netdev - [BUG] IPv6 GRE does not work in multipoint mode

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Date: Mon, 13 May 2024 11:46:30 +0300
From: Antaryo Prospero <jansaley@...il.com>
To: netdev@...r.kernel.org
Subject: [BUG] IPv6 GRE does not work in multipoint mode

I tried to use GRE in multipoint mode, without specifying the remote
address in GRE config.

I use the following configurations for tests:
Host1 - Router - Host2

#Host1 GRE

auto mgre0
iface mgre0 inet6 static
address 2024:10::21/112
pre-up ip tunnel add mgre0 mode ip6gre key 1 tos inherit ttl 64 local
7d:168::2001
pre-up ip link set mgre0 mtu 1400
pre-up ip link set mgre0 multicast on
post-up ethtool -K mgre0 tx-checksumming off > /dev/null
post-down ip link del mgre0

#Host2 GRE

auto mgre0
iface mgre0 inet6 static
address 2024:10::22/112
pre-up ip tunnel add mgre0 mode ip6gre key 1 tos inherit ttl 64 local
7d:168::2002
pre-up ip link set mgre0 mtu 1400
pre-up ip link set mgre0 multicast on
post-up ethtool -K mgre0 tx-checksumming off > /dev/null
post-down ip link del mgre0

where 7d:168::2001 and 7d:168::2002 are the local network addresses of
Host1 and Host2 respectively.

Because iproute2 takes IPv6 addresses as invalid MAC addresses, I have
to use my own tool based on iproute2 to create neighbour records.

But, as the result, the packets are dropped. At the same time, GRE in
IPv4 multipoint mode works and allows to connect two or more nodes.

E.g. this IPv4 GRE config, which differs only by IP address family, works:

#Host1
auto mgre0
iface mgre0 inet static
address 10.10.10.3/24
pre-up ip tunnel add mgre0 mode gre key 1 tos inherit ttl 64 local 172.168.10.3
pre-up ip link set mgre0 mtu 1400
pre-up ip link set mgre0 multicast on
post-up ethtool -K mgre0 tx-checksumming off > /dev/null
post-down ip link del mgre0

#Host2
auto mgre0
iface mgre0 inet static
address 10.10.10.4/24
pre-up ip tunnel add mgre0 mode gre key 1 tos inherit ttl 64 local 172.168.10.4
pre-up ip link set mgre0 mtu 1400
pre-up ip link set mgre0 multicast on
post-up ethtool -K mgre0 tx-checksumming off > /dev/null
post-down ip link del mgre0