Avoid nested direct outbound usage

common/dialer/detour.go

@@ -6,6 +6,7 @@ import (
 	"sync"
 
 	"github.com/sagernet/sing-box/adapter"
+	C "github.com/sagernet/sing-box/constant"
 	E "github.com/sagernet/sing/common/exceptions"
 	M "github.com/sagernet/sing/common/metadata"
 	N "github.com/sagernet/sing/common/network"
@@ -14,13 +15,18 @@ import (
 type DetourDialer struct {
 	router            adapter.Router
 	detour            string
+	allowNestedDirect bool
 	dialer            N.Dialer
 	initOnce          sync.Once
 	initErr           error
 }
 
-func NewDetour(router adapter.Router, detour string) N.Dialer {
+func NewDetour(router adapter.Router, detour string, allowNestedDirect bool) N.Dialer {
-	return &DetourDialer{router: router, detour: detour}
+	return &DetourDialer{
+		router:            router,
+		detour:            detour,
+		allowNestedDirect: allowNestedDirect,
+	}
 }
 
 func (d *DetourDialer) Start() error {
@@ -30,10 +36,17 @@ func (d *DetourDialer) Start() error {
 
 func (d *DetourDialer) Dialer() (N.Dialer, error) {
 	d.initOnce.Do(func() {
-		var loaded bool
+		var (
-		d.dialer, loaded = d.router.Outbound(d.detour)
+			dialer adapter.Outbound
+			loaded bool
+		)
+		dialer, loaded = d.router.Outbound(d.detour)
 		if !loaded {
 			d.initErr = E.New("outbound detour not found: ", d.detour)
+		} else if !d.allowNestedDirect && dialer.Type() == C.TypeDirect {
+			d.initErr = E.New("using a direct outbound as a detour is illegal")
+		} else {
+			d.dialer = dialer
 		}
 	})
 	return d.dialer, d.initErr

common/dialer/dialer.go

@@ -23,7 +23,7 @@ func New(router adapter.Router, options option.DialerOptions) (N.Dialer, error)
 			return nil, err
 		}
 	} else {
-		dialer = NewDetour(router, options.Detour)
+		dialer = NewDetour(router, options.Detour, false)
 	}
 	domainStrategy := dns.DomainStrategy(options.DomainStrategy)
 	if domainStrategy != dns.DomainStrategyAsIS || options.Detour == "" {

outbound/direct.go

@@ -35,6 +35,9 @@ type Direct struct {
 
 func NewDirect(router adapter.Router, logger log.ContextLogger, tag string, options option.DirectOutboundOptions) (*Direct, error) {
 	options.UDPFragmentDefault = true
+	if options.Detour != "" {
+		return nil, E.New("detour option for direct outbound is illegal")
+	}
 	outboundDialer, err := dialer.New(router, options.DialerOptions)
 	if err != nil {
 		return nil, err

route/router.go

@@ -195,7 +195,7 @@ func NewRouter(
 			if server.Detour == "" {
 				detour = dialer.NewRouter(router)
 			} else {
-				detour = dialer.NewDetour(router, server.Detour)
+				detour = dialer.NewDetour(router, server.Detour, true)
 			}
 			switch server.Address {
 			case "local":