Explicitly reject detour to empty direct outbounds

common/dialer/detour.go

@@ -6,26 +6,39 @@ import (
 	"sync"
 
 	"github.com/sagernet/sing-box/adapter"
+	"github.com/sagernet/sing/common"
 	E "github.com/sagernet/sing/common/exceptions"
 	M "github.com/sagernet/sing/common/metadata"
 	N "github.com/sagernet/sing/common/network"
 )
 
+type DirectDialer interface {
+	IsEmpty() bool
+}
+
 type DetourDialer struct {
 	outboundManager adapter.OutboundManager
 	detour          string
+	legacyDNSDialer bool
 	dialer          N.Dialer
 	initOnce        sync.Once
 	initErr         error
 }
 
-func NewDetour(outboundManager adapter.OutboundManager, detour string) N.Dialer {
-	return &DetourDialer{outboundManager: outboundManager, detour: detour}
+func NewDetour(outboundManager adapter.OutboundManager, detour string, legacyDNSDialer bool) N.Dialer {
+	return &DetourDialer{
+		outboundManager: outboundManager,
+		detour:          detour,
+		legacyDNSDialer: legacyDNSDialer,
+	}
 }
 
-func (d *DetourDialer) Start() error {
-	_, err := d.Dialer()
-	return err
+func InitializeDetour(dialer N.Dialer) error {
+	detourDialer, isDetour := common.Cast[*DetourDialer](dialer)
+	if !isDetour {
+		return nil
+	}
+	return common.Error(detourDialer.Dialer())
 }
 
 func (d *DetourDialer) Dialer() (N.Dialer, error) {
@@ -34,11 +47,20 @@ func (d *DetourDialer) Dialer() (N.Dialer, error) {
 }
 
 func (d *DetourDialer) init() {
-	var loaded bool
-	d.dialer, loaded = d.outboundManager.Outbound(d.detour)
+	dialer, loaded := d.outboundManager.Outbound(d.detour)
 	if !loaded {
 		d.initErr = E.New("outbound detour not found: ", d.detour)
+		return
 	}
+	if !d.legacyDNSDialer {
+		if directDialer, isDirect := dialer.(DirectDialer); isDirect {
+			if directDialer.IsEmpty() {
+				d.initErr = E.New("detour to an empty direct outbound makes no sense")
+				return
+			}
+		}
+	}
+	d.dialer = dialer
 }
 
 func (d *DetourDialer) DialContext(ctx context.Context, network string, destination M.Socksaddr) (net.Conn, error) {

common/dialer/dialer.go

@@ -23,6 +23,7 @@ type Options struct {
 	DirectResolver   bool
 	ResolverOnDetour bool
 	NewDialer        bool
+	LegacyDNSDialer  bool
 }
 
 // TODO: merge with NewWithOptions
@@ -45,7 +46,7 @@ func NewWithOptions(options Options) (N.Dialer, error) {
 		if outboundManager == nil {
 			return nil, E.New("missing outbound manager")
 		}
-		dialer = NewDetour(outboundManager, dialOptions.Detour)
+		dialer = NewDetour(outboundManager, dialOptions.Detour, options.LegacyDNSDialer)
 	} else {
 		dialer, err = NewDefault(options.Context, dialOptions)
 		if err != nil {