privacy: Implement a way to disable fallback DNS servers

- Using public DNS servers leads to user's domain queries being known to the
servers, violating privacy. However, it is necessary to address many corner
cases when DNS servers are not known to systemd-resolved but internet
connectivity is working. Allow users to disable fallback DNS servers.

Tests:

- After upgrade to latest version of FreedomBox, the setting is on by default.

- Disabling removes the /etc configuration file and resolvectl shows no fallback
DNS entries.

- Enabling add the /etc configuration file and resolvectl shows fallback
entries. After removing existing DNS servers using resolvectl, one can still
query using fallback servers.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/modules/privacy/forms.py

@@ -22,3 +22,11 @@ class PrivacyForm(forms.Form):
               'target="_blank">popcon.debian.org</a>. Submission happens over '
               'the Tor network for additional anonymity if Tor app is enabled.'
               ), box_name=_(cfg.box_name)))
+
+    dns_fallback = forms.BooleanField(
+        label=_('Allow using fallback DNS servers'), required=False,
+        help_text=_(
+            'Use well-known public DNS servers to resolve domain names in '
+            'unusual circumstances where no DNS servers are known but '
+            'internet connectivity is available. Can be disabled in most '
+            'cases if network connectivity is stable and reliable.'))

plinth/modules/privacy/views.py

@@ -4,6 +4,7 @@
 from django.contrib import messages
 from django.utils.translation import gettext as _
 
+import plinth.modules.names.privileged as names_privileged
 from plinth.modules.privacy.forms import PrivacyForm
 from plinth.views import AppView
 
@@ -20,6 +21,7 @@ class PrivacyAppView(AppView):
         """Return the values to fill in the form."""
         initial = super().get_initial()
         initial.update(privileged.get_configuration())
+        initial.update(names_privileged.get_resolved_configuration())
         return initial
 
     def form_valid(self, form):
@@ -28,11 +30,19 @@ class PrivacyAppView(AppView):
         old_config = form.initial
 
         changes = {}
+        is_changed = False
         if old_config['enable_popcon'] != new_config['enable_popcon']:
             changes['enable_popcon'] = new_config['enable_popcon']
 
+        if old_config['dns_fallback'] != new_config['dns_fallback']:
+            names_privileged.set_resolved_configuration(
+                dns_fallback=new_config['dns_fallback'])
+            is_changed = True
+
         if changes:
             privileged.set_configuration(**changes)
+
+        if changes or is_changed:
             messages.success(self.request, _('Configuration updated'))
 
         return super().form_valid(form)