[Bug 1939255] Re: dhclient triggers systemd-resolved start limit when processing more than 5 interfaces at once

[Dan Streetman]

> It seems this issue may occur not only when receiving addresses on
multiple interfaces, but also when dhcp requests on multiple interfaces
times out and the hook script was called with 'reason' == FAIL.

yep, the try-reload-or-restart is done for those as well and this should
be fixed for those $reason cases; since this bug is already resolved, I
suggest you open a new bug.

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Title:
  dhclient triggers systemd-resolved start limit when processing more
  than 5 interfaces at once

Status in systemd package in Ubuntu:
  Invalid
Status in systemd source package in Bionic:
  Fix Released
Status in systemd source package in Focal:
  Fix Released

Bug description:
  [impact]

  on a system where systemd-resolved is running, if dhclient is used on
  more than 5 interfaces, it calls the '/etc/dhcp/dhclient-enter-
  hooks.d/resolved' script multiple times, which then restarts systemd-
  resolved multiple times, triggering the unit's start-limit throttle
  which results in the unit entering failed state

  [test case]

  on a system with more than 5 available interfaces to run dhclient on
  (where all the interfaces will get a dhcp response), run dhclient with
  the interfaces:

  $ dhclient ens8 ens9 ens10 ens11 ens12 ens13 ens14 ens15

  check if systemd-resolved failed:

  $ journalctl -b -1 -u systemd-resolved
  ...
  Aug 09 00:38:08 sf316232-b systemd[1]: systemd-resolved.service: Start request repeated too quickly.
  Aug 09 00:38:08 sf316232-b systemd[1]: systemd-resolved.service: Failed with result 'start-limit-hit'.
  Aug 09 00:38:08 sf316232-b systemd[1]: Failed to start Network Name Resolution.

  [regression potential]

  failure to start/stop/restart systemd-resolved, or problems adding
  dhclient-provided DNS nameservers to systemd-resolved

  [scope]

  this is needed only for b/f

  the dhclient 'hook' script is provided by the systemd package in focal
  and earlier, and needs fixing in those releases

  in h and later, the 'hook' script is included in the isc-dhcp-client
  package and notifies systemd-resolved in a more direct way without
  requiring restarting, and so doesn't trigger the restart limiting.

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