Anton Danilov wrote:

Add a selftest to verify that FOU-encapsulated packets addressed to a
multicast destination are correctly resubmitted to the inner protocol
handler (GRE) via the UDP multicast delivery path.  Both IPv4 and IPv6
paths are tested.

The test creates two network namespaces connected by a veth pair with
a FOU/GRETAP (IPv4) and FOU/ip6gretap (IPv6) tunnel using multicast
remote addresses (239.0.0.1 and ff0e::1).  Ping is sent through each
tunnel and received packets are counted on the receiver's tunnel
interface.

The veth pair is created directly inside the namespaces to avoid
possible name collisions with devices in the root namespace.

Static neighbor entries are configured on the sender because ARP/ND
replies from the receiver cannot traverse the unidirectional multicast
tunnel back to the sender.

The early demux optimization (net.ipv4.ip_early_demux, which controls
both IPv4 and IPv6) is disabled on the receiver to force packets
through __udp4_lib_mcast_deliver() / __udp6_lib_mcast_deliver(), which
is the code path being tested.

Signed-off-by: Anton Danilov <redacted>
Assisted-by: Claude:claude-opus-4-6

Reviewed-by: Willem de Bruijn <willemb@google.com>

+setup_ipv4() {
+	# IPv4 FOU (CONFIG_NET_FOU) is built in on kernels configured for

Instead of this distinction and modprobe for IPv6, also set those new
CONFIGs to =Y, so that the two are equivalent?

Not important enough to respin.