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

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Message-ID: <d429f28f-f515-405d-b490-9b60e5f95070@6wind.com>
Date: Fri, 20 Jun 2025 18:09:52 +0200
From: Nicolas Dichtel <nicolas.dichtel@...nd.com>
To: Gabriel Goller <g.goller@...xmox.com>,
 "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ipv6: enable per-interface forwarding

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.

> forwarded if either the global setting (conf.all.forwarding) OR the
> interface-specific setting (conf.<interface>.forwarding) is enabled.
> This allows enabling forwarding on individual interfaces without
> setting the global option.
> 
> This change won't allow a `Router`-state interface without forwarding
> capabilities anymore, but (with my limited knowledge) I don't think that
> should be a problem?
> 
> This is quite an impacting change, so I don't really expect this to get
> merged. I'm more interested in hearing your feedback and if something
> like this would even be considered?
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.


Regards,
Nicolas