Re: [PATCH net-next v5] ipv6: add `force_forwarding` sysctl to enable per-interface forwarding

[Paolo Abeni <pabeni@redhat.com>]

On 7/7/25 11:43 AM, Gabriel Goller wrote:
> 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 (on all interfaces)
> to 0 if the net.ipv6.conf.all.forwarding flag is set to 0.
> 
> Add a short selftest that checks if a packet gets forwarded with and
> without `force_forwarding`.
> 
> [0]: https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
> 
> Signed-off-by: Gabriel Goller <g.goller@proxmox.com>

Does not apply cleanly anymore, please rebase and repost.

Also a few nits below...

> @@ -857,6 +859,9 @@ static void addrconf_forward_change(struct net *net, __s32 newf)
>  		idev = __in6_dev_get_rtnl_net(dev);
>  		if (idev) {
>  			int changed = (!idev->cnf.forwarding) ^ (!newf);
> +			/* Disabling all.forwarding sets 0 to force_forwarding for all interfaces */
> +			if (newf == 0)
> +				WRITE_ONCE(idev->cnf.force_forwarding, newf);

You could use:

			WRITE_ONCE(idev->cnf.force_forwarding, 0);
				

>  
>  			WRITE_ONCE(idev->cnf.forwarding, newf);
>  			if (changed)
> @@ -5719,6 +5724,7 @@ static void ipv6_store_devconf(const struct ipv6_devconf *cnf,
>  	array[DEVCONF_ACCEPT_UNTRACKED_NA] =
>  		READ_ONCE(cnf->accept_untracked_na);
>  	array[DEVCONF_ACCEPT_RA_MIN_LFT] = READ_ONCE(cnf->accept_ra_min_lft);
> +	array[DEVCONF_FORCE_FORWARDING] = READ_ONCE(cnf->force_forwarding);
>  }
>  
>  static inline size_t inet6_ifla6_size(void)
> @@ -6747,6 +6753,76 @@ static int addrconf_sysctl_disable_policy(const struct ctl_table *ctl, int write
>  	return ret;
>  }
>  
> +static void addrconf_force_forward_change(struct net *net, __s32 newf)
> +{
> +	struct net_device *dev;
> +	struct inet6_dev *idev;
> +
> +	for_each_netdev(net, dev) {
> +		idev = __in6_dev_get_rtnl_net(dev);
> +		if (idev) {
> +			int changed = (!idev->cnf.force_forwarding) ^ (!newf);
> +
> +			WRITE_ONCE(idev->cnf.force_forwarding, newf);
> +			if (changed) {
> +				inet6_netconf_notify_devconf(dev_net(dev), RTM_NEWNETCONF,
> +							     NETCONFA_FORCE_FORWARDING,
> +							     dev->ifindex, &idev->cnf);
> +			}

Brakets not needed for the above statement. Either drop them or move the
WRITE_ONCE() inside the if ()

> diff --git a/tools/testing/selftests/net/ipv6_force_forwarding.sh b/tools/testing/selftests/net/ipv6_force_forwarding.sh
> new file mode 100644
> index 000000000000..62adc9d4afc9
> --- /dev/null
> +++ b/tools/testing/selftests/net/ipv6_force_forwarding.sh
> @@ -0,0 +1,105 @@
> +#!/bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +#
> +# Test IPv6 force_forwarding interface property
> +#
> +# This test verifies that the force_forwarding property works correctly:
> +# - When global forwarding is disabled, packets are not forwarded normally
> +# - When force_forwarding is enabled on an interface, packets are forwarded
> +#   regardless of the global forwarding setting
> +
> +source lib.sh
> +
> +cleanup() {
> +    cleanup_ns $ns1 $ns2 $ns3
> +}
> +
> +trap cleanup EXIT
> +
> +setup_test() {
> +    # Create three namespaces: sender, router, receiver
> +    setup_ns ns1 ns2 ns3
> +
> +    # Create veth pairs: ns1 <-> ns2 <-> ns3
> +    ip link add name veth12 type veth peer name veth21
> +    ip link add name veth23 type veth peer name veth32
> +
> +    # Move interfaces to namespaces
> +    ip link set veth12 netns $ns1
> +    ip link set veth21 netns $ns2
> +    ip link set veth23 netns $ns2
> +    ip link set veth32 netns $ns3
> +
> +    # Configure interfaces
> +    ip -n $ns1 addr add 2001:db8:1::1/64 dev veth12
> +    ip -n $ns2 addr add 2001:db8:1::2/64 dev veth21
> +    ip -n $ns2 addr add 2001:db8:2::1/64 dev veth23
> +    ip -n $ns3 addr add 2001:db8:2::2/64 dev veth32

The above will trigger DaD...

> +
> +    # Bring up interfaces
> +    ip -n $ns1 link set veth12 up
> +    ip -n $ns2 link set veth21 up
> +    ip -n $ns2 link set veth23 up
> +    ip -n $ns3 link set veth32 up
> +
> +    # Add routes
> +    ip -n $ns1 route add 2001:db8:2::/64 via 2001:db8:1::2
> +    ip -n $ns3 route add 2001:db8:1::/64 via 2001:db8:2::1
> +
> +    # Disable global forwarding
> +    ip netns exec $ns2 sysctl -qw net.ipv6.conf.all.forwarding=0
> +}
> +
> +test_force_forwarding() {
> +    local ret=0
> +
> +    echo "TEST: force_forwarding functionality"
> +
> +    # Check if force_forwarding sysctl exists
> +    if ! ip netns exec $ns2 test -f /proc/sys/net/ipv6/conf/veth21/force_forwarding; then
> +        echo "SKIP: force_forwarding not available"
> +        return $ksft_skip
> +    fi
> +
> +    # Test 1: Without force_forwarding, ping should fail
> +    ip netns exec $ns2 sysctl -qw net.ipv6.conf.veth21.force_forwarding=0
> +    ip netns exec $ns2 sysctl -qw net.ipv6.conf.veth23.force_forwarding=0
> +
> +    if ip netns exec $ns1 ping -6 -c 1 -W 2 2001:db8:2::2 &>/dev/null; then
> +        echo "FAIL: ping succeeded when forwarding disabled"
> +        ret=1
> +    else
> +        echo "PASS: forwarding disabled correctly"
> +    fi
> +
> +    # Test 2: With force_forwarding enabled, ping should succeed
> +    ip netns exec $ns2 sysctl -qw net.ipv6.conf.veth21.force_forwarding=1
> +    ip netns exec $ns2 sysctl -qw net.ipv6.conf.veth23.force_forwarding=1
> +
> +    if ip netns exec $ns1 ping -6 -c 1 -W 2 2001:db8:2::2 &>/dev/null; then
> +        echo "PASS: force_forwarding enabled forwarding"

... I'm wondering if it could sometimes race with the ping and cause
sporadic failures? Possible using 'nodad' option for address creation
could help.

/P