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

[Brian Haley]

Rodolfo - I will post a reproducer, my devstack went sideways that I was
testing on, but it clearly showed the issue. I only had to add a single
route on the external gateway router, and a default route on the nested
router going to the internal interface of that router.

Cascading routers like this is perfectly normal, and something customers
do today. And everything works fine if you attach a floating IP, it's
only default SNAT that is broken.

The other thing to note is that this works fine with ML2/OVS, as the
router with the external gateway will SNAT everything that arrives on
its internal interface, irregardless of the source IP. OVN changes this
by only programming these SNAT rules for subnets directly attached to
the router. I didn't feel like programming a SNAT rule for 0.0.0.0/0 was
a good idea, which is why I proposed the patch.

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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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