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[Bug 2096931] [NEW] Duplicate packets with router betwen flat network and vlan network

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Public bug reported:

Release: 2024.1
Driver: OVN

I have one flat network and one vlan network. I have created a router
between them with the vlan network set as the external interface with
SNAT enabled. When I ping a openstack VM from a machine outside of
openstack I get duplicate responses:

64 bytes from 10.150.5.207: icmp_seq=107 ttl=63 time=0.243 ms
64 bytes from 10.150.5.207: icmp_seq=107 ttl=63 time=0.297 ms (DUP!)
64 bytes from 10.150.5.207: icmp_seq=107 ttl=63 time=0.297 ms (DUP!)

On the VM side I see:

16:32:48.854218 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 100, length 64
16:32:48.854219 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 100, length 64
16:32:48.854219 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 100, length 64
16:32:48.854288 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 100, length 64
16:32:48.854298 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 100, length 64
16:32:48.854303 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 100, length 64
16:32:49.878264 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 101, length 64
16:32:49.878332 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 101, length 64
16:32:49.878400 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 101, length 64
16:32:49.878400 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 101, length 64
16:32:49.878419 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 101, length 64
16:32:49.878431 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 101, length 64

In my setup I have three network nodes which seems to correspond to the
number of duplicated packets. If I block the traffic on one controller
with a firewall, the number of duplicated packets reduces.

Interestingly, if I ping the internal interface of the router from the
VM there is no duplicated packets, but if I ping from outside then I get
duplicates.

** Affects: neutron
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2096931

Title:
  Duplicate packets with router betwen flat network and vlan network

Status in neutron:
  New

Bug description:
  Release: 2024.1
  Driver: OVN

  I have one flat network and one vlan network. I have created a router
  between them with the vlan network set as the external interface with
  SNAT enabled. When I ping a openstack VM from a machine outside of
  openstack I get duplicate responses:

  64 bytes from 10.150.5.207: icmp_seq=107 ttl=63 time=0.243 ms
  64 bytes from 10.150.5.207: icmp_seq=107 ttl=63 time=0.297 ms (DUP!)
  64 bytes from 10.150.5.207: icmp_seq=107 ttl=63 time=0.297 ms (DUP!)

  On the VM side I see:

  16:32:48.854218 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 100, length 64
  16:32:48.854219 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 100, length 64
  16:32:48.854219 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 100, length 64
  16:32:48.854288 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 100, length 64
  16:32:48.854298 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 100, length 64
  16:32:48.854303 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 100, length 64
  16:32:49.878264 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 101, length 64
  16:32:49.878332 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 101, length 64
  16:32:49.878400 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 101, length 64
  16:32:49.878400 IP 10.150.5.199 > 10.150.5.207: ICMP echo request, id 25, seq 101, length 64
  16:32:49.878419 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 101, length 64
  16:32:49.878431 IP 10.150.5.207 > 10.150.5.199: ICMP echo reply, id 25, seq 101, length 64

  In my setup I have three network nodes which seems to correspond to
  the number of duplicated packets. If I block the traffic on one
  controller with a firewall, the number of duplicated packets reduces.

  Interestingly, if I ping the internal interface of the router from the
  VM there is no duplicated packets, but if I ping from outside then I
  get duplicates.

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