netdev - Re: [RFC PATCH] ipv6: enable per-interface forwarding

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Message-Id: <20250620.182649.211302145878462506.davem@davemloft.net>
Date: Fri, 20 Jun 2025 18:26:49 +0100 (BST)
From: David Miller <davem@...emloft.net>
To: nicolas.dichtel@...nd.com
Cc: g.goller@...xmox.com, dsahern@...nel.org, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, horms@...nel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ipv6: enable per-interface forwarding

From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
Date: Fri, 20 Jun 2025 18:09:52 +0200

> Le 20/06/2025 à 17:28, 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.
>> 
>> This patch modifies the forwarding logic to check both the global
>> forwarding flag AND the per-interface forwarding flag. Packets are
> You cannot change this, it will break existing setups.

Therefore.
> The only way I see for this is probably to introduce a new sysctl, say
> net.ipv6.conf.<iface>.fwd_per_iface (there is probably a better name).
> When the user set net.ipv6.conf.all.forwarding to 0, the kernel should reset all
> existing net.ipv6.conf.<iface>.fwd_per_iface entries to keep the backward compat.
I agree.

Thanks.