In this section, we will look at what it takes to actually write your own CNI plugin. First, we will look at the simplest plugin possible – the loopback plugin. Then, we will examine the plugin skeleton that implements most of the boilerplate associated with writing a CNI plugin. Finally, we will review the implementation of the bridge plugin. Before we dive in, here is a quick reminder of what a CNI plugin is:

• A CNI plugin is an executable
• It is responsible for connecting new containers to the network, assigning unique IP addresses to CNI containers, and taking care of routing
• A container is a network namespace (in Kubernetes, a pod is a CNI container)
• Network definitions are managed as JSON files, but stream to the plugin through standard input (no files are being read by the plugin)
• Auxiliary information can be provided via environment variables