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

[Dan Streetman]

ubuntu at lp1939255-b:~$ dpkg -l systemd|grep systemd
ii  systemd        237-3ubuntu10.51 amd64        system and service manager
ubuntu at lp1939255-b:~$ sudo dhclient ens8 ens9 ens10 ens11 ens12 ens13 ens14 ens15
ubuntu at lp1939255-b:~$ journalctl --no-pager -b -u systemd-resolved
-- Logs begin at Sun 2021-09-05 21:36:12 UTC, end at Sun 2021-09-05 21:41:06 UTC. --
...
Sep 05 21:40:57 lp1939255-b systemd[1]: systemd-resolved.service: Start request repeated too quickly.
Sep 05 21:40:57 lp1939255-b systemd[1]: systemd-resolved.service: Failed with result 'start-limit-hit'.
Sep 05 21:40:57 lp1939255-b systemd[1]: Failed to start Network Name Resolution.

ubuntu at lp1939255-b:~$ dpkg -l systemd|grep systemd
ii  systemd        237-3ubuntu10.52 amd64        system and service manager
ubuntu at lp1939255-b:~$ sudo dhclient ens8 ens9 ens10 ens11 ens12 ens13 ens14 ens15
ubuntu at lp1939255-b:~$ journalctl --no-pager -b -u systemd-resolved
-- Logs begin at Sun 2021-09-05 21:36:12 UTC, end at Sun 2021-09-05 21:50:57 UTC. --
...
Sep 05 21:50:57 lp1939255-b systemd[1]: Started Network Name Resolution.


** Tags removed: verification-needed verification-needed-bionic verification-needed-focal
** Tags added: verification-done verification-done-bionic verification-done-focal

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https://bugs.launchpad.net/bugs/1939255

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 Committed
Status in systemd source package in Focal:
  Fix Committed

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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