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  • probably it can't deduce from the initial destination IP what would be the expected reply source IP. has to be expected with multicast Commented Jan 15, 2019 at 21:17
  • That would have been my guess as well. However, isn't this a violation of the IPv6 protocol? In other words is this a bug or a feature? Commented Jan 16, 2019 at 10:16
  • it can still know that an icmp reply has to come after an icmp request. the same with UDP would have instead created a NEW state Commented Jan 16, 2019 at 12:08
  • So you're saying it's a bug? Commented Jan 16, 2019 at 22:04
  • nope. saying it's an impossible feature, or a feature that would allow too much (as in add an expectation for any ping reply). Also a firewall probably violates a lot of protocols, its role is to not allow things expected in the protocol, so saying that a firewall violates a protocol is perhaps a bit too much. IPv4 relies on underlying ARP (which is almost never firewalled anywhere even if it can) for link local discovery etc. IPv6 relies on ICMPv6 ie: IPv6 alone. So some things have to be blindly allowed anyway. Commented Jan 16, 2019 at 22:25