Native Open vSwitch firewall driver

Historically, Open vSwitch (OVS) could not interact directly with iptables to implement security groups. Thus, the OVS agent and Compute service use a Linux bridge between each instance (VM) and the OVS integration bridge br-int to implement security groups. The Linux bridge device contains the iptables rules pertaining to the instance. In general, additional components between instances and physical network infrastructure cause scalability and performance problems. To alleviate such problems, the OVS agent includes an optional firewall driver that natively implements security groups as flows in OVS rather than the Linux bridge device and iptables. This increases scalability and performance.

Configuring heterogeneous firewall drivers

L2 agents can be configured to use differing firewall drivers. There is no requirement that they all be the same. If an agent lacks a firewall driver configuration, it will default to what is configured on its server. This also means there is no requirement that the server has any firewall driver configured at all, as long as the agents are configured correctly.


The native OVS firewall implementation requires kernel and user space support for conntrack, thus requiring minimum versions of the Linux kernel and Open vSwitch. All cases require Open vSwitch version 2.5 or newer.

  • Kernel version 4.3 or newer includes conntrack support.

  • Kernel version 3.3, but less than 4.3, does not include conntrack support and requires building the OVS modules.

Enable the native OVS firewall driver

  • On nodes running the Open vSwitch agent, edit the openvswitch_agent.ini file and enable the firewall driver.

    firewall_driver = openvswitch

For more information, see the Open vSwitch Firewall Driver and the video.

Using GRE tunnels inside VMs with OVS firewall driver

If GRE tunnels from VM to VM are going to be used, the native OVS firewall implementation requires nf_conntrack_proto_gre module to be loaded in the kernel on nodes running the Open vSwitch agent. It can be loaded with the command:

# modprobe nf_conntrack_proto_gre

Some Linux distributions have files that can be used to automatically load kernel modules at boot time, for example, /etc/modules. Check with your distribution for further information.

This isn’t necessary to use gre tunnel network type Neutron.

Differences between OVS and iptables firewall drivers

Both OVS and iptables firewall drivers should always behave in the same way if the same rules are configured for the security group. But in some cases that is not true and there may be slight differences between those drivers.




Traffic marked as INVALID by conntrack but matching some of the SG rules (please check 1 and 2 for details)


Allowed because it first matches SG rule, never reaches rule to drop invalid packets

Multicast traffic sent in the group 224.0.0.X (please check 3 for details)

Allowed always

Blocked, Can be enabled by SG rule.

Open Flow rules processing considerations

The native Open vSwitch firewall driver increases the number of Open Flow rules to be installed in the integration bridge, that could be up to thousands of entries, depending on the number or rules, rule type and number of ports in the compute node.

By default, these rules are written into the integration bridge in batches. The _constants.AGENT_RES_PROCESSING_STEP constant defines how many rules are written in a single operation. It is set to 100.

As seen in LP#1934917, during the Open Flow processing (that could be better displayed during the OVS agent initial transient period), there could be some inconsistencies in the port rules. In order to avoid them, the configuration variable OVS.openflow_processed_per_port allows to process all Open Flow rules related to a single port in a single transaction.

The following script provides a tool to measure, in each deployment, the processing time when using OVS.openflow_processed_per_port or the default _constants.AGENT_RES_PROCESSING_STEP:

# (1) Create a network with a single IPv4 subnet
openstack network create net-scale
openstack subnet create --subnet-range --network net-scale snet-scale

# (2) Create 400 ports bound to one host
for i in {1..400}
    openstack port create \
      --security-group <security_group_id> \
      --device-owner testing:scale \
      --binding-profile host_id=<compute_node_host_name> \
      --network net-scale test-large-scale-port-$i

# (3) Create 1000 security group rules, belonging to the same security
#     group <security_group_id>
for i in {3000..4000}
  curl -g -i -X POST http://controller:9696/v2.0/security-group-rules \
  -H "User-Agent: python-neutronclient" -H "Content-Type: application/json" \
  -H "Accept: application/json" -H "X-Auth-Token: <token>" \
  -d '{
  "security_group_rule": {
    "direction": "ingress", "protocol": "tcp",
    "ethertype": "IPv4", "port_range_max": "'$i'",
    "port_range_min": "3000",
    "security_group_id": <security_group_id>}
  }' 2>&1 > /dev/null

# (4) Setup the port to the host <compute_node_host_name>
# "grep" the test port list into file port_list.
$ for p in `openstack port list -f value -c id -c name -c mac_address -c fixed_ips | grep test-large-scale-port`
      mac=`echo $p | cut -f3 -d" "`
      ip_addr=`echo $p | cut -f7 -d" " | cut -f2 -d"'"`
      dev_id=`echo $p | cut -f1 -d" " | cut -b 1-11`
      echo "===" $mac "===" $ip_addr "===" $dev_id "===" $dev_name
      ovs-vsctl  --may-exist add-port br-int ${dev_name} -- set Interface \
        ${dev_name} type=internal \
        -- set Interface ${dev_name} external-ids:attached-mac="${mac}" \
        -- set Interface ${dev_name} external-ids:iface-id="${p}" \
        -- set Interface ${dev_name} external-ids:iface-status=active
      sleep 0.2

      ip link set dev ${dev_name} address ${mac}
      ip addr add ${ip_addr} dev ${dev_name}
      ip link set ${dev_name} up

# (5) Restart the OVS agent and check that all flows are in place.
# (6) Check the OVS agent restart time, checking the "iteration" time and
#     number.

Permitted ethertypes

The OVS Firewall blocks traffic that does not have either the IPv4 or IPv6 ethertypes at present. This is a behavior change compared to the “iptables_hybrid” firewall, which only operates on IP packets and thus does not address other ethertypes. With the configuration option permitted_ethertypes it is possible to define a set of allowed ethertypes. Any traffic with these allowed ethertypes with destination to a local port or generated from a local port and MAC address, will be allowed.