Use the ICMP error response facility

Add code in IPv6 to send ICMP packets while processing extension headers.

Add some accounting in processing IPV6 Extension headers which
allows us to report meaningful information back in ICMP parameter
problem packets.

IPv4 also needs to send a message when an unsupported protocol
is requested.

Add some tests to generate both ipv4 and ipv6 packets with
various errors and check the responses.

Add some new checkers and cleanup some inconsistencies in
the messages in that file.

Add new error types for the ICMPv4/6 generators.

Fix a bug in the ICMPv4 generator that stopped it from generating
"Unknown protocol" messages.

Updates #2211

PiperOrigin-RevId: 334661716

3 files changed, 374 insertions, 61 deletions

diff --git a/pkg/tcpip/network/ipv6/icmp.go b/pkg/tcpip/network/ipv6/icmp.go
index 4b4b483cc..8e9def6b8 100644
--- a/pkg/tcpip/network/ipv6/icmp.go
+++ b/pkg/tcpip/network/ipv6/icmp.go
@@ -690,6 +690,36 @@ type icmpReason interface {
 	isICMPReason()
 }
 
+// icmpReasonParameterProblem is an error during processing of extension headers
+// or the fixed header defined in RFC 4443 section 3.4.
+type icmpReasonParameterProblem struct {
+	code header.ICMPv6Code
+
+	// respondToMulticast indicates that we are sending a packet that falls under
+	// the exception outlined by RFC 4443 section 2.4 point e.3 exception 2:
+	//
+	//       (e.3) A packet destined to an IPv6 multicast address.  (There are
+	//             two exceptions to this rule: (1) the Packet Too Big Message
+	//             (Section 3.2) to allow Path MTU discovery to work for IPv6
+	//             multicast, and (2) the Parameter Problem Message, Code 2
+	//             (Section 3.4) reporting an unrecognized IPv6 option (see
+	//             Section 4.2 of [IPv6]) that has the Option Type highest-
+	//             order two bits set to 10).
+	respondToMulticast bool
+
+	// pointer is defined in the RFC 4443 setion 3.4 which reads:
+	//
+	//  Pointer         Identifies the octet offset within the invoking packet
+	//                  where the error was detected.
+	//
+	//                  The pointer will point beyond the end of the ICMPv6
+	//                  packet if the field in error is beyond what can fit
+	//                  in the maximum size of an ICMPv6 error message.
+	pointer uint32
+}
+
+func (*icmpReasonParameterProblem) isICMPReason() {}
+
 // icmpReasonPortUnreachable is an error where the transport protocol has no
 // listener and no alternative means to inform the sender.
 type icmpReasonPortUnreachable struct{}
@@ -709,7 +739,7 @@ func returnError(r *stack.Route, reason icmpReason, pkt *stack.PacketBuffer) *tc
 	// Only send ICMP error if the address is not a multicast v6
 	// address and the source is not the unspecified address.
 	//
-	// TODO(b/164522993) There are exceptions to this rule.
+	// There are exceptions to this rule.
 	// See: point e.3) RFC 4443 section-2.4
 	//
 	//	 (e) An ICMPv6 error message MUST NOT be originated as a result of
@@ -727,7 +757,12 @@ func returnError(r *stack.Route, reason icmpReason, pkt *stack.PacketBuffer) *tc
 	//             Section 4.2 of [IPv6]) that has the Option Type highest-
 	//             order two bits set to 10).
 	//
-	if header.IsV6MulticastAddress(r.LocalAddress) || r.RemoteAddress == header.IPv6Any {
+	var allowResponseToMulticast bool
+	if reason, ok := reason.(*icmpReasonParameterProblem); ok {
+		allowResponseToMulticast = reason.respondToMulticast
+	}
+
+	if (!allowResponseToMulticast && header.IsV6MulticastAddress(r.LocalAddress)) || r.RemoteAddress == header.IPv6Any {
 		return nil
 	}
 
@@ -780,10 +815,21 @@ func returnError(r *stack.Route, reason icmpReason, pkt *stack.PacketBuffer) *tc
 	newPkt.TransportProtocolNumber = header.ICMPv6ProtocolNumber
 
 	icmpHdr := header.ICMPv6(newPkt.TransportHeader().Push(header.ICMPv6DstUnreachableMinimumSize))
-	icmpHdr.SetCode(header.ICMPv6PortUnreachable)
-	icmpHdr.SetType(header.ICMPv6DstUnreachable)
+	var counter *tcpip.StatCounter
+	switch reason := reason.(type) {
+	case *icmpReasonParameterProblem:
+		icmpHdr.SetType(header.ICMPv6ParamProblem)
+		icmpHdr.SetCode(reason.code)
+		icmpHdr.SetTypeSpecific(reason.pointer)
+		counter = sent.ParamProblem
+	case *icmpReasonPortUnreachable:
+		icmpHdr.SetType(header.ICMPv6DstUnreachable)
+		icmpHdr.SetCode(header.ICMPv6PortUnreachable)
+		counter = sent.DstUnreachable
+	default:
+		panic(fmt.Sprintf("unsupported ICMP type %T", reason))
+	}
 	icmpHdr.SetChecksum(header.ICMPv6Checksum(icmpHdr, r.LocalAddress, r.RemoteAddress, newPkt.Data))
-	counter := sent.DstUnreachable
 	err := r.WritePacket(nil /* gso */, stack.NetworkHeaderParams{Protocol: header.ICMPv6ProtocolNumber, TTL: r.DefaultTTL(), TOS: stack.DefaultTOS}, newPkt)
 	if err != nil {
 		sent.Dropped.Increment()
diff --git a/pkg/tcpip/network/ipv6/ipv6.go b/pkg/tcpip/network/ipv6/ipv6.go
index aff4e1425..7458c3795 100644
--- a/pkg/tcpip/network/ipv6/ipv6.go
+++ b/pkg/tcpip/network/ipv6/ipv6.go
@@ -1,4 +1,4 @@
-// Copyright 2018 The gVisor Authors.
+// Copyright 2020 The gVisor Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -536,7 +536,10 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt *stack.PacketBuffer) {
 		return
 	}
 
-	for firstHeader := true; ; firstHeader = false {
+	for {
+		// Keep track of the start of the previous header so we can report the
+		// special case of a Hop by Hop at a location other than at the start.
+		previousHeaderStart := it.HeaderOffset()
 		extHdr, done, err := it.Next()
 		if err != nil {
 			r.Stats().IP.MalformedPacketsReceived.Increment()
@@ -550,11 +553,11 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt *stack.PacketBuffer) {
 		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.
-			//
-			// TODO(b/152019344): Send an ICMPv6 Parameter Problem, Code 1
-			// (unrecognized next header) error in response to an extension header's
-			// Next Header field with the Hop By Hop extension header identifier.
-			if !firstHeader {
+			if previousHeaderStart != 0 {
+				_ = returnError(r, &icmpReasonParameterProblem{
+					code:    header.ICMPv6UnknownHeader,
+					pointer: previousHeaderStart,
+				}, pkt)
 				return
 			}
 
@@ -576,13 +579,25 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt *stack.PacketBuffer) {
 				case header.IPv6OptionUnknownActionSkip:
 				case header.IPv6OptionUnknownActionDiscard:
 					return
-				case header.IPv6OptionUnknownActionDiscardSendICMP:
-					// TODO(b/152019344): Send an ICMPv6 Parameter Problem Code 2 for
-					// unrecognized IPv6 extension header options.
-					return
 				case header.IPv6OptionUnknownActionDiscardSendICMPNoMulticastDest:
-					// TODO(b/152019344): Send an ICMPv6 Parameter Problem Code 2 for
-					// unrecognized IPv6 extension header options.
+					if header.IsV6MulticastAddress(r.LocalAddress) {
+						return
+					}
+					fallthrough
+				case header.IPv6OptionUnknownActionDiscardSendICMP:
+					// This case satisfies a requirement of RFC 8200 section 4.2
+					// which states that an unknown option starting with bits [10] should:
+					//
+					//    discard the packet and, regardless of whether or not the
+					//    packet's Destination Address was a multicast address, send an
+					//    ICMP Parameter Problem, Code 2, message to the packet's
+					//    Source Address, pointing to the unrecognized Option Type.
+					//
+					_ = returnError(r, &icmpReasonParameterProblem{
+						code:               header.ICMPv6UnknownOption,
+						pointer:            it.ParseOffset() + optsIt.OptionOffset(),
+						respondToMulticast: true,
+					}, pkt)
 					return
 				default:
 					panic(fmt.Sprintf("unrecognized action for an unrecognized Hop By Hop extension header option = %d", opt))
@@ -593,16 +608,20 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt *stack.PacketBuffer) {
 			// As per RFC 8200 section 4.4, if a node encounters a routing header with
 			// an unrecognized routing type value, with a non-zero Segments Left
 			// value, the node must discard the packet and send an ICMP Parameter
-			// Problem, Code 0. If the Segments Left is 0, the node must ignore the
-			// Routing extension header and process the next header in the packet.
+			// Problem, Code 0 to the packet's Source Address, pointing to the
+			// unrecognized Routing Type.
+			//
+			// If the Segments Left is 0, the node must ignore the Routing extension
+			// header and process the next header in the packet.
 			//
 			// Note, the stack does not yet handle any type of routing extension
 			// header, so we just make sure Segments Left is zero before processing
 			// the next extension header.
-			//
-			// TODO(b/152019344): Send an ICMPv6 Parameter Problem Code 0 for
-			// unrecognized routing types with a non-zero Segments Left value.
 			if extHdr.SegmentsLeft() != 0 {
+				_ = returnError(r, &icmpReasonParameterProblem{
+					code:    header.ICMPv6ErroneousHeader,
+					pointer: it.ParseOffset(),
+				}, pkt)
 				return
 			}
 
@@ -737,13 +756,25 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt *stack.PacketBuffer) {
 				case header.IPv6OptionUnknownActionSkip:
 				case header.IPv6OptionUnknownActionDiscard:
 					return
-				case header.IPv6OptionUnknownActionDiscardSendICMP:
-					// TODO(b/152019344): Send an ICMPv6 Parameter Problem Code 2 for
-					// unrecognized IPv6 extension header options.
-					return
 				case header.IPv6OptionUnknownActionDiscardSendICMPNoMulticastDest:
-					// TODO(b/152019344): Send an ICMPv6 Parameter Problem Code 2 for
-					// unrecognized IPv6 extension header options.
+					if header.IsV6MulticastAddress(r.LocalAddress) {
+						return
+					}
+					fallthrough
+				case header.IPv6OptionUnknownActionDiscardSendICMP:
+					// This case satisfies a requirement of RFC 8200 section 4.2
+					// which states that an unknown option starting with bits [10] should:
+					//
+					//    discard the packet and, regardless of whether or not the
+					//    packet's Destination Address was a multicast address, send an
+					//    ICMP Parameter Problem, Code 2, message to the packet's
+					//    Source Address, pointing to the unrecognized Option Type.
+					//
+					_ = returnError(r, &icmpReasonParameterProblem{
+						code:               header.ICMPv6UnknownOption,
+						pointer:            it.ParseOffset() + optsIt.OptionOffset(),
+						respondToMulticast: true,
+					}, pkt)
 					return
 				default:
 					panic(fmt.Sprintf("unrecognized action for an unrecognized Destination extension header option = %d", opt))
@@ -767,8 +798,7 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt *stack.PacketBuffer) {
 				pkt.TransportProtocolNumber = p
 				e.handleICMP(r, pkt, hasFragmentHeader)
 			} else {
-				// TODO(b/152019344): Send an ICMPv6 Parameter Problem, Code 1 error
-				// in response to unrecognized next header values.
+				r.Stats().IP.PacketsDelivered.Increment()
 				switch res := e.dispatcher.DeliverTransportPacket(r, p, pkt); res {
 				case stack.TransportPacketHandled:
 				case stack.TransportPacketDestinationPortUnreachable:
@@ -778,17 +808,40 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt *stack.PacketBuffer) {
 					//   transport protocol (e.g., UDP) has no listener, if that transport
 					//   protocol has no alternative means to inform the sender.
 					_ = returnError(r, &icmpReasonPortUnreachable{}, pkt)
+				case stack.TransportPacketProtocolUnreachable:
+					// As per RFC 8200 section 4. (page 7):
+					//   Extension headers are numbered from IANA IP Protocol Numbers
+					//   [IANA-PN], the same values used for IPv4 and IPv6.  When
+					//   processing a sequence of Next Header values in a packet, the
+					//   first one that is not an extension header [IANA-EH] indicates
+					//   that the next item in the packet is the corresponding upper-layer
+					//   header.
+					// With more related information on page 8:
+					//   If, as a result of processing a header, the destination node is
+					//   required to proceed to the next header but the Next Header value
+					//   in the current header is unrecognized by the node, it should
+					//   discard the packet and send an ICMP Parameter Problem message to
+					//   the source of the packet, with an ICMP Code value of 1
+					//   ("unrecognized Next Header type encountered") and the ICMP
+					//   Pointer field containing the offset of the unrecognized value
+					//   within the original packet.
+					//
+					// Which when taken together indicate that an unknown protocol should
+					// be treated as an unrecognized next header value.
+					_ = returnError(r, &icmpReasonParameterProblem{
+						code:    header.ICMPv6UnknownHeader,
+						pointer: it.ParseOffset(),
+					}, pkt)
 				default:
 					panic(fmt.Sprintf("unrecognized result from DeliverTransportPacket = %d", res))
 				}
 			}
 
 		default:
-			// If we receive a packet for an extension header we do not yet handle,
-			// drop the packet for now.
-			//
-			// TODO(b/152019344): Send an ICMPv6 Parameter Problem, Code 1 error
-			// in response to unrecognized next header values.
+			_ = returnError(r, &icmpReasonParameterProblem{
+				code:    header.ICMPv6UnknownHeader,
+				pointer: it.ParseOffset(),
+			}, pkt)
 			r.Stats().UnknownProtocolRcvdPackets.Increment()
 			return
 		}
diff --git a/pkg/tcpip/network/ipv6/ipv6_test.go b/pkg/tcpip/network/ipv6/ipv6_test.go
index 94344057e..d7f82973b 100644
--- a/pkg/tcpip/network/ipv6/ipv6_test.go
+++ b/pkg/tcpip/network/ipv6/ipv6_test.go
@@ -21,6 +21,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/checker"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/link/channel"
 	"gvisor.dev/gvisor/pkg/tcpip/network/testutil"
@@ -301,11 +302,18 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 		name         string
 		extHdr       func(nextHdr uint8) ([]byte, uint8)
 		shouldAccept bool
+		// Should we expect an ICMP response and if so, with what contents?
+		expectICMP bool
+		ICMPType   header.ICMPv6Type
+		ICMPCode   header.ICMPv6Code
+		pointer    uint32
+		multicast  bool
 	}{
 		{
 			name:         "None",
 			extHdr:       func(nextHdr uint8) ([]byte, uint8) { return []byte{}, nextHdr },
 			shouldAccept: true,
+			expectICMP:   false,
 		},
 		{
 			name: "hopbyhop with unknown option skippable action",
@@ -336,9 +344,10 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 				}, hopByHopExtHdrID
 			},
 			shouldAccept: false,
+			expectICMP:   false,
 		},
 		{
-			name: "hopbyhop with unknown option discard and send icmp action",
+			name: "hopbyhop with unknown option discard and send icmp action (unicast)",
 			extHdr: func(nextHdr uint8) ([]byte, uint8) {
 				return []byte{
 					nextHdr, 1,
@@ -348,12 +357,58 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 
 					// Discard & send ICMP if option is unknown.
 					191, 6, 1, 2, 3, 4, 5, 6,
+					//^ Unknown option.
 				}, hopByHopExtHdrID
 			},
 			shouldAccept: false,
+			expectICMP:   true,
+			ICMPType:     header.ICMPv6ParamProblem,
+			ICMPCode:     header.ICMPv6UnknownOption,
+			pointer:      header.IPv6FixedHeaderSize + 8,
 		},
 		{
-			name: "hopbyhop with unknown option discard and send icmp action unless multicast dest",
+			name: "hopbyhop with unknown option discard and send icmp action (multicast)",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				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,
+					//^ Unknown option.
+				}, hopByHopExtHdrID
+			},
+			multicast:    true,
+			shouldAccept: false,
+			expectICMP:   true,
+			ICMPType:     header.ICMPv6ParamProblem,
+			ICMPCode:     header.ICMPv6UnknownOption,
+			pointer:      header.IPv6FixedHeaderSize + 8,
+		},
+		{
+			name: "hopbyhop with unknown option discard and send icmp action unless multicast dest (unicast)",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				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,
+					//^ Unknown option.
+				}, hopByHopExtHdrID
+			},
+			expectICMP: true,
+			ICMPType:   header.ICMPv6ParamProblem,
+			ICMPCode:   header.ICMPv6UnknownOption,
+			pointer:    header.IPv6FixedHeaderSize + 8,
+		},
+		{
+			name: "hopbyhop with unknown option discard and send icmp action unless multicast dest (multicast)",
 			extHdr: func(nextHdr uint8) ([]byte, uint8) {
 				return []byte{
 					nextHdr, 1,
@@ -364,39 +419,77 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 					// Discard & send ICMP unless packet is for multicast destination if
 					// option is unknown.
 					255, 6, 1, 2, 3, 4, 5, 6,
+					//^ Unknown option.
 				}, hopByHopExtHdrID
 			},
+			multicast:    true,
 			shouldAccept: false,
+			expectICMP:   false,
 		},
 		{
-			name:         "routing with zero segments left",
-			extHdr:       func(nextHdr uint8) ([]byte, uint8) { return []byte{nextHdr, 0, 1, 0, 2, 3, 4, 5}, routingExtHdrID },
+			name: "routing with zero segments left",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				return []byte{
+					nextHdr, 0,
+					1, 0, 2, 3, 4, 5,
+				}, routingExtHdrID
+			},
 			shouldAccept: true,
 		},
 		{
-			name:         "routing with non-zero segments left",
-			extHdr:       func(nextHdr uint8) ([]byte, uint8) { return []byte{nextHdr, 0, 1, 1, 2, 3, 4, 5}, routingExtHdrID },
+			name: "routing with non-zero segments left",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				return []byte{
+					nextHdr, 0,
+					1, 1, 2, 3, 4, 5,
+				}, routingExtHdrID
+			},
 			shouldAccept: false,
+			expectICMP:   true,
+			ICMPType:     header.ICMPv6ParamProblem,
+			ICMPCode:     header.ICMPv6ErroneousHeader,
+			pointer:      header.IPv6FixedHeaderSize + 2,
 		},
 		{
-			name:         "atomic fragment with zero ID",
-			extHdr:       func(nextHdr uint8) ([]byte, uint8) { return []byte{nextHdr, 0, 0, 0, 0, 0, 0, 0}, fragmentExtHdrID },
+			name: "atomic fragment with zero ID",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				return []byte{
+					nextHdr, 0,
+					0, 0, 0, 0, 0, 0,
+				}, fragmentExtHdrID
+			},
 			shouldAccept: true,
 		},
 		{
-			name:         "atomic fragment with non-zero ID",
-			extHdr:       func(nextHdr uint8) ([]byte, uint8) { return []byte{nextHdr, 0, 0, 0, 1, 2, 3, 4}, fragmentExtHdrID },
+			name: "atomic fragment with non-zero ID",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				return []byte{
+					nextHdr, 0,
+					0, 0, 1, 2, 3, 4,
+				}, fragmentExtHdrID
+			},
 			shouldAccept: true,
+			expectICMP:   false,
 		},
 		{
-			name:         "fragment",
-			extHdr:       func(nextHdr uint8) ([]byte, uint8) { return []byte{nextHdr, 0, 1, 0, 1, 2, 3, 4}, fragmentExtHdrID },
+			name: "fragment",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				return []byte{
+					nextHdr, 0,
+					1, 0, 1, 2, 3, 4,
+				}, fragmentExtHdrID
+			},
 			shouldAccept: false,
+			expectICMP:   false,
 		},
 		{
-			name:         "No next header",
-			extHdr:       func(nextHdr uint8) ([]byte, uint8) { return []byte{}, noNextHdrID },
+			name: "No next header",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				return []byte{},
+					noNextHdrID
+			},
 			shouldAccept: false,
+			expectICMP:   false,
 		},
 		{
 			name: "destination with unknown option skippable action",
@@ -412,6 +505,7 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 				}, destinationExtHdrID
 			},
 			shouldAccept: true,
+			expectICMP:   false,
 		},
 		{
 			name: "destination with unknown option discard action",
@@ -427,9 +521,10 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 				}, destinationExtHdrID
 			},
 			shouldAccept: false,
+			expectICMP:   false,
 		},
 		{
-			name: "destination with unknown option discard and send icmp action",
+			name: "destination with unknown option discard and send icmp action (unicast)",
 			extHdr: func(nextHdr uint8) ([]byte, uint8) {
 				return []byte{
 					nextHdr, 1,
@@ -439,12 +534,38 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 
 					// Discard & send ICMP if option is unknown.
 					191, 6, 1, 2, 3, 4, 5, 6,
+					//^  191 is an unknown option.
 				}, destinationExtHdrID
 			},
 			shouldAccept: false,
+			expectICMP:   true,
+			ICMPType:     header.ICMPv6ParamProblem,
+			ICMPCode:     header.ICMPv6UnknownOption,
+			pointer:      header.IPv6FixedHeaderSize + 8,
 		},
 		{
-			name: "destination with unknown option discard and send icmp action unless multicast dest",
+			name: "destination with unknown option discard and send icmp action (muilticast)",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				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,
+					//^  191 is an unknown option.
+				}, destinationExtHdrID
+			},
+			multicast:    true,
+			shouldAccept: false,
+			expectICMP:   true,
+			ICMPType:     header.ICMPv6ParamProblem,
+			ICMPCode:     header.ICMPv6UnknownOption,
+			pointer:      header.IPv6FixedHeaderSize + 8,
+		},
+		{
+			name: "destination with unknown option discard and send icmp action unless multicast dest (unicast)",
 			extHdr: func(nextHdr uint8) ([]byte, uint8) {
 				return []byte{
 					nextHdr, 1,
@@ -455,22 +576,33 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 					// Discard & send ICMP unless packet is for multicast destination if
 					// option is unknown.
 					255, 6, 1, 2, 3, 4, 5, 6,
+					//^ 255 is unknown.
 				}, destinationExtHdrID
 			},
 			shouldAccept: false,
+			expectICMP:   true,
+			ICMPType:     header.ICMPv6ParamProblem,
+			ICMPCode:     header.ICMPv6UnknownOption,
+			pointer:      header.IPv6FixedHeaderSize + 8,
 		},
 		{
-			name: "routing - atomic fragment",
+			name: "destination with unknown option discard and send icmp action unless multicast dest (multicast)",
 			extHdr: func(nextHdr uint8) ([]byte, uint8) {
 				return []byte{
-					// Routing extension header.
-					fragmentExtHdrID, 0, 1, 0, 2, 3, 4, 5,
+					nextHdr, 1,
 
-					// Fragment extension header.
-					nextHdr, 0, 0, 0, 1, 2, 3, 4,
-				}, routingExtHdrID
+					// 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,
+					//^ 255 is unknown.
+				}, destinationExtHdrID
 			},
-			shouldAccept: true,
+			shouldAccept: false,
+			expectICMP:   false,
+			multicast:    true,
 		},
 		{
 			name: "atomic fragment - routing",
@@ -504,12 +636,42 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 				return []byte{
 					// Routing extension header.
 					hopByHopExtHdrID, 0, 1, 0, 2, 3, 4, 5,
+					// ^^^   The HopByHop extension header may not appear after the first
+					// extension header.
 
 					// Hop By Hop extension header with skippable unknown option.
 					nextHdr, 0, 62, 4, 1, 2, 3, 4,
 				}, routingExtHdrID
 			},
 			shouldAccept: false,
+			expectICMP:   true,
+			ICMPType:     header.ICMPv6ParamProblem,
+			ICMPCode:     header.ICMPv6UnknownHeader,
+			pointer:      header.IPv6FixedHeaderSize,
+		},
+		{
+			name: "routing - hop by hop (with send icmp unknown)",
+			extHdr: func(nextHdr uint8) ([]byte, uint8) {
+				return []byte{
+					// Routing extension header.
+					hopByHopExtHdrID, 0, 1, 0, 2, 3, 4, 5,
+					// ^^^   The HopByHop extension header may not appear after the first
+					// extension header.
+
+					nextHdr, 1,
+
+					// Skippable unknown.
+					63, 4, 1, 2, 3, 4,
+
+					// Skippable unknown.
+					191, 6, 1, 2, 3, 4, 5, 6,
+				}, routingExtHdrID
+			},
+			shouldAccept: false,
+			expectICMP:   true,
+			ICMPType:     header.ICMPv6ParamProblem,
+			ICMPCode:     header.ICMPv6UnknownHeader,
+			pointer:      header.IPv6FixedHeaderSize,
 		},
 		{
 			name:         "No next header",
@@ -553,6 +715,7 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 				}, hopByHopExtHdrID
 			},
 			shouldAccept: false,
+			expectICMP:   false,
 		},
 		{
 			name: "hopbyhop (with skippable unknown) - routing - atomic fragment - destination (with discard unknown)",
@@ -573,6 +736,7 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 				}, hopByHopExtHdrID
 			},
 			shouldAccept: false,
+			expectICMP:   false,
 		},
 	}
 
@@ -582,7 +746,7 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 				NetworkProtocols:   []stack.NetworkProtocolFactory{NewProtocol},
 				TransportProtocols: []stack.TransportProtocolFactory{udp.NewProtocol},
 			})
-			e := channel.New(0, 1280, linkAddr1)
+			e := channel.New(1, 1280, linkAddr1)
 			if err := s.CreateNIC(nicID, e); err != nil {
 				t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
 			}
@@ -590,6 +754,14 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 				t.Fatalf("AddAddress(%d, %d, %s) = %s", nicID, ProtocolNumber, addr2, err)
 			}
 
+			// Add a default route so that a return packet knows where to go.
+			s.SetRouteTable([]tcpip.Route{
+				{
+					Destination: header.IPv6EmptySubnet,
+					NIC:         nicID,
+				},
+			})
+
 			wq := waiter.Queue{}
 			we, ch := waiter.NewChannelEntry(nil)
 			wq.EventRegister(&we, waiter.EventIn)
@@ -631,12 +803,16 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 			// Serialize IPv6 fixed header.
 			payloadLength := hdr.UsedLength()
 			ip := header.IPv6(hdr.Prepend(header.IPv6MinimumSize))
+			dstAddr := tcpip.Address(addr2)
+			if test.multicast {
+				dstAddr = header.IPv6AllNodesMulticastAddress
+			}
 			ip.Encode(&header.IPv6Fields{
 				PayloadLength: uint16(payloadLength),
 				NextHeader:    ipv6NextHdr,
 				HopLimit:      255,
 				SrcAddr:       addr1,
-				DstAddr:       addr2,
+				DstAddr:       dstAddr,
 			})
 
 			e.InjectInbound(ProtocolNumber, stack.NewPacketBuffer(stack.PacketBufferOptions{
@@ -650,6 +826,44 @@ func TestReceiveIPv6ExtHdrs(t *testing.T) {
 					t.Errorf("got UDP Rx Packets = %d, want = 0", got)
 				}
 
+				if !test.expectICMP {
+					if p, ok := e.Read(); ok {
+						t.Fatalf("unexpected packet received: %#v", p)
+					}
+					return
+				}
+
+				// ICMP required.
+				p, ok := e.Read()
+				if !ok {
+					t.Fatalf("expected packet wasn't written out")
+				}
+
+				// Pack the output packet into a single buffer.View as the checkers
+				// assume that.
+				vv := buffer.NewVectorisedView(p.Pkt.Size(), p.Pkt.Views())
+				pkt := vv.ToView()
+				if got, want := len(pkt), header.IPv6FixedHeaderSize+header.ICMPv6MinimumSize+hdr.UsedLength(); got != want {
+					t.Fatalf("got an ICMP packet of size = %d, want = %d", got, want)
+				}
+
+				ipHdr := header.IPv6(pkt)
+				checker.IPv6(t, ipHdr, checker.ICMPv6(
+					checker.ICMPv6Type(test.ICMPType),
+					checker.ICMPv6Code(test.ICMPCode)))
+
+				// We know we are looking at no extension headers in the error ICMP
+				// packets.
+				icmpPkt := header.ICMPv6(ipHdr.Payload())
+				// We know we sent small packets that won't be truncated when reflected
+				// back to us.
+				originalPacket := icmpPkt.Payload()
+				if got, want := icmpPkt.TypeSpecific(), test.pointer; got != want {
+					t.Errorf("unexpected ICMPv6 pointer, got = %d, want = %d\n", got, want)
+				}
+				if diff := cmp.Diff(hdr.View(), buffer.View(originalPacket)); diff != "" {
+					t.Errorf("ICMPv6 payload mismatch (-want +got):\n%s", diff)
+				}
 				return
 			}