Use stack.Route exclusively for writing packets

* Remove stack.Route from incoming packet path.
There is no need to pass around a stack.Route during the incoming path
of a packet. Instead, pass around the packet's link/network layer
information in the packet buffer since all layers may need this
information.

* Support address bound and outgoing packet NIC in routes.
When forwarding is enabled, the source address of a packet may be bound
to a different interface than the outgoing interface. This change
updates stack.Route to hold both NICs so that one can be used to write
packets while the other is used to check if the route's bound address
is valid. Note, we need to hold the address's interface so we can check
if the address is a spoofed address.

* Introduce the concept of a local route.
Local routes are routes where the packet never needs to leave the stack;
the destination is stack-local. We can now route between interfaces
within a stack if the packet never needs to leave the stack, even when
forwarding is disabled.

* Always obtain a route from the stack before sending a packet.
If a packet needs to be sent in response to an incoming packet, a route
must be obtained from the stack to ensure the stack is configured to
send packets to the packet's source from the packet's destination.

* Enable spoofing if a stack may send packets from unowned addresses.
This change required changes to some netgophers since previously,
promiscuous mode was enough to let the netstack respond to all
incoming packets regardless of the packet's destination address. Now
that a stack.Route is not held for each incoming packet, finding a route
may fail with local addresses we don't own but accepted packets for
while in promiscuous mode. Since we also want to be able to send from
any address (in response the received promiscuous mode packets), we need
to enable spoofing.

* Skip transport layer checksum checks for locally generated packets.
If a packet is locally generated, the stack can safely assume that no
errors were introduced while being locally routed since the packet is
never sent out the wire.

Some bugs fixed:
- transport layer checksum was never calculated after NAT.
- handleLocal didn't handle routing across interfaces.
- stack didn't support forwarding across interfaces.
- always consult the routing table before creating an endpoint.

Updates #4688
Fixes #3906

PiperOrigin-RevId: 340943442

1 files changed, 22 insertions, 11 deletions

diff --git a/pkg/tcpip/stack/registration.go b/pkg/tcpip/stack/registration.go
index 203f3b51f..72131ca24 100644
--- a/pkg/tcpip/stack/registration.go
+++ b/pkg/tcpip/stack/registration.go
@@ -63,17 +63,28 @@ const (
 	ControlUnknown
 )
 
+// NetworkPacketInfo holds information about a network layer packet.
+type NetworkPacketInfo struct {
+	// RemoteAddressBroadcast is true if the packet's remote address is a
+	// broadcast address.
+	RemoteAddressBroadcast bool
+
+	// LocalAddressBroadcast is true if the packet's local address is a broadcast
+	// address.
+	LocalAddressBroadcast bool
+}
+
 // TransportEndpoint is the interface that needs to be implemented by transport
 // protocol (e.g., tcp, udp) endpoints that can handle packets.
 type TransportEndpoint interface {
 	// UniqueID returns an unique ID for this transport endpoint.
 	UniqueID() uint64
 
-	// HandlePacket is called by the stack when new packets arrive to
-	// this transport endpoint. It sets pkt.TransportHeader.
+	// HandlePacket is called by the stack when new packets arrive to this
+	// transport endpoint. It sets the packet buffer's transport header.
 	//
-	// HandlePacket takes ownership of pkt.
-	HandlePacket(r *Route, id TransportEndpointID, pkt *PacketBuffer)
+	// HandlePacket takes ownership of the packet.
+	HandlePacket(TransportEndpointID, *PacketBuffer)
 
 	// HandleControlPacket is called by the stack when new control (e.g.
 	// ICMP) packets arrive to this transport endpoint.
@@ -105,8 +116,8 @@ type RawTransportEndpoint interface {
 	// this transport endpoint. The packet contains all data from the link
 	// layer up.
 	//
-	// HandlePacket takes ownership of pkt.
-	HandlePacket(r *Route, pkt *PacketBuffer)
+	// HandlePacket takes ownership of the packet.
+	HandlePacket(*PacketBuffer)
 }
 
 // PacketEndpoint is the interface that needs to be implemented by packet
@@ -172,9 +183,9 @@ type TransportProtocol interface {
 	// protocol that don't match any existing endpoint. For example,
 	// it is targeted at a port that has no listeners.
 	//
-	// HandleUnknownDestinationPacket takes ownership of pkt if it handles
+	// HandleUnknownDestinationPacket takes ownership of the packet if it handles
 	// the issue.
-	HandleUnknownDestinationPacket(r *Route, id TransportEndpointID, pkt *PacketBuffer) UnknownDestinationPacketDisposition
+	HandleUnknownDestinationPacket(TransportEndpointID, *PacketBuffer) UnknownDestinationPacketDisposition
 
 	// SetOption allows enabling/disabling protocol specific features.
 	// SetOption returns an error if the option is not supported or the
@@ -227,8 +238,8 @@ type TransportDispatcher interface {
 	//
 	// pkt.NetworkHeader must be set before calling DeliverTransportPacket.
 	//
-	// DeliverTransportPacket takes ownership of pkt.
-	DeliverTransportPacket(r *Route, protocol tcpip.TransportProtocolNumber, pkt *PacketBuffer) TransportPacketDisposition
+	// DeliverTransportPacket takes ownership of the packet.
+	DeliverTransportPacket(tcpip.TransportProtocolNumber, *PacketBuffer) TransportPacketDisposition
 
 	// DeliverTransportControlPacket delivers control packets to the
 	// appropriate transport protocol endpoint.
@@ -547,7 +558,7 @@ type NetworkEndpoint interface {
 	// this network endpoint. It sets pkt.NetworkHeader.
 	//
 	// HandlePacket takes ownership of pkt.
-	HandlePacket(r *Route, pkt *PacketBuffer)
+	HandlePacket(pkt *PacketBuffer)
 
 	// Close is called when the endpoint is reomved from a stack.
 	Close()