netdev - Re: ipv4: Simplify ARP hash function.

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Message-ID: <CAHXqBFKKRXFjr7y+v12ApuWTpjf-G3H5iBtfQg1gzC5iPfRO=g@mail.gmail.com>
Date:	Fri, 8 Jul 2011 22:34:22 +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.

W dniu 8 lipca 2011 22:10 użytkownik Michał Mirosław <mirqus@...il.com> napisał:
> 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.

BTW, am I correct, that neighbour hash tables never shrink? Looking at
net/core/neighbour.c it seems that after the table reaches gc_thresh3
capacity, it is never reallocated again.

Best Regards,
Michał Mirosław
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