Send ICMP errors when link address resolution fails

Before this change, we would silently drop packets when link resolution
failed. This change brings us into line with RFC 792 (IPv4) and RFC 4443 (IPv6),
both of which specify that gateways should return an ICMP error to the sender
when link resolution fails.

PiperOrigin-RevId: 374699789

2 files changed, 36 insertions, 1 deletions

diff --git a/pkg/tcpip/network/ipv4/icmp.go b/pkg/tcpip/network/ipv4/icmp.go
index d1a82b584..8e27a6677 100644
--- a/pkg/tcpip/network/ipv4/icmp.go
+++ b/pkg/tcpip/network/ipv4/icmp.go
@@ -481,6 +481,22 @@ func (*icmpReasonFragmentationNeeded) isForwarding() bool {
 	return true
 }
 
+// icmpReasonHostUnreachable is an error in which the host specified in the
+// internet destination field of the datagram is unreachable.
+type icmpReasonHostUnreachable struct{}
+
+func (*icmpReasonHostUnreachable) isICMPReason() {}
+func (*icmpReasonHostUnreachable) isForwarding() bool {
+	// If we hit a Host Unreachable error, then we know we are operating as a
+	// router. As per RFC 792 page 5, Destination Unreachable Message,
+	//
+	//   In addition, in some networks, the gateway may be able to determine
+	//   if the internet destination host is unreachable.  Gateways in these
+	//   networks may send destination unreachable messages to the source host
+	//   when the destination host is unreachable.
+	return true
+}
+
 // returnError takes an error descriptor and generates the appropriate ICMP
 // error packet for IPv4 and sends it back to the remote device that sent
 // the problematic packet. It incorporates as much of that packet as
@@ -537,7 +553,12 @@ func (p *protocol) returnError(reason icmpReason, pkt *stack.PacketBuffer) tcpip
 	defer route.Release()
 
 	p.mu.Lock()
-	netEP, ok := p.mu.eps[pkt.NICID]
+	// We retrieve an endpoint using the newly constructed route's NICID rather
+	// than the packet's NICID. The packet's NICID corresponds to the NIC on
+	// which it arrived, which isn't necessarily the same as the NIC on which it
+	// will be transmitted. On the other hand, the route's NIC *is* guaranteed
+	// to be the NIC on which the packet will be transmitted.
+	netEP, ok := p.mu.eps[route.NICID()]
 	p.mu.Unlock()
 	if !ok {
 		return &tcpip.ErrNotConnected{}
@@ -653,6 +674,10 @@ func (p *protocol) returnError(reason icmpReason, pkt *stack.PacketBuffer) tcpip
 		icmpHdr.SetType(header.ICMPv4DstUnreachable)
 		icmpHdr.SetCode(header.ICMPv4NetUnreachable)
 		counter = sent.dstUnreachable
+	case *icmpReasonHostUnreachable:
+		icmpHdr.SetType(header.ICMPv4DstUnreachable)
+		icmpHdr.SetCode(header.ICMPv4HostUnreachable)
+		counter = sent.dstUnreachable
 	case *icmpReasonFragmentationNeeded:
 		icmpHdr.SetType(header.ICMPv4DstUnreachable)
 		icmpHdr.SetCode(header.ICMPv4FragmentationNeeded)
diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index 23178277a..26fb8a871 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -104,6 +104,16 @@ type endpoint struct {
 
 // HandleLinkResolutionFailure implements stack.LinkResolvableNetworkEndpoint.
 func (e *endpoint) HandleLinkResolutionFailure(pkt *stack.PacketBuffer) {
+	// If we are operating as a router, return an ICMP error to the original
+	// packet's sender.
+	if pkt.NetworkPacketInfo.IsForwardedPacket {
+		// TODO(gvisor.dev/issue/6005): Propagate asynchronously generated ICMP
+		// errors to local endpoints.
+		e.protocol.returnError(&icmpReasonHostUnreachable{}, pkt)
+		e.stats.ip.Forwarding.Errors.Increment()
+		e.stats.ip.Forwarding.HostUnreachable.Increment()
+		return
+	}
 	// handleControl expects the entire offending packet to be in the packet
 	// buffer's data field.
 	pkt = stack.NewPacketBuffer(stack.PacketBufferOptions{