htlcswitch+config+server: Adds a rejecthtlc flag that disables forwarded

htlcs

config: Adding RejectHTLC field in config struct

This commit adds a RejectHTLC field in the config struct in config.go.
This allows the user to run lnd as a node that does not accept onward
HTLCs.

htlcswitch/switch: Adding a field RejectHTLC to the switch config

This commit adds a field RejectHTLC to the switch config. When the
switch receives an HTLC it will check this flag and if the HTLC is not
from the source hop, the HTLC will be rejected.

htlcswitch/switch: adding check for RejectHTLC flag and incomingChanID

This commit adds a check when receiving UpdateAddHTLC. The check looks
for the RejectHTLC flag set and whether the HTLC is from the sourceHop
(the local switch). If the HTLC is not from the sourceHop, then we
reject the HTLC and return a FailChannelDisabled error.

server: adding RejectHTLC field to initialization of switch

lnd_test: adding test for RejectHTLC

This commit adds a test which tests that a node with the --rejecthtlc
flag will reject any onward HTLCs but still can receive direct HTLCs and
can send HTLCs.

htlcswitch/switch.go

@@ -171,6 +171,10 @@ type Config struct {
 	// the ChannelNotifier when channels become active and inactive.
 	NotifyActiveChannel   func(wire.OutPoint)
 	NotifyInactiveChannel func(wire.OutPoint)
+
+	// RejectHTLC is a flag that instructs the htlcswitch to reject any
+	// HTLCs that are not from the source hop.
+	RejectHTLC bool
 }
 
 // Switch is the central messaging bus for all incoming/outgoing HTLCs.
@@ -1025,6 +1029,15 @@ func (s *Switch) handlePacketForward(packet *htlcPacket) error {
 	// payment circuit within our internal state so we can properly forward
 	// the ultimate settle message back latter.
 	case *lnwire.UpdateAddHTLC:
+		// Check if the node is set to reject all onward HTLCs and also make
+		// sure that HTLC is not from the source node.
+		if s.cfg.RejectHTLC && packet.incomingChanID != sourceHop {
+			failure := &lnwire.FailChannelDisabled{}
+			addErr := fmt.Errorf("unable to forward any htlcs")
+
+			return s.failAddPacket(packet, failure, addErr)
+		}
+
 		if packet.incomingChanID == sourceHop {
 			// A blank incomingChanID indicates that this is
 			// a pending user-initiated payment.