[RFC PATCH] ipv6: enable per-interface forwarding

[Gabriel Goller]

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
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?

[0]: https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt

Signed-off-by: Gabriel Goller <g.goller@xxxxxxxxxxx>
---
 net/ipv6/ip6_output.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 7bd29a9ff0db..a7e33ab0946c 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -509,7 +509,8 @@ int ip6_forward(struct sk_buff *skb)
 	u32 mtu;
 
 	idev = __in6_dev_get_safely(dev_get_by_index_rcu(net, IP6CB(skb)->iif));
-	if (READ_ONCE(net->ipv6.devconf_all->forwarding) == 0)
+	if ((idev && READ_ONCE(idev->cnf.forwarding) == 0) &&
+	    READ_ONCE(net->ipv6.devconf_all->forwarding) == 0)
 		goto error;
 
 	if (skb->pkt_type != PACKET_HOST)
-- 
2.39.5