dnsforward: fix upstream mode set

CHANGELOG.md

@@ -33,9 +33,14 @@ NOTE: Add new changes BELOW THIS COMMENT.
 - Currently, AdGuard Home uses a best-effort algorithm to fix invalid IDs of
   filtering-rule lists on startup.  This feature is deprecated, and invalid IDs
   will cause errors on startup in a future version.
-- Node.JS 16.  Future versions will require at least Node.JS 18 to build.
+- Node.JS 16.  Future versions will require at least Node.JS 18 to build.\
+
+### Fixed
+
+- Resetting DNS upstream mode when applying unrelated settings ([#6851]).
 
 [#5829]: https://github.com/AdguardTeam/AdGuardHome/issues/5829
+[#6851]: https://github.com/AdguardTeam/AdGuardHome/issues/6851
 
 <!--
 NOTE: Add new changes ABOVE THIS COMMENT.

internal/dnsforward/http.go

@@ -474,8 +474,6 @@ func (s *Server) setConfig(dc *jsonDNSConfig) (shouldRestart bool) {
 
 	if dc.UpstreamMode != nil {
 		s.conf.UpstreamMode = mustParseUpstreamMode(*dc.UpstreamMode)
-	} else {
-		s.conf.UpstreamMode = UpstreamModeLoadBalance
 	}
 
 	if dc.EDNSCSUseCustom != nil && *dc.EDNSCSUseCustom {

internal/dnsforward/http_test.go

@@ -29,6 +29,10 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// TODO(e.burkov):  Use the better approach to testdata with a separate
+// directory for each test, and a separate file for each subtest.  See the
+// [configmigrate] package.
+
 // emptySysResolvers is an empty [SystemResolvers] implementation that always
 // returns nil.
 type emptySysResolvers struct{}