Merge #152699: Revert "nixos/dhcpd: switch to DynamicUser"

A test got broken:
https://hydra.nixos.org/build/162575182
See GitHub discussion on the reverted PR.

nixos/modules/services/networking/dhcpd.nix

···
       }
     '';
 
-  dhcpdService = postfix: cfg:
-    let
-      configFile =
-        if cfg.configFile != null
-          then cfg.configFile
-          else writeConfig cfg;
-      leaseFile = "/var/lib/dhcpd${postfix}/dhcpd.leases";
-      args = [
-        "@${pkgs.dhcp}/sbin/dhcpd" "dhcpd${postfix}" "-${postfix}"
-        "-pf" "/run/dhcpd${postfix}/dhcpd.pid"
-        "-cf" configFile
-        "-lf" leaseFile
-      ] ++ cfg.extraFlags
-        ++ cfg.interfaces;
-    in
-      optionalAttrs cfg.enable {
-        "dhcpd${postfix}" = {
-          description = "DHCPv${postfix} server";
-          wantedBy = [ "multi-user.target" ];
-          after = [ "network.target" ];
+  dhcpdService = postfix: cfg: optionalAttrs cfg.enable {
+    "dhcpd${postfix}" = {
+      description = "DHCPv${postfix} server";
+      wantedBy = [ "multi-user.target" ];
+      after = [ "network.target" ];
+
+      preStart = ''
+        mkdir -m 755 -p ${cfg.stateDir}
+        chown dhcpd:nogroup ${cfg.stateDir}
+        touch ${cfg.stateDir}/dhcpd.leases
+      '';
+
+      serviceConfig =
+        let
+          configFile = if cfg.configFile != null then cfg.configFile else writeConfig cfg;
+          args = [ "@${pkgs.dhcp}/sbin/dhcpd" "dhcpd${postfix}" "-${postfix}"
+                   "-pf" "/run/dhcpd${postfix}/dhcpd.pid"
+                   "-cf" "${configFile}"
+                   "-lf" "${cfg.stateDir}/dhcpd.leases"
+                   "-user" "dhcpd" "-group" "nogroup"
+                 ] ++ cfg.extraFlags
+                   ++ cfg.interfaces;
 
-          preStart = "touch ${leaseFile}";
-          serviceConfig = {
-            ExecStart = concatMapStringsSep " " escapeShellArg args;
-            Type = "forking";
-            Restart = "always";
-            DynamicUser = true;
-            User = "dhcpd";
-            Group = "dhcpd";
-            AmbientCapabilities = [
-              "CAP_NET_RAW"          # to send ICMP messages
-              "CAP_NET_BIND_SERVICE" # to bind on DHCP port (67)
-            ];
-            StateDirectory   = "dhcpd${postfix}";
-            RuntimeDirectory = "dhcpd${postfix}";
-            PIDFile = "/run/dhcpd${postfix}/dhcpd.pid";
-          };
+        in {
+          ExecStart = concatMapStringsSep " " escapeShellArg args;
+          Type = "forking";
+          Restart = "always";
+          RuntimeDirectory = [ "dhcpd${postfix}" ];
+          PIDFile = "/run/dhcpd${postfix}/dhcpd.pid";
         };
-      };
+    };
+  };
 
   machineOpts = { ... }: {
 
···
       default = false;
       description = ''
         Whether to enable the DHCPv${postfix} server.
+      '';
+    };
+
+    stateDir = mkOption {
+      type = types.path;
+      # We use /var/lib/dhcp for DHCPv4 to save backwards compatibility.
+      default = "/var/lib/dhcp${if postfix == "4" then "" else postfix}";
+      description = ''
+        State directory for the DHCP server.
       '';
     };
 
···
 
   imports = [
     (mkRenamedOptionModule [ "services" "dhcpd" ] [ "services" "dhcpd4" ])
-  ] ++ flip map [ "4" "6" ] (postfix:
-    mkRemovedOptionModule [ "services" "dhcpd${postfix}" "stateDir" ] ''
-      The DHCP server state directory is now managed with the systemd's DynamicUser mechanism.
-      This means the directory is named after the service (dhcpd${postfix}), created under
-      /var/lib/private/ and symlinked to /var/lib/.
-    ''
-  );
+  ];
 
   ###### interface
 
···
   ###### implementation
 
   config = mkIf (cfg4.enable || cfg6.enable) {
+
+    users = {
+      users.dhcpd = {
+        isSystemUser = true;
+        group = "dhcpd";
+        description = "DHCP daemon user";
+      };
+      groups.dhcpd = {};
+    };
 
     systemd.services = dhcpdService "4" cfg4 // dhcpdService "6" cfg6;