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 mode called managed, a new “Network” configuration object 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, possibly overriding connectivity mode and host interface for each node group. This is achieved by adding a networks slot to the NodeGroup object and using the networks’ UUIDs as keys.

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, as well as cluster IP addresses (node addresses + cluster master)
external reservations
This field holds all IP addresses that are manually reserved by the administrator, because some other equipment is using them outside the scope of Ganeti.

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).

Ganeti-owned IP addresses (node + master IPs) are reserved automatically 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.

Managed NIC mode

In order to be able to use the new network facility while maintaining compatibility with the current networking model, a new network mode is introduced, called managed to reflect the fact that the given NICs network configuration is managed by Ganeti itself. A managed mode NIC accepts the network it is connected to in its link argument. Userspace tools can refer to networks using their symbolic names, however internally, the link argument stores the network’s UUID.

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 IP address pool of the specified network. This value is only valid for managed-mode NICs, where it is also used as a default instead of constants.VALUE_AUTO. A managed-mode NIC’s IP address can also be specified manually, as long as it is compatible with the network the NIC is connected to.



Add a network to Ganeti

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

Connect a network to a node group. This hook can be used to e.g. configure network interfaces on the group’s nodes.

pre-execution:master node, all nodes in the connected group
post-execution:master node, all nodes in the connected group

Disconnect a network to a node group. This hook can be used to e.g. deconfigure network interfaces on the group’s nodes.

pre-execution:master node, all nodes in the connected group
post-execution:master node, all nodes in the connected group

Remove a network from Ganeti

pre-execution:master node, all nodes
post-execution:master node, all nodes
Hook variables
Non-zero if NIC n is a managed-mode NIC
The friendly name of the network
The network’s UUID
The network’s tags
The subnet in CIDR notation
The subnet’s default gateway

Backend changes

In order 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 link and mode replaced by the connectivity mode and host interface of the given network on the current node group.

The managed mode can be detected by the presence of new environment variables in network configuration scripts:

Network configuration script variables
Non-zero if NIC is a managed-mode NIC
The friendly name of the network
The network’s UUID
The network’s tags
The subnet in CIDR notation
The subnet’s default gateway

Userland interface

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

Network addition/deletion
gnt-network add --cidr= --gateway= \
               --cidr6=2001:db8:2ffc::/64 --gateway6=2001:db8:2ffc::1 \
               --nic_connectivity=bridged --host_interface=br0 public
gnt-network remove public (only allowed if no instances are using the network)
Manual IP address reservation
gnt-network reserve-ips public
gnt-network release-ips public
Network modification
gnt-network modify --cidr= public (only allowed if all current reservations fit in the new network)
gnt-network modify --gateway= public
gnt-network modify --host_interface=test --nic_connectivity=routed public (issues warning about instances that need to be rebooted)
gnt-network rename public public2
Assignment to node groups
gnt-network connect public nodegroup1
gnt-network connect --host_interface=br1 public nodegroup2
gnt-network disconnect public nodegroup1 (only permitted if no instances are currently using this network in the group)
gnt-network add-tags public foo bar:baz
Network listing
gnt-network list
 Name          IPv4 Network    IPv4 Gateway          IPv6 Network             IPv6 Gateway             Connected to
 public       2001:db8:dead:beef::/64    2001:db8:dead:beef::1       nodegroup1:br0
 private       -                     -                              -
Network information
gnt-network info public
 Name: public
 IPv4 Network:
 IPv4 Gateway:
 IPv6 Network: 2001:db8:dead:beef::/64
 IPv6 Gateway: 2001:db8:dead:beef::1
 Total IPv4 count: 256
 Free address count: 201 (80% free)
 IPv4 pool status: XXX.........XXXXXXXXXXXXXX...XX.............
                                           X: occupied  .: free
 Externally reserved IPv4 addresses:,
 Connected to node groups:
  default (link br0), other_group(link br1)
 Used by 22 instances:

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.