Mastering Kubernetes - Second Edition

Flannel

Flannel is a virtual network that gives a subnet to each host for use with container runtimes. It runs a flaneld agent on each host, which allocates a subnet to the node from a reserved address space stored in etcd. Forwarding packets between containers and, ultimately, hosts is done by one of multiple backends. The most common backend uses UDP over a TUN device that tunnels through port 8285 by default (make sure it's open in your firewall).

The following diagram describes in detail the various components of Flannel, the virtual network devices it creates, and how they interact with the host and the pod through the docker0 bridge. It also shows the UDP encapsulation of packets and how they are transmitted between hosts:

Other backends ...