netdev - Re: [PATCH] packet: Use symmetric hash for PACKET_FANOUT

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Date:	Sat, 02 Jul 2016 16:38:25 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	tom@...bertland.com
Cc:	netdev@...r.kernel.org, eric@...it.org, victor@...iniac.net
Subject: Re: [PATCH] packet: Use symmetric hash for PACKET_FANOUT_HASH.

From: Tom Herbert <tom@...bertland.com>
Date: Fri, 1 Jul 2016 14:16:54 -0700

> On Fri, Jul 1, 2016 at 2:07 PM, David Miller <davem@...emloft.net> wrote:
>> From: Tom Herbert <tom@...bertland.com>
>> Date: Fri, 1 Jul 2016 13:52:58 -0700
>>
>>> Why are symmetric hashes required?
>>
>> Because they want load balancing, such that one flow only can overrun
>> one single socket not all of the ones in the fanout.
>>
> I'm still missing it. Why is this any different than what we need with
> something like SO_REUSEPORT?

Because local sockets only demux on RX packets for a flow so they
don't need a symmetric hash, and in fact wouldn't even notice if the
hash was symmetric or not.

Programs like suricata that are operating as a bump in the stack see
both directions of traffic for a flow and therefore for them whether
the hash is symmetric is an issue.