Public IPv6s fail on KVM bridge with "No buffer space available"

[William Gathoye]

On 05/22/2017 02:13 PM, Alarig Le Lay wrote:
> You can try to ping6 the IPv6 multicast address for all the routers on
> the link:
> 
> 14:07 alarig at mew ~ % ping6 -c1 ff02::2%vtnet0
> PING6(56=40+8+8 bytes) fe80::a800:ff:fe93:83a3%vtnet0 --> ff02::2%vtnet0
> 16 bytes from fe80::209%vtnet0, icmp_seq=0 hlim=64 time=0.237 ms
> 
> --- ff02::2%vtnet0 ping6 statistics ---
> 1 packets transmitted, 1 packets received, 0.0% packet loss
> round-trip min/avg/max/std-dev = 0.237/0.237/0.237/0.000 ms

In this use case, you make the assumption that my gateway is actually
the first one to respond, this is why you select only the first answer
using -c1. But as you can see below, if I remove that argument, several
routers are answering to me (seems sensible to me), how can I be sure my
gateway is actually the first device that answers?

PING6(56=40+8+8 bytes) fe80::ff:fec2:e61d%vtnet0 --> ff02::2%vtnet0
16 bytes from fe80::268a:7ff:fe91:e970%vtnet0, icmp_seq=0 hlim=64
time=0.292 ms
16 bytes from fe80::268a:7ff:fe91:ea98%vtnet0, icmp_seq=0 hlim=64
time=0.355 ms(DUP!)
16 bytes from fe80::2ff:ffff:feff:fffd%vtnet0, icmp_seq=0 hlim=64
time=2.970 ms(DUP!)
16 bytes from fe80::2ff:ffff:feff:fffe%vtnet0, icmp_seq=0 hlim=64
time=5.964 ms(DUP!)
16 bytes from fe80::268a:7ff:fe91:e970%vtnet0, icmp_seq=1 hlim=64
time=0.314 ms
16 bytes from fe80::268a:7ff:fe91:ea98%vtnet0, icmp_seq=1 hlim=64
time=0.389 ms(DUP!)
16 bytes from fe80::2ff:ffff:feff:fffd%vtnet0, icmp_seq=1 hlim=64
time=3.222 ms(DUP!)
16 bytes from fe80::2ff:ffff:feff:fffe%vtnet0, icmp_seq=1 hlim=64
time=6.382 ms(DUP!)

How can I understand the "DUP!" statement here? I assume these are due
because we are using multicast here end the ICMP reply are echoes to
each others? Right?

> 
> But, it’s possible that you don’t get any reply if they do some nasty
> things on their network (as Hetzner does). In that case, you’re stuck on
> the /56 setup.
> 

As you can see, it seems there is no restriction / too nasty things at
OVH, as I have got replies, at least :)

Regards,

--
William Gathoye
<william at gathoye.be>

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