Pull request 2174: 6820 Warn local ptrs

Squashed commit of the following:

commit c2319658a49eb750c9c362632697c481ff560c71
Merge: c6162a211 bcd143068
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Mar 14 18:10:20 2024 +0300

    Merge branch 'master' into 6820-warn-local-ptrs

commit c6162a211b96e220271383bd8c84e87ad44ba7f8
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 13 19:35:20 2024 +0300

    dnsforward: fix doc

commit c6cce9644e629a085f3b66cac503e9de0bc9b753
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 13 19:19:49 2024 +0300

    all: fix private conf fail on start

commit c11fc3e7abcd4f592d44a3c163b554017eb305a0
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Mar 13 18:43:31 2024 +0300

    WIP

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

@@ -518,6 +518,29 @@ func (s *Server) prepareLocalResolvers(
 	return uc, nil
 }
 
+// LocalResolversError is an error type for errors during local resolvers setup.
+// This is only needed to distinguish these errors from errors returned by
+// creating the proxy.
+type LocalResolversError struct {
+	Err error
+}
+
+// type check
+var _ error = (*LocalResolversError)(nil)
+
+// Error implements the error interface for *LocalResolversError.
+func (err *LocalResolversError) Error() (s string) {
+	return fmt.Sprintf("creating local resolvers: %s", err.Err)
+}
+
+// type check
+var _ errors.Wrapper = (*LocalResolversError)(nil)
+
+// Unwrap implements the [errors.Wrapper] interface for *LocalResolversError.
+func (err *LocalResolversError) Unwrap() error {
+	return err.Err
+}
+
 // setupLocalResolvers initializes and sets the resolvers for local addresses.
 // It assumes s.serverLock is locked or s not running.  It returns the upstream
 // configuration used for private PTR resolving, or nil if it's disabled.  Note,
@@ -534,13 +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, fmt.Errorf("creating local resolvers: %w", err)
+		return nil, &LocalResolversError{Err: err}
 	}
 
+	s.localResolvers = localResolvers
+
 	// TODO(e.burkov):  Should we also consider the DNS64 usage?
 	return uc, nil
 }
@@ -594,11 +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()
@@ -831,6 +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)

internal/home/dns.go

@@ -18,6 +18,7 @@ import (
 	"github.com/AdguardTeam/AdGuardHome/internal/filtering"
 	"github.com/AdguardTeam/AdGuardHome/internal/querylog"
 	"github.com/AdguardTeam/AdGuardHome/internal/stats"
+	"github.com/AdguardTeam/dnsproxy/upstream"
 	"github.com/AdguardTeam/golibs/errors"
 	"github.com/AdguardTeam/golibs/log"
 	"github.com/AdguardTeam/golibs/netutil"
@@ -157,6 +158,17 @@ func initDNSServer(
 	}
 
 	err = Context.dnsServer.Prepare(dnsConf)
+
+	// TODO(e.burkov):  Recreate the server with private RDNS disabled.  This
+	// should go away once the private RDNS resolution is moved to the proxy.
+	var locResErr *dnsforward.LocalResolversError
+	if errors.As(err, &locResErr) && errors.Is(locResErr.Err, upstream.ErrNoUpstreams) {
+		log.Info("WARNING: no local resolvers configured while private RDNS " +
+			"resolution enabled, trying to disable")
+		dnsConf.UsePrivateRDNS = false
+		err = Context.dnsServer.Prepare(dnsConf)
+	}
+
 	if err != nil {
 		return fmt.Errorf("dnsServer.Prepare: %w", err)
 	}