Re: arp_hash

[David Miller <davem@davemloft.net>]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Mon, 23 Mar 2015 08:56:12 +1100

> On Sun, Mar 22, 2015 at 05:56:21AM -0700, Eric Dumazet wrote:
>> On Sun, 2015-03-22 at 22:42 +1100, Herbert Xu wrote:
>> > 
>> > While googling I found the 2011 discussion on changing the arp_hash
>> > function.  I must say that I'm not really impressed by the new
>> > function that replaced jhash :)
>> > 
>> > 	u32 key = *(const u32 *)pkey;
>> > 	u32 val = key ^ hash32_ptr(dev);
>> > 
>> > 	return val * hash_rnd[0];
> 
> In fact this function is worse than I thought.  Because the IP
> address is stored in big-endian, the low bits correspond to the
> first octet.  So in any network smaller than a /8 everything
> hashes the same value.

I won't argue that there might be some bugs here, and there is some
lost entropy, and that we should fix them.

But I do not agree with the idea that it is in any way possible to
stop someone with root access on the physical network from crudding up
our ARP hash tables.