Support external DHCP providers

https://blueprints.launchpad.net/ironic/+spec/support-external-dhcp

Ironic should allow operators to use an external DHCP server, rather than Neutron’s DHCP server.

Problem description

There are a number of reasons a deployer may wish to use an external DHCP server. These include:

  • Neutron is not available in the deployer’s environment.
  • The deployer may believe Neutron’s DHCP server is not scalable. Neutron’s DHCP server uses dnsmasq under the hood, which is known to be unsuitable for large-scale deployments (see links in references section).

Proposed change

Ironic should have a configuration option dhcp_provider that specifies which provider to use. All DHCP providers will implement a DHCP provider interface, which exposes a method to update a port’s DHCP attributes.

Drivers will no longer call the DHCP provider directly, but instead request a DHCP provider from a factory, which will return the provider specified in the configuration options.

The two initial provider implementations will be “neutron” and “none”.

Neutron functioality that does not relate to DHCP (ie. updating a port’s MAC address) will not be moved.

Alternatives

It would be simpler in the short term to have a boolean use_neutron_for_dhcp config that would tell Ironic to either use Neutron or do nothing. This would duplicate the initial functionality of the proposed change. However, new DHCP providers would be difficult to add, and there would also be more work for the deploy driver.

Data model impact

None.

REST API impact

None.

Driver API impact

None.

Nova driver impact

None.

Security impact

None.

Other end user impact

None.

Scalability impact

This will only make Ironic more scalable, as it will expose a configuration in which Ironic no longer interacts with Neutron.

Performance Impact

None.

Other deployer impact

A config option dhcp_provider will be added that starts with two options, “neutron” and “none”.

There will be no immediate impact on deployers, as dhcp_provider will default to “neutron”, which does not change behavior.

However, deployers that wish to use an external DHCP provider must set the config option and also deploy a DHCP service.

Developer impact

None.

Implementation

Assignee(s)

Primary assignee:
jroll
Other contributors:
JoshNang ellenh

Work Items

  • Add dhcp_provider config option.
  • Move Neutron API behind a DHCP provider interface.
  • Implement “none” DHCP provider.
  • Implement a DHCP provider factory.
  • Add Tempest tests

Dependencies

None.

Testing

Tempest tests should be written that use an external DHCP provider. dnsmasq is already installed in devstack, so this could be accomplished by running dnsmasq standalone, rather than via Neutron.

Documentation Impact

Documentation will need to be written for this config option. This should include:

  • What the config option does.
  • How to configure an external DHCP server to work with Ironic.
  • A diagram of what the control plane looks like for both dhcp_provider options.

References