[Bug 1926139] Re: dhclient: thread concurrency race leads to DHCPOFFER packets not being received

[Matthew Ruffell]

** Summary changed:

- dhclient doesn't receive dhcp offer from kernel
+ dhclient: thread concurrency race leads to DHCPOFFER packets not being received

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https://bugs.launchpad.net/bugs/1926139

Title:
  dhclient: thread concurrency race leads to DHCPOFFER packets not being
  received

Status in bind9-libs package in Ubuntu:
  Fix Released
Status in isc-dhcp package in Ubuntu:
  Invalid
Status in bind9-libs source package in Focal:
  In Progress
Status in bind9-libs source package in Jammy:
  In Progress

Bug description:
  Platform: Qemu/libvirt on AMD64
  Ubuntu version: 20.04
  isc-dhcp-client version: 4.4.1-2.1ubuntu5
  Problem: When dhclient is used during boot every few reboots the DHCP OFFER packets aren't pushed from the kernel to dhclient. The DISCOVER packets can be seen in strace and tcpdump. The OFFER packets can be seen in tcpdump, but no read event is triggered.
  Ubuntu 18.04 doesn't have the problem, neither does Debian 10. Building these dhclient versions on Ubuntu 20.04 alleviates the problem a little, but it still occurs. So this issue might also be kernel related.

  Attached diff shows a strace of all threads and a pcap showing the
  tcpdump output.

  Edit:
  - Sometimes the dhclient command does receive the OFFER packet and connection is restored.
  - In my testing running dhclient manually from the terminal when the OFFERs aren't received will result in a new dhclient session which does receive the OFFER packet and connection is restored.

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