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

3 files changed, 13 insertions, 8 deletions

diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 8ff4310d5..06bb5abc5 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/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()
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)
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 3976f585c..d1d762a8e 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/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
 	}