Make UDP traceroute work.

Adds support to generate Port Unreachable messages for UDP
datagrams received on a port for which there is no valid
endpoint.

Fixes #703

PiperOrigin-RevId: 267034418

2 files changed, 255 insertions, 2 deletions

diff --git a/pkg/tcpip/transport/udp/protocol.go b/pkg/tcpip/transport/udp/protocol.go
index f76e7fbe1..068d9a272 100644
--- a/pkg/tcpip/transport/udp/protocol.go
+++ b/pkg/tcpip/transport/udp/protocol.go
@@ -69,7 +69,106 @@ func (*protocol) ParsePorts(v buffer.View) (src, dst uint16, err *tcpip.Error) {
 
 // HandleUnknownDestinationPacket handles packets targeted at this protocol but
 // that don't match any existing endpoint.
-func (p *protocol) HandleUnknownDestinationPacket(*stack.Route, stack.TransportEndpointID, buffer.VectorisedView) bool {
+func (p *protocol) HandleUnknownDestinationPacket(r *stack.Route, id stack.TransportEndpointID, netHeader buffer.View, vv buffer.VectorisedView) bool {
+	// Get the header then trim it from the view.
+	hdr := header.UDP(vv.First())
+	if int(hdr.Length()) > vv.Size() {
+		// Malformed packet.
+		r.Stack().Stats().UDP.MalformedPacketsReceived.Increment()
+		return true
+	}
+	// TODO(b/129426613): only send an ICMP message if UDP checksum is valid.
+
+	// Only send ICMP error if the address is not a multicast/broadcast
+	// v4/v6 address or the source is not the unspecified address.
+	//
+	// See: point e) in https://tools.ietf.org/html/rfc4443#section-2.4
+	if id.LocalAddress == header.IPv4Broadcast || header.IsV4MulticastAddress(id.LocalAddress) || header.IsV6MulticastAddress(id.LocalAddress) || id.RemoteAddress == header.IPv6Any || id.RemoteAddress == header.IPv4Any {
+		return true
+	}
+
+	// As per RFC: 1122 Section 3.2.2.1 A host SHOULD generate Destination
+	//   Unreachable messages with code:
+	//
+	//     2 (Protocol Unreachable), when the designated transport protocol
+	//     is not supported; or
+	//
+	//     3 (Port Unreachable), when the designated transport protocol
+	//     (e.g., UDP) is unable to demultiplex the datagram but has no
+	//     protocol mechanism to inform the sender.
+	switch len(id.LocalAddress) {
+	case header.IPv4AddressSize:
+		if !r.Stack().AllowICMPMessage() {
+			r.Stack().Stats().ICMP.V4PacketsSent.RateLimited.Increment()
+			return true
+		}
+		// As per RFC 1812 Section 4.3.2.3
+		//
+		//   ICMP datagram SHOULD contain as much of the original
+		//   datagram as possible without the length of the ICMP
+		//   datagram exceeding 576 bytes
+		//
+		// NOTE: The above RFC referenced is different from the original
+		// recommendation in RFC 1122 where it mentioned that at least 8
+		// bytes of the payload must be included. Today linux and other
+		// systems implement the] RFC1812 definition and not the original
+		// RFC 1122 requirement.
+		mtu := int(r.MTU())
+		if mtu > header.IPv4MinimumProcessableDatagramSize {
+			mtu = header.IPv4MinimumProcessableDatagramSize
+		}
+		headerLen := int(r.MaxHeaderLength()) + header.ICMPv4MinimumSize
+		available := int(mtu) - headerLen
+		payloadLen := len(netHeader) + vv.Size()
+		if payloadLen > available {
+			payloadLen = available
+		}
+
+		payload := buffer.NewVectorisedView(len(netHeader), []buffer.View{netHeader})
+		payload.Append(vv)
+		payload.CapLength(payloadLen)
+
+		hdr := buffer.NewPrependable(headerLen)
+		pkt := header.ICMPv4(hdr.Prepend(header.ICMPv4MinimumSize))
+		pkt.SetType(header.ICMPv4DstUnreachable)
+		pkt.SetCode(header.ICMPv4PortUnreachable)
+		pkt.SetChecksum(header.ICMPv4Checksum(pkt, payload))
+		r.WritePacket(nil /* gso */, hdr, payload, header.ICMPv4ProtocolNumber, r.DefaultTTL())
+
+	case header.IPv6AddressSize:
+		if !r.Stack().AllowICMPMessage() {
+			r.Stack().Stats().ICMP.V6PacketsSent.RateLimited.Increment()
+			return true
+		}
+
+		// As per RFC 4443 section 2.4
+		//
+		//    (c) Every ICMPv6 error message (type < 128) MUST include
+		//    as much of the IPv6 offending (invoking) packet (the
+		//    packet that caused the error) as possible without making
+		//    the error message packet exceed the minimum IPv6 MTU
+		//    [IPv6].
+		mtu := int(r.MTU())
+		if mtu > header.IPv6MinimumMTU {
+			mtu = header.IPv6MinimumMTU
+		}
+		headerLen := int(r.MaxHeaderLength()) + header.ICMPv6DstUnreachableMinimumSize
+		available := int(mtu) - headerLen
+		payloadLen := len(netHeader) + vv.Size()
+		if payloadLen > available {
+			payloadLen = available
+		}
+		payload := buffer.NewVectorisedView(len(netHeader), []buffer.View{netHeader})
+		payload.Append(vv)
+		payload.CapLength(payloadLen)
+
+		hdr := buffer.NewPrependable(headerLen)
+		pkt := header.ICMPv6(hdr.Prepend(header.ICMPv6DstUnreachableMinimumSize))
+		pkt.SetType(header.ICMPv6DstUnreachable)
+		pkt.SetCode(header.ICMPv6PortUnreachable)
+		pkt.SetChecksum(header.ICMPv6Checksum(pkt, r.LocalAddress, r.RemoteAddress, payload))
+		r.WritePacket(nil /* gso */, hdr, payload, header.ICMPv6ProtocolNumber, r.DefaultTTL())
+	}
 	return true
 }
 
diff --git a/pkg/tcpip/transport/udp/udp_test.go b/pkg/tcpip/transport/udp/udp_test.go
index 9da6edce2..995d6e8a1 100644
--- a/pkg/tcpip/transport/udp/udp_test.go
+++ b/pkg/tcpip/transport/udp/udp_test.go
@@ -461,7 +461,11 @@ func (c *testContext) injectV4Packet(payload []byte, h *header4Tuple) {
 }
 
 func newPayload() []byte {
-	b := make([]byte, 30+rand.Intn(100))
+	return newMinPayload(30)
+}
+
+func newMinPayload(minSize int) []byte {
+	b := make([]byte, minSize+rand.Intn(100))
 	for i := range b {
 		b[i] = byte(rand.Intn(256))
 	}
@@ -1238,3 +1242,153 @@ func TestMulticastInterfaceOption(t *testing.T) {
 		})
 	}
 }
+
+// TestV4UnknownDestination verifies that we generate an ICMPv4 Destination
+// Unreachable message when a udp datagram is received on ports for which there
+// is no bound udp socket.
+func TestV4UnknownDestination(t *testing.T) {
+	c := newDualTestContext(t, defaultMTU)
+	defer c.cleanup()
+
+	testCases := []struct {
+		flow         testFlow
+		icmpRequired bool
+		// largePayload if true, will result in a payload large enough
+		// so that the final generated IPv4 packet is larger than
+		// header.IPv4MinimumProcessableDatagramSize.
+		largePayload bool
+	}{
+		{unicastV4, true, false},
+		{unicastV4, true, true},
+		{multicastV4, false, false},
+		{multicastV4, false, true},
+		{broadcast, false, false},
+		{broadcast, false, true},
+	}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("flow:%s icmpRequired:%t largePayload:%t", tc.flow, tc.icmpRequired, tc.largePayload), func(t *testing.T) {
+			payload := newPayload()
+			if tc.largePayload {
+				payload = newMinPayload(576)
+			}
+			c.injectPacket(tc.flow, payload)
+			if !tc.icmpRequired {
+				select {
+				case p := <-c.linkEP.C:
+					t.Fatalf("unexpected packet received: %+v", p)
+				case <-time.After(1 * time.Second):
+					return
+				}
+			}
+
+			select {
+			case p := <-c.linkEP.C:
+				var pkt []byte
+				pkt = append(pkt, p.Header...)
+				pkt = append(pkt, p.Payload...)
+				if got, want := len(pkt), header.IPv4MinimumProcessableDatagramSize; got > want {
+					t.Fatalf("got an ICMP packet of size: %d, want: sz <= %d", got, want)
+				}
+
+				hdr := header.IPv4(pkt)
+				checker.IPv4(t, hdr, checker.ICMPv4(
+					checker.ICMPv4Type(header.ICMPv4DstUnreachable),
+					checker.ICMPv4Code(header.ICMPv4PortUnreachable)))
+
+				icmpPkt := header.ICMPv4(hdr.Payload())
+				payloadIPHeader := header.IPv4(icmpPkt.Payload())
+				wantLen := len(payload)
+				if tc.largePayload {
+					wantLen = header.IPv4MinimumProcessableDatagramSize - header.IPv4MinimumSize*2 - header.ICMPv4MinimumSize - header.UDPMinimumSize
+				}
+
+				// In case of large payloads the IP packet may be truncated. Update
+				// the length field before retrieving the udp datagram payload.
+				payloadIPHeader.SetTotalLength(uint16(wantLen + header.UDPMinimumSize + header.IPv4MinimumSize))
+
+				origDgram := header.UDP(payloadIPHeader.Payload())
+				if got, want := len(origDgram.Payload()), wantLen; got != want {
+					t.Fatalf("unexpected payload length got: %d, want: %d", got, want)
+				}
+				if got, want := origDgram.Payload(), payload[:wantLen]; !bytes.Equal(got, want) {
+					t.Fatalf("unexpected payload got: %d, want: %d", got, want)
+				}
+			case <-time.After(1 * time.Second):
+				t.Fatalf("packet wasn't written out")
+			}
+		})
+	}
+}
+
+// TestV6UnknownDestination verifies that we generate an ICMPv6 Destination
+// Unreachable message when a udp datagram is received on ports for which there
+// is no bound udp socket.
+func TestV6UnknownDestination(t *testing.T) {
+	c := newDualTestContext(t, defaultMTU)
+	defer c.cleanup()
+
+	testCases := []struct {
+		flow         testFlow
+		icmpRequired bool
+		// largePayload if true will result in a payload large enough to
+		// create an IPv6 packet > header.IPv6MinimumMTU bytes.
+		largePayload bool
+	}{
+		{unicastV6, true, false},
+		{unicastV6, true, true},
+		{multicastV6, false, false},
+		{multicastV6, false, true},
+	}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("flow:%s icmpRequired:%t largePayload:%t", tc.flow, tc.icmpRequired, tc.largePayload), func(t *testing.T) {
+			payload := newPayload()
+			if tc.largePayload {
+				payload = newMinPayload(1280)
+			}
+			c.injectPacket(tc.flow, payload)
+			if !tc.icmpRequired {
+				select {
+				case p := <-c.linkEP.C:
+					t.Fatalf("unexpected packet received: %+v", p)
+				case <-time.After(1 * time.Second):
+					return
+				}
+			}
+
+			select {
+			case p := <-c.linkEP.C:
+				var pkt []byte
+				pkt = append(pkt, p.Header...)
+				pkt = append(pkt, p.Payload...)
+				if got, want := len(pkt), header.IPv6MinimumMTU; got > want {
+					t.Fatalf("got an ICMP packet of size: %d, want: sz <= %d", got, want)
+				}
+
+				hdr := header.IPv6(pkt)
+				checker.IPv6(t, hdr, checker.ICMPv6(
+					checker.ICMPv6Type(header.ICMPv6DstUnreachable),
+					checker.ICMPv6Code(header.ICMPv6PortUnreachable)))
+
+				icmpPkt := header.ICMPv6(hdr.Payload())
+				payloadIPHeader := header.IPv6(icmpPkt.Payload())
+				wantLen := len(payload)
+				if tc.largePayload {
+					wantLen = header.IPv6MinimumMTU - header.IPv6MinimumSize*2 - header.ICMPv6MinimumSize - header.UDPMinimumSize
+				}
+				// In case of large payloads the IP packet may be truncated. Update
+				// the length field before retrieving the udp datagram payload.
+				payloadIPHeader.SetPayloadLength(uint16(wantLen + header.UDPMinimumSize))
+
+				origDgram := header.UDP(payloadIPHeader.Payload())
+				if got, want := len(origDgram.Payload()), wantLen; got != want {
+					t.Fatalf("unexpected payload length got: %d, want: %d", got, want)
+				}
+				if got, want := origDgram.Payload(), payload[:wantLen]; !bytes.Equal(got, want) {
+					t.Fatalf("unexpected payload got: %v, want: %v", got, want)
+				}
+			case <-time.After(1 * time.Second):
+				t.Fatalf("packet wasn't written out")
+			}
+		})
+	}
+}