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, 11 insertions, 2 deletions

diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 5c8b7977a..0b09e6517 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -59,6 +59,8 @@ func makeRoute(netProto tcpip.NetworkProtocolNumber, localAddr, remoteAddr tcpip
 		loop = PacketLoop
 	} else if multicastLoop && (header.IsV4MulticastAddress(remoteAddr) || header.IsV6MulticastAddress(remoteAddr)) {
 		loop |= PacketLoop
+	} else if remoteAddr == header.IPv4Broadcast {
+		loop |= PacketLoop
 	}
 
 	return Route{
@@ -208,10 +210,17 @@ func (r *Route) Clone() Route {
 	return *r
 }
 
-// MakeLoopedRoute duplicates the given route and tweaks it in case of multicast.
+// MakeLoopedRoute duplicates the given route with special handling for routes
+// used for sending multicast or broadcast packets. In those cases the
+// multicast/broadcast address is the remote address when sending out, but for
+// incoming (looped) packets it becomes the local address. Similarly, the local
+// interface address that was the local address going out becomes the remote
+// address coming in. This is different to unicast routes where local and
+// remote addresses remain the same as they identify location (local vs remote)
+// not direction (source vs destination).
 func (r *Route) MakeLoopedRoute() Route {
 	l := r.Clone()
-	if header.IsV4MulticastAddress(r.RemoteAddress) || header.IsV6MulticastAddress(r.RemoteAddress) {
+	if r.RemoteAddress == header.IPv4Broadcast || header.IsV4MulticastAddress(r.RemoteAddress) || header.IsV6MulticastAddress(r.RemoteAddress) {
 		l.RemoteAddress, l.LocalAddress = l.LocalAddress, l.RemoteAddress
 		l.RemoteLinkAddress = l.LocalLinkAddress
 	}