netdev - Re: When routed to VRF, NF _output

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Message-ID: <ed8f88e7-103a-403b-83ed-c40153e9bef0@6wind.com>
Date: Fri, 20 Jun 2025 18:20:08 +0200
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: Eugene Crosser <crosser@...rage.org>, netdev@...r.kernel.org
Cc: "netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
 David Ahern <dsahern@...nel.org>, Florian Westphal <fw@...len.de>,
 Pablo Neira Ayuso <pablo@...filter.org>
Subject: Re: When routed to VRF, NF _output_ hook is run unexpectedly

Le 20/06/2025 à 18:04, Eugene Crosser a écrit :
> Thanks Nicolas,
> 
> On 20/06/2025 16:56, Nicolas Dichtel wrote:
> 
>>> It is possible, and very useful, to implement "two-stage routing" by
>>> installing a route that points to a VRF device:
>>>
>>>     ip link add vrfNNN type vrf table NNN
>>>     ...
>>>     ip route add xxxxx/yy dev vrfNNN
>>>
>>> however this causes surprising behaviour with relation to netfilter
>>> hooks. Namely, packets taking such path traverse _output_ nftables
>>> chain, with conntracking information reset. So, for example, even
>>> when "notrack" has been set in the prerouting chain, conntrack entries
>>> will still be created. Script attached below demonstrates this behaviour.
>> You can have a look to this commit to better understand this:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8c9c296adfae9
> 
> I've seen this commit.
> My point is that the packets are _not locally generated_ in this case,
> so it seems wrong to pass them to the _output_ hook, doesn't it?
They are, from the POV of the vrf. The first route sends packets to the vrf
device, which acts like a loopback.


Regards,
Nicolas