[Bug 1748894] Re: intermittent dhcp failures

[Salman]

This happens to our cluster as well which is on pike, but somewhat in
different way. Since the cluster is HA, instance do able to get the ip
from one of the dhcp namespaces but dnsmasq does not work later if the
problematic namespace ends up first in resolv.conf. When we delete the
specific port from the network which automatically gets created after a
few seconds, things start working again.

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https://bugs.launchpad.net/bugs/1748894

Title:
  intermittent dhcp failures

Status in neutron:
  Incomplete
Status in neutron package in Ubuntu:
  Confirmed
Status in neutron source package in Xenial:
  Confirmed

Bug description:
  Hi,

  We have observed instances failing to get a DHCP reply, either when booting for the first time, or after a reboot.
  By tcpdumping the traffic on the tap, then qvb and qvo interfaces, we can see the DHCP request leaving, but it doesn't reach the neutron-gateway node where the dhcp agent for the network runs.

  If we remove and then re-add the network to the dhcp agent, it starts
  working fine again.

  I have dumped the output of `ovs-ofctl dump-flows br-tun` on the neutron-gateway side, before and after doing the remove/add.
  One difference I noted, was a rule appearing, with the mac of the VM which had the issue:
   cookie=0xb42e8d404b989e72, duration=13.344s, table=20, n_packets=0, n_bytes=0, hard_timeout=300, idle_age=13, priority=1,vlan_tci=0x0001/0x0fff,dl_dst=fa:16:3e:db:34:a3 actions=load:0->NXM_OF_VLAN_TCI[],load:0x20->NXM_NX_TUN_ID[],output:5201

  However, this is a rule for packets from the dhcp agent to the VM, so
  its absence doesn't explain why DHCP requests don't arrive to
  destination.

  The cloud is running neutron 8.4.0 (Mitaka), on ubuntu (16.04)
  machines.

  Most of the time it works, but this problem comes back often enough
  that it impacts users.

  Thanks.

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