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

[Dan Streetman]

Public bug reported:

[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.

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: Invalid

** Affects: systemd (Ubuntu Bionic)
     Importance: Medium
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Affects: systemd (Ubuntu Focal)
     Importance: Medium
     Assignee: Dan Streetman (ddstreet)
         Status: In Progress

** Also affects: systemd (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Also affects: systemd (Ubuntu Focal)
   Importance: Undecided
       Status: New

** Changed in: systemd (Ubuntu)
       Status: New => Invalid

** Changed in: systemd (Ubuntu Bionic)
       Status: New => In Progress

** Changed in: systemd (Ubuntu Focal)
       Status: New => In Progress

** Changed in: systemd (Ubuntu Bionic)
   Importance: Undecided => Medium

** Changed in: systemd (Ubuntu Focal)
   Importance: Undecided => Medium

** Changed in: systemd (Ubuntu Bionic)
     Assignee: (unassigned) => Dan Streetman (ddstreet)

** Changed in: systemd (Ubuntu Focal)
     Assignee: (unassigned) => Dan Streetman (ddstreet)

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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:
  In Progress
Status in systemd source package in Focal:
  In Progress

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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