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
Change-Id: I31c9f7c501e3f55007f25e1852c27893a16ac6c4

1 files changed, 11 insertions, 5 deletions

diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 12f5efba5..e4f10cfa1 100644
--- a/pkg/tcpip/stack/route.go
+++ b/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)