netstack: allow defaultHandler respond ICMPv4Echo in promiscuous mode

pkg/tcpip/network/ipv4/icmp.go

@@ -356,12 +356,20 @@ func (e *endpoint) handleICMP(pkt *stack.PacketBuffer) {
 		replyData := stack.PayloadSince(pkt.TransportHeader())
 		defer replyData.Release()
 		ipHdr := header.IPv4(pkt.NetworkHeader().Slice())
+		localAddressTemporary := pkt.NetworkPacketInfo.LocalAddressTemporary
 		localAddressBroadcast := pkt.NetworkPacketInfo.LocalAddressBroadcast
 
-		// It's possible that a raw socket expects to receive this.
+		// It's possible that a raw socket or custom defaultHandler expects to
+		// receive this packet.
 		e.dispatcher.DeliverTransportPacket(header.ICMPv4ProtocolNumber, pkt)
 		pkt = nil
 
+		// Skip direct ICMP echo reply if the packet was received with a temporary
+		// address, allowing custom handlers to take over.
+		if localAddressTemporary {
+			return
+		}
+
 		sent := e.stats.icmp.packetsSent
 		if !e.protocol.allowICMPReply(header.ICMPv4EchoReply, header.ICMPv4UnusedCode) {
 			sent.rateLimited.Increment()

pkg/tcpip/network/ipv4/ipv4.go

@@ -1169,6 +1169,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv4, pkt *stack.PacketBuffer,
 	// If the packet is destined for this device, then it should be delivered
 	// locally. Otherwise, if forwarding is enabled, it should be forwarded.
 	if addressEndpoint := e.AcquireAssignedAddress(dstAddr, e.nic.Promiscuous(), stack.CanBePrimaryEndpoint, true /* readOnly */); addressEndpoint != nil {
+		pkt.NetworkPacketInfo.LocalAddressTemporary = addressEndpoint.Temporary()
 		subnet := addressEndpoint.AddressWithPrefix().Subnet()
 		pkt.NetworkPacketInfo.LocalAddressBroadcast = subnet.IsBroadcast(dstAddr) || dstAddr == header.IPv4Broadcast
 		e.deliverPacketLocally(h, pkt, inNICName)

pkg/tcpip/stack/addressable_endpoint_state.go

@@ -602,7 +602,7 @@ func (a *AddressableEndpointState) AcquireAssignedAddressOrMatching(localAddr tc
 
 	// Proceed to add a new temporary endpoint.
 	addr := localAddr.WithPrefix()
-	ep, err := a.addAndAcquireAddressLocked(addr, AddressProperties{PEB: tempPEB}, Temporary)
+	ep, err := a.addAndAcquireAddressLocked(addr, AddressProperties{PEB: tempPEB, Temporary: true}, Temporary)
 	if err != nil {
 		// addAndAcquireAddressLocked only returns an error if the address is
 		// already assigned but we just checked above if the address exists so we

pkg/tcpip/stack/registration.go

@@ -58,6 +58,10 @@ type NetworkPacketInfo struct {
 	// address.
 	LocalAddressBroadcast bool
 
+	// LocalAddressTemporary is true if the packet's local address is a temporary
+	// address.
+	LocalAddressTemporary bool
+
 	// IsForwardedPacket is true if the packet is being forwarded.
 	IsForwardedPacket bool
 }