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Miller" , David Ahern , Eric Dumazet , Kuniyuki Iwashima , Jakub Kicinski , Paolo Abeni , Simon Horman , Shuah Khan , linux-kselftest@vger.kernel.org Subject: [PATCH net-next v6 0/2] udp: fix FOU/GUE over multicast Date: Wed, 8 Jul 2026 03:35:02 +0300 Message-ID: X-Mailer: git-send-email 2.47.3 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit UDP encapsulation (FOU, GUE) has never worked correctly with multicast destination addresses. When a FOU-encapsulated packet arrives at a multicast address, it enters __udp4_lib_mcast_deliver() / __udp6_lib_mcast_deliver() which call consume_skb() on packets that need resubmission to the inner protocol handler, silently dropping them instead. The unicast delivery paths handle this correctly by propagating the return value up to ip[6]_protocol_deliver_rcu() for resubmission, but the multicast paths were never updated to support UDP encapsulation resubmit. This causes silent packet loss for FOU/GRETAP tunnels configured with multicast remote addresses (both IPv4 and IPv6). Reproducing the issue (IPv4): ip netns add ns_a && ip netns add ns_b ip link add veth0 netns ns_a type veth peer name veth1 netns ns_b ip -n ns_a addr add 10.0.0.1/24 dev veth0 && ip -n ns_a link set veth0 up ip -n ns_b addr add 10.0.0.2/24 dev veth1 && ip -n ns_b link set veth1 up ip -n ns_a route add 239.0.0.0/8 dev veth0 ip -n ns_b route add 239.0.0.0/8 dev veth1 # Disable early demux to expose the issue (otherwise it's partially masked) ip netns exec ns_b sysctl -w net.ipv4.ip_early_demux=0 # Join multicast group on receiver ip -n ns_b addr add 239.0.0.1/32 dev veth1 autojoin # Sender: GRETAP with FOU encap ip -n ns_a link add eoudp0 type gretap \ remote 239.0.0.1 local 10.0.0.1 \ encap fou encap-sport 4797 encap-dport 4797 key 239.0.0.1 ip -n ns_a link set eoudp0 up ip -n ns_a addr add 192.168.99.1/24 dev eoudp0 # Receiver: FOU listener + GRETAP ip netns exec ns_b ip fou add port 4797 ipproto 47 ip -n ns_b link add eoudp0 type gretap \ remote 239.0.0.1 local 10.0.0.2 \ encap fou encap-sport 4797 encap-dport 4797 key 239.0.0.1 ip -n ns_b link set eoudp0 up ip -n ns_b addr add 192.168.99.2/24 dev eoudp0 # Static neigh: ARP replies can't traverse unidirectional mcast tunnel recv_mac=$(ip -n ns_b link show eoudp0 | awk '/ether/{print $2}') ip -n ns_a neigh add 192.168.99.2 lladdr $recv_mac dev eoudp0 # Test: ping through the FOU/GRETAP tunnel ip netns exec ns_a ping -c 100 192.168.99.2 # -> without this patch: 0 packets received on eoudp0 # -> with this patch: all packets received on eoudp0 IPv6 (using fou6 + ip6gretap) exhibits the same silent drop with a different fix (see 1/2 for the sign-of-ret difference between ip_protocol_deliver_rcu() and ip6_protocol_deliver_rcu()). AI assistance (Claude, claude-opus-4-6) was used during root cause analysis of the kernel source code (tracing the call chain from udp[6]_queue_rcv_skb through encap_rcv to ip[6]_protocol_deliver_rcu, comparing unicast/GSO/multicast paths) and during patch and selftest authoring. v6: - selftest: use the lib.sh ktap helpers (log_test / log_test_skip) so the IPv6-unavailable case reports KSFT_SKIP properly (Willem de Bruijn) - selftest: add CONFIG_IPV6_FOU / CONFIG_IPV6_FOU_TUNNEL to tools/testing/selftests/net/config (Willem de Bruijn) - selftest: speed up the ping loop with -i 0.01 v5: https://lore.kernel.org/netdev/cover.1783218197.git.littlesmilingcloud@gmail.com/ - Fix IPv6 patch: return ret, not -ret (Willem de Bruijn) - selftest: add IPv6 test case - selftest: create the veth pair inside the namespaces (Willem de Bruijn) v4: https://lore.kernel.org/netdev/cover.1782945956.git.littlesmilingcloud@gmail.com/ - Promoted from RFC to PATCH; no functional changes since v3. v3: https://lore.kernel.org/netdev/cover.1777934869.git.littlesmilingcloud@gmail.com/ - Use return -ret instead of calling ip_protocol_deliver_rcu() directly, matching the unicast path (Kuniyuki Iwashima) - Replace Python packet generator with ping through a real FOU/GRETAP tunnel in the selftest v2: https://lore.kernel.org/netdev/ad_dal164gVmImWl@dau-home-pc/ - Moved inline Python packet generator into a separate helper - Fixed author email typo in Signed-off-by v1 (RFC): https://lore.kernel.org/netdev/ad7MsSJOuUU6EGwS@dau-home-pc/ Anton Danilov (2): udp: fix encapsulation packet resubmit in multicast deliver selftests: net: add FOU multicast encapsulation resubmit test net/ipv4/udp.c | 6 +- net/ipv6/udp.c | 6 +- tools/testing/selftests/net/Makefile | 1 + tools/testing/selftests/net/config | 2 + .../testing/selftests/net/fou_mcast_encap.sh | 172 ++++++++++++++++++ 5 files changed, 183 insertions(+), 4 deletions(-) create mode 100755 tools/testing/selftests/net/fou_mcast_encap.sh -- 2.47.3