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

[Uwe Schindler]

I think this should unset the variables, then error goes away and it
makes sense!

    if ! cmp --silent "$oldstate" "$newstate" 2>/dev/null; then
        unset DNS DNS6 DOMAINS DOMAINS6 DEFAULT_ROUTE

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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:
  New

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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