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Message-ID: <858eb643-bf0a-480e-b7f7-103b7bd94707@redhat.com>
Date: Thu, 10 Jul 2025 15:26:47 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Gabriel Goller <g.goller@...xmox.com>,
Nicolas Dichtel <nicolas.dichtel@...nd.com>,
"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
Jonathan Corbet <corbet@....net>, David Ahern <dsahern@...nel.org>,
Shuah Khan <shuah@...nel.org>
Cc: netdev@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net-next v5] ipv6: add `force_forwarding` sysctl to enable
per-interface forwarding
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@...xmox.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
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