11 @@ let
             // optionalAttrs (hostCfg ? aliases) {
               inherit (hostCfg) aliases;
             }
-          ))
+            // optionalAttrs (hostCfg ? interface) {
+              inherit (hostCfg) interface;
+            }
+          )
+          )
           hostNames))
         subnets)
       locations;

meta.nix

@@ -47,6 +47,7 @@
           deimos = {
             role = "server";
             hw-address = "10:98:36:a0:2c:b2";
+            interface = "eno2";
             aliases = [
               "kasear.net"
               "cloud.kasear.net"

modules/metanix/networkd.nix

@@ -0,0 +1,77 @@
+{ lib, config, ... }:
+
+let
+  inherit (lib) mkOption mkIf types attrValues filterAttrs head;
+
+  metanix = config.metanix or { };
+
+  # For now, tell Metanix "which host am I?"
+  thisHostName = config.metanix.thisHost or config.networking.hostName;
+
+  host = metanix.network.hosts.${thisHostName} or null;
+
+  # Find the subnet this host belongs to by matching IP prefix
+  # VERY SIMPLE: relies on your rigid addressing scheme
+  findSubnetForHost =
+    ip:
+    let
+      subnets = metanix.network.subnets or { };
+      matches =
+        filterAttrs
+          (_: s: lib.strings.hasPrefix (lib.strings.concatStringsSep "." (lib.take 3 (lib.splitString "." s.cidr)))
+            (lib.strings.concatStringsSep "." (lib.take 3 (lib.splitString "." ip))))
+          subnets;
+    in
+    if matches == { }
+    then null
+    else head (attrValues matches);
+
+  cidrPrefix =
+    cidr:
+    let
+      # match "10.1.32.0/19" → [ "19" ]
+      m = builtins.match ".*/([0-9]+)" cidr;
+    in
+    if m == null then 32 else builtins.fromJSON (head m);
+
+  hostSubnet =
+    if host == null then null else findSubnetForHost host."ip-address";
+
+  prefixLen =
+    if hostSubnet == null then 32 else cidrPrefix hostSubnet.cidr;
+
+in
+{
+  options.metanix.thisHost = mkOption {
+    type = types.nullOr types.str;
+    default = null;
+    description = ''
+      Logical Metanix host name for this machine.
+      Used to look up IP/interface in metanix.network.hosts.
+    '';
+  };
+
+  config = mkIf (host != null && host ? interface) {
+    systemd.network.enable = true;
+
+    systemd.network.networks."10-metanix-${host.interface}" = {
+      matchConfig = {
+        Name = host.interface;
+      };
+
+      # Static address from Metanix
+      address = [
+        "${host."ip-address"}/${toString prefixLen}"
+      ];
+
+      # Optional: match MAC if present
+      linkConfig = lib.mkIf (host ? "hw-address") {
+        MACAddress = host."hw-address";
+      };
+
+      # TODO later: gateway & DNS resolution from metanix.network / policy
+      # gateway = "...";
+      # dns = [ ... ];
+    };
+  };
+}