[Bug 1915480] Re: DeviceManager's fill_dhcp_udp_checksums assumes IPv6 available

[Edward Hope-Morley]

** Also affects: cloud-archive/victoria
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1915480

Title:
  DeviceManager's fill_dhcp_udp_checksums assumes IPv6 available

Status in Ubuntu Cloud Archive:
  New
Status in Ubuntu Cloud Archive ussuri series:
  New
Status in Ubuntu Cloud Archive victoria series:
  New
Status in neutron:
  Fix Committed
Status in neutron package in Ubuntu:
  New
Status in neutron source package in Focal:
  New

Bug description:
  The following code in DeviceManager's fill_dhcp_udp_checksums assumes
  IPv6 is always enabled:

  iptables_mgr = iptables_manager.IptablesManager(use_ipv6=True,
                                                  namespace=namespace)

  When iptables_mgr.apply() is later called, an attempt to add the UDP
  checksum rule for DHCP is done via iptables-save/iptables-restore and
  if IPv6 has been disabled on a hypervisor (eg, by setting
  `ipv6.disable=1` on the kernel command line) then an many-line error
  occurs in the DHCP agent logfile.

  There should be a way of telling the agent that IPv6 is disabled and
  as such, it should ignore trying to set up the UDP checksum rule for
  IPv6. This can be easily achieved given that IptablesManager already
  has support for disabling it.

  We've seen this on Rocky on Ubuntu Bionic but it appears the issue
  still exists on the master branch.

  =================================
  Ubuntu SRU details:

  [Impact]
  See above.

  [Test Case]
  Deploy openstack on a hypervisor with IPv6 disabled.
  Create a network which has a subnetwork with DHCP enabled.
  Search the `neutron-dhcp-agent.log` (with debug log enabled) and check if there are any `ip6tables-restore` commands.

  [Regression Potential]
  Minimal.
  Users which were relying on the setting to always be true could be affected.

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