Unable to create unconfigured or DHCP interface alias

> I am unable to create an unconfigured interface alias using the MAAS UI. VMware ESXi deployments require an unconfigured alias for VMs to be associated with.

If I understand correctly, an alias is currently created by linking a subnet:

https://git.launchpad.net/maas/tree/src/maasserver/websockets/handlers/machine.py#n1051

As far as I can see we'd need to add another way to create an alias

> Testing also revealed I am unable to create an alias configured for DHCP.

The ui logic could be wrong here. If the physical interface or none of the interface's aliases (or VLANs?) provide DHCP then we should allow an alias to be configure for DHCP?

Lee, can you expand on how this should work for ESXi?

AFAIK, netplan doesn't support such interface aliases, does it?

How did this work in 2.9?

In previous versions of MAAS I could create an alias which was associated with a subnet but had no IP address configured. When network configuration was applied this would create a VMware PortGroup without a VMK associated with it. This is required for VMs to have access to networking.

I targeted this to 3.0.0RC2 as this is a regression. VMware ESXi 6.7 users will be forced to manually configure a PortGroup in VMware to be able to use VMs.

Pushing this to 3.0.1, since I think this needs some considering. Even in 2.9, you couldn't create an alias with no IP. You'd have to create the alias, and then edit it to make it Unconfigured.

I think that itself was a bug, that was fixed for 3.0. Netplan doesn't support interface alias without IPs, and I couldn't find a way to create it in the API either. It explicitly errors out if you try to do that:

  salvia:~> maas maas29 interface link-subnet 8kpkp6 4 subnet=3 mode=LINK_UP
  {"mode": ["Cannot configure interface to link up (with no IP address) while other links are already configured. Specify force=True to override this behavior and delete all links."]}

Or how do you do this with the API?

Is this still an issue with the current MAAS versions and do we still need to support it?