Pass local link address to DeliverNetworkPacket

This allows a NetworkDispatcher to implement transparent bridging,
assuming all implementations of LinkEndpoint.WritePacket call eth.Encode
with header.EthernetFields.SrcAddr set to the passed
Route.LocalLinkAddress, if it is provided.

PiperOrigin-RevId: 213686651
Change-Id: I446a4ac070970202f0724ef796ff1056ae4dd72a

1 files changed, 19 insertions, 9 deletions

diff --git a/pkg/tcpip/link/fdbased/endpoint.go b/pkg/tcpip/link/fdbased/endpoint.go
index 40a10eb9b..ee99ada07 100644
--- a/pkg/tcpip/link/fdbased/endpoint.go
+++ b/pkg/tcpip/link/fdbased/endpoint.go
@@ -164,17 +164,24 @@ func (e *endpoint) WritePacket(r *stack.Route, hdr buffer.Prependable, payload b
 		views[0] = hdr.View()
 		views = append(views, payload.Views()...)
 		vv := buffer.NewVectorisedView(len(views[0])+payload.Size(), views)
-		e.dispatcher.DeliverNetworkPacket(e, r.RemoteLinkAddress, protocol, vv)
+		e.dispatcher.DeliverNetworkPacket(e, r.RemoteLinkAddress, r.LocalLinkAddress, protocol, vv)
 		return nil
 	}
 	if e.hdrSize > 0 {
 		// Add ethernet header if needed.
 		eth := header.Ethernet(hdr.Prepend(header.EthernetMinimumSize))
-		eth.Encode(&header.EthernetFields{
+		ethHdr := &header.EthernetFields{
 			DstAddr: r.RemoteLinkAddress,
-			SrcAddr: e.addr,
 			Type:    protocol,
-		})
+		}
+
+		// Preserve the src address if it's set in the route.
+		if r.LocalLinkAddress != "" {
+			ethHdr.SrcAddr = r.LocalLinkAddress
+		} else {
+			ethHdr.SrcAddr = e.addr
+		}
+		eth.Encode(ethHdr)
 	}
 
 	if payload.Size() == 0 {
@@ -223,12 +230,15 @@ func (e *endpoint) dispatch(largeV buffer.View) (bool, *tcpip.Error) {
 		return false, nil
 	}
 
-	var p tcpip.NetworkProtocolNumber
-	var addr tcpip.LinkAddress
+	var (
+		p                             tcpip.NetworkProtocolNumber
+		remoteLinkAddr, localLinkAddr tcpip.LinkAddress
+	)
 	if e.hdrSize > 0 {
 		eth := header.Ethernet(e.views[0])
 		p = eth.Type()
-		addr = eth.SourceAddress()
+		remoteLinkAddr = eth.SourceAddress()
+		localLinkAddr = eth.DestinationAddress()
 	} else {
 		// We don't get any indication of what the packet is, so try to guess
 		// if it's an IPv4 or IPv6 packet.
@@ -246,7 +256,7 @@ func (e *endpoint) dispatch(largeV buffer.View) (bool, *tcpip.Error) {
 	vv := buffer.NewVectorisedView(n, e.views[:used])
 	vv.TrimFront(e.hdrSize)
 
-	e.dispatcher.DeliverNetworkPacket(e, addr, p, vv)
+	e.dispatcher.DeliverNetworkPacket(e, remoteLinkAddr, localLinkAddr, p, vv)
 
 	// Prepare e.views for another packet: release used views.
 	for i := 0; i < used; i++ {
@@ -287,7 +297,7 @@ func (e *InjectableEndpoint) Attach(dispatcher stack.NetworkDispatcher) {
 
 // Inject injects an inbound packet.
 func (e *InjectableEndpoint) Inject(protocol tcpip.NetworkProtocolNumber, vv buffer.VectorisedView) {
-	e.dispatcher.DeliverNetworkPacket(e, "", protocol, vv)
+	e.dispatcher.DeliverNetworkPacket(e, "" /* remoteLinkAddr */, "" /* localLinkAddr */, protocol, vv)
 }
 
 // NewInjectable creates a new fd-based InjectableEndpoint.