netdev - Re: Issue with gratuitous arps when new addr is different from cached addr

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Message-ID: <CAO7SqHCUmqNqYwhJtP9iRjG=pyc5Uhnf5eYhXN-mkQLr1oZsWQ@mail.gmail.com>
Date:	Wed, 20 Nov 2013 22:33:17 -0800
From:	Salam Noureddine <noureddine@...stanetworks.com>
To:	Salam Noureddine <noureddine@...stanetworks.com>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Phil Sutter <phil@....cc>, Eric Dumazet <edumazet@...gle.com>,
	netdev@...r.kernel.org
Subject: Re: Issue with gratuitous arps when new addr is different from cached addr

On Wed, Nov 20, 2013 at 10:23 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Wed, Nov 20, 2013 at 10:06:34PM -0800, Salam Noureddine wrote:
>> Isn't locktime useful in that case for limiting the rate of garp
>> changes to the arp cache?
>
> Yes, as I said, it would be nice to have rate-limiting for those, too.
>
> GARP is used in cluster setups to make the switch-over as fast as possible and
> I don't think they accept those lock-down delays, so I guess it would be nice
> to forceful override the lladdr if (sip == tip) && ACCEPT_ARP(dev) is true in
> every case.

I was in fact testing a switch-over scenario and saw that the arp
update was taking
much longer than the locktime period which pointed to this problem.

>
> I guess you are not testing with ACCEPT_ARP?
>

Yes, I am not setting ACCEPT_ARP.

> In that case I am not sure what to do.
>
> override = (...timing...) || sip == tip; could work but does relax the
> protection of the neigh cache.
>
> In IPv6 we have a flag in the packet if we should overwrite the entry.
>
> I'll have to think about that a bit more. Could well be the case that we need
> your proposal, too. But then we would have to validate the change with IPv6,
> too and neighbour cache states are really complex.
>
> Greetings,
>
>   Hannes
>
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