Boot a VM with an unaddressed port

This blueprint aims to allow a VM to boot with an attached port without any IP assigned.

Problem description

Currently Neutron permits users to create a port assigned to a network with corresponding subnets and IP pools, without an IP address assigned. However Nova only allows users to create a VM with a port without an IP only if this address assignment is deferred; that means that the port is expected to have an IP address but Neutron deferred the IP allocation until the host to which the port will be bound is populated.

However, there are some network applications (e.g.: service function forwarding, service function classifier, CMTS) that often forward traffic that is not intended for them. Those applications have an interface without a primary L3 address which may be receiving traffic for so many disparate addresses that configuring all of them in Neutron is a burden.

Use Cases

A typical use case is when a user wishes to deploy a VM which accepts traffic that is neither IPv4 nor IPv6 in nature. For example, a CMTS (Cable Modem Termination System).

Another use case could be a VM that accepts traffic for a very wide address range (for either forwarding or termination) and where the port has no primary address. In such cases, the VM is not a conventional application VM.

Proposed change

This spec proposes to allow to spawn a VM with a manually created port without IP address assignation.

When a port in Neutron is created with the option “–no-fixed-ip”, the port parameter ip_allocation [1] will be populated with “none” [2]. This way Neutron marks a port not to have an IP address. Nova, during the instance creation, validates the build options; in particular the ports provided to be bound to this new VM. To be able to use an unaddressed port, Nova needs to modify the logic where IP assignation is tested [3].


As commented in the use cases, some applications will accept traffic that is neither IPv4 nor IPv6. Having an IP address is irrelevant on those ports but doesn’t affect the application.

In other cases, like in a routing application, there is no alternative. It’s not possible to define in Neutron all the possible IP addresses.

Data model impact


The Neutron port contains the information needed in the ip_allocation parameter and the connectivity parameter inside the binding:vif_details.

REST API impact


Security impact

Those ports without an assigned IP don’t work with the Neutron in-tree firewalls (iptables and OVS Open Flows based). Both firewalls will filter the egress and the ingress traffic depending on several parameters, including the IP address. To let the traffic come into the virtual interface, the firewall should be disabled in the compute node hosting the VM. This mandatory configuration will be documented.

Once the Nova feature is implemented and tested, a new feature will be requested to Neutron, in order to allow those ports without an IP address to work correctly with the in-tree firewalls.

Notifications impact


Other end user impact

To be able to remotely access to the created VM, the user needs to add an addressed port to the VM. This “management” port must have an IP address.

Performance Impact


Other deployer impact

Some L2 driver, like “l2-pop”, may have problems when dealing with this kind of port because they use proxy ARP to answer ARP requests from known IP address.

The [“novnc”] service won’t work with a port without an IP address. This is why it’s recommended to create a VM with at least one management port, with an assigned IP address.

Developer impact


Upgrade impact




Primary assignee:


Other contributors:

Rodolfo Alonso <rodolfo-alonso-hernandez> (

Feature Liaison

Feature liaison:


Work Items

Work items:

  • Change the logic of how the IP assignation is tested [3].

  • Implement the tempest test described.

  • Create a new Neutron feature request to change the in-tree firewalls to work correctly with those ports without IP address assigned.


None. The necessary work in neutron has already been accomplished via two specs. The main neutron change was allowing for the creation of an unaddressed port and mark it, by populating the ip_allocation parameter with none. This was covered by the “Allow vm to boot without l3 address(subnet)” [5] spec. The changes introduced as part of the “Port binding event extended information for Nova” [4] spec means neutron will now provide the type of back-end to which the port is bound, with the parameter connectivity, included now in binding:vif_details. Nova can determine whether a given driver back-end has “l2” connectivity and, if so, know that a port without an IP address can be assigned to a virtual machine.


Apart from the needed functional and unit testing, a tempest test could cover this feature. This tempest test will spawn three VMs, each one with a management port, to be able to SSH to the machine. Then two traffic networks will be created, net1 and net2.

The first machine will have a port, with an IP assigned, connected to net1. The third machine will have a port, with an IP assigned, connected to net2. And finally, the second machine, in the middle of the first and the third one, with be connected to net1 and net2 with two ports without an IP address. The second machine will have the needed iptables rules to NAT the traffic between the first VM and the third VM port.

Both the first and the third machine will need a manual entry in the ARP table to force the traffic going out trough the traffic port.

Documentation Impact

  • Make a reference of this feature in the user document “Launch instances” [6].




Release Name