flannel

flannel

Flannel is a simple and easy way to configure a layer 3 network fabric designed for Kubernetes.

How it works

Flannel runs a small, single binary agent called flanneld on each host, and is responsible for allocating a subnet lease to each host out of a larger, preconfigured address space. Flannel uses either the Kubernetes API or etcd directly to store the network configuration, the allocated subnets, and any auxiliary data (such as the host's public IP). Packets are forwarded using one of several backend mechanisms including VXLAN and various cloud integrations.

Networking details

Platforms like Kubernetes assume that each container (pod) has a unique, routable IP inside the cluster. The advantage of this model is that it removes the port mapping complexities that come from sharing a single host IP.

Flannel is responsible for providing a layer 3 IPv4 network between multiple nodes in a cluster. Flannel does not control how containers are networked to the host, only how the traffic is transported between hosts. However, flannel does provide a CNI plugin for Kubernetes and a guidance on integrating with Docker.

Flannel is focused on networking. For network policy, other projects such as Calico can be used.

Getting started on Kubernetes

The easiest way to deploy flannel with Kubernetes is to use one of several deployment tools and distributions that network clusters with flannel by default. For example, K3s sets up flannel in the Kubernetes clusters it creates using the open source K3s Installer to drive the setup process.

Though not required, it's recommended that flannel uses the Kubernetes API as its backing store which avoids the need to deploy a discrete etcd cluster for flannel. This flannel mode is known as the kube subnet manager.