all: fix private conf fail on start

CHANGELOG.md

@@ -30,6 +30,15 @@ NOTE: Add new changes BELOW THIS COMMENT.
 - Ability to define custom directories for storage of query log files and
   statistics ([#5992]).
 
+### Changed
+
+- Private RDNS resolution (`dns.use_private_ptr_resolvers` in YAML
+  configuration) now requires a valid "Private reverse DNS servers", when
+  enabled ([#6820]).
+
+  **NOTE:** Disabling private RDNS resolution behaves effectively the same as if
+  no private reverse DNS servers provided by user and by the OS.
+
 ### Fixed
 
 - Statistics for 7 days displayed by day on the dashboard graph ([#6712]).
@@ -41,6 +50,7 @@ NOTE: Add new changes BELOW THIS COMMENT.
 [#6711]: https://github.com/AdguardTeam/AdGuardHome/issues/6711
 [#6712]: https://github.com/AdguardTeam/AdGuardHome/issues/6712
 [#6740]: https://github.com/AdguardTeam/AdGuardHome/issues/6740
+[#6820]: https://github.com/AdguardTeam/AdGuardHome/issues/6820
 
 <!--
 NOTE: Add new changes ABOVE THIS COMMENT.

internal/dnsforward/dnsforward.go

@@ -557,13 +557,15 @@ func (s *Server) setupLocalResolvers(boot upstream.Resolver) (uc *proxy.Upstream
 		return nil, err
 	}
 
-	s.localResolvers, err = proxy.New(&proxy.Config{
+	localResolvers, err := proxy.New(&proxy.Config{
 		UpstreamConfig: uc,
 	})
 	if err != nil {
 		return nil, &LocalResolversError{Err: err}
 	}
 
+	s.localResolvers = localResolvers
+
 	// TODO(e.burkov):  Should we also consider the DNS64 usage?
 	return uc, nil
 }
@@ -617,11 +619,13 @@ func (s *Server) Prepare(conf *ServerConfig) (err error) {
 		return fmt.Errorf("setting up fallback dns servers: %w", err)
 	}
 
-	s.dnsProxy, err = proxy.New(proxyConfig)
+	dnsProxy, err := proxy.New(proxyConfig)
 	if err != nil {
 		return fmt.Errorf("creating proxy: %w", err)
 	}
 
+	s.dnsProxy = dnsProxy
+
 	s.recDetector.clear()
 
 	s.setupAddrProc()
@@ -854,6 +858,8 @@ func (s *Server) Reconfigure(conf *ServerConfig) error {
 		}
 	}
 
+	// TODO(e.burkov):  It seems an error here brings the server down, which is
+	// not reliable enough.
 	err = s.Prepare(conf)
 	if err != nil {
 		return fmt.Errorf("could not reconfigure the server: %w", err)