The Container Network Interface (CNI) is a specification that lets a container runtime call network plugins to connect a container to a network and release its networking resources when the container is removed. In Kubernetes, a CNI-compatible cluster network plugin provides pod connectivity; CNI itself is not one particular network or plugin.
What CNI means
The Container Network Interface defines the contract between a container runtime and network plugins. The CNI project supplies the specification, libraries and reference plugins, but CNI is not a single networking product.
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In practice, the runtime passes a plugin JSON configuration and invocation parameters. The plugin performs the requested networking operation and returns a result or an error. The specification defines operations including ADD to set up networking, DEL to remove it, CHECK to check configuration, STATUS for status, VERSION for version negotiation and GC for stale-resource cleanup.
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Kubernetes expects a cluster network plugin to implement its network model. Its documentation requires CNI v0.4.0 or later compatibility and recommends v1.0.0 compatibility. The specification page identifies CNI v1.1.0 as the current specification version; that number is distinct from a plugin or library release. Check the compatibility documentation for the specific plugin and Kubernetes release you plan to use.
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The container runtime, rather than the Kubernetes kubelet, is responsible for loading the required plugins. Kubernetes 1.24 removed the kubelet’s former cni-bin-dir and network-plugin command-line parameters, so older setup guides may describe settings that no longer apply. Follow the current installation instructions for both the runtime and network provider.
Kubernetes also requires a loopback interface in each sandbox. A runtime can use the CNI loopback plugin or provide equivalent behavior. For hostPort, the cluster needs port-mapping support, such as the official CNI portmap plugin or an equivalent plugin capability.
How to choose a CNI implementation
There is no universal best CNI established by the project documentation. Choose against the cluster’s Kubernetes version, runtime, operating system, kernel, cloud environment and workload needs; verify version-specific support before installation.
Connectivity and routing model
Determine whether the implementation uses an overlay or underlay, how it routes or encapsulates traffic, and how it allocates and routes pod addresses. For example, Flannel allocates subnet leases per host and offers forwarding backends including VXLAN. These choices affect the required network configuration and operational troubleshooting.
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Network policy and security
Check whether the selected implementation enforces the network policies you need or requires a separate controller or chained plugin. Flannel’s project documentation says its daemon does not natively enforce Kubernetes NetworkPolicies; it describes using a policy controller or chaining another project for enforcement.
Multiple interfaces and specialized workloads
Most workloads need one pod network, but some require additional attachments or specialized interfaces. Kubernetes describes Multus as supporting multiple network attachments, including integrations for technologies such as SR-IOV, DPDK, OVS-DPDK and VPP. Confirm that the chosen setup supports the hardware, runtime and workload configuration involved.
Compatibility and ongoing operations
Review the provider’s support matrix for your exact Kubernetes version and cloud environment; support can vary by release. Also assess installation and upgrade procedures, IP address capacity, observability, support arrangements and the networking skills needed to operate the system. Project documentation describes capabilities, not independent performance rankings, so do not treat a feature list as a speed comparison.
Troubleshooting CNI networking
Pod networking failures can originate in the runtime, plugin configuration, host networking or the underlying network. Work through these checks in order, and use the documentation for your plugin version and configured backend.
- Confirm runtime configuration. Verify that the runtime has the intended CNI binaries and configuration. Inspect runtime and plugin logs for invocation errors, missing files or failed operations.
- Check pod address ranges. Confirm that nodes have pod CIDRs assigned and that the ranges do not overlap. Flannel’s troubleshooting guide describes inspecting node
podCIDRvalues and checking for overlapping subnet ranges. - Check host permissions and networking support. Confirm that the plugin has the permissions it needs and that required host or kernel networking features are available. Flannel’s documentation discusses permission-related route, VXLAN and masquerading failures and identifies
br_netfilteras a requirement in its project README. - Verify backend firewall rules. Allow the traffic required by the configured backend, based on its current settings. Flannel’s troubleshooting documentation lists UDP 8285 for its UDP backend and UDP 8472 for VXLAN; do not apply these ports blindly to a different backend or configuration.
- Check MTU end to end. Compare the MTU on the physical or underlay interface, across any encapsulation path, and on the pod virtual Ethernet device. Tunnels add overhead, so a mismatch can prevent traffic from passing as expected.
- Investigate host and control-plane health. If new hosts are slow to become reachable, check the control plane and backing datastore or API health as well as CPU and memory availability.
What CNI does not tell you
CNI compatibility alone does not establish that a plugin supports every runtime, Kubernetes release, cloud provider, policy feature or specialized workload. Nor does the specification rank plugins by latency or throughput. Validate compatibility and capabilities for the exact versions and environment you will operate.
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