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, 15 insertions, 9 deletions

diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index cfda7ec3c..047b704e0 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -513,7 +513,7 @@ func (s *Stack) NewRawEndpoint(transport tcpip.TransportProtocolNumber, network
 
 // createNIC creates a NIC with the provided id and link-layer endpoint, and
 // optionally enable it.
-func (s *Stack) createNIC(id tcpip.NICID, name string, linkEP tcpip.LinkEndpointID, enabled bool) *tcpip.Error {
+func (s *Stack) createNIC(id tcpip.NICID, name string, linkEP tcpip.LinkEndpointID, enabled, loopback bool) *tcpip.Error {
 	ep := FindLinkEndpoint(linkEP)
 	if ep == nil {
 		return tcpip.ErrBadLinkEndpoint
@@ -527,7 +527,7 @@ func (s *Stack) createNIC(id tcpip.NICID, name string, linkEP tcpip.LinkEndpoint
 		return tcpip.ErrDuplicateNICID
 	}
 
-	n := newNIC(s, id, name, ep)
+	n := newNIC(s, id, name, ep, loopback)
 
 	s.nics[id] = n
 	if enabled {
@@ -539,26 +539,32 @@ func (s *Stack) createNIC(id tcpip.NICID, name string, linkEP tcpip.LinkEndpoint
 
 // CreateNIC creates a NIC with the provided id and link-layer endpoint.
 func (s *Stack) CreateNIC(id tcpip.NICID, linkEP tcpip.LinkEndpointID) *tcpip.Error {
-	return s.createNIC(id, "", linkEP, true)
+	return s.createNIC(id, "", linkEP, true, false)
 }
 
 // CreateNamedNIC creates a NIC with the provided id and link-layer endpoint,
 // and a human-readable name.
 func (s *Stack) CreateNamedNIC(id tcpip.NICID, name string, linkEP tcpip.LinkEndpointID) *tcpip.Error {
-	return s.createNIC(id, name, linkEP, true)
+	return s.createNIC(id, name, linkEP, true, false)
+}
+
+// CreateNamedLoopbackNIC creates a NIC with the provided id and link-layer
+// endpoint, and a human-readable name.
+func (s *Stack) CreateNamedLoopbackNIC(id tcpip.NICID, name string, linkEP tcpip.LinkEndpointID) *tcpip.Error {
+	return s.createNIC(id, name, linkEP, true, true)
 }
 
 // CreateDisabledNIC creates a NIC with the provided id and link-layer endpoint,
 // but leave it disable. Stack.EnableNIC must be called before the link-layer
 // endpoint starts delivering packets to it.
 func (s *Stack) CreateDisabledNIC(id tcpip.NICID, linkEP tcpip.LinkEndpointID) *tcpip.Error {
-	return s.createNIC(id, "", linkEP, false)
+	return s.createNIC(id, "", linkEP, false, false)
 }
 
 // CreateDisabledNamedNIC is a combination of CreateNamedNIC and
 // CreateDisabledNIC.
 func (s *Stack) CreateDisabledNamedNIC(id tcpip.NICID, name string, linkEP tcpip.LinkEndpointID) *tcpip.Error {
-	return s.createNIC(id, name, linkEP, false)
+	return s.createNIC(id, name, linkEP, false, false)
 }
 
 // EnableNIC enables the given NIC so that the link-layer endpoint can start
@@ -748,7 +754,7 @@ func (s *Stack) getRefEP(nic *NIC, localAddr tcpip.Address, netProto tcpip.Netwo
 
 // FindRoute creates a route to the given destination address, leaving through
 // the given nic and local address (if provided).
-func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address, netProto tcpip.NetworkProtocolNumber) (Route, *tcpip.Error) {
+func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address, netProto tcpip.NetworkProtocolNumber, multicastLoop bool) (Route, *tcpip.Error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
@@ -758,7 +764,7 @@ func (s *Stack) FindRoute(id tcpip.NICID, localAddr, remoteAddr tcpip.Address, n
 	if id != 0 && !needRoute {
 		if nic, ok := s.nics[id]; ok {
 			if ref := s.getRefEP(nic, localAddr, netProto); ref != nil {
-				return makeRoute(netProto, ref.ep.ID().LocalAddress, remoteAddr, nic.linkEP.LinkAddress(), ref), nil
+				return makeRoute(netProto, ref.ep.ID().LocalAddress, remoteAddr, nic.linkEP.LinkAddress(), ref, multicastLoop && !nic.loopback), nil
 			}
 		}
 	} else {
@@ -774,7 +780,7 @@ 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, nic.linkEP.LinkAddress(), ref)
+					r := makeRoute(netProto, ref.ep.ID().LocalAddress, remoteAddr, nic.linkEP.LinkAddress(), ref, multicastLoop && !nic.loopback)
 					if needRoute {
 						r.NextHop = route.Gateway
 					}