Document the fact that blackhole routes normally need to be bound

to lo(4) interfaces to have an effect, and that this is not needed
when using IP fast forwarding.

Sponsored by:	eXtensible Open Router Project <URL:http://www.xorp.org/>
MFC after:	3 weeks

sbin/route/route.8

@@ -412,3 +412,13 @@ utility appeared in
 The first paragraph may have slightly exaggerated
 .Xr routed 8 Ns 's
 abilities.
+.Pp
+Currently, routes with the
+.Dv RTF_BLACKHOLE
+flag set need to have the gateway set to an instance of the
+.Xr lo 4
+driver, using the
+.Fl iface
+option, for the flag to have any effect; unless IP fast forwarding
+is enabled, in which case the meaning of the flag will always
+be honored.