nixos/networking: make /etc/netgroup by default

This will prevent nscd from complaining /etc/netgroup being absent.

Signed-off-by: Masanori Ogino <167209+omasanori@users.noreply.github.com>

nixos/doc/manual/release-notes/rl-2103.xml

@@ -509,6 +509,12 @@ http://some.json-exporter.host:7979/probe?target=https://example.com/some/json/e
        The GNOME desktop manager once again installs <package>gnome3.epiphany</package> by default.
      </para>
    </listitem>
+   <listitem>
+    <para>
+     NixOS now generates empty <literal>/etc/netgroup</literal>.
+     <literal>/etc/netgroup</literal> defines network-wide groups and may affect to setups using NIS.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 </section>

nixos/modules/config/networking.nix

@@ -193,6 +193,10 @@ in
           cat ${escapeShellArgs cfg.hostFiles} > $out
         '';
 
+        # /etc/netgroup: Network-wide groups.
+        netgroup.text = mkDefault ''
+        '';
+
         # /etc/host.conf: resolver configuration file
         "host.conf".text = ''
           multi on