Just tested on devstack deployed on Xenial. The module gets loaded at
some point during neutron configuration on the deployment script.

It seems like a neutron bug to me. It relies on conntrack for the
firewall to work, but never actually loads the module. In most cases
something else will end up loading it, but in the event that no one else
loads it, it will fail.

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https://bugs.launchpad.net/bugs/1834213

Title:
  After kernel upgrade, nf_conntrack_ipv4 module unloaded, no IP traffic
  to instances

Status in OpenStack neutron-openvswitch charm:
  Confirmed
Status in linux package in Ubuntu:
  Confirmed

Bug description:
  With an environment running Xenial-Queens, and having just upgraded
  the linux-image-generic kernel for MDS patching, a few of our
  hypervisor hosts that were rebooted (3 out of 100) ended up dropping
  IP (tcp/udp) ingress traffic.

  It turns out that nf_conntrack module was loaded, but
  nf_conntrack_ipv4 was not loading, and the traffic was being dropped
  by this rule:

   table=72, n_packets=214989, priority=50,ct_state=+inv+trk
  actions=resubmit(,93)

  The ct_state "inv" means invalid conntrack state, which the manpage
  describes as:

                       The state is invalid, meaning that the connection tracker
                       couldn’t identify the connection. This flag is a catch-
                       all for problems in the connection or the connection
                       tracker, such as:

                       • L3/L4 protocol handler is not loaded/unavailable.
                              With the Linux kernel datapath, this may mean that
                              the nf_conntrack_ipv4 or nf_conntrack_ipv6 modules
                              are not loaded.

                       • L3/L4 protocol handler determines that the packet
                              is malformed.

                       • Packets are unexpected length for protocol.

  It appears that there may be an issue when patching the OS of a
  hypervisor not running instances may fail to update initrd to load
  nf_conntrack_ipv4 (and/or _ipv6).

  I couldn't find anywhere in the charm code that this would be loaded
  unless the charm's "harden" option is used on nova-compute charm (see
  charmhelpers contrib/host templates).  It is unset in our environment,
  so we are not using any special module probing.

  Did nf_conntrack_ipv4 get split out from nf_conntrack in recent kernel
  upgrades or is it possible that the charm should define a modprobe
  file if we have the OVS firewall driver configured?

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