linux-kernel - Re: [PATCH v2] ipv6: add `force_forwarding` sysctl to enable per-interface forwarding

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Message-ID: <vtgdrglnu52yech4p244ttzaqetooxphultqigf6dmytnfu6ky@rpx5cl3qmzec>
Date: Wed, 2 Jul 2025 09:42:40 +0200
From: Gabriel Goller <g.goller@...xmox.com>
To: Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc: "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>, netdev@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ipv6: add `force_forwarding` sysctl to enable
 per-interface forwarding

On 01.07.2025 17:58, Nicolas Dichtel wrote:
>Le 01/07/2025 à 16:04, 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 `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.
>>
>> To preserver backwards-compatibility reset the flag (global and on all
>> interfaces) to 0 if the net.ipv6.conf.all.forwarding flag is set to 0.
>>
>> [0]: https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
>>
>> Signed-off-by: Gabriel Goller <g.goller@...xmox.com>
>> ---
>
>[snip]
>
>> @@ -896,6 +907,16 @@ static int addrconf_fixup_forwarding(const struct ctl_table *table, int *p, int
>>  						     NETCONFA_IFINDEX_DEFAULT,
>>  						     net->ipv6.devconf_dflt);
>>
>> +		/*
>> +		 * With the introduction of force_forwarding, we need to be backwards
>> +		 * compatible, so that means we need to set the force_forwarding global
>> +		 * flag to 0 if the global forwarding flag is set to 0. Below in
>> +		 * addrconf_forward_change(), we also set the force_forwarding flag on every
>> +		 * interface to 0 if the global forwarding flag is set to 0.
>> +		 */
>> +		if (newf == 0)
>> +			WRITE_ONCE(net->ipv6.devconf_all->force_forwarding, newf);
>Hmm, is this true? Configuring the default value only impacts new interfaces.
>And before your patch, only the 'all' entry is took into account. In other
>words, configuring the default entry today doesn't change the current behavior,
>so I don't see the backward compat point.

Yes, you're right I didn't think this through.
This only affects new interfaces.
I'll remove it.

>> +
>>  		addrconf_forward_change(net, newf);
>>  		if ((!newf) ^ (!old))
>>  			inet6_netconf_notify_devconf(net, RTM_NEWNETCONF,
>
>[snip]
>
>> +static int addrconf_sysctl_force_forwarding(const struct ctl_table *ctl, int write,
>> +					    void *buffer, size_t *lenp, loff_t *ppos)
>> +{
>> +	int *valp = ctl->data;
>> +	int ret;
>> +	int old, new;
>> +
>> +	old = *valp;
>> +	ret = proc_douintvec(ctl, write, buffer, lenp, ppos);
>> +	new = *valp;
>Maybe you can limit values to 0 and 1, like it was done in the v1.

We use the extra1 and extra2 params for the interfaces when setting
interface-specific options, but I can just copy them out and then add
the min/max options to the table like in `addrconf_sysctl_mtu`.

I'll send a patch soon!

>Regards,
>Nicolas

Thanks for the review!