they are created on the fly. This makes it clear what the route is for
and allows an optimisation in ip_output() by avoiding a call to
in_broadcast() because most of the time we do talk to a host.
It also avoids a needless allocation for the storage of llinfo_arp and
thus vanishes from arp(8) - it showed as incomplete anyway so this
is a nice side effect.
Guard against this and routes marked with RTF_BLACKHOLE in
ip_fastforward().
While here, guard against routes marked with RTF_BLACKHOLE in
ip6_fastforward().
RTF_BROADCAST is IPv4 only, so don't bother checking that here.
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