Merge pull request #82941 from flokli/systemd-network-link-no-networkd-without-lib-pushdown

nixos/systemd: apply .link even when networkd is disabled (without the lib refactor this time)

nixos/doc/manual/release-notes/rl-2003.xml

@@ -768,6 +768,14 @@ auth required pam_succeed_if.so uid >= 1000 quiet
      </itemizedlist>
     </para>
   </listitem>
+  <listitem>
+   <para>
+    The <link linkend="opt-systemd.network.links">systemd.network.links</link> option is now respected
+    even when <link linkend="opt-systemd.network.enable">systemd-networkd</command> is disabled.
+    This mirrors the behaviour of systemd - It's udev that parses <literal>.link</literal> files,
+    not <command>systemd-networkd</command>.
+   </para>
+  </listitem>
   </itemizedlist>
  </section>
 </section>

nixos/modules/services/networking/zerotierone.nix

@@ -69,13 +69,14 @@ in
     environment.systemPackages = [ cfg.package ];
 
     # Prevent systemd from potentially changing the MAC address
-    environment.etc."systemd/network/50-zerotier.link".text = ''
+    systemd.network.links."50-zerotier" = {
-      [Match]
+      matchConfig = {
-      OriginalName=zt*
+        OriginalName = "zt*";
-
+      };
-      [Link]
+      linkConfig = {
-      AutoNegotiation=false
+        AutoNegotiation = false;
-      MACAddressPolicy=none
+        MACAddressPolicy = "none";
-    '';
+      };
+    };
   };
 }

nixos/modules/system/boot/networkd.nix

@@ -355,6 +355,14 @@ let
   };
 
   linkOptions = commonNetworkOptions // {
+    # overwrite enable option from above
+    enable = mkOption {
+      default = true;
+      type = types.bool;
+      description = ''
+        Whether to enable this .link unit. It's handled by udev no matter if <command>systemd-networkd</command> is enabled or not
+      '';
+    };
 
     linkConfig = mkOption {
       default = {};
@@ -1045,7 +1053,14 @@ in
 
   };
 
-  config = mkIf config.systemd.network.enable {
+  config = mkMerge [
+    # .link units are honored by udev, no matter if systemd-networkd is enabled or not.
+    {
+      systemd.network.units = mapAttrs' (n: v: nameValuePair "${n}.link" (linkToUnit n v)) cfg.links;
+      environment.etc = unitFiles;
+    }
+
+    (mkIf config.systemd.network.enable {
 
       users.users.systemd-network.group = "systemd-network";
 
@@ -1053,12 +1068,9 @@ in
         "systemd-networkd.service" "systemd-networkd-wait-online.service"
       ];
 
-    systemd.network.units = mapAttrs' (n: v: nameValuePair "${n}.link" (linkToUnit n v)) cfg.links
+      systemd.network.units = mapAttrs' (n: v: nameValuePair "${n}.netdev" (netdevToUnit n v)) cfg.netdevs
-      // mapAttrs' (n: v: nameValuePair "${n}.netdev" (netdevToUnit n v)) cfg.netdevs
         // mapAttrs' (n: v: nameValuePair "${n}.network" (networkToUnit n v)) cfg.networks;
 
-    environment.etc = unitFiles;
-
       systemd.services.systemd-networkd = {
         wantedBy = [ "multi-user.target" ];
         restartTriggers = attrNames unitFiles;
@@ -1084,5 +1096,6 @@ in
       };
 
       services.resolved.enable = mkDefault true;
-  };
+    })
+  ];
 }

nixos/tests/networking.nix

@@ -655,6 +655,31 @@ let
             ), "The IPv6 routing table has not been properly cleaned:\n{}".format(ipv6Residue)
       '';
     };
+    # even with disabled networkd, systemd.network.links should work
+    # (as it's handled by udev, not networkd)
+    link = {
+      name = "Link";
+      nodes.client = { pkgs, ... }: {
+        virtualisation.vlans = [ 1 ];
+        networking = {
+          useNetworkd = networkd;
+          useDHCP = false;
+        };
+        systemd.network.links."50-foo" = {
+          matchConfig = {
+            Name = "foo";
+            Driver = "dummy";
+          };
+          linkConfig.MTUBytes = "1442";
+        };
+      };
+      testScript = ''
+        print(client.succeed("ip l add name foo type dummy"))
+        print(client.succeed("stat /etc/systemd/network/50-foo.link"))
+        client.succeed("udevadm settle")
+        assert "mtu 1442" in client.succeed("ip l show dummy0")
+      '';
+    };
   };
 
 in mapAttrs (const (attrs: makeTest (attrs // {