[Bug 1853164] Re: systemd: /etc/dhcp/dhclient-enter-hooks.d/resolved error

[Mason Loring Bliss]

sdezial notes this being terser. I do the long form out of superstitious awe
at the notion of a return code of zero being "true", even though it always
is, but this would be terser and also correct:

    if systemctl is-active systemd-resolved > /dev/null 2>&1; then

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Title:
  systemd: /etc/dhcp/dhclient-enter-hooks.d/resolved error

Status in systemd package in Ubuntu:
  New

Bug description:
  The functionality exists to allow users to revert to the traditional ifupdown 
  package for network configuration. Alongside this, systemd's often-buggy 
  resolver can be disabled. However, there's a logic error in the systemd-
  supplied /etc/dhcp/dhclient-enter-hooks.d/resolved that prevents the system
  from populating /etc/resolv.conf properly when systemd-resolved is disabled. 
  The issue is here:

      if [ -x /lib/systemd/systemd-resolved ] ; then

  Instead of checking to see if the systemd-resolved service is enabled or 
  active, which would be the correct behaviour, this checks for the existence of
  a binary, assuming that if it exists it's supposed to be used.

  I've not tested this in the absence of resolvconf, but if systemd-resolved 
  isn't enabled, it's difficult to imagine this code wanting to run. I've tested 
  this with resolvconf and ifupdown driving dhclient, and it corrects the 
  behaviour that was broken with the introduction of systemd-resolved.

  I'm attaching a patch, and am also including it here for easy access:

  *** resolved.broken     2019-11-19 15:01:28.785588838 +0000
  --- resolved    2019-11-19 15:08:06.519430073 +0000
  ***************
  *** 14,20 ****
    #   (D) = master script downs interface
    #   (-) = master script does nothing with this

  ! if [ -x /lib/systemd/systemd-resolved ] ; then
            # For safety, first undefine the nasty default make_resolv_conf()
            make_resolv_conf() { : ; }
            case "$reason" in
  --- 14,21 ----
    #   (D) = master script downs interface
    #   (-) = master script does nothing with this

  ! systemctl is-active systemd-resolved > /dev/null 2>&1
  ! if [ $? -eq 0 ]; then
            # For safety, first undefine the nasty default make_resolv_conf()
            make_resolv_conf() { : ; }
            case "$reason" in

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