Flannel

Flannel is an open-source virtual network fabric for Kubernetes clusters that provides container networking using various backend data plane mechanisms (container networking).

• Implements an overlay or layer 3 network for pods across Kubernetes nodes (container networking)
• Supports multiple backend mechanisms such as Virtual Extensible LAN (VXLAN), host-gw, and others for encapsulation or routing (network transport)
• Allocates subnets to cluster nodes and manages IP address assignment for pod networks (IP address management)
• Integrates with Kubernetes via the Container Network Interface (CNI) to provide pod network connectivity (Kubernetes networking)
• Offers configuration via a central etcd or Kubernetes Application Programming Interface (API) storage for network state and configuration (configuration management)

More About Flannel

Flannel is a virtual networking project focused on providing a simple layer 3 network fabric for containers in Kubernetes clusters (container networking). It addresses the problem of delivering IP connectivity between pods running on different nodes, enabling them to communicate using a unified, cluster-wide IP space. Flannel allocates a subnet lease to each node and ensures that packets destined for pod Intrusion Prevention System (IPS) on remote nodes are delivered through an overlay or routed network.

The project supports multiple backends that define how traffic is transported between nodes (network transport). Common backends described in the official materials include VXLAN, which encapsulates traffic in User Datagram Protocol (UDP) packets, and host-gw, which programs host routing tables to forward traffic directly between nodes. These backends allow operators to choose approaches that align with their infrastructure constraints, such as overlay tunneling or direct routing. Flannel maintains a mapping of node subnets and uses that data to configure the necessary routing or encapsulation rules.

Flannel integrates with Kubernetes through the CNI plugin model (Kubernetes networking). It typically runs a flanneld agent on each node, which communicates with a central data store such as etcd or the Kubernetes API server to obtain network configuration and coordinate subnet assignments (configuration management). This design allows the cluster to maintain a consistent view of the pod network and supports automated provisioning of networking for newly added nodes.

In enterprise environments, Flannel is used as a cluster networking option in Kubernetes distributions and installations, including those provided by Red Hat, to establish basic pod-to-pod connectivity across nodes (enterprise infrastructure). It is often deployed where a straightforward, IP-per-pod overlay or routed network is sufficient and where compatibility with existing infrastructure is desirable. Administrators can configure network ranges, backend types, and MTU settings to align with data center networks or cloud provider environments.

Flannel operates alongside Kubernetes services, ingress controllers, and other networking layers without replacing them (network integration). It focuses on the pod network underlay or overlay, while higher-level constructs such as services or load balancers use the connectivity it provides. The project fits into directories and taxonomies under container networking, Kubernetes networking, and virtual network overlay technologies, as it offers an implementation of an intra-cluster network fabric based on IP routing and encapsulation.