"IPV6 header not found" issue

gistfile1.txt

I am facing some -I think- strange problem with IPV6 and QinQ in a Linux host, and may be someone has faced similar problem (or could provide a hint).

I have some VMs running in a host (KVM), and every time any VM sends an ICMP6 Router Advertisement, we get the following log in syslog:

Aug 10 11:18:36 Hostname kernel: [1722430.045240] IPv6 header not found

For the traffic, I use QinQ (802.1Q in both tags), the inner tag is set with OVS in the tap, and the outer is set with a veth of vlan type, in the following way:

+-------------------------------------------------------+
|                   +-----------+               HOST    |
|                   |           |         Unbuntu 16.04 |
|                   |    VM-1   |         4.4.0-62-generic 
|                   |           |                       |
|                   +----+------+                       |
|                        |                              |
|                        |TAG=1                         |
|              +-----------------------+                |
|              |       OVS             |                |
|              +---------+-------------+                |
|                        | veth1.203                    |
|                        |                              |
|                        |                              |
|                        +veth0                         |
|              +---------------------+                  |
|              |      Bridge         |                  |
|              +-------+-------------+                  |
|                      |                                |
|                      |                                |
|                   +--+----+                           |
|                   |ens11f1|                           |
+-------------------------------------------------------+

'Regular traffic' (non ICMP6) seems to work fine, the problem happens apparently only with the Router Advertisement.

I checked the code writing that log, and I think it's in 'kernel/net/ipv6/exthdrs_core.c'

if (*offset) {
  struct ipv6hdr _ip6, *ip6;

  ip6 = skb_header_pointer(skb, *offset, sizeof(_ip6), &_ip6);
  if (!ip6 || (ip6->version != 6)) {
    printk(KERN_ERR "IPv6 header not found\n");
    return -EBADMSG;
  }
  start = *offset + sizeof(struct ipv6hdr); 
  nexthdr = ip6->nexthdr;
}

but both the protocol and protocol version seem right in tcpdump:

 11:28:38.675686 02:00:40:00:21:31 > 33:33:00:00:00:01, ethertype       802.1Q (0x8100), length 158: vlan 203, p 0, ethertype 802.1Q, vlan 49, p 0, ethertype IPv6, fe80::40ff:fe00:2131 > ff02::1: ICMP6, router advertisement, length 96
`....`:...........@...!1................... @...............@.!1..........@.... ........*.. ..!1................*.. ..!1.............   '.
11:28:39.300076 02:00:40:00:23:2a > 33:33:00:00:00:01, ethertype 802.1Q (0x8100), length 158: vlan 204, p 0, ethertype 802.1Q, vlan 193, p 0, ethertype IPv6, fe80::40ff:fe00:232a > ff02::1: ICMP6, router advertisement, length 96      `....`:...........@...#*...................%@...............@.#*..........@.... ........*..   ..#*................*.. ..#*.............

I already disabled (just in case) all the offloading features in the ethernet card.

Questions:

    Is there any way I can verify which packets are dropped? (which packets are causing the 'header not found' log)
    Is QinQ supported in the kernel? Is there any config param I should change to enable?
    Any hint what could be the reason? QinQ with ICMP6?

Any help/hint is trully appreciated!

Regards.