sysytemd-resolved automatically use an ipv6 dns server on lan

[Max]

lsb_release -rd
Description: Ubuntu Focal Fossa (development branch)
Release: 20.04

i installed focal from the netboot version mini.iso.

i encountered some problem on dsn configuration and i found that systemd-resolved was using an ipv6 dns address

this is my netplan configuration:

network:
  version: 2
  renderer: networkd
  ethernets:
    ens3:
      addresses:
        - 192.168.1.1/24
      gateway4: 192.168.1.212
      nameservers:
          search: [xxx.com]
          addresses: [192.168.1.191, 192.168.1.206]

this is the output of "systemd-resolved --status"

Global
       LLMNR setting: no
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 2 (ens3)
      Current Scopes: DNS
DefaultRoute setting: yes
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
  Current DNS Server: fe80::6670:2ff:feb5:d9c8
         DNS Servers: 192.168.1.191
                      192.168.1.206
                      fe80::6670:2ff:feb5:d9c8
          DNS Domain: xxx.com

--
as you can see systemd is using the ipv6 dns server.

i found that such ip was the address of my openwrt wifi router. dnsmasq is disabled for ipv4 lan interface on the router and i never configured an ipv6 network. Anyway i disabled all the ipv6 dnsmasq feature on the router and after a restart this fixed the problem.
But i think that this is a bug.
In my netplan i never configured an ipv6 dns nor a ipv6 network; furthermore none of my other ubuntu bionic servers showed such a behaviour.
Also another focal netboot installation i made three weeks ago didn't show this problem.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[saturn (saturn-skb)]

I have DDWRT based WiFi router and affected the same bug.
DHCP6 as well as DNS6 in the router are disabled.

My problem was, that local DNS names provided by router stop resolved after few minutes since reconnecting to the router WiFi or LAN, while global domain names seems always resolved fine.

Bypassing systemd-resolve by editing manually nameservers in the /etc/resolv.conf which is linked to the ../run/systemd/resolve/stub-resolv.conf can fix the problem with local DNS names.

First I tried, as it was suggested in some discussions of the similar issue, without success to set DNSSEC=no explicitly. Seems this systemd-resolve setting is default now.

Then I tried to set DNS servers manually in the Network Manager. This didn't help also.
Here the output when local DNS names provided by router stop resolved.
---------------8<----------------
$ systemd-resolve --status
Global
       LLMNR setting: no
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 3 (wlp1s0)
      Current Scopes: DNS
DefaultRoute setting: yes
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
  Current DNS Server: fe80::22cf:30ff:fece:c98
         DNS Servers: 172.20.1.250
                      172.20.1.1
                      8.8.8.8
                      fe80::22cf:30ff:fece:c98
          DNS Domain: ~.
                      lan

Link 2 (enp3s0f0)
      Current Scopes: none
DefaultRoute setting: no
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
---------------8<----------------
Any idea what was ...

[Dan Streetman (ddstreet)]

please reopen if this is still an issue