ipv6: do not create neighbor entries for local delivery

RFC comment:

Please note that I do not fully understand the impacts of this, thus why RFC. I don't know if this is the best way to address the issue.

I have a report that when using TPROXY and IPv6, neighbor cache gets flooded with empty entries, while this does not happen with IPv4. These empty entries looks like:

# ip -6 neigh show nud all   (I masked some bits)
...
20xx::f0xx:x:3xdb dev lo lladdr 00:00:00:00:00:00 NOARP
...

Note that this address was not directly reachable by this host. It was 1 hop away, and still got a neighbor entry.

These entries seems to be not used during input. I disabled their creation and I could not notice any abnormal results.

Bellow the dashes, my original patch description. Applies to net-next.

Please advise.

Thanks,
Marcelo.

----------------

They will be created at output, if ever needed. This avoids creating empty neighbor entries when TPROXYing/Forwarding packets for addresses that are not even directly reachable.

Note that IPv4 already handles it this way. No neighbor entries are created for local input.

---
 net/ipv6/route.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index e229a3bc345dc4138a188282c4ab4f1717882832..e6058ab0bb94233da1eec3349e098175d5abf831 100644
@@ -928,7 +928,7 @@ restart:
 	dst_hold(&rt->dst);
 	read_unlock_bh(&table->tb6_lock);
-	if (!rt->n && !(rt->rt6i_flags & RTF_NONEXTHOP))
+	if (!rt->n && !(rt->rt6i_flags & (RTF_NONEXTHOP|RTF_LOCAL)))
 		nrt = rt6_alloc_cow(rt, &fl6->daddr, &fl6->saddr);
 	else if (!(rt->dst.flags & DST_HOST))
 		nrt = rt6_alloc_clone(rt, &fl6->daddr);

1.7.11.7