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Network Plugins

FEATURE STATE: Kubernetes v1.11 alpha
This feature is currently in a alpha state, meaning:

  • The version names contain alpha (e.g. v1alpha1).
  • Might be buggy. Enabling the feature may expose bugs. Disabled by default.
  • Support for feature may be dropped at any time without notice.
  • The API may change in incompatible ways in a later software release without notice.
  • Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.
Alpha features change rapidly.

Network plugins in Kubernetes come in a few flavors:


The kubelet has a single default network plugin, and a default network common to the entire cluster. It probes for plugins when it starts up, remembers what it found, and executes the selected plugin at appropriate times in the pod lifecycle (this is only true for Docker, as rkt manages its own CNI plugins). There are two Kubelet command line parameters to keep in mind when using plugins:

Network Plugin Requirements

Besides providing the NetworkPlugin interface to configure and clean up pod networking, the plugin may also need specific support for kube-proxy. The iptables proxy obviously depends on iptables, and the plugin may need to ensure that container traffic is made available to iptables. For example, if the plugin connects containers to a Linux bridge, the plugin must set the net/bridge/bridge-nf-call-iptables sysctl to 1 to ensure that the iptables proxy functions correctly. If the plugin does not use a Linux bridge (but instead something like Open vSwitch or some other mechanism) it should ensure container traffic is appropriately routed for the proxy.

By default if no kubelet network plugin is specified, the noop plugin is used, which sets net/bridge/bridge-nf-call-iptables=1 to ensure simple configurations (like Docker with a bridge) work correctly with the iptables proxy.


The CNI plugin is selected by passing Kubelet the --network-plugin=cni command-line option. Kubelet reads a file from --cni-conf-dir (default /etc/cni/net.d) and uses the CNI configuration from that file to set up each pod’s network. The CNI configuration file must match the CNI specification, and any required CNI plugins referenced by the configuration must be present in --cni-bin-dir (default /opt/cni/bin).

If there are multiple CNI configuration files in the directory, the first one in lexicographic order of file name is used.

In addition to the CNI plugin specified by the configuration file, Kubernetes requires the standard CNI lo plugin, at minimum version 0.2.0

Limitation: Due to #31307, HostPort won’t work with CNI networking plugin at the moment. That means all hostPort attribute in pod would be simply ignored.


Kubenet is a very basic, simple network plugin, on Linux only. It does not, of itself, implement more advanced features like cross-node networking or network policy. It is typically used together with a cloud provider that sets up routing rules for communication between nodes, or in single-node environments.

Kubenet creates a Linux bridge named cbr0 and creates a veth pair for each pod with the host end of each pair connected to cbr0. The pod end of the pair is assigned an IP address allocated from a range assigned to the node either through configuration or by the controller-manager. cbr0 is assigned an MTU matching the smallest MTU of an enabled normal interface on the host.

The plugin requires a few things:

Customizing the MTU (with kubenet)

The MTU should always be configured correctly to get the best networking performance. Network plugins will usually try to infer a sensible MTU, but sometimes the logic will not result in an optimal MTU. For example, if the Docker bridge or another interface has a small MTU, kubenet will currently select that MTU. Or if you are using IPSEC encapsulation, the MTU must be reduced, and this calculation is out-of-scope for most network plugins.

Where needed, you can specify the MTU explicitly with the network-plugin-mtu kubelet option. For example, on AWS the eth0 MTU is typically 9001, so you might specify --network-plugin-mtu=9001. If you’re using IPSEC you might reduce it to allow for encapsulation overhead e.g. --network-plugin-mtu=8873.

This option is provided to the network-plugin; currently only kubenet supports network-plugin-mtu.

Usage Summary


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