netdev - Re: ipv4: Simplify ARP hash function.

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Date:	Fri, 8 Jul 2011 22:10:13 +0200
From:	Michał Mirosław <mirqus@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	roland@...estorage.com, johnwheffner@...il.com, mj@....cz,
	netdev@...r.kernel.org
Subject: Re: ipv4: Simplify ARP hash function.

2011/7/8 David Miller <davem@...emloft.net>:
> From: David Miller <davem@...emloft.net>
> Date: Fri, 08 Jul 2011 12:51:18 -0700 (PDT)
>
>> From: Michał Mirosław <mirqus@...il.com>
>> Date: Fri, 8 Jul 2011 21:39:18 +0200
>>
>>> With b[3] = b[0] ^ b[1] ^ b[2] you get 2^24 keys that hash to the same bucket.
>>
>> Ok, I'm convinced, thanks :-)
>
> Although, actually it's not this simple.  The attack doesn't work.
>
> As they "attack" us, the ARP hash table grows and thus the hash mask
> changes to match.  Then his old collisions won't collide any more.
>
> We could even adjust the fold shifts as the table grows to make this
> effect even more pronounced.

There will still be 2^32/n_buckets known values that hash to the same
bucket for every n_buckets. So if the attacker knows when and how the
hash size changes, he can adapt accordingly. It should be easier to
see when you get rid of the XOR [random, but] constant part.

Best Regards,
Michał Mirosław
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