[Bug 1933092] Re: snat arp entry missing in qrouter namespace

[Dan Streetman]

pushed to U/V/W and master branches:

https://git.launchpad.net/~ubuntu-openstack-
dev/ubuntu/+source/neutron/commit/?id=100e69d4593dde0363d8d368c836c2cc49dd2d7b

https://git.launchpad.net/~ubuntu-openstack-
dev/ubuntu/+source/neutron/commit/?id=6348afdd66039ca5365169e315872fb38ad64f3f

https://git.launchpad.net/~ubuntu-openstack-
dev/ubuntu/+source/neutron/commit/?id=5a27fae295fc475ea3d7105850066655c23c1269

https://git.launchpad.net/~ubuntu-openstack-
dev/ubuntu/+source/neutron/commit/?id=22a4f5867c2a44d5ee6636af9fe2f79624a9cd6f

chris plans to start new stable point releases tomorrow so this will get
uploaded along with that

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Title:
  snat arp entry missing in qrouter namespace

Status in Ubuntu Cloud Archive:
  New
Status in Ubuntu Cloud Archive ussuri series:
  New
Status in Ubuntu Cloud Archive victoria series:
  New
Status in Ubuntu Cloud Archive wallaby series:
  New
Status in Ubuntu Cloud Archive xena series:
  New
Status in neutron:
  Fix Released
Status in neutron package in Ubuntu:
  New
Status in neutron source package in Focal:
  New
Status in neutron source package in Groovy:
  New
Status in neutron source package in Hirsute:
  New
Status in neutron source package in Impish:
  New

Bug description:
  [Impact]
  Load Balancers deployed on the cloud are unreachable

  [Test Case]
  1. Deploy openstack with atleast 4 compute nodes with networking features DVR SNAT+L3HA
  2. Execute the script test_snat_arp_entry.sh
  3. The script loops for 20 times creating network, router and connecting router to external, internal network and checking if ARP entries are populated properly on qrouter namespaces
  4. The script stops if arp entries are missing.
  5. If the script runs for 20 loops, then there are no issues.

  [Regression Potential]
  The issue only happens a few times when a router is created, external gateway set and internal subnet attached to router in quick succession. In other cases, the arp entry of snat is already added.
  The fix just adds extra logic to add arp entry retrieving snat information from the router. In working cases, this extra logic will execute commands to add arp entry twice which should not cause further issues.

  [Original Bug Report]
  In one of the cloud environment, the FIP attached to the Octavia Loadbalancer VIP is not reachable. After analysis, we found the ARP entry for SNAT IP is missing in the qrouter namespace where Amphora VM is running. And so the return packets are not forwarded from qrouter to snat on active l3-agent node.

  Version:
  Ubuntu Ussuri packages (16.3.2 point release)
  DVR+SNAT+L3HA enabled

  Expectation is to have PERMANENT arp entry for snat ip on qrouter namespace on all compute nodes
  192.168.33.238 dev qr-4ee692e0-7a lladdr fa:16:3e:25:6a:73 used 38/38/38 probes 0 PERMANENT

  How to reproduce:

  Attaching a script to simulate the problem (without octavia) with following steps
  1. network/subnet/router is created, network attached to router
  2. verify if qrouter on all compute nodes has arp entries related to snat ip
  3. if arp entries exists, delete network/subnet/router
  4. Repeat steps 1,2,3 until missing arp entry is observed.

  I am able to reproduce missing arp entry sometimes in 3rd loop and
  sometimes in 6th loop.

  Observed arp entries for snat ip is updated at the following places
  [1] [2] but get_snat_interfaces() and get_ports_by_subnet() are not
  updated with snat ip in non-working cases.

  [1] https://opendev.org/openstack/neutron/src/commit/dfd04115b059c2263cdd8ac44ccc2ec47614bcc3/neutron/agent/l3/dvr_local_router.py#L570
  [2] https://opendev.org/openstack/neutron/src/commit/dfd04115b059c2263cdd8ac44ccc2ec47614bcc3/neutron/agent/l3/dvr_local_router.py#L317

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