netdev - Re: When routed to VRF, NF _output

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Message-ID: <2211ec87-b794-4d74-9d3d-0c54f166efde@6wind.com>
Date: Fri, 20 Jun 2025 16:56:56 +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 15:38, Eugene Crosser a écrit :
> Hello!
Hello,

> 
> 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

Regards,
Nicolas