[Bug 1988119] Re: Update to systemd 237-3ubuntu10.54 broke dns

[Stefan Zwanenburg]

> Could this be a related issue, when deployment to aks fails, due to a
connection refused when pulling images from azure container
registry(ImagePullBackOff).

I you look closer at the message accompanying the ImagePullBackOff, you should see something like:
  dial tcp: lookup registry-1.docker.io on [::1]:53: read udp [::1]:36288->[::1]:53: read: connection refused

Port 53 is the port a DNS server usually listens on.

If this is what you're seeing, then yes: your problems are caused by the
issue described in here.

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https://bugs.launchpad.net/bugs/1988119

Title:
  Update to systemd 237-3ubuntu10.54 broke dns

Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  Two servers today that updated systemd to "systemd 237-3ubuntu10.54" 
  https://ubuntu.com/security/notices/USN-5583-1

  could not resolve dns anymore.
  no dns servers, normally set through dhcp.

  Ubuntu 18.04

  Temp fix.
   1. Edit /etc/systemd/resolved.conf
   1. Add/Uncomment # FallbackDNS=168.63.129.16
   1. Restart systemd-resolved sudo systemctl restart systemd-resolved.service
   1. Confirm dns working with systemd-resolve google.com

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