netdev - Re: [PATCH] inet: don't call skb_orphan if tproxy happens in layer 2

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Date:   Fri, 16 Feb 2018 16:44:49 +0100
From:   Gregory Vander Schueren <gregory.vanderschueren@...sares.net>
To:     Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>
Cc:     netfilter-devel@...r.kernel.org, gregory.detal@...sares.net,
        Matthieu Baerts <matthieu.baerts@...sares.net>,
        netdev@...r.kernel.org
Subject: Re: [PATCH] inet: don't call skb_orphan if tproxy happens in layer 2

Hi Florian & Pablo,

Thank your very much for your quick feedback.

On 02/16/2018 12:28 PM, Pablo Neira Ayuso wrote:
> On Fri, Feb 16, 2018 at 12:07:06PM +0100, Florian Westphal wrote:
>> Gregory Vander Schueren <gregory.vanderschueren@...sares.net> wrote:
>>
>> [ cc netdev ]
>>
>>> If sysctl bridge-nf-call-iptables is enabled, iptables chains are already
>>> traversed from the bridging code. In such case, tproxy already happened when
>>> reaching ip_rcv. Thus no need to call skb_orphan as this would actually undo
>>> tproxy.
>>
>> I don't like this because it adds yet another test in fastpath, and for
>> a use case that has apparently never worked before.

Agreed. I also thought this was not ideal but I did find another way to 
easily fix this.

>>> We noticed issues when using tproxy with net.bridge.bridge-nf-call-iptables
>>> enabled. In such case, ip_rcv() basically undo tproxy's job. The following
>>> patch proposes a fix.
>>
>> I question wheter its a good idea to mix tproxy with bridges.
>>
>> Tproxy relies on policy routing, but a bridge doesn't route :-)
>>
>> I guess you use bridge snat mac mangling to force local delivery of
>> packets that are otherwise bridged?

Indeed, we use DNAT MAC mangling.

>> If yes, can you use ebtables brouting instead?
>> This would bypass the bridge (so no iptables invocation from bridge
>> prerouting anymore).

We were actually pondering over the usage of MAC DNAT vs brouting. I'll 
thus follow your suggestion and use brouting instead then.

>> We will try to get rid of nf-call-iptables eventually.

Good to know!

>> There might be (more complicated) ways to avoid this problem without
>> adding code in normal network path, but lets check other options first.
> 
> Agreed.
> 
> If there's a fix for this, that should be away from the fast path, not
> in ip_rcv().
> 

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