Comment 0 for bug 1853164

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