Resend packets back to netstack if destined to itself

Add option to redirect packet back to netstack if it's destined to itself.
This fixes the problem where connecting to the local NIC address would
not work, e.g.:
echo bar | nc -l -p 8080 &
echo foo | nc 192.168.0.2 8080

PiperOrigin-RevId: 203157739

Author: fvoznika

pkg/tcpip/link/fdbased/endpoint.go

@@ -45,10 +45,14 @@ type endpoint struct {
 	// its end of the communication pipe.
 	closed func(*tcpip.Error)
 
-	vv       *buffer.VectorisedView
-	iovecs   []syscall.Iovec
-	views    []buffer.View
-	attached bool
+	vv         *buffer.VectorisedView
+	iovecs     []syscall.Iovec
+	views      []buffer.View
+	dispatcher stack.NetworkDispatcher
+
+	// egressLocal indicates whether packets destined to itself should be
+	// forwarded to the FD endpoint (true) or be sent back to netstack (false).
+	egressLocal bool
 }
 
 // Options specify the details about the fd-based endpoint to be created.
@@ -59,6 +63,7 @@ type Options struct {
 	ChecksumOffload bool
 	ClosedFunc      func(*tcpip.Error)
 	Address         tcpip.LinkAddress
+	EgressLocal     bool
 }
 
 // New creates a new fd-based endpoint.
@@ -80,14 +85,15 @@ func New(opts *Options) tcpip.LinkEndpointID {
 	}
 
 	e := &endpoint{
-		fd:      opts.FD,
-		mtu:     opts.MTU,
-		caps:    caps,
-		closed:  opts.ClosedFunc,
-		addr:    opts.Address,
-		hdrSize: hdrSize,
-		views:   make([]buffer.View, len(BufConfig)),
-		iovecs:  make([]syscall.Iovec, len(BufConfig)),
+		fd:          opts.FD,
+		mtu:         opts.MTU,
+		caps:        caps,
+		closed:      opts.ClosedFunc,
+		addr:        opts.Address,
+		hdrSize:     hdrSize,
+		views:       make([]buffer.View, len(BufConfig)),
+		iovecs:      make([]syscall.Iovec, len(BufConfig)),
+		egressLocal: opts.EgressLocal,
 	}
 	vv := buffer.NewVectorisedView(0, e.views)
 	e.vv = &vv
@@ -97,13 +103,13 @@ func New(opts *Options) tcpip.LinkEndpointID {
 // Attach launches the goroutine that reads packets from the file descriptor and
 // dispatches them via the provided dispatcher.
 func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
-	e.attached = true
-	go e.dispatchLoop(dispatcher) // S/R-FIXME
+	e.dispatcher = dispatcher
+	go e.dispatchLoop() // S/R-FIXME
 }
 
 // IsAttached implements stack.LinkEndpoint.IsAttached.
 func (e *endpoint) IsAttached() bool {
-	return e.attached
+	return e.dispatcher != nil
 }
 
 // MTU implements stack.LinkEndpoint.MTU. It returns the value initialized
@@ -130,6 +136,12 @@ func (e *endpoint) LinkAddress() tcpip.LinkAddress {
 // WritePacket writes outbound packets to the file descriptor. If it is not
 // currently writable, the packet is dropped.
 func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload buffer.View, protocol tcpip.NetworkProtocolNumber) *tcpip.Error {
+	if !e.egressLocal && r.LocalAddress != "" && r.LocalAddress == r.RemoteAddress {
+		hdrView := hdr.View()
+		vv := buffer.NewVectorisedView(len(hdrView)+len(payload), []buffer.View{hdrView, payload})
+		e.dispatcher.DeliverNetworkPacket(e, r.RemoteLinkAddress, protocol, &vv)
+		return nil
+	}
 	if e.hdrSize > 0 {
 		// Add ethernet header if needed.
 		eth := header.Ethernet(hdr.Prepend(header.EthernetMinimumSize))
@@ -142,7 +154,6 @@ func (e *endpoint) WritePacket(r *stack.Route, hdr *buffer.Prependable, payload
 
 	if len(payload) == 0 {
 		return rawfile.NonBlockingWrite(e.fd, hdr.UsedBytes())
-
 	}
 
 	return rawfile.NonBlockingWrite2(e.fd, hdr.UsedBytes(), payload)
@@ -175,7 +186,7 @@ func (e *endpoint) allocateViews(bufConfig []int) {
 }
 
 // dispatch reads one packet from the file descriptor and dispatches it.
-func (e *endpoint) dispatch(d stack.NetworkDispatcher, largeV buffer.View) (bool, *tcpip.Error) {
+func (e *endpoint) dispatch(largeV buffer.View) (bool, *tcpip.Error) {
 	e.allocateViews(BufConfig)
 
 	n, err := rawfile.BlockingReadv(e.fd, e.iovecs)
@@ -211,7 +222,7 @@ func (e *endpoint) dispatch(d stack.NetworkDispatcher, largeV buffer.View) (bool
 	e.vv.SetSize(n)
 	e.vv.TrimFront(e.hdrSize)
 
-	d.DeliverNetworkPacket(e, addr, p, e.vv)
+	e.dispatcher.DeliverNetworkPacket(e, addr, p, e.vv)
 
 	// Prepare e.views for another packet: release used views.
 	for i := 0; i < used; i++ {
@@ -223,10 +234,10 @@ func (e *endpoint) dispatch(d stack.NetworkDispatcher, largeV buffer.View) (bool
 
 // dispatchLoop reads packets from the file descriptor in a loop and dispatches
 // them to the network stack.
-func (e *endpoint) dispatchLoop(d stack.NetworkDispatcher) *tcpip.Error {
+func (e *endpoint) dispatchLoop() *tcpip.Error {
 	v := buffer.NewView(header.MaxIPPacketSize)
 	for {
-		cont, err := e.dispatch(d, v)
+		cont, err := e.dispatch(v)
 		if err != nil || !cont {
 			if e.closed != nil {
 				e.closed(err)

pkg/tcpip/stack/nic.go

@@ -317,8 +317,7 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remoteLinkAddr tcpip.Lin
 		return
 	}
 
-	r := makeRoute(protocol, dst, src, ref)
-	r.LocalLinkAddress = linkEP.LinkAddress()
+	r := makeRoute(protocol, dst, src, linkEP.LinkAddress(), ref)
 	r.RemoteLinkAddress = remoteLinkAddr
 	ref.ep.HandlePacket(&r, vv)
 	ref.decRef()

pkg/tcpip/stack/route.go

@@ -40,12 +40,13 @@ type Route struct {
 
 // makeRoute initializes a new route. It takes ownership of the provided
 // reference to a network endpoint.
-func makeRoute(netProto tcpip.NetworkProtocolNumber, localAddr, remoteAddr tcpip.Address, ref *referencedNetworkEndpoint) Route {
+func makeRoute(netProto tcpip.NetworkProtocolNumber, localAddr, remoteAddr tcpip.Address, localLinkAddr tcpip.LinkAddress, ref *referencedNetworkEndpoint) Route {
 	return Route{
-		NetProto:      netProto,
-		LocalAddress:  localAddr,
-		RemoteAddress: remoteAddr,
-		ref:           ref,
+		NetProto:         netProto,
+		LocalAddress:     localAddr,
+		LocalLinkAddress: localLinkAddr,
+		RemoteAddress:    remoteAddr,
+		ref:              ref,
 	}
 }
 
@@ -82,6 +83,11 @@ func (r *Route) Resolve(waker *sleep.Waker) *tcpip.Error {
 
 	nextAddr := r.NextHop
 	if nextAddr == "" {
+		// Local link address is already known.
+		if r.RemoteAddress == r.LocalAddress {
+			r.RemoteLinkAddress = r.LocalLinkAddress
+			return nil
+		}
 		nextAddr = r.RemoteAddress
 	}
 	linkAddr, err := r.ref.linkCache.GetLinkAddress(r.ref.nic.ID(), nextAddr, r.LocalAddress, r.NetProto, waker)

pkg/tcpip/stack/stack.go

@@ -647,7 +647,7 @@ func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address, n
 			remoteAddr = ref.ep.ID().LocalAddress
 		}
 
-		r := makeRoute(netProto, ref.ep.ID().LocalAddress, remoteAddr, ref)
+		r := makeRoute(netProto, ref.ep.ID().LocalAddress, remoteAddr, nic.linkEP.LinkAddress(), ref)
 		r.NextHop = s.routeTable[i].Gateway
 		return r, nil
 	}

runsc/boot/network.go

@@ -134,11 +134,10 @@ func (n *Network) CreateLinksAndRoutes(args *CreateLinksAndRoutesArgs, _ *struct
 		}
 
 		linkEP := fdbased.New(&fdbased.Options{
-			FD:              newFD,
-			MTU:             uint32(link.MTU),
-			ChecksumOffload: false,
-			EthernetHeader:  true,
-			Address:         tcpip.LinkAddress(generateRndMac()),
+			FD:             newFD,
+			MTU:            uint32(link.MTU),
+			EthernetHeader: true,
+			Address:        tcpip.LinkAddress(generateRndMac()),
 		})
 
 		log.Infof("Enabling interface %q with id %d on addresses %+v", link.Name, nicID, link.Addresses)