Implement proper local broadcast behavior

The behavior for sending and receiving local broadcast (255.255.255.255)
traffic is as follows:

Outgoing
--------
* A broadcast packet sent on a socket that is bound to an interface goes out
  that interface
* A broadcast packet sent on an unbound socket follows the route table to
  select the outgoing interface
  + if an explicit route entry exists for 255.255.255.255/32, use that one
  + else use the default route
* Broadcast packets are looped back and delivered following the rules for
  incoming packets (see next). This is the same behavior as for multicast
  packets, except that it cannot be disabled via sockopt.

Incoming
--------
* Sockets wishing to receive broadcast packets must bind to either INADDR_ANY
  (0.0.0.0) or INADDR_BROADCAST (255.255.255.255). No other socket receives
  broadcast packets.
* Broadcast packets are multiplexed to all sockets matching it. This is the
  same behavior as for multicast packets.
* A socket can bind to 255.255.255.255:<port> and then receive its own
  broadcast packets sent to 255.255.255.255:<port>

In addition, this change implicitly fixes an issue with multicast reception. If
two sockets want to receive a given multicast stream and one is bound to ANY
while the other is bound to the multicast address, only one of them will
receive the traffic.

PiperOrigin-RevId: 272792377

1 files changed, 26 insertions, 21 deletions

diff --git a/pkg/tcpip/stack/transport_demuxer.go b/pkg/tcpip/stack/transport_demuxer.go
index 8c768c299..92267ce4d 100644
--- a/pkg/tcpip/stack/transport_demuxer.go
+++ b/pkg/tcpip/stack/transport_demuxer.go
@@ -63,7 +63,7 @@ func (epsByNic *endpointsByNic) handlePacket(r *Route, id TransportEndpointID, v
 
 	// If this is a broadcast or multicast datagram, deliver the datagram to all
 	// endpoints bound to the right device.
-	if id.LocalAddress == header.IPv4Broadcast || header.IsV4MulticastAddress(id.LocalAddress) || header.IsV6MulticastAddress(id.LocalAddress) {
+	if isMulticastOrBroadcast(id.LocalAddress) {
 		mpep.handlePacketAll(r, id, vv)
 		epsByNic.mu.RUnlock() // Don't use defer for performance reasons.
 		return
@@ -338,23 +338,14 @@ func (d *transportDemuxer) deliverPacket(r *Route, protocol tcpip.TransportProto
 		return false
 	}
 
-	// If a sender bound to the Loopback interface sends a broadcast,
-	// that broadcast must not be delivered to the sender.
-	if loopbackSubnet.Contains(r.RemoteAddress) && r.LocalAddress == header.IPv4Broadcast && id.LocalPort == id.RemotePort {
-		return false
-	}
-
-	// If the packet is a broadcast, then find all matching transport endpoints.
-	// Otherwise, try to find a single matching transport endpoint.
-	destEps := make([]*endpointsByNic, 0, 1)
 	eps.mu.RLock()
 
-	if protocol == header.UDPProtocolNumber && id.LocalAddress == header.IPv4Broadcast {
-		for epID, endpoint := range eps.endpoints {
-			if epID.LocalPort == id.LocalPort {
-				destEps = append(destEps, endpoint)
-			}
-		}
+	// Determine which transport endpoint or endpoints to deliver this packet to.
+	// If the packet is a broadcast or multicast, then find all matching
+	// transport endpoints.
+	var destEps []*endpointsByNic
+	if protocol == header.UDPProtocolNumber && isMulticastOrBroadcast(id.LocalAddress) {
+		destEps = d.findAllEndpointsLocked(eps, vv, id)
 	} else if ep := d.findEndpointLocked(eps, vv, id); ep != nil {
 		destEps = append(destEps, ep)
 	}
@@ -426,10 +417,11 @@ func (d *transportDemuxer) deliverControlPacket(n *NIC, net tcpip.NetworkProtoco
 	return true
 }
 
-func (d *transportDemuxer) findEndpointLocked(eps *transportEndpoints, vv buffer.VectorisedView, id TransportEndpointID) *endpointsByNic {
+func (d *transportDemuxer) findAllEndpointsLocked(eps *transportEndpoints, vv buffer.VectorisedView, id TransportEndpointID) []*endpointsByNic {
+	var matchedEPs []*endpointsByNic
 	// Try to find a match with the id as provided.
 	if ep, ok := eps.endpoints[id]; ok {
-		return ep
+		matchedEPs = append(matchedEPs, ep)
 	}
 
 	// Try to find a match with the id minus the local address.
@@ -437,7 +429,7 @@ func (d *transportDemuxer) findEndpointLocked(eps *transportEndpoints, vv buffer
 
 	nid.LocalAddress = ""
 	if ep, ok := eps.endpoints[nid]; ok {
-		return ep
+		matchedEPs = append(matchedEPs, ep)
 	}
 
 	// Try to find a match with the id minus the remote part.
@@ -445,15 +437,24 @@ func (d *transportDemuxer) findEndpointLocked(eps *transportEndpoints, vv buffer
 	nid.RemoteAddress = ""
 	nid.RemotePort = 0
 	if ep, ok := eps.endpoints[nid]; ok {
-		return ep
+		matchedEPs = append(matchedEPs, ep)
 	}
 
 	// Try to find a match with only the local port.
 	nid.LocalAddress = ""
 	if ep, ok := eps.endpoints[nid]; ok {
-		return ep
+		matchedEPs = append(matchedEPs, ep)
 	}
 
+	return matchedEPs
+}
+
+// findEndpointLocked returns the endpoint that most closely matches the given
+// id.
+func (d *transportDemuxer) findEndpointLocked(eps *transportEndpoints, vv buffer.VectorisedView, id TransportEndpointID) *endpointsByNic {
+	if matchedEPs := d.findAllEndpointsLocked(eps, vv, id); len(matchedEPs) > 0 {
+		return matchedEPs[0]
+	}
 	return nil
 }
 
@@ -491,3 +492,7 @@ func (d *transportDemuxer) unregisterRawEndpoint(netProto tcpip.NetworkProtocolN
 		}
 	}
 }
+
+func isMulticastOrBroadcast(addr tcpip.Address) bool {
+	return addr == header.IPv4Broadcast || header.IsV4MulticastAddress(addr) || header.IsV6MulticastAddress(addr)
+}