Add loopback address support

redirect_nftables.go

@@ -46,6 +46,11 @@ func (r *autoRedirect) setupNFTables() error {
 		return err
 	}
 
+	err = r.nftablesCreateLoopbackAddressSets(nft, table)
+	if err != nil {
+		return err
+	}
+
 	skipOutput := len(r.tunOptions.IncludeInterface) > 0 && !common.Contains(r.tunOptions.IncludeInterface, "lo") || common.Contains(r.tunOptions.ExcludeInterface, "lo")
 	if !skipOutput {
 		chainOutput := nft.AddChain(&nftables.Chain{
@@ -61,8 +66,23 @@ func (r *autoRedirect) setupNFTables() error {
 				return err
 			}
 			r.nftablesCreateUnreachable(nft, table, chainOutput)
-			r.nftablesCreateRedirect(nft, table, chainOutput)
-
+			err = r.nftablesCreateRedirect(nft, table, chainOutput)
+			if err != nil {
+				return err
+			}
+			if len(r.tunOptions.Inet4LoopbackAddress) > 0 || len(r.tunOptions.Inet6LoopbackAddress) > 0 {
+				chainOutputRoute := nft.AddChain(&nftables.Chain{
+					Name:     "output_route",
+					Table:    table,
+					Hooknum:  nftables.ChainHookOutput,
+					Priority: nftables.ChainPriorityMangle,
+					Type:     nftables.ChainTypeRoute,
+				})
+				err = r.nftablesCreateLoopbackReroute(nft, table, chainOutputRoute)
+				if err != nil {
+					return err
+				}
+			}
 			chainOutputUDP := nft.AddChain(&nftables.Chain{
 				Name:     "output_udp_icmp",
 				Table:    table,
@@ -77,7 +97,7 @@ func (r *autoRedirect) setupNFTables() error {
 			r.nftablesCreateUnreachable(nft, table, chainOutputUDP)
 			r.nftablesCreateMark(nft, table, chainOutputUDP)
 		} else {
-			r.nftablesCreateRedirect(nft, table, chainOutput, &expr.Meta{
+			err = r.nftablesCreateRedirect(nft, table, chainOutput, &expr.Meta{
 				Key:      expr.MetaKeyOIFNAME,
 				Register: 1,
 			}, &expr.Cmp{
@@ -85,6 +105,9 @@ func (r *autoRedirect) setupNFTables() error {
 				Register: 1,
 				Data:     nftablesIfname(r.tunOptions.Name),
 			})
+			if err != nil {
+				return err
+			}
 		}
 	}
 
@@ -100,12 +123,25 @@ func (r *autoRedirect) setupNFTables() error {
 		return err
 	}
 	r.nftablesCreateUnreachable(nft, table, chainPreRouting)
-	r.nftablesCreateRedirect(nft, table, chainPreRouting)
+	err = r.nftablesCreateRedirect(nft, table, chainPreRouting)
+	if err != nil {
+		return err
+	}
 	if r.tunOptions.AutoRedirectMarkMode {
 		r.nftablesCreateMark(nft, table, chainPreRouting)
-	}
-
-	if r.tunOptions.AutoRedirectMarkMode {
+		if len(r.tunOptions.Inet4LoopbackAddress) > 0 || len(r.tunOptions.Inet6LoopbackAddress) > 0 {
+			chainPreRoutingFilter := nft.AddChain(&nftables.Chain{
+				Name:     "prerouting_filter",
+				Table:    table,
+				Hooknum:  nftables.ChainHookPrerouting,
+				Priority: nftables.ChainPriorityRef(*nftables.ChainPriorityNATDest + 1),
+				Type:     nftables.ChainTypeFilter,
+			})
+			err = r.nftablesCreateLoopbackReroute(nft, table, chainPreRoutingFilter)
+			if err != nil {
+				return err
+			}
+		}
 		chainPreRoutingUDP := nft.AddChain(&nftables.Chain{
 			Name:     "prerouting_udp",
 			Table:    table,

redirect_nftables_exprs.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/metacubex/nftables"
 	"github.com/metacubex/nftables/expr"
+	"github.com/metacubex/sing/common"
 
 	"go4.org/netipx"
 )
@@ -21,6 +22,20 @@ func nftablesCreateExcludeDestinationIPSet(
 	nft *nftables.Conn, table *nftables.Table, chain *nftables.Chain,
 	id uint32, name string, family nftables.TableFamily, invert bool,
 ) {
+	nft.AddRule(&nftables.Rule{
+		Table: table,
+		Chain: chain,
+		Exprs: append(
+			nftablesCreateDestinationIPSetExprs(id, name, family, invert),
+			&expr.Counter{},
+			&expr.Verdict{
+				Kind: expr.VerdictReturn,
+			},
+		),
+	})
+}
+
+func nftablesCreateDestinationIPSetExprs(id uint32, name string, family nftables.TableFamily, invert bool) []expr.Any {
 	exprs := []expr.Any{
 		&expr.Meta{
 			Key:      expr.MetaKeyNFPROTO,
@@ -53,22 +68,63 @@ func nftablesCreateExcludeDestinationIPSet(
 			},
 		)
 	}
-	exprs = append(exprs,
-		&expr.Lookup{
-			SourceRegister: 1,
-			SetID:          id,
-			SetName:        name,
-			Invert:         invert,
-		},
-		&expr.Counter{},
-		&expr.Verdict{
-			Kind: expr.VerdictReturn,
-		})
-	nft.AddRule(&nftables.Rule{
-		Table: table,
-		Chain: chain,
-		Exprs: exprs,
+	exprs = append(exprs, &expr.Lookup{
+		SourceRegister: 1,
+		SetID:          id,
+		SetName:        name,
+		Invert:         invert,
 	})
+	return exprs
+}
+
+func nftablesCreateIPConst(
+	nft *nftables.Conn, table *nftables.Table, id uint32, name string, family nftables.TableFamily, addressList []netip.Addr,
+) (*nftables.Set, error) {
+	var keyType nftables.SetDatatype
+	if family == nftables.TableFamilyIPv4 {
+		keyType = nftables.TypeIPAddr
+	} else {
+		keyType = nftables.TypeIP6Addr
+	}
+	mySet := &nftables.Set{
+		Table:    table,
+		ID:       id,
+		Name:     name,
+		KeyType:  keyType,
+		Constant: true,
+	}
+	if id == 0 {
+		mySet.Anonymous = true
+	}
+	setElements := common.Map(addressList, func(addr netip.Addr) nftables.SetElement { return nftables.SetElement{Key: addr.AsSlice()} })
+	if id == 0 {
+		err := nft.AddSet(mySet, setElements)
+		if err != nil {
+			return nil, err
+		}
+		return mySet, nil
+	} else {
+		err := nft.AddSet(mySet, nil)
+		if err != nil {
+			return nil, err
+		}
+	}
+	for len(setElements) > 0 {
+		toAdd := setElements
+		if len(toAdd) > 1000 {
+			toAdd = toAdd[:1000]
+		}
+		setElements = setElements[len(toAdd):]
+		err := nft.SetAddElements(mySet, toAdd)
+		if err != nil {
+			return nil, err
+		}
+		err = nft.Flush()
+		if err != nil {
+			return nil, err
+		}
+	}
+	return mySet, nil
 }
 
 func nftablesCreateIPSet(

redirect_nftables_rules.go

@@ -117,8 +117,61 @@ func (r *autoRedirect) nftablesCreateLocalAddressSets(
 	return nil
 }
 
+func (r *autoRedirect) nftablesCreateLoopbackAddressSets(
+	nft *nftables.Conn, table *nftables.Table,
+) error {
+	if r.enableIPv4 && len(r.tunOptions.Inet4LoopbackAddress) > 0 {
+		_, err := nftablesCreateIPConst(nft, table, 7, "inet4_local_redirect_address_set", nftables.TableFamilyIPv4, r.tunOptions.Inet4LoopbackAddress)
+		if err != nil {
+			return err
+		}
+	}
+	if r.enableIPv6 && len(r.tunOptions.Inet6LoopbackAddress) > 0 {
+		_, err := nftablesCreateIPConst(nft, table, 8, "inet6_local_redirect_address_set", nftables.TableFamilyIPv6, r.tunOptions.Inet6LoopbackAddress)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func (r *autoRedirect) nftablesCreateExcludeRules(nft *nftables.Conn, table *nftables.Table, chain *nftables.Chain) error {
 	if r.tunOptions.AutoRedirectMarkMode && chain.Hooknum == nftables.ChainHookOutput {
+		if chain.Type == nftables.ChainTypeRoute {
+			ipProto := &nftables.Set{
+				Table:     table,
+				Anonymous: true,
+				Constant:  true,
+				KeyType:   nftables.TypeInetProto,
+			}
+			err := nft.AddSet(ipProto, []nftables.SetElement{
+				{Key: []byte{unix.IPPROTO_UDP}},
+				{Key: []byte{unix.IPPROTO_ICMP}},
+				{Key: []byte{unix.IPPROTO_ICMPV6}},
+			})
+			if err != nil {
+				return err
+			}
+			nft.AddRule(&nftables.Rule{
+				Table: table,
+				Chain: chain,
+				Exprs: []expr.Any{
+					&expr.Meta{
+						Key:      expr.MetaKeyL4PROTO,
+						Register: 1,
+					},
+					&expr.Lookup{
+						SourceRegister: 1,
+						SetID:          ipProto.ID,
+						SetName:        ipProto.Name,
+						Invert:         true,
+					},
+					&expr.Verdict{
+						Kind: expr.VerdictReturn,
+					},
+				},
+			})
+		}
 		nft.AddRule(&nftables.Rule{
 			Table: table,
 			Chain: chain,
@@ -161,6 +214,25 @@ func (r *autoRedirect) nftablesCreateExcludeRules(nft *nftables.Conn, table *nft
 		}
 	}
 	if chain.Hooknum == nftables.ChainHookPrerouting {
+		nft.AddRule(&nftables.Rule{
+			Table: table,
+			Chain: chain,
+			Exprs: []expr.Any{
+				&expr.Meta{
+					Key:      expr.MetaKeyIIFNAME,
+					Register: 1,
+				},
+				&expr.Cmp{
+					Op:       expr.CmpOpEq,
+					Register: 1,
+					Data:     nftablesIfname(r.tunOptions.Name),
+				},
+				&expr.Counter{},
+				&expr.Verdict{
+					Kind: expr.VerdictReturn,
+				},
+			},
+		})
 		if len(r.tunOptions.IncludeInterface) > 0 {
 			if len(r.tunOptions.IncludeInterface) > 1 {
 				includeInterface := &nftables.Set{
@@ -436,44 +508,6 @@ func (r *autoRedirect) nftablesCreateExcludeRules(nft *nftables.Conn, table *nft
 		}
 	}
 
-	if r.tunOptions.AutoRedirectMarkMode &&
-		((chain.Hooknum == nftables.ChainHookOutput && chain.Type == nftables.ChainTypeRoute) ||
-			(chain.Hooknum == nftables.ChainHookPrerouting && chain.Type == nftables.ChainTypeFilter)) {
-		ipProto := &nftables.Set{
-			Table:     table,
-			Anonymous: true,
-			Constant:  true,
-			KeyType:   nftables.TypeInetProto,
-		}
-		err := nft.AddSet(ipProto, []nftables.SetElement{
-			{Key: []byte{unix.IPPROTO_UDP}},
-			{Key: []byte{unix.IPPROTO_ICMP}},
-			{Key: []byte{unix.IPPROTO_ICMPV6}},
-		})
-		if err != nil {
-			return err
-		}
-		nft.AddRule(&nftables.Rule{
-			Table: table,
-			Chain: chain,
-			Exprs: []expr.Any{
-				&expr.Meta{
-					Key:      expr.MetaKeyL4PROTO,
-					Register: 1,
-				},
-				&expr.Lookup{
-					SourceRegister: 1,
-					SetID:          ipProto.ID,
-					SetName:        ipProto.Name,
-					Invert:         true,
-				},
-				&expr.Verdict{
-					Kind: expr.VerdictReturn,
-				},
-			},
-		})
-	}
-
 	if r.enableIPv4 {
 		nftablesCreateExcludeDestinationIPSet(nft, table, chain, 5, "inet4_local_address_set", nftables.TableFamilyIPv4, false)
 	}
@@ -527,6 +561,9 @@ func (r *autoRedirect) nftablesCreateMark(nft *nftables.Conn, table *nftables.Ta
 				SourceRegister: true,
 			},
 			&expr.Counter{},
+			&expr.Verdict{
+				Kind: expr.VerdictReturn,
+			},
 		},
 	})
 }
@@ -534,57 +571,193 @@ func (r *autoRedirect) nftablesCreateMark(nft *nftables.Conn, table *nftables.Ta
 func (r *autoRedirect) nftablesCreateRedirect(
 	nft *nftables.Conn, table *nftables.Table, chain *nftables.Chain,
 	exprs ...expr.Any,
-) {
-	if r.enableIPv4 && !r.enableIPv6 {
-		exprs = append(exprs,
-			&expr.Meta{
-				Key:      expr.MetaKeyNFPROTO,
-				Register: 1,
-			},
-			&expr.Cmp{
-				Op:       expr.CmpOpEq,
-				Register: 1,
-				Data:     []byte{uint8(nftables.TableFamilyIPv4)},
-			})
-	} else if !r.enableIPv4 && r.enableIPv6 {
-		exprs = append(exprs,
-			&expr.Meta{
-				Key:      expr.MetaKeyNFPROTO,
-				Register: 1,
-			},
-			&expr.Cmp{
-				Op:       expr.CmpOpEq,
-				Register: 1,
-				Data:     []byte{uint8(nftables.TableFamilyIPv6)},
-			})
+) error {
+	exprsRedirect := []expr.Any{
+		&expr.Meta{
+			Key:      expr.MetaKeyL4PROTO,
+			Register: 1,
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpEq,
+			Register: 1,
+			Data:     []byte{unix.IPPROTO_TCP},
+		},
+		&expr.Counter{},
+		&expr.Immediate{
+			Register: 1,
+			Data:     binaryutil.BigEndian.PutUint16(r.redirectPort()),
+		},
+		&expr.Redir{
+			RegisterProtoMin: 1,
+			Flags:            unix.NF_NAT_RANGE_PROTO_SPECIFIED,
+		},
+		&expr.Verdict{
+			Kind: expr.VerdictReturn,
+		},
 	}
-	nft.AddRule(&nftables.Rule{
-		Table: table,
-		Chain: chain,
-		Exprs: append(exprs,
-			&expr.Meta{
-				Key:      expr.MetaKeyL4PROTO,
-				Register: 1,
-			},
-			&expr.Cmp{
-				Op:       expr.CmpOpEq,
-				Register: 1,
-				Data:     []byte{unix.IPPROTO_TCP},
-			},
-			&expr.Counter{},
+	if len(r.tunOptions.Inet4LoopbackAddress) == 0 && len(r.tunOptions.Inet6LoopbackAddress) == 0 {
+		if r.enableIPv4 && !r.enableIPv6 {
+			exprs = append(exprs,
+				&expr.Meta{
+					Key:      expr.MetaKeyNFPROTO,
+					Register: 1,
+				},
+				&expr.Cmp{
+					Op:       expr.CmpOpEq,
+					Register: 1,
+					Data:     []byte{uint8(nftables.TableFamilyIPv4)},
+				})
+		} else if !r.enableIPv4 && r.enableIPv6 {
+			exprs = append(exprs,
+				&expr.Meta{
+					Key:      expr.MetaKeyNFPROTO,
+					Register: 1,
+				},
+				&expr.Cmp{
+					Op:       expr.CmpOpEq,
+					Register: 1,
+					Data:     []byte{uint8(nftables.TableFamilyIPv6)},
+				})
+		}
+		nft.AddRule(&nftables.Rule{
+			Table: table,
+			Chain: chain,
+			Exprs: append(exprs, exprsRedirect...),
+		})
+	} else {
+		if r.enableIPv4 {
+			exprs4 := exprs
+			if len(r.tunOptions.Inet4LoopbackAddress) > 0 {
+				exprs4 = append(exprs4, nftablesCreateDestinationIPSetExprs(7, "inet4_local_redirect_address_set", nftables.TableFamilyIPv4, true)...)
+			} else {
+				exprs4 = append(exprs4, &expr.Meta{
+					Key:      expr.MetaKeyNFPROTO,
+					Register: 1,
+				}, &expr.Cmp{
+					Op:       expr.CmpOpEq,
+					Register: 1,
+					Data:     []byte{uint8(nftables.TableFamilyIPv4)},
+				})
+			}
+			nft.AddRule(&nftables.Rule{
+				Table: table,
+				Chain: chain,
+				Exprs: append(exprs4, exprsRedirect...),
+			})
+		}
+		if r.enableIPv6 {
+			exprs6 := exprs
+			if len(r.tunOptions.Inet6LoopbackAddress) > 0 {
+				exprs6 = append(exprs6, nftablesCreateDestinationIPSetExprs(8, "inet6_local_redirect_address_set", nftables.TableFamilyIPv6, true)...)
+			} else {
+				exprs6 = append(exprs6, &expr.Meta{
+					Key:      expr.MetaKeyNFPROTO,
+					Register: 1,
+				}, &expr.Cmp{
+					Op:       expr.CmpOpEq,
+					Register: 1,
+					Data:     []byte{uint8(nftables.TableFamilyIPv6)},
+				})
+			}
+			nft.AddRule(&nftables.Rule{
+				Table: table,
+				Chain: chain,
+				Exprs: append(exprs6, exprsRedirect...),
+			})
+		}
+	}
+	return nil
+}
+
+func (r *autoRedirect) nftablesCreateLoopbackReroute(
+	nft *nftables.Conn, table *nftables.Table, chain *nftables.Chain,
+) error {
+	exprs := []expr.Any{
+		&expr.Meta{
+			Key:      expr.MetaKeyL4PROTO,
+			Register: 1,
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpEq,
+			Register: 1,
+			Data:     []byte{unix.IPPROTO_TCP},
+		},
+		&expr.Meta{
+			Key:      expr.MetaKeyMARK,
+			Register: 1,
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpNeq,
+			Register: 1,
+			Data:     binaryutil.NativeEndian.PutUint32(r.tunOptions.AutoRedirectInputMark),
+		},
+	}
+	var exprs4 []expr.Any
+	if r.enableIPv4 && len(r.tunOptions.Inet4LoopbackAddress) > 0 {
+		exprs4 = append(exprs, nftablesCreateDestinationIPSetExprs(7, "inet4_local_redirect_address_set", nftables.TableFamilyIPv4, false)...)
+	}
+	var exprs6 []expr.Any
+	if r.enableIPv6 && len(r.tunOptions.Inet6LoopbackAddress) > 0 {
+		exprs6 = append(exprs, nftablesCreateDestinationIPSetExprs(8, "inet6_local_redirect_address_set", nftables.TableFamilyIPv6, false)...)
+	}
+	var exprsCreateMark []expr.Any
+	if chain.Hooknum == nftables.ChainHookPrerouting {
+		exprsCreateMark = []expr.Any{
 			&expr.Immediate{
 				Register: 1,
-				Data:     binaryutil.BigEndian.PutUint16(r.redirectPort()),
+				Data:     binaryutil.NativeEndian.PutUint32(r.tunOptions.AutoRedirectInputMark),
 			},
-			&expr.Redir{
-				RegisterProtoMin: 1,
-				Flags:            unix.NF_NAT_RANGE_PROTO_SPECIFIED,
+			&expr.Meta{
+				Key:            expr.MetaKeyMARK,
+				Register:       1,
+				SourceRegister: true,
 			},
-			&expr.Verdict{
-				Kind: expr.VerdictReturn,
+			&expr.Counter{},
+		}
+	} else {
+		exprsCreateMark = []expr.Any{
+			&expr.Immediate{
+				Register: 1,
+				Data:     binaryutil.NativeEndian.PutUint32(r.tunOptions.AutoRedirectInputMark),
 			},
-		),
-	})
+			&expr.Meta{
+				Key:            expr.MetaKeyMARK,
+				Register:       1,
+				SourceRegister: true,
+			},
+			&expr.Meta{
+				Key:      expr.MetaKeyMARK,
+				Register: 1,
+			},
+			&expr.Ct{
+				Key:            expr.CtKeyMARK,
+				Register:       1,
+				SourceRegister: true,
+			},
+			&expr.Counter{},
+		}
+	}
+	if len(exprs4) > 0 {
+		exprs4 = append(exprs4, exprsCreateMark...)
+	}
+	if len(exprs6) > 0 {
+		exprs6 = append(exprs6, exprsCreateMark...)
+	}
+	if len(exprs4) > 0 {
+		nft.AddRule(&nftables.Rule{
+			Table: table,
+			Chain: chain,
+			Exprs: exprs4,
+		})
+	}
+	if len(exprs6) > 0 {
+		nft.AddRule(&nftables.Rule{
+			Table: table,
+			Chain: chain,
+			Exprs: exprs6,
+		})
+	}
+	return nil
 }
 
 func (r *autoRedirect) nftablesCreateDNSHijackRulesForFamily(

stack_gvisor.go

@@ -25,14 +25,16 @@ const WithGVisor = true
 const DefaultNIC tcpip.NICID = 1
 
 type GVisor struct {
-	ctx           context.Context
-	tun           GVisorTun
-	udpTimeout    int64
-	broadcastAddr netip.Addr
-	handler       Handler
-	logger        logger.Logger
-	stack         *stack.Stack
-	endpoint      stack.LinkEndpoint
+	ctx                  context.Context
+	tun                  GVisorTun
+	inet4LoopbackAddress []netip.Addr
+	inet6LoopbackAddress []netip.Addr
+	udpTimeout           int64
+	broadcastAddr        netip.Addr
+	handler              Handler
+	logger               logger.Logger
+	stack                *stack.Stack
+	endpoint             stack.LinkEndpoint
 }
 
 type GVisorTun interface {
@@ -49,12 +51,14 @@ func NewGVisor(
 	}
 
 	gStack := &GVisor{
-		ctx:           options.Context,
-		tun:           gTun,
-		udpTimeout:    options.UDPTimeout,
-		broadcastAddr: BroadcastAddr(options.TunOptions.Inet4Address),
-		handler:       options.Handler,
-		logger:        options.Logger,
+		ctx:                  options.Context,
+		tun:                  gTun,
+		inet4LoopbackAddress: options.TunOptions.Inet4LoopbackAddress,
+		inet6LoopbackAddress: options.TunOptions.Inet6LoopbackAddress,
+		udpTimeout:           options.UDPTimeout,
+		broadcastAddr:        BroadcastAddr(options.TunOptions.Inet4Address),
+		handler:              options.Handler,
+		logger:               options.Logger,
 	}
 	return gStack, nil
 }
@@ -69,7 +73,7 @@ func (t *GVisor) Start() error {
 	if err != nil {
 		return err
 	}
-	ipStack.SetTransportProtocolHandler(tcp.ProtocolNumber, NewTCPForwarder(t.ctx, ipStack, t.handler).HandlePacket)
+	ipStack.SetTransportProtocolHandler(tcp.ProtocolNumber, NewTCPForwarderWithLoopback(t.ctx, ipStack, t.handler, t.inet4LoopbackAddress, t.inet6LoopbackAddress, t.tun).HandlePacket)
 	ipStack.SetTransportProtocolHandler(udp.ProtocolNumber, NewUDPForwarder(t.ctx, ipStack, t.handler).HandlePacket)
 
 	t.stack = ipStack

stack_gvisor_tcp.go

@@ -4,34 +4,77 @@ package tun
 
 import (
 	"context"
+	"net/netip"
 	"time"
 
 	"github.com/metacubex/gvisor/pkg/tcpip"
 	"github.com/metacubex/gvisor/pkg/tcpip/adapters/gonet"
+	"github.com/metacubex/gvisor/pkg/tcpip/header"
 	"github.com/metacubex/gvisor/pkg/tcpip/stack"
 	"github.com/metacubex/gvisor/pkg/tcpip/transport/tcp"
 	"github.com/metacubex/gvisor/pkg/waiter"
+	"github.com/metacubex/sing-tun/internal/gtcpip/checksum"
+	"github.com/metacubex/sing/common"
+	"github.com/metacubex/sing/common/bufio"
 	M "github.com/metacubex/sing/common/metadata"
 )
 
 type TCPForwarder struct {
-	ctx       context.Context
-	stack     *stack.Stack
-	handler   Handler
-	forwarder *tcp.Forwarder
+	ctx                  context.Context
+	stack                *stack.Stack
+	handler              Handler
+	inet4LoopbackAddress []tcpip.Address
+	inet6LoopbackAddress []tcpip.Address
+	tun                  GVisorTun
+	forwarder            *tcp.Forwarder
 }
 
 func NewTCPForwarder(ctx context.Context, stack *stack.Stack, handler Handler) *TCPForwarder {
+	return NewTCPForwarderWithLoopback(ctx, stack, handler, nil, nil, nil)
+}
+
+func NewTCPForwarderWithLoopback(ctx context.Context, stack *stack.Stack, handler Handler, inet4LoopbackAddress []netip.Addr, inet6LoopbackAddress []netip.Addr, tun GVisorTun) *TCPForwarder {
 	forwarder := &TCPForwarder{
-		ctx:     ctx,
-		stack:   stack,
-		handler: handler,
+		ctx:                  ctx,
+		stack:                stack,
+		handler:              handler,
+		inet4LoopbackAddress: common.Map(inet4LoopbackAddress, AddressFromAddr),
+		inet6LoopbackAddress: common.Map(inet6LoopbackAddress, AddressFromAddr),
+		tun:                  tun,
 	}
 	forwarder.forwarder = tcp.NewForwarder(stack, 0, 1024, forwarder.Forward)
 	return forwarder
 }
 
 func (f *TCPForwarder) HandlePacket(id stack.TransportEndpointID, pkt *stack.PacketBuffer) bool {
+	for _, inet4LoopbackAddress := range f.inet4LoopbackAddress {
+		if id.LocalAddress == inet4LoopbackAddress {
+			ipHdr := pkt.Network().(header.IPv4)
+			ipHdr.SetDestinationAddressWithChecksumUpdate(ipHdr.SourceAddress())
+			ipHdr.SetSourceAddressWithChecksumUpdate(inet4LoopbackAddress)
+			tcpHdr := header.TCP(pkt.TransportHeader().Slice())
+			tcpHdr.SetChecksum(0)
+			tcpHdr.SetChecksum(^checksum.Checksum(tcpHdr.Payload(), tcpHdr.CalculateChecksum(
+				header.PseudoHeaderChecksum(header.TCPProtocolNumber, ipHdr.SourceAddress(), ipHdr.DestinationAddress(), ipHdr.PayloadLength()),
+			)))
+			bufio.WriteVectorised(f.tun, pkt.AsSlices())
+			return true
+		}
+	}
+	for _, inet6LoopbackAddress := range f.inet6LoopbackAddress {
+		if id.LocalAddress == inet6LoopbackAddress {
+			ipHdr := pkt.Network().(header.IPv6)
+			ipHdr.SetDestinationAddress(ipHdr.SourceAddress())
+			ipHdr.SetSourceAddress(inet6LoopbackAddress)
+			tcpHdr := header.TCP(pkt.TransportHeader().Slice())
+			tcpHdr.SetChecksum(0)
+			tcpHdr.SetChecksum(^checksum.Checksum(tcpHdr.Payload(), tcpHdr.CalculateChecksum(
+				header.PseudoHeaderChecksum(header.TCPProtocolNumber, ipHdr.SourceAddress(), ipHdr.DestinationAddress(), ipHdr.PayloadLength()),
+			)))
+			bufio.WriteVectorised(f.tun, pkt.AsSlices())
+			return true
+		}
+	}
 	return f.forwarder.HandlePacket(id, pkt)
 }
 

stack_system.go

@@ -21,30 +21,32 @@ import (
 var ErrIncludeAllNetworks = E.New("`system` and `mixed` stack are not available when `includeAllNetworks` is enabled. See https://github.com/SagerNet/sing-tun/issues/25")
 
 type System struct {
-	ctx                context.Context
-	tun                Tun
-	tunName            string
-	mtu                int
-	handler            Handler
-	logger             logger.Logger
-	inet4Prefixes      []netip.Prefix
-	inet6Prefixes      []netip.Prefix
-	inet4ServerAddress netip.Addr
-	inet4Address       netip.Addr
-	inet6ServerAddress netip.Addr
-	inet6Address       netip.Addr
-	broadcastAddr      netip.Addr
-	udpTimeout         int64
-	tcpListener        net.Listener
-	tcpListener6       net.Listener
-	tcpPort            uint16
-	tcpPort6           uint16
-	tcpNat             *TCPNat
-	bindInterface      bool
-	interfaceFinder    control.InterfaceFinder
-	enforceBind        bool
-	frontHeadroom      int
-	txChecksumOffload  bool
+	ctx                  context.Context
+	tun                  Tun
+	tunName              string
+	mtu                  int
+	handler              Handler
+	logger               logger.Logger
+	inet4Prefixes        []netip.Prefix
+	inet6Prefixes        []netip.Prefix
+	inet4ServerAddress   netip.Addr
+	inet4Address         netip.Addr
+	inet6ServerAddress   netip.Addr
+	inet6Address         netip.Addr
+	broadcastAddr        netip.Addr
+	inet4LoopbackAddress []netip.Addr
+	inet6LoopbackAddress []netip.Addr
+	udpTimeout           int64
+	tcpListener          net.Listener
+	tcpListener6         net.Listener
+	tcpPort              uint16
+	tcpPort6             uint16
+	tcpNat               *TCPNat
+	bindInterface        bool
+	interfaceFinder      control.InterfaceFinder
+	enforceBind          bool
+	frontHeadroom        int
+	txChecksumOffload    bool
 }
 
 type Session struct {
@@ -56,19 +58,21 @@ type Session struct {
 
 func NewSystem(options StackOptions) (Stack, error) {
 	stack := &System{
-		ctx:             options.Context,
-		tun:             options.Tun,
-		tunName:         options.TunOptions.Name,
-		mtu:             int(options.TunOptions.MTU),
-		udpTimeout:      options.UDPTimeout,
-		handler:         options.Handler,
-		logger:          options.Logger,
-		inet4Prefixes:   options.TunOptions.Inet4Address,
-		inet6Prefixes:   options.TunOptions.Inet6Address,
-		broadcastAddr:   BroadcastAddr(options.TunOptions.Inet4Address),
-		bindInterface:   options.ForwarderBindInterface,
-		interfaceFinder: options.InterfaceFinder,
-		enforceBind:     options.EnforceBindInterface,
+		ctx:                  options.Context,
+		tun:                  options.Tun,
+		tunName:              options.TunOptions.Name,
+		mtu:                  int(options.TunOptions.MTU),
+		inet4LoopbackAddress: options.TunOptions.Inet4LoopbackAddress,
+		inet6LoopbackAddress: options.TunOptions.Inet6LoopbackAddress,
+		udpTimeout:           options.UDPTimeout,
+		handler:              options.Handler,
+		logger:               options.Logger,
+		inet4Prefixes:        options.TunOptions.Inet4Address,
+		inet6Prefixes:        options.TunOptions.Inet6Address,
+		broadcastAddr:        BroadcastAddr(options.TunOptions.Inet4Address),
+		bindInterface:        options.ForwarderBindInterface,
+		interfaceFinder:      options.InterfaceFinder,
+		enforceBind:          options.EnforceBindInterface,
 	}
 	if len(options.TunOptions.Inet4Address) > 0 {
 		if !HasNextAddress(options.TunOptions.Inet4Address[0], 1) {
@@ -371,11 +375,22 @@ func (s *System) processIPv4TCP(ipHdr header.IPv4, tcpHdr header.TCP) (bool, err
 		ipHdr.SetDestinationAddr(session.Source.Addr())
 		tcpHdr.SetDestinationPort(session.Source.Port())
 	} else {
-		natPort := s.tcpNat.Lookup(source, destination)
-		ipHdr.SetSourceAddr(s.inet4Address)
-		tcpHdr.SetSourcePort(natPort)
-		ipHdr.SetDestinationAddr(s.inet4ServerAddress)
-		tcpHdr.SetDestinationPort(s.tcpPort)
+		var loopback bool
+		for _, inet4LoopbackAddress := range s.inet4LoopbackAddress {
+			if destination.Addr() == inet4LoopbackAddress {
+				ipHdr.SetDestinationAddr(ipHdr.SourceAddr())
+				ipHdr.SetSourceAddr(inet4LoopbackAddress)
+				loopback = true
+				break
+			}
+		}
+		if !loopback {
+			natPort := s.tcpNat.Lookup(source, destination)
+			ipHdr.SetSourceAddr(s.inet4Address)
+			tcpHdr.SetSourcePort(natPort)
+			ipHdr.SetDestinationAddr(s.inet4ServerAddress)
+			tcpHdr.SetDestinationPort(s.tcpPort)
+		}
 	}
 	if !s.txChecksumOffload {
 		tcpHdr.SetChecksum(0)
@@ -451,11 +466,22 @@ func (s *System) processIPv6TCP(ipHdr header.IPv6, tcpHdr header.TCP) (bool, err
 		ipHdr.SetDestinationAddr(session.Source.Addr())
 		tcpHdr.SetDestinationPort(session.Source.Port())
 	} else {
-		natPort := s.tcpNat.Lookup(source, destination)
-		ipHdr.SetSourceAddr(s.inet6Address)
-		tcpHdr.SetSourcePort(natPort)
-		ipHdr.SetDestinationAddr(s.inet6ServerAddress)
-		tcpHdr.SetDestinationPort(s.tcpPort6)
+		var loopback bool
+		for _, inet6LoopbackAddress := range s.inet6LoopbackAddress {
+			if destination.Addr() == inet6LoopbackAddress {
+				ipHdr.SetDestinationAddr(ipHdr.SourceAddr())
+				ipHdr.SetSourceAddr(inet6LoopbackAddress)
+				loopback = true
+				break
+			}
+		}
+		if !loopback {
+			natPort := s.tcpNat.Lookup(source, destination)
+			ipHdr.SetSourceAddr(s.inet6Address)
+			tcpHdr.SetSourcePort(natPort)
+			ipHdr.SetDestinationAddr(s.inet6ServerAddress)
+			tcpHdr.SetDestinationPort(s.tcpPort6)
+		}
 	}
 	if !s.txChecksumOffload {
 		tcpHdr.SetChecksum(0)

tun.go

@@ -68,6 +68,8 @@ type Options struct {
 	AutoRedirectMarkMode     bool
 	AutoRedirectInputMark    uint32
 	AutoRedirectOutputMark   uint32
+	Inet4LoopbackAddress     []netip.Addr
+	Inet6LoopbackAddress     []netip.Addr
 	StrictRoute              bool
 	Inet4RouteAddress        []netip.Prefix
 	Inet6RouteAddress        []netip.Prefix