The post yesterday discussed a method for having segmented multi-tenant networks inside of OpenStack. As a series of test cases were worked through with a setup of this nature, a large gaping hole in OpenStack came into view.
What does the previously described multiple external networks look like inside OpenStack?
In the second and third screenshots, you can see the two tenants see both external networks, but they only see a subnet listed for the external network that was created with their respective tenant-id. At first glance, this would seem to be doing what was intended — each tenant receiving their own external network to consume floating IP addresses from. Unfortunately, it begins to breakdown when a tenant goes to Compute –> Access & Security –> Floating IPs in Horizon.
The above screenshot shows a tenant being assigned an floating IP address from what should have been an external network they did not have access to.
I felt pretty much like Captain Picard after working through the test cases. Surely, OpenStack would allow a design where tenants have segmented external networks — right?
Unfortunately, OpenStack does not honor this type of segmented external networking design — it will allow any tenant consume/claim a floating IP address from any of the other external networks. To read how OpenStack fully implements external networks, you can read the documentation here. At issue here is highlighted here,
Nevertheless, the concept of ‘external’ implies some forms of sharing, and this has some bearing on the topologies that can be achieved. For instance it is not possible at the moment to have an external network which is reserved to a specific tenant.
Essentially, OpenStack Neutron thinks of external networks differently, than I believe most architects. It also does not clearly honor the tenant-id attribute that is specified when the network is created, nor when the shared attribute is not enabled on the external network. The methodology OpenStack Neutron uses is more in-line with the AWS consumption model — everyone drinks from the same pool and there is no segmentation between the tenants. I personally do not believe that model works in a private cloud where there are multiple tenants.
The next post in the series will discuss a potential design for working around the issue inside OpenStack Neutron.