netdev - Re: PROBLEM: Linux 3.9 more-specific ipv6 route ignored until next-hop is in neighbor cache

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Message-ID: <20130703181546.GC12615@order.stressinduktion.org>
Date:	Wed, 3 Jul 2013 20:15:46 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Pierre Emeriaud <petrus.lt@...il.com>, netdev@...r.kernel.org,
	yoshfuji@...ux-ipv6.org
Subject: Re: PROBLEM: Linux 3.9 more-specific ipv6 route ignored until next-hop is in neighbor cache

On Wed, Jul 03, 2013 at 07:00:07AM +0200, Hannes Frederic Sowa wrote:
> [Cc YOSHIFUJI Hideaki because of commit
> 887c95cc1da53f66a5890fdeab13414613010097 ("ipv6: Complete neighbour entry
> removal from dst_entry.")]
> 
> On Wed, Jul 03, 2013 at 12:04:33AM +0200, Pierre Emeriaud wrote:
> > Linux 3.9 more-specific ipv6 route ignored until next-hop is in neighbor cache.
> > 
> > When adding a route to 2000::/3 with a next-hop that is not in the
> > neighbor cache, the route is not preferred over the default.
> 
> Thanks for the report!
> 
> Well.
> 
> We ignore this route because of rt6_score_route returning -1 in this case.
> This traces down to rt6_check_neigh returning false.
> 
> Before the above mentioned commit we kicked off some logic to create a
> neighbour entry in ip6_route_add. Now we end up with neigh == NULL.
> 
> This is a hotfix but I need to do more research regarding
> CONFIG_IPV6_ROUTER_PREF and further expectations of neigh != NULL (you
> can try this at your own risk ;):

I looked up the relevant RFCs and do think this is the proper fix. Could you
give it a test?

[PATCH net] ipv6: rt6_check_neigh should successfully verify neigh if no NUD information are available

After the removal of rt->n we do not create a neighbour entry at route
insertion time (rt6_bind_neighbour is gone). As long as no neighbour is
created because of "useful traffic" we skip this routing entry because
rt6_check_neigh cannot pick up a valid neighbour (neigh == NULL) and
thus returns false.

This change was introduced by commit
887c95cc1da53f66a5890fdeab13414613010097 ("ipv6: Complete neighbour
entry removal from dst_entry.")

To quote RFC4191:
"If the host has no information about the router's reachability, then
the host assumes the router is reachable."

and also:
"A host MUST NOT probe a router's reachability in the absence of useful
traffic that the host would have sent to the router if it were reachable."

So, just assume the router is reachable and let's rt6_probe do the
rest. We don't need to create a neighbour on route insertion time.

If we don't compile with CONFIG_IPV6_ROUTER_PREF (RFC4191 support)
a neighbour is only valid if its nud_state is NUD_VALID. I did not find
any references that we should probe the router on route insertion time
via the other RFCs. So skip this route in that case.

Reported-by: Pierre Emeriaud <petrus.lt@...il.com>
Cc: YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
---
 net/ipv6/route.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index ad0aa6b..450979d 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -547,6 +547,10 @@ static inline bool rt6_check_neigh(struct rt6_info *rt)
 			ret = true;
 #endif
 		read_unlock(&neigh->lock);
+	} else {
+#ifdef CONFIG_IPV6_ROUTER_PREF
+		ret = true;
+#endif
 	}
 	rcu_read_unlock_bh();
 
-- 
1.8.1.4

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