[Bug 1915480] [NEW] DeviceManager's fill_dhcp_udp_checksums assumes IPv6 available

[Launchpad Bug Tracker]

You have been subscribed to a public bug by Ubuntu Foundations Team Bug Bot (crichton):

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.

** Affects: cloud-archive
     Importance: Undecided
         Status: New

** Affects: cloud-archive/ussuri
     Importance: Undecided
         Status: New

** Affects: neutron
     Importance: Medium
     Assignee: Lajos Katona (lajos-katona)
         Status: Fix Committed

** Affects: neutron (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: neutron (Ubuntu Focal)
     Importance: Undecided
         Status: New


** Tags: in-stable-ussuri in-stable-victoria patch
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DeviceManager's fill_dhcp_udp_checksums assumes IPv6 available
https://bugs.launchpad.net/bugs/1915480
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