According to RFC 1812 ICMP Destination unreachable (Network unreachable)
does not quite our case of clients directly behind the router.
If a packet is to be forwarded to a host on a network that is directly
connected to the router (i.e., the router is the last-hop router) and
the router has ascertained that there is no path to the destination
host then the router MUST generate a Destination Unreachable, Code 1
(Host Unreachable) ICMP message.
Issue #4729