Debugging Tool does not use Ping6 correctly

[Paz]

Expected Behaviour:

Successful ping test to IPv6 gateway in debugging mode such as the ping6 test which I carried out manually (independent of pihole -d):

root@odroid:~# ip -6 route | grep default | cut -d ' ' -f 3
fe80::7eff:4dff:fe95:34c4
fe80::7eff:4dff:fe95:34c4
root@odroid:~# ping6 fe80::7eff:4dff:fe95:34c4
connect: Invalid argument
root@odroid:~# ping6 -I eth0 fe80::7eff:4dff:fe95:34c4
PING fe80::7eff:4dff:fe95:34c4(fe80::7eff:4dff:fe95:34c4) from fe80::8a12:dd8e:fb02:5b00%eth0 eth0: 56 data bytes
64 bytes from fe80::7eff:4dff:fe95:34c4%eth0: icmp_seq=1 ttl=64 time=1.35 ms
64 bytes from fe80::7eff:4dff:fe95:34c4%eth0: icmp_seq=2 ttl=64 time=0.896 ms
^C
--- fe80::7eff:4dff:fe95:34c4 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 0.896/1.124/1.352/0.228 ms

I am using
Distributor ID: Ubuntu
Description: Ubuntu 18.04.3 LTS
Release: 18.04
Codename: bionic

Ubuntu 18.04 LTS has switched to Netplan for configuring network interfaces. Netplan has replaced the old configuration file /etc/network/interfaces that was previously used for configuring network interfaces in Ubuntu.

Actual Behaviour:

root@odroid:~# pihole -d
[...]
*** [ DIAGNOSING ]: Networking
[✓] IPv4 address(es) bound to the eth0 interface:
   192.168.178.52/24 matches the IP found in /etc/pihole/setupVars.conf

[✓] IPv6 address(es) bound to the eth0 interface:
  [other IPv6 address] does not match the IP found in /etc/pihole/setupVars.conf (https://discourse.pi-hole.net/t/use-ipv6-ula-addresses-for-pi-hole/2127)
  [other IPv6 address] does not match the IP found in /etc/pihole/setupVars.conf (https://discourse.pi-hole.net/t/use-ipv6-ula-addresses-for-pi-hole/2127)
   fe80::8a12:dd8e:fb02:5b00 matches the IP found in /etc/pihole/setupVars.conf  

   ^ Please note that you may have more than one IP address listed.
   As long as one of them is green, and it matches what is in /etc/pihole/setupVars.conf, there is no need for concern.

   The link to the FAQ is for an issue that sometimes occurs when the IPv6 address changes, which is why we check for it.

[i] Default IPv4 gateway: 192.168.178.1
   * Pinging 192.168.178.1...
[✓] Gateway responded.
[i] Default IPv6 gateway: fe80::7eff:4dff:fe95:34c4
fe80::7eff:4dff:fe95:34c4
   * Pinging fe80::7eff:4dff:fe95:34c4
fe80::7eff:4dff:fe95:34c4...
ping: fe80::7eff:4dff:fe95:34c4
fe80::7eff:4dff:fe95:34c4: No address associated with hostname
[✗] Gateway did not respond. (https://discourse.pi-hole.net/t/why-is-a-default-gateway-important-for-pi-hole/3546)

Corective Action

The ping6 test in the debugging tool needs to add the interface (here: eth0) like:
ping6 -I eth0 fe80::7eff:4dff:fe95:34c4
or
ping6 fe80::7eff:4dff:fe95:34c4%eth0

Additional concern

Does pihole make the same mistake (not providing the interface eth0) while in normal operational mode (when debugging mode is not used)?

[DanSchaper]

The debug is just a quick check for connectivity. This wont affect the operation of Pi-hole in any way.

[Bucking_Horn]

This is not directly related to your assumption of incorrect ping usage, but maybe worth considering:

I've observed your usage of link local addresses (starting with fe80:) (click for more...)

While ping implementation in Ubuntu Bionic requires link specification (i.e. scope or interface) when pinging link-local addresses, Cosmic and Disco mark it as "can be used but it is no longer required". For complete comparison, see Ubuntu man pages for ping(8).

---

You might consider employing ULA addresses (from a random fd00::/8 range) instead, as recommended in Use IPv6 ULA addresses for Pi-hole.

From your post, I assume you are running a recent FritzBox (with Advanced View enabled), so open Home Network|Network|Network Settings|IPv6 Addresses and choose 'Always assign Unique Local Addresses (ULA)' from the list of available ULA options.

After renewing Pi-hole's leases (safest to achieve by a restart), you should edit /etc/pihole/setupVars.conf and /etc/pihole/local.list to reflect your new ULA-prefixed IPv6 address.

As a side note: If my assumption of you using a FritzBox is correct, then I find your gateway's link-local IPv6 address mildly confusing...

It does not seem to be a privacy extension address, yet your interface identifier does not resolve to any known vendor (where I would have expected AVM).
This might be explained perfectly by you obscuring the address when posting it, or other factors I am unaware of like more advanced setups including VPNs.
If you didn't alter your gateway's IP in any way, then you should probably double-check whether that address actually is your FritzBox.

---