diff --git a/internal/dnsforward/http.go b/internal/dnsforward/http.go
index d0fd07167da..b7428334b69 100644
--- a/internal/dnsforward/http.go
+++ b/internal/dnsforward/http.go
@@ -70,7 +70,7 @@ type jsonDNSConfig struct {
 	DisableIPv6 *bool `json:"disable_ipv6"`
 
 	// UpstreamMode defines the way DNS requests are constructed.
-	UpstreamMode *string `json:"upstream_mode"`
+	UpstreamMode *UpstreamMode `json:"upstream_mode"`
 
 	// BlockedResponseTTL is the TTL for blocked responses.
 	BlockedResponseTTL *uint32 `json:"blocked_response_ttl"`
@@ -114,6 +114,22 @@ type jsonDNSConfig struct {
 	DefaultLocalPTRUpstreams []string `json:"default_local_ptr_upstreams,omitempty"`
 }
 
+// UpstreamMode is a enumeration of upstream modes.
+type UpstreamMode string
+
+const (
+	// UpstreamModeEmpty is the default value on frontend, it is used as
+	// UpstreamModeLoadBalance mode.
+	//
+	// Deprecated
+	// TODO(d.kolyshev): Use UpstreamModeLoadBalance instead.
+	UpstreamModeEmpty UpstreamMode = ""
+
+	UpstreamModeLoadBalance UpstreamMode = "load_balance"
+	UpstreamModeParallel    UpstreamMode = "parallel"
+	UpstreamModeFastestAddr UpstreamMode = "fastest_addr"
+)
+
 func (s *Server) getDNSConfig() (c *jsonDNSConfig) {
 	protectionEnabled, protectionDisabledUntil := s.UpdatedProtectionStatus()
 
@@ -145,14 +161,16 @@ func (s *Server) getDNSConfig() (c *jsonDNSConfig) {
 	usePrivateRDNS := s.conf.UsePrivateRDNS
 	localPTRUpstreams := stringutil.CloneSliceOrEmpty(s.conf.LocalPTRResolvers)
 
-	var upstreamMode string
-	// TODO(d.kolyshev): Use 'load_balance' on frontend instead of nil as a
-	// default value.
+	var upstreamMode UpstreamMode
 	switch s.conf.UpstreamMode {
+	case UpstreamModeTypeLoadBalance:
+		// TODO(d.kolyshev): Support UpstreamModeLoadBalance on frontend instead
+		// of UpstreamModeEmpty.
+		upstreamMode = UpstreamModeEmpty
 	case UpstreamModeTypeParallel:
-		upstreamMode = "parallel"
+		upstreamMode = UpstreamModeParallel
 	case UpstreamModeTypeFastestAddr:
-		upstreamMode = "fastest_addr"
+		upstreamMode = UpstreamModeFastestAddr
 	}
 
 	defPTRUps, err := s.defaultLocalPTRUpstreams()
@@ -232,7 +250,11 @@ func (req *jsonDNSConfig) checkUpstreamMode() (err error) {
 	}
 
 	switch um := *req.UpstreamMode; um {
-	case "", "parallel", "fastest_addr", "load_balance":
+	case
+		UpstreamModeEmpty,
+		UpstreamModeLoadBalance,
+		UpstreamModeParallel,
+		UpstreamModeFastestAddr:
 		return nil
 	default:
 		return fmt.Errorf("upstream_mode: incorrect value %q", um)
@@ -466,13 +488,13 @@ func (s *Server) setConfig(dc *jsonDNSConfig) (shouldRestart bool) {
 
 // mustParseUpstreamMode returns an upstream mode parsed from string.  Panics in
 // case of invalid value.
-func mustParseUpstreamMode(mode string) (um string) {
+func mustParseUpstreamMode(mode UpstreamMode) (um string) {
 	switch mode {
-	case "", "load_balance":
+	case UpstreamModeEmpty, UpstreamModeLoadBalance:
 		return UpstreamModeTypeLoadBalance
-	case "parallel":
+	case UpstreamModeParallel:
 		return UpstreamModeTypeParallel
-	case "fastest_addr":
+	case UpstreamModeFastestAddr:
 		return UpstreamModeTypeFastestAddr
 	default:
 		// Should never happen, since the value should be validated.