fix: using a default when upgrading the module

imageroot/bin/expand-config

@@ -46,8 +46,7 @@ with open("dnsmasq.d/00config.conf", "w") as file:
     # write dhcp-server configuration
     if config["dhcp-server"]["enabled"]:
         file.write("dhcp-range=set:default," + config["dhcp-server"]["start"] + "," + config["dhcp-server"]["end"] + "," + str(config["dhcp-server"]["lease"]) + "h\n")
-        # due to gateway field introduced in later releases, we need to check if it's present
-        if 'gateway' in config ['dhcp-server'] and config["dhcp-server"]["gateway"] != "":
+        if config["dhcp-server"]["gateway"] != "":
             file.write("dhcp-option=tag:default,option:router," + config["dhcp-server"]["gateway"] + "\n")
 
     # write dns-server configuration, if no local Samba DC is present

imageroot/update-module.d/20gateway_default

@@ -0,0 +1,19 @@
+#!/usr/bin/env python3
+
+#
+# Copyright (C) 2024 Nethesis S.r.l.
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+
+import json
+
+config = json.load(open('config.json'))
+
+if config['dhcp-server']['enabled']:
+    if 'gateway' not in config['dhcp-server'] and config['dhcp-server']['gateway'] == '':
+        # the field is left empty, to avoid assigning a default value
+        # this is because if in any case the interface the dhcp-server is running
+        # has two or more ips, a default gateway might cause way too many issues
+        config['dhcp-server']['gateway'] = ''
+
+json.dump(config, fp=open('config.json', 'w'))