dhcp server handing out network context for uncontextualized VMs

[FlorianHeigl]

Description
A very simple DHCP server only listening to registered VMs on L3 vnets (not L2 ones!)
It should be possible to enable that.
If a VM is sending a dhcp request the DHCP server hands it the IP assigned by the OpenNebula vnet. The reply would be a unicast reply. The reply would include the network config that overlaps with info that is supported by dhcp. (mask, gw, dns)

Use case
The following scenarios would work out of the box:

• imported VMs
• unsupported OS
• network connectivity for downloading drivers etc.
• fallback configuration when there's errors with the conext
• Rescue CDs etc. would have the correct info, if needed

Currently all of those need manual intervention, without large visible benefits.
The neat part is that it is a graceful behaviour, since both scenarios end up with the same network config (even if other vars would be missing)

Interface Changes
A toggle to turn on / off.

Additional Context
.

Progress Status

• Branch created
• Code committed to development branch
• Testing - QA
• Documentation
• Release notes - resolved issues, compatibility, known issues
• Code committed to upstream release/hotfix branches
• Documentation committed to upstream release/hotfix branches

[hydro-b]

The KEA DHCP server https://www.isc.org/kea/ would be a good candidate to help with this use case as it has a REST-API and can be configured "on the fly" to do whatever needed when a new VM is spun up

[vholer]

DHCP is part of this VNF appliance (#3375). Basically, just a refreshed VR with DHCP (for the start).

We have already KEA there, and it'll have a custom plugin (that's the main reason why KEA) to offer IPs based on OpenNebula MAC<->IP translation. So, it won't talk to ONE directly, but just use same logic to compute addresses.

ETA, beginning of next year.