linux-kernel - Re: [PATCH v3] ipv6: add `force_forwarding` sysctl to enable per-interface forwarding

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Message-ID: <7c47cfb6-c1f1-42a1-8137-37f8f03fa970@6wind.com>
Date: Wed, 2 Jul 2025 17:14:42 +0200
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: Jakub Kicinski <kuba@...nel.org>, Gabriel Goller <g.goller@...xmox.com>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Jonathan Corbet <corbet@....net>,
 David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] ipv6: add `force_forwarding` sysctl to enable
 per-interface forwarding

Le 02/07/2025 à 16:34, Jakub Kicinski a écrit :
> On Wed,  2 Jul 2025 09:46:18 +0200 Gabriel Goller wrote:
>> It is currently impossible to enable ipv6 forwarding on a per-interface
>> basis like in ipv4. To enable forwarding on an ipv6 interface we need to
>> enable it on all interfaces and disable it on the other interfaces using
>> a netfilter rule. This is especially cumbersome if you have lots of
>> interface and only want to enable forwarding on a few. According to the
>> sysctl docs [0] the `net.ipv6.conf.all.forwarding` enables forwarding
>> for all interfaces, while the interface-specific
>> `net.ipv6.conf.<interface>.forwarding` configures the interface
>> Host/Router configuration.
>>
>> Introduce a new sysctl flag `force_forwarding`, which can be set on every
>> interface. The ip6_forwarding function will then check if the global
>> forwarding flag OR the force_forwarding flag is active and forward the
>> packet.
> 
> Should we invert the polarity? It appears that the condition below only
> let's this setting _disable_ forwarding. IMO calling it "force" suggests
> to the user that it will force it to be enabled.
Not sure to follow you. When force_forwarding is set to 1 the forwarding is
always enabled.

sysctl | all.forwarding | iface.force_forwarding | packet processing from iface
       |      0         |           0            |        no forward
       |      0         |           1            |         forward
       |      1         |           0            |         forward
       |      1         |           1            |         forward

> 
> Nicolas, how do you feel about asking for a selftest here? 
> The functionality is fairly trivial from datapath PoV, but feels odd 
> to merge uAPI these days without a selftest..
No problem, let's do it right.