flannel

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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.

Deploying flannel manually

Flannel can be added to any existing Kubernetes cluster though it's simplest to add flannel before any pods using the pod network have been started.

For Kubernetes v1.17+

Deploying Flannel with kubectl

kubectl apply -f https://github.com/flannel-io/flannel/releases/latest/download/kube-flannel.yml

If you use custom podCIDR (not 10.244.0.0/16) you first need to download the above manifest and modify the network to match your one.

Deploying Flannel with helm

# Needs manual creation of namespace to avoid helm error
kubectl create ns kube-flannel
kubectl label --overwrite ns kube-flannel pod-security.kubernetes.io/enforce=privileged

helm repo add flannel https://flannel-io.github.io/flannel/
helm install flannel --set podCidr="10.244.0.0/16" --namespace kube-flannel flannel/flannel

See Kubernetes for more details.

In case a firewall is configured ensure to enable the right port used by the configured backend.

Flannel uses portmap as CNI network plugin by default; when deploying Flannel ensure that the CNI Network plugins are installed in /opt/cni/bin the latest binaries can be downloaded with the following commands:

mkdir -p /opt/cni/bin
curl -O -L https://github.com/containernetworking/plugins/releases/download/v1.2.0/cni-plugins-linux-amd64-v1.2.0.tgz
tar -C /opt/cni/bin -xzf cni-plugins-linux-amd64-v1.2.0.tgz

Getting started on Docker

flannel is also widely used outside of kubernetes. When deployed outside of kubernetes, etcd is always used as the datastore. For more details integrating flannel with Docker see Running

Documentation

Contact

Slack:
- #k3s on Rancher Users Slack
- #flannel-users on Calico Users Slack
Planning/Roadmap: milestones, roadmap
Bugs: issues

Community Meeting

The Flannel Maintainer Community runs a meeting on every other Thursday at 8:30 AM PST. This meeting is used to discuss issues, open pull requests, and other topics related to Flannel should the need arise.

The meeting agenda and Teams link can be found here: Flannel Community Meeting Agenda