Multus CNI is a Kubernetes networking project designed to enable attaching multiple network interfaces to pods. Unlike the standard Kubernetes CNI (Container Network Interface) which typically assigns a single interface per pod, Multus allows for the creation of pods with multiple network namespaces, each with its own IP address and network configuration. This is crucial for advanced networking scenarios like SR-IOV, DPDK, MACVLAN, and other specialized network attachments that require direct hardware access or isolation. It doesn't *replace* the default CNI; instead, it acts as a meta-plugin, delegating the actual network configuration to other CNI plugins.

The core problem Multus solves is the limitation of the standard Kubernetes networking model. Many applications, particularly those focused on high-performance computing, network functions virtualization (NFV), or security, require more than a single network connection. For example, an NFV application might need one interface for management, one for data plane traffic, and another for synchronization. SR-IOV, which allows virtual machines or containers to directly access a physical network interface, necessitates a dedicated interface per pod for optimal performance. Without Multus, achieving these configurations within Kubernetes is complex and often requires workarounds that compromise portability or performance.

Multus operates by creating a "secondary" network namespace for each additional network interface a pod requires. It then utilizes other CNI plugins to configure these secondary namespaces. The primary network namespace remains managed by the default CNI plugin configured in the Kubernetes cluster. This allows Kubernetes to maintain its core networking functionality while extending it to support more complex use cases. Configuration is primarily done through Kubernetes annotations on the pod specification. These annotations specify which CNI plugins should be used to configure each additional interface, along with any necessary configuration parameters for those plugins.

The repository contains the Multus CNI daemon, which is responsible for managing the secondary network namespaces and coordinating with the underlying CNI plugins. It also includes tools for debugging and troubleshooting network configurations. The project supports a wide range of CNI plugins, including those for SR-IOV (e.g., sriov-cni), MACVLAN, DPDK, and others. It's designed to be extensible, allowing new CNI plugins to be easily integrated. The repository also provides comprehensive documentation, examples, and a community forum for support.

Key components within the repository include the Multus daemon itself (written in Go), the configuration schema for pod annotations, and various example configurations for different CNI plugins and use cases. The project is actively maintained and regularly updated with new features and bug fixes. It's a crucial component for many Kubernetes deployments that require advanced networking capabilities beyond the scope of the standard CNI model, enabling a more flexible and powerful networking environment for containerized applications.