[Bug 1972029] Re: dhclient overriding stub-resolv.conf file on Jammy

[Lukas Märdian]

I've uploaded the fix into Kinetic and proposed it for Jammy SRU.

But @vorlon had some reservations: "I want to understand why the partner is invoking dhclient at all, when it is not part of our standard network stack in jammy (or even focal)."
I'll leave the decision to accept/reject this to him.

IMO the bug is real and should be fixed. But at the same time they
should not be using dhclient, which is deprecated:
https://www.isc.org/blogs/dhcp-client-relay-eom/

** Changed in: isc-dhcp (Ubuntu Jammy)
       Status: In Progress => Incomplete

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https://bugs.launchpad.net/bugs/1972029

Title:
  dhclient overriding stub-resolv.conf file on Jammy

Status in isc-dhcp package in Ubuntu:
  Fix Released
Status in isc-dhcp source package in Jammy:
  Incomplete
Status in isc-dhcp source package in Kinetic:
  Fix Released

Bug description:
  [Issue]
  On Jammy, the stub-resolv.conf file on /run/systemd/resolve gets overwritten if dhclient is executed.

  While debugging this, I found the reference on LP#1889068 [0] of the
  move of `resolved` hook from dhclient-enter-hooks.d/ to dhclient-exit-
  hooks.d/, and this new hook [2] has no reference of make_resolv_conf()
  being called.

  [Reproducer]
  -
  I used a cloud-image based Jammy installation (uvt-kvm).  Once in:

  $ cat /run/system/resolve/stub-resolv.conf

  And one will see:

  nameserver 127.0.0.53
  options edns0 trust-ad
  search .

  Then:

  $ sudo dhclient
  $ cat /run/system/resolve/stub-resolv.conf

  You'll see the definition of the nameserver(s) in, and the stub IP
  address no longer there.  To revert this, a `systemctl restart
  systemd-resolved` is needed.

  [Impact]

  This is an improper way to keep nameserver(s) in *resolv* files; the
  override of the file should never happen in these circumstances.

  [Extra]
  -
  As a minor test, I copied over the dhclient-enter-hooks.d/resolved file from a Focal installation to a Jammy one, and the problem _does not happen_ when such hook is in place.

  [ . . . ]

  Let me know if further clarification is needed to proceed.  Thank you.

  BR,
  pprincipeza

  [0] https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1889068
  [1] https://paste.ubuntu.com/p/bvqPZXZZ8w/
  [2] https://paste.ubuntu.com/p/YQdG6z4WS7/

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