How Flannel Manages Subnet Allocation in Kubernetes Docker Clusters

Docker is a representative container platform, and managing Docker clusters can be challenging. This article discusses how the Flannel component in Kubernetes clusters works together with etcd to allocate subnet ranges, helping ensure each Docker container receives a unique IP address.

Flannel and etcd Interaction

When installing a Kubernetes cluster, etcd is configured with a network CIDR that defines the IP address range available to containers. Flannel uses this CIDR to divide a secondary subnet for each minion node, ensuring unique IPs across the cluster.

Flanneld Configuration

The flanneld configuration file typically includes the etcd endpoint and the key where network information is stored:

# Flanneld configuration options
FLANNEL_ETCD="http://192.168.163.152:2379"
FLANNEL_ETCD_KEY="/coreos.com/network"
#FLANNEL_OPTIONS=""

FLANNEL_ETCD points to the etcd server address, allowing flanneld on each minion to communicate with etcd. FLANNEL_ETCD_KEY specifies the key that holds the network configuration, e.g.:

etcdctl -C 192.168.163.148:2379 set /coreos.com/network/config '{ "Network": "192.168.0.0/16" }'

These settings define the IP range that Docker on each minion will use. The Docker daemon receives a --bip parameter derived from Flannel, effectively assigning a unique subnet to each node.

Verification

After configuring Flannel, you can verify the connection status between a minion and etcd, and inspect Docker’s virtual network interface (docker0) to ensure its subnet matches the range defined by FLANNEL_ETCD_KEY. If the docker0 subnet does not fall within the Flannel-assigned range, Docker may fail to start and the node will appear “NotReady” in Kubernetes.

Overall, Flannel, provided by CoreOS for Kubernetes, communicates with etcd to obtain the cluster-wide network CIDR, then performs a secondary subnet allocation for each minion, guaranteeing unique IP addresses for all Docker containers in the cluster.

Flannel connection status on a minion node.

Docker startup parameters on minion1.

Docker startup parameters on minion2.

Network interface information on minion2, showing the Docker virtual network card.