Populate forwarded packet buffer's TransportHeader

Turns out certain features of iptables (e.g. NAT) will not perform
any checks/work unless both the Network and Transport headers are
populated.

With this change, provide the packet directly to the outgoing
network endpoint's `writePacket` method instead of going
through `WriteHeaderIncludedPacket` which expected the headers
to not be set.

PiperOrigin-RevId: 398304004

2 files changed, 19 insertions, 8 deletions

diff --git a/pkg/tcpip/network/ipv6/ipv6.go b/pkg/tcpip/network/ipv6/ipv6.go
index c824e27fa..3799d8245 100644
--- a/pkg/tcpip/network/ipv6/ipv6.go
+++ b/pkg/tcpip/network/ipv6/ipv6.go
@@ -1024,7 +1024,8 @@ func (e *endpoint) forwardPacket(pkt *stack.PacketBuffer) ip.ForwardingError {
 	// We need to do a deep copy of the IP packet because
 	// WriteHeaderIncludedPacket takes ownership of the packet buffer, but we do
 	// not own it.
-	newHdr := header.IPv6(stack.PayloadSince(pkt.NetworkHeader()))
+	newPkt := pkt.DeepCopyForForwarding(int(r.MaxHeaderLength()))
+	newHdr := header.IPv6(newPkt.NetworkHeader().View())
 
 	// As per RFC 8200 section 3,
 	//
@@ -1032,11 +1033,13 @@ func (e *endpoint) forwardPacket(pkt *stack.PacketBuffer) ip.ForwardingError {
 	//                       each node that forwards the packet.
 	newHdr.SetHopLimit(hopLimit - 1)
 
-	switch err := r.WriteHeaderIncludedPacket(stack.NewPacketBuffer(stack.PacketBufferOptions{
-		ReserveHeaderBytes: int(r.MaxHeaderLength()),
-		Data:               buffer.View(newHdr).ToVectorisedView(),
-		IsForwardedPacket:  true,
-	})); err.(type) {
+	forwardToEp, ok := e.protocol.getEndpointForNIC(r.NICID())
+	if !ok {
+		// The interface was removed after we obtained the route.
+		return &ip.ErrOther{Err: &tcpip.ErrUnknownDevice{}}
+	}
+
+	switch err := forwardToEp.writePacket(r, newPkt, newPkt.TransportProtocolNumber, true /* headerIncluded */); err.(type) {
 	case nil:
 		return nil
 	case *tcpip.ErrMessageTooLong:
@@ -2082,6 +2085,13 @@ func (p *protocol) findEndpointWithAddress(addr tcpip.Address) *endpoint {
 	return nil
 }
 
+func (p *protocol) getEndpointForNIC(id tcpip.NICID) (*endpoint, bool) {
+	p.mu.RLock()
+	defer p.mu.RUnlock()
+	ep, ok := p.mu.eps[id]
+	return ep, ok
+}
+
 func (p *protocol) forgetEndpoint(nicID tcpip.NICID) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
diff --git a/pkg/tcpip/network/ipv6/ipv6_test.go b/pkg/tcpip/network/ipv6/ipv6_test.go
index 0735ebb23..74f19900c 100644
--- a/pkg/tcpip/network/ipv6/ipv6_test.go
+++ b/pkg/tcpip/network/ipv6/ipv6_test.go
@@ -3373,7 +3373,8 @@ func TestForwarding(t *testing.T) {
 
 			ipHeaderLength := header.IPv6MinimumSize
 			icmpHeaderLength := header.ICMPv6MinimumSize
-			totalLength := ipHeaderLength + icmpHeaderLength + test.payloadLength + extHdrLen
+			payloadLength := icmpHeaderLength + test.payloadLength + extHdrLen
+			totalLength := ipHeaderLength + payloadLength
 			hdr := buffer.NewPrependable(totalLength)
 			hdr.Prepend(test.payloadLength)
 			icmpH := header.ICMPv6(hdr.Prepend(icmpHeaderLength))
@@ -3391,7 +3392,7 @@ func TestForwarding(t *testing.T) {
 			copy(hdr.Prepend(extHdrLen), extHdrBytes)
 			ip := header.IPv6(hdr.Prepend(ipHeaderLength))
 			ip.Encode(&header.IPv6Fields{
-				PayloadLength:     uint16(header.ICMPv6MinimumSize + test.payloadLength),
+				PayloadLength:     uint16(payloadLength),
 				TransportProtocol: transportProtocol,
 				HopLimit:          test.TTL,
 				SrcAddr:           test.sourceAddr,