ipv6: add `force_forwarding` sysctl to enable per-interface forwarding

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
interfaces 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 preserve 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

Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
Link: https://patch.msgid.link/20250722081847.132632-1-g.goller@proxmox.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 106 insertions, 0 deletions

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 13e2678d418b..b31a71f2b372 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -116,6 +116,7 @@ TEST_GEN_FILES += skf_net_off
 TEST_GEN_FILES += tfo
 TEST_PROGS += tfo_passive.sh
 TEST_PROGS += broadcast_pmtu.sh
+TEST_PROGS += ipv6_force_forwarding.sh
 
 # YNL files, must be before "include ..lib.mk"
 YNL_GEN_FILES := busy_poller netlink-dumps
diff --git a/tools/testing/selftests/net/ipv6_force_forwarding.sh b/tools/testing/selftests/net/ipv6_force_forwarding.sh
new file mode 100755
index 000000000000..bf0243366caa
--- /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 nodad
+    ip -n $ns2 addr add 2001:db8:1::2/64 dev veth21 nodad
+    ip -n $ns2 addr add 2001:db8:2::1/64 dev veth23 nodad
+    ip -n $ns3 addr add 2001:db8:2::2/64 dev veth32 nodad
+
+    # 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"
+    else
+        echo "FAIL: ping failed with force_forwarding enabled"
+        ret=1
+    fi
+
+    return $ret
+}
+
+echo "IPv6 force_forwarding test"
+echo "=========================="
+
+setup_test
+test_force_forwarding
+ret=$?
+
+if [ $ret -eq 0 ]; then
+    echo "OK"
+    exit 0
+elif [ $ret -eq $ksft_skip ]; then
+    echo "SKIP"
+    exit $ksft_skip
+else
+    echo "FAIL"
+    exit 1
+fi