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

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Message-ID: <f674f8ac-8c4a-4c1c-9704-31a3116b56d6@6wind.com>
Date: Thu, 26 Jun 2025 16:51:35 +0200
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: Gabriel Goller <g.goller@...xmox.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Jonathan Corbet <corbet@....net>,
 David Ahern <dsahern@...nel.org>
Cc: netdev@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipv6: add `do_forwarding` sysctl to enable per-interface
 forwarding



Le 25/06/2025 à 16:26, Gabriel Goller a écrit :
> 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 `do_forwarding`, which can be set on every
> interface. The ip6_forwarding function will then check if the global
> forwarding flag OR the do_forwarding flag is active and forward the
> packet. To preserver backwards-compatibility also reset the flag on all
> interfaces when setting the global forwarding flag to 0.
> 
> [0]: https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
> 
> Signed-off-by: Gabriel Goller <g.goller@...xmox.com>
Please, export this sysctl via a NETCONFA_DO_FORWARDING attribute also.

> ---
> 
> * I don't have any hard feelings about the naming, Nicolas Dichtel
>   proposed `fwd_per_iface` but I think `do_forwarding` is a better fit.
What about force_forwarding?

> * I'm also not sure about the reset when setting the global forwarding
>   flag; don't know if I did that right. Feedback is welcome!
It seems correct to me.

> * Thanks for the help!
Maybe you could align ipv6.all.do_forwarding on ipv4.all.forwarding, ie setting
all existing ipv6.*.do_forwarding.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/ipv4/devinet.c#n2423

Regards,
Nicolas