Flannel (software)

Flannel (software)
Name	Flannel
Developer	CoreOS
Released	2016
Programming language	Go
Repository	etcd, Kubernetes
License	Apache License 2.0

Flannel (software) Flannel is an open-source virtual network fabric designed to provide a layer 3 network between containers across multiple hosts, integrating tightly with Kubernetes, Docker, CoreOS, etcd, and cloud providers. It was created to simplify container networking for cluster orchestration systems and has influenced networking solutions used by Red Hat, Canonical, Google, Amazon Web Services, and Microsoft Azure. The project is implemented in Go (programming language) and is distributed under the Apache License.

Overview

Flannel implements an overlay network that assigns each host a dedicated subnet and routes container traffic across a fabric coordinated by etcd, Consul, Zookeeper, and cloud metadata services; it targets container runtimes such as containerd, CRI-O, Docker Engine, and orchestration systems like Kubernetes and Mesos. Designed by CoreOS engineers and used in early Kubernetes deployments at Red Hat and Amazon Web Services, Flannel focuses on simplicity and interoperability with projects including Weave Net, Calico (software), Cilium, Istio, Linkerd, and Envoy (software). The project often appears alongside cluster tooling from HashiCorp and storage platforms such as Ceph, Rook, and GlusterFS in production stacks.

Architecture

Flannel's architecture centers on a small agent that runs on each host and a key-value store that distributes network allocations; the agent interacts with Linux kernel features, iptables, and virtual network devices like VXLAN, VLAN, and IPIP. The control plane uses etcd to coordinate per-host subnet leases and provides integration points for cloud APIs from Google Cloud Platform, Amazon EC2, Microsoft Azure, and DigitalOcean. Components include the flanneld agent, backend drivers, and integration logic for container runtimes such as runc and orchestration controllers in Kubernetes API. Flannel's use of overlays places it in a design lineage with projects such as Open vSwitch, Linux Foundation, and Network Namespace (Linux) research efforts.

Installation and Configuration

Flannel is deployed as a daemonset or system service and is commonly installed via manifests for Kubernetes, packages for Ubuntu, CentOS, and container images hosted by registries used by Docker Hub and Quay.io. Configuration typically requires specifying a network CIDR, backend type, and a key-value store endpoint—options that reference services like etcd, Consul, or cloud metadata endpoints for AWS and GCP. Operators use configuration management tools from Ansible, Puppet, Chef, and Terraform to provision flanneld alongside cluster provisioning tools from kubeadm, Kops, Rancher, and OpenShift. Integration considerations include kernel module availability, MTU tuning for overlays, and compatibility testing with Calico policy enforcement or Cilium eBPF datapaths.

Networking Modes and Backends

Flannel supports multiple backends, including overlay backends like VXLAN and Host-gw, encapsulation backends such as IPIP and simple host routing modes; these backends determine encapsulation, broadcast behavior, and performance characteristics when used with Linux bridge or Open vSwitch. Many operators choose VXLAN for cloud portability, Host-gw for minimal encapsulation in flat networks, or IPIP for compatibility with legacy routers used by Cisco Systems and Juniper Networks deployments. Backends integrate with cloud networking constructs such as VPC, Subnet (networking), and provider-specific routing tables in Amazon VPC and Azure Virtual Network; selection often depends on constraints imposed by firewalld and iptables policies common in enterprise environments.

Security and Performance Considerations

Security models for Flannel depend on backend choice and key-value store hardening: operators secure etcd with TLS certificates issued by authorities like Let's Encrypt or enterprise PKIs managed by HashiCorp Vault and restrict access with role-based controls from Kubernetes RBAC. Overlay modes incur encapsulation overhead affecting packet MTU and throughput, so performance tuning typically involves adjusting MTU, avoiding double encapsulation with IPsec or WireGuard (VPN), and measuring latency with tools such as iperf3 and netperf. Network policy and microsegmentation are often delegated to projects like Calico (software), Cilium, or Kubernetes NetworkPolicy rather than Flannel itself, so secure multi-tenant deployments pair Flannel with policy engines and observability stacks including Prometheus, Grafana, Jaeger, and ELK Stack.

Adoption and Use Cases

Flannel is widely used in small to medium Kubernetes clusters, development environments, and cloud-native stacks where simplicity and predictable host-to-host subnetting are priorities; organizations deploying OpenShift Origin, k3s, Rancher Kubernetes Engine, and custom on-premises Kubernetes clusters have adopted Flannel. It is common in hybrid setups bridging on-premises data centers with AWS Outposts, Google Anthos, and edge computing projects managed by EdgeX Foundry and OpenStack distributions. While enterprises seeking high-performance, policy-rich networking may select Calico, Cilium, or Weave Net, Flannel remains a pragmatic choice for teams using tooling from CoreOS, Red Hat, Canonical, and cloud vendor managed Kubernetes offerings.

Category:Container networking