Comment 5 for bug 1650794

XUbuntu 17.04 (and 16.04/16.10) fail to resolve DNS.

I did some investigations and figured out a little detail which may help to solve issue.

A) Descrption:

With xubuntu 17.04 I cannot resolve dns-names.
Fails with:
- ping
- firefox
- apt-get
- and so on

This was working correctly:
nslookup wwww.web.de

Error-Message:

$ ping google.com
ping: unknown host google.com

$ ping www.web.de
ping: unknown host www.web.de

$ ping 8.8.8.8
works well

DNS-resolving was mostly not working, Only short before updating and once a short time again it was working. I used Lenovo Yoga 300 with wlan.

B) Detected Detail of Difference

But I figured out, that it is depending on different wlan-router and hot-spots.

systemd-resolve --status

No-DNS-Resolving-Problem:

Link 3 (wlp2s0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: allow-downgrade
    DNSSEC supported: no
         DNS Servers: 192.168.43.1

Link 2 (enp3s0)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: allow-downgrade
    DNSSEC supported: yes
xubuntu@xubuntu:~$

With DNS Resolving Problem:

Link 3 (wlp2s0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: allow-downgrade
    DNSSEC supported: yes
         DNS Servers: 192.168.2.1
                      fe80::1
          DNS Domain: Speedport_W_724V_01011603_00_009

Link 2 (enp3s0)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: allow-downgrade
    DNSSEC supported: yes

Link 3 (wlp2s0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: allow-downgrade
    DNSSEC supported: yes
         DNS Servers: 192.168.2.1
          DNS Domain: Speedport_W_724V_01011603_00_009

*******************************************
* The difference I figured out was:
*******************************************

    DNSSEC supported: no

    DNSSEC supported: yes

Hope this will help to solve the problem.

With 16.04 about two month ago I recognized similar problem on wired lan, too.
But there dns resolving was not so long permanently broken for internet use.

Perhaps dns-resolving is disturbed by trying some connections or protocol-pieces with dnssec for secure-dns-https-checking or so on.

*************************************************************************

https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1663031

Appendix:
Similar bugs posted in internet:
https://unix.stackexchange.com/questions/280264/no-dns-resolution-after-upgrade-from-ubuntu-14-04-to-16-04
https://askubuntu.com/questions/838948/16-10-fail-to-resolve-dns
https://askubuntu.com/questions/780689/why-does-dns-resolution-not-work-on-the-first-try
https://askubuntu.com/questions/622470/dns-probe-finished-bad-config-error-in-ubuntu-14-04/622493#622493
Old nearly similar bug:
https://askubuntu.com/questions/368435/how-do-i-fix-dns-resolving-which-doesnt-work-after-upgrading-to-ubuntu-13-10-s