Process Hop-by-Hop header when forwarding IPv6 packets

Currently, we process IPv6 extension headers when receiving packets
but not when forwarding them. This is fine for the most part, with
with one exception: RFC 8200 requires that we process the
Hop-by-Hop headers even while forwarding packets.

This CL adds that support by invoking the Hop-by-hop logic performed
when receiving packets during forwarding as well.

PiperOrigin-RevId: 373145478

6 files changed, 287 insertions, 54 deletions

diff --git a/pkg/tcpip/network/internal/ip/errors.go b/pkg/tcpip/network/internal/ip/errors.go
index 50fabfd79..d3577b377 100644
--- a/pkg/tcpip/network/internal/ip/errors.go
+++ b/pkg/tcpip/network/internal/ip/errors.go
@@ -34,13 +34,13 @@ func (*ErrTTLExceeded) isForwardingError() {}
 
 func (*ErrTTLExceeded) String() string { return "ttl exceeded" }
 
-// ErrIPOptProblem indicates the received packet had a problem with an IP
-// option.
-type ErrIPOptProblem struct{}
+// ErrParameterProblem indicates the received packet had a problem with an IP
+// parameter.
+type ErrParameterProblem struct{}
 
-func (*ErrIPOptProblem) isForwardingError() {}
+func (*ErrParameterProblem) isForwardingError() {}
 
-func (*ErrIPOptProblem) String() string { return "ip option problem" }
+func (*ErrParameterProblem) String() string { return "parameter problem" }
 
 // ErrLinkLocalSourceAddress indicates the received packet had a link-local
 // source address.
diff --git a/pkg/tcpip/network/internal/ip/stats.go b/pkg/tcpip/network/internal/ip/stats.go
index 392f0b0c7..68b8b550e 100644
--- a/pkg/tcpip/network/internal/ip/stats.go
+++ b/pkg/tcpip/network/internal/ip/stats.go
@@ -16,7 +16,7 @@ package ip
 
 import "gvisor.dev/gvisor/pkg/tcpip"
 
-// LINT.IfChange(MultiCounterIPStats)
+// LINT.IfChange(MultiCounterIPForwardingStats)
 
 // MultiCounterIPForwardingStats holds IP forwarding statistics. Each counter
 // may have several versions.
@@ -38,11 +38,30 @@ type MultiCounterIPForwardingStats struct {
 	// because they contained a link-local destination address.
 	LinkLocalDestination tcpip.MultiCounterStat
 
+	// ExtensionHeaderProblem is the number of IP packets which were dropped
+	// because of a problem encountered when processing an IPv6 extension
+	// header.
+	ExtensionHeaderProblem tcpip.MultiCounterStat
+
 	// Errors is the number of IP packets received which could not be
 	// successfully forwarded.
 	Errors tcpip.MultiCounterStat
 }
 
+// Init sets internal counters to track a and b counters.
+func (m *MultiCounterIPForwardingStats) Init(a, b *tcpip.IPForwardingStats) {
+	m.Unrouteable.Init(a.Unrouteable, b.Unrouteable)
+	m.Errors.Init(a.Errors, b.Errors)
+	m.LinkLocalSource.Init(a.LinkLocalSource, b.LinkLocalSource)
+	m.LinkLocalDestination.Init(a.LinkLocalDestination, b.LinkLocalDestination)
+	m.ExtensionHeaderProblem.Init(a.ExtensionHeaderProblem, b.ExtensionHeaderProblem)
+	m.ExhaustedTTL.Init(a.ExhaustedTTL, b.ExhaustedTTL)
+}
+
+// LINT.ThenChange(:MultiCounterIPForwardingStats, ../../../tcpip.go:IPForwardingStats)
+
+// LINT.IfChange(MultiCounterIPStats)
+
 // MultiCounterIPStats holds IP statistics, each counter may have several
 // versions.
 type MultiCounterIPStats struct {
@@ -120,15 +139,6 @@ type MultiCounterIPStats struct {
 }
 
 // Init sets internal counters to track a and b counters.
-func (m *MultiCounterIPForwardingStats) Init(a, b *tcpip.IPForwardingStats) {
-	m.Unrouteable.Init(a.Unrouteable, b.Unrouteable)
-	m.Errors.Init(a.Errors, b.Errors)
-	m.LinkLocalSource.Init(a.LinkLocalSource, b.LinkLocalSource)
-	m.LinkLocalDestination.Init(a.LinkLocalDestination, b.LinkLocalDestination)
-	m.ExhaustedTTL.Init(a.ExhaustedTTL, b.ExhaustedTTL)
-}
-
-// Init sets internal counters to track a and b counters.
 func (m *MultiCounterIPStats) Init(a, b *tcpip.IPStats) {
 	m.PacketsReceived.Init(a.PacketsReceived, b.PacketsReceived)
 	m.DisabledPacketsReceived.Init(a.DisabledPacketsReceived, b.DisabledPacketsReceived)
diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index b11e56c6a..19657fde1 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -645,7 +645,7 @@ func (e *endpoint) forwardPacket(pkt *stack.PacketBuffer) ip.ForwardingError {
 					forwarding: true,
 				}, pkt)
 			}
-			return &ip.ErrIPOptProblem{}
+			return &ip.ErrParameterProblem{}
 		}
 		copied := copy(opts, newOpts)
 		if copied != len(newOpts) {
@@ -827,7 +827,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv4, pkt *stack.PacketBuffer)
 			stats.ip.Forwarding.ExhaustedTTL.Increment()
 		case *ip.ErrNoRoute:
 			stats.ip.Forwarding.Unrouteable.Increment()
-		case *ip.ErrIPOptProblem:
+		case *ip.ErrParameterProblem:
 			e.protocol.stack.Stats().MalformedRcvdPackets.Increment()
 			stats.ip.MalformedPacketsReceived.Increment()
 		default:
diff --git a/pkg/tcpip/network/ipv6/icmp.go b/pkg/tcpip/network/ipv6/icmp.go
index ebb0b73df..247a07dc2 100644
--- a/pkg/tcpip/network/ipv6/icmp.go
+++ b/pkg/tcpip/network/ipv6/icmp.go
@@ -984,11 +984,15 @@ type icmpReasonParameterProblem struct {
 	//                  packet if the field in error is beyond what can fit
 	//                  in the maximum size of an ICMPv6 error message.
 	pointer uint32
+
+	// forwarding indicates that the problem arose while we were trying to forward
+	// a packet.
+	forwarding bool
 }
 
 func (*icmpReasonParameterProblem) isICMPReason() {}
-func (*icmpReasonParameterProblem) isForwarding() bool {
-	return false
+func (p *icmpReasonParameterProblem) isForwarding() bool {
+	return p.forwarding
 }
 
 // icmpReasonPortUnreachable is an error where the transport protocol has no
diff --git a/pkg/tcpip/network/ipv6/ipv6.go b/pkg/tcpip/network/ipv6/ipv6.go
index 659057fa7..4fdb2793d 100644
--- a/pkg/tcpip/network/ipv6/ipv6.go
+++ b/pkg/tcpip/network/ipv6/ipv6.go
@@ -941,6 +941,11 @@ func (e *endpoint) forwardPacket(pkt *stack.PacketBuffer) ip.ForwardingError {
 		return nil
 	}
 
+	// Check extension headers for any errors requiring action during forwarding.
+	if err := e.processExtensionHeaders(h, pkt, true /* forwarding */); err != nil {
+		return &ip.ErrParameterProblem{}
+	}
+
 	r, err := e.protocol.stack.FindRoute(0, "", dstAddr, ProtocolNumber, false /* multicastLoop */)
 	switch err.(type) {
 	case nil:
@@ -1084,6 +1089,8 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 			e.stats.ip.Forwarding.ExhaustedTTL.Increment()
 		case *ip.ErrNoRoute:
 			e.stats.ip.Forwarding.Unrouteable.Increment()
+		case *ip.ErrParameterProblem:
+			e.stats.ip.Forwarding.ExtensionHeaderProblem.Increment()
 		default:
 			panic(fmt.Sprintf("unexpected error %s while trying to forward packet: %#v", err, pkt))
 		}
@@ -1091,6 +1098,28 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 		return
 	}
 
+	// iptables filtering. All packets that reach here are intended for
+	// this machine and need not be forwarded.
+	inNicName := e.protocol.stack.FindNICNameFromID(e.nic.ID())
+	if ok := e.protocol.stack.IPTables().Check(stack.Input, pkt, nil, "" /* preroutingAddr */, inNicName, "" /* outNicName */); !ok {
+		// iptables is telling us to drop the packet.
+		stats.IPTablesInputDropped.Increment()
+		return
+	}
+
+	// Any returned error is only useful for terminating execution early, but
+	// we have nothing left to do, so we can drop it.
+	_ = e.processExtensionHeaders(h, pkt, false /* forwarding */)
+}
+
+// processExtensionHeaders processes the extension headers in the given packet.
+// Returns an error if the processing of a header failed or if the packet should
+// be discarded.
+func (e *endpoint) processExtensionHeaders(h header.IPv6, pkt *stack.PacketBuffer, forwarding bool) error {
+	stats := e.stats.ip
+	srcAddr := h.SourceAddress()
+	dstAddr := h.DestinationAddress()
+
 	// Create a VV to parse the packet. We don't plan to modify anything here.
 	// vv consists of:
 	// - Any IPv6 header bytes after the first 40 (i.e. extensions).
@@ -1101,15 +1130,6 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 	vv.AppendViews(pkt.Data().Views())
 	it := header.MakeIPv6PayloadIterator(header.IPv6ExtensionHeaderIdentifier(h.NextHeader()), vv)
 
-	// iptables filtering. All packets that reach here are intended for
-	// this machine and need not be forwarded.
-	inNicName := e.protocol.stack.FindNICNameFromID(e.nic.ID())
-	if ok := e.protocol.stack.IPTables().Check(stack.Input, pkt, nil, "" /* preroutingAddr */, inNicName, "" /* outNicName */); !ok {
-		// iptables is telling us to drop the packet.
-		stats.IPTablesInputDropped.Increment()
-		return
-	}
-
 	var (
 		hasFragmentHeader bool
 		routerAlert       *header.IPv6RouterAlertOption
@@ -1122,22 +1142,41 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 		extHdr, done, err := it.Next()
 		if err != nil {
 			stats.MalformedPacketsReceived.Increment()
-			return
+			return err
 		}
 		if done {
 			break
 		}
 
+		// As per RFC 8200, section 4:
+		//
+		//   Extension headers (except for the Hop-by-Hop Options header) are
+		//   not processed, inserted, or deleted by any node along a packet's
+		//   delivery path until the packet reaches the node identified in the
+		//   Destination Address field of the IPv6 header.
+		//
+		// Furthermore, as per RFC 8200 section 4.1, the Hop By Hop extension
+		// header is restricted to appear first in the list of extension headers.
+		//
+		// Therefore, we can immediately return once we hit any header other
+		// than the Hop-by-Hop header while forwarding a packet.
+		if forwarding {
+			if _, ok := extHdr.(header.IPv6HopByHopOptionsExtHdr); !ok {
+				return nil
+			}
+		}
+
 		switch extHdr := extHdr.(type) {
 		case header.IPv6HopByHopOptionsExtHdr:
 			// As per RFC 8200 section 4.1, the Hop By Hop extension header is
 			// restricted to appear immediately after an IPv6 fixed header.
 			if previousHeaderStart != 0 {
 				_ = e.protocol.returnError(&icmpReasonParameterProblem{
-					code:    header.ICMPv6UnknownHeader,
-					pointer: previousHeaderStart,
+					code:       header.ICMPv6UnknownHeader,
+					pointer:    previousHeaderStart,
+					forwarding: forwarding,
 				}, pkt)
-				return
+				return fmt.Errorf("found Hop-by-Hop header = %#v with non-zero previous header offset = %d", extHdr, previousHeaderStart)
 			}
 
 			optsIt := extHdr.Iter()
@@ -1146,7 +1185,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 				opt, done, err := optsIt.Next()
 				if err != nil {
 					stats.MalformedPacketsReceived.Increment()
-					return
+					return err
 				}
 				if done {
 					break
@@ -1161,7 +1200,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 						//    There MUST only be one option of this type, regardless of
 						//    value, per Hop-by-Hop header.
 						stats.MalformedPacketsReceived.Increment()
-						return
+						return fmt.Errorf("found multiple Router Alert options (%#v, %#v)", opt, routerAlert)
 					}
 					routerAlert = opt
 					stats.OptionRouterAlertReceived.Increment()
@@ -1169,10 +1208,10 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 					switch opt.UnknownAction() {
 					case header.IPv6OptionUnknownActionSkip:
 					case header.IPv6OptionUnknownActionDiscard:
-						return
+						return fmt.Errorf("found unknown Hop-by-Hop header option = %#v with discard action", opt)
 					case header.IPv6OptionUnknownActionDiscardSendICMPNoMulticastDest:
 						if header.IsV6MulticastAddress(dstAddr) {
-							return
+							return fmt.Errorf("found unknown hop-by-hop header option = %#v with discard action", opt)
 						}
 						fallthrough
 					case header.IPv6OptionUnknownActionDiscardSendICMP:
@@ -1187,10 +1226,11 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 							code:               header.ICMPv6UnknownOption,
 							pointer:            it.ParseOffset() + optsIt.OptionOffset(),
 							respondToMulticast: true,
+							forwarding:         forwarding,
 						}, pkt)
-						return
+						return fmt.Errorf("found unknown hop-by-hop header option = %#v with discard action", opt)
 					default:
-						panic(fmt.Sprintf("unrecognized action for an unrecognized Hop By Hop extension header option = %d", opt))
+						panic(fmt.Sprintf("unrecognized action for an unrecognized Hop By Hop extension header option = %#v", opt))
 					}
 				}
 			}
@@ -1212,8 +1252,13 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 				_ = e.protocol.returnError(&icmpReasonParameterProblem{
 					code:    header.ICMPv6ErroneousHeader,
 					pointer: it.ParseOffset(),
+					// For the sake of consistency, we're using the value of `forwarding`
+					// here, even though it should always be false if we've reached this
+					// point. If `forwarding` is true here, we're executing undefined
+					// behavior no matter what.
+					forwarding: forwarding,
 				}, pkt)
-				return
+				return fmt.Errorf("found unrecognized routing type with non-zero segments left in header = %#v", extHdr)
 			}
 
 		case header.IPv6FragmentExtHdr:
@@ -1248,7 +1293,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 					if err != nil {
 						stats.MalformedPacketsReceived.Increment()
 						stats.MalformedFragmentsReceived.Increment()
-						return
+						return err
 					}
 					if done {
 						break
@@ -1276,7 +1321,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 				default:
 					stats.MalformedPacketsReceived.Increment()
 					stats.MalformedFragmentsReceived.Increment()
-					return
+					return fmt.Errorf("known extension header = %#v present after fragment header in a non-initial fragment", lastHdr)
 				}
 			}
 
@@ -1285,7 +1330,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 				// Drop the packet as it's marked as a fragment but has no payload.
 				stats.MalformedPacketsReceived.Increment()
 				stats.MalformedFragmentsReceived.Increment()
-				return
+				return fmt.Errorf("fragment has no payload")
 			}
 
 			// As per RFC 2460 Section 4.5:
@@ -1303,7 +1348,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 					code:    header.ICMPv6ErroneousHeader,
 					pointer: header.IPv6PayloadLenOffset,
 				}, pkt)
-				return
+				return fmt.Errorf("found fragment length = %d that is not a multiple of 8 octets", fragmentPayloadLen)
 			}
 
 			// The packet is a fragment, let's try to reassemble it.
@@ -1317,14 +1362,15 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 			//    Parameter Problem, Code 0, message should be sent to the source of
 			//    the fragment, pointing to the Fragment Offset field of the fragment
 			//    packet.
-			if int(start)+fragmentPayloadLen > header.IPv6MaximumPayloadSize {
+			lengthAfterReassembly := int(start) + fragmentPayloadLen
+			if lengthAfterReassembly > header.IPv6MaximumPayloadSize {
 				stats.MalformedPacketsReceived.Increment()
 				stats.MalformedFragmentsReceived.Increment()
 				_ = e.protocol.returnError(&icmpReasonParameterProblem{
 					code:    header.ICMPv6ErroneousHeader,
 					pointer: fragmentFieldOffset,
 				}, pkt)
-				return
+				return fmt.Errorf("determined that reassembled packet length = %d would exceed allowed length = %d", lengthAfterReassembly, header.IPv6MaximumPayloadSize)
 			}
 
 			// Note that pkt doesn't have its transport header set after reassembly,
@@ -1346,7 +1392,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 			if err != nil {
 				stats.MalformedPacketsReceived.Increment()
 				stats.MalformedFragmentsReceived.Increment()
-				return
+				return err
 			}
 
 			if ready {
@@ -1368,7 +1414,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 				opt, done, err := optsIt.Next()
 				if err != nil {
 					stats.MalformedPacketsReceived.Increment()
-					return
+					return err
 				}
 				if done {
 					break
@@ -1379,10 +1425,10 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 				switch opt.UnknownAction() {
 				case header.IPv6OptionUnknownActionSkip:
 				case header.IPv6OptionUnknownActionDiscard:
-					return
+					return fmt.Errorf("found unknown destination header option = %#v with discard action", opt)
 				case header.IPv6OptionUnknownActionDiscardSendICMPNoMulticastDest:
 					if header.IsV6MulticastAddress(dstAddr) {
-						return
+						return fmt.Errorf("found unknown destination header option %#v with discard action", opt)
 					}
 					fallthrough
 				case header.IPv6OptionUnknownActionDiscardSendICMP:
@@ -1399,9 +1445,9 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 						pointer:            it.ParseOffset() + optsIt.OptionOffset(),
 						respondToMulticast: true,
 					}, pkt)
-					return
+					return fmt.Errorf("found unknown destination header option %#v with discard action", opt)
 				default:
-					panic(fmt.Sprintf("unrecognized action for an unrecognized Destination extension header option = %d", opt))
+					panic(fmt.Sprintf("unrecognized action for an unrecognized Destination extension header option = %#v", opt))
 				}
 			}
 
@@ -1432,6 +1478,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 					//   transport protocol (e.g., UDP) has no listener, if that transport
 					//   protocol has no alternative means to inform the sender.
 					_ = e.protocol.returnError(&icmpReasonPortUnreachable{}, pkt)
+					return fmt.Errorf("destination port unreachable")
 				case stack.TransportPacketProtocolUnreachable:
 					// As per RFC 8200 section 4. (page 7):
 					//   Extension headers are numbered from IANA IP Protocol Numbers
@@ -1463,6 +1510,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 						code:    header.ICMPv6UnknownHeader,
 						pointer: prevHdrIDOffset,
 					}, pkt)
+					return fmt.Errorf("transport protocol unreachable")
 				default:
 					panic(fmt.Sprintf("unrecognized result from DeliverTransportPacket = %d", res))
 				}
@@ -1476,6 +1524,7 @@ func (e *endpoint) handleValidatedPacket(h header.IPv6, pkt *stack.PacketBuffer)
 
 		}
 	}
+	return nil
 }
 
 // Close cleans up resources associated with the endpoint.
diff --git a/pkg/tcpip/network/ipv6/ipv6_test.go b/pkg/tcpip/network/ipv6/ipv6_test.go
index 4fbe39528..5307516d7 100644
--- a/pkg/tcpip/network/ipv6/ipv6_test.go
+++ b/pkg/tcpip/network/ipv6/ipv6_test.go
@@ -3025,6 +3025,7 @@ func TestForwarding(t *testing.T) {
 
 	tests := []struct {
 		name                         string
+		extHdr                       func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker)
 		TTL                          uint8
 		expectErrorICMP              bool
 		expectPacketForwarded        bool
@@ -3036,6 +3037,7 @@ func TestForwarding(t *testing.T) {
 		expectPacketUnrouteableError bool
 		expectLinkLocalSourceError   bool
 		expectLinkLocalDestError     bool
+		expectExtensionHeaderError   bool
 	}{
 		{
 			name:            "TTL of zero",
@@ -3108,6 +3110,158 @@ func TestForwarding(t *testing.T) {
 			destAddr:                   remoteIPv6Addr2,
 			expectLinkLocalSourceError: true,
 		},
+		{
+			name:       "Hopbyhop with unknown option skippable action",
+			TTL:        2,
+			sourceAddr: remoteIPv6Addr1,
+			destAddr:   remoteIPv6Addr2,
+			extHdr: func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker) {
+				return []byte{
+					nextHdr, 1,
+
+					// Skippable unknown.
+					63, 4, 1, 2, 3, 4,
+
+					// Skippable unknown.
+					62, 6, 1, 2, 3, 4, 5, 6,
+				}, hopByHopExtHdrID, checker.IPv6ExtHdr(checker.IPv6HopByHopExtensionHeader(checker.IPv6UnknownOption(), checker.IPv6UnknownOption()))
+			},
+			expectPacketForwarded: true,
+		},
+		{
+			name:       "Hopbyhop with unknown option discard action",
+			TTL:        2,
+			sourceAddr: remoteIPv6Addr1,
+			destAddr:   remoteIPv6Addr2,
+			extHdr: func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker) {
+				return []byte{
+					nextHdr, 1,
+
+					// Skippable unknown.
+					63, 4, 1, 2, 3, 4,
+
+					// Discard unknown.
+					127, 6, 1, 2, 3, 4, 5, 6,
+				}, hopByHopExtHdrID, nil
+			},
+			expectExtensionHeaderError: true,
+		},
+		{
+			name:       "Hopbyhop with unknown option discard and send icmp action (unicast)",
+			TTL:        2,
+			sourceAddr: remoteIPv6Addr1,
+			destAddr:   remoteIPv6Addr2,
+			extHdr: func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker) {
+				return []byte{
+					nextHdr, 1,
+
+					// Skippable unknown.
+					63, 4, 1, 2, 3, 4,
+
+					// Discard & send ICMP if option is unknown.
+					191, 6, 1, 2, 3, 4, 5, 6,
+				}, hopByHopExtHdrID, nil
+			},
+			expectErrorICMP:            true,
+			icmpType:                   header.ICMPv6ParamProblem,
+			icmpCode:                   header.ICMPv6UnknownOption,
+			expectExtensionHeaderError: true,
+		},
+		{
+			name:       "Hopbyhop with unknown option discard and send icmp action (multicast)",
+			TTL:        2,
+			sourceAddr: remoteIPv6Addr1,
+			destAddr:   multicastIPv6Addr,
+			extHdr: func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker) {
+				return []byte{
+					nextHdr, 1,
+
+					// Skippable unknown.
+					63, 4, 1, 2, 3, 4,
+
+					// Discard & send ICMP if option is unknown.
+					191, 6, 1, 2, 3, 4, 5, 6,
+				}, hopByHopExtHdrID, nil
+			},
+			expectErrorICMP:            true,
+			icmpType:                   header.ICMPv6ParamProblem,
+			icmpCode:                   header.ICMPv6UnknownOption,
+			expectExtensionHeaderError: true,
+		},
+		{
+			name:       "Hopbyhop with unknown option discard and send icmp action unless multicast dest (unicast)",
+			TTL:        2,
+			sourceAddr: remoteIPv6Addr1,
+			destAddr:   remoteIPv6Addr2,
+			extHdr: func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker) {
+				return []byte{
+					nextHdr, 1,
+
+					// Skippable unknown.
+					63, 4, 1, 2, 3, 4,
+
+					// Discard & send ICMP unless packet is for multicast destination if
+					// option is unknown.
+					255, 6, 1, 2, 3, 4, 5, 6,
+				}, hopByHopExtHdrID, nil
+			},
+			expectErrorICMP:            true,
+			icmpType:                   header.ICMPv6ParamProblem,
+			icmpCode:                   header.ICMPv6UnknownOption,
+			expectExtensionHeaderError: true,
+		},
+		{
+			name:       "Hopbyhop with unknown option discard and send icmp action unless multicast dest (multicast)",
+			TTL:        2,
+			sourceAddr: remoteIPv6Addr1,
+			destAddr:   multicastIPv6Addr,
+			extHdr: func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker) {
+				return []byte{
+					nextHdr, 1,
+
+					// Skippable unknown.
+					63, 4, 1, 2, 3, 4,
+
+					// Discard & send ICMP unless packet is for multicast destination if
+					// option is unknown.
+					255, 6, 1, 2, 3, 4, 5, 6,
+				}, hopByHopExtHdrID, nil
+			},
+			expectExtensionHeaderError: true,
+		},
+		{
+			name:       "Hopbyhop with router alert option",
+			TTL:        2,
+			sourceAddr: remoteIPv6Addr1,
+			destAddr:   remoteIPv6Addr2,
+			extHdr: func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker) {
+				return []byte{
+					nextHdr, 0,
+
+					// Router Alert option.
+					5, 2, 0, 0, 0, 0,
+				}, hopByHopExtHdrID, checker.IPv6ExtHdr(checker.IPv6HopByHopExtensionHeader(checker.IPv6RouterAlert(header.IPv6RouterAlertMLD)))
+			},
+			expectPacketForwarded: true,
+		},
+		{
+			name:       "Hopbyhop with two router alert options",
+			TTL:        2,
+			sourceAddr: remoteIPv6Addr1,
+			destAddr:   remoteIPv6Addr2,
+			extHdr: func(nextHdr uint8) ([]byte, uint8, checker.NetworkChecker) {
+				return []byte{
+					nextHdr, 1,
+
+					// Router Alert option.
+					5, 2, 0, 0, 0, 0,
+
+					// Router Alert option.
+					5, 2, 0, 0, 0, 0,
+				}, hopByHopExtHdrID, nil
+			},
+			expectExtensionHeaderError: true,
+		},
 	}
 
 	for _, test := range tests {
@@ -3150,7 +3304,17 @@ func TestForwarding(t *testing.T) {
 				t.Fatalf("SetForwarding(%d, true): %s", ProtocolNumber, err)
 			}
 
-			hdr := buffer.NewPrependable(header.IPv6MinimumSize + header.ICMPv6MinimumSize)
+			transportProtocol := header.ICMPv6ProtocolNumber
+			extHdrBytes := []byte{}
+			extHdrChecker := checker.IPv6ExtHdr()
+			if test.extHdr != nil {
+				nextHdrID := hopByHopExtHdrID
+				extHdrBytes, nextHdrID, extHdrChecker = test.extHdr(uint8(header.ICMPv6ProtocolNumber))
+				transportProtocol = tcpip.TransportProtocolNumber(nextHdrID)
+			}
+			extHdrLen := len(extHdrBytes)
+
+			hdr := buffer.NewPrependable(header.IPv6MinimumSize + header.ICMPv6MinimumSize + extHdrLen)
 			icmp := header.ICMPv6(hdr.Prepend(header.ICMPv6MinimumSize))
 			icmp.SetIdent(randomIdent)
 			icmp.SetSequence(randomSequence)
@@ -3162,10 +3326,11 @@ func TestForwarding(t *testing.T) {
 				Src:    test.sourceAddr,
 				Dst:    test.destAddr,
 			}))
+			copy(hdr.Prepend(extHdrLen), extHdrBytes)
 			ip := header.IPv6(hdr.Prepend(header.IPv6MinimumSize))
 			ip.Encode(&header.IPv6Fields{
 				PayloadLength:     header.ICMPv6MinimumSize,
-				TransportProtocol: header.ICMPv6ProtocolNumber,
+				TransportProtocol: transportProtocol,
 				HopLimit:          test.TTL,
 				SrcAddr:           test.sourceAddr,
 				DstAddr:           test.destAddr,
@@ -3205,10 +3370,11 @@ func TestForwarding(t *testing.T) {
 					t.Fatal("expected ICMP Echo Request packet through outgoing NIC")
 				}
 
-				checker.IPv6(t, header.IPv6(stack.PayloadSince(reply.Pkt.NetworkHeader())),
+				checker.IPv6WithExtHdr(t, header.IPv6(stack.PayloadSince(reply.Pkt.NetworkHeader())),
 					checker.SrcAddr(test.sourceAddr),
 					checker.DstAddr(test.destAddr),
 					checker.TTL(test.TTL-1),
+					extHdrChecker,
 					checker.ICMPv6(
 						checker.ICMPv6Type(header.ICMPv6EchoRequest),
 						checker.ICMPv6Code(header.ICMPv6UnusedCode),
@@ -3249,6 +3415,10 @@ func TestForwarding(t *testing.T) {
 			if got, want := s.Stats().IP.Forwarding.Errors.Value(), boolToInt(!test.expectPacketForwarded); got != want {
 				t.Errorf("got s.Stats().IP.Forwarding.Errors.Value() = %d, want = %d", got, want)
 			}
+
+			if got, want := s.Stats().IP.Forwarding.ExtensionHeaderProblem.Value(), boolToInt(test.expectExtensionHeaderError); got != want {
+				t.Errorf("got s.Stats().IP.Forwarding.ExtensionHeaderProblem.Value() = %d, want = %d", got, want)
+			}
 		})
 	}
 }