[Bug 2051935] Re: [OVN] SNAT only happens for subnets directly connected to a router

[Brian Haley]

Just to provide more info to my last comment. The external_gateway_info
field contains an element called 'enable_snat', which OVN uses to add
SNAT rules for attached subnets. For example (sorry for wrap):

| external_gateway_info     | {"network_id": "dbfb3168-85d5-4577-b221-5168f29760f7", "external_fixed_ips": [{"subnet_id": "c6594685-dfec-4497-8b22-b78f066cb5e4", "ip_address": "172.24.4.187"}, {"subnet_id": "67f8c7c8-7215-4ffb-aa98-f34ca8780efc",          |
|                           | "ip_address": "2001:db8::1"}], "enable_snat": true}

On a "nested" router, this field is empty:

| external_gateway_info   | null |

So OVN assumes it should not provide SNAT for any subnets.

So over-riding the list of returned subnet cidrs downstream of a router
does make the code add more lr-nat-list entries, but there could be more
surgery required to make it work properly.

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https://bugs.launchpad.net/bugs/2051935

Title:
  [OVN] SNAT only happens for subnets directly connected to a router

Status in neutron:
  In Progress
Status in neutron package in Ubuntu:
  New

Bug description:
  I am trying to achieve the following scenario:

  I have a VM attached to a router w/o external gateway (called project-
  router) but with a default route which send all the traffic to another
  router (transit router) which has an external gateway with snat
  enabled and it is connected to a transit network 192.168.100.0/24

  My VM is  on 172.16.100.0/24, traffic hits the project-router thanks
  to the default route gets redirected to the transit-router correctly,
  here it gets into the external gateway but w/o being snat.

  This is because in ovn I see that SNAT on this router is only enabled
  for logical ip in 192.168.100.0/24 which is the subnet directly
  connected to the router

  # ovn-nbctl lr-nat-list neutron-6d1e6bb7-3949-43d1-8dac-dc55155b9ad8
  TYPE             EXTERNAL_IP        EXTERNAL_PORT    LOGICAL_IP            EXTERNAL_MAC         LOGICAL_PORT
  snat             147.22.16.207                       192.168.100.0/24

  But I would like that this router snat all the traffic that hits it,
  even when coming from a subnet not directly connected to it.

  I can achieve this by setting in ovn the snat for 0.0.0.0/0

  # ovn-nbctl lr-nat-add neutron-6d1e6bb7-3949-43d1-8dac-dc55155b9ad8
  snat 147.22.16.207 0.0.0.0/0

  # ovn-nbctl lr-nat-list neutron-6d1e6bb7-3949-43d1-8dac-dc55155b9ad8
  TYPE             EXTERNAL_IP        EXTERNAL_PORT    LOGICAL_IP            EXTERNAL_MAC         LOGICAL_PORT
  snat             147.22.16.207                       0.0.0.0/0
  snat             147.22.16.207                       192.168.100.0/24

  But this workaround can be wiped if I run the neutron-ovn-db-sync-util
  on any of the neutron-api unit.

  Is there a way to achieve this via OpenStack? If not does it make
  sense to have this as a new feature?

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