netdev - Re: [PATCH net 1/2] openvswitch: support asymmetric conntrack

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Date:   Mon, 18 Nov 2019 16:19:29 -0500
From:   Aaron Conole <aconole@...hat.com>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:     netdev@...r.kernel.org, Pravin B Shelar <pshelar@....org>,
        "David S . Miller" <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>, dev@...nvswitch.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/2] openvswitch: support asymmetric conntrack

Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:

> Le 08/11/2019 à 22:07, Aaron Conole a écrit :
>> The openvswitch module shares a common conntrack and NAT infrastructure
>> exposed via netfilter.  It's possible that a packet needs both SNAT and
>> DNAT manipulation, due to e.g. tuple collision.  Netfilter can support
>> this because it runs through the NAT table twice - once on ingress and
>> again after egress.  The openvswitch module doesn't have such capability.
>> 
>> Like netfilter hook infrastructure, we should run through NAT twice to
>> keep the symmetry.
>> 
>> Fixes: 05752523e565 ("openvswitch: Interface with NAT.")
>> Signed-off-by: Aaron Conole <aconole@...hat.com>
> In this case, ovs_ct_find_existing() won't be able to find the
> conntrack, right?

vswitchd normally won't allow both actions to get programmed.  Even the
kernel module won't allow it, so this really will only happen when the
connection gets established via the nf_hook path, and then needs to be
processed via openvswitch.  In those cases, the tuple lookup should be
correct, because the nf_nat table should contain the correct tuple data,
and the skbuff should have the correct tuples in the packet data to
begin with.

> Inverting the tuple to find the conntrack doesn't work anymore with double NAT.
> Am I wrong?

I think since the packet was double-NAT on the way out (via nf_hook
path), then the incoming reply will have the correct NAT tuples and the
lookup will happen just fine.  Just that during processing, both
transformations aren't applied.

Makes sense?

> Regards,
> Nicolas