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

1 files changed, 100 insertions, 1 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
 }