server+routing: pass isAlias to router

This allows the router to determine what is and isn't an alias from
lnd's definition of an alias. Any ChannelAnnouncement that has an
alias ShortChannelID field is not verified on-chain. To prevent a
DoS vector from existing, the gossiper ensures that only the local
lnd node can send its ChannelAnnouncements to the router with an
alias ShortChannelID.

routing/router.go

@@ -358,6 +358,10 @@ type Config struct {
 	// Otherwise, we'll only prune the channel when both edges have a very
 	// dated last update.
 	StrictZombiePruning bool
+
+	// IsAlias returns whether a passed ShortChannelID is an alias. This is
+	// only used for our local channels.
+	IsAlias func(scid lnwire.ShortChannelID) bool
 }
 
 // EdgeLocator is a struct used to identify a specific edge.
@@ -1455,8 +1459,12 @@ func (r *ChannelRouter) processUpdate(msg interface{},
 		// If AssumeChannelValid is present, then we are unable to
 		// perform any of the expensive checks below, so we'll
 		// short-circuit our path straight to adding the edge to our
-		// graph.
-		if r.cfg.AssumeChannelValid {
+		// graph. If the passed ShortChannelID is an alias, then we'll
+		// skip validation as it will not map to a legitimate tx. This
+		// is not a DoS vector as only we can add an alias
+		// ChannelAnnouncement from the gossiper.
+		scid := lnwire.NewShortChanIDFromInt(msg.ChannelID)
+		if r.cfg.AssumeChannelValid || r.cfg.IsAlias(scid) {
 			if err := r.cfg.Graph.AddChannelEdge(msg, op...); err != nil {
 				return fmt.Errorf("unable to add edge: %v", err)
 			}