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I have some dual stack websites. When I open my websites they connect using IPv4 the first time, and then IPv6. If I ping the IP address directly, the first ping fails for IPv6 and then works fine. Wait 24 hours same problem, first ping fails.

2607:9000:0:35::fc3:9749

Is there any cache problem, but it doesn't look like ARP cache problem?

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IPv6 doesn't use ARP, so there isn't an ARP cache. IPv6 uses ND, and a host maintains a Neighbor Cache and a Destination Cache, among others.

This is normal behavior, and you often see it with IPv4, too. Sometimes, it take a little too long for the target device to respond with its MAC address, and a ping will time out. There is really nothing wrong.

  • So it means that my hosting provider removes the IP from the cache, thats why when I open my website it connects using IPv4 and after a couple of times it works with IPv6! – Saud Iqbal Jul 4 '17 at 0:07
  • That is possible. It is also possible that the ICMPv6 process in the server is set at a very low priority, so that the time out happens before you get a response from the server. ICMP is a very low-priority process in the first place. There are other explanations, too, but the subject is really too large to explore on this site. – Ron Maupin Jul 4 '17 at 0:10
  • It's pretty common with IPv6, and reflects a low level of IPv6 traffic and the resulting lack of caching. Not just caching the ND data, but even at the operating system and CPU level the cool execution path means that processes need to be scheduled, code needs to be loaded back into the CPU from RAM. All those startup delays and cache misses in all those layers on all those devices adds up. You get the reverse with IPv4 in IPv6-only networks. – vk5tu Jul 12 '17 at 4:55
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IPv6 use Neighbor Discovery (ND ) .It maintains IPv6 Neighbor cache entry where IPv6 address as well as Ipv6 Link Local address are updated . As per your observation its looks like may be your device is taking some time for neighbor resolution . Ideally when interface came up Router should send NS(neighbor solicitation) and neighbor need to reply its MAC with NA (neighbor advertisement ) message similar to ARP(request and reply) . If Your device is taking more time for resolution Process ,Please check the neighbor state in state machine ,the entry might be in incomplete entry for some time . Its expected for some mili second but not expected if it is taking more time . For more Info Please refer to (RFC 4861).

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I moved to a new VPS provider and it had the same problem, then I fiddled with some settings in

/etc/sysctl.d/99-sysctl.conf

and uncommented

net.ipv6.conf.all.forwarding=1

and now it works fine, ping works even after 12 hours.

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