What is expected behavior when pinging DHCP-PD prefix?

Question

I get a DHCP-PD prefix from ISP and I'm using dynv6 to be able to have dns-entries for my hosts using just the prefix and mac-address for the SLAAC IPv6 address.

https://dynv6.com/

I have a small script that updates the prefix on dynv6 if my prefix has changed.

Dynv6 creates an AAAA record for your zone, for example:

prefix: 2001:db8:dead:beef:: zone: foobar.dynv6.net

Then I get an AAAA for foobar.dynv6.net pointing to 2001:db8:dead:beef::

None of my IPv6 attached interfaces has this IPv6 address but if I do a ping I get an response.

Strangely I get different hosts replying depending on from what device I ping from.

for example:

% ping 2001:db8:dead:beef:: 
PING 2001:db8:dead:beef:: (2001:db8:dead:beef::) 56 data bytes
64 bytes from 2001:db8:dead:beef:1111:2222:3333:4444: icmp_seq=1 ttl=64 time=0.091 ms

And on this machine the responding IPv6 address is the one of itself.

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And on another host:

% ping -6 2001:db8:dead:beef::   
PING 2001:db8:dead:beef:: (2001:db8:dead:beef::) 56 data bytes
64 bytes from 2001:db8:dead:beef:2222:3333:4444:5555: icmp_seq=1 ttl=64 time=7.83 ms

That IPv6 address is another host and also not shown when doing ip -6 neigh

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I have solved it now to explicitly create a dynv6 AAAA for my router's IPv6-address, so now I can ping foobar.dynv6.net and get the router as expected. But I still get strange responses if I ping 2001:db8:dead:beef::

Answer 1

You have not actually given us a good network description or diagram, nor the network device models and configurations. Unless you edit the question with more specific details, it appears what is probably happening is an anycast response.

The network all-zeros address is a required anycast address called the Subnet-Router anycast address.

This is documented in RFC 4291, IP Version 6 Addressing Architecture, Section 2.6. Anycast Addresses

An IPv6 anycast address is an address that is assigned to more than one interface (typically belonging to different nodes), with the property that a packet sent to an anycast address is routed to the "nearest" interface having that address, according to the routing protocols' measure of distance.

-and-

Packets sent to the Subnet-Router anycast address will be delivered to one router on the subnet. All routers are required to support the Subnet-Router anycast addresses for the subnets to which they have interfaces.

The Subnet-Router anycast address is intended to be used for applications where a node needs to communicate with any one of the set of routers.