Network management

This is a design document detailing the implementation of network resource management in Ganeti.

Current state and shortcomings

Currently Ganeti supports two configuration modes for instance NICs: routed and bridged mode. The ip NIC parameter, which is mandatory for routed NICs and optional for bridged ones, holds the given NIC’s IP address and may be filled either manually, or via a DNS lookup for the instance’s hostname.

This approach presents some shortcomings:

  1. It relies on external systems to perform network resource management. Although large organizations may already have IP pool management software in place, this is not usually the case with stand-alone deployments. For smaller installations it makes sense to allocate a pool of IP addresses to Ganeti and let it transparently assign these IPs to instances as appropriate.

  2. The NIC network information is incomplete, lacking netmask and gateway. Operating system providers could for example use the complete network information to fully configure an instance’s network parameters upon its creation.

    Furthermore, having full network configuration information would enable Ganeti nodes to become more self-contained and be able to infer system configuration (e.g. /etc/network/interfaces content) from Ganeti configuration. This should make configuration of newly-added nodes a lot easier and less dependant on external tools/procedures.

  3. Instance placement must explicitly take network availability in different node groups into account; the same link is implicitly expected to connect to the same network across the whole cluster, which may not always be the case with large clusters with multiple node groups.

Proposed changes

In order to deal with the above shortcomings, we propose to extend Ganeti with high-level network management logic, which consists of a new NIC slot called network, a new Network configuration object (cluster level) and logic to perform IP address pool management, i.e. maintain a set of available and occupied IP addresses.

Configuration changes

We propose the introduction of a new high-level Network object, containing (at least) the following data:

  • Symbolic name
  • UUID
  • Network in CIDR notation (IPv4 + IPv6)
  • Default gateway, if one exists (IPv4 + IPv6)
  • IP pool management data (reservations)
  • Default NIC connectivity mode (bridged, routed). This is the functional equivalent of the current NIC mode.
  • Default host interface (e.g. br0). This is the functional equivalent of the current NIC link.
  • Tags

Each network will be connected to any number of node groups. During the connection of a network to a nodegroup, we define the corresponding connectivity mode (bridged or routed) and the host interface (br100 or routing_table_200). This is achieved by adding a networks slot to the NodeGroup object and using the networks’ UUIDs as keys. The value for each key is a dictionary containing the network’s mode and link (netparams). Every NIC assigned to the network will eventually inherit the network’s netparams, as its nicparams.

IP pool management

A new helper library is introduced, wrapping around Network objects to give IP pool management capabilities. A network’s pool is defined by two bitfields, the length of the network size each:

This field holds all IP addresses reserved by Ganeti instances.
external reservations
This field holds all IP addresses that are manually reserved by the administrator (external gateway, IPs of external servers, etc) or automatically by ganeti (the network/broadcast addresses, Cluster IPs (node addresses + cluster master)). These IPs are excluded from the IP pool and cannot be assigned automatically by ganeti to instances (via ip=pool).

The bitfields are implemented using the python-bitarray package for space efficiency and their binary value stored base64-encoded for JSON compatibility. This approach gives relatively compact representations even for large IPv4 networks (e.g. /20).

Cluster IP addresses (node + master IPs) are reserved automatically as external if the cluster’s data network itself is placed under pool management.

Helper ConfigWriter methods provide free IP address generation and reservation, using a TemporaryReservationManager.

It should be noted that IP pool management is performed only for IPv4 networks, as they are expected to be densely populated. IPv6 networks can use different approaches, e.g. sequential address asignment or EUI-64 addresses.

New NIC parameter: network

In order to be able to use the new network facility while maintaining compatibility with the current networking model, a new NIC parameter is introduced, called network to reflect the fact that the given NIC belongs to the given network and its configuration is managed by Ganeti itself. To keep backwards compatibility, existing code is executed if the network value is ‘none’ or omitted during NIC creation. If we want our NIC to be assigned to a network, then only the ip (optional) and the network parameters should be passed. Mode and link are inherited from the network-nodegroup mapping configuration (netparams). This provides the desired abstraction between the VM’s network and the node-specific underlying infrastructure.

We also introduce a new ip address value, constants.NIC_IP_POOL, that specifies that a given NIC’s IP address should be obtained using the first available IP address inside the pool of the specified network. (reservations OR external_reservations). This value is only valid for NICs belonging to a network. A NIC’s IP address can also be specified manually, as long as it is contained in the network the NIC is connected to. In case this IP is externally reserved, Ganeti will produce an error which the user can override if explicitly requested. Of course this IP will be reserved and will not be able to be assigned to another instance.


Introduce new hooks concerning network operations:


Add a network to Ganeti

pre-execution:master node
post-execution:master node

Remove a network from Ganeti

pre-execution:master node
post-execution:master node

Modify a network

pre-execution:master node
post-execution:master node

For connect/disconnect operations use existing:


Modify a nodegroup

pre-execution:master node
post-execution:master node
Hook variables

During instance related operations:

The friendly name of the network

During network related operations:

The friendly name of the network
The ip range of the network
The gateway of the network

During nodegroup related operations:

The friendly name of the network
The mode (bridged or routed) of the netparams
The link of the netparams

Backend changes

To keep the hypervisor-visible changes to a minimum, and maintain compatibility with the existing network configuration scripts, the instance’s hypervisor configuration will have host-level mode and link replaced by the connectivity mode and host interface (netparams) of the given network on the current node group.

Network configuration scripts detect if a NIC is assigned to a Network by the presence of the new environment variable:

Network configuration script variables
The friendly name of the network

Conflicting IPs

To ensure IP uniqueness inside a nodegroup, we introduce the term conflicting ips. Conflicting IPs occur: (a) when creating a networkless NIC with IP contained in a network already connected to the instance’s nodegroup (b) when connecting/disconnecting a network to/from a nodegroup and at the same time instances with IPs inside the network’s range still exist. Conflicting IPs produce prereq errors.

Handling of conflicting IP with –force option:

For case (a) reserve the IP and assign the NIC to the Network. For case (b) during connect same as (a), during disconnect release IP and reset NIC’s network parameter to None

Userland interface

A new client script is introduced, gnt-network, which handles network-related configuration in Ganeti.

Network addition/deletion
gnt-network add --network= --gateway= \
                --network6=2001:db8:2ffc::/64 --gateway6=2001:db8:2ffc::1 \
                --add-reserved-ips=, net100
 (Checks for already exising name and valid IP values)
gnt-network remove network_name
 (Checks if not connected to any nodegroup)
Network modification
gnt-network modify --gateway= net100
 (Changes the gateway only if ip is available)
gnt-network modify --add-reserved-ips= net100
 (Adds externally reserved ip)
gnt-network modify --remove-reserved-ips= net100
 (Removes externally reserved ip)
Assignment to node groups
gnt-network connect net100 nodegroup1 bridged br100
 (Checks for existing bridge among nodegroup)
gnt-network connect net100 nodegroup2 routed rt_table
 (Checks for conflicting IPs)
gnt-network disconnect net101 nodegroup1
 (Checks for conflicting IPs)
Network listing
gnt-network list

Network      Subnet           Gateway       NodeGroups GroupList
net100          1 default(bridged, br100)
net101          1 default(routed, rt_tab)
Network information
gnt-network info testnet1

Network name: testnet1
 size: 16
 free: 10 (62.50%)
 usage map:
       0 XXXXX..........X                                                 63
         (X) used    (.) free
 externally reserved IPs:,,
 connected to node groups:
   default(bridged, br100)
 used by 3 instances:
   test1 : 0:
   test2 : 0:
   test3 : 0:

IAllocator changes

The IAllocator protocol can be made network-aware, i.e. also consider network availability for node group selection. Networks, as well as future shared storage pools, can be seen as constraints used to rule out the placement on certain node groups.