Implement IP_MULTICAST_LOOP.

IP_MULTICAST_LOOP controls whether or not multicast packets sent on the default
route are looped back. In order to implement this switch, support for sending
and looping back multicast packets on the default route had to be implemented.

For now we only support IPv4 multicast.

PiperOrigin-RevId: 237534603
Change-Id: I490ac7ff8e8ebef417c7eb049a919c29d156ac1c

1 files changed, 10 insertions, 2 deletions

diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 2b4185014..c9603ad5e 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -46,17 +46,20 @@ type Route struct {
 	// ref a reference to the network endpoint through which the route
 	// starts.
 	ref *referencedNetworkEndpoint
+
+	multicastLoop bool
 }
 
 // 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, localLinkAddr tcpip.LinkAddress, ref *referencedNetworkEndpoint) Route {
+func makeRoute(netProto tcpip.NetworkProtocolNumber, localAddr, remoteAddr tcpip.Address, localLinkAddr tcpip.LinkAddress, ref *referencedNetworkEndpoint, multicastLoop bool) Route {
 	return Route{
 		NetProto:         netProto,
 		LocalAddress:     localAddr,
 		LocalLinkAddress: localLinkAddr,
 		RemoteAddress:    remoteAddr,
 		ref:              ref,
+		multicastLoop:    multicastLoop,
 	}
 }
 
@@ -134,7 +137,12 @@ func (r *Route) IsResolutionRequired() bool {
 
 // WritePacket writes the packet through the given route.
 func (r *Route) WritePacket(hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error {
-	err := r.ref.ep.WritePacket(r, hdr, payload, protocol, ttl)
+	loop := PacketOut
+	if r.multicastLoop && (header.IsV4MulticastAddress(r.RemoteAddress) || header.IsV6MulticastAddress(r.RemoteAddress)) {
+		loop |= PacketLoop
+	}
+
+	err := r.ref.ep.WritePacket(r, hdr, payload, protocol, ttl, loop)
 	if err == tcpip.ErrNoRoute {
 		r.Stats().IP.OutgoingPacketErrors.Increment()
 	}