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: 207995083
Change-Id: I17adc2a04df48bfea711011a5df206326a1fb8ef

4 files changed, 45 insertions, 28 deletions

diff --git a/pkg/tcpip/link/fdbased/endpoint.go b/pkg/tcpip/link/fdbased/endpoint.go
index 4e20cfbf8..152d8f0b2 100644
--- a/pkg/tcpip/link/fdbased/endpoint.go
+++ b/pkg/tcpip/link/fdbased/endpoint.go
@@ -55,10 +55,15 @@ 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
+
+	// handleLocal indicates whether packets destined to itself should be
+	// handled by the netstack internally (true) or be forwarded to the FD
+	// endpoint (false).
+	handleLocal bool
 }
 
 // Options specify the details about the fd-based endpoint to be created.
@@ -71,6 +76,7 @@ type Options struct {
 	Address         tcpip.LinkAddress
 	SaveRestore     bool
 	DisconnectOk    bool
+	HandleLocal     bool
 }
 
 // New creates a new fd-based endpoint.
@@ -100,14 +106,15 @@ 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)),
+		handleLocal: opts.HandleLocal,
 	}
 	vv := buffer.NewVectorisedView(0, e.views)
 	e.vv = &vv
@@ -117,16 +124,16 @@ 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
+	e.dispatcher = dispatcher
 	// Link endpoints are not savable. When transportation endpoints are
 	// saved, they stop sending outgoing packets and all incoming packets
 	// are rejected.
-	go e.dispatchLoop(dispatcher) // S/R-SAFE: See above.
+	go e.dispatchLoop() // S/R-SAFE: See above.
 }
 
 // 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
@@ -153,6 +160,12 @@ 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.handleLocal && 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))
@@ -165,7 +178,6 @@ 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)
@@ -198,7 +210,7 @@ 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)
@@ -234,7 +246,7 @@ 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++ {
@@ -246,10 +258,10 @@ 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)
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 25c06cba5..c1480f97b 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -327,8 +327,7 @@ 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()
diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 200c39289..423f428df 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -50,12 +50,13 @@ 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,
 	}
 }
 
@@ -92,6 +93,11 @@ 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)
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index fa7aeb051..6c4aa7cc5 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -687,7 +687,7 @@ 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
 	}