KVM host networking

In order to enable KVM host networking features, MAAS must be able to correlate the IP address of a potential KVM host with a device known to MAAS (e.g. a machine or controller). If it cannot, for example, if a machine not known to MAAS is set up as a KVM host, enhanced interface selection features will not be available.

The recommended way of setting up a KVM host is therefore to deploy a machine within MAAS and tick the “Install MAAS-managed KVM Host” checkbox (a full explanation is found here). MAAS will automatically install KVM as well as ensure that the network model is consistent with what is on the machine.

There are other ways of setting up KVM hosts that provide easy management of VMs via the MAAS UI. You can, for example, install KVM manually on a deployed node or on a new or existing rack controller.

Enhanced KVM host networking features may not operate correctly when you install KVM manaully on a deployed node. (E.g. if any of the host interfaces change.)

2.4 and earlier

MAAS requires the use of a DHCP server it can control. Therefore, DHCP must be enabled in MAAS (rather than in libvirt) to allow VMs to use network booting (e.g. PXE). DHCP requests can also be forwarded to MAAS via a DHCP relay.

MAAS first checks for the existence of a libvirt network named maas. The maas network should have DHCP disabled in favor of MAAS-enabled DHCP to allow your VMs to network boot. VMs on the maas libvirt network must be able to reach the wider network. As such, either (1) the bridge the VMs are attached to must include one of the host’s physical network interfaces on the appropriate network, or (2) NAT must be enabled.

If MAAS cannot find a maas network, it will fallback to libvirt’s default network.

Libvirt’s default network has DHCP enabled by default. You must either disable libvirt’s DHCP and enable MAAS DHCP on the default network in libvirt, or create a separate maas network on a VLAN with MAAS DHCP enabled.

2.5+

In MAAS 2.5, you can apply interface-constraints to VMs, which allows you to compose a VM with specific networking requirements (a full-explanation of the feature is found in the following section). When these requirements are present, MAAS is able connect your VMs to the full range of your MAAS-configured network.

With interface constraints

Instead of attaching to a libvirt network like maas or default, MAAS in this case tells the hypervisor on the host to attach the VM directly to a constraints-matching underlying bridge or non-bridge interface via macvtap. In addition, if you provide a specific IP address in the constraint string, MAAS will try to allocate it and assign it to the interface when the VM is created, thereby providing some limited interface configuration upon creating the VM.

With *no* interface constraints

If you do not specify interface constraints, how MAAS attaches the VM to a network depends on how KVM was installed on the host.

If the host was deployed by MAAS for use as a KVM host (recommended), MAAS will skip the libvirt maas and default networks if they are not enabled for DHCP in MAAS, instead preferring a DHCP-enabled MAAS network. This means you don’t have to manually create a MAAS-friendly libvirt network to which to attach the VM as with previous versions of MAAS.

If you’ve instead installed KVM manually on your host machine after deploying via MAAS, MAAS will revert to its 2.4 behavior, namely trying to attach to a suitable maas or default libvirt network, enabling network booting if it detects MAAS-enabled DHCP on either.

Bridges

Macvlan

MAAS uses macvlan if an interfaces constraint specifies a macvlan interface when composing a VM.

You can configure the default macvlan mode of an existing KVM host using the CLI:

maas $PROFILE pod update <pod-id> host=<host> default_macvlan_mode=<mode>

Where:

  • pod-id: The pod’s MAAS ID
  • host: The IP or hostname of the libvirt instance
  • default_macvlan_mode:
    • private: VMs with the same parent interface cannot communicate but can reach destinations outside the parent interface
    • vepa: VM communication is forwarded through the parent interface and reflected back if the source and destination of the packet is local to the VMs. VMs can reach destinations outside the parent interface. Note that this requires a VEPA switching mechanism.
    • bridge: Inter-VM communication is maintained locally (behind the parent interface) and destinations outside are forwarded through the parent interface.
    • passthru: Allows a single VM to be connected directly to the parent interface, which in turn allows the VM to change MAC addresses and other interface parameters.

Bridge vs. macvlan

Unless you have a specific reason to use macvlan, a bridge is the better choice for most situations. Although macvlan is simpler in design than a bridge and therefore can offer better throughput and less demand on CPU, a bridge is typically easier to configure and more likely to result in successful communication.

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