systemd-resolv does not clear IPv6 DNS server entries after disconnect

[rhhoek]

ii systemd 232-21ubuntu3 amd64 system and service manager
ii network-manager 1.4.4-1ubuntu3 amd64 network management framework (daemon and userspace tools)

Problem description:
1)
When connected to a wired network that supports IPv4 and IPv6 with DNS via DHCP 'systemd-resolve --status' reports:
DNS Servers: 10.10.10.1
             fd00::c225:6ff:fe4e:1582
and nmcli (nmcli dev show |grep DNS) reports:
IP4.DNS[1]: 10.10.10.1
IP6.DNS[1]: fd00::c225:6ff:fe4e:1582

2)
After disconnecting this wired network interface it still reports:
DNS Servers: fd00::c225:6ff:fe4e:1582

but nmcli does not report any DNS server, as expected.

3)
After reconnecting to the same wired network, but after changing the IPv4 connection preferences from 'Method: Automatic (DHCP)' to Method: Automatic (DHCP) address only', and for IPv6 from 'Method: Automatic' to 'Method: Automatic, addresses only', 'systemd-resolve --status' still reports:
DNS Servers: fd00::c225:6ff:fe4e:1582

and nmcli still does not report any DNS serv, as expected.

I Have checked with tshark that, although no DNS server should be queried, systemd still did on the IPv6 DNS address (and successfully resolved names and addresses).

---some extra testing---

4)
After disconnecting this wired network interface it still reports:
DNS Servers: fd00::c225:6ff:fe4e:1582

but nmcli does not report any DNS server, as expected.

5)
Connect to a WireLess IPv4 only Network (MiFi) 'systemd-resolve --status' reports:
DNS Servers: 192.168.8.1

and nmcli (nmcli dev show |grep DNS) reports:
IP4.DNS[1]: 192.168.8.1

Both as expected

6)
Network disconnect. 'systemd-resolve --status' reports:
DNS Servers: 8.8.8.8
             8.8.4.4
             2001:4860:4860::8888
             2001:4860:4860::8844
(these are FallbackDNS hostd mentioned in /etc/systemd/resolved.conf)
and nmcli (nmcli dev show |grep DNS) does not report anything as expected.

7)
Reconnect to wired network with IPv4 and IPv6 WITHOUT DNS servers via DHCP (Situation 3)
DNS Servers: 8.8.8.8
             8.8.4.4
             2001:4860:4860::8888
             2001:4860:4860::8844
and nmcli (nmcli dev show |grep DNS) does not report anything as expected.

8)
Network disconnect:
DNS Servers: 8.8.8.8
             8.8.4.4
             2001:4860:4860::8888
             2001:4860:4860::8844
and nmcli (nmcli dev show |grep DNS) does not report anything as expected.

9)
Connect to a wired network that supports IPv4 and IPv6 with DNS via DHCP (Situation 1)
DNS Servers: 10.10.10.1
             fd00::c225:6ff:fe4e:1582
and nmcli (nmcli dev show |grep DNS) reports:
IP4.DNS[1]: 10.10.10.1
IP6.DNS[1]: fd00::c225:6ff:fe4e:1582

[Sebastien Bacher (seb128)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. The issue you are reporting is an upstream one and it would be nice if somebody having it could send the bug to the developers of the software by following the instructions at https://wiki.ubuntu.com/Bugs/Upstream/GNOME. If you have done so, please tell us the number of the upstream bug (or the link), so we can add a bugwatch that will inform us about its status. Thanks in advance.