[Bug 1981103] Re: System with DNS server in /etc/network/interfaces has bogus systemd-resolved config after upgrade to 22.04

[Uwe Schindler]

If you changed away from /etc/network/interfaces file to netplan.io
configuration (/etc/netplan/...) then you won't need ifupdown. There is
a configuartion tool, with simple configs it works: See this info how to
convert your network config. But be sure to have physical access to the
machine's console:
https://gist.github.com/mss/7a8e048dd51e5ef928039f1450ba8f31

I did this for my systems and removed and purged ifupdown. For some
configs it may be very complex, especially if you have many tunnels,
bridges or other special stuff in /etc/network/interfaces. But for
simple single ethernet/wifi device it is easy to migrate using the above
GIST.

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https://bugs.launchpad.net/bugs/1981103

Title:
  System with DNS server in /etc/network/interfaces has bogus systemd-
  resolved config after  upgrade to 22.04

Status in ifupdown package in Ubuntu:
  Confirmed

Bug description:
  Description:    Ubuntu 22.04 LTS
  Release:        22.04

  ifupdown:
    Installed: 0.8.36+nmu1ubuntu3
    Candidate: 0.8.36+nmu1ubuntu3
    Version table:
   *** 0.8.36+nmu1ubuntu3 500
          500 http://de.archive.ubuntu.com/ubuntu jammy/universe amd64 Packages
          100 /var/lib/dpkg/status

  After upgrading a server with classic ifupdown configuration after
  reboot the machine had no valid dns servers anymore.

  The problem is that the state file created by ifupdown using
  /etc/network/if-up.d/resolved looks like this:

  root at pangaea-pm:~# cat /run/network/ifupdown-inet-ens13
  "DNS"="134.102.20.20 134.102.200.14"
  "DOMAINS"="marum.de"

  The script later sources this file and causes the following errors,
  easy to see when you execute this:

  root at pangaea-pm:~# ifdown ens13; ifup ens13
  /etc/network/if-down.d/resolved: 12: mystatedir: not found
  /etc/network/if-up.d/resolved: 12: mystatedir: not found
  /etc/network/if-up.d/resolved: 71: DNS: not found
  /etc/network/if-up.d/resolved: 1: /run/network/ifupdown-inet-ens13: DNS=134.102.20.20 134.102.200.14: not found
  /etc/network/if-up.d/resolved: 2: /run/network/ifupdown-inet-ens13: DOMAINS=marum.de: not found
  Failed to parse DNS server address: DNS
  Failed to set DNS configuration: Invalid argument

  This happened to me on three different servers, so this is a serious
  bug and should be fixed before 22.04 upgrades are allowed for
  everybody. Most servers provided by data centers like Hetzner
  (Germany) are configure like that. After a do-release-upgrade you have
  no working DNS anymore, unless you disable systemd-resolved. I don't
  want to use netplan, so changing to this is no option.

  The fix is easy - remove the quotes in the script on the left side
  "$DNS" => $DNS; same for DOMAINS:

      if  [ -n "$NEW_DNS" ]; then
          cat <<EOF >"$mystatedir/ifupdown-${ADDRFAM}-$interface"
  $DNS="$NEW_DNS"
  EOF
          if  [ -n "$NEW_DOMAINS" ]; then
              cat <<EOF >>"$mystatedir/ifupdown-${ADDRFAM}-$interface"
  $DOMAINS="$NEW_DOMAINS"
  EOF
          fi
      fi

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