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

2 files changed, 14 insertions, 3 deletions

diff --git a/pkg/tcpip/network/ipv6/icmp_test.go b/pkg/tcpip/network/ipv6/icmp_test.go
index 797176243..15574bab1 100644
--- a/pkg/tcpip/network/ipv6/icmp_test.go
+++ b/pkg/tcpip/network/ipv6/icmp_test.go
@@ -161,7 +161,7 @@ func (c *testContext) cleanup() {
 func TestLinkResolution(t *testing.T) {
 	c := newTestContext(t)
 	defer c.cleanup()
-	r, err := c.s0.FindRoute(1, lladdr0, lladdr1, ProtocolNumber)
+	r, err := c.s0.FindRoute(1, lladdr0, lladdr1, ProtocolNumber, false /* multicastLoop */)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/pkg/tcpip/network/ipv6/ipv6.go b/pkg/tcpip/network/ipv6/ipv6.go
index 5f68ef7d5..df3b64c98 100644
--- a/pkg/tcpip/network/ipv6/ipv6.go
+++ b/pkg/tcpip/network/ipv6/ipv6.go
@@ -84,7 +84,7 @@ func (e *endpoint) MaxHeaderLength() uint16 {
 }
 
 // WritePacket writes a packet to the given destination address and protocol.
-func (e *endpoint) WritePacket(r *stack.Route, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8) *tcpip.Error {
+func (e *endpoint) WritePacket(r *stack.Route, hdr buffer.Prependable, payload buffer.VectorisedView, protocol tcpip.TransportProtocolNumber, ttl uint8, loop stack.PacketLooping) *tcpip.Error {
 	length := uint16(hdr.UsedLength() + payload.Size())
 	ip := header.IPv6(hdr.Prepend(header.IPv6MinimumSize))
 	ip.Encode(&header.IPv6Fields{
@@ -94,8 +94,19 @@ func (e *endpoint) WritePacket(r *stack.Route, hdr buffer.Prependable, payload b
 		SrcAddr:       r.LocalAddress,
 		DstAddr:       r.RemoteAddress,
 	})
-	r.Stats().IP.PacketsSent.Increment()
 
+	if loop&stack.PacketLoop != 0 {
+		views := make([]buffer.View, 1, 1+len(payload.Views()))
+		views[0] = hdr.View()
+		views = append(views, payload.Views()...)
+		vv := buffer.NewVectorisedView(len(views[0])+payload.Size(), views)
+		e.HandlePacket(r, vv)
+	}
+	if loop&stack.PacketOut == 0 {
+		return nil
+	}
+
+	r.Stats().IP.PacketsSent.Increment()
 	return e.linkEP.WritePacket(r, hdr, payload, ProtocolNumber)
 }