MAAS should automatically add a PXE interface on pod compose in certain cases

[Mike Pontillo]

When composing a machine from a pod in the UI and specifying custom interfaces, users have some degree of notice about whether or not they are composing a machine that will ultimately be operational: they will be warned if interfaces they add are not capable of PXE booting (and, thus, able to be managed by MAAS).

Users (and API clients) specifying an interface constraint string currently have no such luxury.

As an example, given two spaces, `internal`, and `private`, where `internal` is DHCP-enabled in MAAS and able to PXE boot, and `private` is not DHCP-enabled in MAAS, the following command will raise an error:

$ maas $PROFILE machines allocate interfaces='eth1:space=private'
No available machine matches constraints: [('interfaces', ['eth1:space=private'])] (resolved to "interfaces=eth1:space=private")

This situation is not ideal, because API clients (such as juju) may not recognize the requirement of adding a PXE network.

The complexity here is that MAAS may not guess correctly about which network should be used as a PXE network by default. Therefore, per-pod, users should be able to specify:

(1) Default PXE VLAN (including an option to opt out of auto-selection, if not specified)
(2) Default IP allocation mode (though this might apply not only to the PXE interface)

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Given that (2) above brings up a more general issue, we may want to consider two additional changes to the network model for MAAS 2.6+:

 - VLANs should be able to specify which of their subnet(s) to configure by default for new machines.
 - Subnets should be able to specify the default IP allocation mode for new machines.

Historically these issues have not been as important, since MAAS users have been expected to configure machines for general usage in "Ready" state. But as allocation becomes more automatic and data center configuration becomes more automated, these options will be important for MAAS to have.

[Jerzy Husakowski (jhusakowski)]

Feature request, moved to internal product feedback backlog for further prioritisation.