# OVN OpenStack Tutorial

This tutorial demonstrates how OVN works in an OpenStack “DevStack” environment. It was tested with the “master” branches of DevStack and Open vSwitch near the beginning of May 2017. Anyone using an earlier version is likely to encounter some differences. In particular, we noticed some shortcomings in OVN utilities while writing the tutorial and pushed out some improvements, so it’s best to use recent Open vSwitch at least from that point of view.

The goal of this tutorial is to demonstrate OVN in an end-to-end way, that is, to show how it works from the cloud management system at the top (in this case, OpenStack and specifically its Neutron networking subsystem), through the OVN northbound and southbound databases, to the bottom at the OVN local controller and Open vSwitch data plane. We hope that this demonstration makes it easier for users and potential users to understand how OVN works and how to debug and troubleshoot it.

In addition to new material, this tutorial incorporates content from `ovn_devstack.rst` in OpenStack neutron, by Russell Bryant and others. Without that example, this tutorial could not have been written.

We provide enough details in the tutorial that you should be able to fully follow along, by creating a DevStack VM and cloning DevStack and so on. If you want to do this, start out from Setting Up DevStack below.

## Setting Up DevStack

This section explains how to install DevStack, a kind of OpenStack packaging for developers, in a way that allows you to follow along with the tutorial in full.

Unless you have a spare computer laying about, it’s easiest to install DevStack in a virtual machine. This tutorial was built using a VM implemented by KVM and managed by virt-manager. I recommend configuring the VM configured for the x86-64 architecture, 6 GB RAM, 2 VCPUs, and a 20 GB virtual disk.

Note

Since we will be creating VMs inside this VM, it is important to have nesting configured properly. See https://github.com/openstack/devstack/blob/master/doc/source/guides/devstack-with-nested-kvm.rst for details on that.

Also, if you happen to run your Linux-based host with 32-bit userspace, then you will have some special issues, even if you use a 64-bit kernel:

- You may find that you can get 32-bit DevStack VMs to work to some extent, but I personally got tired of finding workarounds. I recommend running your VMs in 64-bit mode. To get this to work, I had to go to the CPUs tab for the VM configuration in virt-manager and change the CPU model from the one originally listed to “Hypervisor Default’ (it is curious that this is not the default!).

- On a host with 32-bit userspace, KVM supports VMs with at most 2047 MB RAM. This is adequate, barely, to start DevStack, but it is not enough to run multiple (nested) VMs. To prevent out-of-memory failures, set up extra swap space in the guest. For example, to add 2 GB swap:

      $ sudo dd if=/dev/zero of=/swapfile bs=1M count=2048
      $ sudo mkswap /swapfile
      $ sudo swapon /swapfile

  and then add a line like this to `/etc/fstab` to add the new swap automatically upon reboot:

      /swapfile swap swap defaults 0 0

Here are step-by-step instructions to get started:

1. Install a VM.

   I tested these instructions with Centos 7.6. Download the “minimal install” ISO and booted it. The install is straightforward. Be sure to enable networking, and set a host name, such as “ovn-devstack-1”. Add a regular (non-root) user, and check the box “Make this user administrator”. Also, set your time zone.

2. You can SSH into the DevStack VM, instead of running from a console. I recommend it because it’s easier to cut and paste commands into a terminal than a VM console. You might also consider using a very wide terminal, perhaps 160 columns, to keep tables from wrapping.

   To improve convenience further, you can make it easier to log in with the following steps, which are optional:

   1. On your host, edit your `~/.ssh/config`, adding lines like the following:

          Host ovn-devstack-1
                Hostname VMIP
                User VMUSER

      where VMIP is the VM’s IP address and VMUSER is your username inside the VM. (You can omit the `User` line if your username is the same in the host and the VM.) After you do this, you can SSH to the VM by name, e.g. `ssh ovn-devstack-1`, and if command-line completion is set up in your host shell, you can shorten that to something like `ssh ovn` followed by hitting the Tab key.

   2. If you have SSH public key authentication set up, with an SSH agent, run on your host:

          $ ssh-copy-id ovn-devstack-1

      and type your password once. Afterward, you can log in without typing your password again.

      (If you don’t already use SSH public key authentication and an agent, consider looking into it–it will save you time in the long run.)

   3. Optionally, inside the VM, append the following to your `~/.bash_profile`:

          . $HOME/devstack/openrc admin

      It will save you running it by hand each time you log in. But it also prints garbage to the console, which can screw up services like `ssh-copy-id`, so be careful.

2. Boot into the installed system and log in as the regular user, then install Git:

       $ sudo yum install git

   Note

   Support for Centos 7 in Devstack is going away, but you can still use it. Especially while Centos 8 support is not finished. The one important caveat for making Centos 7 work with Devstack is that you will explicitly have to install these packages as well:

       $ sudo yum install python3 python3-devel

   If you installed a 32-bit i386 guest (against the advice above), install a non-PAE kernel and reboot into it at this point:

       $ sudo yum install kernel-core kernel-devel
       $ sudo reboot

   Be sure to select the non-PAE kernel from the list at boot. Without this step, DevStack will fail to install properly later.

3. Get copies of DevStack and Neutron and set them up:

       $ git clone https://git.openstack.org/openstack-dev/devstack.git
       $ git clone https://git.openstack.org/openstack/neutron.git
       $ cd devstack
       $ cp ../neutron/devstack/ovn-local.conf.sample local.conf

   Note

   Depending on the name of the network device used by the VM, devstack may be unable to automatically obtain its IP address. If that happens, edit `local.conf` and explicitly provide it (X marks the spot):

       HOST_IP=X

   If you installed a 32-bit i386 guest (against the advice above), at this point edit `local.conf` to add the following line:

       CIRROS_ARCH=i386

4. Initialize DevStack:

       $ ./stack.sh

   This will spew many screenfuls of text, and the first time you run it, it will download lots of software from the Internet. The output should eventually end with something like this:

       This is your host IP address: 172.16.189.6
       This is your host IPv6 address: ::1
       Horizon is now available at http://172.16.189.6/dashboard
       Keystone is serving at http://172.16.189.6/identity/
       The default users are: admin and demo
       The password: password
       2017-03-09 15:10:54.117 | stack.sh completed in 2110 seconds.

   If there’s some kind of failure, you can restart by running `./stack.sh` again. It won’t restart exactly where it left off, but steps up to the one where it failed will skip the download steps. (Sometimes blindly restarting after a failure will allow it to succeed.) If you reboot your VM, you need to rerun this command. (If you run into trouble with `stack.sh` after rebooting your VM, try running `./unstack.sh`.)

   At this point you can navigate a web browser on your host to the Horizon dashboard URL. Many OpenStack operations can be initiated from this UI. Feel free to explore, but this tutorial focuses on the alternative command-line interfaces because they are easier to explain and to cut and paste.

5. The firewall in the VM by default allows SSH access but not HTTP. You will probably want HTTP access to use the OpenStack web interface. The following command enables that. (It also enables every other kind of network access, so if you’re concerned about security then you might want to find a more targeted approach.)

       $ sudo iptables -F

   (You need to re-run this if you reboot the VM.)

6. To use OpenStack command line utilities in the tutorial, run:

       $ . ~/devstack/openrc admin

   This needs to be re-run each time you log in (but see the following section).

## DevStack preliminaries

Before we really jump in, let’s set up a couple of things in DevStack. This is the first real test that DevStack is working, so if you get errors from any of these commands, it’s a sign that `stack.sh` didn’t finish properly, or perhaps that you didn’t run the `openrc admin` command at the end of the previous instructions.

If you stop and restart DevStack via `unstack.sh` followed by `stack.sh`, you have to rerun these steps.

1. For SSH access to the VMs we’re going to create, we’ll need a SSH keypair. Later on, we’ll get OpenStack to install this keypair into VMs. Create one with:

       $ openstack keypair create demo > ~/id_rsa_demo
       $ chmod 600 ~/id_rsa_demo

2. By default, DevStack security groups drop incoming traffic, but to test networking in a reasonable way we need to enable it. You only need to actually edit one particular security group, but DevStack creates multiple and it’s somewhat difficult to figure out which one is important because all of them are named “default”. So, the following adds rules to allow SSH and ICMP traffic into every security group:

       $ for group in $(openstack security group list -f value -c ID); do \
       openstack security group rule create --ingress --ethertype IPv4 --dst-port 22 --protocol tcp $group; \
       openstack security group rule create --ingress --ethertype IPv4 --protocol ICMP $group; \
       done

3. Later on, we’re going to create some VMs and we’ll need an operating system image to install. DevStack comes with a very simple image built-in, called “cirros”, which works fine. We need to get the UUID for this image. Our later commands assume shell variable `IMAGE_ID` holds this UUID. You can set this by hand, e.g.:

       $ openstack image list
       +--------------------------------------+--------------------------+--------+
       | ID                                   | Name                     | Status |
       +--------------------------------------+--------------------------+--------+
       | 77f37d2c-3d6b-4e99-a01b-1fa5d78d1fa1 | cirros-0.3.5-x86_64-disk | active |
       +--------------------------------------+--------------------------+--------+
       $ IMAGE_ID=73ca34f3-63c4-4c10-a62f-4540afc24eaa

   or by parsing CLI output:

       $ IMAGE_ID=$(openstack image list -f value -c ID)

   Note

   Your image ID will differ from the one above, as will every UUID in this tutorial. They will also change every time you run `stack.sh`. The UUIDs are generated randomly.

## Shortening UUIDs

OpenStack, OVN, and Open vSwitch all really like UUIDs. These are great for uniqueness, but 36-character strings are terrible for readability. Statistically, just the first few characters are enough for uniqueness in small environments, so let’s define a helper to make things more readable:

    $ abbrev() { a='[0-9a-fA-F]' b=$a$a c=$b$b; sed "s/$b-$c-$c-$c-$c$c$c//g"; }

You can use this as a filter to abbreviate UUIDs. For example, use it to abbreviate the above image list:

    $ openstack image list -f yaml | abbrev
    - ID: 77f37d
      Name: cirros-0.3.5-x86_64-disk
      Status: active

The command above also adds `-f yaml` to switch to YAML output format, because abbreviating UUIDs screws up the default table-based formatting and because YAML output doesn’t produce wrap columns across lines and therefore is easier to cut and paste.

## Overview

Now that DevStack is ready, with OVN set up as the networking back-end, here’s an overview of what we’re going to do in the remainder of the demo, all via OpenStack:

1. Switching: Create an OpenStack network `n1` and VMs `a` and `b` attached to it.

   An OpenStack network is a virtual switch; it corresponds to an OVN logical switch.

2. Routing: Create a second OpenStack network `n2` and VM `c` attached to it, then connect it to network `n1` by creating an OpenStack router and attaching `n1` and `n2` to it.

3. Gateways: Make VMs `a` and `b` available via an external network.

4. IPv6: Add IPv6 addresses to our VMs to demonstrate OVN support for IPv6 routing.

5. ACLs: Add and modify OpenStack stateless and stateful rules in security groups.

6. DHCP: How it works in OVN.

7. Further directions: Adding more compute nodes.

At each step, we will take a look at how the features in question work from OpenStack’s Neutron networking layer at the top to the data plane layer at the bottom. From the highest to lowest level, these layers and the software components that connect them are:

- OpenStack Neutron, which as the top level in the system is the authoritative source of the virtual network configuration.

  We will use OpenStack’s `openstack` utility to observe and modify Neutron and other OpenStack configuration.

- networking-ovn, the Neutron driver that interfaces with OVN and translates the internal Neutron representation of the virtual network into OVN’s representation and pushes that representation down the OVN northbound database.

  In this tutorial it’s rarely worth distinguishing Neutron from networking-ovn, so we usually don’t break out this layer separately.

- The OVN Northbound database, aka NB DB. This is an instance of OVSDB, a simple general-purpose database that is used for multiple purposes in Open vSwitch and OVN. The NB DB’s schema is in terms of networking concepts such as switches and routers. The NB DB serves the purpose that in other systems might be filled by some kind of API; for example, in place of calling an API to create or delete a logical switch, networking-ovn performs these operations by inserting or deleting a row in the NB DB’s Logical_Switch table.

  We will use OVN’s `ovn-nbctl` utility to observe the NB DB. (We won’t directly modify data at this layer or below. Because configuration trickles down from Neutron through the stack, the right way to make changes is to use the `openstack` utility or another OpenStack interface and then wait for them to percolate through to lower layers.)

- The ovn-northd daemon, a program that runs centrally and translates the NB DB’s network representation into the lower-level representation used by the OVN Southbound database in the next layer. The details of this daemon are usually not of interest, although without it OVN will not work, so this tutorial does not often mention it.

- The OVN Southbound database, aka SB DB, which is also an OVSDB database. Its schema is very different from the NB DB. Instead of familiar networking concepts, the SB DB defines the network in terms of collections of match-action rules called “logical flows”, which while similar in concept to OpenFlow flows use logical concepts, such as virtual machine instances, in place of physical concepts like physical Ethernet ports.

  We will use OVN’s `ovn-sbctl` utility to observe the SB DB.

- The ovn-controller daemon. A copy of ovn-controller runs on each hypervisor. It reads logical flows from the SB DB, translates them into OpenFlow flows, and sends them to Open vSwitch’s ovs-vswitchd daemon. Like ovn-northd, usually the details of what this daemon are not of interest, even though it’s important to the operation of the system.

- ovs-vswitchd. This program runs on each hypervisor. It is the core of Open vSwitch, which processes packets according to the OpenFlow flows set up by ovn-controller.

- Open vSwitch datapath. This is essentially a cache designed to accelerate packet processing. Open vSwitch includes a few different datapaths but OVN installations typically use one based on the Open vSwitch Linux kernel module.

## Switching

Switching is the basis of networking in the real world and in virtual networking as well. OpenStack calls its concept of a virtual switch a “network”, and OVN calls its corresponding concept a “logical switch”.

In this step, we’ll create an OpenStack network `n1`, then create VMs `a` and `b` and attach them to `n1`.

### Creating network `n1`

Let’s start by creating the network:

    $ openstack network create --provider-network-type geneve n1

OpenStack needs to know the subnets that a network serves. We inform it by creating subnet objects. To keep it simple, let’s give our network a single subnet for the 10.1.1.0/24 network. We have to give it a name, in this case `n1subnet`:

    $ openstack subnet create --subnet-range 10.1.1.0/24 --network n1 n1subnet

If you ask Neutron to show us the available networks, we see `n1` as well as the two networks that DevStack creates by default:

    $ openstack network list -f yaml | abbrev
    - ID: 5b6baf
      Name: n1
      Subnets: 5e67e7
    - ID: c02c4d
      Name: private
      Subnets: d88a34, fd87f9
    - ID: d1ac28
      Name: public
      Subnets: 0b1e79, c87dc1

Neutron pushes this network setup down to the OVN northbound database. We can use `ovn-nbctl show` to see an overview of what’s in the NB DB:

    $ ovn-nbctl show | abbrev
    switch 5b3d5f (neutron-c02c4d) (aka private)
        port b256dd
            type: router
            router-port: lrp-b256dd
        port f264e7
            type: router
            router-port: lrp-f264e7
    switch 2579f4 (neutron-d1ac28) (aka public)
        port provnet-d1ac28
            type: localnet
            addresses: ["unknown"]
        port ae9b52
            type: router
            router-port: lrp-ae9b52
    switch 3eb263 (neutron-5b6baf) (aka n1)
    router c59ad2 (neutron-9b057f) (aka router1)
        port lrp-ae9b52
            mac: "fa:16:3e:b2:d2:67"
            networks: ["172.24.4.9/24", "2001:db8::b/64"]
        port lrp-b256dd
            mac: "fa:16:3e:35:33:db"
            networks: ["fdb0:5860:4ba8::1/64"]
        port lrp-f264e7
            mac: "fa:16:3e:fc:c8:da"
            networks: ["10.0.0.1/26"]
        nat 80914c
            external ip: "172.24.4.9"
            logical ip: "10.0.0.0/26"
            type: "snat"

This output shows that OVN has three logical switches, each of which corresponds to a Neutron network, and a logical router that corresponds to the Neutron router that DevStack creates by default. The logical switch that corresponds to our new network `n1` has no ports yet, because we haven’t added any. The `public` and `private` networks that DevStack creates by default have router ports that connect to the logical router.

Using ovn-northd, OVN translates the NB DB’s high-level switch and router concepts into lower-level concepts of “logical datapaths” and logical flows. There’s one logical datapath for each logical switch or router:

    $ ovn-sbctl list datapath_binding | abbrev
    _uuid               : 0ad69d
    external_ids        : {logical-switch="5b3d5f", name="neutron-c02c4d", "name2"=private}
    tunnel_key          : 1

    _uuid               : a8a758
    external_ids        : {logical-switch="3eb263", name="neutron-5b6baf", "name2"="n1"}
    tunnel_key          : 4

    _uuid               : 191256
    external_ids        : {logical-switch="2579f4", name="neutron-d1ac28", "name2"=public}
    tunnel_key          : 3

    _uuid               : b87bec
    external_ids        : {logical-router="c59ad2", name="neutron-9b057f", "name2"="router1"}
    tunnel_key          : 2

This output lists the NB DB UUIDs in external_ids:logical-switch and Neutron UUIDs in externals_ids:uuid. We can dive in deeper by viewing the OVN logical flows that implement a logical switch. Our new logical switch is a simple and almost pathological example given that it doesn’t yet have any ports attached to it. We’ll look at the details a bit later:

    $ ovn-sbctl lflow-list n1 | abbrev
    Datapath: "neutron-5b6baf" aka "n1" (a8a758)  Pipeline: ingress
      table=0 (ls_in_port_sec_l2  ), priority=100  , match=(eth.src[40]), action=(drop;)
      table=0 (ls_in_port_sec_l2  ), priority=100  , match=(vlan.present), action=(drop;)
    ...
    Datapath: "neutron-5b6baf" aka "n1" (a8a758)  Pipeline: egress
      table=0 (ls_out_pre_lb      ), priority=0    , match=(1), action=(next;)
      table=1 (ls_out_pre_acl     ), priority=0    , match=(1), action=(next;)
    ...

We have one hypervisor (aka “compute node”, in OpenStack parlance), which is the one where we’re running all these commands. On this hypervisor, ovn-controller is translating OVN logical flows into OpenFlow flows (“physical flows”). It makes sense to go deeper, to see the OpenFlow flows that get generated from this datapath. By adding `--ovs` to the `ovn-sbctl` command, we can see OpenFlow flows listed just below their logical flows. We also need to use `sudo` because connecting to Open vSwitch is privileged. Go ahead and try it:

    $ sudo ovn-sbctl --ovs lflow-list n1 | abbrev
    Datapath: "neutron-5b6baf" aka "n1" (a8a758)  Pipeline: ingress
      table=0 (ls_in_port_sec_l2  ), priority=100  , match=(eth.src[40]), action=(drop;)
      table=0 (ls_in_port_sec_l2  ), priority=100  , match=(vlan.present), action=(drop;)
    ...
    Datapath: "neutron-5b6baf" aka "n1" (a8a758)  Pipeline: egress
      table=0 (ls_out_pre_lb      ), priority=0    , match=(1), action=(next;)
      table=1 (ls_out_pre_acl     ), priority=0    , match=(1), action=(next;)
    ...

You were probably disappointed: the output didn’t change, and no OpenFlow flows were printed. That’s because no OpenFlow flows are installed for this logical datapath, which in turn is because there are no VIFs for this logical datapath on the local hypervisor. For a better example, you can try `ovn-sbctl --ovs` on one of the other logical datapaths.

### Attaching VMs

A switch without any ports is not very interesting. Let’s create a couple of VMs and attach them to the switch. Run the following commands, which create VMs named `a` and `b` and attaches them to our network `n1` with IP addresses 10.1.1.5 and 10.1.1.6, respectively. It is not actually necessary to manually assign IP address assignments, since OpenStack is perfectly happy to assign them itself from the subnet’s IP address range, but predictable addresses are useful for our discussion:

    $ openstack server create --nic net-id=n1,v4-fixed-ip=10.1.1.5 --flavor m1.nano --image $IMAGE_ID --key-name demo a
    $ openstack server create --nic net-id=n1,v4-fixed-ip=10.1.1.6 --flavor m1.nano --image $IMAGE_ID --key-name demo b

These commands return before the VMs are really finished being built. You can run `openstack server list` a few times until each of them is shown in the state ACTIVE, which means that they’re not just built but already running on the local hypervisor.

These operations had the side effect of creating separate “port” objects, but without giving those ports any easy-to-read names. It’ll be easier to deal with them later if we can refer to them by name, so let’s name `a`’s port `ap` and `b`’s port `bp`:

    $ openstack port set --name ap $(openstack port list --server a -f value -c ID)
    $ openstack port set --name bp $(openstack port list --server b -f value -c ID)

We’ll need to refer to these ports’ MAC addresses a few times, so let’s put them in variables:

    $ AP_MAC=$(openstack port show -f value -c mac_address ap)
    $ BP_MAC=$(openstack port show -f value -c mac_address bp)

At this point you can log into the consoles of the VMs if you like. You can do that from the OpenStack web interface or get a direct URL to paste into a web browser using a command like:

    $ openstack console url show -f yaml a

(The option `-f yaml` keeps the URL in the output from being broken into noncontiguous pieces on a 80-column console.)

The VMs don’t have many tools in them but `ping` and `ssh` from one to the other should work fine. The VMs do not have any external network access or DNS configuration.

Let’s chase down what’s changed in OVN. Start with the NB DB at the top of the system. It’s clear that our logical switch now has the two logical ports attached to it:

    $ ovn-nbctl show | abbrev
    ...
    switch 3eb263 (neutron-5b6baf) (aka n1)
        port c29d41 (aka bp)
            addresses: ["fa:16:3e:99:7a:17 10.1.1.6"]
        port 820c08 (aka ap)
            addresses: ["fa:16:3e:a9:4c:c7 10.1.1.5"]
    ...

We can get some more details on each of these by looking at their NB DB records in the Logical_Switch_Port table. Each port has addressing information, port security enabled, and a pointer to DHCP configuration (which we’ll look at much later in DHCP):

    $ ovn-nbctl list logical_switch_port ap bp | abbrev
    _uuid               : ef17e5
    addresses           : ["fa:16:3e:a9:4c:c7 10.1.1.5"]
    dhcpv4_options      : 165974
    dhcpv6_options      : []
    dynamic_addresses   : []
    enabled             : true
    external_ids        : {"neutron:port_name"=ap}
    name                : "820c08"
    options             : {}
    parent_name         : []
    port_security       : ["fa:16:3e:a9:4c:c7 10.1.1.5"]
    tag                 : []
    tag_request         : []
    type                : ""
    up                  : true

    _uuid               : e8af12
    addresses           : ["fa:16:3e:99:7a:17 10.1.1.6"]
    dhcpv4_options      : 165974
    dhcpv6_options      : []
    dynamic_addresses   : []
    enabled             : true
    external_ids        : {"neutron:port_name"=bp}
    name                : "c29d41"
    options             : {}
    parent_name         : []
    port_security       : ["fa:16:3e:99:7a:17 10.1.1.6"]
    tag                 : []
    tag_request         : []
    type                : ""
    up                  : true

Now that the logical switch is less pathological, it’s worth taking another look at the SB DB logical flow table. Try a command like this:

    $ ovn-sbctl lflow-list n1 | abbrev | less -S

and then glance through the flows. Packets that egress a VM into the logical switch travel through the flow table’s ingress pipeline starting from table 0. At each table, the switch finds the highest-priority logical flow that matches and executes its actions, or if there’s no matching flow then the packet is dropped. The `ovn-sb`(5) manpage gives all the details, but with a little thought it’s possible to guess a lot without reading the manpage. For example, consider the flows in ingress pipeline table 0, which are the first flows encountered by a packet traversing the switch:

    table=0 (ls_in_port_sec_l2  ), priority=100  , match=(eth.src[40]), action=(drop;)
    table=0 (ls_in_port_sec_l2  ), priority=100  , match=(vlan.present), action=(drop;)
    table=0 (ls_in_port_sec_l2  ), priority=50   , match=(inport == "820c08" && eth.src == {fa:16:3e:a9:4c:c7}), action=(next;)
    table=0 (ls_in_port_sec_l2  ), priority=50   , match=(inport == "c29d41" && eth.src == {fa:16:3e:99:7a:17}), action=(next;)

The first two flows, with priority 100, immediately drop two kinds of invalid packets: those with a multicast or broadcast Ethernet source address (since multicast is only for packet destinations) and those with a VLAN tag (because OVN doesn’t yet support VLAN tags inside logical networks). The next two flows implement L2 port security: they advance to the next table for packets with the correct Ethernet source addresses for their ingress ports. A packet that does not match any flow is implicitly dropped, so there’s no need for flows to deal with mismatches.

The logical flow table includes many other flows, some of which we will look at later. For now, it’s most worth looking at ingress table 13:

    table=13(ls_in_l2_lkup      ), priority=100  , match=(eth.mcast), action=(outport = "_MC_flood"; output;)
    table=13(ls_in_l2_lkup      ), priority=50   , match=(eth.dst == fa:16:3e:99:7a:17), action=(outport = "c29d41"; output;)
    table=13(ls_in_l2_lkup      ), priority=50   , match=(eth.dst == fa:16:3e:a9:4c:c7), action=(outport = "820c08"; output;)

The first flow in table 13 checks whether the packet is an Ethernet multicast or broadcast and, if so, outputs it to a special port that egresses to every logical port (other than the ingress port). Otherwise the packet is output to the port corresponding to its Ethernet destination address. Packets addressed to any other Ethernet destination are implicitly dropped.

(It’s common for an OVN logical switch to know all the MAC addresses supported by its logical ports, like this one. That’s why there’s no logic here for MAC learning or flooding packets to unknown MAC addresses. OVN does support unknown MAC handling but that’s not in play in our example.)

Note

If you’re interested in the details for the multicast group, you can run a command like the following and then look at the row for the correct datapath:

    $ ovn-sbctl find multicast_group name=_MC_flood | abbrev

Now if you want to look at the OpenFlow flows, you can actually see them. For example, here’s the beginning of the output that lists the first four logical flows, which we already looked at above, and their corresponding OpenFlow flows. If you want to know more about the syntax, the `ovs-fields`(7) manpage explains OpenFlow matches and `ovs-ofctl`(8) explains OpenFlow actions:

    $ sudo ovn-sbctl --ovs lflow-list n1 | abbrev
    Datapath: "neutron-5b6baf" aka "n1" (a8a758)  Pipeline: ingress
      table=0 (ls_in_port_sec_l2  ), priority=100  , match=(eth.src[40]), action=(drop;)
        table=8 metadata=0x4,dl_src=01:00:00:00:00:00/01:00:00:00:00:00 actions=drop
      table=0 (ls_in_port_sec_l2  ), priority=100  , match=(vlan.present), action=(drop;)
        table=8 metadata=0x4,vlan_tci=0x1000/0x1000 actions=drop
      table=0 (ls_in_port_sec_l2  ), priority=50   , match=(inport == "820c08" && eth.src == {fa:16:3e:a9:4c:c7}), action=(next;)
        table=8 reg14=0x1,metadata=0x4,dl_src=fa:16:3e:a9:4c:c7 actions=resubmit(,9)
      table=0 (ls_in_port_sec_l2  ), priority=50   , match=(inport == "c29d41" && eth.src == {fa:16:3e:99:7a:17}), action=(next;)
        table=8 reg14=0x2,metadata=0x4,dl_src=fa:16:3e:99:7a:17 actions=resubmit(,9)
    ...

#### Logical Tracing

Let’s go a level deeper. So far, everything we’ve done has been fairly general. We can also look at something more specific: the path that a particular packet would take through OVN, logically, and Open vSwitch, physically.

Let’s use OVN’s ovn-trace utility to see what happens to packets from a logical point of view. The `ovn-trace`(8) manpage has a lot of detail on how to do that, but let’s just start by building up from a simple example. You can start with a command that just specifies the logical datapath, an input port, and nothing else; unspecified fields default to all-zeros. This doesn’t do much:

    $ ovn-trace n1 'inport == "ap"'
    ...
    ingress(dp="n1", inport="ap")
    -----------------------------
     0. ls_in_port_sec_l2: no match (implicit drop)

We see that the packet was dropped in logical table 0, “ls_in_port_sec_l2”, the L2 port security stage (as we discussed earlier). That’s because we didn’t use the right Ethernet source address for `a`. Let’s see what happens if we do:

    $ ovn-trace n1 'inport == "ap" && eth.src == '$AP_MAC
    ...
    ingress(dp="n1", inport="ap")
    -----------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "ap" && eth.src == {fa:16:3e:a9:4c:c7}, priority 50, uuid 6dcc418a
        next;
    13. ls_in_l2_lkup: no match (implicit drop)

Now the packet passes through L2 port security and skips through several other tables until it gets dropped in the L2 lookup stage (because the destination is unknown). Let’s add the Ethernet destination for `b`:

    $ ovn-trace n1 'inport == "ap" && eth.src == '$AP_MAC' && eth.dst == '$BP_MAC
    ...
    ingress(dp="n1", inport="ap")
    -----------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "ap" && eth.src == {fa:16:3e:a9:4c:c7}, priority 50, uuid 6dcc418a
        next;
    13. ls_in_l2_lkup (northd.c:3529): eth.dst == fa:16:3e:99:7a:17, priority 50, uuid 57a4c46f
        outport = "bp";
        output;

    egress(dp="n1", inport="ap", outport="bp")
    ------------------------------------------
     8. ls_out_port_sec_l2 (northd.c:3654): outport == "bp" && eth.dst == {fa:16:3e:99:7a:17}, priority 50, uuid 8aa6426d
        output;
        /* output to "bp", type "" */

You can see that in this case the packet gets properly switched from `a` to `b`.

#### Physical Tracing for Hypothetical Packets

ovn-trace showed us how a hypothetical packet would travel through the system in a logical fashion, that is, without regard to how VMs are distributed across the physical network. This is a convenient representation for understanding how OVN is supposed to work abstractly, but sometimes we might want to know more about how it actually works in the real systems where it is running. For this, we can use the tracing tool that Open vSwitch provides, which traces a hypothetical packet through the OpenFlow tables.

We can actually get two levels of detail. Let’s start with the version that’s easier to interpret, by physically tracing a packet that looks like the one we logically traced before. One obstacle is that we need to know the OpenFlow port number of the input port. One way to do that is to look for a port whose “attached-mac” is the one we expect and print its ofport number:

    $ AP_PORT=$(ovs-vsctl --bare --columns=ofport find  interface external-ids:attached-mac=\"$AP_MAC\")
    $ echo $AP_PORT
    3

(You could also just do a plain `ovs-vsctl list interface` and then look through for the right row and pick its `ofport` value.)

Now we can feed this input port number into `ovs-appctl ofproto/trace` along with the correct Ethernet source and destination addresses and get a physical trace:

    $ sudo ovs-appctl ofproto/trace br-int in_port=$AP_PORT,dl_src=$AP_MAC,dl_dst=$BP_MAC
    Flow: in_port=3,vlan_tci=0x0000,dl_src=fa:16:3e:a9:4c:c7,dl_dst=fa:16:3e:99:7a:17,dl_type=0x0000

    bridge("br-int")
    ----------------
     0. in_port=3, priority 100
        set_field:0x8->reg13
        set_field:0x9->reg11
        set_field:0xa->reg12
        set_field:0x4->metadata
        set_field:0x1->reg14
        resubmit(,8)
     8. reg14=0x1,metadata=0x4,dl_src=fa:16:3e:a9:4c:c7, priority 50, cookie 0x6dcc418a
        resubmit(,9)
     9. metadata=0x4, priority 0, cookie 0x8fe8689e
        resubmit(,10)
    10. metadata=0x4, priority 0, cookie 0x719549d1
        resubmit(,11)
    11. metadata=0x4, priority 0, cookie 0x39c99e6f
        resubmit(,12)
    12. metadata=0x4, priority 0, cookie 0x838152a3
        resubmit(,13)
    13. metadata=0x4, priority 0, cookie 0x918259e3
        resubmit(,14)
    14. metadata=0x4, priority 0, cookie 0xcad14db2
        resubmit(,15)
    15. metadata=0x4, priority 0, cookie 0x7834d912
        resubmit(,16)
    16. metadata=0x4, priority 0, cookie 0x87745210
        resubmit(,17)
    17. metadata=0x4, priority 0, cookie 0x34951929
        resubmit(,18)
    18. metadata=0x4, priority 0, cookie 0xd7a8c9fb
        resubmit(,19)
    19. metadata=0x4, priority 0, cookie 0xd02e9578
        resubmit(,20)
    20. metadata=0x4, priority 0, cookie 0x42d35507
        resubmit(,21)
    21. metadata=0x4,dl_dst=fa:16:3e:99:7a:17, priority 50, cookie 0x57a4c46f
        set_field:0x2->reg15
        resubmit(,32)
    32. priority 0
        resubmit(,33)
    33. reg15=0x2,metadata=0x4, priority 100
        set_field:0xb->reg13
        set_field:0x9->reg11
        set_field:0xa->reg12
        resubmit(,34)
    34. priority 0
        set_field:0->reg0
        set_field:0->reg1
        set_field:0->reg2
        set_field:0->reg3
        set_field:0->reg4
        set_field:0->reg5
        set_field:0->reg6
        set_field:0->reg7
        set_field:0->reg8
        set_field:0->reg9
        resubmit(,40)
    40. metadata=0x4, priority 0, cookie 0xde9f3899
        resubmit(,41)
    41. metadata=0x4, priority 0, cookie 0x74074eff
        resubmit(,42)
    42. metadata=0x4, priority 0, cookie 0x7789c8b1
        resubmit(,43)
    43. metadata=0x4, priority 0, cookie 0xa6b002c0
        resubmit(,44)
    44. metadata=0x4, priority 0, cookie 0xaeab2b45
        resubmit(,45)
    45. metadata=0x4, priority 0, cookie 0x290cc4d4
        resubmit(,46)
    46. metadata=0x4, priority 0, cookie 0xa3223b88
        resubmit(,47)
    47. metadata=0x4, priority 0, cookie 0x7ac2132e
        resubmit(,48)
    48. reg15=0x2,metadata=0x4,dl_dst=fa:16:3e:99:7a:17, priority 50, cookie 0x8aa6426d
        resubmit(,64)
    64. priority 0
        resubmit(,65)
    65. reg15=0x2,metadata=0x4, priority 100
        output:4

    Final flow: reg11=0x9,reg12=0xa,reg13=0xb,reg14=0x1,reg15=0x2,metadata=0x4,in_port=3,vlan_tci=0x0000,dl_src=fa:16:3e:a9:4c:c7,dl_dst=fa:16:3e:99:7a:17,dl_type=0x0000
    Megaflow: recirc_id=0,ct_state=-new-est-rel-rpl-inv-trk,ct_label=0/0x1,in_port=3,vlan_tci=0x0000/0x1000,dl_src=fa:16:3e:a9:4c:c7,dl_dst=fa:16:3e:99:7a:17,dl_type=0x0000
    Datapath actions: 4

There’s a lot there, which you can read through if you like, but the important part is:

    65. reg15=0x2,metadata=0x4, priority 100
        output:4

which means that the packet is ultimately being output to OpenFlow port 4. That’s port `b`, which you can confirm with:

    $ sudo ovs-vsctl find interface ofport=4
    _uuid               : 840a5aca-ea8d-4c16-a11b-a94e0f408091
    admin_state         : up
    bfd                 : {}
    bfd_status          : {}
    cfm_fault           : []
    cfm_fault_status    : []
    cfm_flap_count      : []
    cfm_health          : []
    cfm_mpid            : []
    cfm_remote_mpids    : []
    cfm_remote_opstate  : []
    duplex              : full
    error               : []
    external_ids        : {attached-mac="fa:16:3e:99:7a:17", iface-id="c29d4120-20a4-4c44-bd83-8d91f5f447fd", iface-status=active, vm-id="2db969ca-ca2a-4d9a-b49e-f287d39c5645"}
    ifindex             : 9
    ingress_policing_burst: 0
    ingress_policing_rate: 0
    lacp_current        : []
    link_resets         : 1
    link_speed          : 10000000
    link_state          : up
    lldp                : {}
    mac                 : []
    mac_in_use          : "fe:16:3e:99:7a:17"
    mtu                 : 1500
    mtu_request         : []
    name                : "tapc29d4120-20"
    ofport              : 4
    ofport_request      : []
    options             : {}
    other_config        : {}
    statistics          : {collisions=0, rx_bytes=4254, rx_crc_err=0, rx_dropped=0, rx_errors=0, rx_frame_err=0, rx_over_err=0, rx_packets=39, tx_bytes=4188, tx_dropped=0, tx_errors=0, tx_packets=39}
    status              : {driver_name=tun, driver_version="1.6", firmware_version=""}
    type                : ""

or:

    $ BP_PORT=$(ovs-vsctl --bare --columns=ofport find  interface external-ids:attached-mac=\"$BP_MAC\")
    $ echo $BP_PORT
    4

#### Physical Tracing for Real Packets

In the previous sections we traced a hypothetical L2 packet, one that’s honestly not very realistic: we didn’t even supply an Ethernet type, so it defaulted to zero, which isn’t anything one would see on a real network. We could refine our packet so that it becomes a more realistic TCP or UDP or ICMP, etc. packet, but let’s try a different approach: working from a real packet.

Pull up a console for VM `a` and start `ping 10.1.1.6`, then leave it running for the rest of our experiment.

Now go back to your DevStack session and run:

    $ sudo watch ovs-dpctl dump-flows

We’re working with a new program. ovn-dpctl is an interface to Open vSwitch datapaths, in this case to the Linux kernel datapath. Its `dump-flows` command displays the contents of the in-kernel flow cache, and by running it under the `watch` program we see a new snapshot of the flow table every 2 seconds.

Look through the output for a flow that begins with `recirc_id(0)` and matches the Ethernet source address for `a`. There is one flow per line, but the lines are very long, so it’s easier to read if you make the window very wide. This flow’s packet counter should be increasing at a rate of 1 packet per second. It looks something like this:

    recirc_id(0),in_port(3),eth(src=fa:16:3e:f5:2a:90),eth_type(0x0800),ipv4(src=10.1.1.5,frag=no), packets:388, bytes:38024, used:0.977s, actions:ct(zone=8),recirc(0x18)

Note

Flows in the datapath can expire quickly and the `watch` command mentioned above may be too slow to catch it. If that is your case, stop the `ping 10.1.1.6` session and re-start it a few seconds after this command:

    $ sudo conntrack -F ; rm -f /tmp/flows.txt ; \
         for _ in $(seq 100) ; do \
         sudo ovs-dpctl dump-flows >> /tmp/flows.txt ; \
         sleep 0.1 ; done

Then, look for `recirc_id(0)` in flows.txt after ping command was issued:

    $ sort --uniq /tmp/flows.txt | grep zone

We can hand the first part of this (everything up to the first space) to `ofproto/trace`, and it will tell us what happens:

    $ sudo ovs-appctl ofproto/trace 'recirc_id(0),in_port(3),eth(src=fa:16:3e:a9:4c:c7),eth_type(0x0800),ipv4(src=10.1.1.5,dst=10.1.0.0/255.255.0.0,frag=no)'
    Flow: ip,in_port=3,vlan_tci=0x0000,dl_src=fa:16:3e:a9:4c:c7,dl_dst=00:00:00:00:00:00,nw_src=10.1.1.5,nw_dst=10.1.0.0,nw_proto=0,nw_tos=0,nw_ecn=0,nw_ttl=0

    bridge("br-int")
    ----------------
     0. in_port=3, priority 100
        set_field:0x8->reg13
        set_field:0x9->reg11
        set_field:0xa->reg12
        set_field:0x4->metadata
        set_field:0x1->reg14
        resubmit(,8)
     8. reg14=0x1,metadata=0x4,dl_src=fa:16:3e:a9:4c:c7, priority 50, cookie 0x6dcc418a
        resubmit(,9)
     9. ip,reg14=0x1,metadata=0x4,dl_src=fa:16:3e:a9:4c:c7,nw_src=10.1.1.5, priority 90, cookie 0x343af48c
        resubmit(,10)
    10. metadata=0x4, priority 0, cookie 0x719549d1
        resubmit(,11)
    11. ip,metadata=0x4, priority 100, cookie 0x46c089e6
        load:0x1->NXM_NX_XXREG0[96]
        resubmit(,12)
    12. metadata=0x4, priority 0, cookie 0x838152a3
        resubmit(,13)
    13. ip,reg0=0x1/0x1,metadata=0x4, priority 100, cookie 0xd1941634
        ct(table=22,zone=NXM_NX_REG13[0..15])
        drop

    Final flow: ip,reg0=0x1,reg11=0x9,reg12=0xa,reg13=0x8,reg14=0x1,metadata=0x4,in_port=3,vlan_tci=0x0000,dl_src=fa:16:3e:a9:4c:c7,dl_dst=00:00:00:00:00:00,nw_src=10.1.1.5,nw_dst=10.1.0.0,nw_proto=0,nw_tos=0,nw_ecn=0,nw_ttl=0
    Megaflow: recirc_id=0,ip,in_port=3,vlan_tci=0x0000/0x1000,dl_src=fa:16:3e:a9:4c:c7,nw_src=10.1.1.5,nw_dst=10.1.0.0/16,nw_frag=no
    Datapath actions: ct(zone=8),recirc(0xb)

Note

Be careful cutting and pasting `ovs-dpctl dump-flows` output into `ofproto/trace` because the latter has terrible error reporting. If you add an extra line break, etc., it will likely give you a useless error message.

There’s no `output` action in the output, but there are `ct` and `recirc` actions (which you can see in the `Datapath actions` at the end). The `ct` action tells the kernel to pass the packet through the kernel connection tracking for firewalling purposes and the `recirc` says to go back to the flow cache for another pass based on the firewall results. The `0xb` value inside the `recirc` gives us a hint to look at the kernel flows for a cached flow with `recirc_id(0xb)`. Indeed, there is one:

    recirc_id(0xb),in_port(3),ct_state(-new+est-rel-rpl-inv+trk),ct_label(0/0x1),eth(src=fa:16:3e:a9:4c:c7,dst=fa:16:3e:99:7a:17),eth_type(0x0800),ipv4(dst=10.1.1.4/255.255.255.252,frag=no), packets:171, bytes:16758, used:0.271s, actions:ct(zone=11),recirc(0xc)

We can then repeat our command with the match part of this kernel flow:

    $ sudo ovs-appctl ofproto/trace 'recirc_id(0xb),in_port(3),ct_state(-new+est-rel-rpl-inv+trk),ct_label(0/0x1),eth(src=fa:16:3e:a9:4c:c7,dst=fa:16:3e:99:7a:17),eth_type(0x0800),ipv4(dst=10.1.1.4/255.255.255.252,frag=no)'
    ...
    Datapath actions: ct(zone=11),recirc(0xc)

In other words, the flow passes through the connection tracker a second time. The first time was for `a`’s outgoing firewall; this second time is for `b`’s incoming firewall. Again, we continue tracing with `recirc_id(0xc)`:

    $ sudo ovs-appctl ofproto/trace 'recirc_id(0xc),in_port(3),ct_state(-new+est-rel-rpl-inv+trk),ct_label(0/0x1),eth(src=fa:16:3e:a9:4c:c7,dst=fa:16:3e:99:7a:17),eth_type(0x0800),ipv4(dst=10.1.1.6,proto=1,frag=no)'
    ...
    Datapath actions: 4

It took multiple hops, but we finally came to the end of the line where the packet was output to `b` after passing through both firewalls. The port number here is a datapath port number, which is usually different from an OpenFlow port number. To check that it is `b`’s port, we first list the datapath ports to get the name corresponding to the port number:

    $ sudo ovs-dpctl show
    system@ovs-system:
            lookups: hit:1994 missed:56 lost:0
            flows: 6
            masks: hit:2340 total:4 hit/pkt:1.14
            port 0: ovs-system (internal)
            port 1: br-int (internal)
            port 2: br-ex (internal)
            port 3: tap820c0888-13
            port 4: tapc29d4120-20

and then confirm that this is the port we think it is with a command like this:

    $ ovs-vsctl --columns=external-ids list interface tapc29d4120-20
    external_ids        : {attached-mac="fa:16:3e:99:7a:17", iface-id="c29d4120-20a4-4c44-bd83-8d91f5f447fd", iface-status=active, vm-id="2db969ca-ca2a-4d9a-b49e-f287d39c5645"}

Finally, we can relate the OpenFlow flows from our traces back to OVN logical flows. For individual flows, cut and paste a “cookie” value from `ofproto/trace` output into `ovn-sbctl lflow-list`, e.g.:

    $ ovn-sbctl lflow-list 0x6dcc418a|abbrev
    Datapath: "neutron-5b6baf" aka "n1" (a8a758)  Pipeline: ingress
      table=0 (ls_in_port_sec_l2  ), priority=50   , match=(inport == "820c08" && eth.src == {fa:16:3e:a9:4c:c7}), action=(next;)

Or, you can pipe `ofproto/trace` output through `ovn-detrace` to annotate every flow:

    $ sudo ovs-appctl ofproto/trace 'recirc_id(0xc),in_port(3),ct_state(-new+est-rel-rpl-inv+trk),ct_label(0/0x1),eth(src=fa:16:3e:a9:4c:c7,dst=fa:16:3e:99:7a:17),eth_type(0x0800),ipv4(dst=10.1.1.6,proto=1,frag=no)' | ovn-detrace
    ...

## Routing

Previously we set up a pair of VMs `a` and `b` on a network `n1` and demonstrated how packets make their way between them. In this step, we’ll set up a second network `n2` with a new VM `c`, connect a router `r` to both networks, and demonstrate how routing works in OVN.

There’s nothing really new for the network and the VM so let’s just go ahead and create them:

    $ openstack network create --provider-network-type geneve n2
    $ openstack subnet create --subnet-range 10.1.2.0/24 --network n2 n2subnet
    $ openstack server create --nic net-id=n2,v4-fixed-ip=10.1.2.7 --flavor m1.nano --image $IMAGE_ID --key-name demo c
    $ openstack port set --name cp $(openstack port list --server c -f value -c ID)
    $ CP_MAC=$(openstack port show -f value -c mac_address cp)

The new network `n2` is not yet connected to `n1` in any way. You can try tracing a packet from `a` to see, for example, that it doesn’t make it to `c`. Instead, it will end up as `multicast unknown` in `n1`:

    $ ovn-trace n1 'inport == "ap" && eth.src == '$AP_MAC' && eth.dst == '$CP_MAC
    ...

Now create an OpenStack router and connect it to `n1` and `n2`:

    $ openstack router create r
    $ openstack router add subnet r n1subnet
    $ openstack router add subnet r n2subnet

Now `a`, `b`, and `c` should all be able to reach other. You can get some verification that routing is taking place by running you `ping` between `c` and one of the other VMs: the reported TTL should be one less than between `a` and `b` (63 instead of 64).

Observe via `ovn-nbctl` the new OVN logical switch and router and then ports that connect them together:

    $ ovn-nbctl show|abbrev
    ...
    switch f51234 (neutron-332346) (aka n2)
        port 82b983
            type: router
            router-port: lrp-82b983
        port 2e585f (aka cp)
            addresses: ["fa:16:3e:89:f2:36 10.1.2.7"]
    switch 3eb263 (neutron-5b6baf) (aka n1)
        port c29d41 (aka bp)
            addresses: ["fa:16:3e:99:7a:17 10.1.1.6"]
        port 820c08 (aka ap)
            addresses: ["fa:16:3e:a9:4c:c7 10.1.1.5"]
        port 17d870
            type: router
            router-port: lrp-17d870
    ...
    router dde06c (neutron-f88ebc) (aka r)
        port lrp-82b983
            mac: "fa:16:3e:19:9f:46"
            networks: ["10.1.2.1/24"]
        port lrp-17d870
            mac: "fa:16:3e:f6:e2:8f"
            networks: ["10.1.1.1/24"]

We have not yet looked at the logical flows for an OVN logical router. You might find it of interest to look at them on your own:

    $ ovn-sbctl lflow-list r | abbrev | less -S
    ...

Let’s grab the `n1subnet` router porter MAC address to simplify later commands:

    $ N1SUBNET_MAC=$(ovn-nbctl --bare --columns=mac find logical_router_port networks=10.1.1.1/24)

Let’s see what happens at the logical flow level for an ICMP packet from `a` to `c`. This generates a long trace but an interesting one, so we’ll look at it bit by bit. The first three stanzas in the output show the packet’s ingress into `n1` and processing through the firewall on that side (via the “ct_next” connection-tracking action), and then the selection of the port that leads to router `r` as the output port:

    $ ovn-trace n1 'inport == "ap" && eth.src == '$AP_MAC' && eth.dst == '$N1SUBNET_MAC' && ip4.src == 10.1.1.5 && ip4.dst == 10.1.2.7 && ip.ttl == 64 && icmp4.type == 8'
    ...
    ingress(dp="n1", inport="ap")
    -----------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "ap" && eth.src == {fa:16:3e:a9:4c:c7}, priority 50, uuid 6dcc418a
        next;
     1. ls_in_port_sec_ip (northd.c:2364): inport == "ap" && eth.src == fa:16:3e:a9:4c:c7 && ip4.src == {10.1.1.5}, priority 90, uuid 343af48c
        next;
     3. ls_in_pre_acl (northd.c:2646): ip, priority 100, uuid 46c089e6
        reg0[0] = 1;
        next;
     5. ls_in_pre_stateful (northd.c:2764): reg0[0] == 1, priority 100, uuid d1941634
        ct_next;

    ct_next(ct_state=est|trk /* default (use --ct to customize) */)
    ---------------------------------------------------------------
     6. ls_in_acl (northd.c:2925): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0 && (inport == "ap" && ip4), priority 2002, uuid a12b39f0
        next;
    13. ls_in_l2_lkup (northd.c:3529): eth.dst == fa:16:3e:f6:e2:8f, priority 50, uuid c43ead31
        outport = "17d870";
        output;

    egress(dp="n1", inport="ap", outport="17d870")
    ----------------------------------------------
     1. ls_out_pre_acl (northd.c:2626): ip && outport == "17d870", priority 110, uuid 60395450
        next;
     8. ls_out_port_sec_l2 (northd.c:3654): outport == "17d870", priority 50, uuid 91b5cab0
        output;
        /* output to "17d870", type "patch" */

The next two stanzas represent processing through logical router `r`. The processing in table 5 is the core of the routing implementation: it recognizes that the packet is destined for an attached subnet, decrements the TTL and updates the Ethernet source address. Table 6 then selects the Ethernet destination address based on the IP destination. The packet then passes to switch `n2` via an OVN “logical patch port”:

    ingress(dp="r", inport="lrp-17d870")
    ------------------------------------
     0. lr_in_admission (northd.c:4071): eth.dst == fa:16:3e:f6:e2:8f && inport == "lrp-17d870", priority 50, uuid fa5270b0
        next;
     5. lr_in_ip_routing (northd.c:3782): ip4.dst == 10.1.2.0/24, priority 49, uuid 5f9d469f
        ip.ttl--;
        reg0 = ip4.dst;
        reg1 = 10.1.2.1;
        eth.src = fa:16:3e:19:9f:46;
        outport = "lrp-82b983";
        flags.loopback = 1;
        next;
     6. lr_in_arp_resolve (northd.c:5088): outport == "lrp-82b983" && reg0 == 10.1.2.7, priority 100, uuid 03d506d3
        eth.dst = fa:16:3e:89:f2:36;
        next;
     8. lr_in_arp_request (northd.c:5260): 1, priority 0, uuid 6dacdd82
        output;

    egress(dp="r", inport="lrp-17d870", outport="lrp-82b983")
    ---------------------------------------------------------
     3. lr_out_delivery (northd.c:5288): outport == "lrp-82b983", priority 100, uuid 00bea4f2
        output;
        /* output to "lrp-82b983", type "patch" */

Finally the logical switch for `n2` runs through the same logic as `n1` and the packet is delivered to VM `c`:

    ingress(dp="n2", inport="82b983")
    ---------------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "82b983", priority 50, uuid 9a789e06
        next;
     3. ls_in_pre_acl (northd.c:2624): ip && inport == "82b983", priority 110, uuid ab52f21a
        next;
    13. ls_in_l2_lkup (northd.c:3529): eth.dst == fa:16:3e:89:f2:36, priority 50, uuid dcafb3e9
        outport = "cp";
        output;

    egress(dp="n2", inport="82b983", outport="cp")
    ----------------------------------------------
     1. ls_out_pre_acl (northd.c:2648): ip, priority 100, uuid cd9cfa74
        reg0[0] = 1;
        next;
     2. ls_out_pre_stateful (northd.c:2766): reg0[0] == 1, priority 100, uuid 9e8e22c5
        ct_next;

    ct_next(ct_state=est|trk /* default (use --ct to customize) */)
    ---------------------------------------------------------------
     4. ls_out_acl (northd.c:2925): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0 && (outport == "cp" && ip4 && ip4.src == $as_ip4_0fc1b6cf_f925_49e6_8f00_6dd13beca9dc), priority 2002, uuid a746fa0d
        next;
     7. ls_out_port_sec_ip (northd.c:2364): outport == "cp" && eth.dst == fa:16:3e:89:f2:36 && ip4.dst == {255.255.255.255, 224.0.0.0/4, 10.1.2.7}, priority 90, uuid 4d9862b5
        next;
     8. ls_out_port_sec_l2 (northd.c:3654): outport == "cp" && eth.dst == {fa:16:3e:89:f2:36}, priority 50, uuid 0242cdc3
        output;
        /* output to "cp", type "" */

### Physical Tracing

It’s possible to use `ofproto/trace`, just as before, to trace a packet through OpenFlow tables, either for a hypothetical packet or one that you get from a real test case using `ovs-dpctl`. The process is just the same as before and the output is almost the same, too. Using a router doesn’t actually introduce any interesting new wrinkles, so we’ll skip over this for this case and for the remainder of the tutorial, but you can follow the steps on your own if you like.

## Adding a Gateway

The VMs that we’ve created can access each other but they are isolated from the physical world. In OpenStack, the dominant way to connect a VM to external networks is by creating what is called a “floating IP address”, which uses network address translation to connect an external address to an internal one.

DevStack created a pair of networks named “private” and “public”. To use a floating IP address from a VM, we first add a port to the VM with an IP address from the “private” network, then we create a floating IP address on the “public” network, then we associate the port with the floating IP address.

Let’s add a new VM `d` with a floating IP:

    $ openstack server create --nic net-id=private --flavor m1.nano --image $IMAGE_ID --key-name demo d
    $ openstack port set --name dp $(openstack port list --server d -f value -c ID)
    $ DP_MAC=$(openstack port show -f value -c mac_address dp)
    $ openstack floating ip create --floating-ip-address 172.24.4.8 public
    $ openstack server add floating ip d 172.24.4.8

(We specified a particular floating IP address to make the examples easier to follow, but without that OpenStack will automatically allocate one.)

It’s also necessary to configure the “public” network because DevStack does not do it automatically:

    $ sudo ip link set br-ex up
    $ sudo ip addr add 172.24.4.1/24 dev br-ex

Now you should be able to “ping” VM `d` from the OpenStack host:

    $ ping 172.24.4.8
    PING 172.24.4.8 (172.24.4.8) 56(84) bytes of data.
    64 bytes from 172.24.4.8: icmp_seq=1 ttl=63 time=56.0 ms
    64 bytes from 172.24.4.8: icmp_seq=2 ttl=63 time=1.44 ms
    64 bytes from 172.24.4.8: icmp_seq=3 ttl=63 time=1.04 ms
    64 bytes from 172.24.4.8: icmp_seq=4 ttl=63 time=0.403 ms
    ^C
    --- 172.24.4.8 ping statistics ---
    4 packets transmitted, 4 received, 0% packet loss, time 3003ms
    rtt min/avg/max/mdev = 0.403/14.731/56.028/23.845 ms

You can also SSH in with the key that we created during setup:

    $ ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no \
      -i ~/id_rsa_demo cirros@172.24.4.8

Let’s dive in and see how this gets implemented in OVN. First, the relevant parts of the NB DB for the “public” and “private” networks and the router between them:

    $ ovn-nbctl show | abbrev
    switch 2579f4 (neutron-d1ac28) (aka public)
        port provnet-d1ac28
            type: localnet
            addresses: ["unknown"]
        port ae9b52
            type: router
            router-port: lrp-ae9b52
    switch 5b3d5f (neutron-c02c4d) (aka private)
        port b256dd
            type: router
            router-port: lrp-b256dd
        port f264e7
            type: router
            router-port: lrp-f264e7
        port cae25b (aka dp)
            addresses: ["fa:16:3e:c1:f5:a2 10.0.0.6 fdb0:5860:4ba8:0:f816:3eff:fec1:f5a2"]
    ...
    router c59ad2 (neutron-9b057f) (aka router1)
        port lrp-ae9b52
            mac: "fa:16:3e:b2:d2:67"
            networks: ["172.24.4.9/24", "2001:db8::b/64"]
        port lrp-b256dd
            mac: "fa:16:3e:35:33:db"
            networks: ["fdb0:5860:4ba8::1/64"]
        port lrp-f264e7
            mac: "fa:16:3e:fc:c8:da"
            networks: ["10.0.0.1/26"]
        nat 788c6d
            external ip: "172.24.4.8"
            logical ip: "10.0.0.6"
            type: "dnat_and_snat"
        nat 80914c
            external ip: "172.24.4.9"
            logical ip: "10.0.0.0/26"
            type: "snat"
    ...

What we see is:

- VM `d` is on the “private” switch under its private IP address 10.0.0.6. The “private” switch is connected to “router1” via two router ports (one for IPv4, one for IPv6).

- The “public” switch is connected to “router1” and to the physical network via a “localnet” port.

- “router1” is in the middle between “private” and “public”. In addition to the router ports that connect to these switches, it has “nat” entries that direct network address translation. The translation between floating IP address 172.24.4.8 and private address 10.0.0.6 makes perfect sense.

When the NB DB gets translated into logical flows at the southbound layer, the “nat” entries get translated into IP matches that then invoke “ct_snat” and “ct_dnat” actions. The details are intricate, but you can get some of the idea by just looking for relevant flows:

    $ ovn-sbctl lflow-list router1 | abbrev | grep nat | grep -E '172.24.4.8'
      table=3 (lr_in_unsnat       ), priority=100  , match=(ip && ip4.dst == 172.24.4.8 && inport == "lrp-ae9b52" && is_chassis_resident("cr-lrp-ae9b52")), action=(ct_snat;)
      table=3 (lr_in_unsnat       ), priority=50   , match=(ip && ip4.dst == 172.24.4.8), action=(reg9[0] = 1; next;)
      table=4 (lr_in_dnat         ), priority=100  , match=(ip && ip4.dst == 172.24.4.8 && inport == "lrp-ae9b52" && is_chassis_resident("cr-lrp-ae9b52")), action=(ct_dnat(10.0.0.6);)
      table=4 (lr_in_dnat         ), priority=50   , match=(ip && ip4.dst == 172.24.4.8), action=(reg9[0] = 1; next;)
      table=1 (lr_out_snat        ), priority=33   , match=(ip && ip4.src == 10.0.0.6 && outport == "lrp-ae9b52" && is_chassis_resident("cr-lrp-ae9b52")), action=(ct_snat(172.24.4.8);)

Let’s take a look at how a packet passes through this whole gauntlet. The first two stanzas just show the packet traveling through the “public” network and being forwarded to the “router1” network:

    $ ovn-trace public 'inport == "provnet-d1ac2896-18a7-4bca-8f46-b21e2370e5b1" && eth.src == 00:01:02:03:04:05 && eth.dst == fa:16:3e:b2:d2:67 && ip4.src == 172.24.4.1 && ip4.dst == 172.24.4.8 && ip.ttl == 64 && icmp4.type==8'
    ...
    ingress(dp="public", inport="provnet-d1ac28")
    ---------------------------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "provnet-d1ac28", priority 50, uuid 8d86fb06
        next;
    10. ls_in_arp_rsp (northd.c:3266): inport == "provnet-d1ac28", priority 100, uuid 21313eff
        next;
    13. ls_in_l2_lkup (northd.c:3571): eth.dst == fa:16:3e:b2:d2:67 && is_chassis_resident("cr-lrp-ae9b52"), priority 50, uuid 7f28f51f
        outport = "ae9b52";
        output;

    egress(dp="public", inport="provnet-d1ac28", outport="ae9b52")
    --------------------------------------------------------------
     8. ls_out_port_sec_l2 (northd.c:3654): outport == "ae9b52", priority 50, uuid 72fea396
        output;
        /* output to "ae9b52", type "patch" */

In “router1”, first the `ct_snat` action without an argument attempts to “un-SNAT” the packet. ovn-trace treats this as a no-op, because it doesn’t have any state for tracking connections. As an alternative, it invokes `ct_dnat(10.0.0.6)` to NAT the destination IP:

    ingress(dp="router1", inport="lrp-ae9b52")
    ------------------------------------------
     0. lr_in_admission (northd.c:4071): eth.dst == fa:16:3e:b2:d2:67 && inport == "lrp-ae9b52" && is_chassis_resident("cr-lrp-ae9b52"), priority 50, uuid 8c6945c2
        next;
     3. lr_in_unsnat (northd.c:4591): ip && ip4.dst == 172.24.4.8 && inport == "lrp-ae9b52" && is_chassis_resident("cr-lrp-ae9b52"), priority 100, uuid e922f541
        ct_snat;

    ct_snat /* assuming no un-snat entry, so no change */
    -----------------------------------------------------
     4. lr_in_dnat (northd.c:4649): ip && ip4.dst == 172.24.4.8 && inport == "lrp-ae9b52" && is_chassis_resident("cr-lrp-ae9b52"), priority 100, uuid 02f41b79
        ct_dnat(10.0.0.6);

Still in “router1”, the routing and output steps transmit the packet to the “private” network:

    ct_dnat(ip4.dst=10.0.0.6)
    -------------------------
     5. lr_in_ip_routing (northd.c:3782): ip4.dst == 10.0.0.0/26, priority 53, uuid 86e005b0
        ip.ttl--;
        reg0 = ip4.dst;
        reg1 = 10.0.0.1;
        eth.src = fa:16:3e:fc:c8:da;
        outport = "lrp-f264e7";
        flags.loopback = 1;
        next;
     6. lr_in_arp_resolve (northd.c:5088): outport == "lrp-f264e7" && reg0 == 10.0.0.6, priority 100, uuid 2963d67c
        eth.dst = fa:16:3e:c1:f5:a2;
        next;
     8. lr_in_arp_request (northd.c:5260): 1, priority 0, uuid eea419b7
        output;

    egress(dp="router1", inport="lrp-ae9b52", outport="lrp-f264e7")
    ---------------------------------------------------------------
     3. lr_out_delivery (northd.c:5288): outport == "lrp-f264e7", priority 100, uuid 42dadc23
        output;
        /* output to "lrp-f264e7", type "patch" */

In the “private” network, the packet passes through VM `d`’s firewall and is output to `d`:

    ingress(dp="private", inport="f264e7")
    --------------------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "f264e7", priority 50, uuid 5b721214
        next;
     3. ls_in_pre_acl (northd.c:2624): ip && inport == "f264e7", priority 110, uuid 5bdc3209
        next;
    13. ls_in_l2_lkup (northd.c:3529): eth.dst == fa:16:3e:c1:f5:a2, priority 50, uuid 7957f80f
        outport = "dp";
        output;

    egress(dp="private", inport="f264e7", outport="dp")
    ---------------------------------------------------
     1. ls_out_pre_acl (northd.c:2648): ip, priority 100, uuid 4981c79d
        reg0[0] = 1;
        next;
     2. ls_out_pre_stateful (northd.c:2766): reg0[0] == 1, priority 100, uuid 247e02eb
        ct_next;

    ct_next(ct_state=est|trk /* default (use --ct to customize) */)
    ---------------------------------------------------------------
     4. ls_out_acl (northd.c:2925): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0 && (outport == "dp" && ip4 && ip4.src == 0.0.0.0/0 && icmp4), priority 2002, uuid b860fc9f
        next;
     7. ls_out_port_sec_ip (northd.c:2364): outport == "dp" && eth.dst == fa:16:3e:c1:f5:a2 && ip4.dst == {255.255.255.255, 224.0.0.0/4, 10.0.0.6}, priority 90, uuid 15655a98
        next;
     8. ls_out_port_sec_l2 (northd.c:3654): outport == "dp" && eth.dst == {fa:16:3e:c1:f5:a2}, priority 50, uuid 5916f94b
        output;
        /* output to "dp", type "" */

## IPv6

OVN supports IPv6 logical routing. Let’s try it out.

The first step is to add an IPv6 subnet to networks `n1` and `n2`, then attach those subnets to our router `r`. As usual, though OpenStack can assign addresses itself, we use fixed ones to make the discussion easier:

    $ openstack subnet create --ip-version 6 --subnet-range fc11::/64 --network n1 n1subnet6
    $ openstack subnet create --ip-version 6 --subnet-range fc22::/64 --network n2 n2subnet6
    $ openstack router add subnet r n1subnet6
    $ openstack router add subnet r n2subnet6

Then we add an IPv6 address to each of our VMs:

    $ A_PORT_ID=$(openstack port list --server a -f value -c ID)
    $ openstack port set --fixed-ip subnet=n1subnet6,ip-address=fc11::5 $A_PORT_ID
    $ B_PORT_ID=$(openstack port list --server b -f value -c ID)
    $ openstack port set --fixed-ip subnet=n1subnet6,ip-address=fc11::6 $B_PORT_ID
    $ C_PORT_ID=$(openstack port list --server c -f value -c ID)
    $ openstack port set --fixed-ip subnet=n2subnet6,ip-address=fc22::7 $C_PORT_ID

At least for me, the new IPv6 addresses didn’t automatically get propagated into the VMs. To do it by hand, pull up the console for `a` and run:

    $ sudo ip addr add fc11::5/64 dev eth0
    $ sudo ip route add via fc11::1

Then in `b`:

    $ sudo ip addr add fc11::6/64 dev eth0
    $ sudo ip route add via fc11::1

Finally in `c`:

    $ sudo ip addr add fc22::7/64 dev eth0
    $ sudo ip route add via fc22::1

Now you should have working IPv6 routing through router `r`. The relevant parts of the NB DB look like the following. The interesting parts are the new `fc11::` and `fc22::` addresses on the ports in `n1` and `n2` and the new IPv6 router ports in `r`:

    $ ovn-nbctl show | abbrev
    ...
    switch f51234 (neutron-332346) (aka n2)
        port 1a8162
            type: router
            router-port: lrp-1a8162
        port 82b983
            type: router
            router-port: lrp-82b983
        port 2e585f (aka cp)
            addresses: ["fa:16:3e:89:f2:36 10.1.2.7 fc22::7"]
    switch 3eb263 (neutron-5b6baf) (aka n1)
        port ad952e
            type: router
            router-port: lrp-ad952e
        port c29d41 (aka bp)
            addresses: ["fa:16:3e:99:7a:17 10.1.1.6 fc11::6"]
        port 820c08 (aka ap)
            addresses: ["fa:16:3e:a9:4c:c7 10.1.1.5 fc11::5"]
        port 17d870
            type: router
            router-port: lrp-17d870
    ...
    router dde06c (neutron-f88ebc) (aka r)
        port lrp-1a8162
            mac: "fa:16:3e:06:de:ad"
            networks: ["fc22::1/64"]
        port lrp-82b983
            mac: "fa:16:3e:19:9f:46"
            networks: ["10.1.2.1/24"]
        port lrp-ad952e
            mac: "fa:16:3e:ef:2f:8b"
            networks: ["fc11::1/64"]
        port lrp-17d870
            mac: "fa:16:3e:f6:e2:8f"
            networks: ["10.1.1.1/24"]

Try tracing a packet from `a` to `c`. The results correspond closely to those for IPv4 which we already discussed back under Routing:

    $ N1SUBNET6_MAC=$(ovn-nbctl --bare --columns=mac find logical_router_port networks=\"fc11::1/64\")
    $ ovn-trace n1 'inport == "ap" && eth.src == '$AP_MAC' && eth.dst == '$N1SUBNET6_MAC' && ip6.src == fc11::5 && ip6.dst == fc22::7 && ip.ttl == 64 && icmp6.type == 8'
    ...
    ingress(dp="n1", inport="ap")
    -----------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "ap" && eth.src == {fa:16:3e:a9:4c:c7}, priority 50, uuid 6dcc418a
        next;
     1. ls_in_port_sec_ip (northd.c:2390): inport == "ap" && eth.src == fa:16:3e:a9:4c:c7 && ip6.src == {fe80::f816:3eff:fea9:4cc7, fc11::5}, priority 90, uuid 604810ea
        next;
     3. ls_in_pre_acl (northd.c:2646): ip, priority 100, uuid 46c089e6
        reg0[0] = 1;
        next;
     5. ls_in_pre_stateful (northd.c:2764): reg0[0] == 1, priority 100, uuid d1941634
        ct_next;

    ct_next(ct_state=est|trk /* default (use --ct to customize) */)
    ---------------------------------------------------------------
     6. ls_in_acl (northd.c:2925): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0 && (inport == "ap" && ip6), priority 2002, uuid 7fdd607e
        next;
    13. ls_in_l2_lkup (northd.c:3529): eth.dst == fa:16:3e:ef:2f:8b, priority 50, uuid e1d87fc5
        outport = "ad952e";
        output;

    egress(dp="n1", inport="ap", outport="ad952e")
    ----------------------------------------------
     1. ls_out_pre_acl (northd.c:2626): ip && outport == "ad952e", priority 110, uuid 88f68988
        next;
     8. ls_out_port_sec_l2 (northd.c:3654): outport == "ad952e", priority 50, uuid 5935755e
        output;
        /* output to "ad952e", type "patch" */

    ingress(dp="r", inport="lrp-ad952e")
    ------------------------------------
     0. lr_in_admission (northd.c:4071): eth.dst == fa:16:3e:ef:2f:8b && inport == "lrp-ad952e", priority 50, uuid ddfeb712
        next;
     5. lr_in_ip_routing (northd.c:3782): ip6.dst == fc22::/64, priority 129, uuid cc2130ec
        ip.ttl--;
        xxreg0 = ip6.dst;
        xxreg1 = fc22::1;
        eth.src = fa:16:3e:06:de:ad;
        outport = "lrp-1a8162";
        flags.loopback = 1;
        next;
     6. lr_in_arp_resolve (northd.c:5122): outport == "lrp-1a8162" && xxreg0 == fc22::7, priority 100, uuid bcf75288
        eth.dst = fa:16:3e:89:f2:36;
        next;
     8. lr_in_arp_request (northd.c:5260): 1, priority 0, uuid 6dacdd82
        output;

    egress(dp="r", inport="lrp-ad952e", outport="lrp-1a8162")
    ---------------------------------------------------------
     3. lr_out_delivery (northd.c:5288): outport == "lrp-1a8162", priority 100, uuid 5260dfc5
        output;
        /* output to "lrp-1a8162", type "patch" */

    ingress(dp="n2", inport="1a8162")
    ---------------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "1a8162", priority 50, uuid 10957d1b
        next;
     3. ls_in_pre_acl (northd.c:2624): ip && inport == "1a8162", priority 110, uuid a27ebd00
        next;
    13. ls_in_l2_lkup (northd.c:3529): eth.dst == fa:16:3e:89:f2:36, priority 50, uuid dcafb3e9
        outport = "cp";
        output;

    egress(dp="n2", inport="1a8162", outport="cp")
    ----------------------------------------------
     1. ls_out_pre_acl (northd.c:2648): ip, priority 100, uuid cd9cfa74
        reg0[0] = 1;
        next;
     2. ls_out_pre_stateful (northd.c:2766): reg0[0] == 1, priority 100, uuid 9e8e22c5
        ct_next;

    ct_next(ct_state=est|trk /* default (use --ct to customize) */)
    ---------------------------------------------------------------
     4. ls_out_acl (northd.c:2925): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0 && (outport == "cp" && ip6 && ip6.src == $as_ip6_0fc1b6cf_f925_49e6_8f00_6dd13beca9dc), priority 2002, uuid 12fc96f9
        next;
     7. ls_out_port_sec_ip (northd.c:2390): outport == "cp" && eth.dst == fa:16:3e:89:f2:36 && ip6.dst == {fe80::f816:3eff:fe89:f236, ff00::/8, fc22::7}, priority 90, uuid c622596a
        next;
     8. ls_out_port_sec_l2 (northd.c:3654): outport == "cp" && eth.dst == {fa:16:3e:89:f2:36}, priority 50, uuid 0242cdc3
        output;
        /* output to "cp", type "" */

## ACLs

Let’s explore how ACLs work in OpenStack and OVN. In OpenStack, ACL rules are part of “security groups”, which are “default deny”, that is, packets are not allowed by default and the rules added to security groups serve to allow different classes of packets. The default group (named “default”) that is assigned to each of our VMs so far allows all traffic from our other VMs, which isn’t very interesting for testing. So, let’s create a new security group, which we’ll name “custom”, add rules to it that allow incoming SSH and ICMP traffic, and apply this security group to VM `c`:

    $ openstack security group create custom
    $ openstack security group rule create --dst-port 22 custom
    $ openstack security group rule create --protocol icmp custom
    $ openstack server remove security group c default
    $ openstack server add security group c custom

Now we can do some experiments to test security groups. From the console on `a` or `b`, it should now be possible to “ping” `c` or to SSH to it, but attempts to initiate connections on other ports should be blocked. (You can try to connect on another port with `ssh -p PORT IP` or `nc IP PORT -vv`.) Connection attempts should time out rather than receive the “connection refused” or “connection reset” error that you would see between `a` and `b`.

It’s also possible to test ACLs via ovn-trace, with one new wrinkle. ovn-trace can’t simulate connection tracking state in the network, so by default it assumes that every packet represents an established connection. That’s good enough for what we’ve been doing so far, but for checking properties of security groups we want to look at more detail.

If you look back at the VM-to-VM traces we’ve done until now, you can see that they execute two `ct_next` actions:

- The first of these is for the packet passing outward through the source VM’s firewall. We can tell ovn-trace to treat the packet as starting a new connection or adding to an established connection by adding a `--ct` option: `--ct new` or `--ct est`, respectively. The latter is the default and therefore what we’ve been using so far. We can also use `--ct est,rpl`, which in addition to `--ct est` means that the connection was initiated by the destination VM rather than by the VM sending this packet.

- The second is for the packet passing inward through the destination VM’s firewall. For this one, it makes sense to tell ovn-trace that the packet is starting a new connection, with `--ct new`, or that it is a packet sent in reply to a connection established by the destination VM, with `--ct est,rpl`.

ovn-trace uses the `--ct` options in order, so if we want to override the second `ct_next` behavior we have to specify two options.

Another useful ovn-trace option for this testing is `--minimal`, which reduces the amount of output. In this case we’re really just interested in finding out whether the packet reaches the destination VM, that is, whether there’s an eventual `output` action to `c`, so `--minimal` works fine and the output is easier to read.

Try a few traces. For example:

First, obtain the mac address of logical router’s IPv4 interface on `n1`:

    $ N1SUBNET4_MAC=$(ovn-nbctl --bare --columns=mac \
      find logical_router_port networks=\"10.1.1.1/24\")

- VM `a` initiates a new SSH connection to `c`:

      $ ovn-trace --ct new --ct new --minimal n1 'inport == "ap" &&
        eth.src == '$AP_MAC' && eth.dst == '$N1SUBNET4_MAC' &&
        ip4.src == 10.1.1.5 && ip4.dst == 10.1.2.7 && ip.ttl == 64 &&
        tcp.dst == 22'
      ...
      ct_next(ct_state=new|trk) {
          ip.ttl--;
          eth.src = fa:16:3e:19:9f:46;
          eth.dst = fa:16:3e:89:f2:36;
          ct_next(ct_state=new|trk) {
              output("cp");
          };
      };

  This succeeds, as you can see since there is an `output` action.

- VM `a` initiates a new Telnet connection to `c`:

      $ ovn-trace --ct new --ct new --minimal n1 'inport == "ap" &&
        eth.src == '$AP_MAC' && eth.dst == '$N1SUBNET4_MAC' &&
        ip4.src == 10.1.1.5 && ip4.dst == 10.1.2.7 && ip.ttl == 64 &&
        tcp.dst == 23'
      ct_next(ct_state=new|trk) {
          ip.ttl--;
          eth.src = fa:16:3e:19:9f:46;
          eth.dst = fa:16:3e:89:f2:36;
          ct_next(ct_state=new|trk);
      };

  This fails, as you can see from the lack of an `output` action.

- VM `a` replies to a packet that is part of a Telnet connection originally initiated by `c`:

      $ ovn-trace --ct est,rpl --ct est,rpl --minimal n1 'inport == "ap" &&
        eth.src == '$AP_MAC' && eth.dst == '$N1SUBNET4_MAC' &&
        ip4.src == 10.1.1.5 && ip4.dst == 10.1.2.7 && ip.ttl == 64 &&
        tcp.dst == 23'
      ...
      ct_next(ct_state=est|rpl|trk) {
          ip.ttl--;
          eth.src = fa:16:3e:19:9f:46;
          eth.dst = fa:16:3e:89:f2:36;
          ct_next(ct_state=est|rpl|trk) {
              output("cp");
          };
      };

  This succeeds, as you can see from the `output` action, since traffic received in reply to an outgoing connection is always allowed.

## DHCP

As a final demonstration of the OVN architecture, let’s examine the DHCP implementation. Like switching, routing, and NAT, the OVN implementation of DHCP involves configuration in the NB DB and logical flows in the SB DB.

Let’s look at the DHCP support for `a`’s port `ap`. The port’s Logical_Switch_Port record shows that `ap` has DHCPv4 options:

    $ ovn-nbctl list logical_switch_port ap | abbrev
    _uuid               : ef17e5
    addresses           : ["fa:16:3e:a9:4c:c7 10.1.1.5 fc11::5"]
    dhcpv4_options      : 165974
    dhcpv6_options      : 26f7cd
    dynamic_addresses   : []
    enabled             : true
    external_ids        : {"neutron:port_name"=ap}
    name                : "820c08"
    options             : {}
    parent_name         : []
    port_security       : ["fa:16:3e:a9:4c:c7 10.1.1.5 fc11::5"]
    tag                 : []
    tag_request         : []
    type                : ""
    up                  : true

We can then list them either by UUID or, more easily, by port name:

    $ ovn-nbctl list dhcp_options ap | abbrev
    _uuid               : 165974
    cidr                : "10.1.1.0/24"
    external_ids        : {subnet_id="5e67e7"}
    options             : {lease_time="43200", mtu="1442", router="10.1.1.1", server_id="10.1.1.1", server_mac="fa:16:3e:bb:94:72"}

These options show the basic DHCP configuration for the subnet. They do not include the IP address itself, which comes from the Logical_Switch_Port record. This allows a whole Neutron subnet to share a single DHCP_Options record. You can see this sharing in action, if you like, by listing the record for port `bp`, which is on the same subnet as `ap`, and see that it is the same record as before:

    $ ovn-nbctl list dhcp_options bp | abbrev
    _uuid               : 165974
    cidr                : "10.1.1.0/24"
    external_ids        : {subnet_id="5e67e7"}
    options             : {lease_time="43200", mtu="1442", router="10.1.1.1", server_id="10.1.1.1", server_mac="fa:16:3e:bb:94:72"}

You can take another look at the southbound flow table if you like, but the best demonstration is to trace a DHCP packet. The following is a trace of a DHCP request inbound from `ap`. The first part is just the usual travel through the firewall:

    $ ovn-trace n1 'inport == "ap" && eth.src == '$AP_MAC' && eth.dst == ff:ff:ff:ff:ff:ff && ip4.dst == 255.255.255.255 && udp.src == 68 && udp.dst == 67 && ip.ttl == 1'
    ...
    ingress(dp="n1", inport="ap")
    -----------------------------
     0. ls_in_port_sec_l2 (northd.c:3234): inport == "ap" && eth.src == {fa:16:3e:a9:4c:c7}, priority 50, uuid 6dcc418a
        next;
     1. ls_in_port_sec_ip (northd.c:2325): inport == "ap" && eth.src == fa:16:3e:a9:4c:c7 && ip4.src == 0.0.0.0 && ip4.dst == 255.255.255.255 && udp.src == 68 && udp.dst == 67, priority 90, uuid e46bed6f
        next;
     3. ls_in_pre_acl (northd.c:2646): ip, priority 100, uuid 46c089e6
        reg0[0] = 1;
        next;
     5. ls_in_pre_stateful (northd.c:2764): reg0[0] == 1, priority 100, uuid d1941634
        ct_next;

The next part is the new part. First, an ACL in table 6 allows a DHCP request to pass through. In table 11, the special `put_dhcp_opts` action replaces a DHCPDISCOVER or DHCPREQUEST packet by a reply. Table 12 flips the packet’s source and destination and sends it back the way it came in:

     6. ls_in_acl (northd.c:2925): !ct.new && ct.est && !ct.rpl && ct_label.blocked == 0 && (inport == "ap" && ip4 && ip4.dst == {255.255.255.255, 10.1.1.0/24} && udp && udp.src == 68 && udp.dst == 67), priority 2002, uuid 9c90245d
        next;
    11. ls_in_dhcp_options (northd.c:3409): inport == "ap" && eth.src == fa:16:3e:a9:4c:c7 && ip4.src == 0.0.0.0 && ip4.dst == 255.255.255.255 && udp.src == 68 && udp.dst == 67, priority 100, uuid 8d63f29c
        reg0[3] = put_dhcp_opts(offerip = 10.1.1.5, lease_time = 43200, mtu = 1442, netmask = 255.255.255.0, router = 10.1.1.1, server_id = 10.1.1.1);
        /* We assume that this packet is DHCPDISCOVER or DHCPREQUEST. */
        next;
    12. ls_in_dhcp_response (northd.c:3438): inport == "ap" && eth.src == fa:16:3e:a9:4c:c7 && ip4 && udp.src == 68 && udp.dst == 67 && reg0[3], priority 100, uuid 995eeaa9
        eth.dst = eth.src;
        eth.src = fa:16:3e:bb:94:72;
        ip4.dst = 10.1.1.5;
        ip4.src = 10.1.1.1;
        udp.src = 67;
        udp.dst = 68;
        outport = inport;
        flags.loopback = 1;
        output;

Then the last part is just traveling back through the firewall to VM `a`:

    egress(dp="n1", inport="ap", outport="ap")
    ------------------------------------------
     1. ls_out_pre_acl (northd.c:2648): ip, priority 100, uuid 3752b746
        reg0[0] = 1;
        next;
     2. ls_out_pre_stateful (northd.c:2766): reg0[0] == 1, priority 100, uuid 0c066ea1
        ct_next;

    ct_next(ct_state=est|trk /* default (use --ct to customize) */)
    ---------------------------------------------------------------
     4. ls_out_acl (northd.c:3008): outport == "ap" && eth.src == fa:16:3e:bb:94:72 && ip4.src == 10.1.1.1 && udp && udp.src == 67 && udp.dst == 68, priority 34000, uuid 0b383e77
        ct_commit;
        next;
     7. ls_out_port_sec_ip (northd.c:2364): outport == "ap" && eth.dst == fa:16:3e:a9:4c:c7 && ip4.dst == {255.255.255.255, 224.0.0.0/4, 10.1.1.5}, priority 90, uuid 7b8cbcd5
        next;
     8. ls_out_port_sec_l2 (northd.c:3654): outport == "ap" && eth.dst == {fa:16:3e:a9:4c:c7}, priority 50, uuid b874ece8
        output;
        /* output to "ap", type "" */

## Further Directions

We’ve looked at a fair bit of how OVN works and how it interacts with OpenStack. If you still have some interest, then you might want to explore some of these directions:

- Adding more than one hypervisor (“compute node”, in OpenStack parlance). OVN connects compute nodes by tunneling packets with the STT or Geneve protocols. OVN scales to 1000 compute nodes or more, but two compute nodes demonstrate the principle. All of the tools and techniques we demonstrated also work with multiple compute nodes.

- Container support. OVN supports seamlessly connecting VMs to containers, whether the containers are hosted on “bare metal” or nested inside VMs. OpenStack support for containers, however, is still evolving, and too difficult to incorporate into the tutorial at this point.

- Other kinds of gateways. In addition to floating IPs with NAT, OVN supports directly attaching VMs to a physical network and connecting logical switches to VTEP hardware.